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Three control modes and three control loops of servo motor

Three control modes and three control loops of servo motor

(Summary description)Three control modes and three control loops of servo motor

Three control modes and three control loops of servo motor

(Summary description)Three control modes and three control loops of servo motor

Information
Servo motor speed control and torque control are both controlled by analog, and position control is controlled by sending pulses. The specific control method to be adopted should be selected according to the customer's requirements and which sports functions are met.
 
Next, I will introduce three control methods of servo motors.
 
If you have no requirements on the speed and position of the motor, just output a constant torque, of course, use the torque mode.
 
If there is a certain accuracy requirement for position and speed, but 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 its own requirements are not very high, or there is basically no real-time requirement, the position control method does not have high requirements on the upper controller.
 
As far as the response speed of the servo drive is concerned: the torque mode has the smallest calculation amount and the drive responds the fastest to the control signal; the position mode has the largest calculation amount and the drive has the slowest response to the control signal.
 
When there are relatively high requirements for dynamic performance in motion, the motor needs to be adjusted in real time. If the calculation speed of the controller itself is very slow (such as PLC, or low-end motion controller), use position control.
 
If the controller calculation speed is relatively fast, you can use the speed mode to move the position loop from the driver to the controller to reduce the workload of the driver and improve efficiency; if there is a better upper controller, you can also use torque control. Remove the speed loop from the drive. This is generally only possible for high-end dedicated controllers. Generally speaking, there is a more intuitive way to compare the quality of drive control, called response bandwidth. When torque control or speed control, a square wave signal is given to it through the pulse generator, which makes the motor continuously rotate forward and reverse, and constantly increase the frequency. The oscilloscope displays a frequency sweep signal. When the vertex reaches 70.7% of the highest value, it means that it has been out of step. At this time, the frequency can indicate the quality of the control. Generally, the current loop can achieve more than 1000HZ, while the speed loop can only achieve tens of Hz.
 
 
 
 
1 Torque control
 
The torque control method is to set the external output torque of the motor shaft through the input of the external analog quantity or the direct address assignment. For example, 10V corresponds to 5Nm. When the external analog quantity is set to 5V, the motor shaft The 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 equal to 2.5Nm, the motor will reverse (usually generated under gravity load). The set torque can be changed by instantly changing the analog setting, or the value of the corresponding address can be changed by communication. The application is mainly used in winding and unwinding devices that have strict requirements on the force of the material, such as wire-raising device or fiber-optic equipment. The torque setting should be changed at any time according to the change of the winding radius to ensure that the material is not stressed. Will change as the winding radius changes.
 
 
 
 
2. Position control
 
In the position control mode, the rotation speed is generally determined by the frequency of externally input pulses, and the rotation angle is determined by the number of pulses. Some servos can 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. Application areas such as CNC machine tools, printing machinery, etc.
 
 
 
 
3. Speed ​​mode
 
The rotation speed can be controlled by analog input or pulse frequency, and positioning can also be carried out in the speed mode under the outer loop PID control with the upper control device, but the position signal of the motor or the position signal of the direct load must be sent to the upper Feedback for calculation purposes. The position mode also supports 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 detection device at the final load end. This has the advantage of reducing the intermediate transmission process. The error increases the positioning accuracy of the entire system.
 
 
4. Talk about 3 rings
 
Servo motors are 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 carried out completely inside the servo drive. The output current of each phase of the drive to the motor is detected by the Hall device, and the negative feedback is applied to the current setting for PID adjustment, so that the output current is as close as possible Equal to the set current, the current loop controls the motor torque, so in the torque mode, the driver has the smallest calculation and the fastest dynamic response.
 
The second loop is the speed loop. The negative feedback PID adjustment is carried out through the detected signal of the motor encoder. The 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, any mode must use a current loop. The current loop is the basis of control. While the speed and position are controlled, the system is actually controlling the current (torque) to achieve the corresponding control of the speed and position.
 
The third loop is the position loop, which is the outermost loop. It can be built between the drive 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 has performed all three loop calculations in the position control mode. At this time, the system has the largest amount of calculation and the slowest dynamic response speed.
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