Research on Servo System in CNC Machine Tools
I. Overview <br> <br> servo drive device is a mechanical movement, the control object is an electrical motor, to the controller as the core, power electronic power conversion apparatus of the actuator, under the direction of composition of automatic control theory Transmission automatic control system. 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 machine. Specifically, in the numerical control machine tool, the servo system receives the displacement and speed commands issued by the numerical control system. After transformation, adjustment and rectification, the motor and mechanical transmission mechanism drive the machine tool coordinate axes and spindles to drive the table and the tool holder. The linkage of the axes causes the tool to produce various complex mechanical movements relative to the workpiece, thereby machining the workpiece of complex shapes required by the user.
As the actuator of CNC machine tools, the servo system integrates power electronics, control, drive and protection, and has experienced the progress of digital pulse width modulation technology, special motor material technology, microelectronic technology and modern control technology. From stepping to DC, and then to the development of communication. There are many kinds of servo systems in CNC machine tools. This paper analyzes its structure and simple classification, and briefly discusses its technical status and development trends.
Second, the structure and classification of the servo system
From the basic structure, the servo system is mainly composed of three parts: controller, power drive, feedback device and motor (drawings). The controller adjusts the control amount according to the difference between the given value of the numerical control system and the actual running value detected by the feedback device; the power driving device acts as the main circuit of the system, and on the one hand, the electric energy in the power grid is applied to the electric motor according to the magnitude of the control amount. On the other hand, the magnitude of the motor torque is adjusted. On the other hand, the constant voltage constant frequency power supply is converted into the alternating current or direct current required by the motor according to the requirements of the motor; the motor is driven by the mechanical operation according to the power supply.
The main components in the drawings vary widely, and any change in any of them can constitute a different kind of servo system. According to the type of the drive motor, it can be divided into DC servo and AC servo; according to the different methods of controller implementation, it can be divided into analog servo and digital servo; according to the number of closed loops in the controller, it can be divided into Open loop control system, single loop control system, dual loop control system and multi-loop control system. Considering the application of servo system in CNC machine tools, this paper first divides it into feed servo and spindle servo according to the different transmission machinery in the machine tool, and then discusses the technical characteristics of different servo systems according to other factors.
Third, the current status and prospects of the feed servo system
The feed servo takes the coordinates of the CNC machine as the control object and generates the cutting feed motion of the machine tool. For this reason, the feed servo is required to quickly adjust the movement speed of the coordinate axis and accurately perform position control. The specific requirements include wide speed range, high displacement accuracy, good stability and fast dynamic response. According to the motor used in the system, the feed servo can be subdivided into step servo, DC servo, AC servo and linear servo.
(1) Step servo system Step servo is a control system that uses pulse signals to control and convert pulse signals into corresponding angular displacements. The angular displacement is proportional to the number of pulses, and the rotational speed is proportional to the pulse frequency. The rotational speed of the motor can be adjusted by changing the pulse frequency. If some of the windings remain energized after a shutdown, the system also has a self-locking capability. The stepping motor has a fixed number of steps per revolution, such as 500 steps, 1000 steps, 50 000 steps, etc. In theory, the step error will not be accumulated.
The stepping servo structure is simple and meets the needs of the digital development of the system, but the precision is poor, the energy consumption is high, the speed is low, and the power is higher, the moving speed is lower. In particular, step servos are prone to loss of step, making them mainly used for economical CNC machine tools and old equipment that require less speed and accuracy. However, the constant-wave drive, PWM drive, micro-step drive, ultra-micro step drive and hybrid servo technology developed in recent years have greatly improved the high and low frequency characteristics of stepper motors, especially with intelligent ultra-microsteps. The development of drive technology will take the performance of step servo to a new level.
(2) The working principle of DC servo of DC servo system is based on the law of electromagnetic force. Related to electromagnetic torque are two variables of main magnetic flux and armature current, which respectively control the excitation current and armature current, and can easily control torque and speed. On the other hand, from the control point of view, the DC servo control is a single-input and single-output single-variable control system. The classical control theory is completely applicable to this system. Therefore, the DC servo system has simple control and excellent speed regulation performance. The feed drive has dominated.
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