This invention is an improvement in the type of electronic musical instrument keyboard switch disclosed in U.S. Pat. Nos. 2,630,503 of Larsen and 2,881,293 of Erickson. In those prior art switch mechanisms a coil spring is employed as the movable switch contact member, and is flexed laterally relative to its long dimension to move it into electrical contact with one or more stiff wires or posts serving as the fixed contacts of the switch mechanism. Such use of a coil spring as a movable switch contact has some advantages. From a mechanical point of view, it provides a simple and inexpensive switch movement. One end of the spring is anchored to the frame of the switch mechanism, and the other end, by merely being left free to flex, provides the motion necessary to transfer the movable contact into and out of electrical engagement with the fixed contact or contacts. And the entire mechanism is compact enough to meet the volume constraints of a musical keyboard, which must accommodate large numbers of such switches in shoulder-to-shoulder relationship.
The prior art design also has advantages from an electrical point of view. In particular, the lateral flexing of a coil spring element into contact with a fixed wire or post caused the coil spring to wrap itself partially around the wire or post. As it did so, the individual turns of the coil spring would squirm and scuff abrasively across the surface of the wire or post. The resulting cleaning action would tend to prevent oxide or dirt build-up on the contacts, thus keeping contact resistance low and constant over the life of the switch. In the 1950's, when the Larsen and Erickson patent applications were filed, these switch mechanisms were probably adequate.
But the present day state of the electronic musical instrument art is more demanding. The availability of medium and large scale integrated circuits for special purposes has led to the extensive use of low voltage digital logic in electronic musical instruments. Such circuitry is less tolerant of electrical noise, such as the "hash" generated by the mechanical chattering of switch contacts. A jagged electrical waveform associated with a single closing or opening or the contacts may be difficult to distinguish from a rapid sequence of multiple closings or openings.
For two fundamental reasons, a movable switch contact element formed of a coil spring is prone to produce such electrical noise. The first reason has to do with the fact that small but rapid changes in energy storage conditions apparently take place in the spring itself during actuation, and this inherent "liveliness" of the spring keeps it from being dimensionally stable on a short time scale. The individual coils of the spring seem alternately to bind against, and then abruptly release and skid across, the surface of the fixed contact member, as the tension in the coil spring changes during actuation of the spring mechanism. This abrupt motion causes contact chatter, which results in electrical noise.
The second reason has to do with the inherently rough external configuration which a coil spring has, owing to the fact that consists of an array of individual coils. Even if it were possible, as the coil spring is wrapped around the fixed contact, for each coil to be laid down and picked up smoothly without the skidding and chattering described above, still there would be successive quantum jumps in the amount of contact area as the individual coils came into or out of contact. This effect alone is sufficient to produce discontinuities in the electrical waveform during switch operation.