Body Armor Impact Map System

WORCESTER POLYTECHNIC INSTITUTE

The research, design, and development of a ballistic impact detection, mapping, and biotelemetry system for body armor.

PATENT PENDING U.S. PATENT APPLICATION 62/306,295 Filed March 10, 2016

A Major Qualifying Project Report Completed in Partial Fulfillment of Requirements for the Bachelor of Science Degree in: Electrical and Computer Engineering, Mechanical Engineering, and Computer Science at Worcester Polytechnic Institute, Worcester, MA

Report Submitted by Authors & Inventors: Carolyn Keyes, Mechanical Engineering Nicholas Potvin, Electrical & Computer Engineering Zachary Richards, Computer Science Christopher Tolisano, Mechanical Engineering

April 27, 2016

Report Submitted to the Faculty and Advisors: Professor William Michalson, Electrical & Computer Engineering Professor John Sullivan, Mechanical Engineering

This report represents work of WPI undergraduate students submitted to the faculty as evidence of a degree requirement. WPI routinely publishes these reports on its website without editorial or peer review. For more information about the projects program at WPI, see http://www.wpi.edu/Academics/Projects. Table of Contents Abstract ...... 6 Acknowledgements ...... 7 1. Introduction ...... 8 2. Background ...... 10 2.1 Overview of Current Body Armor ...... 10 2.2 Law Enforcement and Military Body Armor Statistics ...... 11 2.3 Prior Art ...... 12 2.3.1 Patent Application: Bullet-Proof Vest Bullet Hit Detection System ...... 13 2.3.2 Patent: Bullet-Proof Vest with Distress Signaling System ...... 14 2.4 Physiological Effects of Behind Armor Blunt Trauma ...... 15 2.5 Triage of Gunshot Victims ...... 17 3. Methodology ...... 18 3.1 Systems Level Design ...... 18 3.2 Materials ...... 23 3.2.1 Piezoelectric Materials ...... 23 3.2.2 Conductive Fabric ...... 24 3.2.3 Mesh Interlayer ...... 26 3.3 Monitoring Subsystem Design & Architecture ...... 27 3.3.1 Analog to Digital Converter (ADC) ...... 27 3.3.2 Field-Programmable Gate Array (FPGA) ...... 29 3.3.2.1 Control Logic & Memory Architecture ...... 29 3.3.2.2 Clock Dividers ...... 30 3.3.2.3 Circular Memory Buffer ...... 31 3.3.2.4 Detecting a Threshold Exceedance ...... 34 3.3.2.5 Circuit Behavior upon Threshold Exceedance ...... 35 3.3.4 Wireless Data Transmission ...... 36 3.3.5 Constructing a Matrix from Strip Values ...... 36 3.3.6 Android Smartphone Application ...... 37 3.3.7 User Monitoring and Data Aggregation Subsystem ...... 38 3.3.8 Power Specifications and Analysis ...... 39 4. Results ...... 40 4.1 LabVIEW High Speed Data Collection ...... 40 4.2 Construction of Sensor Grid ...... 40 4.3 Small-Scale Sensor Testing ...... 42 4.4 Velostat Testing Without Applied Voltage ...... 46 4.5 Drop Tests ...... 48 4.5.1 Drop Test Set-up ...... 48 4.5.2 Drop Test: Modeled .22 LR ...... 51 4.5.3 Drop Test: Modeled 9x19 ...... 55 4.5.4 Drop Test: Modeled .357 Magnum ...... 58

4.6 First Round of Live-Fire Testing ...... 61 4.6.1 Set up and Plan ...... 62 4.6.2 Slope Threshold Validation ...... 62 4.6.3 Results ...... 63 4.6.4 Strip Construction Performance ...... 68 4.7 Worcester Police Live-Fire Testing ...... 68 4.8 Third Round Live-Fire Testing ...... 72 4.9 Voltage Threshold Exceedance Calculation ...... 77 5.0 Future Development ...... 80 6.0 Appendix ...... 81 6.1 Impact Drop Test Calculations ...... 81 6.2 First Round Live-Fire Test Plan...... 82 6.3 Worcester PD Live-Fire Test Plan ...... 83 6.4 Third Round Live-Fire Test Plan ...... 85 References ...... 87

Table of Figures Figure 1: Ballistic Vest Types4 ...... 10 Figure 2: Ballistic Armor Plates7 ...... 11 Figure 3: Patent Application Vest Diagram17 ...... 13 Figure 4: Patent Vest Diagram ...... 15 Figure 5: System Block Diagram ...... 19 Figure 6: Circuit Block Breakdown ...... 19 Figure 7: Systems View of Impact Detection Grid ...... 20 Figure 8: Side View of Individual Sensor Strip ...... 21 Figure 9: Voltage Divider ...... 22 Figure 10: Piezoelectric Effect in Quartz ...... 23 Figure 11: 3M Velostat ...... 24 Figure 12: Conductive RipStop Silver Fabric ...... 25 Figure 13: Cost of different fabrics ...... 25 Figure 14: pure copper polyester taffeta fabric ...... 26 Figure 15: Various tested mesh interlayers ...... 27 Figure 16: Analog voltage to digital conversion ...... 28 Figure 17: PCB Layout ...... 29 Figure 18: High-Level Systems Architecture (App not shown) ...... 30 Figure 19: Control logic and memory block diagram. (Sensor strips omitted) ...... 30 Figure 20: Synchronized Clock Signals ...... 31 Figure 21: Memory behavior during read operation ...... 32 Figure 22: Circular Memory Buffer ...... 32 Figure 23: Verilog for threshold calculation ...... 34 Figure 24: 4x1 register and threshold comparator ...... 35 Figure 25: Sample vest matrix showing computation ...... 36 Figure 26: Voltage square computation equation ...... 36 Figure 27: Conceptual App Design ...... 38 Figure 28: System power supply breakdown ...... 39 Figure 29: Vertical Sensor Grid ...... 41 Figure 30: Sensor structure with HDPE mesh ...... 41 Figure 31: Static Velostat voltage tests ...... 42 Figure 32: Static tests comparing the effect of multiple layers of Velostat ...... 43 Figure 33: Drop of small weight on test strip. One layer of Velostat ...... 44 Figure 34: Zoomed image of initial peak from Figure 31 ...... 44 Figure 35: Drop of small weight on test strip with ballistics gel ...... 45 Figure 36: Zoomed image of peak from Figure 33 ...... 46 Figure 37: Voltage of Velostat only. Single impact...... 47 Figure 38: Impact of Velostat only - 2 Layers ...... 47 Figure 39: CAD model of rig base ...... 49 Figure 40: Apparatus Base ...... 50 4 ©2016 C. Keyes, N. Potvin, Z. Richards, C. Tolisano. All rights reserved.

Figure 41: Apparatus with Cable Mount Surface and Weight ...... 50 Figure 42: Weight with Steel Rod...... 51 Figure 43: Weight with steel rod...... 52 Figure 44: Impact graph of .22 LR with 1 layer of Velostat ...... 52 Figure 45: Slope of initial impact of .22 LR with 1 layer of Velostat ...... 53 Figure 46: Impact graph of .22 LR with 4 layers of Velostat ...... 54 Figure 47: Slope of initial impact of .22 LR with 4 layers of Velostat ...... 54 Figure 48: Damage to ballistics gel after .22 LR impact ...... 55 Figure 49: Impact graph of 9x19 with 1 layer of Velostat ...... 56 Figure 50: Slope of initial impact of 9x19 with 1 layer of Velostat ...... 56 Figure 51: Impact graph of 9x19 with 4 layers of Velostat ...... 57 Figure 52: Slope of initial impact of 9x19 with 4 layers of Velostat ...... 57 Figure 53: Damage to ballistics gel after 9x19 impact ...... 58 Figure 54: Impact graph of .357 Magnum with 4 layers of Velostat ...... 58 Figure 55: Slope of initial impact of .357 with 4 layers of Velostat ...... 59 Figure 56: Impact graph of .357 Magnum with 2 layers of Velostat ...... 60 Figure 57: Slope of initial impact of .357 with 2 layers of Velostat ...... 60 Figure 58: Damage to ballistics gel after .357 Magnum impact ...... 61 Figure 59: Damage to sensor strip after 0.357 Magnum impact ...... 61 Figure 60: 9x19 Live-fire initial peaks ...... 63 Figure 61: Equation determining sampling period ...... 63 Figure 62: 9x19 Impact slope ...... 64 Figure 63: 9x19 Graph impact and slope combined ...... 65 Figure 64: Bullet impact comparison - Measured Voltage ...... 66 Figure 65: Sensor strip after .44 Mag impact ...... 66 Figure 66: .44 Mag round extracted from vest ...... 67 Figure 67: .44 Mag extracted from vest - front ...... 67 Figure 68: Impact slope comparison ...... 68 Figure 69: Peak slope of tested calibers ...... 68 Figure 72: 9x19 Impact - High resting voltage ...... 70 Figure 73: 9x19 Impact ...... 71 Figure 74: 9x19 Impact ...... 72 Figure 75: .22 LR Impacts Across Sensors ...... 73 Figure 76: 9x19 Impacts Across Sensors ...... 74 Figure 77: .45 ACP Impacts Across Sensors ...... 74 Figure 78: Various Ballistic Impacts on Sensor Three ...... 75 Figure 80: Impact of Punch on Sensor ...... 76 Figure 81: Kevlar insert (yellow) penetrated through shirt and sensor ...... 77 Figure 82: Threshold exceedance equation ...... 77 Figure 83: Impact measured voltage and slope ...... 78 Figure 84: Drop test slope vs. live-fire slope ...... 79 5 ©2016 C. Keyes, N. Potvin, Z. Richards, C. Tolisano. All rights reserved.

Abstract Bullet-resistant vests have saved the lives of many law enforcement officers, however, after being shot; wearers are commonly left with life-threatening injuries. Therefore, users are sometimes unable to call for help, delaying the response time for needed assistance. Reducing the time between the impact and the arrival of medical care is critical to the user’s survival. To assist in ensuring a timely response, a system capable of detecting a ballistic impact, determining its location, and calculating the likely injury sustained by the wearer was created. The system utilizes a custom designed network of sensors and accompanying circuitry. The circuit transmits the collected information of the impact to an application installed on the user's smartphone via Bluetooth. The impact data is then sent along with the GPS location of the smartphone to a monitoring entity, providing an instant medical alert, as well as warning other officers of a potentially dangerous situation.

Acknowledgements We would like to thank the people that played an integral role in taking this project from an initial idea to a prototype. Our advisors, Professor William Michalson and Professor John Sullivan, played an important role in helping us to design our experimentation, but also to help us troubleshoot various issues as they arose throughout the project. Their expertise in experimentation design and circuit design proved to be incredibly valuable in the development of this project. In addition to our advisors, various other groups helped make our project possible.

