Abstract:
A low profile transducer ( 11, 12 ) is provided for use in flow meter for pipes of small diameter. The transducer ( 11, 12 ) has a transducer ( 18 ) or other form of transducer which transmits and receives electrical signals of three volts and 1 MHz and converts between these electrical signals and acoustic waves. In one embodiment, the transducer ( 18 ) produces surface acoustic waves (SAW), while in another embodiment the transducer ( 32 ) produces plate waves known as Lamb waves. These waves are converted to bulk acoustic waves (BAW) transmitted between the pair of transducers ( 11, 12 ).

Description:
TECHNICAL FIELD 
     The field of the invention is ultrasonic transducers for use in flow meters. 
     BACKGROUND OF THE INVENTION 
     This invention is concerned with transducers for use in ultrasonic flow meters. The ultrasonic flow meter is an instrument that uses ultrasonic waves to measure the flow rate of a fluid. In operation, two transducers are placed on opposite inside wall surfaces of a pipe through which the fluid is flowing. Acoustic waves of ultrasonic frequency are generated by a first transducer and travel through the fluid to reach the second transducer. The time taken by the acoustic wave to travel from one transducer to the other, the transit time, is a function of the flow velocity of the fluid. Thus, by measuring the transit time, one can determine the fluid velocity and other relevant flow parameters. 
     Many transducer structures have been developed to generate and detect acoustic waves in a fluid. 
     One of the problems of prior transducers is that they are generally large in size and protrude a considerable distance inward from the pipe wall and into the path of the flowing fluid. This poses a problem, particularly if the diameter of the pipe is small. There is a certain minimum pipe diameter below which the present transducers cannot be used. A second problem is that the protrusion into the flow stream can allow rags, debris, and other large particulate matter to foul the transducer. This is not a problem in large pipes where the transducer does not significantly project into the flow stream, but this can be a problem in pipes of small diameter. A third problem with the current transducers is the effect they have on the flow streams themselves. The sensor can disturb the flow stream sufficiently to create erroneous flow data. 
     Lynnworth, U.S. Pat. No. 4,735,097, illustrates non-invasive type ultrasonic transducers which are supported at an angle in housings attached to the outside wall of a pipe to generate Rayleigh-like surface waves. These transducers are not mounted in the flow stream and the signals generated in this system are attenuated by the pipe wall. 
     Magori, U.S. Pat. No. 4,375,767, discloses a planar transducer using an interdigital array in which the periodicity of the interdigital array must satisfy a condition d=λ/cos α, where d=is the periodicity of the interdigital structure, λ is the wavelength of acoustic waves in the flowing medium and a is the angle of radiation of acoustic waves into the flowing medium, which is dependent on periodicity, d. 
     SUMMARY OF THE INVENTION 
     The invention provides a compact, planar transducer configuration having a transducer which is not limited by the periodicity condition of the prior art. Such a transducer, which should essentially be in the form of a thin, planar device can be mounted flush with the pipe wall resulting in minimal protrusion into the flow channel. The transducer of the present invention utilizes surface acoustic waves (SAW) or plate acoustic waves (Lamb waves) to couple energy into or out of bulk acoustic waves propagating in the fluid. 
     The transducer of the present invention cannot only be fabricated in a small size, but it is also much more efficient than existing transducers. This provides an output signal in response to a much lower voltage applied to the input transducer. This is particularly important in applications where engineering considerations set an upper limit to the maximum driving voltage that can be applied across the input transducer. The lowering of the drive voltage can result in significant cost savings in the packaging of the device. 
     The invention also provides advantageous configurations for packaging the transducer, so that it is sealed from contact with the fluid being measured while providing electrical connections to an instrument outside the pipe. 
