What is a Servo?

In control system terminology, a servo is a closed loop system that consists of a plant, controller, and feedback. Each of these elements can be made up of a few or many parts. These elements work together to make sure that the actual position (or other measure) matches the commanded position (as close as possible).

Plant

A plant is the system under control. MEI customers' plants usually consist of a motor, amplifier, and mechanical structure. Other examples of plants are an amplifier and heating element, or a piezo drive and focusing element. Below is a picture of a motor used in servo systems. The key element of the plant is the actuator (the part of the motor that does work).

Feedback

Feedback is one of two things that separate an open loop system from a closed loop system. Feedback devices send updates about the plant to the controller. MEI customers' feedback devices are usually position encoders. Other examples of feedback devices are resolvers, thermocouples, focus sensors, and strain gauges. The end section of the motor shown above contains an encoder. A tracking number for shipping is an example of feedback. You can send the package open loop (send it and forget it) or you can also send it closed loop (verify that it was received) with feedback (the tracking information).

Controller

The controller takes feedback from the plant and commands such that the plant does what it is commanded to do. In a motion system, the motion controller acts as the controller. The most common application of the controller is sending torque commands to a motor in response to encoder feedback. In addition to commanding a torque to accelerate a motor, the controller will compensate for external disturbances (such as friction) and make sure that the plant stays on course.

Examples of Servos in the Real World

Many industrial servos serve as motion control elements. A semiconductor wafer inspection system that must position a wafer very precisely is a servo. The plant is the linear motor and stage mechanical parts. The linear encoder or laser interferometer is the feedback. The motion controller is the controller.

Another example of a servo is a conveyor belt that must move at a constant speed regardless of load size should be a servo. The motor, belt, and things on the belt are the plant. The sensor on the belt or motor that sense speed is the feedback. The PLC or motion controller is the controller.

Another example of a servo is the cruise control in your car. It presses harder on the gas pedal when the car goes up a hill and lets off the gas when the car goes down a hill. The engine and car are the plant. The speedometer is the feedback. The car's cruise control computer is the controller.

Another example of a servo is a person taking a picture. The persons hands and the camera are the plant. The person's eyes and camera sensors are the feedback. The person's brain is the controller.

In the context of this tuning section, we will discus motion control using motors. Remember, these are not the only kinds of servos. But, the basic principles of tuning servos apply to all kinds of servos. Each implementations have their own unique challenges in addition to the basic servo tuning principles.

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