sensors Article
Development and Characterization a Single-Active-Chamber Piezoelectric Membrane Pump with Multiple Passive Check Valves Ronghui Zhang 1 , Feng You 2,† , Zhihan Lv 3,† , Zhaocheng He 1,† , Haiwei Wang 2, * and Ling Huang 2,† 1 2 3
* †
Research Center of Intelligent Transportation System, School of Engineering, Sun Yat-sen University, Guangzhou 510275, China;
[email protected] (R.Z.);
[email protected] (Z.H.) School of Civil Engineering and Transportation, South China University of Technology, Guangzhou 510640, China;
[email protected] (F.Y.);
[email protected] (L.H.) Department of Computer Science, University College London, 66-72 Gower Street, London WC1E 6EA, UK;
[email protected] Correspondence:
[email protected]; Tel.: +86-135-6010-8489 These authors contributed equally to this work.
Academic Editor: Houbing Song Received: 26 September 2016; Accepted: 5 December 2016; Published: 12 December 2016
Abstract: In order to prevent the backward flow of piezoelectric pumps, this paper presents a single-active-chamber piezoelectric membrane pump with multiple passive check valves. Under the condition of a fixed total number of passive check valves, by means of changing the inlet valves and outlet valves’ configuration, the pumping characteristics in terms of flow rate and backpressure are experimentally investigated. Like the maximum flow rate and backpressure, the testing results show that the optimal frequencies are significantly affected by changes in the number inlet valves and outlet valves. The variation ratios of the maximum flow rate and the maximum backpressure are up to 66% and less than 20%, respectively. Furthermore, the piezoelectric pump generally demonstrates very similar flow rate and backpressure characteristics when the number of inlet valves in one kind of configuration is the same as that of outlet valves in another configuration. The comparison indicates that the backflow from the pumping chamber to inlet is basically the same as the backflow from the outlet to the pumping chamber. No matter whether the number of inlet valves or the number of outlet valves is increased, the backflow can be effectively reduced. In addition, the backpressure fluctuation can be significantly suppressed with an increase of either inlet valves or outlet valves. It also means that the pump can prevent the backflow more effectively at the cost of power consumption. The pump is very suitable for conditions where more accurate flow rates are needed and wear and fatigue of check valves often occur. Keywords: piezoelectric membrane pump; PZT actuator; multiple check valves; flow rate; backpressure
1. Introduction Micropumps are attracting interest in the research community because of their major applications in pharmaceutical and bio-medical drug delivery systems, microelectronics cooling, and their use in chemical and biological substance analysis systems as microfluidic flow control appliances [1–4]. The first micropumps were presented in the 1970s and 1980s [5–8]. In the past three decades, researchers have been designing and developing various micropumps using different actuation methods such as piezoelectric, electromagnetic, electrostatic, thermopneumatic and shape memory alloy (SMA) devices [1–4]. To date, micropumps have already been implemented in the market Sensors 2016, 16, 2108; doi:10.3390/s16122108
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and are widely used in ink jet printers and fuel injection applications [1,4]. Although most micropumps have complex structures and a high level of power consumption, piezoelectric actuation has the advantages of a relatively simple structure, short response time and relatively low power consumption [3,9,10]. Typically, piezoelectrically-actuated pumps use a piezoelectric actuator to move a membrane in a chamber providing fluid delivery with the flow direction being controlled by check valves [11,12]. Applications can be found from micro-dosing drug administration to propelling microspacecraft [13–17]. Presently, piezoelectric pumps can roughly be categorized by their valve types as with check valve or without valve [18–20]. Piezoelectric check-valve pumps are usually comprised of a periodically working membrane and two passive check valves. Valveless micro-pumps make use of diffuse/nozzle structures to replace passive check valves as the flow rectifying elements. Because check-valve piezoelectric pumps can overcome higher flow resistance and be used for both liquids and gases [21,22], check-valve piezoelectric membrane pumps have been the focus of a lot of research and development activities [23–26]. Various piezoelectric actuators like unimorph, bimorph and stack variants were used to drive the membrane pumps [27–30]. More working media such as water, air, oil, alcohol, ethanol and methanol could be delivered. Not only traditional fabrication techniques and materials, but also MEMS technologies and new materials such as PDMS, silicon rubber, plastic and parylene are widely applied in this area. Furthermore, much attention has been paid to the development of passive check valves. A variety of ball-, membrane-, cantilever-, umbrella-, bridge-, and wheel-type passive check valves were presented [1,21,28,31]. Nevertheless, even though great progress has been made during the last thirty years, there is a lot of room for further improving the performance of existing check-valve piezoelectric membrane pumps. There are still some limitations for practical applications, especially the backward leakage of check valves. In fact, not only check-valve piezoelectric pumps suffer from the backflow problem, but this problem is also more serious for valveless piezoelectric pumps because of the lack of blocks. A number of works on pumps and valves were conducted to reduce the backflow and improve the pumping performance [1,22,24,27,32–35]. In this paper, we propose a single-active-chamber piezoelectric membrane pump with multiple passive check valves, which is intended to reduce the backflow by means of increasing the number of passive check valves. Because the single-active-chamber piezoelectric pump has more than one check valve, it is very important to configure the multiple check valves and find out the influence of different configurations of multiple check valves on the pump properties. Through basic experiments on frequency characteristics, the pumping rate and output backpressure of a check-valve piezoelectric membrane pump is investigated by changing the number of inlet valves and outlet valves. 