introduction
CNC machine tools are a general term for numerically controlled machine tools. They are advanced mechatronic products that integrate the latest achievements in modern precision mechanical design and manufacturing technology, computer technology, communication technology, automatic control technology, testing technology, power electronics and microelectronics technology, motor and new materials technology, hydraulic and pneumatic technology, and optoelectronic technology. As machine tools in the processing and manufacturing industry, they are modern equipment that combines high precision, high flexibility, high efficiency, and high automation. CNC machine tools are important basic equipment in national defense and the national economy. Their applications are widespread in all sectors of the social economy. They are pillar industries in machinery, electronics, automobiles, petrochemicals, and construction, and also important tools for the modernization of basic industries such as energy, transportation, materials, and communications.
In fields such as national defense, aerospace, aviation, military equipment, shipbuilding, space technology, and marine engineering, CNC machine tools are irreplaceable, even indispensable key equipment. CNC machine tools, especially high-end CNC machine tools, reflect a nation's comprehensive economic and scientific and technological strength, holding a strategic position and, in some cases, even impacting the survival of the nation and its people. Therefore, all countries attach great importance to them, and my country is no exception, having consistently implemented a series of measures to support the development of the CNC machine tool industry. However, due to various reasons, my country's technological development still lags behind that of technologically advanced developed countries.
In the early 1980s, with the development of computer software and hardware technology, CNC devices capable of interactive automatic programming emerged. These devices became increasingly miniaturized and could be directly installed on machine tools. The automation level of CNC machine tools further improved, featuring functions such as automatic monitoring of tool breakage and automatic workpiece inspection. After decades of development, current CNC machine tools are computer-controlled and widely used in industry, particularly in the mold manufacturing sector. 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 processing processes such as electrical discharge machining and laser processing.
1. Introduction to the Electro-hydraulic Servo Control System of CNC Machine Tools
Electro-hydraulic servo systems are also called follow-up systems. They can be classified in many ways, from different perspectives, such as position control, speed control, force control, valve control systems, pump control systems, high-power systems, low-power systems, open-loop control systems, closed-loop control systems, etc. Based on the form of the input signal, they can also be divided into analog servo systems and digital servo systems.
In an analog servo system, all signals are continuous analog quantities, as shown in Figure 1. In this system, analog quantities can be either DC or AC. The conversion between DC and AC quantities can be accomplished using a modulator or demodulator.
Figure 1. Block diagram of the analog servo system
Analog servo systems offer high repeatability but lower resolution (lower absolute accuracy). The accuracy of a servo system largely depends on the accuracy of the detection device, and analog detection devices are generally less accurate than digital detection devices. Therefore, analog servo systems have lower resolution than digital servo systems. Furthermore, small signals in analog servo systems are easily affected by noise and zero drift; thus, effective control becomes impossible when the input signal is close to or less than the noise and zero drift limits at the input.
In digital servo systems, all or some signals are discrete parameters. Therefore, digital servo systems are further divided into two types: digital servo systems and digital-analog servo systems. In a fully digital servo system, the power components must be able to receive digital signals, and digital valves or electro-hydraulic stepper motors can be used. A hybrid digital-analog servo system is shown in Figure 2. The command pulses issued by the numerical control device are compared with the feedback pulses to generate a digital deviation. This deviation is converted into an analog deviation voltage by a digital-to-analog converter. The subsequent power components remain unchanged and are still analog. The system output is converted into a feedback pulse signal by a digital detector (i.e., an analog-to-digital converter).
Figure 2 Block diagram of digital-analog servo system
2. Establishment of Mathematical Models for Control System Components
Generally speaking, a mathematical model can be described as a mathematical structure obtained by taking a specific object in the real world, making some necessary simplification assumptions based on its unique inherent laws, and using appropriate mathematical tools for a specific purpose.
