Patent Publication Number: US-7900338-B2

Title: Method of making a transducer having a plastic matching layer

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of application Ser. No. 11/746,695 filed May 10, 2007 titled “Systems and methods of a transducer having a plastic matching layer,” now U.S. Pat. No. 7,557,490, and which is incorporated by reference as if reproduced in full below. 
    
    
     BACKGROUND 
     1. Field 
     The various embodiments relate to ultrasonic flow meters and particularly to transducers used in ultrasonic meters. 
     2. Description of the Related Art 
     After hydrocarbons have been removed from the ground, the fluid stream (either in a liquid phase or a gaseous phase) is transported from place to place via pipelines. It is desirable to know with accuracy the amount of fluid flowing in the stream, and particular accuracy is demanded when the fluid is changing hands, or “custody transfer.” Even where custody transfer is not taking place, however, measurement accuracy is desirable, and in these situations ultrasonic flow meters may be used. In an ultrasonic flow meter, ultrasonic signals are sent back and forth across the fluid stream to be measured, and based on various characteristics of the ultrasonic signals a fluid flow may be calculated. Mechanisms which improve the quality of the ultrasonic signals imparted to the fluid may improve measurement accuracy. Moreover, wear and tear (e.g., caused by the corrosivity of the fluid being measured) on the components of the meter can substantially decrease longevity of the device, and thus any method to increase the durability of the meter and its components would be desirable. Finally, ultrasonic flow meters may be installed in harsh environments, and thus any mechanism to reduce maintenance time, and if possible improve performance, would be desirable. 
     SUMMARY 
     The various embodiments are directed to systems and methods of a transducer having a plastic matching layer. At least some of the illustrative embodiments are transducers comprising a housing (having a proximal end, a distal end and an internal volume, the housing configured to couple to a spoolpiece of an ultrasonic meter), a plastic matching layer that has an external surface and an internal surface (the plastic matching layer seals to and occludes the distal end of the housing), and a transducer element abutting the internal surface of the plastic matching layer. 
     Other illustrative embodiments are ultrasonic meters comprising a spoolpiece having an internal flow path for a measured fluid, and a transducer in operational relationship to the spoolpiece. The transducer further comprises a housing that defines an internal volume, a plastic matching layer that separates the internal volume of the housing from the measured fluid (wherein the plastic matching layer has an acoustic impedance between that of a piezoelectric crystal and the measured fluid), and a transducer element abutting an internal surface of the plastic matching layer. 
     Yet still other illustrative embodiments are methods comprising generating an ultrasonic signal, propagating the ultrasonic signal through a plastic matching layer, and imparting the acoustic signal to a fluid within an ultrasonic meter. 
     Finally, other embodiments are methods comprising providing a transducer housing having a proximal end and a distal end, bonding a plastic to the distal end of the transducer housing (the plastic fluidly sealing and occluding the distal end). The bonding further comprises inserting a cylinder at least partially coated with a mold-release chemical into the transducer housing, bonding plastic onto the distal end of the transducer housing, and removing the cylinder when the plastic has hardened. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more detailed description of embodiments, reference will now be made to the accompanying drawings, in which: 
         FIG. 1A  is an elevational cross-sectional view of an ultrasonic flow meter; 
         FIG. 1B  is an elevational end view of a spoolpiece which illustrates chordal paths A, B, C and D; 
         FIG. 1C  is a top view of a spoolpiece housing transducer pairs; 
         FIG. 2  is a perspective view of a transducer in accordance with various embodiments; 
         FIG. 3  is a cross-sectional elevation view of a transducer in accordance with various embodiments; 
         FIG. 4  is a cross-sectional elevation view of a transducer with interior structures not present and prior to molding of the plastic; 
         FIG. 5  (comprising  FIGS. 5A ,  5 B and  5 C) is a cross-sectional elevation view of a transducer after a plastic matching layer has been molded to the distal end; 
         FIG. 6  is a cross-sectional elevation view of a transducer after a plastic matching layer has been machined; 
         FIG. 7  is a flow diagram in accordance with various embodiments of the invention; and 
         FIG. 8  is a flow diagram in accordance with various embodiments. 