Worcester Police Department This project could not have been made possible without the help we received from the Worcester Police Department. Through establishing an early relationship with the WPD through Deputy Chief Mark Roche, we were able to receive valuable insight on the conception of this project, and what is valued by police officers in terms of bullet resistant vest technology. Through our interviews with Deputy Chief Sean Fleming and Sergeant Dan George, we were able to establish some basic product requirements and specifications. The Worcester Police also donated vests for us to use. This allowed us to test the sensor and circuit on four separate occasions to validate our system. The project would not have been feasible financially without these vests, but also the invaluable feedback we received from the officers. We very grateful for all the efforts the WPD made for the benefit of this project.

Philip Camp, MD We spoke with Dr. Philip Camp, of Brigham & Women’s Hospital to seek advice of the necessary aspects of a ballistic impact detection system from a first responder’s perspective. We learned which vital signs are important to first responders and also that a Body Armor Impact Mapping System is valuable, especially to medics in the military. Dr. Camp highlighted the difficulty experienced by medics in taking vital signs in dangerous and unstable conditions and that remote triage capabilities would be a valuable aspect of a complete system. He also noted that being able to immediately know the location of a wounded individual would allow medics to effectively and efficiently allocate their limited resources.

1. Introduction The use of bullet-resistant vests, both in law enforcement and in the military has become more prevalent in recent years. Studies have shown an increase in the number of agencies that require their officers to wear bullet-resistant vests1. Similarly, United States troops serving in Operation Iraqi Freedom and Operation Enduring Freedom have reaped the life-saving benefits of increasingly effective body armor2. Although the use of bullet-resistant vests has saved the lives of both law enforcement officers and military personnel, wearers are commonly left with injuries, many life-threatening or fatal. The purpose of bullet-resistant vests, along with other types of body armor, is to prevent the penetration of a projectile by distributing the force that it exerts on the wearer across a wide area. However, current vests provide no support to the wearer after the shot. When someone is shot while wearing a bullet-resistant vest, they often require immediate medical attention even though the round may not have penetrated the vest. However, first responders might not know that the wearer needs assistance, and might not know their location, especially if the individual is incapacitated or otherwise unable to call for help. In order to make bullet-resistant vests more effective, there exists a need for a bullet detection and alert system. This system would have the ability to determine that the vest had experienced an impact, and notify necessary parties. In the law enforcement arena, many officers do not have their location continuously monitored. In the event of an incident, the officer would need to provide their location to receive help. A bullet detection system would automatically send the officer’s location along with the information about the shooting to dispatch to call for help. This technology would work in a similar manner in a military application, with data being sent upon activation of the vest to a medic. The sensor would identify the relative force of the impact on the wearer, along with the location of impact. The system will have the ability to read and send important vital signs of the wearer, such as heart rate, heart rhythm, and oxygen saturation. If the thoracic cavity sustained a sufficient level of blunt force trauma, the system will then send this data, along with the GPS

location of the wearer to the appropriate response team. This additional information would enable responders to provide more timely medical attention to the wounded individuals.

2. Background Technical advancements in recent years have led to improved effectiveness of bullet- resistant vests. Different types of vests are capable of stopping various calibers of ammunition. Some vests contain rigid plates made of either ceramic or steel, while others are made of soft materials (such as Kevlar). Police officer deaths are currently garnering attention in the public eye and to date there have been patent applications filed for vests that contain monitoring equipment. However, the prior art implements only certain aspects of our design.

2.1 Overview of Current Body Armor There are many different types of ballistic vests available for different applications. For those who wear bullet-resistant vests, breathability, comfort, flexibility, and strength are factors in determining which types of vests to purchase3. The five categories of vests, listed in increasing order of protection provided are: IIA, II, IIIA, III, and IV. Figure 1 below shows the vest categories and the most powerful calibers they are designed to withstand.

Vest Type Firearm Category Caliber Bullet Weight Bullet (grams) Speed

IIA Handgun 9x19, .40 S&W 8.0 g 373 m/s

II Handgun 9x19, .357 Magnum 8.0 g 379 m/s

IIIA Handgun .357 SIG, .44 Magnum 8.1 g 448 m/s

III Rifle 7.62x51 FMJ 9.6 g 847 m/s

IV Armor Piercing .30-06 AP 10.8 g 878 m/s Rifle

Figure 1: Ballistic Vest Types4

Besides their resistance to ballistic impacts, there are other aspects considered when purchasing vests. Level IIIA vests are the strongest vests that are still considered flexible. These

vests are made with the most resistive but flexible materials available, while not containing rigid armor plates4. Vest types III and IV utilize armor plates, which offer greater protection at the expense of significantly limited mobility. This limited mobility is such a significant factor that it deters some police departments from wearing types III and IV. The most popular vest purchased for law enforcement is the type IIIA because of its resistivity, ease of mobility, and versatility5. Many commonly used vests have pockets capable of accommodating various rigid and non-rigid armor plates (Figure 2 below), increasing their effectiveness6. Additionally, type IIIA vests stop the most common rounds used against police officers.

Figure 2: Ballistic Armor Plates7

2.2 Law Enforcement and Military Body Armor Statistics Between 1980 and 2014, an average of 64 police officers each year have been feloniously killed7. In a time where police shootings are at a twenty year high, it is important for police officers to have the best personal protection available8. Research was conducted to determine the extent of the use of bullet-resistant vests in order to justify the feasibility of a bullet detection system. The Federal Bureau of Investigation (FBI) collects situational statistics about all police officer fatalities that occur each year. Military casualty statistics in regards to the use of body armor are not readily available, however qualitative data gathered through interviews with military personnel is used when appropriate.

4 Office of Law Enforcement Standards, 2008 5 Personal Correspondence: Mark Roche, 2015 6 Personal Correspondence: Mark Roche, 2015 7 “FBI Releases 2014 Preliminary Statistics for Law Enforcement Officers Killed in the Line of Duty”, 2015 8 Johnson, 2014 11 ©2016 C. Keyes, N. Potvin, Z. Richards, C. Tolisano. All rights reserved.

In recent years, the use of body armor by law enforcement officers across the country has increased. A study by the Police Executive Research Forum showed that 92% of law enforcement officers who were part of their study responded that they were required to wear body armor at least some of the time9. This is a significant increase from a study done in 2009 which showed that only 59% of law enforcement agencies had a requirement for their officers to wear body armor. The increase over a three year period shows that it is likely that agencies will continue to introduce more formal programs for the use of body armor10. The same report also found that 90% of officers surveyed wear body armor because they recognize the need for the protection that it provides. The United States Federal Bureau of Investigation (FBI) provides tabulated data regarding officer fatalities between 2004 and 2013. During that time, 511 officers were feloniously killed, 330 of whom were wearing body armor. This statistic shows that body armor alone is not enough to keep officers safe. Panic buttons on law enforcement officer’s radios do not provide dispatch with the location of the officer. Therefore, it is possible for an officer to be shot and call for help, but for their location to be unknown. Of the officers killed, 93% were killed by a firearm, and 0.6% were killed by a knife. The most common calibers that police are attacked with were 9x19, followed by .40 caliber, .38 Special, and .45 caliber11. Although it is uncommon for bullets to penetrate law enforcement’s body armor, 90% of officers killed while wearing body armor are killed by rifles. Body armor is also widely used in the United States Military. Although the specific number of lives saved by body armor isn’t available, the increased use of body armor has contributed to improved survival rates in Iraq compared to that of Vietnam, with many soldiers coming home with injuries rather than being killed12.

2.3 Prior Art

Prior patents have explored methods for implementing a bullet detection system. They have recognized the need for such a system, but have fallen short of a design containing all of the pertinent features. Prior art in this field has been studied and the most applicable instances are presented below.

2.3.1 Patent Application: Bullet-Proof Vest Bullet Hit Detection System One patent application, titled: “Bullet-Proof Vest Bullet Hit Detection System” by inventor Charles CHU, of Banciao City (TW), is from 2010. This patent describes the invention of “a detector fitted within a bulletproof vest with a GPS tracker”13. The detector identifies the impact of a bullet and transmits the information to a microprocessor. The GPS tracker receives geographic positioning satellite coordinate signals which are used to determine the position of the bulletproof vest. A drawing of the vest is shown in Figure 3 below. Label 10 points to the original vest itself, label 11 is the detector, 12 is the signal transmission circuit, and 13 is the battery.

Figure 3: Patent Application Vest Diagram17

This patent application depicts a system that detects a bullet impact, however, cannot detect the location on the vest where the bullet made contact. This vest also utilizes a single sensor in the middle of the vest to detect the force of a bullet. Another aspect of this design is the development of a GPS transmission system that can transmit the vest’s location. The GPS unit is

not integrated with any current technology or tracking systems. This design does not include a visual display or way of presenting acquired data to a potential user. The system described by this patent is mounted on an existing ballistic vest and does not come pre-assembled in an independent housing. The vest’s bullet detection system is powered by rechargeable batteries which are connected to solar panels, allowing for continual recharging. The vest detailed in the patent application above was not manufactured on a retail scale. The detection methods and GPS components are undetailed in their design and do not specify how the bullet impact is actually detected. Additionally the means of wireless communication and the acquired data transmitted are not detailed.

2.3.2 Patent: Bullet-Proof Vest with Distress Signaling System Another patent researched was granted in 2002, entitled “Bullet-Proof Vest with Distress Signaling System” by Sean Ford of Medford, NJ. This patent describes a system that can detect the impact of a bullet and can wirelessly transmit the gathered impact data along with the GPS location of the vest to a third party. The general system level design is similar to that of the patent application described in Section 2.3.1, however this design utilizes piezoelectric materials as the means to detect an impact. When pressure is applied to the vest, the resistivity of the piezoelectric material decreases and the sensor is able to measure this change14. Similarly to the patent application described in Section 2.3.1, this system can only detect general impact and cannot detect a specific location of impact on the bulletproof vest. Figure 4 below shows a sketch of the patent. The image on the right shows a front view of a bulletproof vest with the piezoelectric sensor covering the front plane. At the top of the vest are the electrical components including the controller, transmitter, and GPS modules. The image on the left shows a more detailed view of the piezoelectric impact sensor.

Figure 4: Patent Vest Diagram

This patent design presented another approach of a possible way to detect a bullet impact. This system has a relatively straightforward design, and utilizing the piezoelectric material to detect a bullet impact is a possible design choice that can be used to detect specific locations. This patent is general in nature and does not describe how the piezoelectric material interfaces with the controller or by what specific means the acquired data is transmitted from the vest.