    
    
     Other objects and advantages of the invention, besides those discussed above, will be apparent to those of ordinary skill in the art from the description of the preferred embodiments which follow. In the description, reference is made to the accompanying drawings, which form a part hereof, and which illustrate examples of the invention. Such examples, however, are not exhaustive of the various embodiments of the invention, and therefore, reference is made to the claims which follow the description for determining the scope of the invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is longitudinal section view of a pipe with a pair of transducers of the present invention installed in the flow stream; 
     FIG. 2 is a top plan view of a first embodiment of the invention seen in FIG. 1; 
     FIG. 3 is a longitudinal section view taken in the plane indicated by line  3 — 3  in FIG. 2; 
     FIG. 4 is a top plan view of a second embodiment of the invention seen in FIG. 1; 
     FIG. 5 is a top plan view of a third embodiment of the invention seen in FIG. 1; 
     FIG. 6 is a longitudinal section view of a third embodiment of the invention taken in the same plane as FIG. 3; 
     FIG. 7 is a graph of signals related to the embodiment of FIG. 4; and 
     FIG. 8 is a graph of signals related to the embodiment of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a section of pipe  10  in which two transducers  11 ,  12  of the present invention have been positioned on opposite sides of the flow stream  13  against an inside wall of the pipe  10 . A fluid  13  is flowing through the pipe  10 , and the object of the invention is to measure the flow velocity of the fluid. In a first embodiment of the invention, surface acoustic waves (SAW)  16  are generated on the substrate  15  of the transducer  11 , which is seen in detail in FIG.  2 . The transducers  11 ,  12  are fabricated on rectangular substrates  15  of lead zirconate titanate material (Pb Zr x  Ti y  O z , where x, y and z are variables) having a size of approximately 16 mm wide×18 mm long×4.5 mm thickness. This material is further described and referred to in Berlincourt, Curran and Jaffe, “Piezoelectric and Piezomagnetic Materials and their Function in Transducers,”  Physical Acoustics , Vol. I, part A, W. P. Mason, Editor, pp. 169-270, Academic Press, 1964. The material of the substrate  15  is chosen such that its SAW velocity is greater than the velocity of bulk acoustic waves (BAW)  17  transmitted through the fluid  13  between the transducers  11 ,  12 . Under these conditions, the SAW  16  will radiate bulk acoustic waves (BAW)  17  into the fluid  13 . The coherent radiation of bulk acoustic waves  17  will take place at an angle a given by the equation cos α=v −B /v S , where v B =velocity of bulk acoustic waves  17  in the fluid  13 , and v S =velocity of surface acoustic waves  16  in the substrate  15 . 
     As seen in FIG. 2, a transducer  18 , sometimes referred to as an interdigital transducer or “IDT,” is provided by two thin-film conductors  19 ,  20 , each having fingers,  19   a ,  20   a , which are received in spaces between the fingers  19   a ,  20   a , of the opposite conductor  19 ,  20 . The bus bars  19 ,  20  are electrically connected to two respective wires  21 ,  22 , which turn, are connected to the inner and outer conductors of a coaxial cable  23  that is seen in FIGS. 2 and 3. Two holes  29  are provided in the substrate  15 . This allows the routing of wires  21 ,  22  to the back side of the substrate  15 . The wires  21 ,  22  are connected to thin metal films  27   a ,  27   b  (FIGS. 2 and 3) on the underside of the substrate  15 , which is connected via conductors  28  in the two plated through-holes  29  in the substrate  15 . 
     The coaxial cable  23  runs through a hole in the wall of the pipe  10  in FIG. 1, where a seal is formed by a water-insoluble epoxy sealant (not shown). The epoxy sealant  24  is also used as a seal around and under the substrate  15  in FIGS. 2 and 3 and prevents water from getting into the cavity  25   a  under the SAW substrate  15 . The substrate is carried in a carrier  25  of plastic material which forms the cavity  25   a . A cover  26  of Kynar® material, which may be a coating or a separable layer, which is disposed to completely cover the circuit substrate  15  which carries the conductors  19 ,  20  (collectively referred to as “the chip”) to isolate, insulate and protect the chip from fluid  13  in the pipe  10 . The material of the cover  26  is non-conductive. This prevents electrical shorting. The cover  26  has a thickness dimension of less than 0.08 mm thick and is bonded to the substrate  15 . The thickness of the cover  26  is made much less than the acoustic wavelength, so as to minimize its effect on acoustic characteristics of the device. 
     With the proper geometry of the transducer  18  and selection of a length “L” (FIG. 2) between the geometric center and the end of the substrate material  15  in the direction of propagation, the energy of surface acoustic waves  16  can be efficiently converted to BAW energy and vice versa. For example, using lead zirconate titanate as a substrate material, the conversion loss from SAW to BAW and BAW to SAW has been observed to be less than 1.5 dB. A system utilizing the two transducers according to the present invention exhibits an overall electrical insertion loss of less than 15 dB. The voltage applied to the transducer  18  is typically 20 volts RMS at a frequency of 1 MHz. 