     
    
    
     NOTATION AND NOMENCLATURE 
     Certain terms are used throughout the following description and claims to refer to particular system components. This document does not intend to distinguish between components that differ in name but not function. 
     In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ”. Also, the term “couple” or “couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection, or through an indirect connection via other devices and connections. 
     “Fluid” shall mean a liquid (e.g., crude oil or gasoline) or a gas (e.g., methane). 
     DETAILED DESCRIPTION 
       FIG. 1A  is a cross-sectional elevation view of an ultrasonic meter  101  in accordance with various embodiments. Spoolpiece  100 , suitable for placement between sections of a pipeline, is the housing for the meter  101 . The spoolpiece  100  has an internal volume that is a flow path for a measured fluid and also has a predetermined size that defines a measurement section within the meter. A fluid may flow in a direction  150  with a velocity profile  152 . Velocity vectors  153 - 158  illustrate that the fluid velocity through spoolpiece  100  increases toward the center. 
     Transducers  120  and  130  are located on the circumference of the spoolpiece  100 . The transducers  120  and  130  are accommodated by transducer ports  125  and  135 , respectively. The position of transducers  120  and  130  may be defined by the angle θ, a first length L measured between transducers  120  and  130 , a second length X corresponding to the axial distance between points  140  and  145 , and a third length “d” corresponding to the pipe diameter. In most cases distances d, X and L are precisely determined during meter fabrication. Further, transducers such as  120  and  130  may be placed at a specific distance from points  140  and  145 , respectively, regardless of meter size (i.e. spoolpiece size). Although the transducers are illustrated to be recessed slightly, in alternative embodiments the transducers protrude into the spoolpiece. 
     A path  110 , sometimes referred to as a “chord,” exists between transducers  120  and  130  at an angle θ to a centerline  105 . The length L of “chord”  110  is the distance between the face of transducer  120  and the face of transducer  130 . Points  140  and  145  define the locations where acoustic signals generated by transducers  120  and  130  enter and leave fluid flowing through the spoolpiece  100  (i.e. the entrance to the spoolpiece bore). 
     Transducers  120  and  130  are preferably ultrasonic transceivers, meaning that they both generate and receive ultrasonic signals. “Ultrasonic” in this context refers to frequencies above about 20 kilohertz. To generate an ultrasonic signal, a piezoelectric element is stimulated electrically, and it responds by vibrating. The vibration of the piezoelectric element generates an ultrasonic signal that travels through the fluid across the spoolpiece to the corresponding transducer of the transducer pair. Similarly, upon being struck by an ultrasonic signal, the receiving piezoelectric element vibrates and generates an electrical signal that is detected, digitized, and analyzed by electronics associated with the meter. Initially, downstream transducer  120  generates an ultrasonic signal that is then received by upstream transducer  130 . Some time later, the upstream transducer  130  generates a return ultrasonic signal that is subsequently received by the downstream transducer  120 . Thus, the transducers  120  and  130  play “pitch and catch” with ultrasonic signals  115  along chordal path  110 . During operation, this sequence may occur thousands of times per minute. 
     The transit time of the ultrasonic signal  115  between transducers  120  and  130  depends in part upon whether the ultrasonic signal  115  is traveling upstream or downstream with respect to the fluid flow. The transit time for an ultrasonic signal traveling downstream (i.e. in the same direction as the flow) is less than transit time when traveling upstream (i.e. against the flow). The upstream and downstream transit times can be used to calculate the average flow velocity along the signal path, and may also be used to calculate the speed of sound in the fluid. Knowing the cross-sectional area of the meter carrying the fluid and assuming the shape of the velocity profile, the average flow velocity over the area of the meter bore may be used to find the volume of fluid flowing through the meter  101 . 
     Ultrasonic flow meters can have one or more pairs of transducers corresponding to one or more paths.  FIG. 1B  is an elevation end-view of a spoolpiece  100 . In these embodiments, spoolpiece  100  comprises four chordal paths A, B, C, and D at varying levels through the fluid flow. Each chordal path A-D corresponds to two transducers behaving alternately as a transmitter and receiver. Also shown are control electronics  160 , which acquire and process data from the four chordal paths A-D. Hidden from view in  FIG. 1B  are the four pairs of transducers that correspond to chordal paths A-D. 
