A servo system, also known as a follow-up system, is a feedback control system used to accurately follow or reproduce a process. A servo system is an automatic control system that enables the output controlled variable, such as the position, orientation, and state of an object, to follow any change in the input target (or given value). Its main task is to amplify, transform, and regulate power according to control commands, making the torque, speed, and position control of the drive device highly flexible and convenient. In many cases, a servo system specifically refers to a feedback control system where the controlled variable (the system's output) is mechanical displacement or displacement velocity and acceleration. Its function is to ensure that the output mechanical displacement (or rotation angle) accurately tracks the input displacement (or rotation angle). Its structural composition is not fundamentally different from other forms of feedback control systems. Servo systems were initially used in defense and military industries, such as artillery control, ship and aircraft autopilot, and missile launches. Later, they were gradually extended to many sectors of the national economy, such as automatic machine tools and wireless tracking control.
The word "servo" originates from the Greek word for "slave," meaning "to serve" and "to obey." A servo system is a system that can perform desired movements according to external commands, achieving automatic control of output quantities including position, orientation, and status. It is not only a key component of industrial automation but also an essential means to achieve precise positioning and motion. As the execution unit of a servo system, there are many types of servo motors, among which permanent magnet synchronous servo motors have gradually become the mainstream in the market due to their high efficiency, energy saving, and ease of operation. Compared to stepper motors, servo motors have significant advantages in control precision, stable output, and overload capacity, and are widely used in industrial fields. This article will briefly introduce the working principles and classifications of servo systems and servo motors, and explain the commonly used performance indicators of servo motors.
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Servo System Composition and Working Principle
A servo system mainly consists of components such as a servo driver, an encoder, and a servo motor. Its motion principle diagram is shown in the figure below:
A servo system is an automatic control system that enables the controlled output variables, such as the position, orientation, and state of an object, to change in accordance with changes in the input target (or given value). After receiving a control command, the servo driver sends a signal to the servo motor to drive its rotation; simultaneously, an encoder embedded in the motor feeds back the servo motor's motion parameters to the servo driver, which then summarizes, analyzes, and corrects the signals. Thus, the servo system precisely controls the output variables of the actuator (mechanical transmission devices such as motors) in a closed-loop manner.
The functions of servo drivers, servo motors, and encoders are described below:
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servo driver
1. Composition
A servo drive mainly consists of a servo control unit, a power drive unit, and a communication interface unit. The servo control unit includes a position controller, a speed controller, a torque controller, and a current controller.
Servo drives can be broadly divided into two modules: a power board and a control board. The power board is the high-voltage section (high power, high current, low frequency electrical component), comprising two units: an IPM (Integrated Power Module) for driving the motor and a switching power supply unit to provide digital and analog power to the entire system. The control board is the low-voltage section, the core of motor control, and the platform for the core control algorithms of the servo drive technology. The control board outputs pulse width modulation (PWM) or pulse frequency modulation (PFM) signals through corresponding algorithms as drive signals for the drive circuit, thereby changing the inverter's output power to control the AC servo motor. The inverter is a converter that transforms DC power (from batteries or accumulators) into fixed-frequency, fixed-voltage or frequency- and voltage-modulated AC power (typically 220V, 50Hz sine wave).
In servo drive components, IGBTs and DSP chips account for over 50% of the total material cost. IGBTs, or Insulated Gate Bipolar Transistors, are composite, fully controllable, voltage-driven power semiconductor devices composed of bipolar transistors and insulated gate field-effect transistors. They combine the advantages of high input impedance of MOSFETs and low on-state voltage drop of power transistors, and are used in power drive units to assist in the conversion of DC to AC. Foreign companies occupy over 90% of my country's IGBT market, making domestic production of IGBTs in the short term quite difficult.
