This article mainly introduces what a servo system is. First, it introduces the structure and characteristics of servo systems. Second, it introduces the functions, classifications, performance requirements and parameters of servo systems. Finally, it discusses the development trend of servo systems.
What is a servo system?
A servo mechanism, also known as a follow-up system, is a feedback control system used to accurately follow or reproduce a process.
A servo system is an automatic control system that enables the output controlled variables, such as the position, orientation, and state of an object, to follow any changes in the input target (or given value). Its main task is to amplify, transform, and regulate power according to the requirements of control commands, making the torque, speed, and position control of the drive device highly flexible and convenient.
In many cases, a servo system specifically refers to a feedback control system where the controlled variable (the system's output) is mechanical displacement or displacement velocity/acceleration. Its function is to ensure that the output mechanical displacement (or rotation angle) accurately tracks the input displacement (or rotation angle). Its structural composition is not fundamentally different from other forms of feedback control systems. Servo systems were initially used in defense and military industries, such as artillery control, ship and aircraft autopilot, and missile launches. Later, they were gradually extended to many sectors of the national economy, such as automatic machine tools and wireless tracking control.
Structure and characteristics of servo systems
Main structure
A servo system mainly consists of three parts: a controller, a power drive unit, a feedback unit, and a motor. The controller adjusts the control quantity according to the difference between the given value of the CNC system and the actual operating value detected by the feedback unit; the power drive unit, as the main circuit of the system, on the one hand, applies electrical energy from the power grid to the motor according to the magnitude of the control quantity, adjusting the magnitude of the motor torque, and on the other hand, converts the constant voltage and constant frequency power supply from the power grid into the AC or DC power required by the motor; the motor then drives the mechanical operation according to the power supply.
Main features
1. Precise detection device: forming a closed-loop control system for speed and position;
2. Multiple feedback comparison principles and methods: Depending on the principle by which the detection device implements information feedback, the feedback comparison methods of the servo system also differ. The three most commonly used are pulse comparison, phase comparison, and amplitude comparison.
3. High-performance servo motors (or servo motors for short): Used in CNC machine tools for efficient and complex surface machining, servo systems frequently undergo starting and braking. A high ratio of output torque to moment of inertia is required to generate sufficiently large acceleration or braking torque. Servo motors also need to have sufficiently large output torque and smooth operation at low speeds to minimize intermediate links in their connection with moving mechanical parts.
4. Wide-range speed regulation system, i.e., speed servo system: From the perspective of system control structure, the position closed-loop system of a CNC machine tool can be regarded as a dual closed-loop automatic control system with position regulation as the outer loop and speed regulation as the inner loop. Its actual working process involves converting the position control input into a corresponding speed command signal, and then driving the servo motor through the speed regulation system to achieve actual displacement. The main motion of a CNC machine tool requires high speed regulation performance; therefore, the servo system must be a high-performance, wide-range speed regulation system.
Servo System Functions and Classifications
Main function
1. Using low-power command signals to control high-power loads;
2. In the absence of a mechanical connection, the input shaft controls the output shaft located at a remote distance, achieving remote synchronous transmission;
3. To enable the output mechanical displacement to accurately track electrical signals, such as in recording and indicating instruments.
Main categories
From the perspective of the nature of the system components, there are electric servo systems, hydraulic servo systems, electro-hydraulic servo systems, and electro-electric servo systems, etc.
From the perspective of the physical properties of the system output, there are speed or acceleration servo systems and position servo systems, etc.
Based on the characteristics of the components and the signal interaction features within the system, there are analog servo systems and digital servo systems;
From the perspective of system structure, there are single-loop servo systems, multi-loop servo systems, open-loop servo systems, and closed-loop servo systems.
Servo systems can be classified according to their driving components into stepper servo systems, DC motor servo systems, and AC motor servo systems.
Performance requirements and parameters of servo systems
Performance requirements
The basic requirements for servo systems include stability, accuracy, and fast response.
Good stability: After the disturbance acting on the system disappears, the system can return to its original stable state or reach a new stable operating state under the action of input command signals. Under the action of given input or external disturbance, it can reach a new or return to the original equilibrium state after a short adjustment process.
High precision: The precision of a servo system refers to the accuracy with which the output follows the input. As precision machining tools, CNC machine tools typically require high positioning or contour machining accuracy, with allowable deviations generally between 0.01 and 0.001 mm.
Good responsiveness has two aspects: first, it refers to the speed at which the output changes in response to the input command signal during the dynamic response process; second, it refers to the speed at which the dynamic response process ends. Fast responsiveness is one of the hallmarks of a servo system's dynamic quality, requiring a rapid response to tracking command signals. This necessitates a short transition time, typically within 200ms, or even less than tens of milliseconds; second, to meet overshoot requirements, the transition process must have a steep leading edge, i.e., a high rate of ascent.
High energy efficiency: Due to the rapid response of the servo system, the injection molding machine can quickly adjust the supply according to its own needs, which can effectively improve the utilization rate of the injection molding machine's electrical energy, thereby achieving high efficiency and energy saving.