2. Working Principle A piezoelectric pump uses a PZT actuator to move a membrane in a chamber providing fluid entrance and exit with the flow rectification being controlled by check valves. A classic piezoelectric membrane pump is comprised of a piezoelectric membrane, a pumping chamber connected to inlet and outlet valves, and pump body, as illustrated in Figure 1. The working principle can be described as a cyclic process of an expansion stroke and a compression stroke. When an AC voltage is applied to the PZT actuator, the membrane deflection will alternately result in the pumping chamber expansion and compression. During the expansion stroke, with the increase of the pumping chamber volume, under-pressure is generated in the pumping chamber and the inlet valve opens with the corresponding decrease in chamber pressure. Consequently, working medium is sucked into the expanding chamber. During the compression stroke, over-pressure is generated in the pumping chamber because of the chamber volume decrease and thus the outlet valve opens. Working media are discharged through the outlet valve.
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Figure 1. The structure of a classic piezoelectric membrane pump. Figure 1. The structure of a classic piezoelectric membrane pump. Figure 1. The structure of a classic piezoelectric membrane pump.
When the PZT actuator is driven by alternating voltage, a piezoelectric membrane pump can When the driven by aa piezoelectric pump transfer the mechanical energyis membrane deflectionvoltage, into fluid movement. membrane Then the mechanical When the PZT PZT actuator actuator isof driven by alternating alternating voltage, piezoelectric membrane pump can can transfer the mechanical energy of membrane deflection into fluid movement. Then the mechanical vibration of the piezoelectric membrane is transmitted thefluid check valves through themechanical working transfer the mechanical energy of membrane deflection to into movement. Then the vibration of membrane is transmitted to the check valves through theisworking fluid. fluid. Apparently, it takes some time to perform the transmission process. The through valve constrained vibration ofthe thepiezoelectric piezoelectric membrane is transmitted to the check valves the working Apparently, it takes some time to perform the transmission process. The valve is constrained by by the force of inertia caused by the working fluid. Therefore, the actions (open/close) of the passive fluid. Apparently, it takes some time to perform the transmission process. The valve is constrained the force of inertia caused by by the working fluid. the of passive check valves always lagcaused behind the vibration offluid. theTherefore, PZT actuator. Ideally, the outlet valve should be by the force of inertia the working Therefore, theactions actions(open/close) (open/close) of the the passive check valves always lag behind the vibration of the PZT actuator. Ideally, the outlet valve should closed when the inlet valve opens during the expansion stroke. However, because of the outlet check valves always lag behind the vibration of the PZT actuator. Ideally, the outlet valve should be be closed when the inlet valveopens opens during theexpansion expansion stroke. However, because of the the outlet valves’ closing lag, some fluids whichduring have been dischargedstroke. are redrawn intobecause the pump chamber closed when the inlet valve the However, of outlet valves’ lag, some fluids which have discharged are into chamber through the outlet backflow Similarly, because of the closing lag pump from the inlet valves’ closing closing lag,valve, somei.e., fluids which occurs. have been been discharged are redrawn redrawn into the the pump chamber through the outlet valve, i.e., backflow occurs. Similarly, because of the closing lag from the valves during the compression stroke, some fluids which have been sucked are forced out of the through the outlet valve, i.e., backflow occurs. Similarly, because of the closing lag from the inlet inlet valves during the compression stroke, some fluids which have been sucked are forced out of the pump pump through the inletstroke, valve some over fluids again. which So far,have various efforts have mostly valveschamber during the compression beenresearch sucked are forced out of the chamber through the inlet valve over again. So far, various research efforts have mostly focused on focused on adopting new materials and techniques, developing new valve types and optimizing the pump chamber through the inlet valve over again. So far, various research efforts have mostly adopting new materials and techniques, developing new valve types and optimizing the existing check existing check valves. Inspired by the works in [36,37], where Hwang et al. designed a leakage focused on adopting new materials and techniques, developing new valve types and optimizing the valves. Inspired by the to works in [36,37], etinterfering al. where designed a leakage in the barrier in the chamber reduce leakage flow Hwang without with theetnet positive fluid flow, existing check valves. Inspired by thewhere works in [36,37], Hwang al.barrier designed achamber leakage to reduce leakage flow without interfering with the net positive fluid flow, and Ma et al. added and Ma et al. added a secondary chamber to obstruct the stream and reduce the backflow toward barrier in the chamber to reduce leakage flow without interfering with the net positive fluid flow, aand secondary chamber obstruct the streamtoand reducethe themembrane backflow the the primary chamber, piezoelectric pump multiple passive Ma et al. added aatosingle-active-chamber secondary chamber obstruct stream andtoward reducewith theprimary backflowchamber, toward athe single-active-chamber membrane pump with2.multiple passive check is