Assessment of Applications of Force Sensing Materials in Robotics

“Assessment of Applications of Force Sensing Materials in Robotics”

ET-493_01 Senior Design I Cris Koutsougeras / Junkun Ma Advisor: Mohammad Saadeh Spring 2016

Andrew Allemond W0511221 Mechanical Engineering Technology Blake Dufour W0442892 Mechanical Engineering Technology ET-493_01 Andrew Allemond Senior Design I W0511221 Spring 2016 Blake Dufour Cris Koutsougeras / Junkun Ma W0442892

Table Of Contents

Abstract…………………………………………………………………………………………....3

Introduction………………………………………………………………………………………..4

Background………………………………………………………………………………………..5

Piezoelectricity………………………………………………………………………………….6-7

Piezoelectric FDT Sensor ……………………………………………………………………....8-9

Sensor Sampling Materials…………………………………………………………………...10-13

Timeline………………………………………………………………………………………….14

References………………………………………………………………………………....……..15

Abstract:

Create a tactile sensor system into two realistic-looking artificial skin gloves. These artificial skin tactile sensors will use suitable technologies (Research is needed in order to determine the best suitable technology needed) to measure applied force at multiple points on each glove. The sensors will have to be placed in certain areas of the glove in order to receive a full reading of the force on the entire hand. The use of an Open Source Program will be needed in this step (Arduino or Raspberry Pi). The gloves will be created using silicone rubber to simulate both the texture and look of human skin, while maintaining both flexibility and durability (research will be needed in order to determine a suitable silicone rubber product with molding capabilities). Sensor data will first be sent via USB to a computer, where a graphical user display will be created to show the tactile information in real time. The use of an Open

Source Program will be used in this step (Research still needed on a suitable program for this step). Once the gloves are completed, research will be needed in order to wirelessly record the data (XBee’s or Raspberry Pi’s equivalent wireless communication).

Introduction:

Southeastern Louisiana University 500 Western Ave. Hammond, LA 70402 College of Computer Science and Industrial Technology Department of Engineering Technology ET-493_01 Andrew Allemond Senior Design I W0511221 Spring 2016 Blake Dufour Cris Koutsougeras / Junkun Ma W0442892 With technology making ever more advanced mechanical manipulators possible. The field of robotics now has a need for tactile sensors that are more advanced than those already in society. Current tactile sensors consist mainly of rigid or semi-rigid flat sheets that must be conformed to flat grippers. Recent studies have found that greater handling can be achieved by using deformable grippers in order to maintain more contact area and contact friction with the object, much as human skin is able to deform around a hard object. To create a tactile sensor system out of materials not used in robotics today, while sustaining the ability for precise measurement would be beneficial to many different fields of science and engineering.

Background:

Southeastern Louisiana University 500 Western Ave. Hammond, LA 70402 College of Computer Science and Industrial Technology Department of Engineering Technology ET-493_01 Andrew Allemond Senior Design I W0511221 Spring 2016 Blake Dufour Cris Koutsougeras / Junkun Ma W0442892 Often times sensors are used to quantify information about the environment, such as telling temperature or force. Sensors are sometimes used to relay information from an inaccessible location, such as beyond a wall or inside a chamber. Many times they provide information beyond the capability of human senses, such as extremely high speeds. In other cases, the sensor can provide a range of data about the measurement, such as the magnitude of a force value relative to a known measurement. Perhaps the most basic sense, but also the most useful type of sensor is one that provides input about pressure or force. Sensing the magnitude of the force via touch is crucial when dealing with fragile objects, such as picking up an egg or screwing in a light bulb. If too great a force is exerted on a fragile object, the object is likely to break. Today’s society has relied on resistive force sensing modules in robotics, which has limited the advancements of sensing. The use of a new sensing ability will bring light to the advancement in this field of study.

Piezoelectricity:

“cell” of a cubic or rhomboid cage made of atoms. Inside this cell is a single semi-mobile ion which has several stable quantum states. This post-ion state of the ion can be caused to shift by either deforming the cage through applied strain or by applying an electric field.