Main parameters
The main performance indicators for servo systems are bandwidth and accuracy. Bandwidth, defined by the system's frequency response characteristics, reflects the tracking speed of the servo system. A larger bandwidth indicates better speed. The bandwidth of a servo system is primarily limited by the inertia of the controlled object and the actuator. Greater inertia results in a narrower bandwidth. Generally, the bandwidth of a servo system is less than 15 Hz, while the bandwidth of servo systems in large equipment is below 1-2 Hz. Since the 1970s, the development of torque motors and high-sensitivity tachometers has enabled direct drive in servo systems, eliminating or reducing nonlinear factors such as backlash and elastic deformation, achieving bandwidths up to 50 Hz, and leading to successful applications in long-range missiles, artificial satellites, and precision command instruments. The accuracy of a servo system is mainly determined by the accuracy of the measuring elements used.
Therefore, high-precision measuring elements, such as precision potentiometers, synchros, rotary transformers, photoelectric encoders, gratings, magnetic gratings, and ball gratings, must be used in servo systems. Furthermore, additional measures can be taken to improve system accuracy. For example, the measuring axis of the measuring element (such as a synchro) can be connected to the rotating shaft via a reducer, amplifying the rotation angle of the shaft and improving relative measurement accuracy. Servo systems employing this approach are called fine/coarse measurement systems or dual-channel systems. The angle measurement circuit that meshes with the rotating shaft via a reducer is called the fine reading channel, while the angle measurement circuit that takes data directly from the rotating shaft is called the coarse reading channel.
The Development Trend of Servo Systems
1. Communication
Servo technology will continue its rapid shift from DC servo systems to AC servo systems. Currently, almost all new products in the international market are AC servo systems. In industrialized countries, the market share of AC servo motors has exceeded 80%. The number of domestic manufacturers producing AC servo motors is also increasing, gradually surpassing the number of manufacturers producing DC servo motors. It is foreseeable that in the near future, except for certain micro-motor applications, AC servo motors will completely replace DC servo motors.
2. Fully digital
Servo control units employing new high-speed microprocessors and dedicated digital signal processors (DSPs) will completely replace those based primarily on analog electronic devices, thus realizing a fully digital servo system. This full digitalization transforms traditional hardware servo control into software servo control, making it possible to apply advanced algorithms from modern control theory (such as optimal control, artificial intelligence, fuzzy control, and neural networks) within the servo system.
3. Employ new power electronic semiconductor devices
Currently, servo control systems increasingly utilize high-frequency power semiconductor devices for their output circuitry, primarily high-power transistors (GTRs), power MOSFETs, and insulated-gate transistors (IGBTs). The application of these advanced devices significantly reduces power consumption in the servo unit's output circuit, improves system response speed, and reduces operating noise. Particularly noteworthy is the emergence of a new type of module in servo control systems that integrates control circuit functions with high-power electronic switching devices, called Intelligent Power Modules (IPMs). These devices integrate input isolation, energy-saving braking, over-temperature, over-voltage, and over-current protection, as well as fault diagnosis, all within a small module. Their input logic levels are fully compatible with TTL signals and can directly interface with microprocessor outputs. Their application significantly simplifies servo unit design and enables the miniaturization and micro-miniaturization of servo systems.
4. High integration
The new servo system products have changed the traditional approach of dividing servo systems into two modules: a speed servo unit and a position servo unit. Instead, they utilize a single, highly integrated, and multifunctional control unit. This same control unit can have its performance altered simply by setting system parameters through software. It can be used to construct a semi-closed-loop control system using the motor's built-in sensors, or it can be integrated with external position, speed, or torque sensors to form a high-precision fully closed-loop control system. This high level of integration also significantly reduces the overall size of the control system, simplifying installation and commissioning.
5. Intelligent
Intelligentization is a current trend in all industrial control equipment, and servo drive systems, as advanced industrial control devices, are no exception. The latest digital servo control units are typically designed as intelligent products, with intelligent features manifested in the following aspects: First, they all have parameter memory functions. All system operating parameters can be set by software through human-machine interaction and stored internally in the servo unit. These parameters can even be modified by a host computer during operation via a communication interface, making them extremely convenient. Second, they all have fault self-diagnosis and analysis functions. Whenever a system fault occurs, the type of fault and its possible causes are clearly displayed through the user interface, simplifying maintenance and debugging. In addition to these features, some servo systems also have parameter self-tuning capabilities. As is well known, parameter tuning of a closed-loop control system is a crucial step in ensuring system performance and requires significant time and effort. Servo units with self-tuning capabilities can automatically tune the system parameters and achieve optimal performance through several trial runs. For users of servo units, this is one of the most attractive features of new servo systems.
6. Modularization and networking
Abroad, factory automation (FA) engineering technology based on industrial local area network (LAN) technology has developed rapidly in the last decade, showing a strong growth momentum. To adapt to this trend, the latest servo systems are equipped with standard serial communication interfaces (such as RS-232C or RS-422 interfaces) and dedicated LAN interfaces. These interfaces significantly enhance the interconnectivity between servo units and other control devices, making the connection with CNC systems very simple. Only a single cable or fiber optic cable is needed to connect several, or even dozens, of servo units to a host computer to form an entire CNC system. They can also be connected to the CNC modules of programmable logic controllers (PLCs) via serial interfaces.