proposed check valves is proposed in this paper, as shownpiezoelectric in Figure This configuration ofvalves multiple passive primary chamber, apiezoelectric single-active-chamber membrane pump with multiple passive in thisvalves paper,isis asintended shown into Figure 2. This configuration of multiple passive checkof valves is intended check prevent the as backflow toward the working chamber. Then each check check valves proposed in this paper, shown in Figure 2. This configuration multiple passive to prevent the backflow toward the working chamber. Then each check valve can be regarded as valve be regarded as a to leakage barrier. During the expansion stroke, chamber. multiple outlet are checkcan valves is intended prevent the backflow toward the working Then valves each check avalve leakage barrier. expansion stroke, multiple outlet valves are able to reduce thevalves backflow able to can reduce theDuring backflow of the barrier. fluids which have been discharged. Likewise, multiple inlet be regarded asthe a leakage During the expansion stroke, multiple outlet are of the fluids which have been discharged. Likewise, multiple inlet valves can effectively prevent the valves can effectively prevent the fluids which have been sucked into the pump chamber from able to reduce the backflow of the fluids which have been discharged. Likewise, multiple inlet fluids havethe been sucked into pump chamber inlet. flowing back into inlet. valveswhich can effectively prevent the the fluids which have from been flowing sucked back into into the the pump chamber from flowing back into the inlet.
Figure 2. Cont.
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Figure 2. The demonstration structure of a single-active-chamber piezoelectric membrane pump Figure The demonstration demonstrationstructure structureof of a single-active-chamber piezoelectric membrane pump Figure 2. The a single-active-chamber piezoelectric membrane pump with with multiple passive check valves. with multiple passive check valves. multiple passive check valves.
3. Fabrication 3. 3. Fabrication Fabrication In this work, work, six passive passive check valves valves are used used to construct construct the piezoelectric piezoelectric membrane pump. pump. In In this this work, six six passive check check valves are are used to to construct the the piezoelectric membrane membrane pump. As shown in in Figure Figure 2, 2, V Vii (i(i == 1, 1, 2, 2, …, …, 6) 6) represent represent the the passive passive check check valves. valves. The The casting-rubber casting-rubber As As shown shown in Figure 2, V i (i = 1, 2, . . . , 6) represent the passive check valves. The casting-rubber moulding method was adopted to form six umbrella-shaped check valves and six little valve valve moulding method was wasadopted adoptedto to form umbrella-shaped check valves and sixvalve little moulding method form six six umbrella-shaped check valves and six little orifices orifices with with aa diameter diameter of of Φ1.2 mm mm were were designed designed to to enhance enhance reverse-check reverse-check capability. capability. The The orifices with a diameter of Φ1.2 mm Φ1.2 were designed to enhance reverse-check capability. The piezoelectric piezoelectric membrane with a size of Φ35 mm × 0.53 mm is a commercial piezoelectric sound piezoelectric membrane withmm a size of mm Φ35ismm × 0.53 mm is a commercial piezoelectric sound membrane with a size of Φ35 × 0.53 a commercial piezoelectric sound component produced component produced produced by by Murata Murata Manufacturing Manufacturing Co., Co., Ltd. Ltd. (Nagaoka, (Nagaoka, Japan), Japan), where where the the thicknesses thicknesses component by Murata Manufacturing Co., Ltd. (Nagaoka, Japan), where the thicknesses of the piezoelectric of the the piezoelectric piezoelectric component component with the the diameter diameter of of Φ25 mm mm and and the the metal plate plate with with the the diameter of component with the diameter with of Φ25 mm and the Φ25 metal plate with metal the diameter of Φ35diameter mm are of Φ35 mm are 0.23 mm and 0.3 mm, respectively. In fact, we adopted five similar piezoelectric of Φ35 mm are 0.23 mm and 0.3 mm, respectively. In fact, we adopted five similar piezoelectric 0.23 mm and 0.3 mm, respectively. In fact, we adopted five similar piezoelectric membranes to membranes to characterize characterize the the piezoelectric piezoelectric membrane membrane pump pump with with different different numbers numbers of of inlet inlet and and membranes characterizetothe piezoelectric membrane pump with different numbers of inlet and outlet valves. outlet valves. valves. The The pump shell shell and and body body are are made made of of polymethyl polymethyl methacrylate methacrylate which which has has the the outlet The pump shell andpump body are made of polymethyl methacrylate which has the advantages of easy advantages of easy processing, light weight, insulation and easy observation. After two advantages of easy processing, weight, insulation and observation.check Aftervalves two processing, light weight, insulation light and easy observation. After twoeasy umbrella-shaped umbrella-shaped check valves close to the piezoelectric membrane as well as the membrane are umbrella-shaped check valves close as to well the piezoelectric membrane as well asa the membrane are close to the piezoelectric membrane as the membrane are assembled, pumping chamber assembled, a pumping chamber is ready. The room between other two neighbouring check valves assembled, a pumping chamber is ready. The room between other two neighbouring check valves is ready. The room between other two neighbouring check valves forms several fluidic channels. forms several several fluidic fluidic channels. channels. The The prototype prototype photograph photograph of of the the single-active-chamber single-active-chamber piezoelectric piezoelectric forms The prototype photograph of the single-active-chamber piezoelectric membrane pump with multiple membrane pump pump with with multiple passive passive check valves valves is is shown shown in in Figure Figure 3. 3. membrane passive check valves is multiple shown in Figure check 3.
