A feed axis servo drive system (servo system) is an automatic control system that controls the position or angle of a machine tool, such as a CNC machine tool. The drive motor used for feed axis servos requires fast response, accurate positioning, and large moment of inertia. Servo motors used in electromechanical systems have large moments of inertia so that they can be directly connected to mechanical components, such as lead screws.
Feed axis servos utilize a special small-inertia motor to achieve high response speeds. However, this type of motor has low overload capacity and requires acceleration and deceleration devices when used in feed servo systems.
Moment of inertia reflects the acceleration characteristics of the system. When selecting a servo motor, the system's moment of inertia should not exceed three times the motor's moment of inertia. Motors with relatively large inertia, often used in enterprise-specific applications, can be called servo control motors.
Of course, the basic working principle of a feed axis servo is no different from that of a regular AC-DC motor. The special drive unit of this type of motor is called a servo drive unit, sometimes simply called a servo drive module, which typically includes current, speed, and/or position closed loops.
Overview of Feed Axis Servo Systems
Feed axis servo systems use mechanical transmission devices and motors as the controlled objects, with a controller at the core and power electronic conversion devices as the actuators. Electrical drive automatic control systems are composed under the guidance of automatic control theory.
This type of system controls the torque, speed, and angle of the motor, converting electrical energy into mechanical energy to achieve the motion requirements of the moving machinery. In CNC machine tools, the servo system receives the direction of displacement and velocity from the CNC machine tool system. After transformation, adjustment, and refinement, the motor and mechanical transmission mechanism drive the machine tool's coordinate axes, spindle, etc., and drive the worktable and tool post. Through the linkage of the axes, the tool generates various complex mechanical movements relative to the workpiece, thereby machining the complex workpiece shapes required by the user.
As the actuator of a CNC machine tool, the servo system integrates power electronic devices, control, drive, and protection functions. With the advancement of digital pulse width modulation technology, special motor material technology, microelectronics technology, and modern control technology, it has undergone a development process from stepper motors to DC motors, and then to AC motors. There are many types of servo systems in CNC machine tools. This article analyzes the structure and simple classification of feed axis servo systems, briefly discussing their current technological status and development trends.
Structure and Classification of Feed Axis Servo Systems
From a basic structural perspective, a servo system mainly consists of three parts: a controller, a power drive unit, and a feedback unit, and a motor. The controller adjusts the control quantity based on the difference between the given value of the CNC system and the actual operating value detected by the feedback unit. The power driver, as the main drive circuit of the system, applies power to the motor, adjusts the power supply from constant voltage to the motor, and drives the machine according to the power supply.
The main components vary greatly, and any change in any component can constitute different types of servo systems. For example, based on the type of drive motor, they can be divided into DC servo and AC servo; based on the implementation method of the controller, they can be divided into analog servo and digital servo; based on the number of closed loops in the controller, they can be divided into digital servo. It can be further divided into open-loop control systems, single-loop control systems, dual-loop control systems, and multi-loop control systems.
Regarding the application of feed axis servo systems in CNC machine tools, based on the different machine tool transmission mechanisms, servo systems are divided into feed servo systems and spindle servo systems. Then, the technical characteristics of different servo systems are analyzed based on other factors.
Current Status and Future Prospects of Feed Servo Systems
Feed servos control the coordinate axes of a CNC machine tool, generating motions that influence the cutting feed direction.
Therefore, feed servos are needed to rapidly adjust the speed of the coordinate axes and accurately control their position. They require a wide speed range, high displacement accuracy, good stability, and fast dynamic response. Based on the motor used in the system, feed servos can be classified into stepper servos, DC servos, AC servos, and linear servos.