Currently, stability and reliability are paramount for CNC machine tools, and the stability and reliability of the CNC system are crucial for ensuring the stable and reliable operation of CNC machine tools. In the context of economic globalization and China's status as a manufacturing powerhouse, CNC machine tools are typically used continuously for extended periods in industrial environments with harsh electromagnetic conditions, under high intensity and frequency. Therefore, to ensure the normal operation of the CNC system and CNC machine tools, especially given the significant fluctuations in my country's power grid quality, the electromagnetic compatibility (EMC) design of the CNC system is even more critical. Electromagnetic compatibility (EMC) refers to the ability of a CNC system to operate normally in its electromagnetic environment without causing unacceptable electromagnetic interference to anything in that environment. This article will discuss the EMC design of CNC systems using the Huazhong CNC system as an example.
1 Electromagnetic compatibility design requirements for CNC systems
To ensure the CNC system can operate normally for extended periods and continuously in industrial environments with harsh electromagnetic conditions, the system must meet the electromagnetic compatibility requirements in JB/T 8832-2001 "General Technical Conditions for CNC Systems of Machine Tools".
1.1 Electrostatic Discharge Immunity Test
When the CNC system is running, in accordance with the provisions of GB/T 17626.2, electrostatic discharge tests are conducted between all parts frequently touched by the operator and the protective grounding terminal (PE). The contact discharge voltage is 6kV and the air discharge voltage is 8kV. The CNC system can operate normally during the test.
1.2 Electrical Fast Transient/Bulk Immunity Test
When the CNC system is running, in accordance with GB/T 17626.4, a 2kV peak pulse group with a repetition frequency of 5kHz is applied between the AC power supply terminal and the protective ground terminal (PE) for 1 minute; a 1kV peak pulse group with a repetition frequency of 5kHz is applied to the I/O signal, data, and control port cables using coupling clamps for 1 minute. During the test, the CNC system can operate normally.
1.3 Surge (Impact) Immunity Test
During CNC system operation, in accordance with GB/T 17626.5, a surge (impulse) voltage with a peak value of 1kV is superimposed between the phase lines of the AC input power supply; and a surge (impulse) voltage with a peak value of 2kV is superimposed between the phase lines of the AC input power supply and the protective earth (PE) terminal. The surge (impulse) repetition rate is 1 time/min, with positive and negative polarities. The test is performed 5 times each for positive and negative polarities, and the CNC system should operate normally.
1.4 Voltage sag and short-term interruption immunity test
When the CNC system is running, according to GB/T 17626.11, the AC input power supply voltage amplitude drops to 70% of the rated value at any time for a duration of 500ms, with a successive drop interval of 10s; according to GB/T 5226.1—1996, section 4.3, the AC input power supply voltage experiences a short interruption of 3ms at any time, with a successive interruption interval of 10s. The CNC system should operate normally after three voltage dips and three short interruptions.
2 Shielding Technology in Electromagnetic EMC Design of CNC Systems
CNC machine tool systems contain both high-voltage, high-current power equipment and low-voltage, low-current control and signal processing equipment, as well as sensors and other low-voltage devices. The strong electromagnetic interference generated by the power equipment during operation poses a significant threat to the normal operation of the low-voltage equipment. Furthermore, the harsh electromagnetic environment of the production site, including various external power loads, power supply systems, and atmospheric disturbances, can interfere with the control of the low-voltage equipment, causing it to malfunction and ultimately paralyzing the entire system. To ensure the normal operation of the CNC system, shielding technology is one of the three commonly used techniques in CNC system electromagnetic compatibility design.
Shielding technology is used to suppress the propagation of electromagnetic noise through space, that is, to cut off the transmission path of radiated electromagnetic noise. Typically, the area to be shielded is surrounded by metallic or magnetic materials, isolating the "fields" inside and outside the shield. Shielding can be classified according to its mechanism into electric field shielding, magnetic field shielding, and electromagnetic field shielding.