Figure of of thethe single-active-chamber piezoelectric membrane pumppump with Figure 3.3. The Theprototype prototypephotograph photograph single-active-chamber piezoelectric membrane Figure 3.passive The prototype photograph of the single-active-chamber piezoelectric membrane pump multiple check valves. with multiple passive check valves. with multiple passive check valves.
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4. Experimental Setup 4. Experimental Setup The performance of the proposed piezoelectric pump needed to be experimentally measured in The performance of the proposed piezoelectric pump needed to be experimentally measured terms of flow rate and backpressure that the prototype pump could achieve. Figure 4 depicts the in terms of flow rate and backpressure that the prototype pump could achieve. Figure 4 depicts schematic diagram of the experimental setup. The tap water at room temperature, as the most the schematic diagram of the experimental setup. The tap water at room temperature, as the most representative working fluid, is used as working fluid. Two plastic tubes are connected to the inlet representative working fluid, is used as working fluid. Two plastic tubes are connected to the inlet and and outlet of the prototype pump. A digital PZT power supply (P211, SANKI, Nagoya, Japan) is outlet of the prototype pump. A digital PZT power supply (P211, SANKI, Nagoya, Japan) is used to used to drive the piezoelectric membrane pump. The power supply can provide a sinusoidal drive the piezoelectric membrane pump. The power supply can provide a sinusoidal voltage with voltage with a variable frequency of 60–400 Hz and an AC voltage range of 0–240 V. An electronic a variable frequency of 60–400 Hz and an AC voltage range of 0–240 V. An electronic balance with balance with a division value of 0.01 g and a repeatability of 0.01 g is used to measure the outflow. a division value of 0.01 g and a repeatability of 0.01 g is used to measure the outflow. A time relay is A time relay is used to set the time interval to obtain the average flow rate. The time interval of used to set the time interval to obtain the average flow rate. The time interval of sixty seconds is set in sixty seconds is set in each measurement. According to the energy equation, the water column each measurement. According to the energy equation, the water column height which is perpendicular height which is perpendicular to the water free surface is measured by a scale ruler in order to to the water free surface is measured by a scale ruler in order to obtain the pumping pressure value obtain the pumping pressure value (kPa). The measurement of flow rate or backpressure is carried (kPa). The measurement of flow rate or backpressure is carried out through opening and closing out through opening and closing corresponding stop valves. The tee, pressure stop valve and flow corresponding stop valves. The tee, pressure stop valve and flow stop valve are used to choose the stop valve are used to choose the measured object. When the pressure stop valve is closed, the flow measured object. When the pressure stop valve is closed, the flow characteristics can be tested. On the characteristics can be tested. On the contrary, backpressure characteristics can be obtained. Using contrary, backpressure characteristics can be obtained. Using this setup, a series of experiments are this setup, a series of experiments are conducted to explore the performance of the conducted to explore the performance of the single-active-chamber piezoelectric membrane pump single-active-chamber piezoelectric membrane pump with multiple passive check valves. with multiple passive check valves.
Figure Figure 4. 4. Experimental Experimental setup. setup.
