How to Improve the Control Accuracy of Electric Actuator?

Electric actuators are important devices in automatic control systems. Due to the limitation of use conditions, maintenance level and installation location, the control accuracy of electric actuators is generally about 3%, and some even exceed 5%, which seriously restricts the improvement of system adjustment quality. For fast-response control systems, it is even more important to improve the control accuracy of electric actuators. ATO various electric actuators are mainly used to control the working conditions of media such as gas, tar, steam, water, and waste gas. According to different influencing factors, ATO.com has adopted corresponding technical measures to make the control accuracy of electric actuators all reach more than 2%, and some exceed 1.2%, which has played a great role in improving the adjustment quality of the control system.

Main Reasons for the Low Control Accuracy of Electric Actuators

1. Control valve vibration.
2. Electric actuator brake failure.
3. There is a hysteresis link in the output of the electric actuator.
4. The sensitivity of the servo amplifier is too low.
5. The error of the electric actuator valve position feedback signal is large.
6. Inappropriate selection of electric actuators, etc.

Solutions

1st, regulating valve vibration. First of all, find the vibration source. For vibration caused by the outside, keep the control valve away from the vibration source, eliminate or weaken the vibration intensity of the vibration source, and use methods such as increasing the fulcrum to weaken the vibration of the control valve. The vibration caused by the internal medium flow should minimize the resistance of the regulating valve. A bypass line can be added if the adjustment capability allows. For fast response control systems, logarithmic characteristic spools are available. Butterfly valves try to work within the range of 10~70, avoiding the alternating point of the force of the medium on the valve body.

2nd, the electric actuator brake has failed. The brake is used to eliminate the inertia of the rotor and the output shaft and the influence of the load reaction moment after the electric actuator is powered off so that the output shaft can accurately stop at the corresponding position. If the brake fails, it will reduce the control accuracy of the electric actuator. So pay attention to the following 3 points:

• Adjust the braking torque according to the load,
• The gap between the brake wheel and the brake disc should be adjusted appropriately.
• Brake shoes must have a sufficient friction coefficient, and oily substances are strictly prohibited (do not exceed the upper limit of the oil mark when filling with lubricating oil).

3rd, there is a hysteresis link in the output of the electric actuator. The main hysteresis link exists in the connecting part between the output shaft of the quarter-turn electric actuator and the adjustment mechanism. Therefore, there must be no looseness or gaps when installing all connecting rods and levers. Please try to use special couplings instead of linkages. Pay attention to the on-site installation conditions when processing the coupling. It is necessary to ensure that the electric actuator outputs 0~90° rotation angle, the valve body 0~90° angle and the valve position feedback signal 4~20mA correspond to each other.

4th, the sensitivity of the servo amplifier is too low. Excessive sensitivity (≤130A) will cause the electric actuator to oscillate, which will break down the split-phase capacitor of the single-phase servo motor, or burn the servo motor and paralyze the control system. If the sensitivity is too low (≥450A), the action of the actuator will not be timely, which will seriously affect the control accuracy and adjustment quality. Therefore, the sensitivity of the servo amplifier should be adjusted between 180 and 340A according to the response time of the control object and the adjustment characteristics of the control system.

5th, the error of the electric actuator valve position feedback signal is large. Vibration is the main reason for the large error in the valve position feedback signal of the electric actuator. For the differential transformer feedback system, the vibration makes the transformer core run unstable, resulting in displacement and increasing the error of the feedback signal. The potentiometer feedback system causes poor contact with the main potentiometer, and the feedback signal jumps disorderly, resulting in a huge deviation between the real valve position and the feedback valve position of the actuator. For this reason, the electric actuator is changed from being fixed on the platform to being fixed integrally with the butterfly valve, which eliminates the vibration phenomenon of the electric actuator and ensures the stability and reliability of the feedback signal. Therefore, the installation of the electric actuator should avoid the vibration source, be installed on a solid table or integrated with a larger process pipeline, and regularly check the corresponding relationship between the actual valve position and the feedback signal, so that the relative error is controlled within 1%.

6th, the selection of the electric actuator is not suitable. When the electric actuator is used to control a dirty medium or a fluid with a relatively large density, the load capacity of the electric actuator should be considered comprehensively, and the electric actuator with a larger output torque should be selected as much as possible. At the same time, the regulating valve body should be cleaned regularly to ensure the adjustment of the valve shaft rotates flexibly to ensure the control accuracy of the electric actuator. For example, the six sets of electric actuators are used to control ammonia water and tar, and the regulating valve is often covered with tar. For this reason, we need to formulate a strict cleaning system, insist on cleaning with steam once a month, and do a good job of daily inspection to ensure the flexibility, stability and control precision of the electric actuator.