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Scanned Piezoelectric Key Switch Keyboard

IP.com Disclosure Number: IPCOM000088505D
Original Publication Date: 1977-Jun-01
Included in the Prior Art Database: 2005-Mar-04
Document File: 3 page(s) / 43K

Publishing Venue

IBM

Related People

Dahl, JP: AUTHOR

Abstract

This article describes a simple electrical design for a parallel-wired sequentially-scanned piezoelectric keyboard. In Fig. 1, each keyswitch 1 comprises a piezoelectric crystal 2 and a mechanical dampener attached to a movable key stem (not shown). Normally, the mechanical dampener is separated from the crystal to allow it to freely oscillate when electrically excited. The human operator pressing the key stem pushes a key crystal dampener against the crystal to dampen its vibration. Each key switch crystal is chosen to have a unique inherent resonant frequency.

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Scanned Piezoelectric Key Switch Keyboard

This article describes a simple electrical design for a parallel-wired sequentially-scanned piezoelectric keyboard. In Fig. 1, each keyswitch 1 comprises a piezoelectric crystal 2 and a mechanical dampener attached to a movable key stem (not shown). Normally, the mechanical dampener is separated from the crystal to allow it to freely oscillate when electrically excited. The human operator pressing the key stem pushes a key crystal dampener against the crystal to dampen its vibration. Each key switch crystal is chosen to have a unique inherent resonant frequency. If an electrical AC signal is applied to a given key switch at the resonant frequency of the crystal in that key switch, the crystal will resonate and, at resonance, the impedance of the undamped crystal will change very greatly depending upon the quality factor Q of the crystal itself. This changing impedance can be sensed as a voltage change across a resistive voltage-divider network. If the crystal is excited by another AC signal at a different frequency than that at which the crystal will resonate freely, the crystal will resonate but it will resonate at the driving signal frequency, and there will be no great change in the impedance of the undamped crystal.

When the key switch stem is moved, the dampener applies pressure to attenuate oscillations whether they are resonant oscillations or driving frequency oscillations. The impedance of the dampened crystal then approaches that of a crystal having a nonresonant driving signal. In a crystal which is not being driven at its resonant frequency, little change is noticed between the damped or the undamped condition. In a crystal which is being driven at its resonant frequency, there is a great change in impedance between the damped and undamped condition. Therefore, by sensing the amplitude of the voltage output signal at the resonant frequency for a given key, key closure, i.e., dampening of that key, can be sensed.

A series of key switches is wired sequentially together and may be sequentially scanned with an applied variable frequency signal. The applied driving frequency would be varied from the resonant frequency of one crystal to that of another, and by sensing the output for each frequency, each key is sensed...