Patent Publication Number: US-2013243627-A1

Title: Ultrasonic traveling wave micropump for liquid

Description:
The present invention relates to an ultrasonic traveling wave micropump for pumping liquids, the actuation of which micropump depends on the use of two linear piezoelectric transducers at least one of which is used as a linear piezoelectric actuator. 
     Piezoelectric transducers depend on the ability of certain materials—such as quartz, synthetic ceramics, or PZT (lead zirconate titanate)—to become electrically polarized under the action of a mechanical stress, and conversely to deform under the action of an electric field. This reciprocal effect, called the inverse piezoelectric effect, is widely used to produce actuators. 
     Many micropumps using piezoelectric actuators have already been developed, and may be classified, as suggested in the article entitled “Classification and comparison of micropumps in view of operational conditions and restrictions”—Camilo Hernandez, Yves Bernard et al.—ACTUATOR 2008, 11th International Conference on new actuators, Bremen Germany, 9-11 Jun. 2008—pages 818-822, depending on whether or not they use valves in their structures, Among valve-free structures, “peristaltic” micropumps, in which a force, taking the form of a transverse traveling wave, is applied to the walls of a channel containing the liquid, so as to move this liquid in the propagation direction of the wave, are well known. 
     A possible linear structure for a micropump using a transverse traveling wave generated by way of piezoelectric actuators is described in document U.S. Pat. No. 5,961,298. The structure essentially consists of a sealed stack of two rectangular plates that are tightly pressed against each other and placed between an inlet and an outlet of a chamber of the pump. One of the plates is preferably kept stationary whereas the other plate is excited by a series of piezoelectric actuators distributed over one side of the plate opposite the interface between the two plates, over the entire length of the interface. Each actuator is composed of two pairs of control members excited electrically by sinusoidal signals in quadrature phase, each member itself consisting of two linear piezoelectric elements, one dilating under the action of the control signal and the other contracting under the action of the same control signal. The arrangement of the actuators over the entire length of the interface, the choice of the control signals and the sequence of these signals allows the plate to be strained locally so as to create, between the two plates, a closed cavity that receives the fluid, the cavity moving from the inlet toward the outlet of the pump, in the propagation direction of the traveling wave. 
     Such a structure has several drawbacks: thus, since the channel for transporting the fluid is created locally, directly by the cavity formed between the plates, seals must necessarily be provided at the periphery of each of these plates in order to prevent the fluid from escaping and causing problems, especially with the piezoelectric actuators. Furthermore, in such a structure, the amplitude of the strain necessarily dictates the size of the flow channel if a closed cavity is to be created locally. Finally, and above all, this structure requires many piezoelectric elements, which not only increases its cost, but also makes miniaturization of the structure more difficult. 
     The objective of the present invention is to overcome the preceding drawbacks by providing a micropump structure, for pumping liquid, which uses fewer piezoelectric elements and which can be used flexibly. 
     This objective is achieved by the invention, the subject of which is an ultrasonic traveling wave micropump for moving a liquid, characterized in that it comprises: 
     two separate linear piezoelectric transducers; 
     a flexible metal strip each end part of which rests on one of the two linear piezoelectric transducers; 
     a sealed channel made of a deformable material for transporting the liquid from an inlet to an outlet of the micropump, said channel lying on said strip longitudinally between said linear piezoelectric transducers; and 
     excitation means for exciting at least the linear piezoelectric transducer located near the inlet of the micropump so as it generates in the strip and channel a transverse wave vibration that travels toward the outlet of the micropump. 
     According to additional features: 
     in a first embodiment, the linear piezoelectric transducer located near the outlet of the micropump is used to damp the transverse vibration. In this case, it is advantageously connected to an RL load the resistance and inductance of which are chosen so as to reduce and even prevent reflection of the traveling wave; 
     in this case, the linear piezoelectric transducer located near the inlet is preferably positioned a distance of 7λ/8 away from the nearest end of the strip, λ being the wavelength of the traveling wave, whereas the linear piezoelectric transducer located near the outlet is positioned a distance of 7λ/8+nλ/2 away from said left-hand end, n being a positive integer; 
     in a second possible embodiment, the two piezoelectric transducers are used as vibrators in order to excite two consecutive vibration modes of the strip. In this case, preferably the excitation means simultaneously excite both linear piezoelectric transducers, one with a first sinusoidal electrical signal at an intermediate frequency relative to the frequencies of the two consecutive vibration modes, the other with a second sinusoidal electrical signal at the same intermediate frequency, but in quadrature phase with the first signal; 
     the sealed channel is preferably bonded to the strip; 
     the sealed channel is a film made of polydimethylsiloxane; and 
     the two piezoelectric transducers are Langevin structures. 
    
