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In the production site, after the control system is put into operation, various oscillation phenomena of electric actuators are often encountered. The operation of electric actuators and regulating valves in an oscillating state will affect their service life, so oscillation should be eliminated in the control system. There are many reasons for electric actuator oscillation. Today, Shanghai Huquan Valve Co., Ltd., a Sino-foreign joint venture high-tech joint-stock company specializing in research, development, design, production, and sales of "Huquan" brand electric actuators, will analyze in detail some of the reasons for electric actuator oscillation and its elimination methods for our customers.
1. The transmitter output fluctuates due to fluctuations in the measured parameters, causing oscillations
The measured parameters themselves are pulsating signals, such as boiler drum water level, reciprocating compressor outlet pressure, etc., which are prone to cause oscillations. This pulsating fluctuation can cause constant changes in the transmitter output, leading to oscillations in the entire control system without a stable state during the transition process. At this time, measures such as installing a buffer component in the pressure conduit to perform mechanical filtering, installing an electrical damper, adjusting and increasing the filter constants of the transmitter and receiving instrument, or closing the root valve can be considered to eliminate oscillations.
2. Oscillation caused by improper PID parameter setting of the regulator
Improper tuning of the PID parameters of the control system can also cause varying degrees of oscillation in the control system. If the proportional gain of a single loop PID regulator is too large, the integration time is too short, and the differential time and differential gain are too large, the system may oscillate, leading to actuator oscillation. In addition to the above reasons, there is also a problem of interaction between loops in a multi loop control system, such as resonance caused by improper parameter tuning. Aiming at existing problems, PID parameters can be readjusted to ensure a certain stability margin in the control circuit without affecting production and meeting process control requirements.
3. System oscillation caused by flow characteristics of control valves
The flow characteristics of the regulating valve cannot be ignored. The flow characteristics of the regulating valve are too steep. As long as there is a small deviation in the regulated amount, it will cause significant changes in the regulated medium. Even if the controller has a very small output, due to valve reasons, there will be a significant change in the medium flow, resulting in overshooting, resulting in constant amplitude oscillations in the entire system. For the above situations, if the characteristics of the valve cannot be modified, the proportional gain of the controller can be reduced to improve control quality.
4. Vibration caused by mechanical assembly of electric actuator and regulating valve
Poor mechanical connection between the electric actuator and the regulating valve, such as excessive mechanical clearance, can also cause oscillation of the actuator. Therefore, it is necessary to purchase high-quality products. Actuators and regulating valves are supplied by the manufacturer as a complete set, and it is also necessary to ensure installation quality.
5. Oscillations caused by external disturbances in the control system
The oscillations caused by external interference in the control system are often irregular and may be sporadic. They are different from the oscillations of the control system and its own, and are easier to determine, but difficult to eliminate. The measures that can be taken are: connect the ground wire well, take shielding measures for the signal wire, and only one point of grounding is allowed for the shielding layer.
6. Oscillation caused by electric actuator failure
Failure of the braking mechanism of the electric actuator can cause the electric actuator to fall away and also cause oscillation. After the brake mechanism of the electric actuator fails, the holding brake does not engage properly, causing the motor to idle for too long. Even if the deviation of the servo amplifier is already zero, due to excessive valve position adjustment, the deviation of the servo amplifier cannot be zero, resulting in oscillations when the motor rotates back and forth.
