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Dealing with capacitance sensor algorithms in multitouch sensing

Posted: 14 Sep 2009 ?? ?Print Version ?Bookmark and Share

Keywords:capacitance sensor? sensing algorithms? multitouch sensing?

Increasingly, embedded applications must interact directly with their environment and their end users. Consider the best new touchscreen phones, in which the user interface is a large capacitive sensing screen that differentiates a flick from a tap and tracks the motion of your finger but doesn't track your ear.

Sensors are at the heart of these systems. They sense the environment and user behavior, enabling the product to respond in an intuitive but reliable way. However, the sensor films themselves aren't intelligent. They don't even collect data. They only sense. They aren't capable of differentiating between useful and useless data or discriminating between the quality of different types of inputs.

Truth be told, these sensor films hardly sense at all. They really just project an electric field created by an intelligent capacitive sensing chip. This type of capacitive sensing is known as projected capacitive technology, and it's used in the most advanced capacitive touchscreen solutions.

This is not to say that the sensors themselves are not complex. On the contrary, a capacitive touchscreen sensor consists of a large array of indium tin oxide (ITO) conductors on one or more layers of glass or polyethylene terephthalate plastic.

The good optical clarity and low resistivity of ITO make it the perfect conductor for creating a touchscreen. When the ITO sensor is connected to a capacitive sensing chip with a suitably high SNR, it can accurately sense minute changes in capacitance. A finger's presence for instance is on the order of a picoFarad (1012 Farads).

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