The latest generation of handheld personal consumer electronic devices, with their highly sensitive and intuitive user interfaces based around projected capacitive (p-cap) touchscreens, have begun to stimulate greater demand for similar levels of performance and functionality, even in the heavy-duty or high-use and ruggedized interactive displays found in industrial, public information, and self-service applications. However, there are some notable differences in the technical criteria that must be met in order to implement p-cap touch sensing into such systems. Though their speed of response and sensitivity are generally more than adequate for use with smartphones and tablet computers, the often-uncompromising environments of nonconsumer systems put incredible strain on conventional p-cap sensing mechanisms. Therefore, p-cap solutions that are more suited to the demands of these applications are called for.
The phenomenal rate at which p-cap has been embraced as a touch technology is a testament to its all-round ability to offer design engineers user interfaces that are highly resistant to damage and therefore have far greater longevity than rival touch-sensing technologies, while also delivering a high degree of sensitivity, edge-to-edge operation (with no need for use of a bezel around the active area, thereby enhancing the aesthetic), and strong immunity to false-touches, as they react only to the proximity of a finger (or handheld conductive stylus).
P-cap sensing methods
The vast majority of p-cap touch sensors are currently based on mutual capacitance. Here the touch sensor consists of two separate conductive layers (each connected directly to the control electronics); one with the sensing cells that allow the position of the touch event to be determined and one with the driving cells (through which the charge is passed). If a touch event occurs on the touchscreen’s surface, an alteration in the charge held within the local electric field takes place, lowering the mutual capacitance existing between the two layers of the sensor. The cells in the sensing layer pick up this alteration. Detection algorithms within the system’s touch controller then ascertain which cells witnessed the greatest change and are able to subsequently send the exact xy position to the host PC.
Fig. 1: Glass layers with sensing cells that allow the position of the touch to be determined.
Self-capacitance p-cap touch sensing is distinctly different from mutual capacitance, as it requires an xy grid of open-ended copper conductive lines connected to a touch controller which possesses all the necessary detection algorithms. The charge held on the conductive lines can be affected by human body capacitance. So when a user’s finger comes into close proximity with the surface of the touchscreen it is possible to determine which of the lines (in both x and y directions) witnesses the peak change in charge and the PC is then informed of the precise position. However, though it works very well with small format displays, it is much more difficult to apply to larger formats. This is due to the fact that the controller must capture data from each individual sensor cell. When large scale touchscreen implementations are involved, the weight of numbers becomes too great. This normally limits the maximum effective size of display to diagonals of approximately 15 in., as beyond this costly and highly complex control electronics are necessary. In addition, mutual capacitance p-cap sensors are generally based on a cell matrix that makes use of indium tin oxide (ITO) — a near-transparent electrical conductor. ITO is deposited on to the glass using a photolithographic process similar to that employed in semiconductor fabs, but this again has drawbacks with larger display formats and lower volume applications, as the relative inflexibility of the process and up-front investment needed for creating the photo masks required for each new design will impact on unit costs and prototyping time. Furthermore, ITO has a relatively high electrical resistance and for larger displays this, the build of up resistance over the length of a screen significantly impinges on signal integrity levels, and ultimately sensitivity and touch performance.
Advanced self-capacitive p-cap sensors
Zytronic has focussed its engineering resources on developing p-cap sensor technology that can marry together some of the attributes of conventional self-capacitive and mutual-capacitive sensing mechanisms. Its most well-known and proprietary Projected Capacitive Technology (PCT) is self-capacitive in nature and is optimised for larger-screen deployments than sensors based on mutual capacitance. PCT consists of an xy matrix of micro-fine copper capacitor lines, embedded in a laminated substrate. The accompanying touch controller actively scans the capacitor matrix for any changes. Through the modulation of a generated frequency of around 1 MHz in this matrix, it is possible to detect the tiny alterations in capacitance across the individual copper lines, caused by human interaction with the touchscreen. The controller firmware then accurately calculates the position of the touch event and provides xy coordinates to the host system. The high levels of sensitivity supported mean that it can detect touch through thick overlays (10 mm or more) and when the user is wearing gloves – both proving to be huge plus points within industrial or outdoor public-use environments. The copper capacitor lines exhibit far lower resistance than sensor cell structures constructed from ITO. This enables touch detection even on displays with +80-in. diagonals. The copper lines can be deposited directly onto glass surface using a “direct write” process without photo masks being required, thereby keeping both the NRE costs and the development time involved at minimal levels.
As engineers look to bring more intuitive user interfaces, in line with what is readily available in consumer electronics space, to large-form-factor industrial, retail, public-use and medical designs, ITO-based mutual capacitive p-cap technology is being pushed to the edge of its performance boundaries. The manufacturing process employed and the material involved means that, though highly suited to small format, high volume designs, it may not be a good fit when applied elsewhere. The rugged nature of advanced p-cap sensors based on PCT, along with their ability to be applied to large displays and their inherent volume flexibility will allow touch-enabled interaction to be applied to a wider range of electronics designs. This will result in more satisfying user experiences being derived. However, this article certainly does not wish to suggest that mutual capacitance p-cap sensors are without merit. The ease with which (given sufficient cell density and controller IC performance) they can enable the acquisition of data needed for multi-touch functionality means that they will prove invaluable as the demand to support simultaneous multiple user touching or screen, and complex gesture recognition increases.
It is clear that p-cap touch sensing has a bright future, thanks to its all-round performance and design compared to other sensor technologies. Advanced self- and mutual-capacitive sensors (along with ever-improving controller electronics to support them) each have certain attributes that make them highly suitable for different applications — with environment, display format, touch performance, and unit volume all having an influence. Progression in this sector (see Fig. 2) is leading to mutual capacitive, multi-touch sensors with alternative materials to ITO that will offer even greater flexibility and scalability. 
Fig 2: Mutual capacitive, multi-touch sensors offer flexibility and scalability
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