Abstract:
A transit-time flow sensor determines a rate at which fluid flows by measuring a propagation time difference between upstream and downstream acoustic transmissions. This may involve providing an acoustic path consisting of sequentially traversed path segments and a repeating arrangement that uses the energy in a received pulse to repeat the pulse in the next sequential segment.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to acoustic apparatus and methods for measuring the flow rate of a fluid by measuring propagation times of ultrasonic signals transmitted through the fluid. 
         [0002]    Ultrasonic transit time flow sensors, also known as time of flight ultrasonic flow sensors, detect the acoustic propagation time difference between the upstream and downstream ultrasonic transmissions resulting from the movement of a flowing fluid, which may be a liquid or gas, and process this information to derive a fluid flow rate. Increasing the effective flow rate measurement distance along the flow axis between transducers generally provides a longer measured time interval and, correspondingly, a more precise measurement. On the other hand, size constraints and signal strength considerations motivate a designer to use smaller flow meters with a correspondingly shorter measurement distance. Therefore, there is a need for a time-of-flight flow sensor in which the effective flow rate detection distance between transducers is increased for a specified flow meter size. 
       BRIEF SUMMARY OF THE INVENTION 
       [0003]    The present invention relates to apparatus and methods for measuring the flow rate of a flowing fluid using ultrasonic energy transmitted through the flowing fluid. The transducers are separated along the flow direction, and are aligned with and angled to the flow axis of the fluid. In some embodiments the transducers are in directly facing relationships so that they can communicate directly without requiring a reflector. 
         [0004]    A preferred embodiment of the invention provides a transit time flow sensor for determining a flow rate of a fluid from a time difference between upstream and downstream acoustic transmissions. This preferred sensor comprises an acoustic path comprising a sequentially arrayed plurality of acoustic path segments in the fluid, where each segment has a respective transducer at each of its two ends. The sensor also has at least one electrical connection having a positive impedance for selectively connecting a transducer associated with one of the path segments to a transducer associated with another of the path segments so that an acoustic signal received by the transducer associated with the one of the path segments is repeated as an acoustic signal by the transducer associated with the another of the path segments. There is also a signal processing circuit electrically connected to transducers at respective ends of the acoustic path for transmitting and receiving the acoustic transmissions and for calculating the flow rate of the fluid. 
         [0005]    An additional embodiment of the invention provides a transit time flow sensor comprising at least one pair of selectively externally connected transducers for transmitting and receiving the acoustic transmissions, where each of the selectively externally connected transducers defines a respective end of the overall acoustic path, which may consist of a number of segments. In addition to the externally connected transducers, this apparatus comprises a timer; a signal source operable under control of the timer to selectively generate an appropriate acoustic transmission from either one of the pair of selectively externally connected transducers; a switching network made up of a plurality of solid state switches operable under control of the timer to electrically connect the two transducers of the at least one pair thereof together, where the switching network does not provide any amplification; and a signal processing circuit electrically connected to the transducers of the at least one pair thereof for generating and receiving the acoustic transmissions and for calculating the flow rate of the fluid. 
         [0006]    Another preferred embodiment of the invention provides a transit time flow sensor where the acoustic path comprises a plurality of discrete, non-contiguous segments. In this embodiment the sensor comprises at least one pair of externally connected transducers for transmitting and receiving the acoustic transmissions, where each of the at least one pair of transducers defines a respective end of the acoustic path. In addition, the apparatus comprises a signal source to selectively generate, from at least one of the at least one pair of externally connected transducers, the acoustic transmission; and at least one reactive pair of directly electrically connected transducers. In the ‘reactive pair’, the two transducers making up the pair are directly electrically connected, and each transducer defines a respective end of a respective segment of the acoustic path. In addition, this embodiment comprises a signal processing circuit selectively electrically connected to the externally connected transducers for generating and receiving the acoustic transmissions and for calculating the flow rate of the fluid. 
         [0007]    Yet another aspect of the invention is that it provides a transit-time method of determining a rate at which fluid flows through a pipe wherein an ultrasonic signal is directed along a path comprising a serial string of non-contiguous segments. In each of the path segments, the ultrasonic signal is generated by a transducer at one end and received by a transducer at the other end. Although the acoustic signal may be redirected or regenerated and retransmitted several times during each measurement cycle, the method does not provide for intermediate stages of amplification. 
