MAE 435: Engineering Project Management and Design II

MAE 435: Engineering Project Management and Design II Steel Bar Implant - Wireless Transmission

Final Paper

Maureen Loughran Christina Felarca Adham Sobhy Greg Ragosta

Date: 04/25/2013 Instructor: Dr. Bawab Dr. Ringleb Advisor: Dr. Knisley Table of Contents

List of Figures ii Abstract 1 Introduction 2 Method 8 Theoretical Design 10 AutoCAD Analysis 12 Finite Element Analysis 13 Experimental Design 15 Designing the wheatstone Bridge and Amplifier Circuit 16 Software 19 Results 21 Theoretical Results 21 Experimental Results 22 Discussion 23 Conclusion 26 Appendices 27 Budget 32 Gantt Chart 32 References 33

i List of Figures

Fig. 1 Normal v Pectus Excavatum Chest 3 Fig. 2 Pectus Excavatum Chest 3 Fig. 3 Chest CT Top View of Pectus Excavatum Patient 4 Fig. 4 Chest CT Side View of Pectus Excavatum Patient 4 Fig. 5 Front view of patient, after the Nuss procedure 5 Fig. 6 Side view of patient, after the Nuss procedure 5 Fig. 7 Titanium Bar (top) and Steel Bar (bottom) 9 Fig. 8 WISP Data Process 10 Fig. 9 Experimental Setup 15 Fig. 10 Intel Circuit Diagram with 750Ω Potentiometer 17 Fig. 11 Modified Circuit Diagram to increase amplifier gain 18 Fig. 12 Circuit Diagram to further amplify gain 18 Fig. 13 WISP with leads for Vin, Vout, and ground 20 Fig. 14 PATRAN model that shows location of maximum displacement 21 Fig. 15 PATRAN model that shows location of maximum stress 21 Fig. 16 Software Results 22 Fig. A1 AutoCAD Wire Frame Model 27 Fig. A2 Finite Element Model made with PATRAN 27 Fig. A3 AutoCAD Wire frame model (Front and Top Views) 28

ii Abstract

Pectus excavatum is a chest deformity most commonly found in children.

Correcting the deformity involves surgically implanting a steel bar inside the patient.

However, surgeons have no way of determining the correct time to remove the bar and must rely on previous experience. Therefore, the objective of this project is to design and prototype a device that can be implanted within the bar and output the force being applied by the sternum on the bar. The method of achieving this goal involves the

Wireless Identification and Sensing Platform (WISP). WISP is a semi-passive RFID tag developed by Intel, which is able to sense quantities like temperature and acceleration, as well as being able to connect additional sensors (strain) to in/output pins.

The delivery of this project will be an experimental prototype that includes all of the components that are needed to make the system work, as well as an AutoCad computer model that display the simple prototype theoretical design intended for future use. A finite element model was also created in PATRAN, in order to generate data that can be compared to the experimental results gathered during testing.

The results of this project showed on a basic level that the experimental model is functional, as well as provided an ideal model which the system could be miniaturized and fit within the bar itself.

1 Introduction

Pectus excavatum is a deformity of the anterior wall of the chest and rib cage, in which the sternum compresses the heart and lungs. This abnormality results in a sunken chest wall that ranges in severity, as shown in Figures 1 and 2. PE is most prevalent in preadolescent children; however, there are some adults who suffer from it.

The direct cause of this abnormal development of the rib cage is unknown, because the majority of people with this condition have had it since birth or early infancy. One theory suggests that if the ribs grow at a faster rate than the expansion of the heart and lungs, the sternum can sink inward. The expansion of the heart and lungs is one of the main contributors to the sternum being pushed outward, so it is crucial that it occurs simultaneously with rib growth. Another explanation could be that the cartilage holding the ribs to the breastbone developed abnormally, causing the breastbone to be pressed inward .

Pain and fatigue are additional symptoms associated with PE. The chest pain that most patients experience is not severe or long lasting, but any discomfort that decreases their quality of life is concerning. Shortness of breath can also occur. Even though it does not usually affect the patient during normal day-to-day activities, strenuous activity can exacerbate the problem and lead to difficulty breathing. Some cases are so severe that the sternum presses against the heart, not allowing it to perform as efficiently as it should. If the patient does not have the deformity corrected and their body continues to grow, the depression will worsen, causing the severity of the described symptoms to increase .

