Abstract: Servo control systems, as one of the important control systems in modern industrial production equipment, are an indispensable component of current industrial automation. This paper summarizes the current development trends and research and application achievements of servo control systems, and looks forward to their future development.
1. Introduction
With the rapid development of modern science and technology, especially the technological advancements in microelectronics, power semiconductors, computer and motor manufacturing technologies, position servo systems have been widely used in many high-tech fields and have played an important supporting role in automated control systems, such as robots, laser processing, CNC machine tools, large-scale integrated circuit manufacturing, office automation equipment, radar and various military weapon servo systems, as well as flexible manufacturing systems (FMS).
Servo control systems, as one of the control systems in modern industrial production equipment, are an indispensable foundational technology for industrial automation. Position servo control systems generally aim for sufficient position control accuracy (positioning accuracy), position tracking accuracy (position tracking error), and sufficiently fast tracking speed as their primary control objectives. During system operation, the system is required to track changes in commands with a certain level of accuracy in real time; therefore, the operating speed of the servo motor in the system is often constantly changing. Thus, the requirements for tracking performance in servo systems are generally much higher and more stringent than those in ordinary speed control systems. Due to the limitations of brushes and commutators in DC motors, and the high production and maintenance costs of DC servo systems, with the emergence of new control devices and motor control methods, servo systems have widely replaced DC servo systems.
2. Servo System Classification
Servo systems can be broadly categorized into hybrid digital-analog servos, analog servos, and fully digital servos based on their signal processing methods. They can also be classified into three types based on the type of servo motor used: one type uses permanent magnet synchronous servo motors, including square wave permanent magnet synchronous motor (brushless DC) servo systems and sinusoidal wave permanent magnet synchronous motor servo systems; the other type uses squirrel-cage induction motors. The difference lies in the fact that permanent magnet synchronous motor servo systems require magnetic pole position sensors, while induction motor servo systems include a slip frequency calculation component. If microprocessor software is used to implement servo control, both permanent magnet synchronous servo motors and squirrel-cage induction motors can use the same servo amplifier.
Advantages of 3 servo motors
A stepper motor is a discrete motion device, fundamentally linked to modern digital control technology. It is widely used in current domestic digital control systems. With the emergence of fully digital servo systems, servo motors are also increasingly applied in digital control systems. To adapt to the development trend of digital control, most motion control systems use stepper motors or fully digital servo motors as actuators. Although they are similar in control methods (pulse trains and direction signals), they differ significantly in performance and application. The main differences are as follows.
1. Different control precision
The control precision of a servo motor is ensured by a rotary encoder at the rear end of the motor shaft. Taking a Panasonic all-digital servo motor as an example, for a motor with a standard 2500-line encoder, due to the quadruple frequency technology used in the driver, its pulse equivalent is 360°/10000 = 0.036 °. For a motor with a 17-bit encoder, the motor rotates once for every 2^17 = 131072 pulses received by the driver, meaning its pulse equivalent is 360°/131072 = 9.89 seconds. This is 1/655 of the pulse equivalent of a stepper motor with a step angle of 1.8 °.
2. Different moment-frequency characteristics
The output torque of a stepper motor decreases as the speed increases, and drops sharply at higher speeds. Therefore, its maximum operating speed is generally between 300 and 600 RPM. A servo motor, on the other hand, provides constant torque output, meaning it can output rated torque up to its rated speed (generally 2000 or 3000 RPM) and constant power output above the rated speed.
3. Different low-frequency characteristics
Stepper motors are prone to low-frequency vibration at low speeds. The vibration frequency is related to the load and driver performance, and is generally considered to be half of the motor's no-load starting frequency. This low-frequency vibration, determined by the working principle of stepper motors, is very detrimental to the normal operation of the machine. When stepper motors operate at low speeds, damping techniques should generally be used to overcome low-frequency vibration, such as adding a damper to the motor or using microstepping technology in the driver.
The servo motor operates very smoothly, without vibration even at low speeds. The servo system has resonance suppression capabilities to cover insufficient mechanical rigidity, and its internal frequency response time (FFT) function can detect mechanical resonance points, facilitating system adjustments.
4. Different operating performance
Stepper motors are controlled in an open-loop manner. Excessive starting frequency or load can easily lead to missed steps or stalling. Excessive stopping speed can cause overshoot. Therefore, to ensure control accuracy, the acceleration and deceleration issues must be properly addressed. Servo drive systems, on the other hand, use closed-loop control. The driver can directly sample the feedback signal from the motor encoder, internally forming position and speed loops. Generally, the missed steps or overshoot issues of stepper motors are not present, resulting in more reliable control performance.