     An arrangement of the four pairs of transducers may be further understood by reference to  FIG. 1C , showing spool piece  100  and flow direction  150 . Each pair of transducer ports corresponds to a single chordal path of  FIG. 1B . A first pair of transducer ports  125  and  135 , mounted at a non-perpendicular angle θ to centerline  105  of spool piece  100 , houses transducers  120  and  130  ( FIG. 1A ). Another pair of transducer ports  165  and  175  (only partially in view) houses associated transducers so that the chordal path loosely forms an “X” with respect to the chordal path of transducer ports  125  and  135 . Similarly, transducer ports  185  and  195  may be placed parallel to transducer ports  165  and  175  but at a different “level” (i.e. a different elevation in the spoolpiece). Not explicitly shown in  FIG. 1C  is a fourth pair of transducers and transducer ports. Taking  FIGS. 1B and 1C  together, the pairs of transducers are arranged such that the upper two pairs of transducers corresponding to chords A and B, and the lower two pairs of transducers corresponding to chords C and D. The flow velocity of the fluid may be determined at each chord A-D to obtain chordal flow velocities, and the chordal flow velocities combine to determine an average flow velocity over the entire pipe. Although four pairs of transducers are shown forming an X shape, there may be more or less than four pairs. Also, the transducers could be in the same plane or in some other configuration. 
       FIG. 2  is a perspective view of a transducer  210  in accordance with various embodiments. The transducer  210  comprises a cylindrical housing  211 , which in some embodiments is metal (e.g., low carbon stainless steel). In alternative embodiments, any material capable of withstanding the pressure of the fluid within the meter, such as high density plastics or composite materials, may be equivalently used. The transducer  210  comprises a distal end  212  and a proximal end  214 . The distal end  212  is occluded and sealed by a plastic matching layer  216 . Threads  218  on the outside diameter of the transducer housing  210  near the proximal end  214  enable the transducer  210  to be coupled to the spoolpiece  100  ( FIGS. 1A-C ), and an o-ring with groove  220  seals the transducer  210  to the transducer port ( FIGS. 1A-C ). In alternative embodiments, the transducer  210  is welded to the transducer port ( FIGS. 1A-C ) of the spoolpiece, and thus the threads  218  and grove  220  may be omitted. 
       FIG. 3  is a cross-sectional elevation view of a transducer  210  in accordance with various embodiments. In particular, the housing  211 , may, in some embodiments, comprise two individual components. For example, the distal end  212  of the transducer  210  may comprise a first cylindrical outer housing  302 , and the proximal end  214  may comprise a second cylindrical outer housing  304  (comprising the threads  218 ), where the two housings  302 ,  304  are bonded together as part of the construction process. In alternative embodiments, the cylindrical outer housing  211  may comprise a single piece structure, where the various components are installed through one end. 
     The plastic matching layer  216  occludes the distal end  212  and defines an exterior surface  310  and an interior surface  312 . More particularly, the housing  211  defines a circumference around which the plastic matching layer  216  is molded. In some embodiments, the housing  211  comprises circumferential bonding ridges  318  to which the plastic bonds. In alternative embodiments, the housing  211  comprises circumferential bonding grooves ( FIG. 5 ), again to which the plastic bonds. The exterior surface  310  of the plastic matching layer  216  is exposed to fluids flowing through the spoolpiece/meter ( FIGS. 1A-C ), and the interior surface  312  abuts a transducer element  314  (e.g., a piezoelectric element). The volume behind the transducer element  314  comprises a back matching layer  316  and back matching support layer  324 . The back matching layer  316  may be, for example, plastic, metal, glass, ceramic, epoxy, powder-filled epoxy, rubber, or powder-filled rubber. In some embodiments, the transducer element  314  is biased towards the plastic matching layer  216  by way of a conic washer  326 , but any biasing system (e.g., coil springs) may be equivalently used. Biasing the transducer element  314  toward the plastic matching layer  216  helps ensure good acoustic coupling of the transducer element  314  to the plastic matching layer  216 , and further provides structural support for the plastic matching layer  216  by reducing inward deflection of the plastic matching layer caused by high fluid pressures within the meter. 
