Since the rise of the iPhone in the last decade, touch screens have become ubiquitous across all mobile platforms and have extended their reach into other computing platforms. The vast majority of these devices use the capacitive touch screen modality as opposed to the alternative versions which include; resistive, surface acoustic wave and infrared touch screens[1].
Looking more closely at capacitive touch sensors, the interface works off the principle that the human body has a natural capacitance and is able to hold an electric charge. There are a number of different types of capacitive touch screens but the most versatile and ubiquitous version is the projected, mutual capacitance sensor. The touch screen itself is made up of a glass sub-straight, a layer of very small conductive fibers (generally Indium Tin Oxide [2]) arranged in a grid like pattern and a protective coating. A voltage is applied to the conductive layer which generates a uniform electrostatic field across the surface of the screen. When a conductive object is brought in contact with the screen an electrostatic charge is transferred to the object, distorting the electrostatic field. This distortion is detected by a controller and can be interpreted as coordinates corresponding to the point of contact of the object with the screen[3].
Whilst the capacitive touch screen was a major advancement in how we interact with computing platforms, it is still a limited form of interaction. A discrete capacitive sensor has binary sensing capability and can therefore only tell if an object is present or not. The most recent innovation in terms of touch screens has been the introduction of 3D touch by Apple. This advancement extends the dimensionality by one degree since and the screen is able to detect not only the x-y position of a manipulator but also two degrees of pressure, either a hard press of a soft press[4].
Despite this improvement, touch screens still cannot infer the pose or orientation of the manipulator. Furthermore, the manufacturing process for touch screens requires high precision and complex integration which is the hallmark of a mass production item.
Traditional touch screens rely on DC signals in order to detect objects and identify their position on the screen. However, as an alternative form of binary capacitive touch sensing it is possible to gather more information about the manipulator by detecting voltages over a range of frequencies using an AC signal. This idea was first conceptualized in the form of the Touché circuit by Sato et al at the Disney Research Institute [5] and this work expands upon the basic principle.
A driver is required by law to always be in control of the vehicle he or she is driving. A steering wheel was therefore chosen as the primary interface for the touch sensor, thereby enabling the manipulation of the car’s media system whilst always being in contact with the steering wheel. The system can, however, be applied to a number of different scenarios whereby any arbitrary object is used as the sensing interface.
The human body has an external resistance in the range of 1KW - 100KW, an internal resistance of 300W - 1KW and a capacitance in the range of 100 – 200 pF [6]. When a body comes into contact with an electrode, a capacitive link is formed between it and the ionic physiologic fluids in the body[7]. Furthermore, there is a capacitive link that is formed between the user and ground which depends on the shoes that the user is wearing. In this way the human body allows the flow of an AC signal to ground.
The Touché concept utilizes the principle that impedance in a circuit with capacitive and inductive elements is dependent on frequency. Since the human body is capacitive it is possible to arrange circuit elements in such a configuration that by touching an electrically coupled electrode an object creates a detectable change in the voltage profile of the circuit.
There are numerous factors that determine the electrical influence that a human body has on the circuit which include; whether the person is wearing shoes or not, the specific resistance and capacitance of the user and most notably the AC frequency of the signal that is injected into the system. There are a number of systems that exist that utilize the frequency dependent impedance property of the human body in order to detect interaction at a single AC frequency [8][9]. However, by continuously sweeping over a range of frequencies it is possible to gain a large amount of insight into how an object is being grasped.
By sweeping through a range of frequencies the dimensionality of the system is extended from a binary sensor to a two dimensional, spacial and temporal profile with a far larger number of unique data configurations.
There are two notable implementations of the swept frequency capacitive touch sensor the first of which, Touché, has already been mentioned. The second implementation was based on an Arduino MCU[10] and it sought to investigate the feasibility of implementing a sensor using limited hardware. The main difference between the two systems was the quality of the swept frequency signal that was produced. In order to account for the poor signal quality, the Arduino system sampled the voltage levels after the biasing inductor, this reversed the circuit properties and resulted in an inverted notch filter from which peak voltage level analysis was conducted in order to determine manipulator pose. Whilst the implementation was successful it did not have the resolution of the original system and was therefore non-optimal for this implementation.
[1] SSI Electronics, “TOUCH SCREENS : Design Guide,” 2013. [Online]. Available: http://www.ssi-electronics.com/resources/pdf/ssi/Touch Screen Design Guide.pdf. [Accessed: 01-Dec-2016].
[2] LION Precision, “Capacitive Sensor Operation and Optimization,” 2015. [Online]. Available: http://www.lionprecision.com/tech-library/technotes/tech-pdfs/cap-0020-cap-theory.pdf. [Accessed: 01-Dec-2016].
[3] L. K. Baxter, Capacitive Sensors: Design and Applications. John Wiley & Sons, 1996.
[4] J. V Chamary, “3D Touch In iPhone 6S Isn’t Just A Gimmick. Here’s How It Works,” 2015. [Online]. Available: http://www.forbes.com/sites/jvchamary/2015/09/12/3d-touch-iphone-6s/#255d9420abc0. [Accessed: 01-Dec-2016].
[5] M. Sato, I. Poupyrev, and C. Harrison, “Touché: Enhancing Touch Interaction on Humans, Screens, Liquids, and Everyday Objects,” Proc. SIGCHI Conf. Hum. Factors Comput. Syst., no. c, pp. 483–492, 2012.
[6] P. E. Schoen, “CAPACITANCE OF A HUMAN BODY,” Extreme Electronics, 2015. [Online]. Available: http://www.extremeelectronics.co.uk/capacitance-of-a-human-body/. [Accessed: 01-Dec-2016].
[7] H. C. Foster and K. R. Lukaski, “Whole-body impedance - what does it measure?,” he Am. J. Clin. Nutr., vol. 64, no. 3, pp. 388–396, 1996.