Fig. 1 Normal v PE Chest Fig. 2 Pectus Excavatum Chest

There is currently no known cure for PE, but there are exercises that patients with mild cases can perform to minimize the symptoms of pain, fatigue, and appearance. These exercises target the back and chest muscles; aiming to straighten the back, pull the shoulders back, and expand the chest. In order for these exercises to be effective, they must be executed correctly and repeated daily. However, surgery is the patient’s best option for more severe cases.

Before any operation takes place, various tests are performed to indicate whether a case is severe enough for surgery. After a physical examination, investigation of health history records, and chest measurements are taken, the decision of whether or not the patient is a candidate for surgery can be made . The Haller index, which is the ratio of the distance of the inside rib cage over the shortest distance between the vertebrae and the sternum, can be calculated using measurements from a

CT scan, shown in Fig. 3. The measurements required for the Haller index are shown in Fig. 4. D1 is the distance between the vertebrae and the sternum and D2 is the distance inside the rib cage. In normal chests, the Haller index is about 2.5, while severe cases of PE can measure from 3.25 to 5.5 .

3 4 D 1

Fig. 3 Chest CT of Pectus Excavatum How to calculate the Haller index: HI= D /D Fig. 4 2 1

Today, the most commonly performed surgical treatment for correcting PE is the

Nuss Procedure. It was developed in 1987 by Donald Nuss, a pediatric surgeon from

Children’s Hospital of the King’s Daughters, and is described as “minimally invasive” .

The Washington University School of Medicine’s website outlines the Nuss procedure

as follows: “A bar is bent into the desired shape of the chest wall. A large surgical

clamp is passed through one side of the chest, under the sternum and out the other

side. The bar is pulled through using the clamp with the curve of the bar in the opposite

direction. It then is flipped over and, in the process, bends the sternum outward,

stretching the ribs as it does so.” The bar is left inside the patient on an average of two

to three years. An X-ray of the bars stabilized within the body can be seen in Figures 5

and 6.

5 Fig. 5 Front view of patient, after the Fig. 6 Side view of patient, Nuss procedure. after the Nuss procedure. PE is generally not life-threatening, so it often gets overlooked in the medical community for research and funding. However, there is a chance that PE can regress back to its original state after surgery. Approximately 4/100 patients experience PE more than once and the odds of this happening increase when surgery is performed before the patient has hit puberty .

There is currently no way of knowing when the chest deformity has been completely reversed by the procedure. Surgeons depend on past cases and their own experience in order to make the final decision of removing the bar. This gap in the procedure is the driving force behind the senior design project. The objective of the project is to relay information to the doctors, informing them of when it is appropriate to remove the steel bar. The method chosen to accomplish this goal utilizes Radio

Frequency Identification (RFID) technology and strain gauges.

A RFID tag is a miniature device that has the capability to wirelessly transfer it’s stored data when read by a RFID reader. The RFID tag takes in the radio signal and transmits back out the stored information. RFID tags are currently being used for many different applications, such as contactless electronic payment systems, goods tracking,

6 as well as systems monitoring. There are three primary types of RFID: active, passive, and semi-passive RFID. Active are powered by a battery and have the ability to change it’s onboard data. Passive are powered off of an incoming wireless signal, but can not change it’s onboard data. Semi-Passive tags are a combination of active and passive, that can change it’s onboard data while also being powered off of the incoming wireless signal.

Intel’s Wireless Identification and Sensing Platform (WISP) technology, is a specific type of semi-passive RFID tag. The WISP system can be reprogrammed and has a built-in accelerometer, gyroscope, and temperature sensor. Even though the

WISP system can support an impressive amount of sensors, it is entirely powered off of the incoming wireless radio signal. When the radio signal is received by the WISP, it is harvested and stored in the onboard capacitor and then used to carry out it’s programming. Because the WISP system does not require an onboard battery and is powered wirelessly, it is an attractive option for use inside the body. The WISP also has the capability to be attached to various other sensors or controls, such as strain gauges.

Intel has developed a strain gauge solution for the WISP and made their blueprints and schematics for the WISP open source and available. This information provided by Intel was applied to the project.

