servo drives (servo drives), also known as "servo controller", "servo amplifier", is a controller used to control the servo motor, its role is similar to the frequency converter acting on the ordinary AC motor, is a part of the servo system, mainly used in high-precision positioning system. Generally, the servo motor is controlled through three ways of position, speed and torque to achieve high-precision transmission system positioning, which is a high-end product of transmission technology.
Basic introduction
The servo driver is an important component of modern motion control and is widely used in automated equipment such as industrial robots and CNC machining centers. Especially, the servo driver for controlling AC permanent magnet synchronous motors has become a research hotspot both at home and abroad. Currently, the three closed-loop control algorithm based on vector control is commonly adopted in the design of AC servo drivers. In this algorithm, the rationality of the speed closed-loop design plays a crucial role in the performance of the entire servo control system, particularly in the speed control performance. [1]
In the speed closed-loop of the servo driver, the real-time measurement accuracy of the motor rotor speed is crucial for improving the dynamic and static characteristics of the speed loop. To strike a balance between measurement accuracy and system cost, incremental optical encoders are generally used as speed sensors, and the corresponding common speed measurement method is the M/T speed measurement method. Although the M/T speed measurement method has certain measurement accuracy and a wide measurement range, it has inherent drawbacks, mainly including: 1) At least one complete code disk pulse must be detected within the speed measurement period, which limits the lowest measurable speed; 2) The two control system timers used for speed measurement are difficult to maintain strict synchronization, and the speed measurement accuracy cannot be guaranteed in measurement scenarios with large speed variations. Therefore, the traditional speed loop design scheme using this speed measurement method is difficult to improve the speed following and control performance of the servo driver.
Working principle
Most mainstream servo drives adopt digital signal processors (DSPs) as their control cores, enabling the implementation of complex control algorithms and achieving digitalization, networking, and intelligence. Power devices generally use drive circuits centered on intelligent power modules (IPMs), which integrate drive circuits and feature protection circuits for overvoltage, overcurrent, overheating, and undervoltage faults. Soft start circuits are also incorporated in the main circuit to reduce the impact on the drive during the startup process. The power drive unit first rectifies the input three-phase power or mains power through a three-phase full-bridge rectifier circuit to obtain the corresponding DC power. The rectified three-phase power or mains power is then frequency-converted by a three-phase sinusoidal PWM voltage-type inverter to drive a three-phase permanent magnet synchronous AC servo motor. The entire process of the power drive unit can be simply described as an AC-DC-AC process. The main topology of the rectifier unit (AC-DC) is a three-phase full-bridge uncontrolled rectifier circuit.
With the large-scale application of servo systems, the use, debugging and maintenance of servo drives have become important technical issues in today's field. More and more industrial control technology service providers have conducted in-depth technical research on servo drives. Servo drives are an important part of modern motion control and are widely used in industrial robots and CNC machining centers and other automated equipment. Especially the servo drives applied to control AC permanent magnet synchronous motors have become a research hotspot at home and abroad. Currently, the design of AC servo drives generally adopts a three-loop control algorithm based on vector control for current, speed and position. Whether the speed loop design in this algorithm is reasonable plays a key role in the entire servo control system, especially in the performance of speed control.
Working principle
Requirements for Servo Feed Systems
1. Wide speed regulation range
2. High positioning accuracy
3. Sufficient transmission rigidity and high speed stability
4. Fast response without overshoot
To ensure productivity and processing quality, in addition to requiring high positioning accuracy, good fast response characteristics are also required, that is, the response to tracking command signals should be fast. Because in the start-up and braking of the numerical control system, the acceleration and deceleration should be large enough to shorten the transition process time of the feed system and reduce contour transition errors.
5. Low-speed high torque and strong overload capacity
Generally, servo drives have an overload capacity of more than 1.5 times for several minutes or even half an hour, and can withstand overloads of 4 to 6 times for a short period without damage.
6. High reliability
The feed drive system of a numerical control machine tool is required to have high reliability, good working stability, strong environmental adaptability to temperature, humidity, vibration, etc., and strong anti-interference ability.
Requirements for the motor:
1. The motor should operate smoothly from the lowest speed to the highest speed, with minimal torque ripple. Especially at low speeds such as 0.1 r/min or even lower, it should maintain a stable speed without crawling.
2. The motor should have a large overload capacity for an extended period to meet the requirements of low-speed and high-torque operation. Generally, DC servo motors are required to withstand overloads of 4 to 6 times for several minutes without damage.
3. To meet the requirements of rapid response, the motor should have a small moment of inertia and a large locked-rotor torque, and possess the smallest possible time constant and starting voltage.
4. The motor should be capable of withstanding frequent starts, stops, and reversals.
Test platform
There are mainly the following types of test platforms for servo drives: the test platform with mutual feedback and drag between the servo drive and motor, the test platform with adjustable simulated load, the test platform with an actuator motor but no load, the test platform with the actuator motor dragging the inherent load, and the test platform with online testing method.
