Hysteresis curve

When a torque is applied to the output of a Harmonic Drive with the input locked, the torque torsion relationship measured at the output typically follows the hysteresis curve 0-A-B-A'-B'-A, as seen in the figure above.

Lost motion

Harmonic Drive Gears exhibit zero backlash. The term lost motion is used to characterise the hysteresis curve in the low torque region. The value of the displacement B-B' is defined as the lost motion.

Torsional stiffness

The displacement 0-A and A'-0 is the consequence of the torsional stiffness.

Repeatability

The repeatability describes the position difference measured during repeated movement to the same position from the same direction. The repeatability is defined as half of the maximum difference measured, preceded by a ± sign.

Transmission accuracy

The transmission accuracy of the gear represents a linearity error between input and output angle and is measured using a high resolution measurement system. The measurements are carried out without direction reversal. The transmission accuracy is defined as the sum of the maximum positive and negative differences between the theoretical and actual output rotation angle.