2.1 Electric Field Shielding
When the noise source is high voltage and low current, its radiation field is mainly manifested as an electric field. Electric field shielding is used to suppress interference caused by electric field coupling between the noise source and sensitive equipment.
2.1.1 Mechanism of electric field shielding
2.1.2 Key Considerations for Electric Field Shielding Design
When designing electric field shielding, the following should be considered: 1. The metal casings of high-power equipment (servo drives, frequency converters, stepper drives, switching power supplies, motors) in the system should be reliably grounded to achieve active shielding; 2. The casings of sensitive equipment (such as CNC devices) should be reliably grounded to achieve passive shielding; 3. The distance between high-power equipment and sensitive equipment should be as far as possible. Generally, within the electrical cabinet, the distance between high-power and low-power equipment should be at least 30cm, with a minimum distance of 10cm; 4. High-voltage and high-current power lines and signal lines should be routed separately, for example, using their own independent cable trays, with a distance of at least 30cm and a minimum distance of 5-7.5cm. Parallel routing should be avoided as much as possible, and high-power wires and signal lines should not be bundled together; 5. Signal lines should be placed as close as possible to the ground wire (or grounding plate) or surrounded by the ground wire; 6. Shielded cables can provide both passive and active shielding against electric fields, provided that the shielding layer is grounded.
2.2 Magnetic field shielding
When a noise source has low voltage and high current characteristics, its radiation field is mainly a magnetic field. Magnetic field shielding is used to suppress interference caused by magnetic field coupling between the noise source and sensitive equipment.
2.2.1 Mechanism of Magnetic Field Shielding
Magnetic field shielding mainly relies on the low magnetic resistance of highly permeable materials to shunt the magnetic flux, thereby greatly weakening the magnetic field inside the shield. Figure 3 illustrates this principle.
2.2.2 Design Considerations for Magnetic Field Shielding
When designing magnetic field shielding, the following should be considered: 1. Use materials with high magnetic permeability, such as permalloy, and appropriately increase the wall thickness of the shield; 2. Do not place the shielded object too close to the shield to minimize the magnetic flux passing through the object; 3. Pay attention to the structural design of the magnetic shield. For strong magnetic fields, a double-layer magnetic shield structure can be used; 4. Reduce the loop area between the interference source and the sensitive circuit. The best method is to use twisted-pair and shielded cables, twisting the signal line and the grounding line (or current return line) together to minimize the distance between the signal and the ground (or current return line); 5. Increase the distance between the lines to minimize the mutual inductance between the interference source and the induced line; 6. If possible, route the interference source's line and the induced line at a right angle (or close to a right angle) to greatly reduce magnetic field coupling between the two lines; 7. Sensitive equipment should be placed at least 30cm away from interference sources (high-voltage equipment, transformers, etc.).
2.3 Electromagnetic Field Shielding
Electromagnetic shielding is used to suppress interference caused by electromagnetic coupling when noise sources and sensitive equipment are far apart. Electromagnetic shielding must shield both electric and magnetic fields simultaneously, and typically uses high-conductivity materials with low resistivity. When electromagnetic waves strike a metallic surface, they are reflected and absorbed, significantly attenuating their electromagnetic energy. This provides the shielding effect.
3. Conclusion
In the context of economic globalization and China's status as a manufacturing powerhouse, and given the significant fluctuations in my country's power grid quality, as well as the need to ensure the long-term, high-intensity, and continuous use of CNC machine tools in harsh industrial environments, this paper, based on the Huazhong CNC system, describes the electromagnetic compatibility design requirements of CNC systems and how to employ shielding technology to achieve electromagnetic compatibility.
References
[1] Yang Kechong et al. Electrical Control of CNC Machine Tools [M]. Huazhong University of Science and Technology Press, 2005.
[2] Zheng Xiaonian et al. Fault Diagnosis and Repair of CNC Machine Tools [M]. Huazhong University of Science and Technology Press, 2005.
{3} JB/T 8832-2001 General Technical Conditions for CNC Systems of Machine Tools (Mechanical Industry Standard of the People's Republic of China)
