Abstract: With the continuous improvement of automation levels, more and more industrial control applications require precise position control. Therefore, how to achieve position control more conveniently and accurately is an important issue in the field of industrial control. This paper studies a servo control system, taking the servo motor as the controlled object, the controller as the core, and the power electronic power conversion device as the actuator, and constructs an electric drive automatic control system based on automatic control theory. By utilizing the digital and analog output control of a Siemens 200 PLC and the servo controller, precise control of the servo motor speed is achieved, improving the reliability and accuracy of the system control.
Keywords : PLC, servo; control system;
Intermediate Classification Number : TP9 Document Identification Code: B
0 Introduction
Thanks to the continuous advancements in modern microelectronics technology and the excellent control characteristics of power electronic circuits, almost all new control theories and methods have been applied and tested in AC speed control devices. Modern control theory is increasingly penetrating the field of AC speed control, especially with the development of microcomputers and large-scale integrated circuits, which is driving AC motor speed control technology towards higher frequencies, digitalization, and intelligence.
In recent years, research on control technology for power electronic devices has been very active. Various modern control theories, such as adaptive control and sliding mode variable structure control, as well as intelligent control and high dynamic performance control, are all research hotspots. This research will undoubtedly elevate AC speed control technology to a new level. The softening of control systems places higher demands on CPU chips. To achieve high-performance AC speed control, vector coordinate transformations, online calculation of flux vectors and adaptation of flux models to parameter changes, as well as online real-time adjustment of overlapping outer loop control for internal acceleration, velocity, and position are all required. All of these necessitate the storage of various data and the rapid real-time processing of large amounts of information. It is foreseeable that with the increase in computer chip capacity and the acceleration of computing speed, the performance of AC speed control systems will be greatly improved.
Since the 1980s, with the development of integrated circuits, power electronics technology, and AC variable speed drive technology, permanent magnet AC servo drive technology has made remarkable progress. Leading electrical manufacturers worldwide have successively launched their own series of AC servo motors and servo drives, continuously improving and updating them. AC servo systems have become the main development direction of contemporary high-performance servo systems, threatening the obsolescence of traditional DC servos. Since the 1990s, commercially available AC servo systems worldwide have adopted fully digitally controlled sinusoidal wave motor servo drives. The development of AC servo drive devices in the transmission field is progressing rapidly. Currently, the main problems restricting the industrial application of PLC-based control servo systems include: increased initial investment due to intelligent equipment supporting PLC control servo systems; changes in system structure and how to construct a PLC-based servo control system; communication reliability; and limitations in equipment selection, among other user concerns.
1. Basic Structure of Control System
The main components of a servo speed control system are shown in Figure 1. The servo motor is the controlled object and transmission device. The servo driver converts power energy with a certain voltage, current, and frequency into power energy with adjustable voltage, current, or frequency, performing energy conversion and control functions to verify and transform feedback signals. The main feedback quantities in a servo speed control system include voltage, current, speed, torque, magnetic flux, and rotor position angle. The control layer generates the required control commands and deviation signals based on the given and feedback signals. The regulating device controls the energy flow of the converter according to a certain rule, generating algorithms or correction quantities that meet the control requirements through hardware or software to improve or correct the static performance of the system. In applications with less stringent requirements, open-loop control is used without a feedback device, provided that the motor itself has sufficient stability and adjustability.
Figure 1. Basic structural diagram of the servo speed control system
2 servo drives
2.1 Servo Driver
A servo motor controller is a type of controller that controls servo motors. Its function is similar to that of a frequency converter for ordinary AC motors. It is part of a servo system and is primarily used in high-precision positioning systems. Generally, it controls the servo motor through three methods: position, speed, and torque, achieving high-precision positioning of the transmission system. Currently, it represents a high-end product in transmission technology.
2.2 Servo Control System Speed Regulation Control System and its Principle
A servo motor speed control system consists of a servo driver, a motor, and its control system. The servo speed control system changes the synchronous speed of the asynchronous motor by altering the power supply frequency to the stator. Its speed control characteristics largely maintain the inherent mechanical characteristics of a servo motor, such as high mechanical stiffness and low slip, while also offering advantages such as high efficiency, wide speed range, high precision, and smooth speed control. The working principle diagram of servo speed control is shown in Figure 2.
