Abstract: Three rotation sensors circumferentially arranged at 90 degree intervals in a disk code wheel attached to a motor's output shaft respectively output a signal corresponding the rotational speed of the wheel. An error component of one period (one-periodic component) or an error component of two period (two-periodic component) is generated per rotation when the code wheel is eccentric to the output shaft or is elliptically deformed, respectively. Control means averages the signals of the first and second sensors to obtain a first correction signal and subtracts the signal of the third sensor from the signal of the first sensor to obtain a second correction signal while removing the two-periodic component. The control means subtracts or adds the first correction signal relative to the second correction signal upon coinciding these signals in phase and amplitude to calculate a rotation measurement signal from which the periodic components are removed.

Abstract: Three rotation sensors circumferentially arranged at 90 degree intervals in a disk code wheel attached to a motor's output shaft respectively output a signal corresponding the rotational speed of the wheel. An error component of one period (one-periodic component) or an error component of two period (two-periodic component) is generated per rotation when the code wheel is eccentric to the output shaft or is elliptically deformed, respectively. Control means averages the signals of the first and second sensors to obtain a first correction signal and subtracts the signal of the third sensor from the signal of the first sensor to obtain a second correction signal while removing the two-periodic component. The control means subtracts or adds the first correction signal relative to the second correction signal upon coinciding these signals in phase and amplitude to calculate a rotation measurement signal from which the periodic components are removed.