AC servo motor and drive
The DC servo motor has excellent speed control performance. From the early 1980s to the mid 90s, the application of the DC servo motor speed control system has always dominated the applications requiring higher speed control performance. However, it has some inherent disadvantages:
â— Brush and commutator wear and maintenance trouble
â— Complex structure, difficult manufacturing, high cost
AC servo motors do not have the above disadvantages. Especially in the same volume, the output power of the AC servo motor is increased by 10% to 70% compared with the DC motor, and the attainable rotation speed is higher than that of the DC motor. Therefore, people have been looking for AC motor speed control program to replace the DC motor speed control program.
The key to inverter frequency regulation is to obtain the required control waveform (such as SPWM wave) at the inverter control terminal.
â— Control waveform realization method (motor speed control method)
Phase control
Vector transformation control;
PWM control
Magnetic field control;
Closed loop, semi-closed loop feed servo system
â— Classification and characteristics of the implementation of closed-loop feed servo system
Analog system, digital system.
Analog system:
Features:
Strong anti-interference ability, generally do not cause fatal misoperation due to peak error.
You can use conventional instruments (oscilloscopes, multimeters, etc.) to read information directly, making it easy to grasp the basic conditions of the system's work at any time.
Separation of weak signal and noise is difficult, and the improvement of control accuracy is limited.
Near the zero point, it is susceptible to temperature drift, causing drift errors in position control.
The structure and parameter adjustment of the position and speed regulator are difficult, and the ability to adapt to load changes is poor.
The intrinsic deficiencies of analog systems make it difficult to meet the requirements of high-precision position servo control and have been gradually replaced by digital servo systems.
Digital system:
Features: This type of system means that at least its position loop controls and regulates using digital control technology, ie the position command and feedback signals are no longer analog signals to digital signals (logic level pulse signals).
Features:
By increasing the length of the digital information, the required control accuracy can be satisfied.
The drift and noise below the logic power should not be responded to, and the zero positioning accuracy can be fully guaranteed.
It is easy to modify its structure and parameters (according to the control requirements), and it is easy to exchange data with the computer.
When the noise peak value is greater than the logic level, the procedures for the disturbance of the highest and lowest bits of the data are the same. Such errors may cause fatal damage to the system.
Digital circuits that transmit data require a wide frequency band. To ensure that the pulse on the falling edge has enough steepness.
Suppression of interference and prevention of data errors are key to the success of digital servo system design.
Composition: In this system, the position loop control and adjustment operations are mainly performed by the deviation counter (usually a reversible counter) and D/A.
Flexibility difference: The system is made up of hardware, so that its controller parameters are fixed after the electromechanical linkage is adjusted, and it is not easy to change. This is a position servo system with little change in load inertia (such as the lathe tool bed feed control). , Get satisfactory control performance. For some systems with large load inertia, it is difficult to obtain satisfactory control effects over the entire range (load inertia changes). Zero drift will affect accuracy: This type of system relies on D/A to convert the digital output of the position regulation into an analog voltage for the speed command signal. Provided to the speed servo unit, therefore, its zero drift will affect the positioning accuracy.
â— Semi-software servo system
The position control of this kind of system is composed of hardware and software, and the speed control still adopts the simulation mode. The system components are as shown in the figure:
Features:
Strong anti-interference ability, generally do not cause fatal misoperation due to peak error.
You can use conventional instruments (oscilloscopes, multimeters, etc.) to read information directly, making it easy to keep track of the basic conditions of your system's work.
Separation of weak signal and noise is difficult, and the improvement of control accuracy is limited.
Near the zero point, it is susceptible to temperature drift, causing drift errors in position control.
The structure and parameter adjustment of the position and speed regulator are difficult, and the ability to adapt to load changes is poor.
The intrinsic deficiencies of analog systems make it difficult to meet the requirements of high-precision position servo control and have been gradually replaced by digital servo systems.
Digital system:
Features: This type of system means that at least its position loop controls and regulates using digital control technology, ie the position command and feedback signals are no longer analog signals to digital signals (logic level pulse signals).
Features:
By increasing the length of the digital information, the required control accuracy can be satisfied.
