Display Technology
Going T
he widespread adoption of projected capacitive (p-cap) touch sensing has contributed to one of the largest
consumer electronics revolutions in recent years. As devices such as smartphones and tablets have proliferated, so a durable, sensitive touch-enabled user interface has become an almost mandatory feature for product designers in every field. The vast majority of these devices are now based around a p-cap sensor, driving a phenomenal growth rate in this part of the touchscreen sector. Figures from leading industry analyst DisplaySearch show that, though still relatively new, p- cap has rapidly risen to become the most widely used touch sensing technology in the global market, overtaking the long established and increasingly commoditised resistive sensing technology.
This fast uptake has been driven by a compelling feature set, including an effectively unlimited lifespan conferred by a resistant all-glass surface, edge-to-edge design capability (with no requirement for bezels) and high levels of sensitivity. However, as original equipment manufacturers (OEMs) seek to incorporate touch interactivity with a similar style and performance outside the portable consumer domain, there becomes a realization that touch screens which satisfy different set of design criteria are required.
Two choices of technology OEMs can choose between two distinct types of p-cap touch sensing methodologies. The most common now, is mutual capacitance. This uses two separate conductive layers, one of which contains the sensing cells through which the position of the touch event can be identified, while the other has the driving cells through which an electrical signal passes. The cells are usually interlocking and each is connected to the control electronics. When the screen is touched, there is an alteration of the charge held within the local electric field, reducing the mutual capacitance built up between the two layers. This alteration is picked up by the cells in the sensing layer. Detection algorithms within the controller electronics determine the individual cells with the greatest change in charge, and output a corresponding X-Y co-ordinate to the host system.
The second “flavour” of p-cap sensing uses the principle of self-capacitance. In contrast to mutual capacitance, this
32 May 2013
Ian Crosby and Dr. Andrew Morrison look at how self capacitive sensing is helping to bring touch to much larger screen products
technique employs a separated X-Y grid of open ended conductive lines connected to a controller containing the detection algorithms. The charge held on the lines is altered by human body capacitance, as the user’s finger comes closer to the touchscreen surface. The X and Y lines with the peak change in charge are detected and the touch co-ordinate is output to the PC. There are a number of reasons behind the adoption of the mutual capacitance approach in consumer electronics. The technology is particularly capable of providing multi-touch functionality assuming sufficient cell density and controller IC power is available. The high density of individually connected cells makes it possible to gather and interpret the large amounts of touch data required to separate multiple independent touches. However, conventional mutual capacitive
screens can suffer major drawbacks when a designer attempts to move to larger form factors. In order to accurately track multiple touch points, the controller must capture data from each of the small individual cells. The bigger the screen, the larger the amount of information that need to be captured. Eventually the size of the data set becomes overwhelming. In practical terms once the touch display size reaches 15 inches (approx. 380 mm), the number of cell intersections that must be connected to and monitored by the controller becomes a major challenge. The increased complexity in the control electronics and connectivity also adds to the bill of materials and increases the required integration time and effort. For those weighing between mutual and self-capacitive techniques, practical manufacturing issues also take on increasing importance as display size grows. Mutual capacitance solutions are generally based on a matrix of cells made of Indium Tin Oxide (ITO), a conductive, near-transparent material that is deposited and patterned on glass or film using a semiconductor-style photolithographic manufacturing process. ITO is widely used in applications requiring mass produced, small touch displays (such as portable consumer electronic devices), where the volume-friendly production process is a plus. However, if volumes are lower (and this often goes hand in hand with larger screen sizes such as those used in public, self-service applications for example ), the relative inflexibility of the ITO process and
Components in Electronics
the high one-off cost of photo masks become more problematic. Finally, in addition to manufacturing complexity and cost, there is a question of touch performance to be considered. For all its benefits, ITO has a relatively high resistivity. This means that as the display area increases, and the distance between cell and controller grows, the signal to noise ratio decreases rapidly, resulting in progressively lower touch sensitivity and in the worst case an inoperable device.
A self-capacitive alternative Zytronic is a specialist with 30 years experience in developing glass and plastic laminating technology. Based just outside Newcastle the company’s proprietary Projected Capacitive Technology (PCT) is a self-capacitive system that has been proven in deployments all over the world in the last 10 years, particularly in situations
where larger screen size is required. Based on an X-Y matrix of micro-fine capacitors, embedded within a laminated glass substrate, PCT uses frequency modulation to detect minute capacitance changes within the conductive tracks.
A key attribute of this technology is its high sensitivity. It can detect a touch through very thick overlays, protective glass and even heavily gloved hands and therefore has an unsurpassed level of Z-axis sensitivity and control. This makes it eminently suitable for industrial and public access applications, and even for outdoor use. Because PCT requires unique detection algorithms running within the control electronics, Zytronic has also developed its own touch controller hardware and firmware, designed specifically to work with its PCT sensors. The latest controller will output two separate touch co-ordinates, making it
Figure 1: PCT-based touchscreen applied to Microsoft’s Spatial Desk
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