Basic Theory And Experimental Study Of Piezoelectric Directly Dirven Electro-hydraulic Servo Valves

Piezoelectric directly driven electro-hydraulic servo valves are developed in thisthesis with the support of national high technology developed program "Thedevelopment and research of high speed and high precision piezoelectric driven servovalve". This servo valve is used to displace the traditional electro magnetic motor andrealized the high speed and high precision of fluids control by using piezoelectricmultilayered actuator (PMA) which have the characteristics of high power destiny,large output force, fast response and high resolution. The displacement of PMA isamplified by bridge-type and lever-type mechanical amplifiers. This kind of servo valveis studied by theoretical derivation, FEM and experimental test in this paper.1. The research and development of piezoelectric driven fluids control elementsNowadays, the piezoelectric control valves, such as the electrohydraulic ON/OFFvalves, electrohydraulic proportional valves and electrohydraulic servo valves, aremainly research areas for the domestic and foreign researchers. The application ofON/OFF valves mainly concentrates on MEMS active valves and fuel injection controlvalves, while piezoelectric electrohydraulic proportional valves and piezoelectricelectrohydraulic servo valves are mainly used in electrohydraulic proportional systemand electrohydraulic servo system. Piezoelectric electrohydraulic servo valves are thehot research areas because of they have the characteristics of high frequency, highresolution, compact structure and high antipollution ability. The present study is in twoextremes. The high bandwidth valves have small flow rates, while the large flow ratesvalves have low bandwidth. Such cases limits the application occasions. The purpose ofthis paper is to present a kind piezoelectric servo valve which has the high frequencyresponse and larger flowrate at the same time.2. The basic theory of piezoelectric actuationThe piezoelectric actuation principle and the correlative basic theory is thetheoretical start of analyzing PMAs. So it is necessary to expatiate the basic theory ofpiezoelectric actuation. The basic physical properties and main performance parametersare discussed. They includes electromechanical coupling factor, piezoelectric constantand mechanical quality factor etc. The relationship of stress, strain and electric variablesare discussed combined with four groups of piezoelectric equations. The basic theory ofpiezoelectric actuation is clarified and it will indicate the modeling analysis of PMA.3. The analysis and experimental study of piezoelectric multilayered actuatorThe drive elements of Piezoelectric directly driven electro-hydraulic servo valvesare the critical factors, so the PMA is necessary to be analyzed deeply. Themathematical model of PMA is established using energy method and the transferfunction of PMA is obtained;The simplified model of PMA is obtained based on theequivalent circuit theory;The FE model is established by FEM, and the outputdisplacement characteristics, open-circuit stiffness and short-circuit stiffness is analyzed.The natural frequency and modal is analyzed with the modal analysis. The precisedisplacement measure and control system, stiffness testing system is established and thedisplacement and stiffness characteristics are tested and analyzed;The impedanceanalysis of PMA is carried by impedance analysis methods and the impedancecharacteristics is obtained;The frequency response of PMA is tested by establishingdynamic testing system and the dynamic characteristics are measured. The results oftheoretical analysis and experimental testing show that the FEM result of TokinAE0505D16's displacement is 12.56μm. It is closed to the experimental result of11.89μm. So the FEM results are valid. The influence of piezoelectric ceramic platethickness to the output displacement is analyzed by using FEM. The results show thatthe output displacement is increasing corresponding to the decreasing of thickness. TheFEM results of open-circuit and short-circuit stiffness is conformable to theexperimental results. And the results show that the short-circuit stiffness of PMA is lessthan open-circuit stiffness. The FEM results of open-circuit stiffness is 58.6 N/μm andshort-circuit stiffness is 46.5 N/μm;The experimental results of open-circuit stiffness is63.3N/μm and short-circuit stiffness is 48.0 N/μm. These two results meet well and theFEM has good solving accuracy.4. The theoretical modeling and FEA of micro displacement amplifierThe output displacement of PMA needs to be amplified because the stroke ofPMA is not enough compared with the required stroke of spool in piezoelectric servovalves. The current flexure hinge and mechanical amplifier is summarized and analyzed.The principle of bridge type micro displacement amplifier and lever type amplifier isinvestigated deeply with the mathematic model of single-axle flexure hinges, and thetheoretical model of amplification factor, input stiffness and output stiffness isconcluded. The influencing rules of the performance for the amplification mechanismto the structure parameters are obtained. The amplification factor is increased with thelength of connecting rod l;decreased with the angle of