2.4 Physiological Effects of Behind Armor Blunt Trauma The amount of injury sustained when shot while wearing body armor can be severe, even if the round does not perforate the layers of protective material. Behind Armor Blunt Trauma (BABT) is a serious concern of both body armor manufacturers and body armor wearers15. The level of BABT sustained by the individual wearing the armor is dependent on the caliber of round fired and the type of vest worn. BABT sustained by a 12.7x99 cartridge is likely to be highly severe regardless of the type of armor worn by the individual. Conversely, BABT from a 5.56 NATO round is likely to be insignificant regardless of the type of armor. Lastly, BABT from 7.62x51 NATO will vary depending on the protective level of the vest and the location of

impact16. Rigid plate armors generally offer more protection than their flexible counterparts and usually consist of a hard, brittle surface (usually ceramic), an energy absorbent backing (frequently an aramid composite, usually Kevlar), and a layer of adhesive binding the surface and the backing together. The physiological damage of BABT is caused by the stress waves that travel through the thoracic cavity upon impact and then transmit energy to the viscera. When the vest is impacted over the sternum, a bending load is applied to the ribcage and the corresponding local deflection applies a shear to the surrounding tissues, leading to lacerations and contusions. Pressure waves also propagate through the body and reflect off of the rigid structures in the body. The tissue/air boundaries in the parenchyma of the lungs are particularly susceptible to energy transfer. The resulting disruption of the alveolar/capillary boundary causes pulmonary contusions. The propagating stress waves may lead to strain (stretching) between internal organs and their attachment to the body; causing contusions, lacerations and internal bleeding17. The type of Blunt Trauma (BT) characteristic to BABT is different from the type of BT sustained from a slower, high-momentum impact such as a vehicle accident. BT from bullet impacts is due to an extremely fast, low momentum projectile, resulting in high body wall velocities and high stress waves, taking one or two milliseconds to reach their peak levels. The human body is viscoelastic (its stiffness is dependent on the rate of compression). In a study conducted by Cannon (2001), the rate at which the human body is compressed has a much more profound effect than the level of compression. One meter/second compression of 20% of the thickness of the torso yielded no damage in live human test subjects. However, 20 meters/second compression to the same 20% thickness of porcine test subjects resulted in a 100% chance of permanent lung damage. Understanding the way that the body reacts to BABT will be useful to be able to properly map the impact on the vest to the likely damage that the individual sustained.

2.5 Triage of Gunshot Victims Triage is the medical process of prioritizing the severity of a patient’s injuries and conditions. A meeting with Dr. Philip Camp, from Brigham and Women’s Hospital in Boston, Massachusetts, who also has background as an Air Force Medic, revealed that the most useful vital signs for doctors and medics to have access to are heart rate, heart rhythm, and oxygen saturation. These three vital signs can be captured with existing physiological sensors, and when provided to medical personnel can help facilitate proper care in a timely manner18.

3. Methodology To address the needs of wearers of body armor, we have designed a comprehensive system capable of providing assistance to an individual subjected to a ballistic impact. The complete set of features for the production-ready, marketable design will consist of a network of sensor strips with associated data acquisition circuit, biotelemetric sensors for heart rate, heart rhythm, and oxygen saturation, and the software that is responsible for aggregating the data from the sensors and displaying the information to a third party. However, implementing the full range of biotelemetric sensors will be outside the scope of this project and will not be necessary in order to construct a proof-of-concept prototype. Therefore, this project will consist of the sensor network for impact detection and will not focus on the implementation of sensors for capturing vital signs. These sensors would be a beneficial future addition to the product, and would provide both medics and first responders with relevant information of the incoming patient.

3.1 Systems Level Design The impact detection system will be a collection of independent sensors that can detect the location of a ballistic impact and transmit the acquired data remotely to a smartphone application. The application will contain a visual display of the information and will be able to relay the data to a third party along with the GPS location of the smartphone. Figure 5 below is a block diagram of the systems level design. The left side of the Figure represents the data acquisition system that will be worn behind the vest. The first main block, shown in red, is the impact sensor which will consist of a network of individual sensor strips. These individual strips will be able to detect pressure and will be read by the circuitry portion shown in orange on the block diagram. The circuit component will take the analog signal from the individual strips, convert and process the data, and then, if necessary, relay the data to the transmitter. A breakdown of the subparts of the circuit block can be seen in Figure 6. The output of the impact sensor will feed into an Analog to Digital Converter (ADC) then, to a Field-Programmable Gate Array (FPGA), interfacing directly with a transmitter block. The transmitter block, shown in green on the left in Figure 5, will be comprised of a Wireless transmitter. The transmitter will send the information from the sensor system on the vest to the smartphone of the individual who is wearing the vest. The individual’s smartphone is represented by the right side of the block diagram shown 18 ©2016 C. Keyes, N. Potvin, Z. Richards, C. Tolisano. All rights reserved.

in Figure 5. The smartphone will have an installed application that is able to decrypt, process, and display the information to the user. This finalized visual information will be sent by the smartphone application automatically to a third party who will be notified if the wearer is in need of assistance.

Figure 5: System Block Diagram

Figure 6: Circuit Block Breakdown

The production-ready design will consist of an overlapping network of multiple sensors, creating a grid. The more sensors in the network, the more precisely the collected data will model the damage sustained by the wearer. Ideally, a grid of 16 strips will cover the entire front 19 ©2016 C. Keyes, N. Potvin, Z. Richards, C. Tolisano. All rights reserved.

of the bullet-resistant vest and is partially shown in Figure 7 below. The blue stripes represent the vertical sensor strips and the red stripes represent the horizontal. As a pressure is applied to a certain strip, that strip registers the applied force. In Figure 7, the white star represents an example of a point of impact. In this example the fourth blue strip in from the left and the second red strip up from the bottom would both register an impact. This concept of overlapping bidirectional measuring allows the system to locate a specific square area on the vest that was impacted and thus detect the location of impact. The prototype, however, will consist of four sensor strips arranged into a 2x2 grid. The reduced number of sensor strips eliminates some of the complexity required to construct a proof of concept.

Figure 7: Systems View of Impact Detection Grid

Figure 7 also shows the wires that run from the individual strips to the circuit portion of the system. Similarly to the front grid system shown in Figure 7, there will be an identical grid system on the back of the vest. Figure 8 below shows a side view of an individual sensor strip. The strip is made up of three sections. The top and bottom sections are individual layers of conductive fabric. These pieces of conductive fabric are shown in the Figure as gray woven lines. The fabric is then connected at one end to a wire that in turn is connected to the analog to digital converter. The 20 ©2016 C. Keyes, N. Potvin, Z. Richards, C. Tolisano. All rights reserved.

middle section of the strip is comprised of multiple layers of a piezoelectric material (such as Velostat). The layers are shown in the Figure as thick black lines. These pieces of material remain unconnected and are resting between the sections of conductive fabric both above and below.

Figure 8: Side View of Individual Sensor Strip

A voltage will be applied to the top layer of conductive fabric and the layers of Velostat act to resist the flow of electrical current from the top and bottom pieces of conductive fabric. As pressure is applied to the sensor strip, the three sections are compressed together and the resistivity of the Velostat is reduced. The voltage difference between the two sections of conductive fabric is reduced and can be measured by the system. The sensor strips (acting as a resistor) will be paired with a fixed resistor to form a voltage divider (shown in Figure 9). We will measure the voltage across the fixed resistor (points a and b). The voltage across the fixed

푅푓 resistor can be calculated as follows: 푉푓 = 푉푆 ∗ , where 푉푠 is the voltage provided by the 푅푓+푅푠 voltage source, 푅푓 being the fixed resistor and 푅푠 variable resistor, namely the sensor strip. The value of the fixed resistor will be chosen to be close to that of the resting resistance of the sensor strip, giving the sensor the most clarity.

Figure 9: Voltage Divider

In order to be able to detect changes in the pressure applied to the sensor strips, an ADC will be utilized to poll the sensor strips and produce a serialized digital bitstream of the strips. The strips will be polled consecutively, making up a set of four samples. This set of samples provides a snapshot of the vest at that given instant, providing data of the force felt by the wearer. Each snapshot of the vest taken by the circuitry will be analyzed for a cross in a threshold (further described in Section 4.8) in order to allow the system to distinguish from a life threatening impact such as that of a bullet and a non-life threatening impact such as a punch or normal flexing due to the wearer’s movement. We expect this approach to succeed due to the relative simplicity of the individual components of the design. Piezoelectric sensors that are able to measure pressure have been on the market for a few decades. The electronic components behind our design have been time- tested and currently manufactured integrated circuits have the tolerances and speeds necessary for us to measure the quick impact of a bullet. The design allows for flexibility with the number of layers of Velostat used as well as the value of the fixed resistor.

3.2 Materials Our preliminary tests conducted with these materials have shown positive results, it is possible that upon further testing, additional materials will need to be explored.

3.2.1 Piezoelectric Materials Piezoelectric materials have the ability to convert mechanical stress into an electric charge, as well as an electric charge into a mechanical stress19. These materials are dielectric, meaning they can be polarized. The polarization within a given piezoelectric material changes proportionally with the mechanical stress applied to the material20. Piezoelectric materials are comprised of millions of symmetric, non-centric crystal structures, similar to those seen in Figure 10. Piezoelectric materials can create a voltage when a load is applied, or the physical material can experience mechanical stress when a voltage is applied.

Figure 10: Piezoelectric Effect in Quartz21

Velostat (in Figure 11), a 3M product consisting of a carbon-loaded polyolefin polymer film, is suitable for this purpose. Velostat costs approximately $0.66 per square foot22.

Figure 11: 3M Velostat23

To fabricate the sensor we plan on utilizing a sandwich method with the Velostat comprising the middle layers surrounded by conductive fabric. This configuration allows us to take advantage of the semiconductive properties of Velostat and to allow electricity to flow through the material more easily while it is under mechanical stress.

3.2.2 Conductive Fabric Highly conductive fabric placed on either side of the layers of piezoelectric material ensures that the voltage is uniform across the entire sensor and that the change in resistance is independent of where on the strip the force is applied. The presence of the conductive fabric also allows the sensor to be used with a lower supply voltage. The chosen material must have a very low resistivity and be flexible enough to be worn. Although the fabric likely won’t have direct skin contact, the material must be safe for skin contact. One of the materials tested was RipStop Silver Fabric (Figure 12). This fabric has a surface resistivity of less than 0.25 Ω/sq, and is made of a nylon base with silver plating. The material is commonly used for microwave and Radio Frequency shielding purposes, either for clothing or drapes/covers. The material is thin and lightweight, and due to the low resistivity, the voltage across the entire sheet will be close to uniform.

