Applications of E-textile Pressure Sensors

Akshaya Dinesh Hanna Goldfarb akshayadinesh19@gmail.com hgoldfarb@hcrhs.org

Diana Sarrico Catherine Scanlon Sung Hyun (Sunny) Yoo diana.sarrico@yahoo.com cate.scanlon@aol.com sunyoo@bergen.org

Dr. Aaron Mazzeo* aaron.mazzeo@rutgers.edu

New Jersey’s Governor’s School of Engineering and Technology July 21, 2017

*Corresponding Authors

ABSTRACT -- In many cases between civilians and law enforcement officers, there is often a lack of substantial evidence for the court to make an informed decision. By creating an electronic T-shirt that records hits to a policeman’s body, one can determine whether the officer’s forceful action was instigated and justified. Tracking pressure changes on a shirt can also help detect injuries and determine the severity of an attack. The specific technology used in this project is e-textiles (electronic textiles), specifically sensors made using Velostat, a piezoresistive material. Velostat sensors are constructed using conductive thread in a circuit to send data to an Lilypad Arduino. Spikes in pressure data are represented using a graphical interface for other law enforcement officers to view. The testing and design of this project show that a system including Velostat sensors and Arduinos in e-textiles is a viable, effective method of providing external evidence in police brutality cases.

I. INTRODUCTION

E-textiles incorporate circuitry into wearable clothing by replacing large wires with conductive thread, a nylon thread coated in silver to generate electric properties. This allows for an inconspicuous and less intrusive way of using sensors to obtain information such as temperature, motion, pressure, and air contaminants. E-textiles that monitor cardiac pulse for arrhythmias, heart attack, or heart failure are also ideal for analyzing more sensitive areas of the

human skin. There are also wearable muscle sensors, allowing users to know whether or not their muscle is strained or overused in daily life. Thus, e-textiles have potential in recreational clothing, health analytics, geo-tracking, and now police uniforms[1].

E-textile sensors could further improve law enforcement efficacy by creating another layer of transparency and safety.

In cases regarding violent acts between a police officer and a civilian, it can be difficult to ascertain the exact events that occurred at the scene. Often times, the testimonies offered by the police officer differ from the civilians’.Wearable technology is an optimal solution to create more transparency in such cases. Ideally, judges and courts could get a closer look into the incident and determine whether or not any force inflicted on a civilian was provoked and justified. To do so, Velostat textiles can be incorporated into a policeman’s uniform in order to measure any force used against the officer. These are a type of e-textile sensor that can monitor applied pressure. The pressure data can be time-stamped and used alongside video footage to provide more conclusive evidence in trials and more accurate indictments[2].

An equally important application for these Velostat sensors involves an alert system to aid law enforcement agents who are injured in the line of duty. If pressure exceeds a particular threshold that would indicate injury, ideally other squadrons could

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be alerted and come to the aid of the law enforcement agent in question.

This study focuses on creating a wearable garment that can track applied pressure, allowing for a measurement of physical aggression against police officers to settle confusion surrounding contradicting testimonies in court cases and also facilitate a safety alert system.

II. BACKGROUND 2.1 Materials and Tools

The specific material incorporated into the e-textiles sensors constructed throughout this study is called Velostat. Velostat is the brand name of a black, carbon-impregnated polyolefin foil originally created by Custom Materials, of the company 3M. It is typically used to protect components sensitive to electrostatic discharge, and its properties are generally not affected by age or humidity. This piezoresistive material can be used in a wide range of pressure and bend sensors, resistive sensors, and position sensors. This is because it changes electrical resistance in response to pressure. As force is applied, the resistance in the polyolefin textile decreases because the material is compressed and the carbon molecules become closer together, increasing the conductivity[3]. This is due to Ohm’s Law, which states that current and resistance are inversely proportional when voltage is constant. Thus, when placed between two non-conductive cotton layers and wired using conductive thread, the Velostat becomes similar to a variable resistor in a traditional circuit.

For the exact cutting of the Velostat material, a Silhouette Cameo vinyl cutter is used. A corresponding program is used to create exact measurements on a diagram, which the cutter is then capable of replicating.

Throughout the study, the Model SE-400 Brother sewing machine is used to construct the Velostat sensor circuit[4]. Also, both conductive thread and non conductive cotton thread are used in conjunction with cotton fabric to simultaneously secure the sensor unit and establish contact points on the Velostat resistor where the current can travel through. The conductive thread utilized is silver-coated nylon, which allows it to conduct electricity.