5. Different speed response performance
Stepper motors require 200–400 milliseconds to accelerate from a standstill to their operating speed (typically several hundred revolutions per minute). Servo systems offer better acceleration performance; for example, the Panasonic MSMA400W servo motor accelerates from a standstill to its rated speed of 3000 RPM in just a few milliseconds, making it suitable for control applications requiring rapid start and stop.
4. Advantages of Servo Systems and Digital Control
Currently, most digital control systems for servo systems employ a combination of hardware and software control methods, with the software control typically implemented using a microcomputer. This is because microcomputer-based digital servo controllers offer the following advantages over analog servo controllers:
(1) It can significantly improve the reliability of control. The mean time between failures (MTBF) of integrated circuits and large-scale integrated circuits is much longer than that of discrete component electronic circuits.
(2) It can significantly reduce the hardware cost of the controller. With the continuous emergence of new generations of microprocessors with faster speeds and newer functions, hardware costs will become very cheap. Small size, light weight, and low power consumption are common advantages.
(3) Digital circuits have small temperature drift and no parameter influence, resulting in good stability.
(4) The use of microprocessor digital control greatly enhances the bidirectional information transmission capability, makes it easy to connect with the host system, and allows for changes in control parameters at any time.
(5) The hardware circuit is easy to standardize. Some shielding measures are adopted in the circuit integration process, which can avoid electromagnetic interference caused by excessive transient current and voltage in power electronic circuits, thus the reliability is relatively high.
(6) A unified hardware circuit suitable for numerous power electronic systems can be designed, in which the software can be modularly designed and assembled to form control algorithms applicable to various applications; to meet different uses. Software modules can be easily added, modified, deleted, or completely updated when the actual system changes.
(7) With the continuous improvement of the computing speed and memory capacity of microcomputer chips, the control strategy with excellent performance but complex algorithm has a basis for implementation.
(8) It improves the ability of information storage, monitoring, diagnosis and hierarchical control , making the servo system more intelligent.
5. Current Status and Future Prospects of Servo Systems
Over the past decade, the performance of permanent magnet synchronous motors has improved rapidly. Compared with induction motors and ordinary synchronous motors, their advantages such as simple control, good low-speed operation performance, and high cost-effectiveness have made permanent magnet brushless synchronous motors the mainstream actuator motors in servo systems, especially in the field of small and medium power servo applications requiring high precision and high performance. Meanwhile, AC asynchronous servo systems are still mainly concentrated in the field of high-power servo applications with lower performance requirements.
Servo motors are inherently nonlinear, strongly coupled, and time-varying systems. Furthermore, the servo object itself exhibits significant uncertainty and nonlinearity. Coupled with varying degrees of interference during system operation, conventional control strategies are insufficient to meet the control requirements of high-performance servo systems. Therefore, incorporating new developments in control theory and introducing advanced composite control strategies to improve controller performance is a key breakthrough in the development of high-performance servo systems.
Since the late 1980s, with the rapid development of modern industry, increasingly higher demands have been placed on servo systems, one of the important driving sources for industrial equipment. Researching and developing high-performance servo systems has become a consensus among colleagues both domestically and internationally. Some efforts have yielded significant results. On the "hard" side, research directions and efforts include improving the performance of motor materials, refining motor structures, and enhancing the performance and accuracy of inverters and sensing elements. On the "soft" side, research and exploration focus on improving servo system performance from the perspective of control strategies. Examples include "sensorless" estimation of rotor speed and position using the Kalman filter method; improving the structure and performance of PMSM rotors using high-performance permanent magnet materials and processing technologies to eliminate/reduce the impact of PMSM torque ripple caused by cogging torque on system performance; employing robust sliding mode control strategies based on modern control theory to improve the system's adaptability to parameter perturbations; introducing nonlinear and adaptive design methods on top of traditional PID control to improve the system's adjustment and adaptability to nonlinear loads; and intelligent control-based motor parameter and model identification, as well as load characteristic identification.
For the development of high-performance servo systems, under certain conditions, the performance improvement of servo motors, inverters, and corresponding feedback detection devices, which exist in a "hard form," is constrained by many objective factors. On the other hand, control strategies, which exist in a "soft form," have greater flexibility. In recent years, with the new development of control theory, especially the rise and continuous maturation of intelligent control, coupled with the rapid development of computer technology and microelectronics technology, the "integration" of advanced control strategies based on intelligent control and traditional control strategies based on traditional control theory has been realized, laying the material foundation for their practical application.
6 Conclusions
The 21st century will be a century of rapid development in various sciences and technologies. With the rapid development of materials technology, power electronics technology, control science and theoretical technology, microelectronics technology, computer technology, and the gradual improvement of motor manufacturing processes, along with the continuous upgrading of the manufacturing industry and the rapid development of "flexible manufacturing technology," it is believed that "servo drive control technology," one of the core technologies of "flexible processing and manufacturing technology," will inevitably move towards a better development direction.
