Three control modes of servo motor

Introduce three control modes of servo motor

If you have no requirements for the speed and position of the motor, just output a constant torque, which of course is torque mode.
If you have certain accuracy requirements for position and speed, but you don't care much about real-time torque, it is better to use speed or position mode.

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

In terms of the response speed of the servo driver, the torque mode has the smallest amount of calculation, and the driver has the fastest response to the control signal; The position mode has the largest amount of computation, and the driver has the slowest response to the control signal.

When there are high requirements for dynamic performance in motion, the motor needs to be adjusted in real time.
If the operation speed of the controller itself is very slow (such as PLC, or low-end motion controller), the position control is used.
If the controller has a relatively fast operation speed, the position loop can be moved from the driver to the controller in the speed mode to reduce the workload of the driver and improve the efficiency;

If there is a better upper controller, you can also use torque control to remove the speed loop from the drive, which is generally only available for high-end dedicated controllers.

Generally speaking, there is an intuitive way to compare the quality of drive control, called response bandwidth.

When torque control or speed control, a square wave signal is sent to it through the pulse generator to make the motor continuously rotate forward and reverse, and continuously adjust the frequency. The oscilloscope displays a sweep signal. When the peak of the envelope reaches 70.7% of the maximum value, it means that the step has been lost. At this time, the frequency level can indicate the quality of control. Generally, the current loop can achieve more than 1000HZ, while the speed loop can only achieve tens of hertz.

1. Torque control
The torque control mode is to set the external output torque of the motor shaft through the input of the external analog quantity or the assignment of the direct address. For example, if 10V corresponds to 5Nm, when the external analog quantity is set to 5V, the motor shaft output is 2.5Nm: if the motor shaft load is lower than 2.5Nm, the motor will rotate forward, when the external load is equal to 2.5Nm, the motor will not rotate, and when the external load is greater than 2.5Nm, the motor will reverse (usually generated under the condition of gravity load). The set torque can be changed by changing the setting of analog quantity in real time, or by changing the value of corresponding address through communication.

The application is mainly in the winding and unwinding devices with strict requirements on the stress of the material, such as the winding device or the optical fiber pulling device. The torque setting should be changed at any time according to the change of the winding radius to ensure that the stress of the material will not change with the change of the winding radius.

2. Position control
The position control mode generally determines the rotation speed through the frequency of the externally input pulse, and the rotation angle through the number of pulses. Some servo systems can directly assign the speed and displacement through communication. Because the position mode can control the speed and position strictly, it is generally applied to the positioning device.

Application fields such as CNC machine tools, printing machinery, etc.

3. Speed mode
Rotation speed can be controlled by analog input or pulse frequency. The speed mode can also be positioned when there is an outer loop PID control of the upper control device, but the position signal of the motor or the position signal of the direct load must be fed back to the upper controller for calculation. The position mode also supports the detection of the position signal by the direct load outer ring. 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 advantage is that it can reduce the error in the intermediate transmission process and increase the positioning accuracy of the whole system.

4. Talk about three rings
The servo motor is generally controlled by three loops. The so-called three loops are three closed-loop negative feedback PID control systems. The innermost PID loop is the current loop, which is completely carried out inside the servo driver. The output current of the driver to the motor is detected by the Hall device, and the negative feedback to the current setting is PID adjusted, so that the output current is as close as possible to the set current. The current loop is used to control the motor torque, so the driver has the smallest calculation and the fastest dynamic response in the torque mode.

The second loop is the speed loop, which performs negative feedback PID regulation through the detected signal of the motor encoder. Its PID output in the loop is directly the setting of the current loop, so the speed loop control includes the speed loop and the current loop. In other words, the current loop must be used in any mode, and the current loop is the basis of control, At the same time of speed and position control, the system is actually controlling the current (torque) to achieve the corresponding control of speed and position.

The third ring is the position ring, which is the outermost ring. It can be built between the driver and the motor encoder, or between the external controller and the motor encoder or the final load, depending on the actual situation. Since the internal output of the position control loop is the setting of the speed loop, the system performs the operation of all three loops in the position control mode. At this time, the system has the largest amount of calculation and the slowest dynamic response speed.

Source: PLCJSYY