In order to know when it is time to remove the steel bar from the patient, it is necessary to be able to measure the amount of force applied on the bar, by the sternum. Removal should only take place when there is no force on the bar. For the purpose of measuring the force, a strain gauge needs to be placed on the bar at the location that undergoes the most force. Intel’s strain gauge solution requires building a

7 wheatstone bridge and amplifier and connecting them to the WISP. The strain gauge needs to be connected to the WISP, so the WISP, wheatstone bridge and amplifier will also need to be attached to the steel bar somehow. The goal of this project is to develop a working prototype that incorporates the described WISP/strain gauge system to measure force on a steel bar. A theoretical model will also be designed, because the prototype developed for this project will be too large and will need to be scaled down for use in the real world.

8 Method

In order to provide a visual representation of the ideal model and the prototype that will be delivered, AutoCAD and PATRAN models were created. The AutoCAD model was designed to show the placement of the strain gauge on the bar. As printed circuit board (PCB) manufacturing and miniaturizing or rearranging of the WISP was outside the scope of this project, the prototype developed in this project will have all the working components on the outside of the bar. The AutoCAD model shows where a strain gauge would be placed, as well as the channels for the wires to be run on the inside of the bar. Similarly, the wheatstone bridge, amplifier, and WISP are shown as black boxes, with their volumes recalculated to fit around the curve of the bar and within the smaller height of the bar. Finally, a PATRAN model was designed to show how the forces should look within the bar, so as to check if the experimental results are accurate.

The WISP/strain gauge system consists of a WISP unit that was provided by

Intel, RFID reader and antenna, a strain gauge, a wheatstone bridge, and amplifier.

However, the most important element, which the WISP will be attached to, is a nine-inch steel bar (Fig. 7) provided by Dr. Robert Kelly, a pediatric surgeon at CHKD. The steel bar was previously used and the shape was no longer ideal; therefore, it was necessary to re-bend the bar to make it suitable for the project. Stainless steel was chosen instead of an alternative material, titanium, because it is more commonly used in surgeries, the process of adding strain gauges is simpler, and titanium is far more expensive than steel.

9 Fig. 7 Titanium Bar (Top) and Steel Bar (Bottom)

The WISP has a range of up to 15 feet and is completely powered by a RFID reader, collecting the radio signals that are emitted from the reader, converting into DC power which is stored in an onboard capacitor and used for transmitting the programmed data back out. For the purpose of this project, the Impinj Speedway 1000

RFID Reader was purchased. Fig. 8 depicts the process of how the WISP technology works. A computer running the reader software, connects through the network to the

RFID reader. The commands given in the software are then converted to radio frequency signals by the reader and absorbed by the antenna of the WISP. After data is processed by the WISP it sends the reader antenna a return signal. The antenna then sends the data back to the reader, which relays the data back to the computer to be analyzed.

10 For the sake of simplifying the design and an attempt to avoid potential power issues, there will only be one strain gauge attached to the bottom of the bar, directly in the center. The strain gauge that will be used is a SGD-13/1000-LY11 which uses 1000 ohms. This particular gauge was chosen because it was used with previous success with the WISP as demonstrated by the Lamborghini Team at Univ. Washington .

Theoretical Design

Part of the objective of this project was to create a theoretical design that allows the WISP and wheatstone bridge/amplifier to be implanted within the steel bar. Unlike the experimental design that could only accommodate one strain gauge, the theoretical model has two strain gauges, in order to reduce electromagnetic noise as well as cancel the tension / compression effect of the bar in bending. Even though new components are being introduced to the steel bar, it is important that the procedure and the effects on the body do not change because of the additions. Therefore, the theoretical design requires the strain gauges, WISP, wheatstone bridge and amplifier to be implanted within the steel bar itself. By encasing these components within the steel, all biocompatibility issues are bypassed. The WISP antenna will have to be run outside of

11 the bar in order to avoid interference from the steel itself, so the bar will need to be coated with a medical grade epoxy. However, the provided WISP, wheatstone bridge and amplifier that were built are too large to be implanted within the steel bar. So, it was necessary to perform theoretical calculations in order to determine what size the

WISP, wheatstone bridge, and amplifier should be. According to images in Gasco's publication , the wheatstone bridge and amplifier that are used in accordance with the

WISP amount to about the same size as the WISP, so the volume calculations of the wheatstone bridge and amplifier are assumed to equal that of the WISP.