5. Results and Discussion In this six six similar passive check check valves were adopted conducttobecause the piezoelectric thiswork, work, similar passive valves were to adopted conduct because the pump cannot properly function if the number of check valves is too many. Actually, a piezoelectric piezoelectric pump cannot properly function if the number of check valves is too many. Actually, a membrane pump can havepump a different number of inlet and outletofvalves. Theoutlet number and configuration piezoelectric membrane can have a different number inlet and valves. The number of check valves must different influence the piezoelectric pump with multiple and configuration of have checka valves must have aon different influence membrane on the piezoelectric membrane valves. Therefore, we mainly focus on the influence of multiple check valves’ various configuration on pump with multiple valves. Therefore, we mainly focus on the influence of multiple check valves’ the performance of the on piezoelectric membrane pump in this paper. As mentioned orderAs to various configuration the performance of the piezoelectric membrane pump before, in this in paper. characterize and fully compare the different performances of the piezoelectric pump with multiple mentioned before, in order to characterize and fully compare the different performances of the check valves, five piezoelectric membranes are used. Then the configuration of inlet outlet valves piezoelectric pump with multiple check valves, five piezoelectric membranes areand used. Then the can be changedofbyinlet means activating anycan onebe PZT actuator,bymeanwhile, inactiveany piezoelectric configuration andof outlet valves changed means of the activating one PZT membranes consists ofthe fluidic channels. Thereby,membranes we realize the configuration changes without altering actuator, meanwhile, inactive piezoelectric consists of fluidic channels. Thereby, we any of the structure, ensuring thealtering same measurement conditions. structure, ensuring the realize themechanical configuration changes without any of the mechanical same measurement conditions.
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5.1. The Results of the Pump with One Inlet Valve and Five Outlet Valves 5.1. The of theofPump with One Inlet Valve and Five Outlet TheResults structure the piezoelectric membrane pump withValves one inlet valve and five outlet valves is shown in Figure 2b. We connected the PZT actuator demonstrated Figure with the valves power The structure of the piezoelectric membrane pump with one inlet in valve and2b five outlet supply toin actuate Through the adjusting the actuation frequency by2b step, thethe flow rate is shown Figurethe 2b.pump. We connected PZT actuator demonstrated in step Figure with power (mL/min) versus frequency (Hz) characteristics is tested at a fixed driving voltage of 150 V. During supply to actuate the pump. Through adjusting the actuation frequency step by step, the flow rate the measurement of the flow rate, the backpressure difference between inlet and theV.outlet is (mL/min) versus frequency (Hz) characteristics is tested at a fixed drivingthe voltage of 150 During maintained to be of zero, thethe flow rate againstdifference zero pressure. Figure shows the measurement thenamely flow rate, backpressure between the 5inlet andexperimental the outlet is results of the pump performance in terms of flow rate versus actuation frequency, keeping the maintained to be zero, namely the flow rate against zero pressure. Figure 5 shows experimental results applied voltage fixed at 150 V. The flow rate is changed with the change of the frequency of the pump performance in terms of flow rate versus actuation frequency, keeping the applied voltage characteristics. There arerate twoispeaks of flow at 100 Hz andfrequency 220 Hz within the whole frequency fixed at 150 V. The flow changed withrates the change of the characteristics. There are range from 60 Hz to 400 Hz and the peak flow rates are almost the same. The maximum flow rateHz of two peaks of flow rates at 100 Hz and 220 Hz within the whole frequency range from 60 Hz to 400 100 the mL/min is achieved the frequency 220maximum Hz and aflow driving of 150 V. The minimum and peak flow rates areatalmost the same.ofThe rate voltage of 100 mL/min is achieved at the flow rate occurs at the frequency of 340 Hz and the valley value of the flow rate is 46.4 mL/min. frequency of 220 Hz and a driving voltage of 150 V. The minimum flow rate occurs at the frequency of Apart thevalley flow value rate, of backpressure important representing the pump 340 Hz from and the the flow rateisis another 46.4 mL/min. Apartparameter from the flow rate, backpressure is performance. Next,parameter we will perform the testing of the frequency-dependent backpressure order another important representing the pump performance. Next, we will perform theintesting to the further examine the piezoelectric membrane pump with multiple valves. Figure 6 shows of frequency-dependent backpressure in order to further examinecheck the piezoelectric membrane the relationship between backpressure as a function of operation frequency at a fixed actuation pump with multiple check valves. Figure 6 shows the relationship between backpressure as a function voltage of 150 V. A atrelatively big backpressure fluctuation is observed in Figure 6. Four of operation frequency a fixed actuation voltage of 150 V. A relatively big backpressure fluctuation backpressure peaks, two high peaks and two low peaks, are observed from the backpressure versus is observed in Figure 6. Four backpressure peaks, two high peaks and two low peaks, are observed within frequency range ofwithin 60–400the Hz.frequency The maximum 3.45 kPa is achieved at 240 from thethe backpressure versus range backpressure of 60–400 Hz. of The maximum backpressure Hz and the minimum backpressure of 2.26 kPa is achieved at 340 Hz. of 3.45 kPa is achieved at 240 Hz and the minimum backpressure of 2.26 kPa is achieved at 340 Hz.
