Analysis of anti-interference problem in motion control system

As the core part of some automation equipment, the reliability and stability of the motion control system directly affect the performance of the equipment, and one of the main factors affecting its reliability and stability is anti-interference. Therefore, how to effectively solve the problem of interference is an issue that cannot be ignored in the design of motion control systems.

1、 Interference phenomenon

In applications, the following main interference phenomena are often encountered:

The first type: When the control system does not issue instructions, the motor rotates irregularly.

The second type: When the servo motor stops moving and the motion controller reads the motor position, the numerical feedback from the photoelectric encoder at the end of the motor jumps irregularly.

The third type: When the servo motor is running, the value read by the encoder does not match the command value issued, and the error value is random and irregular.

The fourth type: When the servo motor is running, the difference between the value read by the encoder and the command value issued is a stable value or varies periodically.

The fifth type: Devices that share the same power supply as the AC servo system (such as displays) are not functioning properly.

2、 Interference source analysis

There are two main channels through which interference enters the motion control system:

1. Signal transmission channel interference, which enters through the signal input channel and output channel connected to the system;

2. Power supply system interference.

The signal transmission channel is the way for the control system or driver to receive feedback signals and issue control signals, because pulse waves can experience delay, distortion, attenuation, and channel interference on the transmission line. During the transmission process, interference from long lines is the main factor. Any power supply and transmission line has internal resistance, which causes noise interference from the power supply. Without internal resistance, any type of noise will be absorbed by the power supply short circuit, and no interference voltage will be established in the line. In addition, the AC servo system driver itself is also a strong interference source, which can interfere with other devices through the power supply.

3、 Anti interference measures

1. Anti interference design of power supply system

(1) Perform power supply grouping, for example, separate the driving power supply of the motor from the control power supply to prevent interference between devices

(2) The use of noise filters can also effectively suppress the interference of AC servo drives on other devices. This measure can effectively suppress the above-mentioned interference phenomena.

(3) Adopting an isolation transformer, considering that high-frequency noise mainly passes through the transformer not through mutual inductance coupling between primary and secondary coils, but through parasitic capacitance coupling between primary and secondary coils, the primary stages of the isolation transformer are isolated with shielding layers to reduce their distributed capacitance and improve their ability to resist common mode interference.

2. Anti interference design of signal transmission channel

(1) Optoelectronic coupling isolation measures

During long-distance transmission, the use of optocouplers can cut off the connection between the control system and the input and output channels, as well as the input and output channels of servo drives. If optoelectronic isolation is not used in the circuit, external spike interference signals will enter the system or directly enter the servo drive device, resulting in the first type of interference phenomenon. The main advantage of optoelectronic coupling is that it can effectively suppress peak pulses and various noise interferences, thereby greatly improving the signal-to-noise ratio during signal transmission. The main reason is that although interference noise has a large voltage amplitude, its energy is small and can only form weak currents. The light emitting diode in the input part of the optocoupler works in a current state, with a general conduction current of 10-15mA. Therefore, even if there is high amplitude interference, it is suppressed due to the inability to provide sufficient current.

(2) Long line transmission with twisted pair shielded wire

During the transmission process, signals are affected by interference factors such as electric fields, magnetic fields, and ground impedance. The use of grounding shielding wires can reduce the interference of electric fields. Compared with coaxial cables, twisted pair cables have lower frequency bands but higher wave impedance and stronger resistance to common mode noise, which can cancel out electromagnetic induction interference in various small links. In addition, differential signal transmission is generally used in long-distance transmission to improve anti-interference performance. The use of twisted pair shielded wires for long-distance transmission can effectively suppress the occurrence of the second, third, and fourth types of interference phenomena.

(3) Grounding

Grounding can eliminate the noise voltage generated when current flows through the ground wire. In addition to grounding the servo system, the signal shielding wire should also be grounded to prevent electrostatic induction and electromagnetic interference. If there is no correct grounding, the second type of interference phenomenon may occur.