Figure 12: Conductive RipStop Silver Fabric24

In an effort to improve the quality of the sensor and ultimately to reduce fabrication costs, pure copper polyester taffeta fabric (PCPF) was tested (Figure 14). The Ripstop Silver Fabric (RSF) costs $3.65 per square foot, while the PCPF costs $3.09 per square foot. This small difference in cost per square foot equates to approximately $5 savings per vest, as seen below in Figure 13. Vest Front sq ft Cost/sq ft Cost per vest Copper 9.52 $3.09 $29.4168 Silver 9.52 $3.65 $34.748 Figure 13: Cost of different fabrics25 PCPF is 0.08mm thick, weighs 80 g/m² and consists of about 35% Copper. It has a surface resistivity of 0.05 Ω/sq. The PCPF has a lower resistivity compared to the RSF, which has a surface resistivity of 0.25 Ω/sq.

Figure 14: pure copper polyester taffeta fabric26

The PCPF was tested in the lab via multiple small scale drop tests alongside the RSF. The results of these drops were found to be almost identical. When punctured, and repeatedly impacted, the material responded similarly to RSF, indicating that this material would most likely succeed in live-fire testing. This material also proved to cut cleaner than the RSF, and did not fray. Having a clean cut decreases the chances that the sensor would short. Additionally, the PCPF fabric is more durable.

3.2.3 Mesh Interlayer Through experimentation, it was determined that as bullet resistant vests were tightened around the test mannequin, the resting voltage across the sensor was significantly higher than if the sensor was sitting without a uniform load distributed over it. Considering that under ideal conditions, the resting voltage should be at 0 and peak to the excitation voltage upon impact, a method to reduce the sensitivity of the sensor under light loading was needed. In an effort to lower the resting voltage and ultimately increase the precision of the sensor, various forms of plastic meshes in between the piezoelectric layers were tested. Figure 15 shows the various meshes experimented with in order to see which would provide the optimal sensor response under impacts, if at all, while lowering the resting voltage across the sensor.

Figure 15: Various tested mesh interlayers

3.3 Monitoring Subsystem Design & Architecture In order to detect whether or not the sensor network has been subjected to an impact, a circuit will be rapidly sampling each sensor for abnormalities. When detected, the likely damage sustained by the wearer will be calculated and a third party will be notified.

3.3.1 Analog to Digital Converter (ADC) The output voltage values from the individual strips of the sensor grid will range from 0 to 3.3 Volts. The values of these strips will be rapidly changing during the impact of the bullet and therefore the system acquiring the changing data values must be sufficiently fast. Each changing analog signal from a strip must be converted to a digital signal before it can be processed. In order to reduce the amount of components needed to build the system, a four channel analog to digital converter (ADC) was selected. Each input channel of the ADC will correspond to one sensor strip. The four input channels will be sampled sequentially for analysis. The selected chip is an ADC084S101 manufactured by Texas Instruments. This ADC can convert with 8-bit precision, yielding a possible range of 256 discrete Voltage readings (from 0 to 255). Figure 16 below, illustrates how an analog voltage is converted to a digital value. 27 ©2016 C. Keyes, N. Potvin, Z. Richards, C. Tolisano. All rights reserved.

V S = ⌊ ∗ 255⌉ 3.3 Figure 16: Analog voltage to digital conversion This converter can sample at 1 MHz, which must be split across the input channels, resulting in each sensor strip being polled at 250 kHz. After the polling of all four strips and the completion of the conversion, the chip repeats the process. Testing with ballistic fire on a bullet- resistant vest confirmed that a sample rate of 250 kHz sufficiently maps the impact of the bullet. This sampling rate results in a sample being taken once every 4 µs (and one snapshot taken every 16µs). In order to function, the ADC requires two clock signals, a 1 MHz chip select (CS) and a 16 MHz serial clock (SCLK). The CS instructs the ADC when to take a sample and the SCLK controls the output of the digital bitstream into the FPGA. The ADC receives inputs from the four sensor strips that comprise the grid as well as a 3.3 V power supply. The chip also needs an input dictating which channel to sample in the next cycle, written as a binary number into the chip’s control register. The ADC then outputs a bitstream of the digital voltage values to the FPGA for data processing. The ADC is mounted on a custom designed printed circuit board (PCB). The PCB allows for short connecting traces between the ADC and other components in the circuit, allowing operation at high frequencies. Figure 17, below shows the schematic layout of the PCB.

Figure 17: Schematic of PCB Layout

3.3.2 Field-Programmable Gate Array (FPGA) The serial output of the ADC will be read into the FPGA for further processing. The FPGA reads the serial output of the ADC, saves the sensor values into memory, and simultaneously analyzes the data for potential impacts. An FPGA was chosen because it has the capability of being able to be rapidly prototyped and redeployed, allowing for quick and easy development of the system and correction of any errors.

3.3.2.1 Control Logic & Memory Architecture The FPGA is programmed via a hardware description language (HDL) to describe its behavior. The systems-level design uses the Spartan-6 as the main controller for the Control Logic and Memory section of the subsystem. The FPGA controls all interactions between the circuitry peripherals (shown in Figure 18 Below).

Figure 18: High-Level Systems Architecture (App not shown)

The entire circuitry block diagram is shown below in Figure 18. The dotted line encloses the modules implemented via the FPGA.

Figure 19: Control logic and memory block diagram. (Sensor strips omitted)

3.3.2.2 Clock Dividers The control logic component requires three different clock signals for controlling the timing of the different peripheral elements: a 1 MHz Chip Select (CS), 16 MHz Serial Clock (SCLK) and 500 kHz memory clock (MEMCLK). These clock signals (shown in Figure 20) must be synchronized (rising edges aligned) for the circuit to function properly.

Figure 20: Synchronized Clock Signals

The CS and SCLK signals control the interface between the FPGA and the ADC. The MEMCLK controls the interface between the FPGA and the memory.

3.3.2.3 Circular Memory Buffer The data acquisition speed of the sensor strip (1 MHz) exceeds the Bluetooth transfer rate (approximately 900 kbps). It is therefore necessary to store the data in a temporary location so that it may be transmitted after a threshold exceedance has been detected (see Section 4.8). A circular memory buffer is used for this purpose (Figure 22). The memory chip used is a 16 MB high-speed pseudo-static CMOS Dynamic Random Access Memory (DRAM) organized as 8 Meg by 16-bits, capable of supporting a maximum clock speed of 133 MHz. Therefore, the circular memory buffer is 16 MB in size, and able to store approximately 16 seconds worth of data samples. The memory IC has a 23-bit input address bus and a 16-bit input/output data bus (and various other control inputs). On each rising clock edge, the memory chip conducts either a read or write to the 23-bit location on the chip specified by the address bus (timing diagram shown in Figure 21). The values of the address bus are latched on the first rising clock edge.

Figure 21: Memory behavior during read operation27

The 23-bit address, controlled by the 23-bit binary counter, is indexed at each rising edge of MEMCLK. A register in the FPGA stores the last address that was written to in order to ensure that data isn’t overwritten before it can be transmitted.

Figure 22: Circular Memory Buffer

During normal operation (no voltage threshold cross), the last-written-to address pointer is indexed with each rising edge of MEMCLK by the 23-bit binary counter shown in Figure 19. The address is stored in the register.

3.3.2.4 Detecting a Threshold Exceedance The circuit threshold detection module can be implemented via a binary adder and a comparator. The module (Verilog, Figure 23) will compare the difference in voltage between two consecutive samples of the same sensor strip, and determine whether or not that difference is greater than a predetermined threshold. Because the Cellular RAM stores data two bytes at a time, the threshold detection module was implemented to compare two strips simultaneously and indicate that the threshold has been crossed if either strip0 or strip1 has crossed the threshold. If the threshold is being exceeded, the vest is currently being subjected to an impact.

module ThresholdDetection( input [7:0] vC0, //current voltage strip 0 input [7:0] vP0, //previous voltage strip 0 input [7:0] vC1, //current voltage strip 1 input [7:0] vP1, //previous voltage strip 1 input clk, //clock output tc //1 if threshold of either strip crossed, else 0 );

reg s1; //if strip 1 crossed reg s2; //if strip 2 crossed reg [7:0] st = *TBD*; // Threshold value

always @(posedge clk) begin s1 <= (vP0 > vC0) ? 1'b0 : (((vC0 - vP0) > st) ? 1'b1 : 1'b0); s2 <= (vP1 > vC1) ? 1'b0 : (((vC1 - vP1) > st) ? 1'b1 : 1'b0); end

assign tc = s1 | s2;

endmodule Figure 23: Verilog for threshold calculation

In order to be able to determine whether the threshold has been crossed, it is necessary to simultaneously compare two samples. However, the Cellular RAM cannot be read from and written to simultaneously. Therefore, we are unable to use the strip samples previously stored into memory for threshold exceedance analysis. Instead, each sample, in addition to being stored in the RAM, is loaded into a 4x1-byte shift register (Shown in Figure 24). The 4x1 register allows the byte to be delayed one clock cycle (P0 and P1 in figure). This allows the previous sample to arrive at the threshold comparator at the same time as the current sample. This approach allows us to simplify the memory controller because the memory will not need to

alternate between read and write cycles. The register will alternate between sending samples from strip 0 and 1, and sending samples from strip 2 and 3 into the comparator.

Figure 24: 4x1 register and threshold comparator

3.3.2.5 Circuit Behavior upon Threshold Exceedance Each time the sensor strips are sampled, their values are stored into the circular buffer. Simultaneously, the values are analyzed to determine whether or not they have crossed the threshold as described in Section 4.8. Upon the detection of a threshold cross, the last-written-to address pointer stops advancing in order to mark the start of the impact. Then, additional samples are taken for a predetermined amount of time to ensure that the impact has been adequately captured (the exact duration will need to be determined through research and experimentation). The current hardware allows for the collection of 16 seconds of impact data without any samples being overwritten.

3.3.4 Wireless Data Transmission A wireless transmission method, such as Bluetooth, will be used to transmit the impact data from the data collection circuit to the smartphone application. The data will be encrypted to ensure that it cannot be viewed, transmitted, or modified by any device outside of the system in order to protect the privacy of the wearer.

3.3.5 Constructing a Matrix from Strip Values In order to determine the location of the impact on the vest, the values of the squares of the grid must be computed from the overlapping sensor strips. The raw voltage values read from the sensor strip grid provide the longitudinal and latitudinal strip values, not the voltage readings of the individual squares on the vest. Each individual sensor strip will record the maximum impact to which it was subjected. For example, the outlined box in the matrix in Figure 25 shows that the horizontal strip has read a voltage of 4.8 while the vertical strip read a voltage of 3.5. Due to the center of impact being in the center of the vest (a voltage reading of 4.8), that entire strip will read 4.8. In order to compute the voltage reading at a single square, the minimum of the two readings must be taken.

Figure 25: Sample vest matrix showing computation

The equation, shown below in Figure 26, is used to determine the voltage reading of a square using the voltage readings of the two strips that intersect at that square. This process will be handled by the smartphone application.

