SAWs for touch panels

Fujitsu Laboratories has succeeded in developing a touch panel screen for PDAs with a light transparency ratio of 98%.

The newly developed touch panel employs surface acoustic waves (SAWs) that are generated by a piezoelectric thin-film transducer housed on a single glass substrate 0.7mm thick in a 1.4mm-wide frame.

In the widely used resistive film method of touch screen manufacture, a touch panel is formed from multiple layers, and light becomes scattered as it passes through each layer. Consequently, the light transparency ratio is usually around 82%. Even by applying a coating treatment on each layer, this ratio has only been able to be improved to about 86%.

Although it has been recognised that the SAW method could be applied to achieve a touch panel with a single transparent substrate, until now this has required a wide frame to house the piezoelectric transducers that generate the acoustic waves, thus making it difficult to implement such a system in a small-size panel.

To solve the problem, the new touch screen panel has made use of an array of consecutive V-shaped comb-type electrodes. In addition, specialised control circuitry was developed to achieve a position resolution of 100 dpi (dots per inch) and a sampling rate of 100 times per second – suitable for hand written input.

More details of the technology will be disclosed at the meeting of the Ultrasonic Technical Group of the Institute of Electronics, Information and Communication Engineers to be held on November 29 at Shizuoka University in Japan.

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