What is a servo control system?
A servo control system is an operating system that automatically controls the mechanical motion of a testing device according to predetermined requirements. In many cases, a servo system specifically refers to a feedback control system where the controlled variable (the system's output) is mechanical displacement, displacement velocity, or acceleration. Its function is to ensure that the output mechanical displacement (or rotation angle) accurately tracks the input displacement (or rotation angle). The structural composition of a servo system is not fundamentally different from other forms of feedback control systems.
Key Indicators of Servo Control Systems
The main indicators for evaluating the performance of a servo control system are system accuracy, stability, response characteristics, and operating frequency, with particular emphasis on bandwidth and accuracy.
Frequency bandwidth, or simply bandwidth, is defined by the frequency response characteristics of a system and reflects the tracking speed of a 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 a large-scale equipment servo system 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, and rotary transformers, must be used in servo systems. Additionally, further measures can be taken to improve system accuracy, such as connecting the measuring axis of a measuring element (e.g., a synchro) to a rotating shaft via a reducer, thus amplifying the rotation angle and improving relative measurement accuracy. Servo systems employing this approach are called precision and coarse measurement systems or dual-channel systems. The angle measurement circuit that meshes with the shaft via a reducer is called the precision reading channel, while the angle measurement circuit that takes data directly from the shaft is called the coarse reading channel.
Applications of servo control systems
In CNC machine tools, the servo control system is an indispensable part. Its task is to convert CNC information into machine tool feed motion, thereby achieving precise control. Because CNC machine tools have high requirements for product processing, the servo control system used is crucial.
The advantages of servo control systems used in CNC machine tools mainly include: high precision (the accuracy of a servo system refers to the degree to which the output can accurately reproduce the input, including positioning accuracy and contour machining accuracy); good stability (stability means that under given input or external disturbances, the system can reach a new or return to its original equilibrium state after a short adjustment process, directly affecting the accuracy and surface roughness of CNC machining); fast response (fast response is an important indicator of the dynamic quality of a servo system, reflecting its tracking accuracy); wide speed range (its speed range can reach 0-30m /min) ; and high torque at low speeds (the servo control of the feed coordinate is constant torque control, maintaining this torque throughout the entire speed range; the servo control of the spindle coordinate is constant torque control at low speeds, providing a large torque, and constant power control at high speeds, with sufficiently large output power).
The primary feed servo systems used in machine tools are permanent magnet synchronous AC servo systems, which come in three types: analog, digital, and software. Analog servos have a single purpose, receiving only analog signals, and position control is typically implemented by a host computer. Digital servos can perform multiple functions, such as speed, torque, and position control. They can receive analog and pulse commands, and all parameters are set digitally, offering good stability. They also have rich self-diagnostic and alarm functions. Software servos are microprocessor-based fully digital servo systems. They implement the monitoring programs for various control methods and servo motors of different specifications and power levels in software. During use, the user sets codes and related data, and the system automatically enters the working state. Equipped with a digital interface, changing the operating mode or replacing the motor specifications only requires resetting the code; hence, they are also called universal servos.
AC servos have become the dominant force in machine tool feed servos and are constantly improving with the development of new technologies, specifically in three aspects. First, the power electronic devices in the system power drive are constantly developing towards higher frequencies, and intelligent power modules are becoming more widespread and applied. Second, the maturity of microprocessor-based embedded platform technology will promote the application of advanced control algorithms. Third, the promotion of networked manufacturing models and the maturity of fieldbus technology will make network-based servo control possible.
