The Liquid Level System's Sensor Design And Fault Diagnosis And Fault Tolerance Control Research

The liquid level control system plays an important role in many industry productions.The traditional PID(proportion-integration-differential)control method occupies the main position in liquid level process control.Due to the fact that liquid level control system,especially its sensor is easy to be in fault,PID control method has the weakness of lacking the ability to anti fault,which may cause huge loss.So it's needful to judge the state of liquid level control system's running state,abnormal conditions and make the diagnosis result as the basis of system's fault recovering.Adopt fault tolerance control system to ensure liquid level control system still works relatively normally when the system's sensor is in fault is also very necessary.Currently fault diagnosis has the drawback of low fault resolution,low ability to deal with uncertain knowledge.Meanwhile,the fault tolerance control is more to be in depth study because its late appearance and immature theory system.So it's very necessary for this paper to study on the liquid level sensor fault diagnosis and system's fault tolerance control.This paper mainly does the research on these three aspects below:(1)Use the electrode distance variance capacitor style pressure sensor to measure the liquid level and design this electrode distance variance capacitor style sensor's measuring circuit on the basis of its operational principle.Obtain the most fitting material parameters according to the sensor configuration parameters' desired values and the parameter restrict on production processing,practicability and cost;also obtain the main circuit elements' parameters according to the sensor measuring circuit's amplifier characteristics,bridge characteristics,output signal acquisitions and so on.Then design the modules such as sensor measuring circuit input signal's transformer circuit,output signal's filter circuit,data acquisition frame,fault notification buzzer.These modules form the complete liquid level sensor.(2)Acquire the sensor fault existence criterion according to the principle that the sensor's measuring circuit signal must be in the rated range when in normaland the liquid level can't change sharply.Take the advantage of the different features in different sensor fault elements and different fault type to distinguish the complete faults and the numerical drift faults and realize the fault location accurate to measuring circuit's single element.Then realize the fault evaluation on the basis of the relation expressions of the measuring circuit's elements and the circuit output signal.Make research on the processing method of simulation data to improve accuracy of the fault evaluation.The simulations prove that the fault location of the liquid level sensor can be accurate to the measuring circuit's single elements.The errors of numerical drift fault evaluation are mostly below 5% and the evaluation errors with elements value smaller more than 50% are below 10%.(3)Use linear parameter varying observer to estimate the sensor liquid level fault values and the states of the system.Suspend the sensor to control the nominal controller and replace the sensor by the states estimated by the observer.Thus the liquid level control system can't be influenced by the fault sensor and now has the function of fault tolerance.Under the fault tolerance,the liquid level transform is below 1.5cm/s relative to the reference signal.