Moreover, for testing purposes, an INSTRON 44-11 machine is test pressure.[5] The Instron machine has an anvil that extends down and applies force. An Impedance Analyzer is also used during testing to measure changes in resistance as more pressure is continually applied to the sensor.

Finally, an Arduino Uno and an Lilypad Arduino, as seen in Fig. 1 and 2, are both used during the process to test the viability of the Velostat sensor[6]. The Arduino Uno is a microcontroller board based on the ATmega328, which features high performance and low power. It has 14 digital input/output pins, 6 analog inputs, and a USB connector to upload Arduino-C code. Since the Arduino Uno is bulky and not suited for sewing into a shirt, a Lilypad is used for its ability to be sewn into e-textiles and incorporated within a wearable garment. The Lilypad Arduino is smaller and compact, so a breakout board, which distributes power and code, is required to connect it to a computer.

Fig. 1: Lilypad Arduino

Fig. 2: Arduino UNO

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There are multiple different electrical

components used throughout the construction, including a breadboard, various resistors, and a battery. A breadboard is a board used for making an experimental model of an electric circuit. Resistors are two-terminal electrical components that are used to reduce current flow. Finally, a battery is used to supply voltage through the Velostat circuit and back to the Arduino. 2.2 Software Used To process data and display the information in a user interface, different programming languages and software was used. Here is the list of all the software that was used in the creation of the project:

● MATLAB: MATLAB is a numerical computing environment and programming language used to process the data from testing to be used for graphing.

● Arduino IDE: This was where the Arduino code is written to create a connection between the Arduino microcontroller and the software.

● Processing: This software development environment was used to write Java code that displayed the sensor values in a graphical user interface, or GUI.

● giCentre: A Processing library used to create a graph of the data and update it in real time.

2.3 Equations

The main relationship used in analyzing the circuitry and designing the product is Ohm’s Law:

V=IR

The variable “V” is voltage, “I” is current,

and “R” is resistance. Ohm’s Law is used to determine the amount of power required by the Lilypad Arduino and the Arduino UNO. It is also used to calculate the optimal resistors used in series with the Velostat in the voltage divider.

Fig. 3: Voltage divider diagram

In Fig. 3, R1 represents the variable resistances given by the Velostat. The Vout represents the Analog input going to the Arduino, which shows voltage values that are then converted to resistance.

There is an inverse relationship between pressure applied and resistance from the Velostat, although it is not predictably quantifiable.

III. DESIGN PROCESS 3.1 Summary of Design/Creation

The prototype of the wearable was designed in multiple steps. Multiple smaller prototypes were created to test different configurations of the larger design. During the process, visual diagrams as well as electrical schematics were made to map out the design of the product. The Velostat sensors were created using a sewing machine and conductive thread. One side of the sensor was connected to ground, and the other was connected to the analog pin that establishes the electric signals as input in the Arduino hardware. The Arduino is programmed so the resistance values were translated into a pressure and force reading.

3.2 First Proof of Concept and General Construction

In order to test the viability of Velostat as a pressure sensor in electronics, an initial prototype was created with a large, single Velostat sensor with only one point of contact between the conductive thread on both pieces of fabric. The type of silver coated nylon conductive thread that was used in this research project was untwisted from 4 ply to produce a 1 ply product in order to facilitate the sensor array

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construction process. Thinning the thread prevented entanglements in the bobbin of the sewing machine and allowed the conductive thread to be sewn using the embroidery machine. Once the thread was completely untwisted, it was threaded around one bobbin on the sewing machine. The other bobbin contained nonconductive cotton thread. Thus, when each cotton outer layer was sewn with the circuit design, there was only one conductive thread running through. In the first model, a single line was sewn diagonally across a cotton fabric square. These two squares were then arranged with the Velostat in the middle so that the lines crossed each other to create one contact point. One line became the input lead and one became the output lead, and both lines of conductive thread were tied onto two test wires, which were then connected to the Arduino Uno for testing.

Additionally, several other stitches of nonconductive thread around the circuit design were used to further secure the Velostat within the two cotton layers. This was to ensure that when force was applied, the cotton layer would not slip and provide an inaccurate reading. However, the stitches could not be sewn too close to the circuitry, otherwise the punctures in the Velostat would decrease the accuracy of the resistance readings.

