Servo motor control method selection

There are three main control modes for servo motors, namely speed control mode, torque control mode and position control mode. Each control mode is described in detail below.

1. Speed ​​control method

The rotation speed can be controlled by the analog input or the frequency of the pulse. When there is the outer ring PID control of the upper computer control device, the speed mode can also be positioned, but the position signal of the motor or the position signal of the direct load must be used. Give feedback to the host computer for calculation. The speed mode also supports the direct load outer loop detection position signal. At this time, the encoder at the motor shaft end only detects the motor speed, and the position signal is provided by the direct final load end detecting device. This has the advantage of reducing the intermediate transmission process. The error increases the positioning accuracy of the entire system.

2. Torque control mode

The torque control mode is to set the external output torque of the motor shaft by the external analog input or the assignment of the direct address. The specific performance is as follows: for example, if 10V corresponds to 5Nm, when the external analog quantity is set to 5V, The output of the motor shaft 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. When the motor is larger than 2.5Nm, the motor will reverse. The set torque can be changed by changing the analog setting in real time, or by changing the value of the corresponding address by communication. Applications are mainly in winding and unwinding devices where the stress of the material is critical, such as winding devices or fiber-optic devices.

3. Position control method

The position control method generally determines the rotation speed by the frequency of the externally input pulse, and determines the rotation angle by the number of pulses. Some servo drivers can directly assign speed and displacement by communication. Since the position mode has strict control of speed and position, it is generally applied to positioning devices, such as CNC machine tools, printing machines, and the like.

How to choose the control mode of the servo motor? In terms of the response speed of the servo drive, the torque mode calculation amount is the smallest, the driver has the fastest response to the control signal; the position mode calculation amount is the largest, and the driver has the slowest response to the control signal.

If you do not have any requirements on the speed and position of the motor, just output a constant torque, of course, using the torque mode.

If there is a certain accuracy requirement for position and speed, and the real-time torque is not very concerned, it is not convenient to use the torque mode, and the speed or position mode is better. If the host controller has a better 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, there is no high requirement for the upper controller by the position control method.

If there is a high requirement for dynamic performance in motion, the motor needs to be adjusted in real time. Then if the controller itself is slow (such as plc, or low-end motion controller), it is controlled by position. If the controller is running faster, you can use the speed mode to move the position loop from the drive to the controller, reducing the workload of the drive and improving efficiency (such as most mid-to-high-end motion controllers); if there is better upper control The torque can also be controlled by the torque mode, and the speed loop is also removed from the drive. Moreover, the servo motor is not required at all.