     Instead of the transducer  18 , an optical source of energy and a pattern etched or deposited on the substrate  15  in the pattern of transducer  18 , or a similar pattern of closely spaced parallel conductors, may be utilized. 
     Another approach that can be used to develop flat, planar transducers is to use the coupling between Lamb waves in a plate, and bulk waves in the fluid. Ultrasonic Lamb waves, sometimes also referred to as acoustic plate waves, are elastic waves propagating in plates of finite thickness. A given plate can support a number of modes of these waves, depending on the value of the ratio h/α, where h is the plate thickness, and α is the acoustic wavelength. It is well known that there are three all pass modes which can propagate down to h/α=0. These are the lowest order symmetric mode (the S O  mode), the lowest order antisymmetric mode (the A O  mode), and the lowest order quasi-shear horizontal mode (the QSH O  mode) The velocity of the A O  mode asymptotically tends to zero as h/α tends to zero. This is the mode that is utilized in the present application. The coupling between the A O , mode and bulk waves propagating in the fluid as a function of the h/α has been tested for a number of different materials. A strong coupling can be obtained by a proper choice of the h/α ratio. A wider range of substrate materials is suitable for use with the Lamb wave devices as compared to what could be used for the SAW device. In particular, efficient Lamb wave transducers can be realized using the widely used, strong piezoelectric material, lithium niobate. 
     A basic structure of a Lamb wave device  30  for generating acoustic waves in water is shown in FIG.  5 . FIG. 5 is a top plan view and FIG. 6 is a cross-sectional view of the Lamb wave device. The device consists of a piezoelectric substrate  31  of thickness “h”. The transducer  32  is similar to the transducer  18  in FIGS. 2 and 4. Transducer  32  generates Lamb waves in the substrate  31 . Under proper conditions, the Lamb wave device will efficiently generate bulk acoustic waves  17  (BAW) in the fluid  13 . In FIG. 6 the transducer  32  is located on a surface  33  of the substrate  31  opposite to the one  35  which is in contact with the fluid  13 , thus simplifying the protection of the transducer  32  from the fluid  13 . The electrical connection of a coaxial cable  41  to the transducer  32  can be made without the need for plated-through holes or other types of arrangements needed when the IDT array  32  is on the side of the substrate facing into the flow stream. The Lamb wave device  30  can be fabricated on a crystal of lithium niobate (LiNbO 3 ) as opposed to the lead zirconate titanate used for the substrate  15  in the first embodiment. This provides easier fabrication and lower device cost. Because the transducer  32  is not immersed in fluid  13  in the pipe  10 , the Lamb wave device  30  will be more efficient than the SAW device  11 ,  12  of the first embodiment. In the Lamb wave embodiment  30 , the holes in the substrate  15  of the previous embodiments and the cover  26  of Kynar® material are not necessary. 
     In the SAW device of FIGS. 1-3, the substrate  15  can be provided with corrugated edges  15   b  as shown in FIG.  4 . This is done in order to suppress the acoustic reflections from the edges  15   b . The corrugation operation, however, adds to the cost of the device. An alternative and preferred approach, which eliminates the extra cost, is to provide the substrate  15  with angled edges  15   a  as shown in FIG.  2 . If the edge angle θ is properly selected, the signal due to a wave reflected from the edge will cancel out over the aperture, W, of the receiving transducer. The value of θ can be calculated from the equation 
     
       
         tan(2θ)= nλ/W   
       
     
     where λ=SAW wavelength, W=IDT aperture which is the overlap dimension of the fingers  19   a ,  20   a  (FIG. 2) and n is any integer. Testing has shown that this approach is suitable, and the results are illustrated in FIGS. 7 and 8. FIG. 7 shows the output  43  of a device provided on a substrate  15  with corrugated edges  15   b , while FIG. 8 shows the output  44  of a device having a substrate  15  with angled edges  15   a . The upper wave form  42  in both FIG.  7  and FIG. 8 is the electrical signal applied to the input transducer. It can be seen that the spurious signals are reduced in the received signal (lower wave form) in FIG. 8 as compared with the received signal (lower wave form) in FIG.  7 . The modification of the edges of the substrate  15  is also advantageous for use in the Lamb wave devices of FIG.  5 . 
     The above has been a description of the detailed, preferred embodiments of the apparatus of the present invention. Various modifications to the details which are described above, which will be apparent to those of ordinary skill in the art, are included within the scope of the invention, as will become apparent from the following claims.