Principles of Position Control

For controlling a heavy load, one could, (as illustrated in Figure 20.2) use three nested feedback loops viz. a position loop, a velocity loop and a current loop. This configuration allows independent tunning of the loop parameters without affecting the adjacent loop. A current amplifier is used to amplify the current for driving the motor. The position is sensed by a suitable transducer. The velocity of the antenna is generally sensed by the tachometers mounted on the motor shaft.

Figure 20.2: Three nested feed back loop.

The block diagram shown above can not be directly used in all position control applications. The back-lash which is inherent in any gear box, introduces a non-linearity in the position loop. Such a system exhibits a phenomena called as ``limit cycle hunting''. This affects the positioning accuracy of the antenna.