The WISP’s actual volume (0.54 cm3) was calculated using the following dimensions:

· Height = 1.8 cm · Length = 1.5 cm · Thickness = 0.2 cm

However, the steel bar cannot accommodate this volume because the area that will house the components only has a height of 1 cm and a thickness of 0.3 cm. To ensure that the bar remains strong enough to withstand the pressure being applied by the sternum, the design requires only half of the thickness to be occupied by the WISP, wheatstone bridge and amplifier. Knowing that the WISP must fit within a height of 1 cm, a new length of 3.6 cm (1.42 in) was calculated. Therefore, a theoretical dimensions were calculated as such:

· Height = 1 cm · Length = 3.6 cm · Thickness = 0.15 cm

This gets the same volume as the original WISP volume. Assuming a design factor of

1.2 for unexpected design limitations of the individual electronic components, the volume of the WISP is 0.648 cm3. With this new volume, the two constrained

12 dimensions can not change (height and thickness), thus the new length is 4.32 cm

(1.701 in). Also assuming that the wheatstone bridge and amplifier are the same size as the WISP (or smaller), the total volume of both black boxes is 1.296 cm3.

AutoCAD Analysis

An ideal wireframe model was created in AutoCAD (Autodesk, San Rafael,

California) to show the dimensions of the ideal model prototype (Fig. A.1.). The ideal model shows two strain gauges inlaid into the bar itself, to provide more accurate results, as well as a channel for the wires for the strain gauges. There is also a minimum volume which was be needed to fit the WISP, wheatstone bridge, and amplifier within the bar without compromising the structural integrity of the bar. Finally, the antenna of the WISP was run to the outside of the bar to prevent interference with the steel.

The wireframe was developed using the dimensions provided as well as the actual steel and titanium bars provided. The X-Y projection was first drawn as a base and then copied 1.5 cm in the Z direction to create the top and bottom of the bar. Then the fillets were added to either end based on measurements taken, as well as the hole through either end. Finally, the black box for the strain gauge was drawn in the center of the backside of the bar. This completed the real model and provided the basis of the ideal model. The ideal model has the blackbox of the strain gauge moved to the inside of the bar and mirrored across the centerline of the bar. A square prism is then extended off of the black box and connected at the bottom of the strain gauges. This is to provide a channel for the cables off of the strain gauges. A similar sized square area is extended along the arc of the bar for the cables to run through.

13 Finite Element Analysis

A 2-dimensional model of the steel bar (Fig. A.2.) was created using PATRAN

(MSC Software Corporation, Santa Ana, California), a finite element analysis software.

This was done to generate theoretical results, which would be compared to the experimental values found in testing and to determine ideal strain gauge placement.

First, dimensions of the bar were taken and the significant locations were plotted on an

X-Y-Z coordinate system. Lines were created to connect the points and fillets were implemented to represent the curvature of the bar. All of the lines were then translated

1.2 centimeters in the Z direction (width of bar), which allowed five surfaces to be created. After producing this simple outline of the bar, material properties of steel were applied and a thickness of 0.3 centimeters was assigned. In order to create quad elements and bypass connectivity issues, mesh seeds were strategically assigned.

Four seeds were placed on the smaller surfaces of the bar and twelve seeds were placed onto the larger surfaces. To assure that all the nodes were connected and in order, the bar was then equivalenced and optimized.

When the steel bar is surgically implanted into the body, one side has a stabilizer attached to it in order to prevent the bar from flipping back. To represent this in the

PATRAN model, one end of the bar was constrained to prevent translation in the X, Y and Z directions and rotation in the Y direction. Rotation was allowed in the X and Z directions in the case of bending due to the load applied by the sternum. A downward force was applied in the center of the bar to represent the sternum and two upward forces were applied to represent the ribs. A Young’s modulus value of 1.92 GPa was used for this type of stainless steel, along with a Poisson’s ratio of 0.265.

14 A .bdf file was created through PATRAN, so different values of the forces could be analyzed using NASTRAN (MSC Software Corporation, Santa Ana, California), a finite element analysis program. Finally, NASTRAN created an .op2 file, ran it back through PATRAN.

