Appliance Design - February 2008 - (Page 19) CONTROLS & SENSORS given key count, according to Ard. QMatrix uses a pair of sensing electrodes. One is an emitting electrode into which a charge consisting of logic pulses is driven in burst mode. The other is a receive electrode that couples to the emitter via the overlying panel dielectric. When a finger touches the panel the field coupling is reduced, and touch is detected. Electrodes are typically areas of copper on a printed circuit board, but can also be areas of clear conductive indium tin oxide (ITO) on a glass or plastic touch screen. A single QMatrix device can drive a large number of keys, enabling a very low costper-key to be achieved, Ard says. High key counts, he says, are made possible via the configuration of pulse-driven rows and charge-receiving columns. For each key, a row couples into the driven electrode and a column connects to the receive electrode. If there are 8 rows and 4 columns, 32 possible keys can be wired into the matrix. Quantum’s QMatrix devices currently go up to an array of 8 rows and 6 columns, for a maximum of 48 keys in one chip. But, it is not just the number of keys that are important, says Ard, it is the ability of the designer to place them where needed. QMatrix based keys can be located anywhere on a panel up to a meter apart from each other, all controlled by a single chip. They can also be a variety of sizes and shapes, mixing small and large keys together in one design. A single QMatrix chip can also be used for sense sliders, wheels and touch screens along with discrete buttons. To help ensure that unintentional touches do not occur, the company has developed a number of features. A change in signal strength can indicate a valid touch or it could be a false actuation – for example a glitch due to electrical noise. To be certain one way or the other, the signal needs to be verified repeatedly, but quickly, as soon as a likely detection event has occurred. To make sure the signal is not merely electrical noise, Quantum’s chips operate at multiple frequencies, using digital spread spectrum technology to skip around noise frequencies and make sure that there really is a touch. The process of noise suppression uses what Quantum calls a “detection integrator,” a type of filter that employs consensus over a number of signal samples, for example at different frequencies, to prevent false detections. This type of filter can require that 10 consecutive signal samples all confirm a touch; even one drop out among the ten samples will cause the signal confirmation to fail. Ard says that when the system is turned on, the chip takes a measurement of its “capacitive surroundings” on each key. The result is considered the resting count or signal reference level. “Signal changes relative to the reference level are the important things to look for. They are a function of moisture, interference, temperature drift, and of course touch,” he says. If the long term signal level varies in a particular direction from the reference level for a period of time, for example due to temperature effects, the system will Reach out and touch As effortless as running your finger over this page, Quantum’s touch sensor solutions free your product from mechanical controls. The proven technology behind many of the world’s leading brands can easily be integrated into your product. Touch screens, buttons, sliders and wheels, or customised solutions, with short lead times and reliability in your application. Speak with the world leader in touch sensor technology. Speak with Quantum. www.qprox.com For more Information Enter 106 AD02084Quant.indd 1 www.applianceDESIGN.com applianceDESIGN 1/10/08 1:55:42 19 February 2008 PM http://www.qprox.com http://www.qprox.com http://www.appliancedesign.com
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