Servo motor (servomotor) refers to the servo control of mechanical components in the system operation of the engine, is a kind of subsidy motor indirect variable speed device. Servo motor can make the control speed, position accuracy is very accurate, can convert voltage signal into torque and rotational speed to drive control object. Servo motor rotor speed controlled by the input signal, and can be rapid response, in the automatic control system, is used as actuators, and has a small electromechanical time constant, high linearity, initiating character such as voltage, can convert the received electrical signal to the motor shaft angular displacement or angular velocity on output. Divided into two major categories of dc and ac servo motor, its main characteristic is, when the signal voltage is zero no rotation, speed decreases with the increase of torque and uniform.

Function of CNC machine tool servo system is to accept instructions from the nc device signal, drive the machine moving parts to follow instruction pulse movement, and make sure the action is rapid and accurate, which requires the high quality of speed and position servo. The above refers to the main feed servo control, and the servo control of the main movement, but not as good as the former high control requirements. The precision and speed of the nc machine tools and other technical indicators tend to mainly depends on the servo system.

One, the basic requirements and characteristics of servo system

1. The basic requirements of servo system

(1) have good stability, stability refers to the system under the action of a given input or interference, can arrive only after a short adjustment process, new or revert to the original state of equilibrium.

(2) high precision: the accuracy of the servo system refers to the amount of output can follow the input is accurate. As precision machining CNC machine tool, the requirements of the position precision and contour machining precision is higher, usually allow deviation is generally between 0.01 ~ 0.00 LMM.

(3) the characteristic of fast response, quick responsiveness is one of the symbols of servo system dynamic quality, which requires command signal tracking faster response, on the one hand require short transient time, generally within 200 ms, even less than tens of milliseconds. On the other hand, to meet the requirements of overshoot, the forefront of the steep transition process, the rate of climb.

2, the main characteristic of servo system

(1) accurate detection device: closed-loop control of speed and position.

(2) has a variety of feedback comparison principle and methods: according to the principle of detection device to realize the information feedback, feedback servo system comparison method is not the same. At present commonly used pulse comparison, comparison of phase and amplitude comparing 3 kinds.

(3) the high performance of the servo motor (hereinafter referred to as servo motor) : used for efficient and complexity of surface processing of CNC machine tool, servo system will often in frequent starting and braking process. For the output torque of the motor and the ratio of the moment of inertia is big, to generate large enough acceleration or braking torque. For servo motor at low speed are sufficiently large output torque and smooth operation, so that as far as possible in the connected to the mechanical part of the decrease of the intermediate links.

4) wide speed range of speed regulation system, namely the speed servo system: from the system control structure, numerical control machine tool the location of the closed-loop system is considered to be a position adjustment for the outer ring, speed adjustment for the inner ring double closed-loop automatic control system, the actual working process of the internal position control is put into the corresponding speed for a given signal, again through the control of motor speed system driven by servo motor, to realize the actual displacement. The main movement requires high performance CNC machine tool is more high, so the servo system for high performance wide speed regulation system.

Second, the classification of the servo system

Servo system according to its driving element division, step by step) (hereinafter referred to as dc motor servo system, dc motor, ac motor servo system (hereinafter referred to as ac motor servo system. Divided according to the control mode, the open loop servo system, a closed-loop servo system and half closed loop servo system and so on, in fact CNC system is divided into open loop and closed loop and half closed loop three types, is associated with servo system these three ways.

1, the open-loop system

Open loop system, which is mainly composed of drive circuit, actuators and machine 3 major components. Commonly used actuators is stepping motor, often said by step motor open loop components as the implementation of the system for the stepping servo system, in this system, if it is a high power drive, with stepping motor as executive component. Drive circuit is the main task of the command pulse can be converted to drive components of the signal.

2, the closed loop system

Closed loop system is mainly composed of actuators, detection unit, compare the link, driving circuit and machine tool of five parts. The structure diagram is shown in figure 2. In a closed loop system, detecting element to actual location of the machine moving parts detected translates into electrical signal feedback to compare. Common detecting element are rotating transformer, inductosyn, grating, the magnetic grid and coded disk, etc. Usually installed on the screw of the servo system is called a half closed loop system composed of detecting element; The installation on the workbench servo system of detecting element is called a closed-loop system. Due to the presence of a transmission error between screw and the workbench, the precision of half closed loop servo system to lower than the accuracy of the closed loop servo system.

Is the role of comparative link command signal and feedback signal of comparison, the difference as follow error of servo system, the drive circuit, control actuators driven worktable moving, until the following error to zero. According to entering the comparison signal and feedback test way, in the form of closed loop (closed loop) system can be divided into pulse compare servo system, the phase comparison servo system and three kinds of amplitude comparison of servo system.

Because more links of the output signal is weak, not enough to drive actuators, provide for the large, driver circuit is set up for it.

The function of actuators is according to the control signal, that is, from the comparison of following error signal, the displacement of the said electrical signal can be converted to mechanical displacement. Commonly used actuators have wide speed dc motor, ac motor, etc. Actuators is an essential part of in the servo system, drive circuit is varies with the actuators.

Third, the development direction of servo system

As productivity development, requires the servo system to develop in the direction of high precision, high speed, high power.

(1) make full use of rapid development

Electronic and computer technology, USES the digital servo system, using microcomputer to realize adjusting control, strengthen the function of software control, rule out the nonlinear error and adjusting error of analog circuit and the influence of factors such as temperature drift, it can greatly improve the performance of servo system, and to realize the optimal control and adaptive control.

(2) the development of high precision, fast detecting element.

(3) the development of high-performance servo motor (actuators). The ac servo motor speed ratio is 1:10000, use is increasing. Brushless motor without brush and commutator segment parts, accelerating performance is higher than the dc servo motor twice, maintenance more convenient, also often used in high speed nc machine tools.