External encoders are used to determine the position and movement of shafts inside a machine or device. Encoders typically produce a stream of encoder pulses as the encoder shaft rotates with respect to the encoder body. There are two general types of external encoders: through shaft designs and ridged mount designs. In general the encoders using the ridged mount designs are typically of higher quality and have better accuracy or higher resolution. Ridged mount designs are attached directly to the device with a ridged mount and use a flexible or compliant coupling between the encoder shaft and the device shaft. Through shaft designs typically use a ridged coupling that attaches the encoder directly to the shaft of the device. A compliant mount couples the encoder body to the side of the device.
Ideally, for both types of encoders, the center of rotation of the encoder will be aligned with the center of rotation of the shaft in the device. But in reality there is always some misalignment between the two different centers of rotation. The compliant coupling between the shafts in the ridged mount encoder and the compliant coupling between the encoder body and the device for the through shaft encoder both compensate for the inherent offset between the center of rotation of the encoder and the center of rotation of the shaft in the device. Compliant designs (compliant shaft coupling for ridged mount, and single compliant tether for through shaft mount) have the disadvantage of inducing small inconsistencies in the encoder pulse stream timing. The inconsistencies manifest themselves as cyclic increases and decreases in encoder pulse timing with each revolution of the encoder due to the geometric limitations of these existing designs.