         [0008]    A further aspect of the invention is that it provides a transit-time method of determining a rate at which fluid flows by measuring a propagation time difference between upstream and downstream acoustic transmissions. This method involves providing an acoustic path consisting of sequentially traversed path segments, each of which has a respective transducer located at each of its two ends. An acoustic pulse is generated at a first end of a selected path segment and received at the second end of the selected acoustic path segment. The energy contained in the received pulse is then used to generate another acoustic pulse in a sequentially related path segment. The steps of generating, receiving and repeating are used to pass an acoustic transmission from one end of the acoustic path to the other end so that a suitable signal processing circuit can determine the rate at which fluid flows from the measured times of flight. 
         [0009]    In one embodiment, the method employs a switching network having a positive impedance to repeatedly transmit a signal along a path segment lying between two externally connected transducers. In another embodiment, the method employs one or more reactive pairs of transducers that are directly connected to each other. These transducers are arranged along the path so that a selectively externally connected transducer is disposed at a respective end of the overall path and so that each reactive pair is disposed between two of the path segments. This ensures that each of its transducers is at the end of a different one of the path segments. To make a measurement according to this method, one electrically energizes one of the externally connectable transducers by means of the signal source to generate an acoustic transmission which is then sequentially received and regenerated by each of reactive pair of transducers so that the acoustic transmission traverses all of the path segments. The acoustic transmission is then received at the second one of the externally connectable transducers. The rate at which the fluid flows is a function of the time of flight of the acoustic transmission summed along all the path segments. 
         [0010]    Although it is believed that the foregoing description may be of use to one who is skilled in the art and wishes to learn how to practice the invention, it will be recognized that the foregoing recital is not intended to list all the features and advantages. Those skilled in the art will appreciate that they may readily use both the underlying ideas and the specific embodiments disclosed in the in the following Detailed description as a basis for designing other arrangements to achieve the same purposes of the present invention and that such equivalent constructions are within the spirit and scope of the invention in its broadest form. Moreover, it may be noted that different embodiments of the invention may provide various combinations of the recited features and advantages of the invention, and that less than all of the recited features and advantages may be provided by some embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic depiction of an embodiment of the invention employing a passive reflector. 
           [0012]      FIG. 2  is a schematic depiction of an embodiment of the invention employing a reactive pair of transducers. 
           [0013]      FIG. 3  is a schematic depiction of an embodiment of the invention employing both a passive reflector and a reactive pair of transducers. 
           [0014]      FIG. 4  is a partially schematic longitudinal cross-section through an in-line flow sensor of the invention that employs a single reactive pair of transducers and a single externally connected pair of transducers. In the interest of clarity of presentation, all four transducers are shown as being in the same plane. Those skilled in the flow measurement arts will recognize that many other settings are possible. 
           [0015]      FIG. 5  is a schematic depiction of an array of transducers usable in an in-line flow meter incorporating a single externally connected pair of transducers and two reactive pairs of transducers for a total of six transducers. 
           [0016]      FIG. 6  is a schematic circuit diagram of a control and signal processing circuit. 
           [0017]      FIG. 7  is a schematic circuit diagram of a recycling switch used in some embodiments of the invention. 
           [0018]      FIG. 8  is a timing diagram depicting operation of the circuit of  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0019]    Turning now to  FIG. 1 , one finds a schematic representation of a portion of an ultrasonic transit time flow sensor  10  comprising two transducers  12 ,  14  positioned to communicate with each other along an acoustic propagation path shown with arrowed lines  16 . In this embodiment the path involves reflection from a reflector  18  that may be a wall of a pipe (for an in-line flow meter) or a component of a probe. The direction of flow of fluid is indicated with an arrow  20 . 
         [0020]    The reader who is familiar with the transit time flow measurement art will recognize that if the schematic box labeled “recycle switch”  22  were removed,  FIG. 1  would depict a configuration familiar in the prior art. In this prior art flow meter the downstream acoustic transmissions are generated with one  12  of the two depicted transducers and received by the other  14 . The function of the transducers would, of course, be reversed for the upstream transmissions. Those skilled in the flow measurement arts will recognize that one can elect to either generate the upstream and downstream pulses sequentially or in unison. 
