With the advent of all-digital AC servo systems, AC servo motors are increasingly being used in digital control systems. In order to adapt to the development trend of digital control, most of the motion control systems use an all-digital AC servo motor as the execution motor. It is implemented in the control mode with pulse train and direction signals.
The basic concept of servo is accurate, accurate and fast positioning. Frequency conversion is a necessary internal link of servo control, and there is also frequency conversion in the servo drive (for stepless speed regulation). However, the servo controls the current loop speed loop or the position loop to be closed, which is a big difference. In addition, the construction of the servo motor is different from the ordinary motor, to meet the fast response and accurate positioning. The AC servo motors currently on the market are mostly permanent magnet synchronous AC servos, but such motors are limited by the process, it is difficult to achieve a large power, and the synchronous servo price of more than ten KW is expensive, so that it is allowed in the field. In the case of AC asynchronous servo, many drives are high-end inverters with encoder feedback closed-loop control. The so-called servo is to meet the accuracy, accuracy and rapid positioning. As long as it is satisfied, there is no competition for servo frequency conversion.
2, the difference between the three control methodsThe general servo has three control modes: speed control mode, torque control mode, and position control mode.
Both speed control and torque control are controlled by analog quantities. Position control is controlled by pulse. The specific control method should be selected according to the customer's requirements and what kind of sports function is satisfied. If you do not have any requirements on the speed and position of the motor, just output a constant torque, of course, using the torque mode. If there is a certain accuracy requirement for position and speed, and the real-time torque is not very concerned, it is not convenient to use the torque mode, and the speed or position mode is better. If the host controller has a better closed-loop control function, the speed control effect will be better. If the requirements are not very high, or there is basically no real-time requirement, there is no high requirement for the upper controller by the position control method. In terms of the response speed of the servo drive, the torque mode calculation amount is the smallest, the driver has the fastest response to the control signal; the position mode calculation amount is the largest, and the driver has the slowest response to the control signal. When there is a high requirement for dynamic performance in motion, the motor needs to be adjusted in real time. Then if the controller itself is slow (such as PLC, or low-end motion controller), it is controlled by position. If the controller is running faster, you can use the speed mode to move the position loop from the drive to the controller, reducing the workload of the drive and improving efficiency (such as most mid-to-high-end motion controllers); if there is better upper control It can also be controlled by torque, and the speed loop is also removed from the drive. This is generally only a high-end dedicated controller to do this, and at this time, there is no need to use a servo motor.
1. Torque control: The torque control mode is to set the external output torque of the motor shaft through the external analog input or the assignment of the direct address. The specific performance is, for example, 10V corresponds to 5Nm, when the external analog quantity is set. When the motor is rated at 5V, the motor shaft output is 2.5Nm: if the motor shaft load is lower than 2.5Nm, the motor rotates forward, the motor does not rotate when the external load is equal to 2.5Nm, and the motor reverses when it is greater than 2.5Nm (usually generated under gravity load) ). The set torque can be changed by changing the analog setting in real time, or by changing the value of the corresponding address by communication. The application is mainly used in the winding and unwinding devices which have strict requirements on the material stress, such as the wire drawing device or the fiber drawing device. The torque setting is changed according to the change of the winding radius to ensure the stress of the material is not It will change as the winding radius changes.
2. Position control: The position control mode generally determines the rotation speed by the frequency of the externally input pulse. The number of pulses is used to determine the angle of rotation. Some servos can directly assign speed and displacement through communication. Since the position mode has strict control over speed and position, it is generally applied to positioning devices. Applications such as CNC machine tools, printing machinery, etc.
3. Speed ​​mode: The rotation speed can be controlled by the analog input or the frequency of the pulse. The speed mode can also be positioned when the outer ring PID of the upper control device is controlled, but the position signal or direct load of the motor must be used. The position signal is given to the upper feedback for calculation. The position mode also supports the 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 final load end detection device. This has the advantage of reducing the intermediate transmission process. The error increases the positioning accuracy of the entire system.
