Servo drive (top)

I. Overview

Feed servo system consists of: servo drive circuit, servo drive device, mechanical transmission mechanism and execution components.

Servo Drive Circuit Servo Drive Mechanical Transmission

The role is: to receive the feedrate and displacement command signal issued by the CNC system, converted and amplified by the servo drive circuit, after the servo drive device (DC, AC servo motor, power stepper motor, electro-hydraulic pulse motor, etc.) and machinery The transmission mechanism, which drives the worktables and spindle heads of the machine tool, implements work feeds and rapid movements. The feed servo system of CNC machine tools is essentially different from the feed system of general machine tools. It can precisely control the movement speed and position of the executing parts according to the command signal, and the movement trajectories of several execution parts moving according to certain rules. .

Various motors:

The CNC feed servo system is classified according to presence or absence of position detection and feedback, and there are the following two types.

1. Open loop servo system

Figure 4-30 is a schematic diagram of an open-loop servo system. It consists of a stepper motor and its drive lines. Its function is to input a command pulse every time, the stepping motor rotates a certain angle, the rotation speed of the stepping motor depends on the frequency of the command pulse. The size of the corner is determined by the number of command pulses. The open-loop system is poor in accuracy, but due to its simple structure and easy adjustment, it is still widely used in applications where accuracy is not so high.

2 . Closed loop servo system

Composition: It consists of servo motor, detection device, comparison circuit, servo amplifier system and other components. It controls the speed and position based on the comparison of the feedback signal from the detection device with the command signal.
For some CNC machine tools, the detection feedback signal is obtained from the servo motor shaft or ball screw. For high-precision or large-scale machine tools, feedback signals are directly obtained from a detection device mounted on a moving part such as a table. To distinguish the two, the former is called a semi-closed-loop system. The closed loop system directly measures the displacement of the moving parts such as the table so as to realize highly accurate feedback control. However, the cost of such measuring devices is high, and installation and adjustment are complicated and difficult to maintain. In contrast, the measurement of the rotation angle in the semi-closed loop system is relatively easy to implement, but due to the influence of the transmission chain error of the subsequent transmission chain, the measurement compensation accuracy is worse than that of the closed loop system. The semi-closed loop system has been widely applied to CNC machine tools because of its simple system and easy adjustment.
How to select the servo drive system must be determined according to the requirements of the machine tool in practice. Various CNC machine tools have different processing tasks, so their requirements for feed drive are not the same, but they can be summarized as follows:

(1) Accuracy The servo system must ensure the positioning accuracy and machining accuracy of the machine tool. For low-grade CNC systems, the drive control accuracy is generally 0.01mm; for high-performance CNC systems, the drive control accuracy is 1 μm, or even 0.1 μm.

(2) Response speed In order to ensure the contour cutting shape accuracy and low machining surface roughness, in addition to requiring a higher positioning accuracy, there must be a good fast response characteristics, that is, the response to the tracking command signal needs to be faster.

(3) Speed ​​range Refers to the maximum speed that the production machinery requires the motor to provide , and minimum speed Ratio. usually


In a variety of CNC machine tools, due to the different processing tools, materials being processed and parts processing requirements, in order to ensure the best cutting conditions in any case, the feed drive system must have a wide enough range of speed .

(4) Low speed, high torque According to the processing characteristics of the machine tool, heavy cutting is often performed at low speed, ie, the feed system must have large torque output at low speed.

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