Anti-interference analysis of 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 the problem of anti-interference. Therefore, how to effectively solve the interference problem is a problem that cannot be ignored in the design of the motion control system.

1. Interference phenomenon

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

1. When the control system does not issue instructions, the motor rotates irregularly.

2. The servo motor stops moving, and when the motion controller reads the motor position, the value fed back by the photoelectric encoder at the end of the motor jumps randomly.

3. When the servo motor is running, the value of the encoder 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 read encoder value and the issued command value is a stable value or changes periodically.

5. The equipment (such as display, etc.) that shares the same power supply with the AC servo system does not work properly.

Two, interference source analysis

There are two main types of channels that interfere with entering the motion control system:

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

2. Interference of the power supply system.

The signal transmission channel is the way for the control system or driver to receive the feedback signal and send out the control signal, because the pulse wave will have delay, distortion, attenuation and channel interference on the transmission line. In the transmission process, the long-term interference is the main factor.

There are internal resistances in any power supply and transmission lines. It is these internal resistances that cause the noise interference of the power supply. If there is no internal resistance, no matter what kind of noise will be absorbed by the power supply short circuit, no interference voltage will be established in the line. , AC servo system drive itself is also a strong source of interference, it can interfere with other equipment through the power supply.

Three, anti-interference measures

1. Anti-interference design of power supply system

(1) Implement power supply grouping, for example, separate the drive power of the executive motor from the control power to prevent interference between devices.

(2) The noise filter can also effectively suppress the interference of the AC servo drive to other equipment. This measure can effectively suppress the above several interference phenomena.

(3) The isolation transformer is adopted. Considering that high-frequency noise passes through the transformer mainly not by the mutual inductance coupling of the primary and secondary coils, but by the primary and secondary parasitic capacitance coupling, the primary and secondary sides of the isolation transformer are isolated by a shielding layer to reduce its Distributed capacitance to improve the ability to resist common mode interference.

2. Anti-interference design of signal transmission channel

(1) Photoelectric coupling isolation measures

In the process of long-distance transmission, the use of photocouplers can cut off the connection between the control system and the input channel, output channel, and the input and output channels of the servo drive. If photoelectric isolation is not used in the circuit, the external spike interference signal will enter the system or directly enter the servo drive device, causing the first interference phenomenon.

The main advantage of photoelectric coupling is that it can effectively suppress spikes and various noise interference, thereby greatly improving the signal-to-noise ratio during signal transmission. The main reason is: although the interference noise has a large voltage amplitude, but the energy is small, it can only form a weak current, and the light-emitting diode of the input part of the photocoupler works under the current state, and the general conduction current is 10-15mA, so Even if there is a high amplitude interference, it can be suppressed because it cannot provide enough current.

(2) Long wire transmission with twisted-pair shielded wire

Signals will be affected by interference factors such as electric field, magnetic field, and ground impedance during transmission. The use of grounded shielding wires can reduce the interference of electric fields.

Compared with coaxial cable, although the frequency band is poor, twisted-pair cable has high wave impedance and strong resistance to common mode noise, which can offset the electromagnetic induction interference of each small link.

In addition, in the process of long-distance transmission, differential signal transmission is generally used to improve the anti-interference performance. The use of twisted-pair shielded wire for long-wire transmission can effectively suppress the second, third, and fourth interference phenomena.

(3) Grounding

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