Figure Figure 5. 5. The The flow flow rate rate versus versus frequency frequency characteristics characteristics of the the pump with different different configurations configurations of of inlet inlet valves valves and and outlet outlet valves. valves.
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backpressure versus frequency characteristics of the pump configurations Figure 6.6.The The backpressure versus frequency characteristics of with the different pump with different of inlet valves and outlet valves. configurations of inlet valves and outlet valves.
5.2. The The Results Results of of the the Pump Pump with Two Inlet Valves and Four Outlet Valves Figure 2c 2c demonstrates demonstratesthe thestructure structure of the piezoelectric membrane withinlet twovalves inlet of the piezoelectric membrane pumppump with two valves and four valves. outlet valves. In to order to evaluate this structure, therate flow rate and backpressure and four outlet In order evaluate this structure, the flow and backpressure versus versus frequency characteristics of the are pump are at tested at adriving fixed driving of In 150this V. section In this frequency characteristics of the pump tested a fixed voltagevoltage of 150 V. section nextsections, three sections, the experimental procedures are totally theas same asof those of the and theand nextthe three the experimental procedures are totally the same those the pump pump with one inletand valve fivevalves. outlet Figures valves. 5Figures 5 andthe 6 show the frequency-dependent with one inlet valve fiveand outlet and 6 show frequency-dependent flow rate flow rate and backpressure curves, respectively. For the frequency-dependent flow rates, and backpressure curves, respectively. For the frequency-dependent flow rates, only oneonly peakone of peak of the rate is observed at 360 Hzapplied for an applied 150 aVmaximum and a maximum flow the flow rateflow is observed at 360 Hz for an voltage voltage of 150 Vofand flow rate of rate 140.8 mL/min at 360 Hz is recorded. Before the frequency of 360 Hz, the rate of the 140.8ofmL/min at 360 Hz is recorded. Before the frequency of 360 Hz, the flow rateflow of the pump is pump almost monotonically with increasing the frequency. The of flow rate is almostismonotonically increasedincreased with increasing the frequency. The valley of valley flow rate is recorded recorded at the 60 minimum Hz and the flow rateis of 38 mL/min measured. As for the at 60 Hz and flowminimum rate of 38 mL/min measured. As foristhe frequency-dependent frequency-dependent backpressures, the backpressure fluctuation much smaller that the backpressures, the backpressure fluctuation is much smaller than isthat of the pumpthan with oneofinlet pump with one inlet valve and five outlet valves, especially beyond the frequency of 160 Hz. Figure valve and five outlet valves, especially beyond the frequency of 160 Hz. Figure 6 illustrates that only 6one illustrates that only one peak backpressure occurs. The of maximum backpressure of 3.06 kPa and peak backpressure occurs. The maximum backpressure 3.06 kPa and the minimum backpressure the minimum backpressure ofHz 2.6and kPa100 are Hz, recorded at 240 Hz and 100 Hz, respectively. of 2.6 kPa are recorded at 240 respectively. 5.3. The The Results Results of of the the Pump Pump with with Three Three Inlet Inlet Valves Valves and and Three Three Outlet 5.3. Outlet Valves Valves The structure structure of the piezoelectric piezoelectric membrane membrane pump pump with with three three inlet inlet valves valves and and three outlet The valves is shown in Figure 2a. The The experimental experimental results results of of the pump are directly presented because the experimental experimental procedures are very similar to Figures 55 and and 66 show show the the to last last experiments. experiments. Figures frequency-dependent flow rate and backpressure curves, respectively. As observed in Figure 5, frequency-dependent flow rate and backpressure curves, respectively. As observed in Figure 5, the the flow rate initially increases with the excitation frequency. After the optimal frequency, the flow rate flow rate initially increases with the excitation frequency. After the optimal frequency, the flow rate tends to decrease. The The resonant resonant phenomenon phenomenon occurs occurs in in the the pump pump chamber chamber because because of the working tends fluid vibration with the pump components (six check valves and one diaphragm). Within Within the whole frequency range range from from 60 60 to to 400 Hz, the pump with with three inlet valves and three outlet valves valves has frequency
only one optimal frequency of 200 Hz where the excitation frequency reaches the pump resonance frequency. It is similar to the frequency characteristics of the displacement amplification system.