푉푖푗 = min(푉푖, 푉푗)

Figure 26: Voltage square computation equation

3.3.6 Android Smartphone Application Android is a largely open-source operating system for smartphones. It is free to develop applications on the Android platform (the same cannot be said for Apple’s iOS). Android smartphones have built-in Bluetooth transceivers and GPS transmitters; therefore it is not necessary to have a standalone GPS transmitter in order to relay the location of the wearer. Most police officers carry their personal smartphone with them while they are on duty. It is likely feasible for a smartphone application of our Body Armor Impact Map System to be a marketable final design. Using an Android application eliminates the need to construct an application- specific integrated circuit; implementing a GPS transmitter, encryption, etc. to produce a working prototype. The visual display can be viewed from the individual vest wearer’s smartphone as well as remotely by a monitoring third party. Figure 27 below shows an artist’s rendition of the visual display. There are three tabs at the top of the screen. The first tab on the far left has an image of a single person. When this tab is selected the window shown in Figure 27 will appear. This tab will contain the individual's name, heart rate, heart rhythm, oxygen saturation, a map with coordinates and current location of the smartphone, and an image of the vest with a visual indication of the location of the bullet impact. The second tab will allow the user to navigate to a screen where they can view the individual information for other members in their unit. The third tab navigates to settings in which the user can turn off certain display features if they do not wish to transmit portions of their individual information. The GPS location of the smartphone will not be sent to the third party until the sensor strips are activated (i.e. when the vest is subjected to an impact severe enough to have been caused by a bullet). This will reduce power consumption and ensure that the officer’s whereabouts are not being continuously monitored, alleviating privacy concerns from the Police Union regarding officer’s positions being continuously monitored28.

Figure 27: Conceptual App Design

3.3.7 User Monitoring and Data Aggregation Subsystem In order for the ballistic impact to trigger an immediate call for help, a system that will respond to the detection of an impact by the smartphone application is necessary. Though the design and implementation of this subsystem is outside of the scope of this project, various aspects have nevertheless been considered. The system will be aware of the users and will be able to send out a system-wide alert to nearby officers if necessary. The system would also have the capability to determine the officers nearest to the victim and would be able to automatically send them a special alert, allowing them to act quickly. Though the officer’s locations will not be continuously monitored, it may be desirable to include a special option that would allow, at the user’s discretion, their location to be constantly reported to other nearby officers in the event of a hostile situation where the officer may need immediate backup (foot pursuit, crime in-progress, etc.).

3.3.8 Power Specifications and Analysis The final design for the ballistic impact mapping system will require a power source located on the vest. This power source must be capable of providing enough power to support the FPGA, ADC, Bluetooth, and sensor strips. The power system design will be centered on a rechargeable battery unit. The entire vest-mounted system will have a master switch to control power up and power down for all system components. The FPGA is powered by an external power supply with a recommended limit of 5.5 VDC. Voltage regulator circuits will be used to provide the required 3.3 V, 2.5 V, 1.8 V and 1.2 V supplies. These voltages will be supplied by the main power input. Figure 28 shows the power requirements of the individual modules.

Component Power Usage (Typ.) (mW) Power Usage (Max.) (mW) Sensor Network 42 85 ADC 3.2 3.2 FPGA TBD TBD Bluetooth 39.6 99 Figure 28: System power supply breakdown The 4-channel ADC has a typical power dissipation of 3.2 mW at 1 MHz. The device accepts a wide analog power supply range from 2.7 V to 5.25 V. The low power consumption of these devices suits them well for battery-powered applications. Recommended design specifications for this chip utilize a 1 μF and 0.1 μF decoupling capacitor at the supply pin located as close to the device as possible. The Bluetooth component is powered by a 3.3 V. The Bluetooth module uses a maximum current of 30 mA during data transmission, and 12 mA while idle. Because the amount of time that data will be transmitted will be relatively low, it can be assumed that the average power consumption of the Bluetooth module will be 39.6 mW. The power requirements of the designed sensor grid system are calculated assuming two layers of Velostat. The sensor strip, with two layers of Velostat, has a resting resistance of approximately 510 Ω. The 3.3 V source and the 510 Ω fixed resistor yield a current draw of 3.1 mA per strip, or 12.6 mA for the four strip grid. The power consumed by the sensor grid will be therefore be approximately 41.5 mW while idle, and 85 mW while under impact.

4. Results In order to determine if the current sensor design will be adequate, numerous tests were conducted on the materials on both a small scale, and with ballistic impacts. The tests became increasingly realistic, starting with static tests, then progressing to low velocity drop tests and finally subjecting the sensor to live-fire testing.

4.1 LabVIEW High Speed Data Collection In order to collect data fast enough to read the sensor at the speed of a bullet, a LabVIEW program was developed to collect sufficient data within the ballistic impact time range. LabVIEW and a data acquisition box were utilized in order to develop a voltmeter capable of sampling at a rate of 250 kHz. The LabVIEW program also has the ability to provide power to the sensor strip.

4.2 Construction of Sensor Grid The sensor network is comprised of several sensor strips arranged in a grid. The sensor is constructed with two rectangular sensor sheets, one with strips running vertically, and the other with horizontal strips. Each of the sensor sheets is constructed with two rectangular pieces of fabric. A strong, woven fabric is used for the construction of the prototype. The fabric pieces may need to be replaced with a more breathable, wicking fabric. The purpose of this fabric is to keep the strips of Velostat and conductive fabric in place. The strips of conductive fabric are first glued to the woven base layer, and then the fabric sheets are glued together, sandwiching the layers of Velostat and mesh in between. The construction of the sensor sheets must be completed carefully to avoid contact between the strips of conductive fabric on each side of the Velostat. In order to ensure that the strips of conductive fabric do not touch, the Velostat which separates the layers are slightly larger. Figure 29 shows a sample sensor sheet consisting of five vertical sensor strips. The red dotted lines show where glue has been applied. The adhesive allows the materials to flex and move like a shirt, while also preventing the layers from shifting. A flexible and clear adhesive, ECLECTIC E6000, was used, which bonded strongly to the chosen materials.

Figure 29: Vertical Sensor Grid

The current sensor setup consists of a layer of pure copper polyester taffeta fabric (PCPF), a layer of Velostat, High-Density Polyethylene mesh, and then another layer of Velostat, and PCPF. This setup is illustrated in the figure 30 below.

Figure 30: Sensor structure with HDPE mesh Our testing was centered on level IIA and level III vests, due to their popularity. Because of the variability of the levels of body armor protection, a system that is worn close to the body more accurately maps the impact felt by the wearer. Our system will therefore be located behind the protective layers of the vest, near the wearer’s skin. This location of the system’s detection sensors allows close monitoring of the trauma felt by the wearer and eliminates any need to recalibrate the sensor if armor plates are inserted into the vest. Our system can be affixed to nearly any size and style of vest that are currently on the market.

4.3 Small-Scale Sensor Testing Initial testing of the sensor was conducted in order to determine if the Velostat and conductive fabric combination would be able to detect a range of impacts. The sensor strips were tested with a LabVIEW program (described in Section 4.1) and the corresponding National Instruments data acquisition box. Initially, static tests were conducted to determine the force at which the measured voltage would plateau. The tests were performed with varying fixed resistors, weights, and layers of Velostat. It was found that the voltage spike plateaued at a force much lower than was expected, even with an increase in the number of layers of Velostat. Figure 31 below, shows the voltages measured when the varying static weights were applied to the sensor strip.

0 g 50 g 150 g 250 g 350 g 450 g 550 g 650 g 750 g 850 g 950 g 1050 g

10 Ω 0.10 0.12 0.35 0.53 0.62 0.62 0.63 0.63 0.63 0.63 0.64 0.63

100 Ω 0.07 1.06 2.44 3.07 3.42 3.66 3.91 4.05 4.18 4.26 4.30 4.34

1 kΩ 0.51 3.60 4.50 4.68 4.78 4.81 4.85 4.88 4.89 4.90 4.91 4.92

10 kΩ 2.40 4.81 4.95 4.97 4.98 4.98 4.98 4.99 4.99 4.99 4.99 4.99

100 4.59 4.98 4.99 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 kΩ

1 MΩ 4.94 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00

Figure 31: Static Velostat voltage tests

It can be seen that with each of the six resistors used, the voltage values began to plateau at or below 1 kg. As more layers of Velostat were added, an incrementally larger force was required to reach the voltage plateau. A practical number of layers of Velostat would not provide adequate measuring capabilities for the sensor. These results conclude that the magnitude of the voltage spike alone will not be able to detect a bullet impact. The below Figure (32) illustrates the weight at which the number of layers plateaus.

Figure 32: Static tests comparing the effect of multiple layers of Velostat

Following the static tests, small scale drop tests were performed. The goal of the drop test was to determine how quickly the Velostat could react to an impact, and if the data acquisition could adequately measure and record the event of the impact. When a small object was dropped on the sensor strip, which was resting directly on the lab table, several peaks were shown in the data (Figure 33). The peaks represent the object bouncing as it hits the surface.

Figure 33: Drop of small weight on test strip. One layer of Velostat

The zoomed peak of the initial impact is shown in Figure 34. The curve of the impact took place over about 200 data points.

Figure 34: Zoomed image of initial peak from Figure 31

A test was conducted using ballistics gel under the sensor strip, to more closely mimic the eventual use of the sensor on a human torso. It was found that the gel absorbed the energy of the impact, and did not allow the weight to bounce as it did in the prior test. Additionally, the use of the gel increased the time and distance over which the dropped weight came to rest, resulting in more data points being collected during the impact. The peak on the image below (Figure 35) shows the impact hitting the test strip.

Figure 35: Drop of small weight on test strip with ballistics gel

Figure 36 below shows the zoomed curve of the peak above. With the use of the ballistics gel, the curve consists of almost 3,000 data points.

Figure 36: Zoomed image of peak from Figure 33

4.4 Velostat Testing Without Applied Voltage Due to the properties of Velostat, as pressure is applied to the silicon crystals in the material, the change in the crystal structure alters the polarity of the material, thus enabling the Velostat to produce a voltage. Through initial testing, it was found that the measured voltage will plateau with a minimal force applied to the Velostat. However, the sensor had never been tested without an excitation voltage. The Velostat should be able to sense changes in pressure applied to the material, and it was hypothesized that without an applied excitation voltage, the sensitivity of the sensor would decrease. This hypothesis proved to be principally correct, but the use of this method in the design would be too unreliable. The initial test seen in Figure 37 shows a single impact on the Velostat, simply measuring the voltage generated by two layers of Velostat. The impact caused the voltage to spike to 2.6 volts. The two peaks represent the weight bouncing before coming to rest.