15 Experimental Design

Strain gauges, are used to measure strain by the macroscopic change of resistance by the small changes on a conductive grid. This gives a direct correspondence of physical strain to electrical signals. Typically a strain gauge has a couple volt input while only outputting changes in millivolts.

Where:

∆R = Change in Resistance due to strain

Rg = Resistance of the gauge without deformity

GF = Gauge factor of the gauge.

ϵ = Strain.

Strain gauges are normally matched with a wheatstone bridge, which are used to measure an unknown resistance.

16 Fig. 9 Experimental Setup

Designing the wheatstone Bridge and Amplifier Circuit for the Strain Gauge

The input from the wheatstone bridge strain gauge is small, specifically at the low

1.2V voltage the WISP provides. The input potential difference at the center of the bridge will need to be amplified to increase the accuracy of readings due to the noise interfering with the RFID signal from the WISP to the reader. A publication of a research project led by Federico Gasco at “Automobili Lamborghini Advanced

Composite Structures Laboratory, University of Washington, Seattle, WA” regarding a strain gauge solution for the WISP was used by the team as a reference.

A challenge for implementing this circuit consists of getting an accurate end result that can be reliably used by physicians, considering that noise levels from the several devices at a physician’s clinic will be considerably higher than the noise in the lab. This includes the following:

17 i) Finding an operational amplifier that will operate on low voltages. ii) Creating a suitable gain from the operational amplifier circuit. iii) Assuring resistance accuracy. iv) Soldering the wheatstone bridge.

Hardware Solution

The resistors needed to be checked using an ohmmeter as they usually didn’t match the figures listed on the package. In order to get exact resistances as the Gasco publication circuit we had to use 2 resistances in series for each resistance on the circuit.

For the wheatstone bridge, initially the resistors were assembled on a circuit builder board, but was extremely inaccurate as the board connections had high resistances. It was opted for a soldered wheatstone bridge consisting of three pairs of

1999Ω resistors connected in parallel then soldered to the strain gauge wire. Final resistances are 999.5Ω for each pair of resistors and RSG, initial of 1001Ω. In the testing of the wheatstone bridge, a voltmeter was attached to the leads going to the WISP, so as to monitor the voltage and prevent the WISP from being overloaded.

For the operational amplifier circuit we had 2 different designs, the circuit developed by

Gasco that proved unsuitable for our strain levels and a circuit designed by the team.

The following is the original circuit designed by Gasco; the potentiometer is set to 750Ω according to their publication.

18 The following is a modified version of the circuit designed to increase amplifier gain. The 200Ω resistance has been removed, the resistances of the wheatstone bridge have been reduced to 1kΩ, the 2000Ω resistance was removed, and the potentiometer was set down to 50Ω.

The following circuit was designed to further amplify the gain for our specific low strain setup.

19 It was not possible to find a low voltage operational amplifier at the labs of

Kaufmann building. The technology is new, yet it is possible to attain such a low voltage amplifier, but exceeded the timeframe and budget of this project. The current setup on external higher DC voltage from a power supply 10-15 DC and the exclusive use of the wheatstone bridge has provided reasonably accurate results on the WISP

GUI software. However, for the ideal design a low voltage operational amplifier is required in order to show more accurate results and amplify the current mV output to a

Voltage.

Software and Firmware

There are two different portions of code that went into this project, the firmware of the WISP unit itself, and the GUI (Graphical User Interface) that controls the RFID reader and interprets the data.

20 By default, the WISP outputs quick acceleration as its data. In order to change this, the firmware on the WISP itself must be updated. The WISP outputs its data via the EPC (Electronic Product Code), so it can be read by any standard RFID reader.

The EPC format is as follows:

[ 1 byte | tag type] + [ 8 bytes | data] + [ 1 byte | WISP HW Version] + [ 2 bytes | HW Serial #]

where the tag type defines what type of sensor is being used on the WISP to the software. The data is variable, but the HW Version (WISP 4.1) and hardware serial number do not change.

The internal temperature code for the WISP was used as a base model to output Analog DC

Voltage as a reading (INCH6 - Input Channel 6)(code in appendix). Pin 3.6 on the WISP is the default for the input for this reading, as well as pin 3.3 is voltage output, which would be used in the theoretical model with the full circuit of the wheatstone bridge and amplifier powered off the

WISP itself.

