| The power piezoelectric hydraulic pumps utilize the inverse piezoelectric effect of the piezoelectric material,combined with the rectification effect of the valve,to convert the high-frequency small stroke of the piezoelectric material into a one-way flow of the liquid,and push the cylinder shaft and load to perform external work,with the advantages of high power density,fast response,easy miniaturization,etc.,which have broad application prospects in aerospace,automotive and robotics field,etc.However,the power of piezoelectric hydraulic pumps is generally low at present,the main reason is that the operating frequency is not high,and the liquid inertia,the dynamic characteristics of the valve,and the characteristics of the piezoelectric actuator,such as the inherent loss,limit the working frequency.As a result,the advantage of high power density of piezoelectric material has not been brought into full play.For this reason,this paper starts from the working principle of piezoelectric hydraulic pumps,and analyzes in detail the main factors restricting the increase of the working frequency of piezoelectric hydraulic pumps,and the improvement measurements are proposed from the aspects of creating a high-pressure liquid environment to create conditions for highfrequency work,introducing a vibration filter to reduce the liquid inertia,introducing a mechanical elastic structure to compensate for the liquid and piston inertia,and using liquid elasticity to compensate for the liquid inertia.The main work and conclusions of this paper are as follows:It theoretically analyzes the factors restricting the improvement of the working frequency of power piezoelectric hydraulic pump,and puts forward that creating a high bias pressure liquid environment can create conditions for the high-frequency operation of piezoelectric hydraulic pump.Firstly,the effects of liquid inertia,dynamic characteristics of valve and characteristics of piezoelectric actuator at high frequency are analyzed,and the problems to be considered at high frequency are put forward.Then it analyzes the advantages of increasing bias pressure,which can inhibit cavitation and improve the maximum working frequency of piezoelectric hydraulic pump.At the same time,high bias pressure can also increase the liquid stiffness,reduce the energy loss caused by liquid compressibility,and increase the maximum output pressure of piezoelectric hydraulic pump,which is conducive to reducing the damping loss of liquid in pipeline and realizing high-efficiency liquid transportation.The effects of high bias pressure on the piezoelectric actuator,the effective bulk modulus of hydraulic oil and the performance of piezoelectric hydraulic pump are studied.Firstly,the performances of piezoelectric actuator under high stress are studied.The results show that the piezoelectric performance of piezoelectric actuator under appropriate high stress does not decrease,which is beneficial to improving the bias pressure of piezoelectric hydraulic pump.At the same time,the measurement method of liquid effective bulk modulus based on piezoelectric actuator and liquid resonance is proposed for the first time,and the change of effective bulk modulus of hydraulic oil in the range of 7 MPa is studied.Finally,a piezoelectric hydraulic pump system driven by piezoelectric actuator is designed to verify the influence of increasing the bias pressure.The experimental results show that within a certain pressure range,the higher the bias pressure,the greater the maximum blocked force of the piezoelectric hydraulic pump.A method of applying vibration filter to piezoelectric hydraulic pump to reduce the liquid inertia is proposed.Firstly,based on solid-liquid analogy,the effect of the vibration filter is analyzed,and then the influence of the vibration filter on the discharge speed and flow rate of the piezoelectric hydraulic pump is studied through the fluidsolid coupling simulation.The simulation results show that adding an elastic cavity can improve the liquid discharge speed and flow,and the closer the elastic cavity is to the liquid discharge port,the better the effect.It shows that the introduction of the elastic cavity makes the directly pushed liquid confined in the elastic cavity,thereby reducing the liquid inertia.Finally,according to the characteristics of the high-pressure environment of the power piezoelectric hydraulic pump,a gas cavity with good elasticity and high pressure resistance is proposed as a vibration filter.A prototype was designed and produced,and good experimental results are obtained.A mechanical elastic structure is introduced to compensate for the inertia of the liquid and piston,and high-frequency piezoelectric hydraulic pumps driven by the resonant vibrator are designed.The introduction of a mechanical elastic structure to drive the liquid can convert the large output force and small displacement of the piezoelectric actuator into appropriate output force and large displacement,which is beneficial to improving the efficiency of the valves.At the same time,the mechanical elastic structure can largely offset the inductive impedance caused by the liquid and the piston mass during resonance,which can greatly improve the power factor of the piezoelectric actuator power.Two kinds of resonant vibrators are designed using the rhombic micro-displacement amplifier as the mechanical elastic structure.The resonant vibrator based on the double micro-displacement amplifiers has two working modes.When the bias pressure is 2 MPa and the exciting voltage is 500 VPP,the resonant frequency of the system is 720 Hz when driven by the same direction vibration mode,the maximum output velocity and blocked force are 11.3 mm/s and 47.5 N,respectively,and the corresponding maximum flow and pressure are 85.7 mL/min and 0.38 MPa,respectively;the resonant frequency of the system is about 1590 Hz when driven by the reverse vibration mode,the maximum output velocity and blocked force of the system are 20 mm/s and 66 N,respectively,and the corresponding maximum output flow and pressure are 151.7 mL/min and 0.52 MPa,respectively.When the piezoelectric hydraulic pump is driven by the resonant vibrator based on the single microdisplacement amplifier,the resonant frequency is about 650 Hz under no-load condition,and when the exciting voltage is 600 Vpp,the maximum no-load velocity and blocked force are 10.22 mm/s and 165 N,respectively,and the corresponding maximum output flow and pressure are 77.5 mL/min and 1.3 MPa,respectively.The use of liquid elasticity to compensate for the liquid inertia is proposed,and a high-pressure and high-frequency piezoelectric hydraulic pump driven by liquid resonance is designed.Based on electro-hydraulic analogy and acoustic analogy,the resonant frequency of liquid is deduced theoretically,and the characteristic frequency of liquid is simulated.The working principle of piezoelectric hydraulic pump driven by liquid resonance is analyzed.The experimental device is designed and built for verification.The experimental results show that the designed piezoelectric hydraulic pump can work well at about 2.4 kHz,and within the bias pressure of 6 MPa,the higher the pressure,the greater the no-load velocity,indicating improving the bias pressure is conducive to improving the performance of the piezoelectric hydraulic pump.When the exciting voltage is 400 VPP and the bias pressure is 5 MPa,the maximum no-load velocity of the system is 5.4 mm/s and the maximum blocked force is 181 N,and the corresponding maximum output flow and pressure are 40.8 mL/min and 1.43 MPa,respectively.When the working frequency is 2.44 kHz,the efficiency can reach 9.7%. |
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