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issue: July 2006 APPLIANCE Magazine

Supplier Solutions: Switches and Switching Devices
Charge-Transfer Sensing Revolutionizes Appliance Control Design, Functionality


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by Jill Russell, Senior Associate Editor

Designers of appliances are continually challenged to differentiate their products not only by functionality, but also by appearance. Conventional electromechanical switches and membrane switch panels may limit design freedom and may wear out. By contrast, capacitive touch controls that drive electronic switches eliminate mechanical movement. What’s more, the sense electrodes can be placed behind any insulating layer to make an environmentally sealed touch pad.
Early capacitive touch panels suffered from electrostatic discharge and electromagnetic interference, causing false triggering and temperature changes affecting calibration. Moisture and other contaminants on the surface of the panels affected reliability of operation. A new variation on capacitive sensing, known as charge-transfer, or QT sensing, overcomes these challenges-without adding cost.
The QT sensor is essentially a microcontroller programmed to charge a sense plate, which can be anything conductive, from a printed circuit board (PCB) pad to an area of optically-clear indium tin oxide (ITO) laid underneath, or on top of, a display screen. The construction of a common type of control panel is illustrated in Figure 1. An approaching finger affects the flow of charge to permit sensing and create a switching action via a transistor. The QT sensor is programmed to eliminate false triggering due to electrostatic discharge or momentary unintentional touch. It is available as single or multiple keys, matrix keyboards, touch sliders, touch wheels (like the iPod), touch screens, and combinations of these. Where multiple keys are used, each key can be set for an individual sensitivity level. This enables keys of different sizes and shapes to meet both functional and aesthetic requirements. Furthermore, the QT sensor has automatic drift compensation to account for slow changes due to ageing or changing environmental conditions and, unlike traditional capacitive sensors, do not require coils, oscillators, RF components, special cable, RC networks, or a lot of discrete parts. As an engineering solution it is simple, robust, elegant, and affordable.

This information was provided by Hal Philipp, CEO, Quantum Research Ltd.

Suppliers mentioned in this article:
Quantum Research Group Ltd.
 

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