Application Background
Interference is a frequent problem in servo applications, causing unpredictable malfunctions or alarms, preventing the servo from operating normally as intended, and even burning out the driver. Furthermore, troubleshooting these problems can be difficult. Based on research and experience in handling interference issues, and combined with recent interference problems encountered with TIG welding machines, this paper discusses how to address interference.
Interference sources generated by argon arc welding machines
Argon arc welding machines use high-frequency arc ignition. During arc ignition, a small gap is maintained between the tungsten electrode tip and the welding surface. Then, the high-frequency oscillator pulse arc ignition circuit is activated, causing the gap to break down and discharge, igniting the arc. After the high-voltage current breaks down, a large current is used to stabilize the arc. When tungsten arc welding machines use high-frequency arc ignition, the welding machine utilizes high-frequency high voltage (hundreds of thousands of hertz) to break down the air gap and form an arc. Therefore, high-frequency arc ignition is a strong source of harmonic interference. The arc welding inverter power supply is essentially a large rectifier power supply to the power grid. Because the power electronic devices generate pulses with very steep leading and trailing edges during the commutation process, severe harmonic interference is caused. The input current of the inverter power supply is a sharp-angle wave, resulting in a large number of high-order harmonics in the power grid. A severe phase shift exists between voltage and current harmonics, leading to a very low power factor for the welding machine.
Measures to deal with interference
In this case, the power supplies for the TIG welding machine and the servo driver are separate. The TIG welding machine power supply is isolated by an isolation transformer. The servo power supply line is fitted with a choke coil before being fed into an active power filter. After filtering, it is then sent to the switching power supply. The servo driver's power supply and control power supply are supplied by two separate switching power supplies, each fitted with a choke coil before being connected to the driver. Both the driver power supply and the control (encoder) power supply are connected with shielded cables, ensuring reliable single-end grounding. The attached diagram shows the system wiring diagram.
Interference sources at the application site
Radiation interference from space
Space-radiated electromagnetic fields are mainly generated by power grids, lightning, radio broadcasts, and radar, and are commonly referred to as radiated interference. Their impact primarily occurs through two paths: one is direct radiation into the servo circuitry, causing interference through circuit induction;
Secondly, there is the radiation from the servo communication network, which can cause interference induced by the communication lines. This type of interference is relatively rare and is generally protected by using shielded cables.
Interference from system external leads
This type of interference is mainly generated through power and signal lines and is commonly referred to as conducted interference. This interference is quite serious in industrial settings in my country and mainly falls into the following three categories:
The first type is interference from the power supply. Experience has shown that many servo control systems malfunction due to power supply interference, which is generally resolved by adding devices such as voltage regulators.
The second type is interference introduced from signal lines. This interference mainly has two pathways: one is mains interference from the power grid introduced through the transmitter's power supply or the power supply of shared signal instruments, which is often overlooked;
Secondly, interference caused by electromagnetic radiation from space, i.e., external induced interference on signal lines, is often very serious. Interference introduced by signals can cause abnormal operation of circuit board components, and in severe cases, damage to components. For systems with poor isolation performance, it can also lead to mutual interference between signals, causing backflow in the common ground system bus, resulting in changes in logic data, malfunctions, and system crashes. Many control systems also experience system failures due to damage to internal components caused by signal-introduced interference. This type of interference frequently occurs in applications with long signal distances, and is often addressed by adding relays for isolation to shield against induced voltage and resolve the interference problem.
The third type of interference comes from a disordered grounding system. As is well known, proper grounding is one of the effective ways to improve the anti-interference capabilities of electronic equipment; correct grounding can suppress interference emitted by the equipment.
However, incorrect grounding can introduce severe interference signals, rendering the system malfunction. Generally, the grounding of a control system includes system ground, shield ground, AC ground, and protective ground. If the grounding system is chaotic, the interference to the servo system is mainly due to uneven potential distribution at various grounding points, creating potential differences between different grounding points and causing ground loop currents, which affect the normal operation of the system. For example, if both ends a and b of the cable shield are grounded, a potential difference exists, and current flows through the shield. When abnormal conditions such as lightning strikes occur, the ground current will be even greater. In addition, the shield, grounding wire, and earth may form a closed loop. Under the influence of a changing magnetic field, induced currents will appear in the shield, interfering with the signal loop. If the system ground and other grounding methods are confused, the resulting ground loop currents may create unequal potential distributions on the ground wire, affecting the normal operation of the servo circuit. The key to solving this type of interference lies in distinguishing the grounding method and providing good grounding performance for the system.
Interference from within the system
This is mainly caused by electromagnetic radiation between internal components and circuits, such as mutual radiation between logic circuits, mutual influence between analog and logic grounds, and mismatched use of components. These are all part of the electromagnetic compatibility design of the controller system by the manufacturer and rarely occur in mature systems.
measures to handle interference
Add filters and reactors
Filtering is a method to suppress conducted interference. Connecting a filter to the power input can suppress the damage to the circuit from noise from the power grid, as well as suppress interference generated by the circuit and fed back to the power grid. As a crucial unit for suppressing conducted interference on power lines, power filters play an extremely important role in the electromagnetic compatibility design of equipment or systems. They not only suppress conducted interference on transmission lines but also have a significant suppression effect on radiated emissions from transmission lines.
All power filters must be grounded because the filter's common-mode bypass capacitor only functions when grounded. A common grounding method is to connect the filter to its metal casing and then connect the filter casing to the equipment's grounding point using a thicker wire. Lower grounding impedance results in better filtering. The filter should be installed as close as possible to the power input. The filter's input and output terminals should be kept as far apart as possible to prevent interference signals from directly coupling from the input to the output.
Install isolation transformer
The primary voltage of an isolation transformer is the grid voltage, which is the voltage relative to the ground. The secondary voltage of the isolation transformer, however, is not connected to the ground. If the coil ratio is 1 : 1, the current on both sides is the same, achieving electrical isolation; this can eliminate some harmonics and effectively reduce the zero-to-ground voltage.
External shielded cable
Shielding is an important and effective means of solving radiation interference problems, aiming to cut off the propagation path of electromagnetic waves. Most radiation interference problems can be solved through electromagnetic shielding, and using electromagnetic shielding to solve electromagnetic interference problems will not affect the normal operation of the circuit.
Conclusion
The anti-interference capability of a control system is crucial to the reliable operation of the entire system, and the reliability of the system directly affects the production efficiency of an enterprise, as well as the safe and economical operation of equipment. With the increasing automation of industrial equipment, control technology issues are becoming increasingly important. Effectively preventing interference can improve system reliability and allow equipment to operate more stably.
