Generally speaking, there are three servo control modes: speed control mode, torque control mode and position control mode.
Both speed control and torque control are controlled by simulation. Position control is controlled by sending pulses. The specific control mode should be selected according to the customer's requirements and what kind of motion function it meets.
If you have no requirements for the speed and position of the motor, just output a constant torque, of course, in torque mode.
If you have certain accuracy requirements for position and speed, but you don't care much 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 better closed-loop control function, the speed control effect will be better. If the requirement itself is not very high, or there is basically no real-time requirement, the position control does not have high requirements for the upper controller. As far as the response speed of the servo driver is concerned, the torque mode operation is the least, and the driver responds to the control signal the fastest. The operation amount of position mode is the largest, and the response of the driver to the control signal is the slowest.
When there is a high demand for dynamic performance in motion, it is necessary to adjust the motor in real time. If the running speed of the controller itself is slow (such as PLC or low-end motion controller), it is controlled by position mode. If the operation speed of the controller is fast, the positioning ring can be moved from the driver to the controller by speed, which reduces the workload of the driver and improves the efficiency (for example, most high-end motion controllers); If you have a better upper controller, you can also use torque control to move the speed loop away from the driver, which can only be done by high-end dedicated controllers, and there is no need to use servo motors at this time.
In other words:
1. torque control: the torque control mode is to set the external output torque of the motor shaft through the input of external analog or the distribution of direct address. For example, 10V corresponds to 5Nm, and 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, and when the external load is equal to 2.5Nm, the motor reverses (usually, the set torque can be changed by changing the analog quantity setting in real time or changing the value of the corresponding address through communication. It is mainly used for winding and unwinding equipment with strict requirements on material stress, such as winding equipment or optical fiber drawing equipment. The setting of torque should be changed at any time according to the change of winding radius to ensure that the stress of the material will not 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 external input pulses, and the rotation angle is determined by the number of pulses. Some servo systems can directly assign values to speed and displacement through communication. Because 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, etc.
3. Speed mode: The speed can be controlled by analog input or pulse frequency, and the speed mode can also be located when the PID control of the outer ring of the upper control device is provided, but the position signal of the motor or the position signal of the direct load must be fed back to the upper control for calculation. The position mode also supports directly loading the 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 detection device at the direct final load end. The advantage of this is to reduce the error in the intermediate transmission process and increase the positioning accuracy of the whole system.
The basic concept of servo is accuracy, precision and fast positioning. Frequency conversion is a necessary internal link of servo control, and there is also frequency conversion (stepless speed regulation) in servo driver. But the servo control is current loop, speed loop or position loop, which is very different. In addition, the structure of servo motor is different from that of ordinary motor, which should meet the requirements of fast response and accurate positioning. At present, most AC servo motors circulating in the market are permanent magnet synchronous AC servo, but this kind of motor is limited by technology and it is difficult to achieve high power. Synchronous servo above 10 KW is extremely expensive, so AC asynchronous servo is often used when field application allows. At this time, many drivers are high-end frequency converters controlled by encoder feedback closed loop. The so-called servo is to meet the requirements of accuracy, precision and rapid positioning. As long as it meets, there will be no servo frequency conversion dispute.
I. Similarities between the two:
The technology of AC servo itself refers to and applies frequency conversion technology, which is realized by imitating the control mode of DC motor through frequency conversion PWM mode on the basis of DC motor servo control, that is to say, AC servo motor must have a frequency conversion link: frequency conversion is to rectify the frequency of 50 and 60HZ AC power supply into DC power supply, and then convert it into a waveform with adjustable frequency through various transistor (IGBT, IGCT, etc.) inverters by adjusting carrier frequency and PWM, similar to sine and cosine pulse electricity. Because the frequency can be adjusted, the speed of AC motor can be adjusted (n=60f/p, n-speed, f-frequency, p-pole logarithm).
