Abstract:
A transit time ultrasonic flow sensor uses a pair of transducers that alternate between transmitting and receiving operational states, the operating frequency of a reference oscillator driving the transmitting transducer is controllably varied so as to maintain a constant phase relationship between the transmitted and received signals. A second oscillator is slaved, in frequency to the reference oscillator just before the alternation in operational states takes place and retains that frequency for most of the next operational state, and so forth. Fluid flowing along a line between the two transducers causes one of the two oscillators to swing to a higher frequency, and the other to swing low with respect to each other. A difference frequency between the two oscillators is detected and used as a basis for calculating the rate of fluid flow. In preferred versions of the invention the pair of transducers and an acoustic reflector are configured as a probe that can be inserted into the flowing fluid.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to fluid flow rate measurement devices, and more particularly to an apparatus and a method for measuring the flow rate of a fluid whereby the propagation times of ultrasonic signals transmitted through the fluid vary with the flow rate of the fluid and can be detected to determine fluid flow rate. 
     2. Discussion 
     Transient-Time ultrasonic flow sensors, also known as time-of-flight ultrasonic flow sensors, detect the propagation time difference between the upstream and downstream ultrasonic transmissions resulting from the movement of the flowing fluid and process this information to derive fluid flow rate. These sensors typically use a pair of multiplexed transducers to permit each to alternately provide both transmit and receive functions. Relatively complex multiplexing and transit-time detection electronic circuitry is necessary for achieving usable measurement precision because the change in transit-time due to the flowing fluid is typically a very small part of the total transit time. The transducers are most often mounted on the outside of the pipe which affords the convenience of avoiding penetration of the pipe. However, the uncertainty of the pipe wall uniformity and surface condition, and the variabilities of locating and attaching the transducers, often constitute unfavorable conditions which can lead to substantial measurement error. These sensors are relatively complex and expensive, and have a reputation for sometimes producing erroneous readings. 
     It is therefore an object of the present invention to provide improved means for processing the signals, reducing the costs and improving the reliability of transit time flow sensors. 
     SUMMARY OF THE INVENTION 
     The present invention relates to an apparatus and a method for measuring the low rate of a flowing fluid through the use of ultrasonic energy transmitted through the flowing fluid, and by monitoring the shift in frequency of transmitted and received ultrasonic signals caused by the flowing fluid. One preferred embodiment of the present invention comprises an insertion probe whereby two flow sensing ultrasonic transducers are permanently mounted on the probe which enters the pipe carrying the flowing fluid. The transducers are located one upstream and one downstream, in line with and at an angle to the flowing fluid, and are directly wetted by the fluid. The transducers are also mounted and well insulated acoustically to minimize the transfer of acoustical energy between them which has not been shifted in time by the fluid flow. A probe-mounted acoustically reflective surface is also preferably provided to enable an acoustic path between the transducers to be completed. 
     In a basic form of the present invention, one transducer transmits a high frequency acoustic signal which passes through the flowing fluid to the reflector and then again through the flowing fluid to the receiving transducer. The transmitted and received signals are compared in a phase detector which provides an output signal controlling the frequency of the transmitted signal to maintain a constant phase difference between the two signals, as in a phase locked loop. The change in the transit time of the acoustic signal caused by a flowing fluid causes an incremental phase shift between the transmitted and received signals which is also detected by the phase detector. The phase detector output varies the frequency of the transmitted signal which in turn varies the wavelength of the acoustic signal and therefore the acoustic phase shift between the transducers, to minimize the change in detected phase shift. That change in frequency is representative of the fluid flow rate. Since the acoustic phase shifts due to fluid flow rate are typically on the same order of magnitude as those from error sources, such as transducer, mechanical and electronic drifts, the elementary form of the present invention may likely suffer from instability and might have limited utility. 
