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). 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/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 ship autopilots, gun control, and command instruments , and have since been extended to many other fields , particularly automatic lathes, antenna position control, and missile and spacecraft guidance. The main purposes of using servo systems are: ① To control high-power loads with low-power command signals. Gun control and rudder control are typical examples. ② To achieve remote synchronous transmission by controlling a remotely located output shaft from an input shaft without mechanical connection. ③ To enable precise tracking of electrical signals by the output mechanical displacement, such as in recording and indicating instruments.
The main performance indicators for servo systems are bandwidth and accuracy. Bandwidth, defined by the system's frequency response characteristics, reflects the tracking speed of the servo system. A larger bandwidth indicates better speed. The figure shows typical frequency response characteristics of a servo system. Typically, the bandwidth (ωb) is taken as the frequency value corresponding to the logarithm of the frequency response characteristic dropping to -3 dB (starting from the origin ω=0). The bandwidth of a servo system is mainly limited by the inertia of the controlled object and the actuator. The greater the inertia, the narrower the bandwidth. Generally, the bandwidth of a servo system is less than 15 Hz, while the bandwidth of a large equipment servo system is below 1-2 Hz. The accuracy of a servo system is mainly determined by the accuracy of the measuring elements used. Therefore, high-precision measuring elements, such as precision potentiometers, synchros, and rotary transformers, must be used in servo systems. In addition, additional measures can be taken to improve system accuracy, such as connecting the measuring axis of the measuring element (e.g., the synchro) to the rotating shaft through a reducer, thus amplifying the rotation angle of the shaft and improving relative measurement accuracy. Servo systems employing this approach are called precision and coarse measurement systems or dual-channel systems. The angle measurement circuit that meshes with the shaft via a reducer is called the precision reading channel, while the angle measurement circuit that takes data directly from the shaft is called the coarse reading channel.
Servo System - Main Structure
A servo system mainly consists of three parts: a controller, a power drive unit, a feedback unit, and a motor. The controller adjusts the control quantity according to the difference between the given value of the CNC system and the actual operating value detected by the feedback unit. The power drive unit, as the main circuit of the system, on the one hand, applies electrical energy from the power grid to the motor according to the magnitude of the control quantity, adjusting the motor torque; on the other hand, it converts the constant voltage and frequency power supply from the power grid into the AC or DC power required by the motor. The motor then drives the mechanical operation according to the power supply.
Main features
1. Precise detection device: forming a closed-loop control for speed and position;
2. Multiple feedback comparison principles and methods: Depending on the principle by which the detection device implements information feedback, the feedback comparison methods of the servo system also differ. The three most commonly used are pulse comparison, phase comparison, and amplitude comparison.
3. High-performance servo motors (or servo motors for short): Used in CNC machine tools for efficient and complex surface machining, servo systems are frequently in the process of starting and braking. A high ratio of output torque to moment of inertia is required to generate sufficiently large acceleration or braking torque. Servo motors also need to have sufficiently large output torque and smooth operation at low speeds to minimize intermediate links in the connection with moving mechanical parts.
4. Wide-range speed regulation system, i.e., speed servo system: From the perspective of system control structure, the position closed-loop system of a CNC machine tool can be regarded as a dual closed-loop automatic control system with position regulation as the outer loop and speed regulation as the inner loop. Its actual working process involves converting the position control input into a corresponding speed command signal, and then driving the servo motor through the speed regulation system to achieve actual displacement. The main motion of a CNC machine tool requires high speed regulation performance; therefore, the servo system must be a high-performance, wide-range speed regulation system.
Technical Requirements
1. System accuracy
Servo system accuracy refers to the degree to which the output reproduces the input signal with the required precision . It is expressed in the form of error and can be summarized into three aspects: dynamic error , steady-state error, and static error .
2. Stability
The stability of a servo system refers to its ability to return to its original stable state after the disturbance acting on the system disappears; or its ability to reach a new stable operating state after a new input command is given to the system .
3. Response characteristics
Response characteristics refer to the speed at which the output changes in response to input commands , and determine the efficiency of the system . Response speed is related to many factors , such as the computer's operating speed , the damping of the motion system, and its mass .
4. Operating frequency
Operating frequency typically refers to the range of frequencies of input signals that the system is allowed to operate . When a signal at the operating frequency is input , the system can operate normally according to technical requirements; however, when other frequency signals are input , the system cannot operate normally.
Servo Systems - Classification
Servo systems can be categorized by their driving components into stepper servo systems, DC motor servo systems, and AC motor servo systems. They can also be categorized by their control method into open-loop servo systems, closed-loop servo systems, and semi-closed-loop servo systems. In fact, CNC systems are also divided into these three types, which are related to the different control methods of servo systems.
1. Open-loop system
An open-loop system mainly consists of three parts: a drive circuit, an actuator, and the controlled object. The commonly used actuator is a stepper motor; an open-loop system using a stepper motor as the actuator is usually called a stepper servo system. In such systems, stepper motors are used as actuators for high-power driving. The main task of the drive circuit is to convert command pulses into signals required to drive the actuator.
2. Closed-loop system
A closed-loop system mainly consists of five parts: an actuator, a detection unit, a comparison circuit, a drive circuit, and the controlled object. In a closed-loop system, the detection element detects the actual position of the moving part of the controlled object and converts it into an electrical signal, which is then fed back to the comparison circuit. Common detection elements include rotary transformers, inductive synchronizers, optical gratings, magnetic gratings, and encoders. A servo system with a detection element mounted on the motor shaft is usually called a semi-closed-loop system; a servo system with a detection element mounted on the controlled object is called a closed-loop system. Due to transmission errors between the motor shaft and the controlled object, the accuracy of a semi-closed-loop servo system is lower than that of a closed-loop servo system.
The function of the comparison circuit is to compare the command signal and the feedback signal. The difference between the two is used as the following error of the servo system. This error is then transmitted through the drive circuit to control the actuator to move the worktable until the following error is zero. Based on the form of the signal entering the comparison circuit and the feedback detection method, closed-loop (semi-closed-loop) systems can be divided into three types: pulse comparison servo systems, phase comparison servo systems, and amplitude comparison servo systems.
Since the signal output by the comparison circuit is relatively weak and insufficient to drive the actuator, it needs to be amplified, and the drive circuit is designed for this purpose.
The function of an actuator is to convert an electrical signal representing displacement into mechanical displacement based on a control signal, specifically a following error signal from the comparison circuit. Commonly used actuators include DC wide-range speed-regulating motors and AC motors.
3. DC servo drive and AC servo drive
In the 1970s and early 1980s, CNC machine tools mostly adopted DC servo drives. High-inertia DC servo motors have excellent wide speed range performance, high output torque, and strong overload capacity. Moreover, because the motor's inertia is comparable to that of the machine tool's transmission components, it is easy to adjust after forming a closed loop. Meanwhile, low-to-medium inertia DC servo motors and their high-power transistor pulse-width modulation (PWM) drive devices are more suitable for the frequent starts, stops, and rapid positioning and cutting requirements of CNC machine tools. However, a major characteristic of DC motors is the presence of brushes and mechanical commutators, which limits their development towards larger capacity, higher voltage, and higher speeds, thus restricting their application.