(a) Schematic diagram of servo driver operation (b) Block diagram of servo speed control operation
Figure 2. Working principle diagram of servo speed regulation
As the formula shows, changing the motor frequency and the number of poles can both change the motor speed. Therefore, changing the motor frequency can achieve speed regulation.
The main component of a servo drive system is the servo drive that provides variable frequency power. Servo drives can be divided into two main categories: AC-DC-AC drives and AC-DC inverters. Currently, most domestic systems use AC-DC-AC drives. Their advantages include high efficiency, no additional losses during speed regulation, wide application range, large speed range, and high precision.
Changing the stator power supply frequency can change the synchronous speed and the motor speed. Furthermore, according to the electric potential formula for a motor, the applied voltage is approximately proportional to the product of the frequency and the magnetic flux.
As shown in the above formula, if the applied voltage remains constant, the magnetic flux changes with the frequency. Generally, in the design of motors, the magnetic flux Φ is selected close to the magnetic saturation value to fully utilize the core material. Therefore, if the frequency is reduced from the rated value, the magnetic flux will increase, causing magnetic circuit oversaturation, increased excitation current, and core overheating, which is unacceptable. Therefore, we need to reduce the voltage while reducing the frequency, which requires coordinated control of frequency and voltage.
3. System Analysis
The circuit diagram of the electric motor bus-type speed control system is shown in Figure 3:
Figure 3 Servo motor speed control system circuit
Control Circuit Analysis: The PLC is the main controller of this system. The PLC is connected to a servo driver via a special cable; the driver is then connected to a servo motor; the servo motor sends a feedback signal to the PLC through the servo driver, and this feedback signal is connected to the PLC's analog input terminal. This facilitates more precise and faster control. The user program controls the PLC's actions, which in turn trigger the servo driver's response, thereby controlling the motor speed. The encoder is connected to a 24V DC power supply. The servo driver is connected to a 220V AC power supply. The industrial control computer is connected to the PLC via a PPI cable, allowing the user to observe the control system's operation through configuration software. This enables remote control of the motor speed regulation system.
4 Software Design
When programming, based on the fundamental principle that PLCs execute programs sequentially in a cyclic scanning manner, the ladder diagram is drawn line by line from top to bottom according to the order of actions. This is often clearer and easier to understand than the ladder diagram program drawn from relay control circuits.
A screenshot of the subroutine SBR_0 is shown in Figure 4:
Figure 4 shows a portion of the SBR_0 program.
Program Analysis :
DB1 is the main data block, storing the main data during system program execution. The user defines the names and storage addresses of the main data in the symbol definition table. SBR_0 is a user-written program block whose main functions are: SBR_0 performs analog-to-digital conversion, facilitating PLC calculations since PLCs can only process digital quantities; and it also allows for easy observation using configuration software. When the system powers on, the program scans sequentially. When SBR_0 is encountered, it first calls the subroutine SBR_0 to convert the input analog quantity into a digital quantity, which is then sent to the PLC for calculation. The data is stored in the main data block DB1. After some inverter control commands and data transmission instructions, the subroutine FC2 is finally called to convert the PLC's calculation result back into an analog output, which is also stored in the symbol definition table, allowing the user to observe the system's operation and data changes using their custom configuration software.
5. Conclusion
Servo motors, PLCs, contactors, etc., can be installed in a single control cabinet, allowing for local or remote operation with simple and flexible operation. However, attention must be paid to the heat dissipation and electromagnetic interference issues of the servo controller and motor. Motor startup may cause interference to the servo controller. When switching between speed-regulating power supply modes, the closing and opening sequence of each contactor must be carefully observed. The contactors should have sufficient delay to prevent the induced electromotive force generated by the motor windings from being applied to the output inverter bridge of the servo driver, causing damage to the motor. Siemens PLCs offer advantages such as comprehensive functionality, modular programming, simplicity, and reliability. Furthermore, their configuration screens facilitate debugging, making them convenient for on-site operators.