     In one preferred form of the present invention the receive and transmit functions of two transducers are interchanged at a low frequency rate. The transmitted frequency from a reference oscillator is compared against the frequency of a variable frequency slave oscillator. The slave oscillator frequency is controlled to be the same as that of the reference oscillator and to retain that frequency just prior to the reversal of the transducer functions. For most of the operating period in each mode, the slave oscillator frequency is that of the reference oscillator during the prior mode of operation. A fluid flow direction which in one mode of operation causes the reference oscillator to increase in frequency, results in the slave oscillator producing, during almost all of the same period, a corresponding reduction in oscillator frequency. Both oscillators alternately swing high and low in frequency out of phase tending to cancel their error contributions. The two frequencies are combined, whereby the difference frequency is detected and provided as an output signal indicative of flow rate. A relatively low frequency oscillator is also used to occasionally reset the reference oscillator to the nominal center of its operating range to correct for transducer mechanical or electronic related drifts over a period of time. 
     At zero fluid flow rate, the reference and slave oscillator frequencies are equal and the output frequency is therefore automatically zero. When fluid flow occurs, a continuous output frequency signal is produced representative of the flow rate. Since the output frequency range can easily be several kilohertz or even tens of kilohertz, a very wide dynamic range of operation is inherent with this electronic processing means. Furthermore, as phase detection is used in a feedback system over a narrow range, its error contribution to the fluid flow rate measurement error is small compared to that of a phase detector used to detect the full range of time difference between the transmitted and received signals. Such processing means are therefore applicable to a wide range of transit-time ultrasonic flow sensors and, in particular, to insertion probe and 2-measurement axis sensor configurations. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various advantages of the present invention will become apparent to one skilled in the art by reading the following specification and subjoined claims and by referencing the following drawings in which: 
     FIG. 1 illustrates a simplified cross-sectional view of a preferred embodiment of the transit time ultrasonic flow sensor of the present invention; 
     FIG. 1A illustrates a simplified end cross-sectional view of the sensor of FIG. 1; 
     FIG. 2 illustrates a block diagram indicating the major functional blocks of electronic circuitry in accordance with an embodiment of the invention; 
     FIG. 3 illustrates a block diagram indicating the functional blocks of electronic circuitry in accordance with a preferred embodiment of the invention; and 
     FIG. 4 illustrates an arrangement of two pairs of transducer elements in accordance with an alternative preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG.  1  and FIG. 1A, a flow sensor  10  in accordance with a preferred embodiment of the present invention is shown. Fluid flow represented by arrow  12  enters pipe  14  and passes between a pair of piezoelectric transducers  16  and  18 , and a reflector  20 . Transducers  16  and  18  are each located and surrounded by an acoustic absorbing material  34  for example, cork, to minimize the transfer of acoustic energy between them within a housing  24 , and to maximize the transfer of acoustic energy beamed along the signal path lines  22 . This enables the transmitted and received signals to be well isolated from each other so that they may be phase compared. The housing  24  is used for mounting the transducers  16  and  18 , and joins with stem  26  upon which is mounted an electronics enclosure  28 . The reflector is supported by posts  36  (FIG. 1 a ). A stem  26  coupled to housing  24  extends through a hot tap fitting  30  to a pipe mounted fitting  32 . 
     Transducers  16  and  18  are angled so that acoustic energy which is not reflected between them is propagated away from sensor  10  and does not interfere with its operation. Since transducers  16  and  18  are not acoustically connected to the pipe  14 , the pipe is no longer a medium for conveying acoustic signals between them. Operation is very clean and there is little signal interaction between the transducers. 
     In prior art devices, the transmitted signal usually consists of a few high magnitude carrier cycles followed by a relatively long period of inactivity both prior to and after the acoustic signal reaches the receiving transducer. A narrow reception window of time is opened for the receiving transducer to be active to minimize the effects of interfering signals from direct conduction, reflection and reverberation of acoustic energy. With the present invention, it is permissible to continue transmitting with the transmitting transducer while the receiving transducer is transmitting and phase comparison is taking place, since the interfering signals are of relatively low magnitude with respect to the desired signal. Alternatively, the reference and slave oscillators may continue to operate continuously while the transducers are switched to actively transmit or receive only during short periods of time so that the desired acoustic energy which is travelling along the path between the transducers, while also passing through the fluid, is detected. This enables flow sensing operation to proceed under less than ideal conditions, for example, when the transducers are mounted on the outside surface of a metal tube carrying a flowing fluid. This also helps to significantly reduce the power consumption of the transducers. 