     Still referring to  FIG. 3 , on the proximal end  214  of the housing  211  is a pin recess  328  within which resides two connection pins  321  and  322 . The two connection pins  321 ,  322  are arranged at the desired spacing and exposed to enable the pins to couple to the external electronics of the meter by way of a cable. Interior of the transducer  210  the pins mate with the connector  320  within the back matching support layer  324 , which connector  320  provides an electrical coupling of the pins  321 ,  322  to the transducer element  314 . In some embodiments, the pins  321 ,  322  seal to the housing  211  (in area  325 ), such as by a glass-to-metal seal. The sealing of the pins  321 ,  322  along with the seal provided by the plastic matching layer  216  isolates the internal components of the transducer  210  both from the fluid and meter and atmosphere. In the event the seal provided by the plastic matching layer fails, the sealing of the pins  321 ,  322  reduces the possibility of escape of fluid in the meter through the transducer. The level of protection provided by sealing the pins against escape of the fluid through the transducer is particularly important in situations where the fluid in the meter contains poisonous substances (e.g., the fluid is a hydrocarbon stream containing hydrogen sulfide). 
     In addition to sealing an interior volume of the transducer  210  from fluids in the meter, the plastic matching layer  216  provides acoustical coupling between the transducer element  314  and fluid in the meter. In accordance with the various embodiments, the plastic matching layer has acoustic impedance between that of the transducer element  314  and fluid in the meter. With the acoustic impedance of the matching layer between that of the transducer element and the fluid in the meter, the quality of the ultrasonic signal is improved (e.g., larger amplitude and faster rise time). In some embodiments the plastic matching layer  216  is thermoplastic, which is corrosion resistance. Depending on the pressure to which the transducer  210  will be exposed and the characteristics of the fluid in the meter (e.g., how corrosive), other plastics may be equivalently used. Plastic matching layers have the desired acoustic impedance to provide good acoustic coupling while being strong enough to resist the pressure of the fluid within the meter so that the transducer element can be isolated from the fluid in the within the meter. In some embodiments, the acoustic impedance of the plastic matching layer  216  is between about 1 and about 30 Mega-rayl (MRayl), and particularly between about 2 and about 4 MRayl. Comparatively, the acoustic impedance of a matching layer comprising substantially stainless steel is more than the acoustic impedance of the piezoelectric element, and therefore provides poor acoustic coupling. 
     The plastic matching layer  216  has a thickness (along an axis shared with the remaining portions of the housing  211 ) that in some embodiments is substantially equal to an odd multiple of one-quarter (¼, ¾, 5/4, 7/4, etc.) wavelength of the sound generated by the transducer element  314 . For example, consider a transducer element  314  operating at a frequency of 125 kHz and a plastic matching layer  216  with a speed of sound of 2,500 m/s. The wavelength of the sound in the matching layer is approximately 0.788 inches. In these embodiments the plastic matching layer may be 0.197, 0.590, 0.984, 1.378 and so on, inches thick. A thinner plastic matching layer gives better acoustical performance, but making the plastic matching layer thicker enables the transducer  210  to withstand higher pressures. Picking the optimal matching layer thickness involves choosing the thinnest matching layer that can hold the highest pressures expected inside the meter. 
     The discussion now turns to various embodiments of constructing a transducer  210  having a plastic matching layer. In particular,  FIG. 4  is a cross-sectional elevation view of a portion of housing  211 , with interior structures not present and prior to molding of the plastic to create the plastic matching layer. Before the plastic matching layer is applied, a telescoping cylinder  412  having a outside diameter slightly smaller than the inside diameter  410  of the housing  211  is inserted into the housing  211 . The telescoping cylinder  412  is at least partially coated with a mold release chemical to facilitate the removal of the cylinder after the plastic matching layer has hardened. In some embodiments (and as shown in  FIG. 4 ), the end of the telescoping cylinder is recessed slightly from the distal end  212  of the housing  211 , enabling the plastic to partially fill an interior volume of the housing  211 . In alternative embodiments, the cylinder  412  maybe positioned such that the end of the cylinder  412  and the distal end of the housing  211  form a plane, and thus when formed the plastic of the plastic matching layer will not extend any appreciable distance into the interior volume of housing  211 . 
