A servo drive is a special electric amplifier used to power electric servo motors. It monitors feedback signals from the motor and continually adjusts for deviation from expected behavior. A servo drive receives a command signal from a control system, amplifies the signal, and transmits electric current to a servo motor in order to produce motion proportional to the command signal. Typically the command signal represents a desired velocity, but can also represent a desired torque or position. A velocity sensor attached to the servo motor reports the motor's actual velocity back to the servo drive. The servo drive then compares the actual motor velocity with the commanded motor velocity. It then alters the voltage frequency to the motor so as to correct for any error in the velocity. In a properly configured system, the servo motor rotates at a velocity that very closely approximates the velocity signal being received by the servo drive from the control system. Several parameters, such as stiffness (also known as proportional gain), damping (also known as derivative gain), and feedback gain, can be adjusted to achieve this desired performance. The process of adjusting these parameters is called tuning. Servo systems can be used in CNC machining, factory automation, and robotics, among other uses.