The speed control and torque control of the servo motor are controlled by analog, and the position control is controlled by sending pulses. The specific control method should be selected according to the customer's requirements and the motion function to be met.
Next, I will introduce the three control methods of the servo motor.
If you have no requirements for the speed and position of the motor, you only need to output a constant torque, of course, use the torque mode.
If there is a certain accuracy requirement for the position and speed, but the real-time torque is not very concerned, it is better to use the speed or position mode.
If the upper controller has a better closed-loop control function, the effect of speed control will be better. If the requirement itself is not very high, or there is basically no real-time requirement, the position control method does not have high requirements for the upper controller.
In terms of the response speed of the servo drive: the torque mode has the smallest calculation amount, and the driver has the fastest response to the control signal; the position mode has the largest calculation amount, and the driver has the slowest response to the control signal.
When there are relatively high requirements for dynamic performance in motion, it is necessary to adjust the motor 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 operation speed of the controller is relatively fast, you can use the speed method 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, torque control can also be used to remove the speed loop from the driver. 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, which is called response bandwidth.
During torque control or speed control, a square wave signal is given to it through the pulse generator, so that the motor rotates forward and reverse continuously, and the frequency is continuously increased. The oscilloscope displays a frequency sweep signal. When the envelope curve When the apex reaches 70.7% of the highest value, it means that the synchronization has been lost. At this time, the frequency can indicate whether the control is good or bad. 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 assignment of the direct address. For example, if 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, the motor will not rotate when the external load is equal to 2.5Nm, and the motor will reverse when the external load is greater than 2.5Nm (usually occurs under the condition of gravity load). The set torque can be changed by changing the setting of the analog quantity in real time, and it can also be realized by changing the value of the corresponding address through communication.
It is mainly used in winding and unwinding devices that have strict requirements on the force of the material, such as wire rafting devices or optical fiber pulling 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 with the change of winding radius.
2. Position control:
In the position control mode, the rotation speed is generally determined by the frequency of the external input pulse, and the rotation angle is determined by the number of pulses. Some servos can also directly assign speed and displacement through communication. Since the position mode can strictly control the speed and position, it is generally used in positioning devices.
Applications such as CNC machine tools, printing machinery and so on.
3. Speed mode
The rotation speed can be controlled by analog input or pulse frequency, and the speed mode can also be used for positioning 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 given to the upper. Feedback is used for computation. The position mode also supports direct load outer ring 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 whole system.
4. Talk about the 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 completely carried out 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 given to the setting of the current for PID adjustment, so as to achieve the output current as close as possible to It is equal to the set current, and the current loop controls the torque of the motor, so in the torque mode, the operation of the driver is the smallest and the dynamic response is the fastest.
The second loop is the speed loop. Negative feedback PID adjustment is performed 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 the current loop. The current loop is the basis of control. While controlling the speed and position, the system is actually controlling the current (torque) to achieve the corresponding control of the speed and position.
The third ring is the position ring, which is the outermost ring. It can be constructed 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 calculations 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.
