Motion reducer

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Keyboard switches typically have between 2.5 and 4 millimetres of travel. This is by design: a large amount of travel gives the keys "swing", allows for the operator to comfortably overcome the tactile peak, and affords the operator with good feedback (the keys move far enough to clearly feel that they were pressed).

The keystroke sensing method chosen may not, however, support this much motion. For example, the centre membrane in a membrane keyboard is under 0.1 mm thick, which is as little as one fortieth of the total travel of the keyboard.

Some form of motion reducer is required to condense 2.5–4 mm of motion into the smaller range of motion of the sensing system. For example, membrane keyboards are paired with a number of motion reducers including:

  • Buckling spring: the spring buckles after it has already absorbed over 2 mm of compression, and can be compressed further once buckled.
  • Prong over membrane: the return spring sits over a stiff plastic prong, which deflects at a slower rate than the return spring; again, the return spring can be compressed further once the prong is deflected as far as it can be.

The Datanetics elastic diaphragm array actuator refers to the motion reduction as "lost motion", a term that normally means backlash and is more commonly used to refer to hysteresis. Elastic diaphragm array has its own special arrangement to provide motion reduction over a membrane.

Stacked spring switches that use a snap-action inverting plate spring typically place a helical spring above the plate spring, which amongst other objectives provides the motion reduction required.