     After placing cylinder  412 , the plastic is molded to the distal end of the housing  211 . In particular, the plastic matching layer is molded onto the housing at high temperature. In some embodiments, the plastic of the plastic matching layer has a coefficient of thermal expansion greater than that of the housing. As the plastic matching layer cools, it contracts more than the housing, thus forming a hermetic seal on at least the outside diameter of the housing.  FIG. 5  (comprising  FIGS. 5A ,  5 B and  5 C) is a cross-sectional elevation view of a transducer  211  after the plastic has been applied to the distal end  212  and the telescoping cylinder  412  has been removed. In particular, in some embodiments the plastic is set in a mold having an inside diameter larger than the outside diameter  512  of the housing  211 . As the plastic cools and shrinks the plastic bonds to the housing  211 . Although in some embodiments the plastic may bond to a smooth surface on the outside diameter of the housing  211 , in other embodiments the bonding of the plastic is aided by features on the outside diameter of the housing.  FIG. 5A  illustrates the plastic bonding to circumferential bonding grooves  514 .  FIG. 5B  illustrates the plastic bonding to circumferential bonding ridges  318 .  FIG. 5C  illustrates the plastic bonding to a tapered distal end  520  of the hosing  211 . Moreover, the grooves, ridges and tapers need not be mutually exclusive, and may be combined in any combination (e.g., tapered with bonding grooves, tapered with bonding ridges). As illustrated, the plastic matching layer  510  occludes and seals the distal end  212  of the housing  211 . 
     After rough forming of the plastic of the matching layer to encompass the distal end of the housing  211 , the plastic is machined to its final form.  FIG. 6  is a cross-sectional elevation view of a transducer  210  after machining of the plastic, and comprising illustrative circumferential bonding ridges  318 . In some embodiments, the plastic matching layer  216  is machined to have an outside diameter substantially equal to the outside diameter  512  of the housing  211 . In the area delimited by the inner diameter  410  of the housing  211 , the interior surface  312  and the exterior surface  310  are substantially flat and parallel. 
       FIG. 7  is a flow diagram of construction of a transducer in accordance with at least some embodiments. In particular, the method starts (block  700 ) and the plastic matching layer is molded around the distal end of the housing (block  702 ). In some embodiments, molding the plastic matching layer around the distal end of the housing comprises inserting a cylinder within the housing, and then molding the plastic matching layer around the distal end of the housing. The cylinder within the housing controls the depth at which the plastic matching layer protrudes into the interior volume of the housing. After the plastic matching layer has hardened, the cylinder may be removed from the housing (block  704 ). In embodiments where the plastic is molded to an outside diameter larger than the outside diameter of the housing, the plastic is machined to have an outside diameter substantially equal to an outside diameter of the housing (block  706 ), and the illustrative method ends (block  708 ). 
       FIG. 8  is a flow diagram in accordance with at least some embodiments. In particular, the method starts (block  800 ) and an ultrasonic signal is generated (block  802 ) by way of the transducer. The ultrasonic signal is propagated through the plastic matching layer (block  804 ) and imparted to the fluid traveling through the meter (block  806 ). Thereafter, the illustrative method ends (block  808 ). 
     The above discussion is meant to be illustrative of the principles and various embodiments of the present invention. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. For example, in molding the plastic matching layer to encompass the distal end of the housing, a cylinder need not be used; rather, the plastic may be allowed to free-flow into the interior volume of the housing, an then the excess may be machined away. Further still, in embodiments where a cylinder is used to limit flow of the plastic into the interior volume during molding, the cylinder need not specifically define interior surface. The plastic may be allowed to flow into the interior volume beyond that desired, and then machine away to define the interior surface. Moreover, while the various embodiments are discussed in terms of molding the plastic matching layer to initial have a larger outside diameter than the housing and machining the plastic matching layer, in other embodiments the plastic matching layer may be molded to have an outside diameter approximately the same such that no machining with respect to outside diameter is needed; however, the exterior face  310  may be machined to ensure a smooth surface, and a surface substantially parallel to the interior surface  312 . It is intended that the following claims be interpreted to embrace all such variations and modifications.