The function of the Rate Loop Amplifier (RLA) is to control the velocity of the antenna. In position control applications, the rate loop improves the transient response of the position loop by adding a pole in the position loop.
The output of the PLA which acts as a velocity command, is applied to the one input while tachometer signal is applied to the other input. The RLA subtracts both the input signals and generates an error signal which is then applied to the compensator. For position control applications like the GMRT the rate loop compensator can be of phase flag type (Type O) which avoids limit cycle hunting. The electro-mechanical time constant of the combined motor and load determines the bandwidth of the compensator. The output of the RLA acts as a command to the current loop. If the command speed is more than the actual speed, then the error at the output of the RLA becomes large, which commands the current loop to pass more current through the motor.
For GMRT antennas, where a dual drive system is used, the rate loop controls the antenna velocity by sensing the tacho signal from both the motors. Both these tacho signals are averaged and then applied to RLA as feedback. A voltage corresponding to torque bias is added/subtracted at the output of the rate loop, to generate two current commands. These two current commands are applied to the two current loop amplifiers, for controlling currents in accordance with the rate loop.