    
     
       The present invention and its advantages will be better understood on reading the following description of an embodiment according to the present invention, given with reference to the appended figures, in which: 
         FIG. 1  illustrates schematically an elevation view of a micropump according to the invention; 
         FIG. 2  shows the shape of the progressive wave generated in the strip according to the principles of the invention; 
         FIG. 3  shows a linear piezoelectric transducer having a Langevin structure; 
         FIG. 4  illustrates an improvement to the preceding Langevin structure, particularly suited to the micropump according to the invention; 
         FIG. 5  illustrates a first way of exciting the micropump according to the invention; and 
         FIG. 6  shows a partially exploded view of the structure of a demonstration unit used to confirm the operation of the micropump according to the invention. 
     
    
    
     The simplification of the micropump according to the invention over the prior art is based on experimental studies and trials carried out in the laboratory, which studies and trials confirmed not only the fact that it is possible to produce a traveling wave in a metal strip using only two linear piezoelectric transducers, but also that this traveling wave may strain the wall of a sealed film or channel deposited on the strip sufficiently to transport a liquid between an inlet and outlet of the canal. 
     As is shown schematically in  FIG. 1 , a micropump  1  according to the invention, for pumping a liquid, essentially comprises two linear piezoelectric transducers  2  and  3 , a flexible metal strip  4  resting on and connecting the two transducers  2  and  3 , and a sealed channel  5  made of a deformable material for transporting the liquid between an inlet E and an outlet of this channel  5 . The assembly preferably rests on a base  6  made of a material having a high acoustic impedance, so as to prevent vibrations from passing through said base. An electronic module  7  connected to the two linear piezoelectric transducers  2  and  3  comprises means for exciting these transducers. 
     The transducers  2  and  3  are used to generate a transverse traveling wave in the strip  4 , which traveling wave moves along the strip from the transducer  2  toward the transducer  3 .  FIG. 2  illustrates various curves representing, at five successive times, the shape of this wave. The wave thus generated is a periodic function at the resonant frequency of the strip and has antinodes (high and low points of the various curves) that move, in time, from left to right. The traveling wave may be obtained by exciting the two transducers using various strategies, which will be described below. 
     Whatever the strategy adopted, linear piezoelectric transducers with a Langevin structure, or what are called “Tonpilz” structures after the German terminology, will preferably be used. These structures, the construction of which is shown in  FIG. 3 , essentially comprise at least two piezoelectric ceramics  20  that are pressed and prestressed, between two metal bodies, namely an upper body  21  and a lower body  22 , by means of a fastening element such as a metal screw  23 . The metal bodies  21  and  22  serve, on the one hand, to protect the ceramics  20 , and on the other hand, to calibrate the acoustic transducer thus formed to a predefined frequency. In such a structure, when a sinusoidal electrical signal is applied to the two ceramics  20 , the latter contract. Since the ceramics  20  are mechanically connected in series and electrically connected in parallel, longitudinal acoustic waves propagate from the ceramics into the bodies. Maximal movement of the bodies used, in the vertical direction, may be obtained when the frequency of the sinusoidal electrical signal matches the mechanical resonant frequency of the structure. Langevin structures typically have resonant frequencies possibly ranging from 20 to 200 kHz. 
     To pump liquid in a micropump application it is necessary, in order to adjust the resonant frequency of the assembly, for the lower body  22  and the upper body  21 , respectively, to be of the correct size and shape. Furthermore, the shape of the upper body  21  must be modified so as, on the one hand, to amplify the deformation obtained at the top of the upper body, in the location where the strip  4  is fastened, and on the other hand, to minimize the region of contact between the transducer and the strip  4 .  FIG. 4  illustrates an upper body  21  with a conical shape particularly suited to achieving the two aforementioned objectives. The lower body  22  is preferably made of a heavy and stiff material, such as tungsten or steel, which inhibits the propagation of vibrations. In contrast, the upper body  21  is preferably made of a light and flexible material, such as aluminum for example, in order to have a low acoustic impedance and promote propagation of the vibration toward the top of the structure. 