         [0021]    The embodiment of the invention depicted in  FIG. 1  also comprises a ‘recycle switch’  22  network, an embodiment of which is schematically depicted in  FIG. 7 , that functions to allow the acoustic path segment  16  to be traversed at least twice, thereby at least doubling the overall acoustic path length and correspondingly increasing the precision of measurement. In this embodiment, after a pulse is generated by one of the transducers  12 ,  14  the recycle switch  22  connects the two transducers  12 ,  14  together so that when the pulse arrives at the receiving transducer  14 ,  12  it is redirected back to the transducer that originally generated it, which uses the energy from that pulse to generate a second acoustic pulse that follows the same path as the first one did. Because there is no amplification provided in the recycle switch network, the amplitude of the second pulse is lower than that of the first one. Thus, if this recycling process is repeated several times, eventually the amplitude of a received pulse will be too small to be detected. 
         [0022]    Turning now to  FIG. 2  and  FIG. 4 , one finds schematic depictions of another embodiment of the invention. In this embodiment acoustic signals are generated and received by means of a pair of transducers  12 ,  14 , each of which is connected to an output of a direction switch  40  used to selectively connect it to a transmitter  36  or receiver  38 . Each of the selectively externally connected transducers  12 ,  14  is at one end of a path segment and in a facing arrangement with another transducer  24 ,  26  at the opposite end of the same path segment. The transducers facing the externally connected ones comprise a reactive pair  28  that are permanently directly electrically connected to each other. This connection  30  is usually by metal wires, but may be of any sort that has a positive impedance, however low. In the operation of the embodiment depicted in  FIG. 2 , a carrier burst generated at a first of the externally connected transducers  12  is received at the other end of the first path segment  16   a  by one  24  of the reactive pair  28  of transducers and is repeated by the other member  26  of the reactive pair so that the carrier burst can traverse the second acoustic path  16   b  segment and be received by the second of the externally connected transducers  14 . 
         [0023]    It will be recognized that arrangements for repeating the acoustic signal are not limited to a single repetition. As noted above with respect to the embodiment of  FIG. 1 , the combination of a single pair of transducers  12 ,  14 , a reflector  18 , and a recycle switch  22  can be used for multiple transmissions along a path segment  16 . Correspondingly, one can configure a flow meter comprising multiple reactive pairs  28  of transducers to allow for multiply repeated signals.  FIG. 5 , for example, is a highly schematic depiction a portion of a transit time flow meter having an acoustic path having three path segments  16   a ,  16   b ,  16   c  and two reactive pairs  28   a ,  28   b . It may be noted that in an in-line flow meter made this way the three upstream transducers  12 ,  26   a ,  26   b  might be equiangularly spaced around the inside of one end of the in-line meter and the three downstream transducers  24   a ,  24   b ,  14  might be equiangularly spaced around the other end, probably with different angular settings, with the three flow segments  16   a ,  16   b ,  16   c  sampling different portions of the flowing fluid. This relationship might be visualized as being generated by taking the depiction of  FIG. 5 , rolling it about a cylinder and then twisting the cylinder about its axis. This is a difficult geometrical arrangement to visualize or to draw. Hence, in the interest of clarity of presentation, the transducer array is shown as being more or less unrolled and laid out in the plane of the drawing. 
         [0024]    One can use both a recycle switch  22  and at least one reactive pair  28  of transducers in a single sensor, as is schematically depicted in  FIG. 3 . In this embodiment a downstream pulse is created by one of the selectively externally connected transducers  12 ; is detected by a transducer that faces it  24  and that is a member of a reactive pair  28 . The pulse is repeated by the other  26  of the reactive pair and detected by the second selectively externally connected transducer  14  (that is, by this time in the operating cycle, disconnected from the signal generation and processing circuitry  32  and connected to the first selectively externally connected transducer which is also disconnected from the external circuitry). The pulse is then again repeated by the first selectively externally connected transducer  12 . Following this repetition the acoustic transmission again traverses the acoustic path and is detected by the second selectively externally connected transducer  14  as an electric signal input to the signal processing circuitry  32 . Tests have shown that either recycle switch arrangement depicted in  FIG. 1 , the multiple reactive pair arrangement of  FIG. 5 , or a combination of the two can provide at least two repetitions before the acoustic signal becomes too weak for reliable detection. 