Fig. 13 WISP with leads for V , V , and Ground in out After the firmware of the WISP was proven to be working, the WISP GUI had to be modified in order to output at least proper voltage, if not a calculated and calibrated

21 strain value. Using the temperature sensor tab of the interface, as well as its background code, the voltage output was added into the code under the 0x37 tag type, a type added into the code to specify the additions in this project. The temperature conversion was removed, so as to get raw data output of the AVDC input.

22 Results

Theoretical

The finite element model created in PATRAN generated stress (Fig. 15), displacement (Fig. 14) and strain values for the forces applied.

23 Experimental

Fig. 16 Software Results

24 In order to test the output of the WISP, the firmware of the WISP was modified to act as an external temperature sensor (0x0E tag type). This would show the change of the voltage sensor on the WISP without having to modify the GUI code of the reader software, even if the numbered reading out was not accurate. Fig. 16 shows the results from one such test (note the

Ext Temp under sensor), with the first part without voltage applied to the wheatstone bridge, the second part with voltage applied, the third part with force applied to the bar and strain gauge, and finally with the force removed. This test was done without the amplifier circuit and shows that the system is functional in the crudest sense.

25 Discussion

The purpose of this project was to create a device that can alert physicians of when the sternum is no longer applying any force onto the pectus bar; therefore, inhibiting the sternum from regressing back to it’s original state after the pectus bar is removed. This was done through the use of WISP technology and a strain gauge.

These devices were attached to a nine inch bar.

There were significant limitations in the testing of the experimental model, including limited time, limited knowledge of electronic components and code, and hardware limitations. The amount of time for programming, learning the code for the

WISP, as well as the time it took to receive equipment was underestimated. However, despite the listed issues, it was proven that the ideal system was a possibility.

If testing had gotten a value of strain which the bar was under, the PATRAN model would have been used as a baseline to compare the experimental values to; therefore, verifying the results of testing. Should the results have been accurate, the

PATRAN model would then have been used to predict the steel's strain with the portions hollowed out for the various blackboxes, to show that the bar could still withstand the forces of the body without failure due to stress.

It is known that the output of a strain gauge and wheatstone bridge system is in millivolts, which must be amplified in order to be read easily. Unfortunately, the WISP puts out a maximum of 1.8 V which is not enough to power the wheatstone bridge or the operational amplifier provided. In order to provide accurate test results, an external power supply was used for testing purposes, providing 10V to wheatstone bridge. The amplifier designed for the system was using an operational amplifier that required a

26 minimum of 10V to function, which was higher than the input coming in from the wheatstone bridge and the WISP. This caused random fluctuations in the amplifier circuit, thus it was abandoned for initial testing. In addition to prototype board issues of the amplifier, there were issues getting all software necessary on the computer provided, as well as hardware limitations of the computer itself. Ideally, a lighter program would be written, that would simply output the strain values after initial calibration, which would report via email to surgeons when the bar is no longer under strain. A smaller program would be needed to be able to run on a microcomputer such as the Raspberry Pi ($25 microcomputer). This is assuming that a computer system for continuous reporting was used. A desktop computer could be used so long as the patient was scanned by the RFID reader at least once a day.

While, there were issues with the experimentation, the fact remains that the system is viable and would help provide accurate data to surgeons. The ideal model can be used to implement the system, after the WISP, wheatstone bridge, and amplifier are redesigned to fit the model dimensions. If this system is implemented as such, the bar would have to be bent at the factory and the electronic components built into it, or a plug and play system would have to be setup. Though a system being built into the bar should not be an issue for bars such as titanium that must be bent at the factory, a plug and play system would be more viable for steel bars which are bent on-site. A plug and play system would involve the backside of the bar being hollowed out for the WISP and assorted PCBs, the PCBs printed on flexible backing, and plugs for the strain gauge wires to be plugged into the PCB. After the bar was bent into the right shape, the PCBs would be plugged in, attached with an epoxy and covered with a biocompatible material.

