 Categories of control theory are defined as open-loop and closed-loop control. In an open-loop system, the output is not measured or monitored for comparison with the system set point. Open-loop systems are commonly referred to as non-feedback systems. Closed-loop systems, on the other hand, are designed to automatically maintain the desired output by comparing it with the actual condition. Feedback devices are used to control the outcome and are referred to as closed-loop. A closed-loop motion control system is comprised of the following elements. The operator interface allows the user to command signals to the controller. Examples of an interface are industrial computers or an operator interface terminal. The motion controller takes the user input from the operator interface and creates a motion profile for the actuator to follow. A simple motion profile consists of position, velocity, and torque. Position parameters require a start and a finish position to move a load. The velocity profile resembles a trapezoidal shape. In this example, the acceleration, velocity, and deceleration is constant. In the torque profile, acceleration is positive, constant, or negative to meet the target velocity defined by the motion profile. Amplifiers, also referred to as drives, take the motion profile from the controller and generate the current required to drive the motor or other actuator type. The actuator is defined as a motor, cylinder, or other electromechanical device used to create motion. Linear actuators used in industrial applications convert rotary motion to linear motion by turning the actuator nut to move the screw in a straight line. The feedback device, usually an encoder or position-sensing device, recognizes the motor position and reports the result back to the controller. As the encoder rotates, position and speed information are relayed to the controller and compared to the user input.