1. The test platform with mutual feedback and drag between the servo drive and motor
This test system consists of four parts: three-phase PWM rectifier, the servo drive-motor system under test, the load servo drive-motor system and the upper computer. The two motors are connected through a coupling. The motor under test operates in the electric state, while the load motor operates in the generating state. The servo drive-motor system under test operates in the speed closed-loop state to control the speed of the entire test platform, and the load servo drive-motor system operates in the torque closed-loop state to change the torque of the load motor by controlling its current, simulating the load variation of the motor under test. Thus, the mutual feedback and drag test platform can flexibly adjust the speed and torque and complete various test functions. The upper computer is used to monitor the operation of the entire system, send control instructions to the two servo drives according to the test requirements, and receive their operation data, as well as save, analyze and display the data.
For this test system, high-performance vector control is adopted to control the speed and torque of the motor under test and the load equipment respectively, which can simulate the dynamic and static performance of the servo drive under various load conditions and complete comprehensive and accurate tests on the servo drive. However, due to the use of two sets of servo drive-motor systems, this test system is bulky and cannot meet the portable requirements. Moreover, the measurement and control circuits of the system are also relatively complex and costly.
2. The test platform with adjustable simulated load
This test system consists of three parts: the servo drive-motor system under test, the adjustable simulated load and the upper computer. The adjustable simulated load, such as magnetic powder brake and electric dynamometer, is connected to the motor under test on the same axis. The upper computer and data acquisition card control the load torque by controlling the adjustable simulated load, and simultaneously collect the operation data of the servo system, as well as save, analyze and display the data. For this test system, by controlling the adjustable simulated load, the dynamic and static performance of the servo drive under various load conditions can also be simulated, and comprehensive and accurate tests on the servo drive can be completed. However, this test system is still relatively large and cannot meet the portable requirements. Moreover, the measurement and control circuits of the system are also relatively complex and costly.
3. The test platform with an actuator motor but no load
This test system consists of two parts: the servo drive-motor system under test and the upper computer. The upper computer sends the speed command signal to the servo drive, and the servo drive starts to operate according to the command. During the operation, the upper computer and data acquisition circuit collect the operation data of the servo system and save, analyze and display the data. Since the motor in this test system does not carry a load, compared with the previous two test systems, the system volume is relatively reduced, and the measurement and control circuits of the system are also relatively simple. However, this also makes the system unable to simulate the actual operation of the servo drive. Generally, this type of test system is only used for testing the speed and angular displacement of the system under no-load conditions, and cannot conduct comprehensive and accurate tests on the servo drive.
4. The test platform with the actuator motor dragging the inherent load
This test system consists of three parts: the servo drive-motor system under test, the inherent load of the system and the upper computer. The host computer sends the speed command signal to the servo driver, and the servo system starts to operate according to the command. During the operation, the host computer and the data acquisition circuit collect the operation data of the servo system and save, analyze and display the data.
For this kind of test system, the load adopts the inherent load of the tested system, so the test process is close to the actual working condition of the servo driver, and the test results are relatively accurate. However, since the inherent load of some tested systems is not easy to remove from the equipment, the test process can only be carried out on the equipment, which is not very convenient.
5. Test Platform Using Online Testing Method
This test system only has a data acquisition system and a data processing unit. The digital acquisition system collects and conditions the real-time operation status signals of the servo driver in the equipment, and then sends them to the data processing unit for processing and analysis. Finally, the data processing unit makes the test conclusion. Since the online testing method is adopted, this test system has a relatively simple structure and does not need to separate the servo driver from the equipment, making the test more convenient. This kind of test system is completely tested according to the actual operation of the servo driver, so the test conclusion is closer to the actual situation. However, due to the characteristics of many servo drivers in manufacturing and assembly, the installation positions of various sensors and signal measurement components in this kind of test system are difficult to choose. Moreover, if other parts of the equipment fail, it will also have a negative impact on the working state of the servo driver, ultimately affecting its test results.
Relevant parameter
Position Proportional Gain
1. Set the proportional gain of the position loop regulator;
2. The larger the set value, the higher the gain, the greater the stiffness, and the smaller the position lag under the same frequency command pulse conditions. However, a value that is too large may cause oscillation or overshoot;
3. The parameter value is determined by the specific servo system model and load conditions.
Position Feedforward Gain
1. Set the feedforward gain of the position loop;
2. The larger the set value, the smaller the position lag under any frequency command pulse;
3. A large feedforward gain of the position loop improves the high-speed response characteristics of the control system, but it may cause position instability and oscillation in the system;
