Modulating electric control devices

Design procedure for a precision electric controller-positioner is described. Minimization of on-off device null zones, and inherent increase in positioning accuracy, are achieved by incorporating a modulating electric actuator. The actuator design is divided into two major portions: mechanical and electrical. The mechanical portion is essentially a torque amplification device, where the high speed, low torque of a motor is converted to a low speed, high torque at the actuator output. For large amplification and high efficiency with a minimum of components and increased reliability, a positive drive belt and pulley in combination with a ball-bearing screw is used. The result is a series of actuators with output torques from 100 ft-lb to 3200 ft-lb with speed reductions of 500:1 to 1600:1 and mechanical efficiencies of 80 percent The electrical portion of the design incorporates a dc motor with good control characteristics. Control of the motor is accomplished with phase-controlled silicon controlled rectifiers. The motor speed is varied in proportion to an error signal, permitting the actuator to “creep” to within 0.2 percent of a position dictated by the control signal.