The solution to the problem of anti-interference of CNC system

Interference generally refers to those that have nothing to do with the signal, and some uncertain and harmful electrical transients that occur in the process of signal input, transmission and output, which will make the data in the numerical control system change in the transmission process, increase the error, and cause abnormal conditions in the local device or the whole system, causing failures.

The main causes of interference are as follows

1. Power interference

Due to the wide coverage of the power grid, there are a variety of equipment to share a power grid, especially the changes within the power grid, power switch operation, lightning surge, large power equipment start and stop, AC and DC transmission device caused by harmonics, power grid short circuit transient impact, etc., are transmitted to the primary side of the power supply through the transmission line, so that the voltage changes temporarily, resulting in grid voltage fluctuations. In addition, the power line can also generate noise and fast transient bursts during transmission, which can pollute the power grid.

2. Radiation interference

Electromagnetic or electric fields are ubiquitous in nature. In addition to the action of the electric field, the EDM piercing machine in the work is also affected by the magnetic field. During the operation of the EDM perforator, due to the harshness of the working environment, it will inevitably be subject to electromagnetic interference

3. Interference between digital signals and analog signals

In the working process of the EDM perforator, due to the large number of devices involved in the whole set of equipment, there are AC380V, AC220V AC signals, and DV24V, DC5V various low-voltage DC signals. The cable used to transmit the signal, in the process of wiring, sometimes due to the analog signal output equipment or the interference generated by the servo drive or frequency converter, causes malfunction, affecting the normal operation of the equipment; The frequency used to transmit I/O input/output signals is interfered by clock frequencies and harmonics, coupled with improper wiring of the line, so that the digital signal line and the analog signal line will inevitably be interfered by external interference signals, and various signal lines will also interfere with each other through interline coupling.

◆ Anti-interference measures

These measures mainly include shielding, isolation, filtering, grounding, and software handling.

◇ Shielding technology

Shielding is currently the most used and most effective method. Shielding technology cuts off the transmission path of radiated electromagnetic noise, commonly used metal materials or magnetic materials to surround the area required to shield, so that the fields inside and outside the shield are isolated from each other, cut off the electromagnetic radiation signal, in order to protect the shielded body from interference, shielding is divided into electric field shielding, magnetic field shielding and electromagnetic shielding. In practical engineering applications, when the electric field is disturbed, the metal shells of the strong current equipment in the system (servo drives, inverters, drives, switching power supplies, motors, etc.) are reliably grounded to achieve active shielding; Enclosures for sensitive equipment, such as intelligent error correction devices, should be reliably grounded to achieve passive shielding. The distance between the high-voltage equipment and the sensitive equipment is as far as possible; The high-voltage and high-current power line and the signal line should be routed separately, and the cable with shielding layer should be selected, and for magnetic field interference, materials with high permeability, such as glass mold alloy, etc., should be selected, and the wall thickness of the shield should be increased appropriately. With twisted pair and shielded wires, twist the signal line with the ground wire or current-carrying return wire so that the distance between the signal and the ground or current-carrying return wire is the closest; Increase the distance between the lines, so that the mutual inductance between the interference source and the induced line is as small as possible, and the sensitive equipment should be kept away from the interference source, strong current equipment, transformers, etc.

◇ Isolation technology

Isolation is to isolate the interference source with isolated components to prevent interference from entering the equipment and ensure the normal operation of the EDM machine. Common isolation methods include photoelectric isolation, transformer isolation, and relay isolation.

1. Photoelectric isolation

Photoelectric isolation can effectively suppress system noise and eliminate the interference of the ground loop. At the input and output ends of the intelligent error correction system, optocouplers are used as interfaces to isolate signals and noises. In the motor drive control circuit, the optocoupler is used to isolate the control circuit from the motor high-voltage circuit.

2. Transformer isolation

This is a kind of power line anti-interference component that is widely used, and its most basic function is to realize the electrical isolation between circuits and circuits, so as to solve the interference between devices and equipment caused by the ground loop current, and the isolation transformer also has a certain effect on anti-common mode interference. The isolation transformer can play a good role in suppressing transient pulse train and lightning surge interference, and for the transmission of AC signals, the method of isolating the interference signal is generally used.

3. Relay isolation

There is no electrical connection between the coil and the contacts of the relay. Therefore, the coil of the relay can be used to receive the electrical signal, and the contact can be used to send and output the signal, so as to avoid the direct connection between the strong and weak current signals, and realize the anti-interference isolation.

◇ Filtering technology

Filtering technology is an effective measure to suppress interference. The filter is composed of lumped parameters R, L, and C to form an equivalent circuit. It has functions such as separating signals, suppressing interference, impedance conversion and impedance matching, and delaying signals. The use of filters can well filter the harmful components in the equipment circuit and improve the reliability of the equipment. On CNC machine tools, in order to suppress the interference of high frequencies to intelligent control devices. A low-pass filter can be used to filter out high-frequency components in the circuit and improve power quality. For all kinds of contacts or switches, the interference caused by contact jitter at the moment of closing or disconnection, and suppressing the reverse potential generated by the inductive load at the moment of cutting off the power supply, can be eliminated by resistor-capacitance filtering, so that the ability to release the magnetic field of the inductor coil can be converted into the energy of the electric field of the capacitor and stored to reduce energy consumption. The use of L-C filter will reduce the load impedance, thereby increasing the filtering effect, playing the role of the filter, and reducing interference.

◇ Grounding treatment

Grounding is a low-impedance connection between an electrical circuit, equipment case, etc., and a common reference point (ground) that serves as zero potential. The main ways of grounding are: protective grounding, working grounding, and shielding grounding. The purpose of grounding is twofold, one is to reduce interference. The second is for personal safety. In order to reduce the occurrence of safety accidents, the safety grounding protection grounding terminal and the chassis of the electrical equipment should achieve a good overlap, so as to truly connect with the earth. In the electrical cabinet of CNC machine tools, the thickness of the grounding row shall not be less than 3mm (copper plate), and the grounding resistance connected to the ground shall be 4 ohms smaller. The protective ground wire in the system should be connected to the grounding row with as thick and short yellow and green two-color wire as possible, and avoid forming a loop; It can reduce mutual electromagnetic interference with other equipment. In order to avoid the common ground impedance interference, ground loop interference and common mode current radiation interference in the working process of CNC machine tools, working grounding is extremely important. The working grounding methods include floating, single-point grounding, multi-point grounding and mixed grounding.

◇ Software anti-interference

The method of using software to identify desirable and interfering signals and filter out interfering signals is called software filtering. Generally, the time, space, and properties of the signal are used to determine whether it is a useful signal or an interfering signal. When electromagnetic interference causes the program of the CNC system to run away, the watchdog can help the system to automatically resume normal operation.

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