As a core component of some automated equipment, the reliability and stability of motion control systems directly affect the performance of the equipment. One of the main factors affecting its reliability and stability is interference resistance. Therefore, how to effectively solve interference problems is an issue that cannot be ignored in the design of motion control systems.
Part 01
Interference phenomenon
In applications, the following main interference phenomena are often encountered:
1. The motor rotates erratically when the control system does not issue a command.
2. When the servo motor stops moving and the motion controller reads the motor position, the value fed back by the photoelectric encoder at the end of the motor jumps erratically.
3. When the servo motor is running, the encoder value read does not match the command value issued, and the error value is random and irregular.
4. When the servo motor is running, the difference between the encoder value read and the command value issued is a stable value or changes periodically.
5. Devices sharing the same power supply as the AC servo system (such as monitors) are malfunctioning.
Part 02
Interference source analysis
There are two main channels through which interference enters the motion control system:
1. Signal transmission channel interference: Interference enters through the signal input and output channels connected to the system.
2. Power supply system interference.
The signal transmission channel is the path through which a control system or driver receives feedback signals and sends control signals. Because pulse waves experience delay, distortion, attenuation, and channel interference on transmission lines, interference along long lines is a major factor during transmission. All power supplies and transmission lines have internal resistance, and it is this internal resistance that causes power supply noise interference. Without internal resistance, any noise would be absorbed by a short circuit in the power supply, and no interference voltage would be established in the line. Furthermore, the AC servo system driver itself is a strong source of interference, which can interfere with other devices through the power supply.
Part 03
Anti-interference measures
1. Anti-interference design of the power supply system
( 1 ) Group power supply, for example, separate the drive power supply of the actuator motor from the control power supply to prevent interference between devices.
( 2 ) Using a noise filter can also effectively suppress the interference of the AC servo drive to other devices. This measure can effectively suppress the above-mentioned interference phenomena.
( 3 ) An isolation transformer is used. Considering that high-frequency noise is mainly coupled through the primary and secondary parasitic capacitances of the transformer, rather than through the mutual inductance of the primary and secondary coils, the primary and secondary windings of the isolation transformer are isolated by a shielding layer to reduce their distributed capacitance and improve the common-mode interference immunity.
2. Anti-interference design of signal transmission channel
( 1 ) Opto-coupled isolation measures
In long-distance transmission, optocouplers can be used to disconnect the control system from the input and output channels, as well as the input and output channels of the servo driver. Without opto-isolation in the circuit, external spike interference signals can enter the system or directly enter the servo drive, causing the first type of interference. The main advantage of optocouplers is their ability to effectively suppress spike pulses and various noise interferences, thereby significantly improving the signal-to-noise ratio during signal transmission. This is primarily because although interference noise has a large voltage amplitude, its energy is small, only forming a weak current. The LEDs at the input of the optocoupler operate under current, typically with a conduction current of 10-15mA . Therefore, even with high-amplitude interference, it is suppressed because sufficient current cannot be provided.
( 2 ) Long-distance transmission of twisted-pair shielded cable
Signals are susceptible to interference from electric fields, magnetic fields, and ground impedance during transmission. Using grounded shielded cables can reduce electric field interference. Compared to coaxial cables, twisted-pair cables, although having a narrower bandwidth, have higher impedance and stronger common-mode noise immunity, allowing electromagnetic interference from various components to cancel each other out. Furthermore, differential signal transmission is generally used for long-distance transmission to improve anti-interference performance. Using twisted-pair shielded cables for long-distance transmission can effectively suppress the second, third, and fourth types of interference.
( 3 ) Grounding
Grounding eliminates noise voltage generated when current flows through the ground wire. In addition to grounding the servo system, the signal shielding wire must also be grounded to prevent electrostatic induction and electromagnetic interference. Without proper grounding, a second type of interference may occur.
