A servo motor (servomotor) is an engine that controls the operation of a mechanical component in a servo system. It is a kind of auxiliary motor indirect transmission. The servo motor can make the control speed, the position precision is very accurate, the voltage signal can be converted to torque and speed to drive the control object. The servo motor rotor speed control by the input signal, and can respond quickly, in the automatic control system, used as actuators, and electromechanical time constant, high linearity, initiating voltage characteristics, to convert signals received from the motor shaft angular displacement or angular velocity output. It can be divided into two main categories: DC and AC servo motors. The 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 servo system of NC machine is to accept the instruction signal from NC device, drive the moving parts of machine tool to follow the instruction pulse and ensure the speed and accuracy of action, which requires high speed and position servo. The above refers to the main feed servo control, in addition to the main motion of the servo control, but the control requirements are not as high as the former. The precision and speed of CNC machine tools often depend on the servo system.
The basic requirements and characteristics of the servo system
1. basic requirements for servo system
(1) stability is good: stability means that the system can arrive at the new or return to the original equilibrium state after a short regulation process under given input or external interference.
(2) high precision: the precision of the servo system is the accuracy of the output measure to follow the input. As a CNC machine tool for precision machining, the required positioning precision or contour machining precision is usually high, and the allowable deviations are generally between 0.01 and 0.00lmm.
(3) fast response: fast response is one of the symbols of the servo system's dynamic quality, which requires the response tracking command signal to be fast, demanding shorter transition time, generally less than 200ms, or even less than tens of milliseconds; on the other hand, in order to meet the requirements for the overshoot, the steep transition process that is, the rate of rise of larger.
2. The main features of the servo system
(1) an accurate detection device: closed loop control with the composition of speed and position.
(2) there are a variety of feedback comparison principles and methods: the principle of information feedback is different according to the detection device, and the feedback comparison method of the servo system is also different. There are 3 kinds of pulse comparison, phase comparison and amplitude comparison commonly used at present.
(3) high-performance servo motors (referred to as servo motors): CNC machine tools for high efficiency and complex surface processing, servo systems will often be in frequent start-up and braking process. The ratio of the output torque to the moment of inertia of the motor is required to produce enough acceleration or braking torque. It is required that the servo motor have large enough output torque at low speed and run smoothly so as to minimize the intermediate link in the connection with the mechanical motion part.
4) adjust the system wide speed range, speed, the speed servo system: from the control structure of the system, the position closed-loop system of CNC machine tool can be regarded as the position adjusting loop, speed regulation automatic control system of double closed loop, the actual working process of the internal control is the position input into a given speed signal the corresponding, then drive the servo motor speed control system, realize the actual displacement. The main motion of CNC machine tool requires high speed performance, so the servo system is required as a high performance wide speed control system.
Two. Classification of servo system
The servo system is divided into stepper servo system, DC motor servo system and AC motor servo system according to its driving components. According to control mode, there are open loop servo system, closed loop servo system and semi closed loop servo system. In fact, CNC system is also divided into 3 types: open loop, closed loop and half closed loop, which are related to 3 ways of servo system.
1. Open loop system
Open loop system, it is mainly composed of 3 parts of the driving circuit, the executive component and the machine tool. The commonly used executive element is stepper motor, usually called stepper motor as executive element of the open loop system is stepping servo system, in this system, if the high-power drive, stepper motor as executive element. The main task of the drive circuit is to convert the instruction pulse into the signal needed to drive the actuator.
2. Closed loop system
The closed loop system is mainly composed of 5 parts: the executive element, the detection unit, the comparison link, the driving circuit and the machine tool. The block diagram is shown in Figure 2. In a closed loop system, the detection element detects the actual position of the moving parts of the machine tool and converts it into a signal to feed back to the comparison link. The common detection components are the rotating transformer, the induction synchronizer, the grating, the magnetic grid and the coding disk. Usually the servo system consisting of detecting elements installed on the screw rod is called a semi closed loop system, and the servo system consisting of detecting elements installed on the workbench is called a closed loop system. Because of the transmission error between the screw and the worktable, the precision of the semi closed loop servo system is lower than the precision of the closed loop servo system.
The role of comparison is to compare the instruction signal and the feedback signal. The difference between them is the following error of the servo system. After driving circuit, the executive element is controlled to drive the workbench to move until the following error is zero. According to the form of the entry signal and the feedback detectionAccording to the form of signal and the way of feedback detection, the closed loop (half closed loop) system can be divided into 3 kinds: pulse comparison servo system, phase comparison servo system and amplitude comparison servo system.
Because the signal output in the comparison link is weak, it is not enough to drive the actuator, so it needs to be magnified, and the drive circuit is set for this.
The function of the actuator is based on the control signal, that is, the following error signal from the comparison link, and transforms the electrical signal representing the displacement to the mechanical displacement. The commonly used executive components are DC wide speed motor, AC motor and so on. The actuating element is an essential part of the servo system, and the driving circuit is different with the difference of the actuating element.
Three, the direction of the development of servo system
With the continuous development of the productive forces, the servo system is required to develop in the direction of high precision, high speed and high power.
(1) make full use of rapid development
Electronic and computer technology, the digital servo system, realize the regulation and control by using computer software to enhance the control function, and adjust the effect of nonlinearity error and temperature drift and other factors out of analog circuit, which can greatly improve the performance of the servo system, and to achieve the optimal control conditions.
(2) developing high precision and rapid detection components.
(3) developing high performance servo motor (actuating element). At present, the speed ratio of AC servo motor has reached 1: 10000, and the use of AC servo motor is increasing. The speed performance of brushless motor is two times higher than that of DC servo motor because of no brush and commutator parts. It is more convenient to maintain and is often used in high speed CNC machine tools.