This sensor was then placed on one side of a voltage divider in series with a 10k Ohm resistor. The center of the voltage divider was then connected to Analog Input A0 on the Arduino to measure the voltage across the Velostat sensor. Preliminary tests were conducted to determine the changes in resistance in the Velostat sensor as pressure was applied. Then, once the initial hypothesis was confirmed, the basic Velostat circuit was redesigned with more sensors and was connected with conductive thread rather than physical wires (Fig. 4).

Fig. 4: Initial sensor set-up with voltage divider

3.3 Sensor Design

One major factor of the circuit design to consider was the number of contact points between the patches of fabric. Multiple contact points decrease the base resistance and too many points of contact can actually reduce the range of resistance values, which was previously noted during testing of sensors constructed by Jillian Maling and Mandev Singh. Therefore, the goal was to create as large of a surface area for detection without compromising the range of possible pressure values that could be extrapolated from the sensor by creating too many points of contact. For this reason, testing was crucial to ensure that making a larger Velostat pressure sensor would still allow for a significant change in resistance during pressure testing. Two different sensor patterns, as shown in Fig. 5 and Fig. 6, were designed to have different arrangements of contact points and tested to evaluate the viability of each design over the other. The first sensor design featured a boxed grid pattern when overlapped and each lead was continuous with no branches along the circuit path. One prediction with this design was that the resistance would increase along the circuit because it was a continuous pathway and thus the pressure values that could be extrapolated from the sensor would be slightly different across different locations on the sensor . The second sensor featured a finger grid pattern with several branches originating from the source lead. It was designed in parallel in an attempt to reduce linear resistance because each branch was individually shorter than the circuit path in Sensor 1. Testing was then necessary to validate these predictions and determine which one would be optimal in the final design.

Fig. 5: 1st Sensor Design

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Fig. 6: 2nd Sensor Design

3.4 Sensor Testing

For sensing, both Velostat pressure sensor designs were analyzed using an impedance analyzer and an INSTRON pressure machine. First, the sensors were attached to the impedance analyzer machine to track the changes in the resistance of the sensor over time. The sensors were also placed under the top compressive plate of the Instron machine, which recorded the extension and force over time.

The Instron machine was set to a maximum load around 100 Newtons, and the max extension value was recorded. The two machines simultaneously recorded the values every 0.02 mm for time, force, extension, and resistance. The values for each measurement were then inputted into a MATLAB script that coordinated the time values of both sets of data. The outputted data was inputted into another MATLAB script that interpolated the force data and outputted the final set of data. The last step of the testing process was to input data into OriginLab and produce graphs. Then, using the Instron pressure sensor and an impedance analyzer, a graph was generated for each sensor showing the inverse relationship between pressure applied and resistance for up to 100 N of force, shown in Fig. 7 and 8. Testing data was gathered for five locations on each sensor, including the centers and all four corners. This was done to determine if the sensors were consistent and if the resistance drops were independent of the location on the sensor where pressure was applied.

Fig. 7: 1st Sensor Graph

Fig. 8: 2nd Sensor Graph 3.5 Analysis of Sensors

As shown in the testing, both sensors produce an inverse relationship between applied force and resistance. Figures 7 and 8 demonstrate that this inverse relationship was consistent across 3 different trials for Sensor 1, but it was also consistent for Sensor 2. As a result, both can be used to measure pressure. Once this was established, the graphs were then visually compared to determine the differences in force and resistance ranges in order to choose the optimal design to include in the shirt. In Sensor 1, which corresponds to Fig. 7, resistance and force are more closely related to one another; the relationship was more linear than that of Sensor 2. For Sensor 2 there is no discernable peak in the resistance to correspond with the peak in the force, making it so that it is unclear as to when the low point of the resistance is reached. Also, in Sensor 2, the force that the Instron machine applied to the sensor was unsteady as the force graph rises quickly, plateaus,

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and then shows a steep peak again. Similarly, when no force is applied, the peaks in resistance gradually increases with each cycle. This is most likely caused by the Velostat material being misshaped by the heavy force and exhibiting a spring-like effect when compressed.

Once a specific design was chosen, the test sensors were replicated to scale. The measurements of each larger sensor were 4.5 inches by 5 inches, with 6 sensors on both the front and back, as shown in Fig. 9.

