A servo motor is a motor that controls the operation of mechanical components in a servo system; it is an auxiliary motor with indirect speed control. 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, it is used as an actuator and possesses characteristics such as a small electromechanical time constant, high linearity, and low starting voltage. It can convert received electrical signals into angular displacement or angular velocity output on the motor shaft. Servo motors are divided into two main categories: DC and AC. 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 function of a CNC machine tool servo system is to receive command signals from the CNC device, drive the machine tool's moving parts to follow the command pulses, and ensure the speed and accuracy of the movements. This requires high-quality speed and position servo control. The above mainly refers to feed servo control; there is also servo control for the main motion, but the control requirements are not as high as the former. The accuracy and speed of a CNC machine tool often depend primarily on the servo system.
I. Basic Requirements and Characteristics of Servo Systems
1. Basic requirements for servo systems
(1) Good stability: Stability means that the system can reach a new or return to the original equilibrium state after a short adjustment process under a given input or external disturbance.
(2) High precision: The precision of a servo system refers to the degree to which the output can follow the input. As a precision machining CNC machine tool, the required positioning accuracy or contour machining accuracy is usually relatively high, and the allowable deviation is generally between 0.01 and 0.001 mm.
(3) Good fast response: Fast response is one of the indicators of the dynamic quality of a servo system, that is, the response of the tracking command signal must be fast. On the one hand, the transition process time is required to be short, generally within 200ms, or even less than tens of milliseconds; on the other hand, in order to meet the overshoot requirement, the leading edge of the transition process must be steep, that is, the rise rate must be large.
2. Main characteristics of servo systems
(1) Precise detection device: to form a closed-loop control of speed and position.
(2) There are multiple feedback comparison principles and methods: Depending on the principle of information feedback implemented by the detection device, the feedback comparison methods of the servo system are also different. Currently, the three commonly used methods are pulse comparison, phase comparison and amplitude comparison.
(3) High-performance servo motors (hereinafter referred to as servo motors): Used in CNC machine tools for high-efficiency and complex surface machining, the servo system will frequently be in the process of starting and braking. The ratio of the motor's output torque to its moment of inertia is required to generate a sufficiently large acceleration or braking torque. The servo motor is required to have a sufficiently large output torque at low speeds and to run smoothly so as to minimize intermediate links in the connection with the mechanical moving parts.
(4) Wide speed range speed regulation system, i.e. speed servo system: From the perspective of the system's 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 is to convert the position control input into the corresponding speed command signal, and then drive the servo motor through the speed regulation system to achieve the actual displacement. The main motion of the CNC machine tool also requires high speed regulation performance, so the servo system is required to be a high-performance wide speed regulation system.
II. Classification of Servo Systems
Servo systems can be classified according to their driving components, such as stepper servo systems, DC motor servo systems, and AC motor servo systems. They can also be classified according to their control methods, such as open-loop servo systems, closed-loop servo systems, and semi-closed-loop servo systems. In fact, CNC systems are also divided into three types: open-loop, closed-loop, and semi-closed-loop, which are related to these three control methods of servo systems.
1. Open-loop system
An open-loop system mainly consists of three parts: a drive circuit, an actuator, and a machine tool. The commonly used actuator is a stepper motor; an open-loop system using a stepper motor as the actuator is usually called a stepper servo system. In such systems, stepper motors are used as actuators for high-power drives. The main task of the drive circuit is to convert command pulses into signals required to drive the actuator.
2. Closed-loop system
A closed-loop system mainly consists of five parts: an actuator, a detection unit, a comparison circuit, a drive circuit, and a machine tool. Its block diagram is shown in Figure 2. In a closed-loop system, the detection element detects the actual position of the machine tool's moving parts and converts it into an electrical signal, which is then fed back to the comparison circuit. Common detection elements include rotary transformers, inductive synchronizers, optical gratings, magnetic gratings, and encoder disks. A servo system with a detection element mounted on the leadscrew is usually called a semi-closed-loop system; a servo system with a detection element mounted on the worktable is called a closed-loop system. Due to the transmission error between the leadscrew and the worktable, the accuracy of a semi-closed-loop servo system is lower than that of a closed-loop servo system.
The function of the comparison circuit is to compare the command signal and the feedback signal. The difference between the two is used as the following error of the servo system. This error is then transmitted through the drive circuit to control the actuator to move the worktable until the following error is zero. Based on the form of the signal entering the comparison circuit and the feedback detection method, closed-loop (semi-closed-loop) systems can be divided into three types: pulse comparison servo systems, phase comparison servo systems, and amplitude comparison servo systems.
Since the signal output by the comparison circuit is relatively weak and insufficient to drive the actuator, it needs to be amplified, and the drive circuit is designed for this purpose.
The function of an actuator is to convert an electrical signal representing displacement into mechanical displacement based on a control signal, specifically a tracking error signal from the comparison circuit. Commonly used actuators include DC wide-range speed-adjustable motors and AC motors. Actuators are an essential part of a servo system, and the drive circuit varies depending on the actuator used.
Recently, our school has developed a high-performance AC servo (CNC machine tool) control system. This system is stable and reliable, and can be widely used in industrial production applications such as digital engraving, packaging machinery, and mold production. It is also suitable for experimental research in production internships and course design for students (graduate students) majoring in mechatronics, electronics, and electrical automation in higher education institutions.
III. Development Direction of Servo Systems
With the continuous development of productivity, servo systems are required to develop towards higher precision, higher speed, and higher power.
(1) Make full use of the rapidly developing electronic and computer technologies, adopt digital servo systems, use microcomputers to realize adjustment and control, enhance software control functions, eliminate the influence of nonlinear errors and adjustment errors of analog circuits and temperature drift, which can greatly improve the performance of servo systems and create conditions for achieving optimal control and adaptive control.
(2) Develop high-precision, fast detection components.
(3) Develop high-performance servo motors (actuators). Currently, the speed ratio of AC servo motors has reached 1:10000, and their use is increasing. Brushless motors, because they have no brushes and commutator components, have acceleration performance twice as high as DC servo motors and are easier to maintain, and are often used in high-speed CNC machine tools.
