Principle and application of servo motion control system and CNC machining

Mechatronics technology is developing rapidly with the continuous development of science and technology, and the production process constantly puts forward new requirements. The control method is mainly from manual to automatic; the control function is from simple to complex; and the operation is from heavy to light. With the emergence of new control theory and new types of electrical appliances and electronic devices, it has opened up new ways for the development of electrical control technology.

The traditional machine tool electrical control is a relay contact control system, which is composed of relays, contactors, buttons, travel switches, etc., to realize the control of machine tool start, stop, and extreme speed regulation. The advantages of the relay contact control system are simple structure, convenient maintenance, strong anti-interference, and low price, so it is widely used in various machine tools and mechanical equipment. At present, relay contact control in China is still one of the most basic forms of electrical control for machine tools and other mechanical equipment.

In actual production, due to the existence of a large number of simple program control processes controlled by the switch, and the actual production process and process are often changing, the traditional relay contact control system often cannot meet this requirement, so it has appeared A control device that combines relay contact control and electronic technology is called a sequence controller. It can change the control program according to the production needs, but it is far simpler than the electronic computer structure, and the price is low. It realizes the contact control of the relay by combining or programming the logic elements. However, its device is bulky and its functions are also limited. With the development and application of large-scale integrated circuits and microprocessor technology, the above-mentioned control technology has also undergone fundamental changes. In the 1970s, the storage technology of computers was introduced into sequential controllers, resulting in new industrial controllers. ——Programmable Programmable Controller (PLC), which combines the advantages of computer control and relay control system, so it has been widely used in industrial control as a standardized general device in countries around the world.

In order to solve the automation problem of single piece and small batch production which accounts for about 80% of the total machining volume of the machine, CNC machine tools appeared in the 1950s. It comprehensively applies the latest technological achievements in various technical fields such as electronics, computers, inspection, automatic control and machine tool structural design. It is a typical mechatronics product. After 40 years of development of CNC machine tools, the variety is increasing and the performance is constantly improving. Among them, CNC machine tools controlled by contours and CNC machining centers with automatic tool changers and automatic indexing of worktables have developed more rapidly. The CNC machine tool is composed of control medium, CNC device, servo system and machine tool body. The performance of the servo system is one of the main factors that determine the machining accuracy and productivity of the CNC machine tool.

2. The role and composition of the servo system in CNC machining

In the automatic control system, the system whose output value can follow the change of the input value with a certain accuracy is called the servo system, also known as the servo system. The servo system of the CNC machine tool refers to an automatic control system that uses the position and speed of the moving parts of the machine tool as a control amount, and is also called a servo system.

The servo system is composed of a servo drive device and a drive element (or actuator servo motor). The high-performance servo system also has a detection device to feed back the actual output status.

The function of the CNC machine tool servo system is to receive the command signal from the CNC device, drive the moving parts of the machine tool to follow the command pulse movement, and ensure the fast and accurate action, which requires high-quality speed and position servo. The above refers to the feed servo control, in addition to the servo control of the main motion, but the control requirements are not as high as the former. Technical indicators such as precision and speed of CNC machine tools often depend on the servo system.

3. The basic requirements and characteristics of the servo system

1. Basic requirements for the servo system

(1) Good stability: Stability means that the system can reach a new one or return to the original equilibrium state after a short adjustment process under the action of a given input or external interference.

(2) High accuracy: The accuracy of the servo system refers to the accuracy with which the output can follow the input. As a CNC machine tool for precision machining, the required positioning accuracy or contour machining accuracy is usually relatively high, and the allowable deviations are generally between 0.01 and 0.00 lmm.

(3) Good fast response: fast response is one of the signs of the dynamic quality of the servo system, that is, the response of the tracking command signal is required to be fast, on the one hand, the transition process time is 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 is required to be steep, that is, the rate of rise is large.

2. The main features of the servo system

(1) Accurate detection device: closed loop control with composition speed and position.

(2) There are a variety of feedback comparison principles and methods: According to the different principles of information feedback realized by the detection device, the servo system feedback comparison method is also different. At present, there are three kinds of pulse comparison, phase comparison and amplitude comparison.