The technology of AC servo itself is a technology that draws on and applies frequency conversion. It is realized by the PWM method of DC motor to simulate the control mode of DC motor based on the servo control of DC motor. That is to say, AC servo motor must have frequency conversion. Link: The frequency conversion is to rectify the AC frequency of 50, 60HZ of the power frequency into DC power, and then adjust the inverter to the frequency adjustable waveform through the carrier frequency and PWM through various types of transistors (IGBT, IGCT, etc.) that can control the gate. The sinusoidal pulsating power, because the frequency is adjustable, the speed of the AC motor can be adjusted (n=60f/p, n speed, f frequency, p pole logarithm)
1.2.1 Inverter:
The simple inverter can only adjust the speed of the AC motor. At this time, it can be opened or closed depending on the control mode and the inverter. This is the V/F control method in the traditional sense. Nowadays, many frequency conversions have been established through mathematical models, and the phase of the stator magnetic field UVW3 of the AC motor is converted into two current components that can control the motor speed and torque. Now most of the famous brands of inverters that can perform torque control are In this way, the torque is controlled in such a way that the output of each phase of the UVW is added with a Hall effect current detecting device, and the PID feedback of the current loop of the closed loop negative feedback is formed after the sampling feedback; the ABB frequency conversion proposes a direct torque control technique different from this manner. Please refer to the relevant information for details. This can control both the speed of the motor and the torque of the motor, and the speed control accuracy is better than the v/f control. The encoder feedback can also be added or not. The control accuracy and response characteristics are much better when added.
1.2.2 Servo Drive:
Drive aspect: Under the premise of developing the frequency conversion technology, the servo drive has more precise control technology and algorithm operation than the general frequency conversion in the current loop, speed loop and position loop (the inverter does not have the loop) inside the drive. It is also much more powerful than traditional inverters. The main point is precise position control. The speed and position are controlled by the pulse sequence sent by the host controller (of course, some servos integrate the control unit or directly set the position and speed parameters in the drive through the bus communication), the internal algorithm of the drive and faster. More accurate calculations and better performance electronics make it superior to frequency converters.
Motor: The material, structure and processing technology of servo motor are much higher than AC motor driven by inverter (general AC motor or constant torque, constant power and other variable frequency motors), that is, when the drive output current, voltage, When the frequency changes rapidly, the servo motor can respond to changes in the power supply according to the power supply change. The response characteristics and overload resistance are much higher than those of the AC motor driven by the inverter. The serious difference in the motor is also the fundamental difference between the two. . That is to say, the inverter does not output the power signal as fast as the change, but the motor itself cannot respond. Therefore, in the internal algorithm setting of the inverter, the corresponding overload setting is made to protect the motor. Of course, even if the output capacity of the inverter is not limited, some inverters with excellent performance can directly drive the servo motor! ! !
1.2.3 AC motor:
AC motors are generally divided into synchronous and asynchronous motors
1. AC synchronous motor: the rotor is made of permanent magnet material. Therefore, after the rotation, as the rotating magnetic field of the stator of the motor changes, the rotor also changes the speed of the response frequency, and the rotor speed = stator speed, so it is called "synchronous". .
2. AC asynchronous motor: The rotor is composed of induction coil and material. After the rotation, the stator generates a rotating magnetic field, the magnetic field cuts the induction coil of the stator, the rotor coil generates an induced current, and the rotor generates an induced magnetic field, and the induced magnetic field follows the change of the rotating magnetic field of the stator, but the magnetic field change of the rotor is always smaller than the change of the stator, once equal to The magnetic field of the rotor is cut by the magnetic field without change, and there is no induced current in the rotor coil. The rotor magnetic field disappears, and the rotor stall speed and the stator generate a speed difference and regain the induced current. . . So a key parameter in AC asynchronous motors is that the slip ratio is the ratio of the rotor to stator speed difference.
3. Corresponding AC synchronous and asynchronous motor inverters have synchronous inverters and asynchronous inverters. The servo motors also have AC synchronous servo and AC asynchronous servo. Of course, AC asynchronous inverter is common in the inverter, and servo synchronous servo is common.
4, applicationDue to the difference in performance and function between the inverter and the servo, the application is not the same:
1. In the case of speed control and torque control, the general-purpose inverter is not very high, and the position feedback signal is added in the upper position to form a closed-loop frequency conversion for position control, and the accuracy and response are not high. Some existing frequency conversions also accept the pulse sequence signal to control the speed, but it seems that the position cannot be directly controlled.
2. In the case of strict position control requirements, only servo can be used. The servo response speed is much larger than the frequency conversion. Some precision and response requirements are also servo control, and can be controlled by frequency conversion. The sports occasion can almost be replaced by servo. The key is two points: First, the price servo is much higher than the frequency conversion, and the second is the power reason: the maximum frequency conversion can achieve several hundred KW, or even higher, and the servo is the largest. Dozens of KW. However, with the continuous improvement of servo motor technology, the power can gradually reach several hundred KW.
Control Cable,Control Wire,Shielded Control Cable,Power Control Cable
HENAN QIFAN ELECTRIC CO., LTD. , https://www.hnqifancable.com