Created on:2021-08-27 15:30

Fault diagnosis and maintenance of marine hydraulic equipment_ Fault of electro-hydraulic servo control system

Fault diagnosis and maintenance of marine hydraulic equipment_ Fault of electro-hydraulic servo control system

As an effective fault protection device, hydraulic fault protection module has been widely used in electro-hydraulic servo control system. Especially after the electro-hydraulic servo system has developed from an independent and single system to a complex coordinated loading system with multi-channel and composite control, the hydraulic fault protection module shows its advantages in improving the safety and reliability of the system.

The hydraulic fault protection module is an integrated block installed with various hydraulic valves and fixed with the hydraulic cylinder. When the system has a general fault, the system is in a safe state through the function of the hydraulic valve to achieve the purpose of fault protection.

Common fault forms of electro-hydraulic servo control system

① Overload is one of the most common faults in the electro-hydraulic servo control system. Overload may cause misoperation of the system or damage the test piece, which directly affects the safety and reliability of the system. Therefore, the overload hydraulic protection is the most commonly used fault protection device. Generally, the overflow valve is used to limit the maximum pressure of the load chamber of the hydraulic cylinder. Common overload faults are as follows.

a. Servo valve failure. Failure is often encountered in servo system, especially in system commissioning and initial operation stage, which is caused by system pollution. Failure of the servo valve will result in overload of the loading system. Therefore, some anti pollution measures shall be taken for the servo system before use. For example, the servo valve shall be replaced by the flushing cover plate. The hydraulic oil shall circulate for a certain time. A filter with high precision shall be installed in the circuit, and the commissioning and operation shall be started after flushing.

b. Closed loop control failure. In the closed-loop control system, once the feedback loop fails, it becomes open-loop control, which will lead to overload; If the feedback signal is reversed or the command signal is too large, it will also cause system overload. These two situations are easy to occur during system debugging.

c. Force coupling overload. In a multi-channel electro-hydraulic servo system, the channels interact with each other to produce a coupling force. This coupling force may cause system overload, which is undesirable to the system. The solution is usually to eliminate it by decoupling method. In some systems, such as multi-channel coordinated loading system of structural fatigue and electro-hydraulic servo system of wave maker, hydraulic load limiting is used to eliminate the overload caused by coupling force between channels, which is easy to realize.

② Power failure is often encountered in the operation of hydraulic system. Power system failure; Manually cut off the system power supply in case of emergency failure; In addition, the protection devices set on the hydraulic oil source, such as motor overload, overheating, system pressure drop and high oil temperature, may cut off the power of the system. In some electro-hydraulic servo control systems, when the system is powered off, the servo valve control system is also powered off. For the hydraulic cylinder in the loading state, due to the power off, the servo valve loses the control signal, the valve core automatically returns to the middle position, and the hydraulic cylinder is maintained in the loading state. This situation is undesirable. After the power-off protection device is adopted, any power failure of the system occurs, and the load chamber of the hydraulic cylinder communicates with the oil return circuit to make the system in the unloading state.

③ The influence of cavitation on the working quality of cavitation hydraulic system has attracted great attention. In the servo control system, the cavitation caused by the control load is easy to be ignored, but this cavitation is often more serious. For example, in the large inertia servo loading system, when the servo valve returns to the zero position or reversing control, the hydraulic cylinder piston cannot stop or reverse immediately due to the inertia load, resulting in instantaneous overload in one chamber and cavitation in the other chamber. In addition, in the servo control system with random load change, the air flow disturbance and wave impact in the ship rudder control may also empty the load chamber of the hydraulic cylinder and produce cavitation. Such cavitation in the system will damage the inner hole of the cylinder barrel, the surface quality of the piston and the working sharp edge of the fine valve pair of the servo valve.

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