The characteristics, classification and development direction of the servo system
Servomotor (servomotor) refers to the engine that controls the operation of mechanical components in the servo system, and is an auxiliary motor indirect transmission device. The servo motor can control the speed and the position accuracy is very accurate, and can convert the voltage signal into torque and speed to drive the control object. The rotor speed of the servo motor is controlled by the input signal and can react quickly. It is used as an actuator in an automatic control system and has the characteristics of small electromechanical time constant, high linearity, starting voltage, etc. Converted to angular displacement or angular velocity output on the motor shaft. It is divided into two categories of DC and AC servo motors. Its main feature is that there is no rotation phenomenon when the signal voltage is zero, and the speed decreases with the increase of torque. 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. 1. 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 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 0.00 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 principle 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. Second, the classification of the servo system The servo system is divided according to its driving components, 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. 1. Open loop system Open loop system, it is mainly composed of drive circuit, actuator and machine tool 3 parts. 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, mold production, etc., and is more suitable for production internships and courses design for students (graduate students) in mechanical and electrical integration of higher schools, electronics and electrical appliances, electrical automation Experimental research on other courses. 3. The development direction of the 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, adopt 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 rapid 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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