Abstract: This paper studies a servo control system, using a servo motor as the controlled object, a controller as the core, and a power electronic power conversion device as the actuator. Based on automatic control theory, an automatic control system for electric drives is constructed. By utilizing the digital and analog output control of a Siemens 300 PLC and the servo controller, precise control of the servo motor speed is achieved. This improves the reliability and accuracy of the system control and meets the needs of industrial production.
1 Introduction
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 transformation, online calculation of magnetic flux vectors and correction of the magnetic flux model to adapt to parameter changes, and online real-time adjustment of overlapping outer loop control of internal acceleration, speed, 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. AC servo technology—AC servo motors and AC servo control systems—is gradually becoming the dominant products. AC servo drive technology has become one of the fundamental technologies for automation in the industrial field and will gradually replace DC servo systems.
The rapid development of science today has blurred the lines between disciplines, leading to mutual influence and penetration. From a developmental perspective, applying modern control theories such as neural networks, fuzzy control, and sliding mode variable structure control to servo motor speed control technology is of paramount importance and holds broad promise. This can be considered one of the development directions for servo motor speed control technology. Furthermore, other new technologies in the control field, such as fieldbus, adaptive control, and genetic algorithms, will also be introduced into the AC drive field, significantly impacting servo motor speed control technology.
Siemens PLCs are developed by internationally renowned automation companies and are specifically designed for industrial automation applications. Servo systems are used in highly automated industries, characterized by high reliability requirements, numerous and complex monitoring devices and objects, and high real-time performance demands. With the increasing maturity of servo technology and remote control, and the growing demand for information technology, fieldbus-based control systems will provide a better option for power industry automation systems. Currently, the main challenges hindering the industrial application of PLC-based servo control systems include: increased initial investment due to the availability of intelligent devices supporting PLC-based servo control systems; changes in system architecture and how to construct a PLC-based servo control system; communication reliability; and limitations in equipment selection.
2. Composition and Drivers of AC Servo
2.1 Composition of AC Servo Motor
Servo motors enable highly accurate speed and position control, converting voltage signals into torque and speed to drive the controlled object. The rotor speed of a servo motor is controlled by the input signal and can respond quickly. In automatic control systems, they are used as actuators and possess characteristics such as a small electromechanical time constant, high linearity, and low starting voltage. They can convert received electrical signals into angular displacement or angular velocity output on the motor shaft. Servo motors are broadly classified into DC and AC servo motors. Their main characteristic is that they do not rotate when the signal voltage is zero, and their speed decreases uniformly as the torque increases.
The structure of an AC servo motor can be mainly divided into two parts: the stator and the rotor. The structure of the stator is basically the same as that of a rotary transformer. Two-phase windings with a spatial electrical angle of 90° are also placed in the stator core, as shown in Figure 1. L1-L2 are called the excitation windings, and k1-k2 are called the control windings. Therefore, an AC servo motor is a two-phase AC motor.
The commonly used rotor structures include squirrel-cage rotors and non-magnetic cup-shaped rotors. The structure of a squirrel-cage AC servo motor is shown in Figure 2. Its rotor consists of a shaft, a rotor core, and rotor windings. The rotor core is made of stacked silicon steel sheets, each stamped into a toothed and slotted shape, as shown in Figure 3. These sheets are then stacked to press the shaft into the shaft hole. A conductor bar is placed in each slot of the core, and all conductor bars are connected at both ends by two short-circuit rings, which constitutes the rotor windings.
2.2 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.
A servo motor speed control system consists of a servo driver, a motor, and its control system. By changing the power supply frequency to the stator of the asynchronous motor, the servo speed control system alters the synchronous speed of the motor. Its speed control characteristics largely maintain the inherent mechanical characteristics of servo motors, 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 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.
3 Control System Design
3.1 System Hardware Design
The PLC controller serves as the main controller of the system. The servo drive can be easily connected to the PLC control network via its built-in communication module. A ladder diagram program is designed using the S7-300 programming software Step7 and downloaded to the PLC controller, enabling remote control of the servo drive based on the S7-300, thereby achieving motor speed regulation. The computer is used for system configuration, monitoring, and programming. A PPI cable handles communication between the PLC controller and the computer. The PLC controller performs sequential and drive control, transmitting controller commands to the servo drive via a dedicated servo drive line, while also receiving servo drive status and providing real-time feedback. The control program transmits speed setpoint commands to the PLC controller in control word data format, enabling automatic control of the servo drive. Figure 4 shows the block diagram of the servo motor control system.
3.2 Control Circuit
The PLC serves as 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. 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.
3.3 System Composition
The Siemens S7-300 series PLC features a modular design, allowing for extensive combination and expansion of individual modules. Its system composition includes a power supply module (PS), a central processing unit (CPU), a signal module (SM), and function modules (FM). It connects directly to the programmer (PG), operator panel (OP), and other S7 PLCs via a PPI network interface.
Load power supply module (PS): Used to connect Siemens S7-300 to a 120/230V AC power supply or a 24/48/60/110V DC power supply.
Central Processing Unit (CPU): Different CPUs have different performance characteristics and are generally selected based on industrial needs.
Signal Module (SM): Used for digital and analog input/output.
Functional Module (FM): Used for high-speed calculation, positioning operations (open-loop and closed-loop positioning), and closed-loop control.
4 System Simulation
In WinCC, a dialog box is set for the speed input value, allowing a 4-digit value to be entered. The data attributes in this dialog box are then set to the corresponding integer variable data in the PLC (e.g., AIW0). The purpose is that after a value is entered in the dialog box, the motor should reach the corresponding speed. The PLC outputs an analog signal of 0-10V, corresponding to an integer value of 0-32000; while the servo motor's analog input is 0-10V, corresponding to a speed of 0-6500 RPM. Since these values are theoretical, and the ultimate goal is for the input value to correspond to the speed, the analog signal is only used as an intermediate reference. The key considerations are the input value, the integer value, and the actual speed. Direct testing data is shown in Table 1.
As shown in Table 1, the input value differs significantly from the actual rotational speed. The only solution is to convert the input value into an integer value that corresponds to the actual rotational speed through calculation. However, it is necessary to first find the values corresponding to the highest and lowest rotational speeds. Experiments revealed the corresponding relationship, as shown in Table 2.
The analog output of the PLC and the speed output of the servo motor are both linear. Based on the data in Table 2, a system of linear equations can be listed to calculate the relationship between the input value and the integer value.
5. Conclusion
Servo motors, PLCs, contactors, and other components can be installed in a single control cabinet, allowing for local or remote operation with simple and flexible operation. Siemens PLCs offer advantages such as comprehensive functionality, modular programming, simplicity, and reliability. Furthermore, their configuration screens are easy to debug, facilitating on-site operations. Configuration software, as a user-customizable platform tool, leverages its excellent communication capabilities and human-machine interface to develop a user-friendly interface on a PC, enabling real-time monitoring and providing system fault alarms and report printing. The S7-300-based servo motor speed control system described in this paper fully utilizes these features of Siemens PLCs.