         [0025]    The foregoing discussion describes several embodiments of the invention in which an acoustic signal is repeated so that it can traverse a segmented overall acoustic path  16 . In some cases (e.g.,  FIG. 1 ) the segmented path  16  comprises a single physical path that the signal traverses more than once. In other cases (e.g.,  FIG. 2 ) the path  16  comprises a plurality of physically distinct segments  16   a ,  16   b , each of which is traversed once by the acoustic transmission. In yet other cases, (e.g.,  FIG. 3 ) the acoustic path comprises physically distinct segments  16   a ,  16   b , each of which is traversed more than once. That is, the transmission process is inherently sequential but may or may not involve physically distinct path segments. In reading this disclosure, the reader will understand that when terms such as a “sequentially arrayed plurality of acoustic path segments” are encountered, although the sequence is inherently temporal, the ‘plurality’ referred to may be spatial, temporal, or any combination of the two. 
         [0026]    Turning now to  FIG. 6 , one finds an exemplar signal processing and control circuit  32  usable with various embodiments of the invention. Timing circuits  34  control operation of a burst transmitter  36  and receiver  38  and of a direction switch  40  used to alternately provide an upstream and a downstream acoustic transmission. The timing circuits  34  also optionally control a recycle switch  22 , and provide a timing input to the signal processing portion  42  of the circuitry. 
         [0027]    As mentioned above, prior art transit time flow meters are sometimes arranged to have both the upstream and downstream acoustic transmissions generated sequentially, and sometimes are designed for simultaneous transmissions in both directions. The latter approach allows for a more direct comparison of the time difference between the upstream and downstream transmissions and is thus sometimes preferred. In the embodiments of the present invention employing a reactive pair of transducers, a straightforward application of the simultaneous transmission approach can lead to a problem if the pulses received by the two transducers of the reactive pair overlap, which would lead to a deleterious linear summing of the two pulses. If the pulses do not overlap, however, this problem does not arise. Hence, preferred embodiments of the present invention using a reactive pair and simultaneous transmission are configured so that the physical lengths of various path segments differ. In  FIG. 2 , for example, this is done by moving one of the transducers  12   a  inwardly from a position that it would occupy if the physical path segments were of equal length. It will be recognized that other approaches to changing the effective path length can be used. These include, without limit, translating one of the transducers perpendicular to the flow direction and changing its angular orientation so that it remains aimed at whatever transducer is at the other end of its acoustic path segment. 
         [0028]    Turning now to  FIG. 7  one finds a schematic depiction of a switching network combining the recycle switch  22  and direction switch  40  functions discussed above. A sequence of timing pulses from the signal processing and control circuit  32  are applied to electrically controlled switches, which may conveniently be MOSFET transistors or other solid state switches, to sequentially generate and receive upstream and downstream transmissions, as depicted by the associated timing diagram of  FIG. 8 , where time windows T 1 , T 2 , RT, R 1 , R 2  correspond to timing signals shown in  FIG. 7 . At an initial instant in the cycle, T 1 , a timing signal causes a switch  44   a  to close so that a first  12  of two selectively externally connected transducers can receive an electrical carrier burst from the transmitter  36 . This switch is then opened and another switch  44   b , which provides the recycle function, is closed for an interval labeled RT to connect the first and second transducers together during a time window in which the acoustic transmission is expected to arrive at the second transducer  14 . This received transmission is repeated by the first transducer. At the conclusion of the RT time window the switch  44   b  is opened and, at a later time R 2 , at which the repeated transmission is expected to arrive at the second selectively externally connected transducer  14 , another switch  44   c  is closed to connect the second transducer  14  to the signal processing circuits  42 . This sequence of events is then repeated with the functions of the first and second transducers interchanged in order to measure the transit time in the reverse direction. One may provide the same functionality with respect to the RT interval by closing switches  44   a  and  44 d simultaneously and not having the openable switch indicated as  44   b.    
         [0029]    Although the immediately foregoing discussion involved only the two selectively externally connected transducers, it will be recognized that the presence of repeating pairs in a measurement configuration affects only the expected time intervals between switch closures. That is, there are no external electrical connections to transducers in a repeating pair, so their presence or absence does not affect the circuit topology. 
         [0030]    Although the present invention has been described with respect to several preferred embodiments, many modifications and alterations can be made without departing from the invention. Accordingly it is intended that all such modifications and alterations be considered as within the spirit and scope of the invention as defined in the attached claims.