CNC machine tool servo systems, also known as position follow-up systems or simply servo systems, are actuators that convert numerical control information into machine tool feed motions. They are widely used in many fields of automation control. The variety of CNC machine tool servo systems and their unique technical principles can pose significant challenges to their application. This article provides a brief introduction to the classification of CNC machine tool servo systems and their application requirements.
I. Classification of CNC Machine Tool Servo Systems
CNC machine tool servo systems are classified according to their application and function into feed drive systems and spindle drive systems; according to their control principle and the presence or absence of position detection feedback loops into open-loop systems and closed-loop systems; and according to the operating principle of the drive actuators into electro-hydraulic servo drive systems and electric servo drive systems. Electric servo drive systems are further divided into DC servo drive systems and AC servo drive systems.
1. Feed drive and spindle drive
Feed drives are control systems used in the coordinate systems of CNC machine tool worktables or tool holders. They control the cutting feed motion of each coordinate axis of the machine tool and provide the torque required for the cutting process. Spindle drives control the rotational motion of the machine tool spindle, providing drive power and the required cutting force. Generally, for feed drive systems, the main concerns are the magnitude of the torque, the size of the adjustment range, the level of adjustment accuracy, and the speed of dynamic response. For spindle drive systems, the main concerns are whether they have sufficient power, a wide constant power adjustment range, and a wide speed adjustment range.
2. Open-loop control and closed-loop control
CNC machine tool servo drive systems are classified into two basic control structures based on the presence or absence of position feedback: open-loop control and closed-loop control, as shown in Figure 5-1. This results in open-loop and closed-loop position control systems. Closed-loop control systems can be further divided into semi-closed-loop and fully closed-loop servo drive control systems depending on the installation position of the position detection device on the machine tool. If the position detection device is installed on the machine tool's worktable, the resulting servo drive control system is a fully closed-loop control system; if the position detection device is installed on the machine tool's leadscrew, the resulting servo drive control system is a semi-closed-loop control system. Modern CNC machine tools mostly employ closed-loop control systems for their servo drives. Open-loop control systems are commonly used for economical CNC systems or for retrofitting older equipment.
3. DC servo drive and AC servo drive
In the 1970s and early 1980s, CNC machine tools mostly adopted DC servo drives. High-inertia DC servo motors have excellent wide speed range performance, high output torque, and strong overload capacity. Moreover, because the motor's inertia is comparable to that of the machine tool's transmission components, it is easy to adjust after forming a closed loop. Meanwhile, low-to-medium inertia DC servo motors and their high-power transistor pulse-width modulation (PWM) drive devices are more suitable for the frequent starts, stops, and rapid positioning and cutting requirements of CNC machine tools. However, a major characteristic of DC motors is the presence of brushes and mechanical commutators, which limits their development towards larger capacity, higher voltage, and higher speeds, thus restricting their application.
In the 1980s, AC motor speed control technology made breakthrough progress in the field of motor control, and AC servo drive systems were widely adopted in various fields of electrical drive speed control. The biggest advantage of AC servo drive systems is that AC motors are easy to maintain, simple to manufacture, and easy to develop towards large capacity and high speed, making them suitable for use in harsh environments. At the same time, from the perspective of reducing the size of servo drive systems and improving reliability, using AC motors is more reasonable than DC motors.
II. Application Requirements of CNC Machine Tool Servo Systems
(1) High precision: The precision of a servo system refers to the degree to which the output can accurately reproduce the input. This includes positioning accuracy and contour machining accuracy.
(2) 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. It directly affects the accuracy and surface roughness of CNC machining.
(3) Fast response Fast response is an important indicator of the dynamic quality of a servo system, reflecting the tracking accuracy of the system.
(4) Wide speed range. The speed range refers to the ratio of the highest speed to the lowest speed that the motor can provide, which is required by the production machinery. 0~30m/min.
(5) Servo control for low-speed, high-torque feed coordinates is constant torque control, which maintains this torque throughout the entire speed range; servo control for spindle coordinates is constant torque control at low speeds, providing a large torque. At high speeds, it is constant power control, with sufficiently large output power.
