Three control methods for servo motor

Servo motor speed control and torque control are controlled by analog quantity, and position control is controlled by pulse. The specific control method should be selected according to the requirements of the customer and what kind of sports function is satisfied.
Next, I will introduce you to the three control methods of the servo motor.
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 it is not very concerned about real-time torque, it is better to use speed or position mode.
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.
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.
When there is a high requirement for dynamic performance in motion, the motor needs to be adjusted in real time.
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.
If there is a better host controller, you can also use the torque control to remove the speed loop from the drive. This is usually only a high-end dedicated controller.
Generally speaking, the control of the drive is good or bad, and there is a relatively straightforward comparison method called response bandwidth.
When torque control or speed control, a square wave signal is given to it by the pulse generator, so that the motor continuously rotates and reverses continuously, 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 it has lost the step. At this time, the frequency can explain the quality of the control. Generally, the current loop can achieve 1000HZ or more, and 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 by external analog input or direct address assignment, which is expressed as, for example, 10V corresponding to 5Nm, when the external analog is set to 5V, the motor shaft The output is 2.5Nm: if the motor shaft load is lower than 2.5Nm, the motor rotates forward, the motor does not rotate when the external load is equal to 2.5Nm, and the motor reverses when it is greater than 2.5Nm (usually generated under gravity load). 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.
The application is mainly used in the winding and unwinding devices which have strict requirements on the material stress, such as the wire drawing device or the fiber drawing device. The torque setting is changed according to the change of the winding radius to ensure the stress of the material is not It will change as the winding radius changes.
2. Position control:
The position control mode generally determines the rotation speed by the frequency of the externally input pulse, and determines the angle of rotation by the number of pulses. Some servos can directly assign speed and displacement by communication. Since the position mode has strict control over speed and position, it is generally applied to positioning devices.
Applications such as CNC machine tools, printing machinery, etc.
3. Speed mode
The rotation speed can be controlled by the analog input or the frequency of the pulse. The speed mode can also be positioned when the outer ring PID of the upper control device is controlled, but the position signal of the motor or the position signal of the direct load must be given to the upper position. Feedback is used for calculations. The position 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 detection device. This has the advantage of reducing the intermediate transmission process. The error increases the positioning accuracy of the entire system.
The servo motor is generally controlled by three loops. The so-called three loops are three closed loop negative feedback PID adjustment systems. The innermost PID loop is the current loop. This loop is completely carried out inside the servo driver. The Hall device detects the output current of each phase of the driver to the motor, and the negative feedback gives the current setting PID adjustment, so that the output current is as close as possible. Equal to the set current, the current loop is to control the motor torque, so in the torque mode the drive has the smallest operation and the fastest dynamic response.
The second ring is the speed loop. The negative feedback PID is adjusted by the detected signal of the motor encoder. The PID output in the ring is directly set by 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. The current loop is the fundamental 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 ring. It can be built between the drive and the motor encoder. It can also be built 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 all three loop operations in the position control mode. At this time, the system has the largest amount of calculation and the slowest dynamic response speed.