There are three servo motor control modes: pulse, analog and communication control. How to choose the servo motor control mode in different application scenarios?
1. Servo motor pulse control mode
In some small stand-alone equipment, the use of pulse control to achieve motor positioning, should be the most common application, this control mode is simple, easy to understand. Basic control idea: the total pulse determines the motor displacement, pulse frequency determines the motor speed. The pulse is selected to realize the control of the servo motor. Open the user manual of the servo motor, there will be a table as follows:
Both are pulse controlled, but the implementation is different:
First, the driver receives two high-speed pulses (A, B), and determines the rotation direction of the motor through the phase difference between the two pulses. In the figure above, if B is 90 degrees faster than phase A, it is positive. If B is 90 degrees slower than A, it is inverted. In operation, the two phase pulses of this control are alternating, so we also call this control mode differential control. It has the difference characteristic, which also shows that this control mode, the control pulse has higher anti-interference ability, in some application scenarios with strong interference, this mode is preferred. But in this way, a motor shaft needs to occupy two high-speed pulse ports, which is awkward for the tense situation of 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. When one pulse is in output state, the other one must be in invalid state. When choosing this control mode, it is important to ensure that only one pulse is output at a time. Two pulses, one output for the positive direction, the other for the negative direction. As in the case above, this mode 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 positive and negative operation of the motor is determined by the IO signal in one direction. This control mode is simpler and occupies the least resources of high speed pulse port. In a typical small system, this is preferred.
2. Servo motor simulation control mode
In the application scenario that needs to use servo motor to realize speed control, we can choose the analog quantity to realize speed control of the motor, the value of the analog quantity determines the running speed of the motor. The analog quantity can be selected in two ways, current or voltage. Voltage mode, only need to add a certain amount of voltage in the control signal end. The implementation is simple, in some scenarios using a potentiometer to achieve control. However, when the voltage is used as the control signal, the voltage is easy to be interfered in the complex environment, resulting in unstable control. Current mode: The corresponding current output module is required. But the current signal has strong anti-jamming ability and can be used in complex scenes.
3. Servo motor communication control mode
CAN, EtherCAT, Modbus and Profibus are common ways to realize servo motor control by means of communication. To control the motor by means of communication is the preferred control method in some complex and large system application scenarios. Using communication mode, the size of the system, the number of motor shaft are easy to cut, no complex control wiring. The built system is extremely flexible.
The speed control and torque control of servo motor are controlled by analog quantity. Position control is controlled by sending pulses. The specific control mode should be selected according to the requirements of customers and meet the movement function. If you have no requirements on the speed and position of the motor, as long as the output of a constant torque, of course, is the torque mode.
If the position and speed have certain accuracy requirements, and the real-time torque is not very concerned, the torque mode is not very convenient, the speed or position mode is better. If the upper controller has a good closed-loop control function, the effect of speed control will be better. If the requirements are not very high, or there is no real time requirement, the position control mode does not have high requirements on the upper controller.
In terms of the response speed of the servo driver, the torque mode requires the least computation and the driver responds the fastest to the control signal. The position mode has the most computation and the driver's response to the control signal is the slowest.
It is necessary to adjust the motor in real time when the dynamic performance in motion is required. So if the controller itself is slow (such as PLC, or low-end motion controller), use position control. If the controller has a fast computing speed, the position ring can be moved from the driver to the controller in a speed way to reduce the workload of the driver and improve efficiency (such as most middle and high-end motion controllers); If you have a better upper controller, you can also use torque control, the speed loop is also removed from the drive, this is generally only high-end dedicated controller can do this, and, at this time, do not need to use a servo motor.
Generally speaking, driver control is not good, each manufacturer says that they do the best, but now there is a more intuitive comparison way called response bandwidth. When torque control or speed control, a square wave signal is given to the pulse generator to make the motor continuously turn and reverse, and constantly adjust the frequency. What is displayed on the oscilloscope is a sweep frequency signal. When the apex of the envelope reaches 70.7% of the highest value, it indicates that the step has gone out of step. The average current loop can operate at more than 1000Hz, while the speed loop can only operate at tens of Hertz.
To put it in a more technical way:
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 or direct address assignment. The specific performance is as follows: for example, if 10V corresponds to 5Nm, when the external analog is set to 5V, the output of the motor shaft is
2.5Nm: If the motor shaft load is less than 2.5Nm, the motor will turn positive; if the external load is equal to 2.5Nm, the motor will not turn; if the motor is greater than 2.5Nm, the motor will reverse (usually generated when there is a gravity load). The torque can be changed by immediately changing the setting of the analog quantity, and the corresponding address value can also be changed by means of communication.
It is mainly used in winding and unwinding devices that have strict requirements on the force of the material, such as wire device or fiber pulling equipment. The setting of torque should be changed at any time according to the change of winding radius to ensure that the force of the material will not change with the change of winding radius.
2. Position control of servo motor:
The control mode is generally through the external input pulse frequency to determine the size of the rotation speed, through the number of pulses to determine the rotation Angle, some servo can also be directly through the communication mode of speed and displacement assignment. Because the position mode can have a very strict control of speed and position, so it is generally used in positioning devices. Applications such as CNC machine tools, printing machinery and so on.
3. Servo motor speed mode:
Over analog input or pulse frequency can be controlled for rotation speed, in the upper control device of the outer loop PID control speed mode can also be positioned, but must the motor position signal or direct load position signal to the upper feedback for calculation. The position mode also supports the direct load outer ring to detect the position signal. In this case, 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. The advantage of this mode is that the error in the intermediate transmission process can be reduced and the positioning accuracy of the whole system can be increased.
4. Talk about 3 rings
The servo is generally controlled by three rings, and the so-called three rings are three closed-loop negative feedback PID regulating systems. The innermost PID ring is the current ring, which 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 gives negative feedback to the setting of the current for PID regulation, so as to achieve the output current as close to equal to the set current as possible. The current ring is to control the motor torque, so the operation of the driver in the torque mode is minimal.
Dynamic response is the fastest.
The second ring is the speed ring, which is adjusted by negative feedback PID through the signal of the motor encoder detected. The PID output in the ring is directly the setting of the current ring, so the speed ring control includes the speed ring and the current ring, in other words, any mode must use the current ring, the current ring is the root of the control. At the same time of speed and position control, current (torque) control is also carried out in the system to achieve the corresponding control of speed and position.
The third ring is the position ring, which is the outermost ring and 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 situation. Since the internal output of the position control ring is the setting of the speed ring, the system performs the operation of all three rings in the position control mode, and at this time the system has the largest amount of computation and the slowest dynamic response speed.