2. Working principle
The control unit is the core of the entire AC servo system, responsible for position control, speed control, torque and current control. The digital signal processor (DSP) used in the control board, in addition to its fast data processing capabilities, integrates a wealth of dedicated integrated circuits for motor control, such as AC/DC converters, PWM generators, timer/counter circuits, asynchronous communication circuits, CAN (Control Area Network) bus transceivers, high-speed programmable static RAM (Random Access Memory), and large-capacity program memory. This allows for the implementation of complex control algorithms, achieving digitalization, networking, and intelligence.
Power devices commonly employ drive circuits designed around intelligent power modules (IPMs). The IPM integrates the drive circuitry and includes fault detection and protection circuits for overvoltage, overcurrent, overheating, and undervoltage. The power drive unit first rectifies the input three-phase power or mains power through a three-phase full-bridge rectifier circuit to obtain the corresponding DC power. The rectified three-phase power is then converted to AC power by an inverter to drive the servo motor. The entire process of the power drive unit can be simply described as an AC-DC-AC cycle.
A servo loop provides proportional control of the motor based on input command signals. A simple servo drive contains a single servo loop for torque control. More advanced servo drives can add a speed loop and may also include a position loop. In a complete servo drive system, digital signals from the motion controller command the desired motion trajectory using these three servo loops to optimize performance. Each loop sends signals to subsequent loops and monitors appropriate feedback elements for real-time corrections to match the command parameters.
Components of a servo system
A servo system mainly consists of a servo controller, a drive circuit, a servo motor, and corresponding feedback detection devices.
(1) Servo motor
A servo motor is a motor used in servo systems to control the operation of mechanical components. Servo motors have internal encoders that provide real-time motion data feedback to the servo driver. Servo motors enable highly accurate speed and position control, converting voltage signals into torque and speed to drive the controlled object. The rotor speed of a servo motor is controlled by the input signal and can respond quickly. In automatic control systems, it is used as an actuator, converting received electrical signals into angular displacement or angular velocity output on the motor shaft.
(2) Servo driver
A servo driver is a controller used to control a servo motor. Its function is similar to that of a frequency converter for a regular AC motor. It is part of a servo system. Its main function is to amplify, transform, and regulate the power according to the control commands and then transmit it to the servo motor.
(3) Sensing device
The most commonly used sensing device is the encoder. Servo motors typically have built-in encoders to feed back the collected actual motion data to the driver, thereby achieving closed-loop motion control.
(4) Servo driver control of servo motor
Servo drives typically control servo motors through three methods: position, speed, and torque, to achieve high-precision positioning of the transmission system.
① Position control: Position control mode generally determines the rotation speed by the frequency of externally input pulses and determines the rotation angle by the number of pulses. Some servos can also directly assign values to speed and displacement. Position mode can have very strict control over both speed and position.
②Speed Mode: Speed mode controls rotation speed through analog input or pulse frequency input. When there is an outer loop PID control with a host control device, speed mode can also be used for positioning, but the position signal of the motor or the position signal of the load must be fed back to the host computer for calculation.
③ Torque control: Torque control is a method that sets the output torque of the motor shaft by inputting an external analog signal or assigning a direct address. The torque can be changed by changing the setting of the analog signal in real time.
3. Servo System Principles
When a control signal is given manually and received by the servo control system, the actuator will perform a series of movements and actions according to the instructions of the control signal; if no signal is received, the controlled transmission device will stop moving until the control signal arrives.
4. Classification of Servo Systems
(1) Based on their different objects of action, they can be divided into two main categories: position servo systems and speed servo systems.
1) Position Servo System
A position servo system is a servo system capable of accurately tracking and locating a target command position. Based on the presence or absence of feedback, position servo systems are divided into two types: open-loop control and closed-loop control.
Open-loop position servo systems have the advantages of simple structure and low cost, but they do not have position and speed feedback functions. Their position control accuracy depends on the step angle of the stepper motor and the accuracy of the transmission mechanism.