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only one optimal frequency of 200 Hz where the excitation frequency reaches the pump resonance frequency. It is similar to the frequency characteristics of the displacement amplification system. The resonance frequency determines where the maximum flow rate locates. Under resonance, the flow rate reaches the highest value of 132.4 mL/min at the excitation voltages of 150 V. The minimum flow rate of 36 mL/min occurs at 60 Hz. For the frequency-dependent backpressure characteristics, it is seen from Figure 6 that there is also basically a similar trend between this pump and the pump with two inlet valves and four outlet valves. Nevertheless, the maximum backpressure as well as the minimum backpressure is higher than those of the pump with two inlet valves and four outlet valves correspondingly. They are 3.65 kPa at 240 Hz and 2.87 kPa at 100 Hz, respectively. 5.4. The Results of the Pump with Four Inlet Valves and Two Outlet Valves The structure of the piezoelectric membrane pump with four inlet valves and two outlet valves is shown in Figure 2d. Figures 5 and 6 show the frequency-dependent flow rate and backpressure curves, respectively. Experimental results show that the flow rate versus frequency curve of the pump with three inlet valves and three outlet valves takes on a similar trend with the pump with two inlet valves and four outlet valves. The flow rate of the pump is also almost monotonically increased with increasing the frequency until 360 Hz where one peak of the flow rate is measured. Furthermore, the minimum flow rate of the pump also occurs at 60 Hz. However, the maximum flow rate as well as the minimum flow rate is greater than those of the pump with two inlet valves and four outlet valves correspondingly. They are 165.2 mL/min and 42.4 mL/min with an applied voltage of 150 V, respectively. For the frequency-dependent backpressure characteristics, it is observed that the backpressure versus frequency curve of the pump is quite similar to that of the pump with two inlet valves and four outlet valves. The maximum backpressure and the minimum backpressure also occur at 240 Hz and 100 Hz, respectively. Moreover, the maximum and minimum values of output backpressures are 3.05 kPa and 2.54 kPa, respectively, where they are close to the corresponding values of the pump with two inlet valves and four outlet valves. In other words, the backpressure characteristic of the pump with four inlet valves and two outlet valves is basically the same as that of the pump with two inlet valves and four outlet valves. 5.5. The Results of the Pump with Five Inlet Valves and One Outlet Valve The structure of the piezoelectric membrane pump with five inlet valves and one outlet valve is illustrated in Figure 2e. Figures 5 and 6 show the frequency-dependent flow rate and backpressure curves, respectively. The flow rate versus frequency curve in Figure 5 shows a very similar change trend with the pump with one inlet valve and five outlet valves. Two peaks of flow rates are measured at 100 Hz and 240 Hz, respectively. The maximum flow rate occurs at 240 Hz and the pump achieves the highest value of 116.8 mL/min at the excitation voltages of 150 V. The minimum flow rate of 50.8 mL/min occurs at 60 Hz, which is almost consistent with the pump with one inlet valve and five outlet valves. As for the frequency-dependent backpressures, there is a fairly big backpressure fluctuation within the whole frequency range from 60 Hz to 400 Hz. Two peaks of the backpressures are observed at 120 Hz and 320 Hz and a maximum backpressure of 3.1 kPa at 120 Hz is measured with a driving voltage of 150 V. In addition, a minimum backpressure of 1.6 kPa is achieved at 180 Hz. Meanwhile, the pump also achieves a backpressure which is close to the minimum value at 400 Hz. 6. Discussion It is found from the experimental results that different configurations of multiple check valves have a strong influence on the flow rate and backpressure of the piezoelectric membrane pump. In order to demonstrate the performance differences caused by different numbers of inlet and outlet valves intuitively, all the aforementioned frequency-dependent flow rate curves are plotted and compared in one figure, and so are the backpressure curves. They are illustrated in Figures 5 and 6, respectively.
