Three servo motor control methods

Three servo motor control methods

Next, we will introduce three kinds of servo motor control methods.

If you have no requirement on the speed and position of the motor, as long as you output a constant torque, of course, use the torque mode.

If there is a certain accuracy requirement for position and speed, and not very much concern with real-time torque, it is better to use speed or position mode.

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

Looking at the response speed of the servo driver, the torque calculation mode has the smallest amount of calculation, and the driver responds to the control signal the fastest; the position mode calculation amount is the largest, and the driver responds to the control signal the slowest.

When there is a relatively high requirement for the dynamic performance in sports, the motor needs to be adjusted in real time.

If the operation speed of the controller itself is slow (such as PLC, or low-end motion controller), it is controlled by the position method.

If the speed of the controller is faster, the speed can be used to move the position loop from the drive to the controller to reduce the workload of the drive and increase the efficiency.

If there is a better upper controller, torque control can also be used to remove the speed ring from the drive, which is generally only the high-end dedicated controller to do so.

Generally speaking, the driver control is good or bad, there is a more intuitive way to compare, called the response bandwidth.

When the torque control or the speed control, a square wave signal is given to it by the pulse generator, so that the motor continuously forwards and reverses, and the frequency is constantly increased. The oscilloscope displays a sweep signal when the envelope is When the apex reaches 70.7% of the highest value, it means that the step has been lost. At this time, the frequency can tell whether the control is good or bad. Generally, the current loop can be more than 1000HZ, and the speed loop can only be dozens of Hertz.

1 

Torque control:

The torque control method is to set the external output torque of the motor shaft through the input of an external analog input or a direct address. Specifically, for example, when 10 V corresponds to 5 Nm, the external axis is set to 5 V. The output is 2.5Nm: if the motor shaft load is less than 2.5Nm, the motor rotates forward, the external load is equal to 2.5Nm, the motor does not rotate, and when the load is greater than 2.5Nm, the motor rotates (generally under gravity load). You can change the set torque by changing the analog setting immediately, or you can change the value of the corresponding address by means of communication.

The application is mainly used in winding and unwinding devices that have strict requirements on the material's force, such as a wire-raising device or a pulling fiber device. The setting of the torque must be changed at any time according to the change of the winding radius to ensure that the material is not stressed. It will change with the winding radius.

2 

Position control:

The position control mode generally determines the rotation speed by the frequency of the externally input pulse, and determines the rotation angle through the number of pulses, and some servos can directly assign the speed and the displacement through the communication mode. Because the position mode can have very strict control on speed and position, it is generally applied to a positioning device.

Application areas such as CNC machine tools, printing machinery and so on.

3

Speed mode

Through the analog input or pulse frequency can be controlled by the rotation speed, in the presence of the upper control device of the outer ring PID control speed mode can also be positioned, but must be the motor position signal or direct load position signal to the upper Feedback to do calculations. The position mode also supports the direct load outer ring detection position signal. At this time, the motor shaft end encoder only detects the motor rotation speed, and the position signal is provided by the direct final load end detection device. This has the advantage that the intermediate transmission process can be reduced. The error increases the positioning accuracy of the entire system.

4

Talk about 3 rings

The servo motor is generally controlled by three rings. The so-called three rings are three closed-loop negative feedback PID adjusting systems. The innermost PID loop is the current loop. This loop is fully implemented inside the servo driver. The output current of each phase of the motor provided by the driver is detected by the Hall device, and the negative feedback is used to adjust the current setting to achieve the output current as close as possible. Equal to the set current, the current loop is to control the motor torque, so the driver in the torque mode is the smallest and the dynamic response is the fastest.

The second loop is the speed loop. Negative feedback PID adjustment is performed by the detected motor encoder signal. Its loop PID output is directly the setting of the current loop, so the speed loop control includes the speed loop and the current loop. In other words, any mode must use the current loop. The current loop is the root of control. At the same time of speed and position control, the system actually performs current (torque) control to achieve corresponding control of speed and position.

The third ring is the position loop, which is the outermost loop. It can be constructed between the drive and the motor encoder, or it can be built between the external controller and the motor encoder or the final load. It must be determined according to the actual situation. Since the output of the position control loop is the setting of the speed loop, the system performs all three loop operations in the position control mode. At this time, the system has the largest amount of computation and the slowest dynamic response speed.