Closed-loop control is divided into full closed-loop control and semi-closed-loop control. In full closed-loop control, the sensing element directly detects the displacement of the controlled object on the worktable and feeds this displacement back to the controller, thus forming a full closed-loop control. Because the controller can control based on the actual displacement of the controlled object, full closed-loop control has high positioning accuracy and can eliminate errors throughout the entire process from the motor to the mechanical transmission mechanism and then to the controlled object. However, closed-loop control structures are relatively complex, costly, and difficult to implement.
2) Speed servo system
The load torque of the driven machinery is usually constantly changing, as are the voltage and frequency of the power supply. Consequently, the running speed of the driven object is also typically variable. Therefore, the primary task of a speed servo system is to maintain the driven machinery (or load) at a stable operating speed (not just one specific speed) that is precisely required.
(2) According to the different motors they use, they can be divided into DC servo systems and AC servo systems.
1) DC servo system
A DC servo system refers to a servo system in which the servo motor uses a DC motor.
2) AC servo system
An AC servo system mainly consists of an AC servo driver (or controller) and an AC servo motor. The system is centered around the driver, which controls the operation of the AC servo motor. Closed-loop control of torque, speed, or position ensures excellent dynamic and static performance. Industrial robots have four main components: the body, the servo motor, the reducer, and the controller. The typical structure of an industrial robot's electric servo system involves three closed-loop controls: a current loop, a speed loop, and a position loop. Generally, for AC servo drivers, multiple functions such as position control, speed control, and torque control can be achieved by manually setting their internal parameters.
A servo system, also known as a follow-up system, is a feedback control system used to accurately follow or reproduce a process. A servo system is an automatic control system that enables the output controlled variables, such as the position, orientation, and state of an object, to follow any changes in the input target (or given value).
Servo systems are products developed based on frequency conversion technology. They are automatic control systems that use mechanical position or angle as the controlled object. In addition to speed and torque control, servo systems can also perform precise, fast, and stable position control.
In a broad sense, a servo system is a control system that accurately tracks or reproduces a given process; it can also be called a follow-up system.
A narrow-sense servo system, also known as a position follow-up system, controls the linear or angular displacement of the load's mechanical spatial position as its controlled variable (output). When the position setpoint (input) changes arbitrarily, the system's main task is to make the output quickly and accurately reproduce the change of the setpoint.
The structure of a servo system
Mechatronics servo control systems come in a variety of structures and types, but from the perspective of automatic control theory, a servo control system generally includes five parts: controller, controlled object, execution link, detection link, and comparison link.
Servo System Composition Principle Block Diagram
1. Comparison Section
The comparison stage is a stage that compares the input command signal with the system feedback signal to obtain the deviation signal between the output and the input. It is usually implemented by a dedicated circuit or computer.
2. Controller
The controller is usually a computer or a PID (proportional, integral, and derivative) control circuit. Its main task is to transform and process the deviation signal output by the comparator to control the actuator to act as required.
3. Execution phase
The function of the actuator is to convert various forms of energy into mechanical energy according to the requirements of the control signal, thereby driving the controlled object to work. In mechatronics systems, actuators generally refer to various motors or hydraulic/pneumatic servo mechanisms, etc.
4. Controlled object
The controlled object refers to the object being controlled, such as a robotic arm or a mechanical work platform.
5. Testing process
The detection stage refers to the device that can measure the output and convert it into the dimensions required by the comparison stage. It generally includes sensors and conversion circuits.
Features and functions of servo systems
Servo systems differ fundamentally from the feed systems of general machine tools. They can precisely control the speed and position of the actuators based on command signals. The servo system is the link between the CNC device and the machine tool, and is an important component of the CNC system, possessing the following characteristics:
It must have a high-precision sensor that can accurately provide the electrical signal of the output quantity.
Both the power amplifier and the control system must be reversible.
It has a sufficiently large speed range and sufficiently strong low-speed load-carrying capacity.
It has a fast response capability and strong anti-interference capability.