The relationship between applied force and the resultant response depends on the piezoelectric properties of the ceramic, the size and shape of the piece, and the direction of the electrical or mechanical load/stress. In order to characterize a piezo, a number of property coefficients are defined, many of which feature single or double subscripts that link electrical and mechanical quantities. The first of these subscripts indicates the direction (as defined in the Figure above) of the electrical field associated with the voltage applied, or charge produced. The second subscript gives the direction of the mechanical stress or strain. The most important property constant to discuss is the voltage coefficient, “G,” which is used to relate the electric field produced by the mechanical stress, having units of volts/meter per Newton/square

Piezoelectric FDT Sensor:

The “F” in FDT Series stands for “‘Flexible Leads”. These are rectangle elements of Piezo film with silver ink screen printed electrodes. Rather than making the lead attachment near the sensor, the Piezo polymer tail extends Southeastern Louisiana University 500 Western Ave. Hammond, LA 70402 College of Computer Science and Industrial Technology Department of Engineering Technology ET-493_01 Andrew Allemond Senior Design I W0511221 Spring 2016 Blake Dufour Cris Koutsougeras / Junkun Ma W0442892 from the active sensor area as flex circuit material with offset traces. This gives a very flat, flexible lead with a connector at the end. The FDT elements are available in a variety of different sizes and thicknesses. They are available without a laminate (FDT), with a laminated (0.005” mylar) on one side (FLDT) or with tape release layer adhesive (FDT with adh) in the sensor area.

The connector pins on the FDT sensors can be directly soldered to a PCB with a reasonable level of care.

This component cannot withstand high temperatures (>80°C) and therefore soldering of the pins to a PCB must be done quickly. A heat sink clamped to the interface area between the film and the crimps will take the heat away from the film. Pre-tin the pins and then quickly solder them to the board. Do not allow the soldering iron to touch the film and do not use a dwell time of more than 5 seconds on the pins. Low temperature solder can also be used.

The idea of using piezoelectricity as a force sensor is to use the electrical charge generated when polyvinylidene fluoride (PVDF) film is deformed on impact, i.e. pressed, bent, etc. The measured voltage is proportional to the applied force in an area of the film. These come in different sizes and thicknesses to fit the application at hand. The physical dimensions and properties of the films available to us are listed in reference to each individual sensor. Piezoelectric materials are anisotropic, meaning that the response to pressure will be dependent on the axis on which the pressure is applied. As we will be applying pressure on the thickness of the film, i.e. the third axis the voltage will be directly proportional to the force applied.

The lower-right inset shows that the time sequential electrical output was 11 mV at less than 30 kPa pressure. We applied the pressure for 2 s and measured the electrical output under six different pressure conditions. The pressure range converted from the applied force was calculated as a range from 3 kPa to 30 kPa. A gentle touch, reportedly, refers to a Southeastern Louisiana University 500 Western Ave. Hammond, LA 70402 College of Computer Science and Industrial Technology Department of Engineering Technology ET-493_01 Andrew Allemond Senior Design I W0511221 Spring 2016 Blake Dufour Cris Koutsougeras / Junkun Ma W0442892 pressure below 10 kPa [2], which is included in the pressure range we measured. For the discrete run, we acquired a linear sensor response from 3 kPa to 30 kPa pressure ranges and the output voltage was linearly proportional to the applied pressures from 1.8 mV to 11 mV. The upper-left inset shows the P-E hysteresis of PZT films, showing good ferroelectric properties.

Sensor Sampling Materials:

A multiplexer, or mux, is another common component of many circuit designs. A multiplexer is an electronic device capable of selecting one out of a number of analog or digital input signals and

Piezoelectric sensors simultaneously throughout the many different data analysis components of the project.

Operational amplifiers are high-gain electronic amplifiers feature a differential input and usually a single-ended output, capable of amplifying some voltage hundreds to thousands of times larger than the voltage difference between the inputs. Ideally, an op-amp has an input offset voltage of zero, as well as infinite input resistance. While ideal op-amps do not exist in real life, present day op-amps have come so close to ideal that they can usually be assumed as ideal for analysis purposes. A multitude of circuits can be created using op-amps, including noninverting amplifiers, inverting amplifiers, differential amplifiers, adding circuits, and many others, each serving a different purpose. When operating at unity gain (no amplification), the noninverting amplifier reduces to a voltage follower, where the output voltage is identical to the input voltage. By using an op-amp to create a voltage follower circuit, a high impedance input source can be buffered to a low impedance output. When used with a piezoelectric sensor, it allows for measuring of the voltage difference, while greatly reducing charge leakage.

