Since the release of the original iPhone, the previously small market technology of capacitive touch screens has rapidly grown into one of the most definitive technologies of the mobile era. [1,2] Touch technology has rapidly become a standard in human computer interaction, largely because of this growth in the development of capacitive interfaces. As the demand for this technologically increases, a technical person should benefit from understanding the underlying physics. Projected Capacitive Touch Screens (pro-cap) holds the title of the most commonly utilized method in smartphones today, and thus will be the focus henceforth. [1]
As in all capacitive sensors, pro-cap relies on the basic principle of capacitance. Older forms of touch sensing use a variance in resistance to detect touch, but this is limiting and has been avoided as capacitive technology has started to become more affordable. [2] Before examining the method of sensing, let us first explain the technical layout of the screen.
Above the actual display (e.g. LCD), there are three layers: glass substrate, sensing layer, and protective glass. [3] In pro-cap, the sensing layer consists of a two-layer grid of transparent thin film conductors, one with a driving voltage and the other with a sensing voltage. [4] This grid can be set onto a single layer, with the grid etched into the layer, or it can be composed of two separate layers, as described above. [3] The wires in these films are most often made of Indium Tin Oxide, and have a thin separation layer. [4] It is at the intersections of this grid that the sensing occurs. This two dimensional grid allows for adequate sensing resolution.
The sensing layer of the pro-cap screen acts as a single plate in a capacitor, holding a charge at the surface. Similarly, a person's finger acts as a conductor which, when put in close proximity to the charge on the sensing layer, will disrupt the expected capacitance. [2] The current in the sensing lines reflect the changing capacitance, which acts as a variable voltage source. [4] Interestingly, by increasing the sensitivity threshold of a pro-cap screen, a touch sensor can easily be transformed into a proximity sensor. [4]
As with any technology, the art behind the science lies in optimizing the variables. In pro-cap screens, these variables are the area of the sensing areas, the sensitivity threshold, the protective screen layer width, and the sensing algorithms. [4] To increase sensitivity, the capacitive area can be increased (as can be determined by basic electromagnetism), the layers between the sensing layer and the finger can be decreased (again, basic electromagnetism), or the sensing algorithms/threshold can be changed. It is in these variables that the competition lies.
Projected Capacitive Sensing is currently the leading method due to its advantages. The component layers have all been made to be thin and transparent, which equates to optimal optical properties. [4] The grid of wires creates an array of sensors, which easily allows for multi-touch sensing. [4] Most importantly, pro-cap technology and the surface layers that sit above the electrical sensing circuit allows for resiliency. In the consumer electronics market, resiliency and longevity greatly influence the success of a product, making this technology a current favorite.
As with any technology, improvements can be made on the disadvantages, and it is in correcting these pitfalls that the future lies. The biggest issue to solve, currently, is the price. Pro-cap sensing costs more to make than other similar technologies, but as time goes on and this technology remains a preference, the cost can be expected to drop. [1,2] Pro-cap is also limited to sensing touch by fingers and other similarly conductive objects. [4] As the technology and demand for improvements continue to grow, staggering progress and innovation is inevitable. With new technology being developed all the time, one cannot say how long projected capacitive touch technology will be the standard in the smartphone industry - but up to this point it remains the best method available.
© Evan Lee. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] J. Colegrove, "The State of the Touch-Screen Market in 2010," Information Display 26, No. 8, 22 (2010).
[2] J. Leigh, A. Johnson, and L. Renambot, "Chapter 2 Advances in Computer Displays," Adv. Computers 77, 57 (2009).
[3] M. R. Bhalla and A. V. Bhalla, "Comparative Study of Various Touchscreen Technologies," Int. J. Computer Appl. 6, No. 8, 3 (2010).
[4] G. Barrett and R. Omote, "Projected Capacitive Touch Technology," Information Display 26, No. 3, 16 (2010).