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The comparison results show that the output flow rate of the pump changes with different numbers of inlet and outlet valves. Not only the optimal frequency but also the maximum flow rate are significantly affected by the number change of inlet valves and outlet valves. The reason is very complicated. The pump must work on the condition of consistent movement between the membrane and the check valve. Especially for multiple check valves, because there are different time lags between membrane and valve movement, this will lead to different resonant behaviors for the pump with multiple check valves, Among the five tested structures, the variation of the optimal frequency ranges from 200 Hz to 360 Hz, meanwhile, the variation of the maximum flow rate ranges from 100 mL/min to 165.2 mL/min. The variation ratio of the maximum flow rate is up to 66%. However, it is observed from the curve that the pump displays very similar flow rate characteristics between the pump with one inlet valve & five outlet valves and the pump with five inlet valves & one outlet valve. In addition, the flow rate characteristics of the pump are similar when the pump has two inlet valves & four outlet valves as well as four inlet valves and two outlet valves. The flow rate property of the pump with three inlet valves and three outlet valves is totally different from other four types of structures. Nevertheless, there is a very similar backpressure property between the pump with three inlet valves & three outlet valves, two inlet valves and four outlet valves and four inlet valves and two outlet valves, especially the latter two. The variation of the maximum backpressure ranges from 3.05 kPa to 3.65 kPa in all the tested structures. The variation ratio of the maximum backpressure is less than 20%. It can be inferred that the different configurations influence of inlet valves and outlet valves on the flow rate is much greater than the influence on the backpressure. No matter whether a check-valve piezoelectric pump or a valveless pump is considered, back flow is inevitable. The backflow characteristics of a check-valve piezoelectric membrane pump are investigated by changing the number of inlet and outlet valves. As described above, when the number of inlet valves in one kind of configuration is the same as that of outlet valves in another configuration, the pump generally shows very similar flow rate and backpressure characteristics. Consequently, we can infer that the backflow from pumping chamber to inlet is basically the same as the backflow from outlet to pumping chamber. It means that either increasing the number of inlet valves or increasing the number of outlet valves can effectively reduce the backflow. However, the frequency characteristics of the pump are changed with the different numbers of inlet valves and outlet valves. In addition, the increase of inlet valves or outlet valves can significantly suppress the backpressure fluctuation. It is found that there is a relatively big backpressure fluctuation when just one inlet valve or outlet valve is involved. On the contrary, the backpressure fluctuation is relatively small when more than two inlet valves or outlet valves are involved. For example, the pump with one inlet valve and five outlet valves has a backpressure fluctuation of 1.19 kPa; and the backpressure fluctuation of the pump with two inlet valve and four outlet valves is only 0.46 kPa. It is helpful to improving the stability of the pump, so the pump can retain the backpressure stability across a wider frequency range. However, the increasing number of passive valves and buffer chambers will increase the power consumption. The pump with multiple check valves has a higher power consumption compared with the pump with single inlet valve and outlet valve. In other words, the pump can prevent the backflow more effectively at the cost of power consumption. 7. Conclusions At present, research on check valves of piezoelectric pumps is mostly focused on valve design, development and optimization. Unlike previous research, we mainly explore the effect of check-valve number on the properties of a piezoelectric membrane pump, especially the backflow characteristics of the pump. Because each check valve is not only used as a flow-rectifying element but also regarded as a leakage barrier, multiple check valves could effectively reduce the backward flow. In this paper, a single-active-chamber piezoelectric membrane pump with multiple passive check valves is proposed. The influence on the pumping performance is experimentally studied by altering the number of inlet valves and outlet valves, keeping the total number of check valves constant.
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A piezoelectric membrane pump with six passive check valves is fabricated and the prototype pump is experimentally tested in terms of flow rate and backpressure. Experimental results indicate that different configurations of multiple check valves have a strong influence on the output capabilities of the piezoelectric membrane pump. Not only the optimal frequency but also the maximum flow rate is significantly affected by the number change of inlet valves and outlet valves. Among the five tested structures, the variations of the optimal frequency and the maximum flow rate range from 200 Hz to 360 Hz and from 100 mL/min to 165.2 mL/min, respectively. The variation of the maximum backpressure ranges from 3.05 kPa to 3.65 kPa in all the tested structures. Although the flow rate and backpressure are changed with different configurations of inlet valves and outlet valves, it is found that the piezoelectric pump generally demonstrates very similar flow rates and backpressure characteristics when the number of inlet valves in one kind of configuration is the same as that of outlet valves in another configuration. This indicates that the backflow from the pumping chamber to the inlet is basically the same as the backflow from the outlet to the pumping chamber. It is also found that an increase in the number of inlet valves or outlet valves can significantly suppress the backpressure fluctuation. The pump is very suitable for the conditions where a more accurate flow rate is required and wear and fatigue of check valves often occur. It will provide a new method for preventing the backflow in order to get accurate flow rates. We expect that this study will offer a better understanding of the time lag between diaphragm and valve movement. It is also expected that this work will provide new ideas about check-valve equipped piezoelectric membrane pumps. Acknowledgments: This work was partially supported by the Supported by Science and Technology Planning Project of Guangdong Province (Granted No. 2014B010118002), National Natural Science Foundation of China (Granted No. 51408237, 51108192 and 51208500), National Key Technology R&D Program (Granted No. 2014BAG01B04), Chinese Postdoctoral Science Foundation (Granted No. 2012M521824 and No. 2013T60904), and Public Welfare Research and Capacity Building Project of Guangdong Province (Granted No. B2161520). Author Contributions: Ronghui Zhang and Feng You proposed the method and designed the overall system. In addition, they wrote the paper. Zhihan Lv, Zhaocheng He, Haiwei Wang and Ling Huang contributed to the experiment and statistical analysis of the data. Besides, Haiwei Wang, Ronghui Zhang and Feng You finished additional experiments and data analysis. Conflicts of Interest: The authors declare no conflict of interest.
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