27 It is expected that the center of the bar would not be bent (outlines of strain gauges could be added to show where the bar could not be bent), as well as extra wire coiled inside the bar will allow the bar to be bent without effecting the internals. The theoretical blackbox calculations were designed with a design factor of 1.2, and with that factor the design length of the blackbox barely fit within the 9in bar, though the original volume of the WISP, wheatstone bridge, and amplifier circuit could fit. Since the

PCB of the WISP was designed for developers, it can be expected that a system that only has the needed outputs (no accelerometer, temp sensor, ect.) would be significantly smaller than the original. It was decided that this was acceptable, as it is not recommended to have this procedure done on a patient before they have gone through puberty, as a growth spurt can cause a recurrence of Pectus Excavatum.

28 Conclusion

While there were issues in the experimental setup, the system was proven to work in the rawest sense. After several different attempts to build an amplifier circuit, it was opted to only use the wheatstone bridge for testing. Several different pieces of firmware were attempted for use with the WISP, but the third piece (test_run.c)(see appendix), which went back to the basic temperature code and was modified, was the only successful example. Due to time constraints and limitations of the testing equipment, the code for outputting voltage or strain was not finished.

Despite these issues, the system is still viable, and the developed ideal AutoCAD model shows how each of the individual components could fit within the bar itself. The system would provide doctors with real-time information, when the bar is no longer under strain, and thus ready to be removed.

29 Appendix

Fig. A.1 AutoCAD Wire Frame Model

30 Fig. A.3 AutoCAD Wire frame model (Front and Top Views)

31 Code test_run.c (modified WISP firmware code):

/* See license.txt for license information. */ #include "mywisp.h" #if (ACTIVE_SENSOR == TEST_RUN)

#include "dlwisp41.h" #include "rfid.h" #include "test_run.h" unsigned char sensor_busy = 0; void init_sensor() { return; } void read_sensor(unsigned char volatile *target) {

#if MONITOR_DEBUG_ON // for monitor - set READ_SENSOR_STATE debug line - 00101 - 5 P1OUT |= wisp_debug_1; P1OUT &= ~wisp_debug_1; P2OUT &= ~wisp_debug_2; P3OUT = 0x21; #endif

P3OUT |= BIT3;

// slow down clock BCSCTL1 = XT2OFF + RSEL1; // select internal resistor (still has effect when DCOR=1) DCOCTL = DCO1+DCO0; // set DCO step.

if(!is_power_good()) sleep();

// already off. Only needs to be done when READ has set P1OUT &= ~RX_EN_PIN; // turn off comparator

// Set up ADC for internal temperature sensor ADC10CTL0 &= ~ENC; // make sure this is off otherwise settings are locked. ADC10CTL1 = INCH_6 + ADC10DIV_3; // Volt Sensor ADC10CLK/6 ADC10CTL0 = SREF_1 + ADC10SHT_3 + REFON + ADC10ON;

// a little time for regulator to stabilize active mode current AND // filter caps to settle. for (int i = 0; i < 100; i++); // Doubled settling time to filter noise

// start conversion unsigned int k = 0; ADC10CTL0 |= ENC + ADC10SC; // Sampling and conversion start

while (ADC10CTL1 & ADC10BUSY); // wait while ADC finished work

*(target + k + 1 ) = (ADC10MEM & 0xff); // grab msb bits and store it *(target + k) = (ADC10MEM & 0x0300) >> 8;

// Power off sensor and adc ADC10CTL0 &= ~ENC; ADC10CTL1 = 0; // turn adc off ADC10CTL0 = 0; // turn adc off

// Store sensor read count sensor_counter++; ackReply[10] = (sensor_counter & 0x00ff); // grab msb bits and store it ackReply[9] = (sensor_counter & 0xff00) >> 8;

return; } unsigned char is_sensor_sampling() { if ( sensor_busy ) return 1; return 0; }

#endif // (ACTIVE_SENSOR == TEST_RUN) test_run.h (header file for c code):

/* See license.txt for license information. */

#ifndef TEST_RUN_H #define TEST_RUN_H

33 #define SENSOR_DATA_TYPE_ID 0x0E // Use 0x0E for proof of function (fake temp) // Use 0x37 for test code for voltage

#endif // TEST_RUN_H

34 Budget

Budget $750.00 Impinj Speedway 1000 RFID Reader $200.00 Antenna (900-925 MHz) $35.00 Strain Gauge (SGD-13/1000-LY11) $125.00 WISP Development Kit $0.00 Stainless Steel Lorenz Pectus Bar $0.00 Total $360.00

Gantt Chart

35 References

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