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Threshold Capacitive Key

IP.com Disclosure Number: IPCOM000074292D
Original Publication Date: 1971-Apr-01
Included in the Prior Art Database: 2005-Feb-23
Document File: 2 page(s) / 32K

Publishing Venue

IBM

Related People

Abbatecola, R: AUTHOR [+4]

Abstract

Fig. A shows a threshold key arrangement for capacitively making electrical contact between conductors 1 and 3. The force versus displacement characteristics of the key arrangement of Fig. A is shown in Fig. B. The characteristics of Fig. B approximate the "feel" of a keyboard of a SELECTRIC Typewriter, thus meeting the human factors requirement of tactile feedback. To approximate this "feel" the force-displacement characteristic of a key arrangement must exhibit a firm switching action in the middle range of key travel while providing the components of first, a pretravel, and then, a positive threshold and negative impedance (toggle action) and finally,, a post travel. In addition, practical key arrangements must also have provision for restoring the key to its original position.

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Threshold Capacitive Key

Fig. A shows a threshold key arrangement for capacitively making electrical contact between conductors 1 and 3. The force versus displacement characteristics of the key arrangement of Fig. A is shown in Fig. B. The characteristics of Fig. B approximate the "feel" of a keyboard of a SELECTRIC Typewriter, thus meeting the human factors requirement of tactile feedback. To approximate this "feel" the force-displacement characteristic of a key arrangement must exhibit a firm switching action in the middle range of key travel while providing the components of first, a pretravel, and then, a positive threshold and negative impedance (toggle action) and finally,, a post travel. In addition, practical key arrangements must also have provision for restoring the key to its original position.

The key arrangement of Fig. A meets these requirements by employing a pair of springs 5 and 7, respectively, and a magnet 9. The springs and magnet act to impart the required forces upon key 11 and its corresponding key stem 13. Spring 5 acts as a return spring while spring 7 acts as a compression-expansion spring. Magnetic metal plate 1~ is held in position by spring 5 and magnet 9 when the key is in its normal position. The force imparted by spring 7 upon plate 15, during the travel of key 11 from a displacement of zero to PT, as shown in Fig. B, is less than the magnetic force of attraction between magnet 9 and plate
15. Thus, pin 17 is allowed to travel the...