Project development: control of a DC motor
The circuit is an analog regulator used to regulate DC motor speed. The amplifier is essentially a power amplifier like a power operational amplifier. It would usually be implemented using a controlled rectifier or pulse width modulation scheme.

A DC motor is basically controlled by controlling the armature terminal voltage. However regulating armature voltage does not exactly regulate the speed. Speed is proportional to the motor's counter emf. Counter EMF is less than the terminal voltage by the amount of voltage drop across the armature resistance (IR drop). That is proportional to the armature current which is proportional to torque.

To prevent the motor speed from dropping when the torque increases, it is necessary tor provide additional feedback proportional to the IR drop. That is called IR compensation. The Rs in the diagram is a shunt resistor that provides IR compensation feedback.

The amplifier should have some kind of current limiting circuit provided internally to limit the maximum current supplied during acceleration or mechanical overload.