Buke servo motor - a detailed explanation of the three control modes of the servo motor

1. Servo motor pulse control mode

In some small stand-alone equipment, the use of pulse control to achieve 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 pulse amount 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 servo motor manual, and there will generally be a table like this:

Buke servo motor - a detailed explanation of the three control modes of the servo motor

　　It's all pulse control, but the implementation is not the same:

First, the driver receives two high-speed pulses (A and B), and determines the direction of rotation of the motor through the phase difference of the two pulses. As shown in the figure above, if B is 90 degrees faster than phase A, it is a positive rotation; Then B is 90 degrees slower than phase A, then it is reversed. During operation, the two-phase pulses of this control are alternate, 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 a higher anti-interference ability, and in some application scenarios with strong interference, this method is preferred. However, in this way, a motor shaft needs to occupy two high-speed pulse ports, and it is embarrassing to be nervous about the high-speed pulse port.

Second, the driver still receives two high-speed pulses, but the two high-speed pulses do not exist at the same time, and when one pulse is in the output state, the other must be in an invalid state. When choosing this control method, it is important to ensure that only one pulse is output at the same time. Two pulses, one output runs in the positive direction and the other runs in the negative direction. As in the case above, this method is also a motor shaft that needs to occupy two high-speed pulse ports.

Third, only one pulse signal needs to be given to the driver, and the forward and reverse operation of the motor is determined by the IO signal in one direction. This control method is simpler to control, and the high-speed pulse port occupies the least amount of resources. In general, this method can be preferred in small systems.

2. Servo motor analog control mode

In the application scenario that requires the use of servo motor to achieve speed control, we can choose analog quantity to achieve speed control of the motor, and the value of analog quantity determines the operating speed of the motor. There are two ways to choose from analogues, current or voltage. In the voltage mode, only a certain amount of voltage needs to be added to the control signal terminal. It is easy to implement, and in some cases it can be controlled with a single potentiometer. However, the voltage is used as the control signal, and in the scene of complex environment, the voltage is easy to be interfered with, resulting in unstable control; For the current mode, the corresponding current output module is required. However, the current signal has strong anti-interference ability and can be used in complex scenarios.

3. Servo motor communication control mode

Common ways to realize servo motor control by communication mode are CAN, EtherCAT, Modbus, and Profibus. The use of communication to control the motor is the preferred control method for some complex and large system application scenarios. Using the communication method, the size of the system and the number of motor shafts are easy to cut, and there is no complicated control wiring. The system is built with a high degree of flexibility.

The speed control and torque control of the servo motor are controlled by analog quantities. Position control is controlled by generating pulses. The specific control mode should be selected according to the customer's requirements and what kind of motion function is satisfied. If you don't have any requirements for the speed or position of the motor, just output a constant torque, of course, in torque mode.

If you have certain accuracy requirements for position and speed, and you are not very concerned about real-time torque, it is not convenient to use torque mode, and it is better to use speed or position mode. If the upper controller has a better closed-loop control function, the effect will be better with speed control. If the requirements are not very high, or there is basically no real-time requirements, there is no high requirement for the upper controller with position control.

In terms of the response speed of the servo drive, the torque mode has the least amount of calculation, and the driver responds the fastest to the control signal. The position mode is the most computationally intensive, and the driver is the slowest to respond 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 controller itself is very slow (such as a PLC, or a low-end motion controller), it can be controlled by position. If the computing speed of the controller 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 (such as most mid-to-high-end motion controllers); If there is a better upper controller, it can also be controlled by torque and the speed ring can also be removed from the drive, which is generally only possible with high-end dedicated controllers, and there is no need to use a servo motor at all.

Generally speaking, whether the drive is well controlled or not, each manufacturer says that it does the best, but now there is a more intuitive comparison method called response bandwidth. When the torque control or speed control through the pulse generator to give him a square wave signal, so that the motor continues to rotate forward, reverse, and constantly adjust the frequency, the oscilloscope is displayed is a sweeping signal, when the apex of the envelope reaches the highest value of 70.7%, it means that it has lost step, the frequency at this time, you can show whose product is bull, the general current loop can be made to more than 1000Hz, and the speed loop can only be made to tens of hertz.

To put it more professionally:

1. Servo motor torque control

The torque control mode is to set the output torque of the motor shaft through the input of the external analog quantity or the assignment of the direct address, which is specifically manifested in the case of 10V corresponding 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 does not rotate when the external load is equal to 2.5Nm, and the motor is reversed when it is greater than 2.5Nm (usually generated under the condition of gravity load). The set torque can be changed by changing the setting of the analog quantity in real time, and the value of the corresponding address can also be changed by means of communication.

The application is mainly in the winding and unwinding device that has strict requirements for the force of the material, such as the wire rao device or the optical fiber pulling equipment, the setting of torque 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.

2. Servo motor position control:

The control mode is generally to determine the magnitude of the rotation speed through the frequency of the external input pulse, and to determine the rotation angle through the number of pulses, and some servos can directly assign the speed and displacement through communication. Since the position mode can have a very tight control of speed and position, it is generally used in positioning devices. Application fields such as CNC machine tools, printing machinery and so on.

3. Servo motor speed mode:

The speed can be controlled by the input of the analog quantity or the frequency of the pulse, and the speed mode can also be positioned when the PID control of the outer ring with the upper control device is located, but the position signal of the motor or the position signal of the direct load must be fed back to the upper position for calculation. The position mode also supports the direct load outer ring to detect the position signal, the encoder at the motor shaft end only detects the motor speed, and the position signal is provided by the detection device at the direct final load end, which has the advantage that the error in the intermediate transmission process can be reduced, and the positioning accuracy of the whole system is increased.

4. Talk about the 3 rings

The servo is generally controlled by three rings, and the so-called three rings are three closed-loop negative feedback PID adjustment systems. The innermost PID ring is the current loop, which is completely carried out inside the servo drive, and the output current of each phase of the driver to the motor is detected by the Hall device, and the PID is adjusted by the setting of the negative feedback current, so as to achieve the output current as close as possible to the set current, and the current loop is to control the torque of the motor, so the operation of the driver in the torque mode is the smallest, and the dynamic response is the fastest.

The second ring is the speed loop, through the signal of the motor encoder detected to carry out negative feedback PID adjustment, its loop PID output 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 root of the control, in the speed and position control at the same time the system is actually carrying out the current (torque) control to achieve the corresponding control of speed and position.

The third ring is the position ring, which is the outermost ring, which can be built 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 the calculation of all three rings in the position control mode, and the system calculation amount is the largest and the dynamic response speed is the slowest.

Buke servo motor - a detailed explanation of the three control modes of the servo motor

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