introduction
CNC machine tools, short for Computer Numerical Control Machine Tools, are automated machine tools equipped with a program control system. This control system logically processes and decodes programs containing control codes or other symbolic instructions, thereby enabling the machine tool to move and process parts. CNC machine tools have wide adaptability; when the workpiece changes, only the input program instructions need to be changed. Their machining performance is higher than that of general automatic machine tools, and they can accurately machine complex surfaces. Therefore, they are suitable for processing small to medium batches of workpieces that require frequent modifications, high precision, and complex shapes, while also achieving good economic results.
With the development of CNC technology, the variety of machine tools using CNC systems is increasing, including lathes, milling machines, boring machines, drilling machines, gear processing machines, and electrical discharge machining (EDM) machines. In addition, there are machining centers and turning centers that can automatically change tools and perform multiple machining operations in a single setup. After decades of development, current CNC machine tools have achieved computer control and are widely used in industry, especially in the mold manufacturing industry. Various categories of CNC machine tools have been developed to meet the needs of metal cutting processes such as turning, milling, grinding, drilling, and planing, as well as special machining processes such as EDM and laser processing.
Position servo control systems are a type of automatic control system. Therefore, when analyzing and designing such control systems, it is necessary to use automatic control principles as their theoretical foundation to study the dynamic performance of the entire system, and further investigate how to assemble various components into a stable control system with stable performance indicators. If the original system is unstable, it can be stabilized by adjusting the proportional parameters and using hysteresis compensation, and appropriate parameters can be selected to ensure the system meets design requirements.
1. Composition of a position servo system
1.1 Input Elements
Also known as a command element, it provides an input signal (command signal) to the system's input terminal. This element can be mechanical, electrical, pneumatic, etc.
1.2 Feedback Measurement Element
The system measures the output of the measurement system and converts it into a feedback signal; it is a position sensor, similar to the human eye.
1.3 Comparison Element
The feedback signal is compared with the input signal to give the deviation signal.
1.4 Amplification and Conversion Components
An adjustment action is generated and applied to the controlled object to achieve the adjustment task.
1.5 Actuating Element
Adjustment actions are applied to the controlled object to achieve the adjustment task. Examples include hydraulic cylinders and hydraulic motors.
In addition, there may be various correction devices, as well as hydraulic power devices not included in the control circuit.
2. Working principle of position servo system
Based on the design requirements of the CNC machine tool, the system has relatively low control power and a relatively large table stroke, so a valve-controlled hydraulic motor system is adopted. The system block diagram is shown in Figure 1.
A CNC machine tool table position servo system is an automatic control system that controls the movement of the CNC machine tool table. As the actuator of the CNC machine tool, the position servo system integrates power electronic devices, control, drive, and protection. The output displacement of the CNC machine tool table position servo system can automatically, quickly, and accurately reproduce changes in the input displacement.
Because the worktable output has a displacement detection device (displacement sensor) that converts the displacement signal into an electrical signal and feeds it back to the input, forming a negative feedback closed-loop control system. The feedback signal is compared with the input signal to obtain a differential pressure signal. This differential pressure signal is then converted into a current signal by a servo amplifier and sent to an electro-hydraulic servo valve (electro-hydraulic converter and power amplifier element) to convert it into a high-power hydraulic signal (flow and pressure) output. This causes the four-way spool valve of the hydraulic motor to open, resulting in pressurized oil output to the hydraulic motor, which drives the hydraulic motor to rotate the reduction gear, thereby driving the ball screw. Since the ball screw is connected to the worktable, the worktable also undergoes a corresponding displacement when the ball screw moves.
3. Establishment of Mathematical Model for CNC Machine Tools
3.1 Calculation of Total Load Force of Machine Tool
Figure 3 System dynamic response curve
6 Conclusions