Second, talk about frequency converter:
Simple inverter can only adjust the speed of AC motor, so it can be open-loop or closed-loop, depending on the control mode and inverter. This is the traditional V/F control mode. At present, many frequency converters convert the stator magnetic field UVW3 phase of AC motor into two current components that can control the motor speed and torque by establishing mathematical models. At present, most of the well-known brand frequency converters that can control torque adopt this method. Each phase output of UVW needs a Hall effect current detection device, and after sampling and feedback, a closed-loop negative feedback current loop PID adjustment is formed. ABB's frequency conversion also puts forward a direct torque control technology different from this way. Please refer to relevant materials for details. In this way, the speed and torque of the motor can be controlled, and the speed control accuracy is better than v/f control. The encoder feedback can be added or not, and the control accuracy and response characteristics are much better with the addition of control.
Third, talk about servo:
Driver: On the premise of developing frequency conversion technology, servo driver has carried out more accurate control technology and algorithm operation than general frequency conversion in current loop, speed loop and position loop (frequency converter does not have this loop), and it is also much more powerful in function than traditional frequency conversion. The most important point is that it can carry out accurate position control. The speed and position are controlled by the pulse sequence sent by the upper controller (of course, some servos integrate the control unit or directly set the parameters such as position and speed in the driver through bus communication). The algorithm inside the driver, faster and more accurate calculation and electronic devices with better performance make it superior to the frequency converter.
Motor: The material, structure and processing technology of servo motor are much higher than that of AC motor driven by inverter (general AC motor or inverter motor with constant torque and constant power), that is to say, when the driver outputs power with rapidly changing current, voltage and frequency, servo motor can respond to the change of power supply, and its response characteristics and overload resistance are much higher than that of AC motor driven by inverter. The serious difference of motor performance is also the root of the difference. That is to say, it's not that the inverter can't output the power signal that changes so fast, but that the motor itself doesn't respond, so the corresponding overload setting is made to protect the motor when the internal algorithm of the inverter is set. Of course, even if the output capacity of the inverter is not set, some inverters with excellent performance can directly drive the servo motor! ! !
Fourth, talk about AC motors:
Ac motors are generally divided into synchronous motors and asynchronous motors.
1, AC synchronous motor: that is, the rotor is made of permanent magnet material, so after rotation, with the change of the rotating magnetic field of the motor stator, the rotor also changes the speed of response frequency, and the rotor speed = the stator speed, so it is called "synchronization".
2. AC asynchronous motor: the rotor consists of induction coil and material. After the stator rotates, it produces a rotating magnetic field, which cuts the induction coil of the stator, and the rotor coil produces an induced current, which follows the rotating magnetic field of the stator, but the magnetic field change of the rotor is always smaller than that of the stator. Once they are equal, there will be no induced current in the rotor coil, the magnetic field of the rotor will disappear, and the rotor will stall, resulting in a speed difference with the stator, and the induced current will be regained. . . Therefore, there is a key parameter in AC asynchronous motor, that is, slip ratio is the ratio of speed difference between rotor and stator.
3. Corresponding to AC synchronous and asynchronous motor frequency converters, there are corresponding synchronous and asynchronous frequency converters, and servo motors also have AC synchronous servo and AC asynchronous servo. Of course, AC asynchronous frequency conversion is common in frequency converters, and AC synchronous servo is common in servo.
Verb (abbreviation of verb) application
Because frequency converter and servo are different in performance and function, their applications are also different:
1, in the occasions of speed control and torque control, frequency converters are generally used, and some are used to control the position by adding position feedback signals to the upper position, with low accuracy and response. Now some frequency converters also accept pulse sequence signals to control the speed, but it seems that they can't directly control the position.
2. Servo can only be used in situations where position control is strict, and the response speed of servo is much faster than frequency conversion. Servo control is also used in some occasions with high precision and response requirements, and servo can be used instead of almost all occasions where variable frequency control can be used. The key points are two points: first, the price servo is much higher than frequency conversion, and second, the power reason: the maximum frequency conversion can be hundreds of KW or even higher, while the maximum servo is tens of KW. However, with the continuous improvement of servo motor technology, the power can gradually reach several hundred KW.