connecting rodα . The inputstiffness is increased with the minimum thick t;decreased with the length of connectingrod l andα . The output stiffness is increased with the minimum thick t;decreasedwith the length of connecting rod l. The amplification factor of lever type amplifier lieson the ratio of the two arms of force and the ratio of input and output stiffness equals tothe square of amplification factor. The amplification factor, input stiffness, outputstiffness of the two kinds of amplifiers and influencing factors of the performance areanalyzed using FEM. Compared with the theoretical results, the two results ofamplification factor and output stiffness meet well, while the result of input stiffnesshas a bigger difference between the two kinds of results.5. The analysis and modeling of spool movement mechanismThe spool movement mechanism is the hardcore of the piezoelectric directlydriven servo valves. It is a generic term of movement components which realized theshuttling of spool in piezoelectric servo valves. It is composed of PMA(act as powersource), flexure hinged mechanical amplifier, spool, comeback spring plate andconnecting pieces. The structure parameters of the spool movement mechanism aredetermined elementarily by theoretical calculations. The optimized structure parametersis obtained by direct cross factor analysis method combined with FEM. The dynamicmodel of bridge type spool movement mechanism is established and the transferfunction is deduced. The model is simulated by Matlab Control Toolbox and the stepresponse and frequency response is obtained. The influences of damping ratio andequivalent mass to the mechanism performance are analyzed. The first naturalfrequency of the mechanism is gained by model analysis and harmonic analysis andthis verified the validity of theoretical model. The results of FEM show that the spoolstroke of the movement mechanism reached 61.34μm, and the first natural frequencyreached 1107Hz for the Tokin AE0505D16 PMA. According to the above steps, thelever type spool movement mechanism is modeled and the structure parameters aregained. The dynamic model and FEM model are established. The static and dynamiccharacteristics are simulated by using Matlab simulation toolbox and Ansys FEsoftware. The results show that the spool stroke of lever type spool movementmechanism which composed of PSt150/7/80 PMA and comeback spring plate reaches202μm and natural frequency reaches 1050Hz.6. The experimental study on the trial-manufacture prototype of piezoelectricdirectly driven servo valvesThe prototypes of piezoelectric servo valves are trial manufactured based on thetheoretical analysis of the former spool movement mechanism. The experimental studyhas been carried on the basis of prototypes. The displacement resolution, displacementoutput characteristics, flow rates, step response and frequency response are tested. Theexperimental testing system is established aims at the piezoelectric directly driven servovalves and it consists static testing system and dynamic testing system. Thedisplacement characteristics and flow rate characteristics can be tested automatically.The amplifier mechanism is manufactured by electric spark method. This mechanismand Tokin AE0505D16 PMA compose the prototype of bridge type piezoelectricdirectly driven servo valve. The static and dynamic characteristics are tested on theprototype. The results show that this servo valve's bandwidth can reach 1050Hz, thespool stroke reaches 56μm and the maximum control flow rate reaches 1.44 L/min. ThePSt150/7/80 PMA is adopted to form a lever amplification piezoelectric directly drivenservo valve combines with lever type amplifier in order to enhance the maximumcontrol flow rates. This valve adopts the build-in manner of spool and the bidirectionalmovement of spool is realized by single PMA;The radial force is reduced by steel ballconnection;The precise adjustment of spool position is realized by using wedge typenull adjustment mechanism;The pre-tight force is adjustable by using screw pairadjustment device. The lever amplified piezoelectric servo valve is assembled and trialmanufactured. The experimental results show that the spool stoke reaches 185.85μm,the maximum control flow rate reached 5.9L/min at the pressure of 7MPa, the responsetime is 1. 08mS and bandwidth is larger than 1000Hz.7. The control mechod of piezoelectric directly driven servo valvesThe servo valve controller is developed aims at the characteristics of piezoelectricdirectly driven servo valve. This controller is composed of signal processing unit,power amplification unit, displacement measuring unit and the information display unitof system. The signal processing unit processes the signal by MSC51 microprocessor.The close-loop control of spool stroke is realized by the processor and PID controlalgorithm. The hysteresis is eliminated by the close-loop control. The position testingunit is the transforming unit of strain gauge sensor and the high speed and precisemeasurement of position signal is realized. The combined PID control algorithm basedon the dynamic feedforward Prersach model is adopted aims at the character of thehysteresis which changes with the frequency. The control effect is nice.The maximumcontrol error is 0.45μm and the control accuracy reaches 0.25%.