Oscillations due to oscillations in the output of the transmitter due to fluctuations in the measured parameters
The measured parameter itself is a pulsation signal, such as boiler drum water level, reciprocating compressor outlet pressure, etc. It is easy to cause oscillation. This fluctuating pulse will cause constant changes in the output of the transmitter, which will cause the entire control system to oscillate without a stable state during the transition. At this time, consider installing a buffer component in the pressure tube to perform mechanical filtering, or install an electric damper, adjust and increase the filter constants of the transmitter and the receiving instrument, or close the valve of the small root to eliminate the oscillation.
Improper Tuning of Regulator PID Parameters
Improper setting of the PID parameters of the control system may also cause the control system to generate different degrees of oscillation. If the proportional gain of the single-loop PID regulator is too large, the integral time is too short, and the differential time and differential gain are too large, the system may oscillate and cause the actuator to oscillate. For the multi-loop control system, in addition to the above reasons, there is also a mutual influence between the loops, due to the resonance problem caused by improper parameter setting. For the existing problems, the PID parameters can be adjusted again, so that the control loop has a certain stability margin without affecting the production and meeting the requirements of the process control.
System oscillation caused by regulating valve flow characteristics
The flow characteristics of the regulating valve cannot be ignored and the flow characteristics of the regulating valve are too steep. If the adjusted variable has a small deviation, it will cause a large change in the regulated medium. Even if the controller has a small output, due to the valve's cause will cause a great change in the media flow, resulting in overshoot, and the entire system appears equal amplitude oscillation. For the above cases, if the valve characteristics cannot be modified, the proportional gain of the controller can be reduced to improve the control quality.
Vibration caused by mechanical assembly of electric actuator and regulating valve
The mechanical connection between the electric actuator and the regulating valve is not good, and if the mechanical clearance is too large, it may cause the actuator to oscillate. Therefore, we must buy good quality, the actuator and the regulating valve are preferably the products supplied by the manufacturer, and there is the need to ensure the installation quality.
Control system external disturbance caused by the oscillation
The oscillations caused by external disturbances in the control system are often irregular oscillations, and may be sporadic. They are different from the control system and its own oscillations, or are easier to judge, but it is difficult to eliminate them. The measures that can be taken include: connecting the ground wire, shielding the signal wire, and shielding the wire only at one point.
Electric actuator failure causes oscillation
Failure of the brake mechanism of the electric actuator can cause the electric actuator to trip and also cause oscillation. After the brake mechanism of the electric actuator fails, the brake is not good enough to pull the motor, causing the motor to rotate too long. Even if the deviation of the servo amplifier is zero, the deviation of the servo amplifier may not be zero due to the over-adjustment of the valve position. Causes the motor to oscillate back and forth.