Figure 37: Voltage of Velostat only. Single impact.

Figure 38 below shows a second drop test without a voltage applied. This test was to verify the repeatability of the initial drop test. The resting voltage in this case was lower after the impact than before, showing again that the system without an excitation voltage is inconsistent.

Figure 38: Impact of Velostat only - 2 Layers

Monitoring our system in real time through our LabVIEW program showed that as the Velostat shifts when it is moved, the voltage across it spiked and switched polarity rapidly. This would result in our sensor giving false positives caused by normal movement. This proved that 47 ©2016 C. Keyes, N. Potvin, Z. Richards, C. Tolisano. All rights reserved.

we need the excitation voltage and the voltage divider to stabilize the voltage being read across the Velostat.

4.5 Drop Tests The calibers that were modeled for testing in the first round of full-scale drop tests were .22 LR, 9x19, and .357 Magnum. These calibers were chosen due to their various muzzle energies and their popularity amongst law enforcement and civilians. From these tests enough sample data was collected to determine that Velostat is able to respond accurately to an impact of the equivalent energy of a bullet.

4.5.1 Drop Test Set-up With the circuit capable of reading the voltage across the sensor strip as described in Section 3.1, the performance of the sensor strip under the kinetic energies of various caliber rounds could be tested. The method for testing the sensor strip with varying kinetic energies was to drop a dumbbell from three different heights onto the sensor strip. In order to more accurately model the impact of a bullet, a steel rod was threaded into the bottom of the dumbbell. The opposite end of the rod was ground to match the diameters of the bullet sizes to be tested (.22 LR, 9x19, and .357 Magnum). An apparatus was constructed in order to provide control to the falling dumbbell, and to ensure that it landed on the sensor strip. A system, shown in Figure 39 was developed which consisted of plywood layers, ballistics gel, the sensor strip, and the bullet- resistant vest. The apparatus also contains an elevated platform with a cavity cut out of the center.

Figure 39: CAD model of rig base

This cavity contained the ballistics gel that absorbs the impact of the bullet similar to that of a human. Figure 40 below shows the cavity that was created by using a bottom sheet of plywood, a layer of 2 x 4’s, and covered by an additional layer of plywood. The ballistics gel rests in the cavity, and the sensor and vest rest above, flush with the gel and plywood. Two guide cables were fastened 24 feet above the base in order to guide the weight vertically down to the impact zone. The cable is threaded through eye hooks attached to the sides of the dumbbell. The elevation for the cable connection can be seen in Figure 41 where the cables connected to the 2x4’s that are hovering over the location of the vest.

Figure 40: Apparatus Base

Figure 41: Apparatus with Cable Mount Surface and Weight

In order to stop the eye hooks attached to the weight from colliding with the 2 x 4’s holding the cables, the length of the impact rod (Figure 42) was chosen to be long enough as to not interfere with the free-fall of the weight. 50 ©2016 C. Keyes, N. Potvin, Z. Richards, C. Tolisano. All rights reserved.

Figure 42: Weight with Steel Rod

At the top of the dumbbell was an additional eyehook that a third cable, separate from the two guide cables, was used to hoist the weight up to the appropriate heights to match the kinetic energies of the calibers tested. This cable was run through a pulley above the drop rig.

4.5.2 Drop Test: Modeled .22 LR A drop was designed to mimic the kinetic energy of a .22 LR caliber round (40 gr. solid at 1200 fps), having a muzzle energy of 174 J29. The 30 pound dumbbell was dropped from a height of 4’1”. A sensor strip was constructed with one layer of Velostat sandwiched between two layers of conductive fabric. This first test punctured the 9”x9”x1.5” ballistics gel block, and dented a one half inch sheet of plywood under the gel. The sensor strip itself held up well to the impact, and didn’t show any visible damage. Figure 43 below shows the final position of the weight after the fall. Figure 44 shows the raw data from the drop, with the data points on the x-axis and the voltage being read across the fixed resistor on the y-axis. The average slope over the course of the initial impact was 0.0027 Volts/4 μs. Figure 45 is zoomed to show the slope of that impact.

Figure 43: Weight with steel rod

Figure 44: Impact graph of .22 LR with 1 layer of Velostat

Figure 45: Slope of initial impact of .22 LR with 1 layer of Velostat

The next drop was conducted with the same weight, steel rod, and height as the previous test, but with a different sensor strip consisting of four layers of Velostat rather than one. Similarly to the previous drop, this drop penetrated all the way through the ballistics gel block. This drop also punctured the cotton layer of the sensor strip. As seen in Figure 46, after the voltage spike caused by the impact, the force of the impact pulled the alligator clip off the sensor strip, causing the measured voltage to fall to 0 V. Figure 47 shows the slope of the impact. Figure 46 shows the ballistic gel after the impact.

Figure 46: Impact graph of .22 LR with 4 layers of Velostat

Figure 47: Slope of initial impact of .22 LR with 4 layers of Velostat

Figure 48: Damage to ballistics gel after .22 LR impact

4.5.3 Drop Test: Modeled 9x19 The following test was modeled to mimic the kinetic energy of a 9x19 round. The 9x19 has a kinetic energy of 494 J (124 gr. FMJ at 1150 fps30). This required the weight to be dropped from 11’7”. This drop was almost three times the height of the previous two drops, and left a large dent in the plywood under the gel block. The drop also tore the cotton and once again pulled the alligator clip off of the sensor strip, as seen in Figure 49. Figure 50 shows the slope of the impact.

Figure 49: Impact graph of 9x19 with 1 layer of Velostat

Figure 50: Slope of initial impact of 9x19 with 1 layer of Velostat

The next drop was from the same height, and modeled the same caliber as the previous drop. This test instead used a sensor strip containing four layers of Velostat. Figure 51 (below) shows that the sensor strip was shorted. The force of the steel rod hitting the sensor strip caused the Velostat to puncture, and allowed contact between the two layers of conductive fabric. However, the sensor still recorded the actual impact of the weight. The weight then bounced which can be seen by the section of the graph hovering at 1 V. When the weight landed the

second time, it shorted the circuit, and the sensor read 3.3 V (the excitation voltage). Figure 52 shows the slope of the impact, and Figure 53 shows the ballistic gel after the impact.

Figure 51: Impact graph of 9x19 with 4 layers of Velostat

Figure 52: Slope of initial impact of 9x19 with 4 layers of Velostat

Figure 53: Damage to ballistics gel after 9x19 impact

4.5.4 Drop Test: Modeled .357 Magnum The next drop modeled the kinetic energy of a .357 Magnum round. To achieve 731 J (158 gr. JSP at 1240 fps31), the weight to be dropped from 17’1”. The sensor strip used in this test contained four layers of Velostat. Figures 54 and 55 show the raw data and the zoomed image of the initial impact, respectively.

Figure 54: Impact graph of .357 Magnum with 4 layers of Velostat

Figure 55: Slope of initial impact of .357 with 4 layers of Velostat

The final drop was done using a sensor grid with two layers of Velostat. The weight was dropped from 17’1” to reach the same kinetic energy of a .357 Magnum. The weight bounced almost two feet high after the initial impact. The initial impact is shown in Figure 56 while Figure 57 shows the slope of the impact. The weight penetrated the ballistics gel, and caused a large amount of damage (Figure 58). Figure 59 shows the damage caused to the sensor strip.

Figure 56: Impact graph of .357 Magnum with 2 layers of Velostat

Figure 57: Slope of initial impact of .357 with 2 layers of Velostat

Figure 58: Damage to ballistics gel after .357 Magnum impact

Figure 59: Damage to sensor strip after 0.357 Magnum impact

4.6 First Round of Live-Fire Testing

Live-fire tests were conducted in order to test the behavior of the sensor strips at the speed of a ballistic impact. A variety of rounds were tested to see if it would be possible to distinguish between different calibers.

4.6.1 Set up and Plan A National Instruments data acquisition box was used to collect the impact data of the shots. The data acquisition box worked in conjunction with the LabView program as described in Section 4.1. The circuit received an excitation voltage of 5 V from the data acquisition box, and utilized a 510 Ω fixed resistor. The test apparatus was constructed with plywood and 2x4’s, with an empty cavity in the center. The cavity was filled with approximately three inches of ballistic gel, similar to the rig used in the drop tests. The sensor strip was affixed to the backside of the bullet-resistant vest, which was hung from the test rig. The sensor strips for the testing were constructed with two layers of Velostat. Sensor strips were created with cutouts for easy connection of the alligator clips. A variety of firearms and ammunition was used in the tests, as shown in the test schedule in Appendix 6.2.

4.6.2 Slope Threshold Validation An important aspect to the test was validating the method for determining whether an impact was caused by a bullet or by a less dangerous impact (such as a punch). The initial idea of using a threshold of measured voltage to detect a bullet was found to be ineffective, as the threshold could be easily reached with a light impact. Instead, it was hypothesized that measuring the change of the voltage over a fixed interval of time could determine the type of impact. There are several variables that affect the shape of an impact curve. As the bullet diameter increases, the area of the sensor that is compressed also increases, increasing the peak slope of the impact. Additionally, because the change in voltage is measured over a specific time interval, the velocity of the bullet is a factor of the slope of the impact curve. Lastly, the kinetic energy of the bullet, which takes into account the mass, could also impact the peak slope. Further testing needs to be conducted to determine which of these factors have the strongest correlation

to the slope of the impact. It is important to understand what factors determine the slope in order to predict the amount of injury that the wearer of the vest has incurred.

4.6.3 Results Figure 61 below shows the impact curve of a 9x19 round. It is important to note that the resting voltage of the sensor is approximately 2 V, while the excitation voltage is 5 V. Figure 60 shows the impact of the bullet, where the measured voltage increases from 2 V to just under 5 V. After the impact, the sensor returned to its resting voltage. This graph shows only the initial portion of the impact, however the voltage remains high over the course of about 100 μs. The graph shows that the initial impact of the bullet takes place over a relatively small amount of time compared to the complete time in which it takes the vest to stop the bullet.

9x19 Impact 6.0000

5.0000

4.0000

3.0000

Voltage (V) 2.0000

1.0000

0.0000

0 4 8

16 12 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80 84 88 92 96

104 100 108 Time (μs)

Figure 60: 9x19 Live-fire initial peaks

Figure 62, below, shows the slope of the above impact. The slope was calculated by determining the change in voltage for a time interval between consecutive samples (4 μs, see Figure 61). 1 time between data points = = 4 휇푠 250 푘퐻푧 Figure 61: Equation determining sampling period 63 ©2016 C. Keyes, N. Potvin, Z. Richards, C. Tolisano. All rights reserved.

The peak slope of the 9x19 impact was 0.961 V/4 μs, as seen in Figure 62. The spike in slope was captured over about six data points.