     The materials used for the various elements of the micropump  1 , the dimensions, especially of the strip  4  and the channel  5 , the type of linear transducers used and their position relative to the strip must all be correctly chosen depending on the application envisioned, the point of commonality between one application and another being that a mechanical vibration is generated by way of the transducers, which vibration is transmitted, with as few losses as possible, to the strip  4  in the form of a traveling wave that moves between the inlet and the outlet, and that moves a certain amount of liquid at a given speed between the inlet and the outlet of the micropump. 
     Each application will thus impose many and very varied constraints, which range from flow-rate and counter-pressure values, for a particular liquid, to requirements in terms of power consumption, bulk, biocompatibility, etc. Thus, a detailed study of the fluid dynamics specific to each application must be carried out before the micropump is developed. 
     The various trials carried out by the Applicant have allowed certain choice criteria to be identified, which will now be detailed. 
     The pumping performance essentially depends on the specific characteristics of the traveling wave liable to be generated in the strip, such as the frequency of the wave, its wavenumber and its amplitude. These characteristics are intimately related to the dynamic characteristics of the piezoelectric transducers. The exact choice of the transducers thus depends on prior knowledge of the deformation amplitude and frequency that it is desired to obtain in the strip, and consequently, in the channel for transporting the fluid. 
     Various strategies may be used to excite the transducers  2  and  3  in order to generate the traveling wave: according to a first possible embodiment shown schematically in  FIG. 5 , only the transducer located near the inlet of the channel, here the transducer  2 , is used as a vibrator, the transducer  3  being, for its part, used to damp the vibration. To do this, the means  7  for exciting the transducer  2  will apply, to this transducer, a periodic, preferably sinusoidal, excitation voltage. By analogy with the theory of transmission in an electrical line, the transducer  2  acts as a wave generator, the strip  4  is the electrical transmission line, and the transducer  3  represents the load on this line. Said transducer is, for example, connected to an RL circuit comprising a resistance R in parallel with an inductance L. The values of R and L are chosen so as to match the load to the acoustic impedance of the strip. Thus, the wave propagates in the strip and the zero (ideal case) or negligible reflection at the transition between the strip and the transducer  3  does not affect the traveling nature of the wave. Specifically, in the case where the coupling between the impedances of the strip  4  and the transducer  3  is inadequate, the addition of the wave generated by the transducer  2  and that reflected by the transducer  3  could lead to a standing wave being generated. Furthermore, the transducers  2  and  3  must be correctly positioned relative to the strip  4  if the impedances are to match. Trials carried out by the Applicant have shown that, if λ represents the wavelength of the traveling wave, the transducer  2  used as the vibrator must preferably be positioned a distance of 7λ/8 away from the left-hand end of the strip  4 , whereas the transducer  3  used as a damper must be positioned a distance of 7λ/8+nλ/2 away from the same left-hand end, in which expression n is a positive integer. It should be noted that the operation of the micropump can easily be inverted by ensuring that the transducers,  2  and  3 , used are the same. To obtain inverted operation, the excitation means  7  will excite the transducer  3 , whereas the transducer  2  will be used as a damper. The positions of the inlet E and the outlet S will then be inverted relative to those shown in the figures. 
     In a second possible excitation strategy, the two transducers  2  and  3  are both used as vibrators in order to excite two consecutive vibration modes of the strip  4 . More precisely, because a pure traveling wave is the sum of two stationary waves shifted by 90° both in time and space, the excitation means  7  will simultaneously excite the two linear piezoelectric transducers  2  and  3 , one with a first sinusoidal electrical signal at an intermediate frequency relative to the frequencies of the two consecutive vibration modes, and the other with a second sinusoidal electrical signal at the same intermediate frequency, but in phase quadrature with the first signal. The resulting vibration is then a traveling wave having a variable amplitude and phase speed. Here, the position of the transducers  2  and  3  along the strip must be precisely identified in order to effectively obtain a traveling wave. 
     More precisely, it is possible to show that the strain of the strip is given by the following equation: 
         u ( x,t )= D   1 [ sin(γ n   x )−cos(γ n   x )] cos(Ω t )+ D   2 [ sin(γ n+1   x )−cos(γ n+1   x )] sin(Ω t )+ D   3 [ sin(γ n+1   x )−cos(γ n+1   x )] cos(Ω t )+ D   4 [ sin(γ n   x )−cos(γ n   x )] sin(Ω t )
 
     where:
 
x is the position along the strip;
 
γ n  and γ n+1  represent the wavenumber of two consecutive modes;
 
Ω is the intermediate frequency applied; and
 
D 1  to D 4  are constants that depend on the excitation frequency, on the material used for the strip  4 , on the boundary conditions and on the position of the transducers.
 
