How Digital Calipers Work ?

With multiple plates the digital calipers can form a capacitive array which can senses motion precisely. There is a stator and slider (“rotor�? plates in a digital caliper. The stator is embedded in the metallic ruler on which electronic housing slides, and the electronic housing contains the slider.

The stator pattern is fabricated on the top copper layer of a standard glass-epoxy laminate and glued to the stainless steel bar of the caliper. The slider pattern is similarly fabricated on PC laminate, drives a 100 kHz signal through the sin/cos plates to the stator electrodes, and picks up AC voltages at the two central pickup plates which describe sin(displacement) and cos(displacement) signals.

Moreover, the separate sin and cos signals can determine the direction of motion. The combination of plate-counting digital circuits and analog interpolation between plates yields 0.0002�?accuracy over 6�?with standard PC fabrication methods. This application uses a small watch battery, and shows the microamp-level current consumption possible with the technology.