4. When a high response characteristic is not required, this parameter is usually set to 0. Range: 0~100%.
Speed Proportional Gain
1. Set the proportional gain of the speed regulator;
2. The larger the set value, the higher the gain and the greater the stiffness. The parameter value is determined by the specific servo drive system model and load value. Generally, the larger the load inertia, the larger the set value;
3. Set the value as large as possible without causing oscillation in the system.
Speed Integral Time Constant
1. Set the integral time constant of the speed regulator;
2. The smaller the set value, the faster the integral speed. The parameter value is determined by the specific servo drive system model and load conditions. Generally, the larger the load inertia, the larger the set value;
3. Set the value as small as possible without causing oscillation in the system.
Speed Feedback Filter Factor
1. Set the characteristics of the speed feedback low-pass filter;
2. The larger the value, the lower the cut-off frequency, and the less noise generated by the motor. If the load inertia is large, the set value can be appropriately reduced. A value that is too large will cause a slower response and may cause oscillation;
3. The smaller the value, the higher the cut-off frequency, and the faster the speed feedback response. If a higher speed response is required, the set value can be appropriately reduced.
Maximum Output Torque Setting
1. Set the internal torque limit value of the servo motor;
2. The set value is a percentage of the rated torque;
3. This limit is always effective.
Position Completion Range
4. Set the position completion pulse range in the position control mode;
5. This parameter provides the basis for the driver to determine whether the positioning is completed in the position control mode. When the remaining pulse count in the position deviation counter is less than or equal to the set value of this parameter, the driver considers the positioning to be completed, and the position switch signal is ON; otherwise, it is OFF;
6. In the position control mode, output the position completion signal, acceleration and deceleration time constants;
7. The set value represents the acceleration time from 0 to 2000 r/min or the deceleration time from 2000 to 0 r/min;
8. Acceleration and deceleration characteristics are linear;
9. Set the arrival speed;
10. In non-position control mode, if the motor speed exceeds this set value, the speed arrival switch signal is ON; otherwise, it is OFF;
11. This parameter is not used in position control mode;
12. It is independent of the rotation direction.
1. Set the proportional gain of the position loop regulator;
2. The larger the set value, the higher the gain, the greater the stiffness, and the smaller the position lag under the same frequency command pulse conditions. However, a value that is too large may cause oscillation or overshoot;
3. The parameter value is determined by the specific servo system model and load conditions.
Position Feedforward Gain
1. Set the feedforward gain of the position loop;
2. The larger the set value, the smaller the position lag under any frequency command pulse;
3. A large feedforward gain of the position loop improves the high-speed response characteristics of the control system, but it may cause position instability and oscillation in the system;
4. When a high response characteristic is not required, this parameter is usually set to 0. Range: 0~100%.
Speed Proportional Gain
1. Set the proportional gain of the speed regulator;
2. The larger the set value, the higher the gain and the greater the stiffness. The parameter value is determined by the specific servo drive system model and load value. Generally, the larger the load inertia, the larger the set value;
3. Set the value as large as possible without causing oscillation in the system.
Speed Integral Time Constant
1. Set the integral time constant of the speed regulator;
2. The smaller the set value, the faster the integral speed. The parameter value is determined by the specific servo drive system model and load conditions. Generally, the larger the load inertia, the larger the set value;
3. Set the value as small as possible without causing oscillation in the system.
Speed Feedback Filter Factor
1. Set the characteristics of the speed feedback low-pass filter;
2. The larger the value, the lower the cut-off frequency, and the less noise generated by the motor. If the load inertia is large, the set value can be appropriately reduced. A value that is too large will cause a slower response and may cause oscillation;
3. The smaller the value, the higher the cut-off frequency, and the faster the speed feedback response. If a higher speed response is required, the set value can be appropriately reduced.
Maximum Output Torque Setting
1. Set the internal torque limit value of the servo motor;
2. The set value is a percentage of the rated torque;
3. This limit is always effective.
Position Completion Range
4. Set the position completion pulse range in the position control mode;
5. This parameter provides the basis for the driver to determine whether the positioning is completed in the position control mode. When the remaining pulse count in the position deviation counter is less than or equal to the set value of this parameter, the driver considers the positioning to be completed, and the position switch signal is ON; otherwise, it is OFF;
6. In the position control mode, output the position completion signal, acceleration and deceleration time constants;
7. The set value represents the acceleration time from 0 to 2000 r/min or the deceleration time from 2000 to 0 r/min;
8. Acceleration and deceleration characteristics are linear;
9. Set the arrival speed;
10. In non-position control mode, if the motor speed exceeds this set value, the speed arrival switch signal is ON; otherwise, it is OFF;
11. This parameter is not used in position control mode;
12. It is independent of the rotation direction.
Application field
Servo drive is widely used in the field of injection molding machine, textile machinery, packaging machinery, CNC machine tools, etc.
Controller characteristics
Speed adjustment ratio 1:500
Revolution ratio 0.3:1500
Position control
Zero speed lock
Overload capacity 200[%] - 300[%]
High starting torque
The speed is not affected by the load
Triple closed loop control