Patent ID: 8118276

Claim:
A valve actuator comprising: an electric drive motor with a motor shaft; an output shaft linked to the motor shaft to drive movement of a valve; mechanical drive train coupling the motor shaft to the output shaft; a control system arranged to provide a first closed loop control of the actuator output shaft and including a first position sensor operatively coupled to sense positions of the output shaft, a second position sensor operatively coupled to sense positions of the motor shaft, and a controller coupled to the first and second position sensors and in response to the second position sensor determining a speed of the motor shaft, the control system being configured to respond to sensed signals from the first and second sensors by adjusting at least one of speed and position of the motor shaft, and wherein when the electric drive motor reverses direction, the controller is configured to measure a backlash within the mechanical drive train by reference to an amount of change in position indicated by the second sensor that occurs before a change in position is indicated by the first sensor.