     Studies carried out by the Applicant have demonstrated that appropriate positions for the transducers are those for which the constants D 1  to D 4  have equal absolute values, at least one of these constants being of opposite sign to the other three. 
     Here again, the micropump can be reversed by inverting the manner of excitation each of the transducers  2  and  3 . 
     As regards the flexible metal strip  4 , the metal used, especially its density and its elastic coefficient, and the length, width and thickness of the strip, must be chosen depending on the wavelength, the frequency and the amplitude of the traveling wave that it is desired to obtain. The strip  4  must meet the following three criteria: 
     the material used for the strip must be a good acoustic conductor since it must be able to transmit, ideally without loss, the vibration generated by the piezoelectric transducers; 
     the acoustic impedance of the strip at the excitation frequency must correspond to that of the piezoelectric transducers  2  and  3 , which especially restricts the length, width and thickness of the strip; and 
     the strip must also be of such a size that the resulting weight of the channel transporting the fluid is negligible with respect to the transverse stress generated by the strip, in order not to interfere with the progression of the wave. 
     As regards the base  6 , the latter must be a poor conductor of acoustic waves, here again in order to promote transmission of the vibrations generated by the transducer  2  (vibrator/damper mode) or by the two transducers  2  and  3  (vibrator/vibrator mode) to the strip  4 . This may be obtained by various means, especially just by choosing a material that is a poor acoustic conductor. As a variant, it may be arranged for the acoustic impedance of the base  6  to be much higher than the acoustic impedance of the strip  4 . Knowing that the acoustic impedance Z 0  may be defined by the relationship: 
     
       
      
       Z 
       0 
       =ρ×C×A 
       0  
      
     
     where ρ is the density of the material, C is the speed of sound and A 0  is the area of contact between the base and each of the transducers  2  and  3 , a base  6  made of a material with a high density, and/or large contact areas, will possibly be chosen. 
     Finally, the material of the channel  5  must be sufficiently deformable that the strain generated by the traveling wave in the channel wall making contact with the strip  4  effectively moves the liquid. The sealed channel may for example be produced in the form of a PDMS (polydimethylsiloxane) film. 
     A demonstration unit, schematically illustrated in  FIG. 6 , was produced in order to confirm the operation of the micropump according to the principles indicated above. In this demonstration unit, two commercially available Langevin transducers having resonant frequencies of 28 kHz were used, aluminum cones suited to the resonant frequency of the transducers being added thereto in the upper portion. 
     The strip  5  of the demonstration unit was made of aluminum (duralium) 
     The sealed channel (not shown in  FIG. 6 ) incorporated, at its ends, two reservoirs, respectively at its inlet and outlet. The channel confined the liquid to be transported and formed an interface with the strip. 
     To confirm the operating principle of the micropump, a small amount of liquid was placed in the inlet reservoir. Enough time was allowed to pass to confirm that the liquid did not move via a capillary action. Next, the transducers were excited, and the filling of the outlet reservoir was observed, thus confirming that the liquid was transported by the traveling wave. 
     The micropump according to the invention has all the advantages associated with its (piezoelectric) technology, radiated magnetic fields in particular being absent and the actuators containing no moving parts. In addition, the micropump according to the invention possesses a channel that completely covers the fluid and allows activation of channels the deformation of which will cause a flow, in contrast to techniques known in the art. The results of the study demonstrate that it is not necessary to generate a wave the amplitude of which corresponds to the height of the channel in order to pump the liquid, in contrast to the teachings of document U.S. Pat. No. 5,961,298. 
     The optimal dimensions of the various constituents of the micropump may be determined for each envisioned application using a numerical model. 
     Especially with regard to the micropump described in document U.S. Pat. No. 5,961,298, here the entire top part of the pump according to the invention is unencumbered. The assembly may thus easily be equipped with sensors allowing it to adapt in operation to variations in the behavior of the pump.