Servo systems consist of a motor, controller, feedback device, and drive. The controller determines what torque, velocity, or position is needed from the motor and then triggers the drive to send the electrical energy necessary to let the motor move as commanded. No wonder servo drives sometimes called amplifiers for how the controller’s modestly powered control signal informs the shaping of far-larger motor-powering voltage and current.
To use the human body’s neuromuscular system as an analogy: A servo system’s controller is akin to the brain - taking information from feedback devices and sending the appropriate voltage signals to the drive. In turn the drive acts as the neuromuscular system’s motor neurons to send the necessary amount of current to the motor being commanded to move. This process of reading and responding to feedback makes the system closed loop - the defining characteristic of a servo system.
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