Compared with the general computer control system, the most important point of the CNC plasma cutting system is to have a high anti-interference ability. The CNC plasma cutting system can resist the interference of plasma ignition and other equipment. If the CNC plasma cutting system is disturbed, the system cannot work normally, which may cause damage to the CNC system and affect the service life of the system. In the design of the CNC plasma cutting system, various possible interferences are comprehensively considered, and corresponding measures are taken to eliminate or reduce the interference and ensure the normal use of the machine and the full performance of the machine.
This article focuses on the reliability measures on the periphery of the numerical control device (CNC), excluding the software and circuit anti-interference methods of the CNC system and the servo unit itself. Starting from the interference source, transmission path, and the interfered equipment, various hardware measures are taken to suppress the interference of the plasma cutting system and ensure the normal operation of the CNC plasma cutting system.
Inhibition measures of plasma power supply
The main source of interference in the CNC plasma cutting system is the power supply. It generally uses a high-frequency arc starter to ignite the arc. The voltage on the secondary side of the high-frequency transformer is as high as 3000-6000V, and the pulse frequency may reach several kilohertz, which has strong pollution (interference) to the power grid. There are also high-current AC and DC contactors and various relays that are turned off, which will also cause surges to the power grid. When the usual plasma cutting machine starts the arc, it will cause chaos inside the computer and cannot cut normally. The earliest CNC cutting machines even required the user to start the arc first and then start the computer to run the CNC system program. This makes the cutting operation cumbersome, the computer is not fully utilized and has a serious impact on the life of the computer. Therefore, suppressing the interference of the plasma generation power supply and reducing the pollution to the power grid are the primary issues.
a. Add a shield to the high-frequency arc starter to reduce high-frequency radiation.
b. Modify the control circuit of the plasma power supply.
The control power supply of the original KLG-260 plasma cutting machine is directly derived from the grid voltage. The starting/extinguishing arc control line is directly led from the cutting machine to the CNC electric cabinet. The power supply system of CNC spans a cable rack of more than 10 meters, so the high-frequency interference caused by plasma initiation/extinguishing and the electromagnetic interference caused by the large current is directly connected to the power grid. The measure is to add an isolation transformer to the strong current control line, and at the same time, the arc start/extinguish control signal is isolated into a relatively weak AC through the relay and enters the CNC electrical cabinet through the control line.
c. Other anti-interference measures.
RC absorption circuit and varistor are added to the primary and secondary sides of the main transformer, the high-current contactor and the relay coil are connected in parallel with a resistance-capacitance circuit, and the DC part is equipped with a high-frequency bypass capacitor. The purpose of these measures is to suppress interference sources and reduce pollution to the grid voltage.
Anti-jamming measures of the numerical control device (CNC) and servo unit
The CNC device and servo unit of the CNC plasma cutting system are the core parts of the system, and its power supply is the main way for interference to enter. Power source interference is mainly generated by impedance coupling of power supply lines, and various high-power electrical equipment is the main source of interference.
a. Use shielded wire for power supply
The power line of the CNC plasma cutting system, the cathode line of the cutting torch and the arc start/extinguish control line are hung on the sliding bracket together. The arc starting/extinguishing control line and the cathode line of the cutting tool come from the plasma power supply, and the DC on the cathode line of the cutting torch is hundreds of amperes. The electromagnetic field and the high-frequency signal of the high-frequency arc starter are coupled through an impedance, which will cause electromagnetic interference to the power supply of the numerical control device (CNC) and the servo unit. The shielded cable with copper and aluminum as the shielding layer can effectively suppress high-frequency electromagnetic interference. After the shielding layer is grounded, it can also suppress the electrostatic induction of the core wire by the changing electric field.
b. Use of power filter
The power filter is an indispensable anti-interference component, which has beneficial interference suppression performance in both high frequency and low-frequency bands. It should be noted that the filter should be installed on a conductive metal surface or connected to the grounding point through a braided grounding strap. The installation position of the filter should be as close as possible to the entrance of the power cord. The input and output of the filter are best to use shielded wire or twisted pair. To avoid coupling between input and output wires, it is forbidden to bundle input and output wires together and use shielded wires.
c. Use of power transformer
Use a shielded power transformer. The shielding layer must be connected to the AC neutral line of the primary winding to prevent interference from entering the secondary side of the electrical transformer. The shielded power transformer used in the CNC device and the servo unit can be separated to prevent mutual interference. Among them, the numerical control device can be replaced with a purifying AC stabilizer, or an interference suppressor made by the principle of the spectrum equalization method can be added, which will enhance its ability to resist interference from the power grid.
d. Strictly separate the strong and weak current in the cabinet
The voltage and current in the strong wire have a large rate of change, resulting in drastic electric field changes, forming electromagnetic wave interference, and seriously affecting nearby signal wires and weak current control wires. Keep signal wires away from strong wires, and use shielded wires and twisted-pair wires reasonably to avoid interference signals in signal transmission.
e. The signal line between the cabinet and the cabinet adopts shielded cable
The use of shielded cables can suppress the interference caused by stray electric floating magnetic fields into the transmission line through electromagnetic induction and electrostatic induction. And the grounding of the shielding layer uses the correct single-ended grounding method.
f. Reliable grounding system
Enough attention should be paid to the grounding process of the CNC plasma cutting system. Because its CNC part and servo unit both make the moving part on the track, the interference strength has a great relationship with the grounding mode of the system.
The separation of AC ground and DC ground prevents the interference introduced by the AC power line from being transmitted to the control device due to ground resistance. This not only ensures the safety of the internal components of the control system device, but also improves the reliability and stability of the system, and reduces the ground current interference of high-current equipment.
The cabinet is well and grounded. The CNC plasma cutting machine occupies a large area, and it is best to lay the grounding device separately. The grounding device should be reliably connected to the machine's rails, cabinets, and even cable sliding support. In this way, the induced high-frequency interference voltage on the case can have a low-impedance leakage channel, and the case voltage will not increase due to the accumulation of charges, which is beneficial to suppress the interference impact.
Through the comprehensive anti-interference design of the system, the reliability of the CNC plasma cutting system has been greatly improved. When the automatic arc starting/extinguishing control is realized, the failure rate is reduced, and the system has achieved the strong anti-interference performance required by the plasma arc cutting system.