     FIG. 2 illustrates a block diagram of a representative electronic circuit for the present invention. An oscillator  146  generates a carrier signal, typically several megahertz, which is amplified by amplifier  148 , and which powers transmitting transducer  118  corresponding to transducer  18  in FIG.  1 . This signal is converted into a representative acoustic energy signal which is received by transducer  116 , corresponding to transducer  16  in FIG.  1 . The corresponding signal produced by transducer  116  enters amplifier  142  for amplification, and then phase detector  144 . The signal from oscillator  146  also enters phase detector  144 . The output from phase detector  144  is a DC level signal corresponding to the phase relationship between its two input signals. This signal is filtered in low pass filter  140  to remove the carrier frequencies prior to entering frequency controller  150 . Frequency controller  150  varies the frequency of oscillator  146  that provides the output signal. The DC level signal produced by the phase detector  144  ultimately shifts the frequency of oscillator  146  in the direction necessary to maintain the phase relationship of its two input signals constant, as in a typical phase locked loop. 
     When fluid flow is present, as represented by flow arrow  12  in FIG. 1, the acoustic transit time between transducers  118  and  116  varies in proportion to the velocity contribution of the fluid flow. The incremental change in the transit time, which is converted by the components of FIG. 2 to an incremental frequency change of oscillator  146 , is representative of the fluid flow rate. That incremental frequency change may be extracted and referenced to zero hertz, for example, by combining the oscillator  146  output with that of another signal source having a stable frequency equal to that of oscillator  146  when the fluid is stationary. The difference frequency between the two signal sources may then be detected. The difference frequency will vary between zero, corresponding to zero fluid flow rate, and some finite frequency corresponding to the fluid flow rate. 
     The utility of the embodiment represented by FIG. 1 is quite limited because the incremental phase shift, usually represented by the fluid flow  12 , is very small and generally on the same order as the phase shift drifts and uncertainties exhibited by the associated transducers, mechanical structure and electronics. These problems are substantially reduced by using electronic circuitry in accordance with the circuit of FIG. 3, whereby the FIG. 3 blocks having functions similar as those in FIG. 2 have reference numerals increased by 100. 
     FIG. 3 illustrates the block diagram of a preferred embodiment of an electronic circuit of the present invention. Reference oscillator  246  generates a carrier signal, typically several megahertz, which is amplified by amplifier  248  and powers transmitting transducer  216  corresponding to transducer  16  in FIG.  1 . This signal is converted to representative acoustic energy which is received by transducer  218 , which corresponds to transducer  18  in FIG.  1 . The corresponding signal produced by transducer  218  enters amplifier  242  for amplification and then phase detector  244 . The signal from reference oscillator  246  also enters phase detector  244 . The output from phase detector  244  is a DC level signal corresponding to the phase relationship between its two input signals. This signal is filtered in low pass filter  240  to remove the carrier frequencies prior to entering frequency controller  250 . Frequency controller  250  varies the frequency of reference oscillator  246 . The DC level signal produced by the phase detector  244  ultimately shifts the frequency of reference oscillator  246  in the direction needed to maintain the phase relationship of its two input signals constant, as in a typical phase locked loop. 
     Mode reversal oscillator  252  provides a low frequency, typically a 10 to 100 hertz square wave signal, which is delayed a small amount by time delay  254  to change the state of relay  274  so that transducers  216  and  218  are alternately switched between transmitting and receiving functions. The mode reversal oscillator  252  signal also triggers a one-shot multivibrator  256  to produce a short duration pulse from each transition of the square wave from mode reversal oscillator  252 . These pulses enable the sample and hold circuit  262  to thereby detect and retain the DC level at the output of low pass filter  260  that was produced at phase detector  258  from the phase detection of the signals from oscillators  246  and  264  just before relay  274  changes state. That DC level signal is used by frequency control  276  to control the frequency of slave oscillator  264  so that during the period of the pulses provided by the one-shot multivibrator  256 , the slave oscillator  264  frequency is controlled as in the manner of a typical phase locked loop to produce a frequency exactly the same as that of reference oscillator  246 . During the interval between the pulses, the slave oscillator  264  frequency is maintained at its most recent setting as determined by the DC level signal established by the sample and hold circuit  262 . 
     Signals from oscillators  246  and  264  enter detector  268  which detects their sum to provide a signal with a difference frequency representative of fluid flow rate. Low pass filter  270  removes carrier signal components while amplifier  272  provides the magnitude of the output signal as desired. 
