Abstract : CNC machine tools typically have high grounding requirements. A complete grounding and shielding system is a prerequisite for ensuring the safe and reliable operation of CNC machine tools. Grounding and shielding are important methods for suppressing interference. Good grounding and proper use of shielding are crucial measures for stable operation of CNC systems and elimination of interference. The grounding of CNC machine tools is not only related to the design of the machine tool system itself, but also directly related to the user company. This paper analyzes and discusses the grounding, shielding, and interference suppression of CNC machine tools, aiming to enable machine tool designers and user companies to better understand and master the methods of grounding, shielding, and interference suppression for CNC machine tools, and to make timely and accurate analysis and judgment of machine tool problems and take corresponding countermeasures.
Keywords : CNC machine tool grounding shielding interference suppression
CNC machine tools, grounding, shielding and interference suppression
YAN En-gang
(Tianshui Spark Machine Tool Co., Ltd.,Tianshui, Gansu 741024,China)
Abstract：CNC machine tools on the ground usually have higher requirements, a complete grounding and shielding system is to ensure that the prerequisite for CNC machine tools safe and reliable operation. Grounding and shielding is an important way to suppress interference, a good grounding and proper use of shielding is an important measure of the numerical control system stable operation, to eliminate interference. Grounding of CNC machine tools is not only related to the design with the machine tool system, directly linked to the enterprise and machine tool applications. In this paper, the grounding of CNC machine tools, shielding and interference suppression analysis, the study discusses the purpose is to make the machine designers and companies, more understanding and knowledge of CNC machine tools grounding, shielding and interference suppression methods, the machine appears. to achieve timely and accurate analysis and judgment and take the appropriate treatment response.
Key words：CNC machine tools Ground Shielded Interference Suppression
I. Overview
The anti-interference capability of a system is a crucial factor affecting the normal operation of a CNC machine tool and is key to the overall reliability of the machine. Interference is largely caused by improper grounding and shielding. CNC systems are characterized by low operating signal voltage (generally around 10V) and poor anti-interference capability. For CNC machine tools, this interference superimposed on the signal can cause signal measurement and control distortion and malfunctions, position control drift, and direct impact on measurement and control accuracy. Clock signals, reset signals, interrupt signals, and control signals are highly sensitive to noise interference, leading to system instability and equipment malfunction. Reasonably and effectively suppressing interference sources and researching effective, economical, and applicable suppression methods to achieve electromagnetic compatibility (EMC) is crucial for the safe and efficient operation of CNC machine tools in electromagnetic interference environments. A robust grounding and shielding system is an important measure for resisting EMC. A good grounding method can significantly suppress the coupling of internal noise, prevent the intrusion of external interference, and ensure the EMC and operational reliability of the system. This is the basic principle of interference suppression and one of the effective means to improve the EMC design of CNC equipment.
II. Types of Grounding and Shielding and Their Responsiveness to Interference
Grounding is one of the effective means to improve the electromagnetic compatibility (EMC) of electronic equipment. Grounding is the process of establishing a low-impedance connection between circuits, equipment housings, electrical control cabinets, etc., and a common reference point (earth) that is at zero potential. Grounding in CNC machine tools has strict requirements; if it is not grounded according to these requirements, interference signals will cause interference to the machine tool through the ground wire. The main grounding methods for CNC systems are: protective grounding, functional grounding, and shielding grounding.
1. Protective grounding
Ensuring a reliable connection between the protective grounding system and the CNC equipment's casing, and connecting it to the earth, can effectively reduce the occurrence of safety accidents. In the electrical cabinet of a CNC machine tool, the protective grounding busbar has strict design standards and requirements; the grounding resistance connected to the earth should ideally be less than 1 ohm. The protective grounding wire within the system adopts a standard design, using a yellow-green double-color wire connected to the grounding busbar, while avoiding loop formation. This reduces electromagnetic interference with other equipment and enhances the system's interference suppression capabilities.
2. Working grounding
To avoid common-mode current radiation interference, grounding loop interference, and common-mode current radiation interference during CNC machine tool operation, working grounding is extremely important. Working grounding methods include floating grounding, single-point wiring, multi-point grounding, and hybrid grounding.
