How Do Servo Systems Work?
Servo Systems includes four main components, a Servo Drive, a Servo Motor, a motion Controller, and a Feedback Device (usually referred to as an Encoder).
Basically, a Servo Drive takes a command signal for position, speed or torque requirement and compares it with feedback from a servomechanism to provide the required voltage and/or current to a Servo Motor to correct any deviation from the commanded status (closed loop control).
Role of the Motion Controller
The motion Controller is responsible for higher-level computation and decision-making (sometimes referred to as the ‘brains’). A motion Controller handles many supervisory and processing tasks, but the most basic functions are command tracking and disturbance rejection (execution of these functions are often referred to as “closing the loop”). In basic terms, the motion Controller’s role is to determine what needs to be done to achieve the desired position, velocity or torque and sends this information to the Servo Drive, which produces the voltage that energises the motor windings and causes the motor to rotate or produce torque. The Controller thus moves information from the Feedback Device and sends the necessary signals to the Servo Drive. The motion Controller (the ‘brains’) has basically two tasks to perform as follows:
- Command Tracking: Track the commanded input (Reference Tracking) – This ensures the Servo Motor follows the application’s motion profile without error, or with minimal error (deals with errors that occur continuously throughout the motion). The motion Controller does this by continuously comparing the actual values (position, velocity, or torque) of the Servo Motor (based on feedback from the Feedback Device) with the desired values and sending signals to the Servo Drive to correct possible errors (mismatches between actual and desired values).
- Disturbance Rejection: Improve the system’s disturbance rejection – This deal with errors caused by external forces that cause the Servo Motor to deviate from the target value (position, speed, or torque). These errors are processed, and commands are issued to correct them.
Command Tracking and Disturbance Rejection functions are enabled by the Servo System’s control loop capability. Servo Tuning involves finding the appropriate corrections (amount and type of change for the various error conditions) to allow Command Tracking and Disturbance Rejection without causing the Servo System to become unstable.
Proportional Integral Derivative (PID) controllers are powerful devices used for motion control. A PID helps understand an error signal (difference between a commanded value and the actual value) and allows driving the error to zero. Communication to and from Controllers and other devices can be via a simple Digital and Analog i/o or Digital Fieldbus Communication (e.g., EtherCAT, CANopen and other industry standard protocols).
Role of the Servo Drive
The role of the Servo Drive is to translate low power command signals from the Controller into high power voltage and current to the Servo Motor. In essence, the Servo Drive (the ‘nervous system’) sends the required amount of current to the Servo Motor. Depending on the application, the Servo Drive can regulate and properly coordinate the Servo Motor’s desired position, speed, torque, etc. This is an ongoing process of reading and responding to the feedback which created a closed loop system and is the defining feature of a Servo System, allowing it to improve the transient response times, reduce any steady-state errors, and reduce the sensitivity of the system to load parameters.
Servo Drives can account for expected errors through a feedback monitoring device that leverages negative feedback to send a signal back through its own control loop and/or to the main Controller. In motion control, the Feedback Device evaluates the relation of the control input to the actual position of the mechanism or control shaft. By understanding the relationship between the actual value and “wanted value” of the shaft’s position, the Feedback Device sends a signal to the Servo Drive for corrective action in the Servo Motor.
Servo Drives are Digital or Analog, with Digital Servo Drives including a microprocessor (or computer) to analyse incoming signals while controlling the system. The microprocessor receives pulse streams from an Encoder which determines parameter values (e.g. velocity). Varying the pulse (or blip) allows the system to adjust accordingly (e.g. change the speed). The repetitive tasks performed by the processor allows a Digital Servo Drive to self-adjust effortlessly and quickly (convenient in cases where systems need to adapt to many conditions). A Digital Servo Drive is thus like an Analog Servo Drive, except that a microprocessor uses algorithms to predict system conditions. Analog Servo Drive drives provide control through various electrical inputs (usually ±10 Volts) and are often adjusted with potentiometers. Analog Servo Drive generally consume less energy than Digital Servo Drives and can offer high performance and consistency in certain situations.
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