What is a servo system?
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 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 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.
Main functions of servo systems
1. Using low-power command signals to control high-power loads;
2. In the absence of a mechanical connection, the input shaft controls the output shaft located at a remote distance, achieving remote synchronous transmission;
3. To enable the output mechanical displacement to accurately track electrical signals, such as in recording and indicating instruments.
Classification of servo systems
From the perspective of the nature of the system components, there are electric servo systems, hydraulic servo systems, electro-hydraulic servo systems, and electro-electric servo systems, etc.
From the perspective of the physical properties of the system output, there are speed or acceleration servo systems and position servo systems, etc.
Based on the characteristics of the components and the signal interaction features within the system, there are analog servo systems and digital servo systems;
From the perspective of system structure, there are single-loop servo systems, multi-loop servo systems, open-loop servo systems, and closed-loop servo systems.
Servo systems can be classified according to their driving components into stepper servo systems, DC motor servo systems, and AC motor servo systems.
Basic requirements of servo systems
(1) Good stability: Stability means that the system can reach a new or return to the original equilibrium state after a short adjustment process under a given input or external disturbance.
(2) High precision: The precision of a servo system refers to the degree to which the output can accurately follow the input. As a precision machining CNC machine tool, the required positioning accuracy or contour machining accuracy is usually relatively high, and the allowable deviation is generally between 0.01 and 0.001 mm.
(3) Good fast response: Fast response is one of the indicators of the dynamic quality of a servo system, that is, the response of the tracking command signal must be fast. On the one hand, the transition process time is required to be short, generally within 200ms, or even less than tens of milliseconds; on the other hand, in order to meet the overshoot requirements, the leading edge of the transition process must be steep, that is, the rise rate must be large.
Main features of servo systems
1. Precise detection device: forming a closed-loop control system 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 frequently undergo 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 their 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.
Basic components of a servo system
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 magnitude of the motor torque, and on the other hand, converts the constant voltage and constant 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.
Application of servo systems in automated production
In CNC machine tools, the servo control system is an indispensable component. Its task is to convert CNC information into machine tool feed motion, thereby achieving precise control. Servo systems are widely used in various automated mechanical equipment, and their application is becoming increasingly important, especially given the growing trend of automated production. Currently, my country is promoting the automation of its industrial manufacturing sector, which requires a large number of industrial robots and machine tools. In these devices, servo systems play a crucial role in overall control.
The role of the servo system in a CNC machine tool is to receive command signals from the CNC device, drive the moving parts of the machine tool to follow the command pulses, and ensure the speed and accuracy of the movements. This requires high-quality speed and position servo control. The above mainly refers to feed servo control; there is also servo control for the main motion, but the control requirements are not as high as the former. The accuracy and speed of a CNC machine tool often depend primarily on the servo system.
Because CNC machine tools have high requirements for product processing, the servo control system used is crucial. Currently, the advantages of servo control systems used in CNC machine tools mainly include: high precision (the accuracy of a servo system refers to the degree to which the output can accurately reproduce the input, including positioning accuracy and contour machining accuracy); and good stability (stability means that under given input or external disturbances, the system can reach a new or return to its original equilibrium state after a short adjustment process).
