CNC machine tool servo drive system

I. Overview (familiar)

The servo drive system, referred to as the servo system, is an automatic control system that uses a mechanical position or angle as a control object.

The performance of the servo system largely determines the performance of CNC machine tools.

(a) Classification of servo drive system

According to the control object and purpose of use, the feed servo system is used to control the cutting feed motion of each coordinate axis of the machine tool. It is a precise position tracking and positioning system, which includes speed control and position control. The spindle servo system is used to control the torque and power in the rotary motion and cutting process of the spindle of the machine tool, and is generally based only on speed control. The auxiliary servo system is used in various types of machining centers or multi-functional numerical control machine tools to control auxiliary systems such as magazines and magazines. In general, simple position control is used. According to the servo system adjustment theory The feed servo system of CNC machine tools can be divided into open loop, closed loop and semi-closed loop systems. The electro-hydraulic control system and the electrical control system are based on the principle of the operation of the driving parts, wherein the electrical control system is divided into a stepping motor, a DC servo motor and an AC servo motor driving system. According to the feedback comparison control mode pulse comparison, phase comparison, amplitude comparison and all-digital servo system.

(II) Requirements of CNC Machine Tools for Servo Drive System

Requirements for feed servo drive system:

High accuracy requires accurate positioning of the servo system, that is, the positioning error, especially the repeated positioning error, is small, and the following precision of the servo system is high, ie, the following error is small. General positioning accuracy of μm level, high requirements of ± 0.01 ~ ± 0.005μm. Fast response, no overshoot In the process of overshooting, in order to increase productivity and ensure the quality of the process, the acceleration (subtraction) speed is required to be large enough to shorten the transition time of the servo system. Generally, the motor speed changes from zero to the highest speed, or from the highest speed to zero, and the time is less than 200 milliseconds and even less than tens of milliseconds. The wide speed range ensures that optimum cutting conditions are achieved under all circumstances. The feed drive must have a sufficiently wide speed range (at least 1:1000, and some high-performance systems can achieve 1:100000). And it is usually stepless. Low speed and high torque have strong overload capacity. Torque fluctuations are small, and the general allowable fluctuation is about 3%. High reliability High adaptability to the environment, stable performance, long life, and long average time between failures.

Requirements for the spindle drive system:

The spindle drive system should not only have a wide speed range, but also maintain a constant power output over the widest possible speed range. In order to meet the processing requirements of different CNC machine tools, some special requirements are also put forward for the spindle drive system, such as:

In order to be able to machine threads on CNC lathes and other machine tools, the spindle drive and feed drive are required to implement synchronous control;

In order to ensure the surface roughness of the end surface processing, it is required that the spindle drive system of CNC lathes, grinders and other machine tools has a constant wire speed cutting function;

In the machining center, due to the need of automatic tool change, the spindle drive system is required to have a high-precision stop position control function;

Some CNC machine tools also require the spindle drive system to have an angular control function.

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