1 ) Floating ground is used to prevent interference from external common-mode noise on the internal shielded ground line. Here, the working ground line is floating and is not connected to the chassis insulation working ground.
2 ) Single-point wiring: Single-point grounding means that only one physical point in a circuit or device is defined as the grounding reference point, and all other points that need to be grounded are connected to this point.
3 ) Multi-point grounding: Multi-point grounding refers to connecting each grounding point in a device or system directly to the nearest grounding plane to minimize the length of the grounding wire. The grounding plane can be the device's dedicated grounding wire or the device's frame.
4 ) Hybrid grounding: Hybrid grounding connects only the points that need to be grounded nearby and those that need to be grounded at high frequency to the ground plane through bypass capacitors.
3. Shielding grounding
Shielding is also an effective measure to suppress magnetic field coupling interference. When current flows through certain components or circuits in a system device, a magnetic field is established around it; simultaneously, the charge stored in a certain part of the circuit also establishes a magnetic field in the surrounding space; the mutual conversion of electrical energy and magnetic field will form electromagnetic interference. This electric and magnetic field is an endogenous interference to the equipment itself, reducing the system's anti-interference tolerance, and in severe cases, causing equipment failure. Similarly, the operation of equipment such as welding machines and high-frequency quenching machines, as well as the sudden start-up and shutdown of large electrical equipment, will all interfere with the CNC system, which is an exogenous interference. In order to limit the generated electric or magnetic field within a certain specified tolerance range, or to protect the CNC system and components from the influence of external electromagnetic fields, isolation or shielding measures are often used to prevent electric and magnetic fields from penetrating these shielding materials.
Shielding ground is used to suppress various interference signals. There are many types of shielding, but reliable grounding is essential. Figure 1 illustrates the connection between the shielding connector and the CNC system power module. Shielding ground is the grounding of the shielding network. Connecting the cable shielding layers together using clamps achieves good shielding. To suppress noise, the shielding layers of cables, transformers, etc., need to be connected to a corresponding ground wire, called the shielding ground wire. For low-frequency signals, single-end grounding of the shielding layer is usually used to reduce interference. For high-frequency signals, double-end grounded shielded cables are typically used, and the shielding layer must also be grounded at both ends.
Figure 1 shows the connection between the shielding bracket and the power module.
Electromagnetic compatibility design requirements for grounding and shielding connections: In principle, shielded cables should be connected to the metal housing at both ends, ensuring a large contact area with the metal surface to withstand instantaneous high-frequency interference. Signal lines between the CNC system and servo drives, frequency converters, power modules, and motors in the machine tool should, in principle, use shielded twisted-pair cables with the shielding layer grounded at both ends.
Electrostatic shielding: Electrostatic shielding is mainly used to eliminate interference caused by distributed capacitance coupling between two or more circuits. The grounded shielding layer between the primary and secondary coils of a transformer mostly falls into this category. Electromagnetic compatibility regulations stipulate that these types of transformers are mostly isolation transformers.
4. Grounding system of CNC machine tools
Figure 2 shows a schematic diagram of the grounding connection of the CNC system.
A chaotic grounding system can lead to uneven potential distribution at each grounding point, causing interference to the CNC system . Potential differences between different grounding points can induce ground loop currents, affecting normal system operation. The shielding layer, grounding wire, and earth can potentially form a closed loop. Under the influence of a changing magnetic field, induced currents will appear within the shielding layer, forming interference signal loops through coupling between the shielding layer and the core wire. To prevent common ground impedance interference, each device may have multiple grounding wires, but they can be broadly categorized into three types: signal ground, chassis ground, and system ground.
(1) Signal ground: also known as DC system ground (logic ground, working ground), it is the reference potential (0V) used by CNC machine tools to provide electrical signals. This ground can be connected to earth ground or a common point. If the system ground is not connected to earth ground, that is, the system ground is in a floating working state, it is called floating ground. There are three grounding methods for signal ground: floating ground, direct grounding and capacitor grounding. Since the CNC system is a high-speed, low-level control system, direct grounding is used in principle. Due to the influence of distributed capacitance of signal cables and filtering of input devices, the signal exchange frequency between CNC devices is generally a low-frequency signal, so the grounding wire of the CNC system is generally grounded at a single point.