9x19 Slope of Voltage 1.2000

1.0000

0.8000 s) μ 0.6000

0.4000 Slope (V/4

0.2000

0.0000 0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80 84 88 92 96 100104108

-0.2000 Time (μs)

Figure 62: 9x19 Impact slope

Figure 63 below shows the measured voltage in blue, on the primary y-axis, and the slope of the voltage in red, on the secondary y-axis. The peak slope correlates to the steepest portion of the voltage graph.

9x19 Slope and Impact 6.0000 1.2000

5.0000 1.0000 s)

0.8000 μ 4.0000 0.6000 3.0000 Measured Voltage 0.4000

Voltage (V) Slope of Voltage 2.0000

0.2000 Slopeof impact (V/4 1.0000 0.0000

0.0000 -0.2000

0 4 8

12 80 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 84 88 92 96

100 104 108 Time (μs)

Figure 63: 9x19 Graph impact and slope combined The impacts from five different calibers were collected and charted below (Figure 64). The behavior of the .22 LR, 9x19, and .45 ACP were relatively similar, and as hypothesized. The resting voltage of the sensor strips were either 2.0 or 2.5 V, as shown below. It is likely that the cause of the difference in resting voltage between the tests is based on how tightly the vest was secured over the sensor, as well as any variability in the sensor strip construction process. The data from the .44 Rem Magnum is potentially inaccurate due to the location of the impact. As seen below (Figure 65), the sensor was shot on the edge, causing the alligator clip to detach from the sensor. The impact caused significant deformation to the vest, resulting in damage to the sensor. The .44 Mag round is the limit to what this vest is rated to stop. The bullet penetrated several layers of the vest, and mushroomed to more than double its original size (Figures 66 and 67). Figure 67 shows the weave pattern of the Kevlar material imprinted in the bullet. The 5.56 NATO round is armor piercing, and therefore went completely through the vest and sensor. This is likely the reason why the plot increases and decreases rapidly, rather than remaining high.

Bullet Impact Comparison 6.0000

5.0000

4.0000 .22 LR 3.0000 9x19 .45 ACP 2.0000

.44 Rem Mag Measured Measured Voltage (V) 1.0000 5.56 NATO

0.0000 0 8 16 24 32 40 48 56 64 72 80 88 96 104 Elapsed Time (μs)

Figure 64: Bullet impact comparison - Measured Voltage

Figure 65: Sensor strip after .44 Mag impact

Figure 66: .44 Mag round extracted from vest

Figure 67: .44 Mag extracted from vest - front

The graph and table below (Figures 68 and 69) show the slopes of the above impacts (Figure 64). The .45 ACP and 9x19 rounds had the highest peak slope values, both close to 1 V/4 μs (shown in green and red, respectively). Not surprisingly, due to the energy and size of the round, the .22 LR had the lowest peak slope of 0.6693 V/4 μs. The .44 Rem Mag and the 5.56 NATO rounds had peak slopes of 0.8205 V/4 μs and 0.7369 V/4 μs, respectively.

Various Caliber Slope Comparison 1.5000

1.0000

0.5000

s) 0.0000

8 0 4

48 12 16 20 24 28 32 36 40 44 52 56 60 64 68 72 76 80 84 88 92 96

104 108 -0.5000 100 .22 LR

Slope(V/4 -1.0000 9x19 -1.5000 .45 ACP .44 Rem Mag -2.0000 5.56 NATO -2.5000 Elapsed Time (μs)

Figure 68: Impact slope comparison

Round Peak Slope (V/4 μs) .22 LR 0.6693 9x19 0.9610 .45 ACP 1.0764 .44 Rem Mag 0.8205 5.56 NATO 0.7369 Figure 69: Peak slope of tested calibers 4.6.4 Strip Construction Performance Throughout the testing, the sensors were able to consistently detect the impact of the bullet. The sensor was replaced frequently to ensure that a working sensor was being used. The test plan in Appendix 6.2 shows the schedule of replacement of the sensors. Prior to each test, the sensors were tested to determine if they were damaged from a previous impact. It was found that the majority of the strips were still functional after being impacted and for this reason, some strips were reused to conduct extra tests after the scheduled testing was complete.

4.7 Worcester Police Live-Fire Testing Additional live-fire tests were conducted with the Worcester Police Department (WPD) utilizing the custom built data acquisition circuit as described in Section 3.3. This test data was the first data collected using the custom circuit, and with mannequins wearing the vests. 68 ©2016 C. Keyes, N. Potvin, Z. Richards, C. Tolisano. All rights reserved.

Additionally, for this round of testing, sensors were integrated into shirts that were worn by the mannequins (Figure 70). This setup is similar to how the sensor will be assembled in an official prototype.

Figure 70: Mannequin with sensor shirt and vest

The full test procedure for this round of testing can be found in Appendix 5.3. In this round of testing, the vests were firmly strapped to the mannequin, rather than hung in front of the test rig. This difference reduced the sensitivity and repeatability of the tests. Because the vest was tightly fastened around the mannequin, the threshold that could be measured by the custom circuitry was reduced. The pressure of the vest resting on the mannequin caused the resting voltage (the voltage measured across the sensor before the impact) to be approximately 237 steps out of 255 (see Section 3.3.1). Because the sensor was already partially compressed, the available sampling range was reduced. This did not allow the sensor to accurately respond to the impact as seen in the first round of live-fire testing. The Figure 71 below shows the performance of the sensor with reduced range. In order to address this issue, additional layers of Velostat, or another filler material, must be added to reduce the sensitivity of the sensor so that it responds in the same manner as the first round of live-fire testing.

Figure 71: .22 LR Impact – Reduced range

Figure 72 shows the 9x19 impact. The slope of the impact does not have enough room to create an accurate depiction, although the impact is nevertheless registered.

Figure 70: 9x19 Impact - High resting voltage

In order to reduce the resting voltage of the sensor, the straps on the vests were loosened. This increased the number of steps between the resting voltage and the voltage rail. Figure 73, shows that the resting voltage was reduced to 180 steps from the 237 steps seen previously.

Figure 713: 9x19 Impact

An impact with a IIIA tactical vest, is shown in Figure 74 below. The resting voltage was approximately 100 steps and peaked at 255 steps.

Figure 724: 9x19 Impact

This round of testing proved that the custom circuitry works as expected. Testing the system in a more similar manner to its eventual end use illustrated some shortcomings of the initial design. In order to improve the accuracy of the sensor, additional layers of Velostat or another filler material will need to be added.

4.8 Third Round of Live-Fire Testing A third round of live-fire testing was conducted in order to address the sensitivity problem with the sensor that was found during the testing with the Worcester Police Department. Eight sensors were created, each with a different plastic mesh filler material. The sensors were built into t-shirts, to allow for them to be worn by the mannequin. The mannequin was filled with sand, dressed with the t-shirt and bullet-resistant vest, and then wired to the circuitry. The vest was firmly strapped to the mannequin, to imitate the way it would likely be worn by an officer. The detailed test plan describing each filler material can be found in Appendix 6.4. Before the testing began, the resting voltage was measured to determine if the filler material was addressing the sensitivity problem as expected. The resting voltage of the sensors ranged from 2 mV to 0.6 V, with six of the eight sensors resting below 100 mV. The low resting 72 ©2016 C. Keyes, N. Potvin, Z. Richards, C. Tolisano. All rights reserved.

voltage allows for the majority of the 256 available voltage steps to measure the impact. This compares to previous testing, where the resting voltage was sometimes over 200 steps (corresponding to 2.5 V out of a 3.3 V excitation voltage). Therefore, all filler materials tested were successful in lowering the resting voltage. It is important for the sensors to have low resting voltages and low sensitivity to prevent the sensor from measuring a high voltage during everyday activity. Each sensor was tested with a .22 LR revolver, a 9mm pistol, and a .45 pistol. The range in energies and velocities was important to ensure that each sensor was capable of detecting different types of ballistic impacts. The following three graphs show how four of the sensors performed when impacted with each of the three rounds. Figure 75, below, shows the four sensors being shot with a .22 LR.

.22 LR Impacts Across Sensors 4

3.5

2.5 Sensor 2 2 Sensor 3 1.5

Sensor 4 Voltage Voltage (V) 1 Sensor 5 0.5

0 0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 -0.5 Elapsed Time (μs)

Figure 735: .22 LR Impacts Across Sensors

The graph below (Figure 76) shows 9x19 impacts on the same four sensors shown above.

9x19 Impacts Across Sensors 4.5

3.5

2.5 Sensor 2 2 Sensor 3 Sensor 4 Voltage Voltage (V) 1.5 Sensor 5 1

0.5

0 0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 -0.5 Elapsed Time (μs)

Figure 746: 9x19 Impacts Across Sensors

Figure 77 below shows .45 ACP impacts on the same four sensors as above.

.45 ACP Impacts Across Sensors 4.5 4 3.5 3 2.5 Sensor 2 2 Sensor 3 Sensor 4

Voltage Voltage (V) 1.5 1 Sensor 5 0.5 0 0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 -0.5 Elapsed Time (μs)

Figure 757: .45 ACP Impacts Across Sensors

As seen in the three graphs above, sensor three (Part #XB1130 from Industrial Netting) consistently performed well when impacted with different rounds. Sensor three is made of High- Density Polyethylene, has square 0.12 inch holes, and a thickness of 0.04 inches. Figure 78 below shows the three impacts on sensor three. As seen on the graph, it had a resting voltage of 0.22 V.

Sensor 3 Impacts 4.5 4 3.5 3 2.5 .22 LR Voltage 2 .45 ACP Voltage Voltage Voltage (V) 1.5 9x19 Voltage 1 0.5 0 0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 Elapsed Time (μs)

Figure 768: Various Ballistic Impacts on Sensor Three Figure 79, below, shows the peak slopes that occurred during each of the impacts. Round Peak Slope (V/4 μs) .22 LR 1.62 9x19 2.07 .45 ACP 1.55 Figure 79: Peak Slopes of Impacts The slopes of the impacts are determined by a combination of the velocity of the round, and the cross sectional area of the bullet. Future repetitive testing will allow for estimation of which bullet impacted the sensor. It is important that the sensor and circuitry can differentiate between a ballistic impact and a less dangerous impact, such as a punch. Tests were conducted where the sensor was punched to determine how it would react. Figure 80, below, shows the impact of a punch on the sensor. It was found that because the punch is slow compared to the impact of a bullet, the maximum slope is much lower. The maximum slope of the punch was 0.04 V/4 μs, about 40 75 ©2016 C. Keyes, N. Potvin, Z. Richards, C. Tolisano. All rights reserved.

times lower than the .45 ACP maximum slope. The graph shows that the duration of the punch was approximately 25,000 μs (25ms), whereas the ballistic impact’s duration was approximately 60 μs.

