Over the last decade the uptake of projected capacitive (p-cap) touch sensor technology has been phenomenal. It has helped shape a new era of human-machine interaction and enabled the derivation of more fulfilling user experiences. Further proliferation of p-cap touchscreens seems certain, with the enormous potential for it to benefit many different application scenarios. If this is to be achieved, however, advances in the supporting touch control electronics will almost certainly be required.
Almost everyone, from adults to small children, is now very familiar with touchscreen interaction in handheld electronic devices, and increasingly in domestic appliances, gym equipment, self-service terminals and bank ATMs. It has become so pervasive that people, presented with a display will often attempt to control it by touch first, whether or not it is capable of this. Given the intuitive control that touchscreens offer in comparison to push buttons and keyboards, there is now a growing demand to apply the technology in more and more areas, and in increasingly demanding environments. This, as we will discuss, puts certain operational pressures on the touch controller electronics that accompany them.
The implications of touch control
There are two basic sensing methodologies associated with p-cap touchscreens. The most widely used to date have been based on mutual capacitive touch sensing – which is seen extensively in the portable consumer market. This has been the foundation, for example, of the touchscreens found in tablet computers, and smartphones. Most mutual capacitive touch sensors have two separate patterned conductive layers (with a matrix of discrete cells formed from Indium Tin Oxide, or ITO), which are each directly connected to the touch control electronics. A small charge is applied to one layer of cells and passes to the second layer through capacitive coupling. Touch events, caused by the approach of a finger or suitable conductive stylus will draw some of the charge passing between the layers. Detection algorithms embedded within the touch controller firmware are then able to mathematically determine the cells on the matrix where the most acute capacitance change occurs and supply the host PC with details in the form of XY coordinates.
In contrast, the less widely used self-capacitive sensing technique relies upon the detection of minute changes in frequency. A known oscillation frequency is applied to the XY grid on the rear side of a suitable substrate. The known oscillation frequency is affected by the presence of the capacitance of the human body or a suitable conductive stylus. If a user’s finger comes into proximity with the touchscreen’s surface it is possible to determine which conductors (in both X and Y axes) experience the greatest alteration in frequency, and a suitable set of output coordinates are provided to the host computer.
Mutual capacitive touch technology is very well suited to touchscreen applications requiring multitouch functionality, as each cell within the conductive matrix is capable of registering a touch. However, at relatively low voltages, it is difficult for this approach to detect a touch through more than a couple of millimetres of glass. Whereas self-capacitive touch sensing is an inherently more sensitive method, and therefore can typically detect touch through far greater thicknesses of overlying material. However, it has the drawback of normally only being able to detect one or two touch points, as the entire conductor in each axis is monitored, rather than each individual cell in a matrix.
Key issues defining next generation P-Cap touch controllers
Whether the touchscreen is for consumer electronics or commercial/industrial use, there are four fundamental areas where research and development has been focused to advance both touch performance and user interface design. These are:
Faster speed – Particularly with larger touchscreen systems the size of the conductive matrix in the touch sensor will have a negative effect on the registration of touch coordinates. To counter this, the associated touch control electronics must possess substantial processing capabilities – otherwise lag or latency will be evident as a touch point is moved across the screen, detracting from users’ overall satisfaction.
Heightened accuracy – If the touch points cannot be determined with suitable precision then user frustration may result. It is important that both the design of the touch sensor matrix and its controller be optimized for the screen size and application.
Improved immunity to EMI – Though it might be assumed that this was only an issue for touchscreen systems placed into industrial environments, there are in fact a wide variety of commercial applications where exposure to electro-magnetic interference (EMI) can have a detrimental effect on touch operation. For example, self-service kiosks such as ticket and vending machines located in train stations will be subjected to surges in EMI as trains pass. Similarly, touchscreens deployed in areas where the power supply is inconsistent or not well regulated will also be affected by transient interference coming up the power cable from the mains supply. Major improvements to the electronic design and touch detection firmware employed by the touch controller are needed in these circumstances to ensure that signal integrity is maintained at a high level.
Greater integration – For compact touchscreen designs, there is a clear advantage if the footprint of the touch controller can be kept to a minimum. Reducing the PCB size is therefore important, as is making available the controller chip-set, so that designers can consider embedding the touch controller onto an existing system motherboard.
Commercial applications now demand a new generation of touch sensor systems which are capable of achieving higher degrees of responsiveness and precision, as well as maintaining greater EMI stability and overall compactness.
To meet the challenges raised by increasing demand for larger format touchscreens that both match the fast touch performance of small consumer devices and continue to function effectively in environments with high levels of electrical interference, companies will need to be smarter about the control electronics they employ in systems. Fortunately, touch sensor developers are continuing to push the boundaries of performance and resilience and can provide solutions for even the most demanding applications.