(3) High-performance servo motor (referred to as servo motor): CNC machine tool used for efficient and complex surface processing, the servo system will often be in the process of frequent starting and braking. The ratio of the output torque of the motor to the moment of inertia is required to be large to generate a sufficiently large acceleration or braking torque. It is required that the servo motor has a large enough output torque at low speed and runs smoothly, so as to minimize the intermediate links in the connection with the mechanical moving part.

(4) Speed ​​adjustment system with wide speed adjustment range, namely speed servo system: from the control structure of the system, the position closed-loop system of CNC machine tool can be regarded as double closed-loop automatic control with position adjustment as outer ring and speed adjustment as inner ring The actual working process of the system is to convert the position control input into the corresponding speed reference signal, and then drive the servo motor through the speed control system to achieve the actual displacement. The main motion of the CNC machine tool requires relatively high speed control performance, so the servo system is required to be a high-performance wide speed control system.

Four. The classification of the servo system

The servo system is divided according to its driving elements, including stepping servo system, DC motor (DC motor) servo system, and AC motor (AC motor) servo system. According to the control method, there are open-loop servo system, closed-loop servo system and semi-closed-loop servo system. In fact, the numerical control system is also divided into three types: open-loop, closed-loop and semi-closed-loop, which are related to these three modes of servo system.

Open loop system

Figure 1 is a diagram of the open-loop system, which is mainly composed of the drive circuit, the actuator and the machine tool 3. The commonly used actuator is a stepper motor. The open-loop system with a stepper motor as the actuator is usually called a stepped servo system. In this system, if it is driven by high power, the stepper motor is used as the actuator. The main task of the drive circuit is to convert the command pulse into the signal required to drive the actuator.

2. Closed loop system

The closed-loop system is mainly composed of five parts: actuator, detection unit, comparison link, drive circuit and machine tool. Its block diagram is shown in Figure 2. In the closed-loop system, the detection element detects the actual position of the moving parts of the machine tool and converts it into an electrical signal to feed back to the comparison link. Common detection components include resolvers, inductive synchronizers, gratings, magnetic grids, and encoding disks. Generally, the servo system composed of the detection elements installed on the screw is called a semi-closed loop system; the servo system composed of the detection elements installed on the worktable is called a closed loop system. Due to the transmission error between the lead screw and the table, the accuracy of the semi-closed-loop servo system is lower than that of the closed-loop servo system.

The function of the comparison link 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. After the drive circuit, the control actuator drives the workbench to continue moving until the following error is zero. The closed-loop (semi-closed loop) system can be divided into three types: pulse comparison servo system, phase comparison servo system and amplitude comparison servo system according to the form of the signal entering the comparison link and the feedback detection method.

Because the signal output from the comparison link is relatively weak and insufficient to drive the actuator, it needs to be amplified. The drive circuit is designed for this purpose.

The function of the actuator is to convert the electrical signal representing the displacement into mechanical displacement according to the control signal, that is, the following error signal from the comparison link. Commonly used actuators are DC wide speed motors and AC motors. The actuator is an indispensable part of the servo system, and the drive circuit varies with the actuator.

Recently, our school has developed a high-performance AC servo (CNC machine tool) control system. The system has stable performance and reliable quality, and can be widely used in industrial production applications such as digital engraving, packaging machinery, and mold production. Experimental research on other courses.

V. Development direction of servo system

With the continuous development of productivity, the servo system is required to develop in the direction of high precision, high speed and high power.

(1) Make full use of the rapidly developing electronic and computer technology, use digital servo system, use microcomputer to realize adjustment control, enhance software control function, exclude the influence of non-linear error and adjustment error of analog circuit and temperature drift, etc. Greatly improve the performance of the servo system, and create conditions for achieving optimal control and adaptive control.

(2) Develop high-precision and fast detection components.

(3) Develop high-performance servo motors (actuators). At present, the speed ratio of the AC servo motor has reached 1: 10000, and the use is increasing. Because of brushless motors and commutator parts, the acceleration performance of brushless motors is two times higher than that of DC servo motors, and maintenance is also more convenient. It is often used in high-speed CNC machine tools.

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