Fig. 9: Shirt Diagram

3.6 Circuit System Design

Designing the circuit was initially a challenge, because the Lilypad Arduino only has 6 analog inputs and there were 12 sensors in the T-shirt final design. Furthermore, there aren’t enough 5V and GND pins on the Lilypad Arduino or Uno to connect each sensor to power and ground. The first problem was solved by using two Lilypad Arduinos in the final design. Half of the front sensors and half the back sensors were connected to one Lilypad, and the rest were connected to the other Lilypad. The second problem was solved by placing six voltage dividers in parallel with each other. Each voltage divider has a 10k Ohm resistor in series with a Velostat sensor to separate the voltage between both components. Wires were used to detect Analog input voltage at the points between the resistors and sensors in the circuit. Fig. 10 shows the complete design of the circuit used in the final product[7].

Fig. 10: Circuit Design

3.7 Arduino Programming

The sensor system was connected to an Arduino Uno, which needed to be programmed to receive sensor values and process them. The code that read the sensor values through Analog input was written in Arduino-C. These values were then printed to a Serial port that transmitted the data through a USB port to a computer. The next part of the coding process took place in Processing. Processing is a visual style-based programming language that is derived from Java. In this project, Processing was used to display the values from the sensors in a graphical user interface (GUI) for easier viewing. In the code, the values from 6 sensors were first collected from the Serial port. The Serial Monitor displayed information relayed between the Arduino and the computer, such as the input voltages from Analog pins. Then, the maximum of the values was taken. This maximum was compared against a pre-defined pressure threshold to determine if the force applied was significant enough to be measured. The threshold was chosen by testing pressure and voltage values and choosing a significant pressure. Finally, a short algorithm determines the “spikes,” or relative maxima, of sensor values. These relative maxima are recorded into the interface and are added as points in a graph, as shown in Fig. 11. Since the values are recorded every second, the graph and interface are dynamically changing as more data flows through. This allows for live, real-time data.

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Fig. 11: Graphical User Interface

IV. RESULTS 4.1 Key Findings & Final Testing

It was reaffirmed that force and resistance are inversely proportional on the Velostat pressure sensor. Data from Fig. 7 and 8 indicates that as force is applied to the shirt, resistance decreases consistently, producing visible change. This allows for reliable, repeatable data, which makes it easy to extrapolate information about how much force was applied to the wearer of the shirt. However, the results of the final shirt testing showed slightly different results. The sensor values coming from the Lilypad Arduino had randomized starting values, and the pressure did not have a strong correlation to resistance. The Arduino Uno, however, had much more consistent values that confirmed the initial hypothesis and testing. This is likely due to bad connections with conductive thread and resistors on the Lilypad.

Overall, the shirt sensor system confirmed the initial hypothesis that Velostat pressure sensors could be used to detect pressure applied to one’s body. With the Lilypad Arduino, the sensor values coming through Analog inputs were consistent and therefore were able to be used to monitor the pressure applied to the garment.

Fig. 12: Final Shirt

4.2 Discussion of Results

Some possible improvements in the sensors include using a better conductive thread for the shirt. Since the thread came as 4-ply, it had to be unraveled into four separate threads, so 1-ply would be used on the shirt. This process was difficult, and often resulted in knotting or fraying of the thread. This could lead to an inaccurate measure of voltage/resistance in the material, because the thread was not uniform throughout the sensors. Fortunately, such effects were mitigated by placing the conductive thread on the bottom bobbin of the sewing machine.

In this study, the final shirt prototype consisted of twelve sensors, six in the front and six in the back. However, a future improvement would feature a broader array of much smaller sensors. Not only would this create a greater detection surface area, but less points of contact per sensor would create more precise pressure readings.

Finally, by utilizing a sewing software such as Embrilliance to create a computer-generated stitch pattern, the stitch pattern could be made more precise throughout the sensors. Embrilliance is capable of directly sewing a computer pattern without additional human measurements. This would reduce human error in manually sewing the product and would make the testing results more consistent.

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V. RESULTS 5.1 Summary of Findings

The final shirt design consists of twelve sensors on the shirt - six on the back and six on the front, connected to an Lilypad Arduino on either side of the shirt. After testing two possible sensors, a final sensor was chosen. In particular, the one with the most points of contact would ensure greater accuracy on where the police officer was hit. This final sensor was chosen through tests done with an Instron machine, that exerted pressure on this sensor. Next, the sensor was also connected to an Impedance Analyzer machine which received the values of resistance from the sensor to be interpreted.

This project demonstrated that a Velostat pressure sensor is a simple, cost-effective way of measuring pressure. However, these sensors have some drawbacks, such as being too sensitive to pressure and recording inessential changes in resistance. In addition, the tests were unable to apply over 120 N to the Velostat, so there is no data to indicate what would occur if this force was exceeded. However, with more improvement in Velostat sensors, they can be revolutionary in providing evidence for law enforcement cases.

