Touchscreen-based interactivity has rapidly progressed from being a desired feature to an almost mandatory requirement for displays utilized in all manner of equipment. Vending machines, home appliances, vehicle control consoles, and industrial instruments increasingly feature a touchscreen. The evolution of human-machine interfaces (HMIs) and computer interfaces (HCIs) is underway, with simple button on/button off controls giving way to advanced gesture-based screen interaction requiring so called multi-touch operation. This presents system designers with a way to add functionality and differentiate their products.
Multi-touch technology enables more complex gestures, such as a pinch, zoom, a rotate or a flick, to navigate intuitively through menus and pages on a suitably designed graphic user interface (GUI). It also has the potential to allow multiple users of a screen to share information collaboratively. Already a great deal of progress has been made in applying multi-touch operation in portable and home computing devices, such as smart phones, tablets and all-in-one PCs. However, these devices are primarily designed for personal use and, therefore, the number of touches actually required to operate most applications is two or possibly four – how many fingers can one user put on a 3-inch screen? The need for more points of touch detection becomes critical as devices with larger displays add touch interactivity, such as demonstrated by Samsung’s SUR40 PixelSense – a 40” touch table for home/office use, and the follow up to Microsoft’s groundbreaking original Surface tablet.
Out of home
With the increasingly widespread acceptance of multi-touch enabled consumer electronic devices, supported by computing operating systems optimized for touch and running suitable application software, a new range of non-consumer applications are opening up which could benefit from multi-touch operation if an effective solution for larger format displays could be found. Among these applications are; point of sale (PoS) systems such as vending machines, public information terminals, gaming and entertainment units, interactive digital signage, and touchscreen tables for retail, banking and educational use. The large form factor multi-touch screens to be found on the market at the moment are suitable for office and home environments, but there are serious question marks about their operational durability if deployed in more demanding environments.
Systems using cameras or infrared (IR) technology rely on protruding bezels or frames around the display (where the optical sensor elements are housed). Whereas this makes them relatively easy to fit to existing systems, from a design aesthetic, one of their drawbacks is that edge-to-edge flat glass front designs are not possible. The exposed frame also renders them less suitable for use outside their domestic ‘comfort zone’, since they are susceptible to various forms of mechanical damage from external forces, as well as the build-up of dust/dirt around the bezel, impairing operational performance levels. Furthermore, such screens are prone to ‘false touches’ – particularly crucial in table applications, where users will naturally lean across the surface, catching sleeves, ties and elbows on the touchscreen and causing inadvertent activation.
Current multi-touch technology based on P-Cap and its limitations
The vast majority of multi-touch screens used globally are based on projective capacitance (p-cap) sensing, as the technology allows feather-light touch detection and flat, edge-to-edge glass surfaces. There are two principle variants of p-cap touchscreens. First, there is the self-capacitive approach – which detects minute frequency changes along discrete electrodes and is particularly suited to ultra-ruggedized systems, due to its exceptional Z-axis or depth sensitivity allowing the screens to be mounted behind very thick overlays (in some cases behind up to 20mm of toughened glass). This technology can support dual touch operation, but is not capable of true multi-touch functionality. Conversely, the mutual capacitive approach (very popular in consumer electronics) - which measures the charge transfer between layered cells in an XY grid has a weaker through-glass performance, but boasts the capacity to determine a larger number of individual touch points provided suitable data processing capability is employed.
The conductive matrix in these sensors is usually a chemically processed, virtually transparent metal compound called Indium Tin Oxide (ITO). There are, however, issues when migrating such sensors to larger devices, as ITO , has a relatively high electrical resistance. Therefore, signal impedance builds up over the total length of the sensor’s conductive tracks, generally causing the system’s signal-to-noise ratio to deteriorate when applied to displays of 22 inches or more (unless complex and costly tiling arrangements are used). Furthermore, the manufacturing process required to product an ITO touch sensor requires a different set of tooling (photo masks) for each different size of screen. Whereas this is not a problem for the higher volume, standardized designs typically required for consumer electronic projects, this process lacks the necessary flexibility when it comes to the smaller production runs and customized designs - which are the norm in enterprise or industrial applications.
The mask-less, plotting process used to produce both mutual and self-capacitive PCT touch sensors is flexible can be quickly scaled up, without the need for any financial outlay on photo masks allowing small batch quantities and ready customization..
The value proposition that multi-touch offers as a means of sophisticated user interaction with computing devices, is too great to be restricted to just hand-held consumer electronics. As a result the number of p-cap solutions capable of supporting multi-touch operation within the consumer arena is already starting to be mirrored in other applications, with the result that in the near future the general public will begin to see more touchscreens on the street, in the workplace and at leisure venues with the functionality they currently enjoy on their smartphone.