The drift and noise below the logic power should not be matched, and the zero positioning accuracy can be fully guaranteed.
It is easy to modify its structure and parameters (according to the control requirements), and it is easy to exchange data with the computer.
When the noise peak value is greater than the logic level, the procedures for the disturbance of the highest and lowest bits of the data are the same. Such errors may cause fatal damage to the system.
Digital circuits that transmit data require a wide frequency band. To ensure that the pulse on the falling edge has enough steepness.
Suppression of interference and prevention of data errors are key to the success of digital servo system design.
Composition: In this system, the position loop control and adjustment operations are mainly performed by the deviation counter (usually a reversible counter) and D/A.
Flexibility difference: The system is made up of hardware, so that its controller parameters are fixed after the electromechanical linkage is adjusted, and it is not easy to change. This is a position servo system with little change in load inertia (such as the lathe tool bed feed control). , Get satisfactory control performance. For some systems with large load inertia, it is difficult to obtain satisfactory control effects over the entire range (load inertia changes). Zero drift will affect accuracy: This type of system relies on D/A to convert the digital output of the position regulation into an analog voltage for the speed command signal. Provided to the speed servo unit, therefore, its zero drift will affect the positioning accuracy.
â— Semi-software servo system
The position control of this kind of system is composed of hardware and software, and the speed control still adopts the simulation mode. The system components are as shown in the figure:
The position-controlled software can now be implemented by the CPU of the NC device, or it can be implemented by the CPU that comes with the position control board.
The position control adjustments are implemented by software, which adds flexibility:
(1) The parameters of the regulator can be modified and set;
(2) The adjustment algorithm can use more complex algorithms to improve the control performance (variable structure, gain);
(3) Many auxiliary functions (failure diagnosis, pulse equivalent transformation, etc.) can be added;
(4) Zero drift can be compensated by software;
Since the speed unit of this type of system is still analog, the problems in the full hardware system are not clearly solved, such as its inner loop parameters (speed, current) and the number of digits of the D/A converter in the position loop. It is fixed. Therefore, it is difficult to balance changes in load inertia. However, due to some compensation measures using software, the position control accuracy and control performance of the semi-software position servo system is higher than that of the full hardware position servo system.
The position and speed control of the microcomputer in the figure can be either a single microcomputer or dual microcomputers (one for position control and the other for speed control). However, the microcomputer in the system often consists of a single-chip microcomputer.
Due to the application of the microcomputer, the control of the system is more flexible. Its features are:
The structure and parameters of the position and speed regulator can be automatically switched according to the working environment, so that the ability to adapt to the load changes is significantly enhanced. The application of the optimization theory can also automate the parameters of the regulator, so that the system can drive different execution machines and generalize The degree has greatly increased.
The other half of the software system.
The output of this system is converted into analog voltage by D/A and sent to the analog current loop as a current command. In this way, the zero drift of the analog will only make a slight change in the current command. Generally, this kind of change is not enough to generate servo motor motion. The torque does not adversely affect the position control accuracy.
Because the structure and parameters of the current loop are still fixed, it is still not possible to change the control strategy through the microcomputer to obtain a better control effect.
Because the system is reliable and compact, the control performance is also superior to the above two systems, making it gradually occupy the dominant position in the AC and DC position servo system products since the mid-1980s, becoming the preferred solution for the position servo system.
â— All-digital position servo system
Since the birth of a software position servo system, software has been used to replace hardware work as much as possible to reduce costs and improve performance. With the emergence of electronic semiconductor devices that can directly control the on and off of logic levels, power transistors, the commercialization of power field rate, and the emergence of high-performance microcontrollers, the realization of all-digital position servo systems has become a reality.
All the control and adjustment of the system is completed by software. Finally, the pulse width modulation control signal of the logic level is directly driven to drive the power transistor amplifier, and the servo motor is controlled to complete the position control task.
All the software of the controller enables many new control methods such as: robust control, adaptive control, variable parameter control, variable structure control, neural network control, fuzzy control, expert system control, etc. in the control theory. and many more. The automatic optimization of parameters and the automatic diagnosis of faults can also be completed so that the system control performance can be further improved.