For each sensor, the force applied to it is inversely proportional to the resistance. However, when the data from the Instron machine and Impedance Analyzer machine is graphed, it is not perfectly proportional (See Fig. 7 and 8). This shows that there could be some sources of error in the resistance values from the sensors. The correlation between force and resistance is repeatable, as seen by the graph in Figure 13.

Fig. 13: Force and Resistance Vs. Extension

5.2 Future of E-Textiles in Law Enforcement

E-textiles can further enhance an officer’s uniform for more complete evidence in court cases. However, while effective, a t-shirt that only detects the presence of force may not be completely reliable. This can be rectified with additional equipment, such as a heart rate monitor that can determine if the time of impact was consistent with a sudden change in heart rate. Also, such a monitor could be implemented as a safety device to identify if an officer suddenly becomes unconscious in the line of duty. To measure this, a pulse sensor or electrocardiogram (ECG) sensor could be incorporated into e-textiles in a manner similar to the Velostat pressure sensor. Moreover, such a uniform can be made more effective with garments that detect changes in motion. In particular, changes in the flexion and extension of muscles can be analyzed to determine the wearer’s precise movements, which can be important for injury and physical therapy applications. Also, an accelerometer could be incorporated to track distance traveled.

Moreover, current technology can be improved upon through adding a web interface and wireless control. While the research detects impact on an officer, it requires connection with a computer. A wireless module could be used to present the results continuously, and a web interface can keep a running tab on changes of pressure on an officer. Such a web interface can also include running video footage, which can be monitored at all times. Combined with an alert system, a sudden change in impact can be used to contact other officers to report to the site, which can decrease feedback time in serious situations. 5.3 Future of E-Textiles Pressure Sensors

Pressure sensitive e-textiles can also be used to approach issues in other fields also, simply by connecting already existing sensors into a wearable garment and an Lilypad Arduino, which is capable of analyzing results through a web interface. For instance, pressure sensitive e-textiles can be incorporated into sports equipment, such as football helmets, in order to measure the force of an impact. Doctors can utilize this data in order to better assess

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injuries, especially in patients who may be concussed. This can be incredibly valuable in dangerous occupations, such as working with heavy machinery. Another possible application of this technology is as a discrete locking mechanism, which can utilize hidden buttons in fabric to unlock backpacks and prevent pickpocketing. There are countless other recreational applications, such as incorporating buttons into clothing for remote control of smartphones and other devices.

ACKNOWLEDGEMENTS A great thanks is in order to the many

mentors who gave help and aid throughout this project, and to the extremely generous supporters who allowed this project and program to be possible. In particular, special gratitude towards Dean Ilene Rosen, Director of the New Jersey Governor’s School of Engineering and Technology (GSET) and Dean Jean Patrick Antoine, Associate Director of the New Jersey Governor’s School of Engineering and Technology (GSET), who made it possible for research to be conducted at such a high level. Moreover, thanks to Dr. Aaron Mazzeo, Professor of Mechanical and Aerospace Engineering along with Jillian Maling, a Rutgers Class of 2020 Mechanical Engineering Major, and Mandev Singh, a Rutgers Class of 2019 Aerospace Engineering Major. They were extremely important to the research and creation of this project, and gave up countless hours to ensure any aid needed would be received. Another special thanks to the Governor’s School Residential Teaching Assistants, in particular, Jacob Battipaglia, who oversaw the progress of the project, and our research coordinator Jamie Swartz. Finally, there is much gratitude to the many sponsors of Governor’s School: Rutgers University, Rutgers School of Engineering, The State of New Jersey, Silverline Windows, Lockheed Martin, and the Alumni of the NJ Governor’s School of Engineering and Technology.

REFERENCES [1] “Electronic Textile (E-Textile),” in Techopedia. [2] Feeney, Matthew. PoliceMisconduct.net. Proc. of Police Body Cameras. CATO Institute, 2015. Web.

[3] M. Satomi and H. Perner-Wilson, “Velostat,” in How To Get What You Want, 2009. [4] “SE400,” in Brother. [5] “Instron Machine 44-11,” in Instron. [6] Lilypad Arduino Main Board. Proc. of Arduino. 2017 Arduino, n.d. Web. [7] “How to Make a Basic E-Textile LED Circuit,” in Kitronik.

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