Working principle and control method of servo drive

Servo control system

A servo control system is an operating system that can automatically control the mechanical movement of a test device according to predetermined requirements. In many cases, a servo system specifically refers to a feedback control system in which the controlled quantity (the output of the system) is a mechanical displacement or displacement velocity or acceleration. Its function is to make the output mechanical displacement (or rotation angle) accurately track the input displacement (or rotation angle). The structural composition of a servo system is not fundamentally different from other forms of feedback control systems.

Working principle of servo drive

First, the power drive unit rectifies the input three-phase power or mains power through a three-phase full-bridge rectifier circuit to obtain the corresponding direct current. After the rectified three-phase power or mains power, the three-phase sinusoidal PWM voltage inverter is used to drive the AC servo motor. The entire process of the power drive unit can be simply described as an AC-DC-AC process. The main topology circuit of the rectifier unit (AC-DC) is a three-phase full-bridge uncontrolled rectifier circuit.

Servo drive control mode

Generally, servos have three control modes: position control, torque control, and speed control.

1. Position control: The position control mode generally determines the rotation speed by the frequency of the external input pulse, and determines the rotation angle by the number of pulses. Some servos can also directly assign speed and displacement through communication. Since the position mode can have very strict control over speed and position, it is generally used in positioning devices.

When there is an outer loop PID control of the upper control device, the speed mode can also be used for positioning, but the position signal of the motor or the position signal of the direct load must be fed back to the upper control device for calculation. The position mode also supports direct load

The outer ring detects the position signal, and the encoder at the motor shaft end only detects the motor speed. Since the position mode has very strict control over both speed and position, it is mainly used in positioning devices, such as CNC machine tools, printing machinery, etc.

2. Torque control: The torque control method is to set the output torque of the motor shaft through external analog input or direct address assignment. The set torque size can be changed by instantly changing the analog setting, or by changing the corresponding address value through communication.

It is mainly used in winding and unwinding devices that have strict requirements on the material's grip, such as winding devices or optical fiber pulling equipment. The torque setting must be changed at any time according to the change of the winding radius to ensure that the material's force does not change with the change of the winding radius.

转矩控制方式实际上就是通过外部模拟量的输入或直接的地址賦值来设定电动机轴 输出转矩。例如10V对应5N • m的话，当外部模拟量设定为5V时，电动机轴输出为 2.5N • m.如果电动机轴负载低于2.5N.m时电动机正转，外部负载等于2.5N • m时电 动机不转，大于2.5N*m时电动机反转（通常在有重力负载情况下产生）。可以通过即 时改变模拟量的设定来改变设定力矩大小，也可通过通信方式改变对应的地址的数值来 实现。转矩控制主要应用在对材质的受力有严格要求的缠绕和放卷的装置中，例如绕线 装置或拉光纤设备。

3. Speed ​​mode: The rotation speed can be controlled by analog input or pulse frequency. The speed mode can also be used for positioning when there is an outer loop PID control of the upper control device, but the motor position signal or the direct load position signal must be fed back to the upper control device for calculation. The position mode also supports direct load outer loop detection of position signals. At this time, the encoder at the motor shaft end only detects the motor speed, and the position signal is provided by the detection device at the direct final load end. This has the advantage of reducing the error in the intermediate transmission process and increasing the positioning accuracy of the entire system.

Figure 4-27 shows the schematic diagram of the PWM speed regulation system. The main circuit consists of an uncontrolled rectifier UR, a smoothing capacitor C and an inverter UI. The inverter input is a fixed DC voltage. The voltage and frequency regulation are achieved by adjusting the pulse width and frequency of the inverter output voltage. At the same time, the output of the three-phase current waveform distortion is reduced. The main circuit characteristics of this form are as follows.

① Since the main circuit has only one power control stage UI, the structure is simple.

② Due to the use of an uncontrolled rectifier bridge, the power factor of the power grid has nothing to do with the output size of the inverter. ③ The inverter achieves voltage regulation during frequency modulation, which is independent of the component parameters of the intermediate DC link, thereby accelerating the dynamic response of the system. The actual variable frequency speed regulation system generally requires perfect protection to ensure the safe operation of the system.

If there is no requirement for the speed and position of the motor, and only a constant torque needs to be output, the torque mode should be used.

If you have certain accuracy requirements for position and speed, but are not very concerned about real-time torque, it is not convenient to use the torque mode, and it is better to use the speed or position mode.

If the upper controller has a better closed-loop control function, speed control will be better. If the requirements are not very high, or there is basically no real-time requirement, position control is used.