(2) Frame ground: Frame ground is a grounding system designed to prevent external and internal noise. It can be the equipment panel, the unit shell, the control panel, and the shielded wire connecting the various devices.
(3) Systematic: connects the framework ground and the earth.
Figure 2 System grounding connection
5. Principles to be followed in CNC system grounding design: The entire system should only be grounded at one point, because neither the system grounding nor the grounding resistance can be zero. Furthermore, when a large current flows from the ground wire into the earth, the ground potential at and near the grounding electrode rises. If there are multiple grounding points, potential differences will occur at the grounding points, causing interference to the CNC system. Even the system grounding wire within the same device should adhere to the single-point grounding principle; otherwise, a grounding loop will be formed, creating potential differences between the grounding points, and interference will be introduced into other circuits.
6. Grounding, shielding, and interference suppression measures
① When the signal source is grounded, the shielding layer should be grounded on the signal side. When there is a joint in the middle of the signal line, the shielding layer should be firmly connected and insulated. Multiple grounding points must be avoided. When the shielded twisted-pair cables of multiple measurement point signals are connected to the multi-core twisted-pair shielded cable, each shielding layer should be interconnected and insulated. A suitable grounding point should be selected for single-point grounding. Simultaneously, using single-point grounding , increasing the diameter of the grounding wire , and reducing the grounding resistance can all achieve good interference suppression. ② CNC systems have many signal transmission lines , and interference can easily be transmitted from the signal lines into the system . Using twisted-pair cables for the signal lines or adding ground wires between the signal lines are very effective measures to suppress interference . These measures can reduce the sensitivity of the CNC system to noise. A well-designed CNC device should be sensitive to useful signals while suppressing noise interference. ③ The earth is a conductor with a large electrostatic capacitance and serves as a reference point for potential. In practice, there is always a certain grounding resistance, which creates a potential difference between different grounding points, causing current to flow in the conductor, known as a ground loop current. Grounding interference suppression mainly avoids ground loop current and reduces coupling interference from the common ground impedance. Single-point grounding can effectively avoid ground loop current. ④ The grounding busbar and PE should be connected metal-to-metal and also connected through the protective grounding electrode to avoid safety release and bypass interference. ⑤ The working ground of the power supply neutral point refers to the potential ground of the neutral point in a stable power supply system. There are many ways to handle neutral and ground wires. Sometimes the neutral wire is not connected, sometimes the ground wire is not connected, and sometimes the neutral wire and ground wire are connected together. If the neutral wire and ground wire are connected, an unbalanced current will be generated in the neutral wire. This current may flow back into the machine tool through the ground wire, causing instability in the system operation. The usual practice is to build a grounding network, i.e., a grounding busbar, outside the equipment plant, and then connect the machine tool's grounding wire directly to the protective grounding busbar. Some companies believe that the factory already has a system grounding and there is no need to build a grounding busbar. In fact, the factory's grounding system already connects to many devices, such as welding machines, EDM machines, lifting equipment, high-power thyristor equipment, high-frequency quenching machines, and high-electromagnetic equipment, which may generate high-frequency harmonics. These devices can severely interfere with the CNC machine tool system, making the machine tool unable to work properly.
III. Conclusion
With the widespread application of CNC machine tools, a thorough understanding and mastery of grounding, shielding, and interference suppression methods for machine tool systems is crucial for reducing failure rates and improving reliability during machine tool use. From another perspective, interference sources in CNC systems are multi-faceted, requiring different handling methods for different environments, making it a highly complex issue. Therefore, interference suppression design should comprehensively consider various factors. Only by correctly and rationally applying grounding and shielding suppression methods can the machine tool system operate stably and reliably.
References
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[2] Wang Ailing et al., Numerical Control Principles and Numerical Control Systems, Beijing: Machinery Industry Press, 2006 .
[3] Ye Beihua, Digital Control Technology, Beijing: Tsinghua University Press, 2002 .