Invented in 1833 by Samuel Hunter Christie and later improved upon by Charles Wheatstone in

1843, the Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit where one leg includes the unknown component. The main benefit of the Wheatstone bridge is that it can provide extremely accurate measurements unlike a simple voltage divider. A Wheatstone bridge’s ability to measure is similar to an original potentiometer; however, its initial purpose was for soils analysis and comparison.

An example of the Wheatstone bridge is featured in the example above. Rx symbolizes the unknown resistance to be measured with R1, R2, and R3 are the resistors with a known value of resistance and R2 is the resistance that is adjustable. The theory states that if the ratio of the two resistances in the known leg (R2, R1) is equal to the ratio of the two in the unknown leg (Rx,R3), then the voltage between the two midpoints (Band D) will be zero and no current will flow through the galvanometer Vg. Therefore, when the bridge is unbalanced, the direction of the current indicates whether

R2 is too low or too high. Since R2 is variable, the galvanometer reads zero once there is no current

At the point where balance is achieved, the ratio of R2/R1=Rx/R3 or it can be done as Rx=R2/R1 x R3. In other words, if R1, R2, R3 are known and R2 is not adjustable, the difference of voltage across or current flow through the meter can be used to calculate the value of Rx, using Kirchhoff’s circuit laws.

This is used frequently in resistance thermometer and strain gauge measures, since it is faster to read a voltage level off a meter than to adjust a resistance to zero voltage.

What all of this means is that the Wheatstone bridge demonstrates the concept of a difference measurement, which can be extremely accurate. Variances on the Wheatstone bridge can be used to measure capacitance, inductance, impedance and other quantities. The Wheatstone bridge was specially adapted and the Kelvin bridge was developed from the Wheatstone bridge to measure very low resistance.

This is important because measuring the unknown resistance is related to measuring the impact of a certain physical event such as temperature, pressure, or force and this allows the use of the Wheatstone bridge in measuring these elements indirectly. Later developments upon the Wheatstone bridge was conducted to measure alternating current measurements by James Clark Maxwell in 1865 and again by

Alan Blumlein in 1926.

Processing is an open source programming language and environment meant to create images, animations, and interactions. Because the Arduino programming environment is based on the Processing programming environment, Processing was chosen to create a graphical user display. This graphical user display would enable viewing the data from each sensor in real-time using a computer screen. Several sample programs included in the Arduino programming environment provided instruction for how to read

Timeline: February 10th – 24th:  Research piezoelectric sensors.  Research method for sampling multiple sensors.  Research data acquisition method. March 3rd – 16th:  Order Materials for testing purposes.  Research piezoelectric equations. March 16th – 23rd:  Code microcontroller for sampling sensors.  Receiving materials.

References:

Analog/Digital MUX Breakout. (2010). Retrieved February 16, 2016, from SparkFun Electronics:

http:// www.sparkfun.com/commerce/product_info.php?products_id=9056

FingerTipMulti-Touch Screen Controller with Ultra-Low Power. (n.d.). Retrieved March 02, 2016, from

http://www.st.com/web/catalog/sense_power/FM89/SC1717/SS990/PF251157

Operational Amplifier Basics - Op-amp tutorial. (2013). Retrieved February 27, 2016, from

http://www.electronics-tutorials.ws/opamp/opamp_1.html

Mancini, R. (2002, August). Op Amps For Everyone. Retrieved January 20, 2016, from Texas

Instruments: http:// focus.ti.com/lit/an/slod006b/slod006b.pdf

Piezo Systems, I. (2008). Introduction of Piezo Transducers. Retrieved January 9, 2016, from Piezo.com: Southeastern Louisiana University 500 Western Ave. Hammond, LA 70402 College of Computer Science and Industrial Technology Department of Engineering Technology ET-493_01 Andrew Allemond Senior Design I W0511221 Spring 2016 Blake Dufour Cris Koutsougeras / Junkun Ma W0442892 http:// www.piezo.com/catalog7C.pdf%20files/Cat7C.20,21,22,23,60,61&62.pdf

Stutz, M. (2001). Multiplexers. Retrieved February 16, 2016, from

http ://www.allaboutcircuits.com/textbook/digital/chpt-9/multiplexers /

Southeastern Louisiana University 500 Western Ave. Hammond, LA 70402 College of Computer Science and Industrial Technology Department of Engineering Technology