There are three control modes of servo motor: pulse, analog and communication. How should we choose the control mode of servo motor in different application scenarios?
1. Pulse control mode of servo motor
In some small stand-alone equipment, the use of pulse control to realize the positioning of the motor should be the most common application method. This control method is simple and easy to understand.
The basic control idea: the total amount of pulses determines the motor displacement, and the pulse frequency determines the motor speed. The pulse is selected to realize the control of the servo motor, open the manual of the servo motor, and generally there will be a table like the following:
Both are pulse control, but the implementation is different:
The first is that the driver receives two high-speed pulses (A and B), and determines the rotation direction of the motor through the phase difference between the two pulses. As shown in the figure above, if phase B is 90 degrees faster than phase A, it is forward rotation; then phase B is 90 degrees slower than phase A, it is reverse rotation.
During operation, the two-phase pulses of this control are alternating, so we also call this control method differential control. It has the characteristics of differential, which also shows that this control method, the control pulse has higher anti-interference ability, in some application scenarios with strong interference, this method is preferred. However, in this way, one motor shaft needs to occupy two high-speed pulse ports, which is not suitable for the situation where the high-speed pulse ports are tight
Second, the driver still receives two high-speed pulses, but the two high-speed pulses do not exist at the same time. When one pulse is in the output state, the other must be in an invalid state. When this control method is selected, it must be ensured that there is only one pulse output at the same time. Two pulses, one output runs in positive direction and the other runs in negative direction. As in the above case, this method also requires two high-speed pulse ports for one motor shaft.
The third type is that only one pulse signal needs to be given to the driver, and the forward and reverse operation of the motor is determined by one direction IO signal. This control method is simpler to control, and the resource occupation of the high-speed pulse port is also the least. In general small systems, this method can be preferred.
Second, the servo motor analog control method
In the application scenario that needs to use the servo motor to realize speed control, we can choose the analog value to realize the speed control of the motor, and the value of the analog value determines the running speed of the motor.
There are two ways to choose the analog quantity, current or voltage.
Voltage mode: You only need to add a certain voltage to the control signal terminal. In some scenarios, you can even use a potentiometer to achieve control, which is very simple. However, the voltage is selected as the control signal. In a complex environment, the voltage is easily disturbed, resulting in unstable control.
Current mode: The corresponding current output module is required, but the current signal has strong anti-interference ability and can be used in complex scenarios.
3. Communication control mode of servo motor
Common ways to realize servo motor control by communication are CAN, EtherCAT, Modbus, and Profibus. Using the communication method to control the motor is the preferred control method for some complex and large system application scenarios. In this way, the size of the system and the number of motor shafts can be easily tailored without complicated control wiring. The system built is extremely flexible.
Fourth, the expansion part
1. Servo motor 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. The specific performance is that, for example, if 10V corresponds to 5Nm, when the external analog quantity is set to 5V, the motor shaft is The output is 2.5Nm. If the motor shaft load is lower than 2.5Nm, the motor is in the acceleration state; when the external load is equal to 2.5Nm, the motor is in a constant speed or stop state; when the external load is higher than 2.5Nm, the motor is in a deceleration or reverse acceleration state. The set torque can be changed by changing the setting of the analog quantity in real time, or the value of the corresponding address can be changed through communication.
It is mainly used in winding and unwinding devices that have strict requirements on the force of the material, such as winding 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 force of the material will not change with the change of the winding radius. changes with the winding radius.
2. Servo motor 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 the speed and position, it is generally used in positioning devices, CNC machine tools, printing machinery and so on.
3. Servo motor speed mode
The rotation speed can be controlled through the input of analog quantity or pulse frequency. The speed mode can also be used for positioning when the outer loop PID control of the upper control device is provided, but the position signal of the motor or the position signal of the direct load must be sent to the upper computer. Feedback for operational use. The position mode also supports the direct load outer loop to detect the 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. The advantage of this is that it can reduce the intermediate transmission process. The error increases the positioning accuracy of the entire system.
4. Talk about the three rings
The servo 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, which is completely carried out inside the servo driver. The output current of each phase of the motor to the motor is detected by the Hall device, and the negative feedback is used to adjust the current setting for PID adjustment, so as to achieve the output current 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 operation and the fastest dynamic response.
The second loop is the speed loop. The negative feedback PID adjustment is performed through the detected signal of the motor encoder. The PID output in its 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 foundation of the 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 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, in the position control mode, the system performs the operations of all three loops. At this time, the system has the largest amount of calculation and the slowest dynamic response speed.
Above come from Chengzhou News
Post time: May-31-2022