The position loop amplifier (PLA) has two inputs viz. command input and feedback input. In an automatic position control system, the output of the position sensor is filtered, scaled and then applied to the PLA. The command signal is applied to the other input of the PLA. The PLA (which can be either analog or digital) subtracts its two inputs to generate an error signal. This error signal is then applied to the compensator.
A compensator is designed depending on the application. For example the GMRT antennas are used for tracking of stellar radio sources which are moving at constant speed in the sky (/hr, the speed of the earth's rotation). For such an application, a position system having type II response is required. With a type I position compensator and with the use of rate loop in the position control, the overall system response is of type II .
|Type of position system||Pointing Error||Tracking Error|
Parameters like the structural natural resonant frequency (Wc) and the frictional (Bc) constants of the structure are required for the design of the position loop compensator . The main objective while designing the position compensator is that it should offer enough attenuation at the natural resonant frequency of the structure.
The output of the PLA acts as velocity command. If the target's angular position is far removed from the current position, then the error is very large and could saturate the PLA . The saturation of the PLA is considered as a fixed velocity command to the rate loop. The rate loop moves the antenna with a constant velocity towards the target position. As the antenna approaches the target position, the error at the output of the PLA goes on reducing, which commands the rate loop to reduce the speed of the antenna. When the antenna is at the target position the error at the output of the PLA goes to zero, which translates to a zero speed command to the rate loop. The sign of the error signal at the output of the PLA decides whether the antenna is to be moved forward or reverse .