Sunday, January 13, 2013

Servomotors

Srvomotors Part1
       Aservosystem is one in which the output is some mechanical variable like position, velocity or acceleration. Such systems are automatic control systems in which output is some mechanical function such as controlling the position of the shaft, controlling angular speed of the shaft etc.

       As seen earlier, the motors used in such control systems are driven by the signal which is derived based on the error information supplied to the controller. These motors used in such servosystems or servomechanism are called as servomotors. These motors are low power rating motors and can drive the load directly, hence these motors are usually coupled to the load through a gear train for power matching purpose.
1.1 Requirement of  a Good Servomotor
       The servomotors which are designed for use in feedback control systems must have following requirements :
1. Linear relationship between electrical control signal and rotor speed, over a wide range.
2. Inertia of rotor should be as small as possible. A servomotor must stop running without any time delay, if control signal to it is removed.
       For low inertia, it is designed with large length to diameter ratio, for rotors. Compared to its frame size, the rotor of a servo motor has very small diameter. From its low diameter rotor only, the servomotor is distinguished from other motors in practice.
3. Its response should be very fast. For quickly changing error signals, it must react with good response.
       This is achieved by keeping torque to weight ratio high. Hence these motors can be started, stopped or reversed very quickly compared to normal motors.
4. It should be quickly reversible.
5. It should have linear torque-speed characteristics.
6. The output torque at any speed should be roughly proportional to the applied control signal.
7. Its operation should be stable without any oscillations or overshoots.

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