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Capacitive Sensing 3D-Printed Wristband for Enriched Hand Gesture Recognition

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Capacitive Sensing 3D-Printed Wristband for Enriched Hand Gesture Recognition Capacitive Sensing 3D-printed Wristband for Enriched Hand Gesture Recognition Hoang Truong, Phuc Nguyen, Anh Nguyen, Nam Bui and Tam Vu University of Colorado Boulder {hoang.truong, vp.nguyen, anh.tl.nguyen, nam.bui, tam.vu}@colorado.edu ABSTRACT Silicon Bracelet Engineered From Scratch In this work, we design a wearable-form hand gesture recognition system using capacitive sensing technique. Our proposed system Flexible Circuit Board Embedded into Silicon includes a 3D printed wristband, capacitive sensors arrays in a flex- Flexible Circuit Board ible circuit board, a low-cost micro-controller unit and wireless Bottom View communication module (BLE). In particular, the wristband manip- ulates the changes in capacitance from multiple capacitive sensors Integrated Integrated Header Sensors to recognize and detect users’ hand gestures. The software stack Top View translates the detected gestures into control command for applica- Integrated Wires tion layer, together with an user-friendly web interface that sup- ports both data communication and training between the wristband and the host PC. We also release an open API of our designed sys- Figure 1: Overview of the proposed system tem for future applications. Lastly, we envision our system open user experience, and (3) the high cost of the some commercial or API will be available for developers to customize vast range of hand DIY systems. Existing techniques have had their own pros and cons gesture and integrate the wristband into various applications, from in which some of the stated drawbacks have been solved but not all. command on remote computer to video game controller. The ultimate aim for each products should lie on optimizing system in order to satisfy user experiences and open-source DIY devices Keywords for wide range of applications. capacitive sensing; hand gesture recognition; 3D-printed wearable In this work, we introduce a hand gesture recognition system consisting of a capacitive-sensing-based wristband to sense the users’ 1. INTRODUCTION hand movement, a low-cost micro-controller to record and manip- ulate measured data from sensors, a BLE component to commu- As human-computer-interaction experience evolves and IoT get nicate with application client and a software stack to train hand smarter, consumers tend to prefer a more intuitive way to interact gesture patterns and export control command to application layer. with computers other than just tapping or swiping on their smart- There are many challenges in realizing such a system. First of all, devices. Thus, more and more big names have joined the play- the sensitivity of capacitive sensors makes the sensor reading fluc- ground of voice or gesture control technology. Among these in- tuate over time and be susceptible to noise. Second, the design teresting trend, hand gesture recognition have been researched and sensors (i.e. number and arrangement of sensors, distance to users’ manufactured in various forms of wearable devices such as wrist- skin) must be aligned precisely. Too short distance leads to near- band, gloves, and rings. The techniques behind those utilize wide identical readings for different gestures, and too far distance results range usage of sensors (e.g. motion [5], camera-based sensor [24], in low resolution of measurement. Third, real-time communication and etc.) or the measurement the signatures of human body such as is needed to determine users’ current gesture. impedance or muscle electrical signal, so called electromyography To address these challenges, we design a 3D printed wristband (EMG) [17]. model with a complete system stack, and exploit support vector ma- While various manufactures and researches have been in the field chine (SVM) method to differentiate the hand gestures. We make for long time, each of them have to face up with a different num- the following contributions in this work: bers of drawbacks that make them modestly adopted universal. The drawbacks include (1) the need of easy to access and replicate the Providing a system which includes both hardware and soft- • already published research, (2) the convenient and friendly UI for ware designs with low-cost hardware components and open- source software stack for developers. Permission to make digital or hard copies of all or part of this work for personal or Customizing sensor board design and layout using flexible classroom use is granted without fee provided that copies are not made or distributed • for profit or commercial advantage and that copies bear this notice and the full cita- circuit for precise measurement of sensor signal. tion on the first page. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, or re- Designing a capacitive sensing wristband and prototyping publish, to post on servers or to redistribute to lists, requires prior specific permission • the flexible version by using 3D-printed molds. and/or a fee. Request permissions from [email protected]. WearSys’17, June 19, 2017, Niagara Falls, NY, USA In the following sections, we first present the fundamentals of c 2017 ACM. ISBN 978-1-4503-4959-8/17/06. $15.00 capacitive sensing technique in Section 2. In Section 3 and Sec- DOI: http://dx.doi.org/10.1145/3089351.3089359 tion 4, we describe in details the architecture, hardware design, and software stack of our proposed technique respectively. We provide ference or frequency changing) to process further in order to give the role of support vector machines in recognizing different trained the decision of capacitive sensor measurement. gestures, and also present the application implementation based on In the open capacitor design, the sensor is considered as one side our API. We conclude the paper with a discussion of related work of the capacitor plate, the external contacting point will act as the and a summary. second one (skin of the user in most of the cases). In our scenario, the users’ hand movement causes the compression of the silicone wristband, which lead to a change in distance from the user’s skin 2. CAPACITIVE SENSING BACKGROUND to the sensors and changes in capacitance. Nowadays, capacitive sensing have become more and more per- vasive, which has the wide range of presence, from daily life tools (control board buttons, car door handle and so on) to enormous use 3. SYSTEM DESIGN in smart phone touchscreen; and various application from touch recognition, proximity sensor to user identification and authentica- 3.1 System architecture tion [22, 12, 7]. In this section, we present the key principle of capacitive sensing technique. The capacitance change from sensor Micro- Wristband External App is usually measured indirectly from the changing in voltage, cur- controller rent, frequency or pulse width which are usually integrated into an Command out application specific integrated circuit (ASIC). The variance in volt- Serial out Command age, current, frequency or pulses is used to infer the sensor mea- Serial service service surement. Serial in In so-called ‘direct’ capacity sensing method, the key idea is to charge the capacitor from a defined current source for a known time and the voltage will be measured across the capacitor subsequently. Web App API Command in This method needs a current source at very low and highly precise source. The next technique is to create an RC oscillator using the capacitor, and after that measures the time constant, frequency, or Figure 3: Overview of the proposed system period. Although this method does not always generate high accu- Our system envisions a wearable wristband that is able to de- rate results; it is easy and straightforward. Another method worth termine the user’s current hand gesture, thus control external ap- mentioning here is the one exploiting capacitor’s AC impedance. A plication or remote devices. The system architecture can be seen sine-wave source excites the capacitance, and the capacitor’s cur- in Figure 3. Each movement of users’ hand will cause the change rent and voltage are measured. Nevertheless, the circuit is very in capacitance which is measured from sensors on the wristband. complicated and experiences the highest part count. In addition, The wristband with micro-controller unit and sensors are controlled applying a charge amplifier that converts the ratio of the sensor ca- via a wired serial connection. The micro-controller receives the pacitor to a reference capacitor into a voltage is also a common way single-byte command from the host device and processes one of for measuring of high precision, low-capacitance sensors. the following action: response connection checking, update base- line measurements of all capacitive sensors, collect data from sen- Output to sors and transfer to host device. The host device then processes the processing MCU streamed data based on two modes: training gesture or consuming S S 1 2 gesture events. The training gesture process takes the input capac- itive measurement from the sensors, and then extract its features using SVM technique. We implement a web interface to make the Source Comparator training process more friendly to user while the key classification sensor bleed C integrated algorithm is run in the background server of the web app. Then, the R C recognized gesture will be consumed and translated to correspond- ing actions for specific applications (e.g. gestures can be translated into key presses in Linux operating system). Figure 3 illustrates the overview of our system. Figure 2: Capacitive sensing under-layer circuit To provide a robust working prototype, the following compo- nents are carefully designed and tested: While surveying multiple capacitve sensing methods such as Mi- Silicone wristband created using 3D-printed molds. crochip Capacitive Sensing Module [9], Charge Time Measure- • ment Unit [11], Capacitive VoltageDivider [10]; Cypress CapSense Custom sensor design and layout using flexible circuit board. • Sigma Delta Modulation [4] or Silicon Labs Relaxation Oscilla- Micro-controller serial communication protocol to stream sen- tor [16]; we generalize the most common pattern of the underlying • sensing circuit.
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