Punch Impact 2.5

1.5

1 Voltage Voltage (V)

0.5

1056 2112 3168 4224 5280 6336 7392 8448 9504

21120 33792 10560 11616 12672 13728 14784 15840 16896 17952 19008 20064 22176 23232 24288 25344 26400 27456 28512 29568 30624 31680 32736 34848 35904 36960 38016 Elapsed Time (μs)

Figure 770: Impact of Punch on Sensor

The sensors were also tested with a 12 gauge shotgun slug to determine if they could register such a large impact. The sensor detected the impact of the slug, with a peak slope of 1.79 V/4 μs. The bullet-resistant vest stopped the slug, but the vest itself penetrated the shirt, sensor, and mannequin (Figure 81).

Figure 781: Kevlar insert (yellow) penetrated through shirt and sensor

4.9 Voltage Threshold Exceedance Calculation It is possible for non-life-threatening impacts, such as being punched, to cause the voltage measured by the sensor network to spike. It is therefore necessary that the monitoring circuitry be able to detect the difference between a non-life-threatening impact and a life- threatening impact. This detection will be used to determine if the collected data should be analyzed and transmitted to a third party to call for assistance. We constructed our circuit to detect whether the threshold of the slope of voltage increase has been exceeded (the philosophy being that it is possible to punch with the same kinetic energy of a bullet, but impossible to punch as quickly as a bullet’s impact). The equation used to determine a threshold exceedance is shown below (Figure 82). ΔV Threshold Crossed = > 푆 = 푉 − 푉 > 푆 dt 푡 current previous 푡 Figure 792: Threshold exceedance equation

The equation illustrates that if the change in voltage (푉current − 푉previous) occurs quickly enough relative to a slope threshold (푆푡), then the threshold has been crossed. The exact value of 77 ©2016 C. Keyes, N. Potvin, Z. Richards, C. Tolisano. All rights reserved.

the threshold will need to be determined through additional testing. It is unlikely that it will be possible to determine exactly which caliber of bullet struck the vest, but we expect to be able place impacts into categories based on the level of damage, such as red, yellow, and green.

Figure 803: Impact measured voltage and slope

Figure 83, above, displays the first 26 measured samples (104 μs elapsed) of an impact of a .45 ACP round. The Velostat initially reacts quickly, with the voltage having a peak slope of approximately 1.15 V/4 μs.

Live Fire vs. Drop Test 9x19 Slope 1.200

1.000 Drop Test Slope Live Fire Slope

0.800 s) μ 0.600 Peak Slope: Drop: 0.074

0.400 Live Fire: 0.961 Slope(V/4 0.200

0.000 0 8 16 24 32 40 48 56 64 72 80 88 96 104 112 120 128 136 144 152 160 168 176 -0.200 Elapsed Time (μs)

Figure 814: Drop test slope vs. live-fire slope Figure 84, above, shows a comparison of the slopes of the measured voltages from a 9x19 simulated impact from the drop tests, and a 9x19 live-fire impact. The peak slope for the drop test was 0.074 V/4 μs while the peak slope for the actual bullet is 0.961 V/4 μs, an increase of 1200%. The circuitry will have the capability of differentiating between impacts using the threshold method.

5.0 Future Development The work done on the system during the course of the project proves that it is possible, with current materials, to create a sensor capable of responding to the impact of a ballistic projectile. The design of the end-to-end system has been completed under the scope of this project, but future and further development will be needed for the implementation of the complete system. The system and methods that have been developed can detect a ballistic impact, and by applying the slope of the impact curve to a threshold, estimate the likely injury. The system also determines the location on the vest where the impact took place. The design of the system then allows the impact data and the officer’s geographical location to be automatically sent to first responders for timely assistance. The provisional patent acquired will allow us to conduct further market research and determine the best way to commercialize this product and how to turn the proof-of-concept prototype into a marketable system. Discussions with more police officers, the police union, medical professionals, and other stakeholders will allow us to obtain more feedback on our system from an end-user’s perspective.

6.0 Appendix

6.1 Impact Drop Test Calculations The following calculations were used to find the heights that the weight must be dropped from in order to reach the muzzle energy (kinetic energy) of various calibers.

Calculations determining the height at 30 lb. (13.608 kg) weight would need to be dropped: Determine the velocity of a given mass with a given energy: 1 푚푣2 = 퐾푖푛푒푡푖푐 퐸푛푒푟푔푦 2 1 (13.608푘푔)푣2 = 1400 퐽표푢푙푒푠 → 푣 = 14.34휇푚/푠 2 Determining the height at which the mass must be dropped to achieve the velocity found above: 2 2 푣푓 = 푣푖 + 2푎̅훥푥 푚 14.3442 = 0 + 2(9.81 ) ∗ 훥푥 → 훥푥 = 10.48 푚 푠2

6.2 First Round Live-Fire Test Plan

Ammunition types used for first round live-fire testing Caliber Weight (gr) Manufacturer Type Velocity (fps) Energy (J) .22 LR 40 Federal Solid 1200 174 9x19 115 Federal FMJ RN 1125 438 .45 ACP 230 Blazer FMJ 830 477 .44 Rem Mag 240 Federal American JHP 1230 1091 Eagle 5.56 NATO 62 PMC Green Tip-LAP 3100 1793

First round live-fire testing schedule and observations Test No. Caliber Vest Strip Notes 1 .22 LR 1 1 No damage to sensor. 2 9x19 1 1 Minimal visible damage to sensor. 3 .45 ACP 1 2 Visible damage to strip, strip shorted after impact, cannot be reused. 4 9x19 2 3 Rapid fire attempt, only first shot was recorded. Sensor fell from test structure after first shot, before second shot was recorded. Visible damage to strip. 5 .44 Rem Mag 1 4 Bullet hit bottom edge of strip, strip destroyed. 6 9x19 None 5 Test with no vest in front of strip. Bullet created hole through the sensor, sensor still worked after test. 7 5.56 NATO 1 6 Bullet completely penetrated vest and sensor strip. 8 9x19 1 Silver Silver sensor strip. 9 9x19 4 8 9x19 repeatability test. Sensor still functioning after test, minimal visible damage. 10 .45 ACP 4 8 .45 ACT repeatability test.

6.3 Worcester PD Live-Fire Test Plan

Wednesday, February 17, 2016

Sampling Rate: 233.072 kHz

Ammunition Types:

Caliber Weight (gr.) Manufacturer Type Velocity (fps) Energy (J)

.22 LR 40 CCI LRN 1235 183

9x19 115 Speer Lawman TMJ 1200 485

.38 Special 150 Winchester USA LRN 845 323

5.56 NATO 55 Winchester FMJ 3240 1738

Tests:

Test # Caliber Shirt # Vest # Notes

1 .22 LR 1 1 11:27 Just high of top dot; No visible damage to shirt. Strip still works

2 .22 LR 2 1 11:31 Near bottom dot; No visible damage to vest. Sensor still works

3 .22 LR 3 2 11:37 Top dot; No visible damage. Didn’t test sensor

4 9x19 4 2 11:42 Bottom dot; Surprising amount of vest deformation

5 9x19 5 3 11:47 Top dot; both clips came off of sensor

6 9x19 6 3 11:57 One clip came off of sensor

7 9x19 7 Tactical 12:02 Rapid fire 3 rounds; Clips came off after 3rd (IIIA) round

8 5.56 NATO 8 4 12:07 Bullet went through everything; Sensor still works

9 5.56 NATO 9 4 12:12 With flexible plate; through everything; Sensor still works

10 5.56 NATO 9 Tactical 12:16 Two rounds rapid fire; through everything; Clips did not detach

11 9x19 1 N/A 12:20 Strip still works post test; Clips remained on

12 .38 Special 2 5 12:26 Shot was at the top of the top dot, edge of sensor

13 .38 Special 2 5 12:28 Right on bottom dot

14 .38 Special 2 6 12:30 On dot; Clips detached from strip

6.4 Third Round Live-Fire Test Plan

Note: These are the first set of tests conducted using different filler materials between the layers of Velostat

Sensor strip setup (in order): Copper fabric, Velostat, Filler Material, Velostat, Copper Fabric

Testing done on mannequin with sensors built into t-shirt. National Instruments DAQ box with 510Ω fixed resistor

Sensors used Sensor Mesh Part # Hole Dim. (in.) Hole Area (in².) Thickness (in.) Resting (Ω) (In Lab) 2 XB1131 0.33x0.33 0.109 0.100 9.0M 3 XB1130 0.12x0.12 0.014 0.040 11.5M 4 XN1678 0.16x0.10 0.016 0.025 6.5M 5 Red 0.12x0.34 0.041 0.060 5.0M 6 XV1672 0.13x0.15 0.020 0.050 7.0M 7 XN2950 0.15x0.23 0.035 0.050 5.9M 10 XN4700 0.25x0.25 0.063 0.100 2.8M 11 XB1132 0.38x0.58 0.220 0.075 6.7M

Ammunition types used Caliber Weight (gr.) Manufacturer Type Velocity (fps) Energy (J) .22 LR 40 Federal Solid 1200 174 9x19 115 Federal FMJ RN 1125 438 .45 ACP 230 Blazer FMJ 830 477 5.56 NATO 62 PMC Green Tip-LAP 3100 1793 12 Ga. 437.5 Federal Hydra Shok® Rifled 1300 2230 Slug

Testing schedule and observations Test Caliber Vest Sensor Time Notes 1 .22 LR 1 2 10:36 2 .45 ACP 1 2 10:38 Clip came off 3 9x19 1 2 10:41 Didn’t work, clip was off from previous 4 9x19 1 2 10:45 Clip came off 5 .22 LR 2 3 10:50 6 .45 ACP 2 3 10:52 7 9x19 2 3 10:54 9 .22 LR 3 4 10:59 10 .45 ACP 3 4 11:00 Clip came off 11 9x19 3 4 11:02 13 .22 LR 4 5 11:05 14 .45 ACP 4 5 11:07 Clip came off 15 9x19 4 5 11:09 85 ©2016 C. Keyes, N. Potvin, Z. Richards, C. Tolisano. All rights reserved.

17 .22 LR 5 6 11:15 18 .45 ACP 5 6 11:18 Clip came off 19 9x19 5 6 11:23 Clip came off 21 .22 LR 6 7 11:31 22 .45 ACP 6 7 11:34 Clip came off 23 9x19 6 7 11:37 25 .22 LR 6 10 11:41 26 .45 ACP 6 10 11:43 29 .22 LR 6 11 11:45 30 .45 ACP 6 11 11:46 33 5.56 NATO 3 11 11:49 34 5.56 NATO 3 5 11:53 37 12 Ga. 3 5 11:57 Clips came off 38 12 Ga. 3 4 12:03 Clips came off 41 9x19 3 5 11:55

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