     Reset oscillator  252  produces pulses typically at a very low frequency rate such as once every few seconds, which are synchronized to be coincident with the leading edge of the output from time delay  254 . These pulses enable a switch to momentarily connect a reference voltage to the frequency controller  250  so that reference oscillator  246  is momentarily forced to change its frequency to the nominal center of its operating range. Once phase locked near its nominal center, it will tend to remain in that general location. In this way drifts from any source, or operating or startup transients which may have caused a phase lock to occur at some frequency considerably away from the center of the operating range, are either corrected within a short time or prevented from occurring. 
     Transducer  216  transmits continuously for a half cycle as determined by oscillator  275  while transducer  218  receives during the same period. For the next half cycle the transducers reverse functions and so forth. In each mode, the flowing fluid causes a shift in the transit-time, essentially a phase shift in the frequency of the transmitted acoustic energy reaching the receiving transducer. The electronic processing means detects the acoustic phase shift for each mode of operation due to the flowing fluid and changes the frequency of the transmitted acoustic energy to minimize that change in phase shift. 
     The electronic processing means uses a variable frequency reference oscillator  246  which determines the transmitted frequency. The received signal is compared in phase detector  244  with that of the transmitted frequency whereby its detected output is fed back to the reference oscillator  246  to shift its frequency so as to minimize any phase change, as in a phase locked loop. This action results in the reference oscillator  246  frequency changing in response to flow rate changes. In one mode of operation the reference oscillator  246  frequency will be reduced by the action of the flowing fluid while in the other mode of operation, when the transducers  218  and  218  have changed functions, the frequency will be increased. By incorporating a detector to determine whether the reference oscillator frequency is higher or lower than the slave oscillator  264  when referenced to the mode reversal oscillator, flow direction may be determined. The electronic processing means if therefore capable of being broadly applied for ultrasonic flow detection, including the detection of Karman vortex induced flows. 
     The reference oscillator  246  frequency is compared against the frequency of a variable frequency slave oscillator  264 , whereby the slave oscillator  264  frequency is forced to be the same as that of the reference oscillator  246  and to retain that frequency just prior to the mode change which reverses the transducer functions. For most of the operating period in each mode, the slave oscillator  264  frequency is that of the reference oscillator  246  at the end of the prior mode of operation because of the time delay introduced by time delay  254 . A flow direction which in one mode of operation causes the reference oscillator  246  to increase in frequency results in the slave oscillator  264  producing, during that same period, a corresponding reduction in oscillator frequency. Both oscillators alternately swing high and low in frequency out of phase so as to maintain their difference frequency constant. The two oscillator signals are combined in detector  268  whereby their difference frequency is detected, low pass filtered in filter  270  and amplified in amplifier  272  to provide a continuous output signal with a frequency representative of fluid flow rate. 
     The flow sensor configuration of probe  10  is also suitable for use in open channels and in large bodies of water, for example, as it provides for the complete acoustic reflective path within itself. Furthermore, a second set of transducers located in an enlarged form of housing  24 , and mounted orthogonally to transducers  16  and  18 , which similarly beam to and receive from reflector  20 , will provide a measurement of flow rate in a direction orthogonal to the first set whereby their rate and directional components enable a resultant flow rate and angle to be determined by electronic computation. Such a transducer arrangement is illustrated in FIG. 4 whereby transducers  216  and  218 , corresponding to transducers  16  and  18  of probe  10 , establish an acoustic energy beam line  222 . The beam line  222  is reflected by reflector  220  to sense the component of the fluid flow which moves horizontally across the page, in the direction of arrow  212 . Transducers  224   a  and  224   b,  mounted in the same housing  224 , also establish an acoustic energy beam, represented by lines  225 , to sense the component of fluid flow moving orthogonally to energy beam  222 . It is possible for both sets of transducers  216 ,  218  and  224   a,    224   b  to operate at the same time when the acoustic beam angles are narrow. Should excessive interaction occur, however, the sets of transducers can operate sequentially or even at different frequencies. Each set of transducers  216  and  218 , and  224   a  and  224   b  is supported by its own electronics as in the FIG. 2 example or, alternatively, they may be multiplexed to share the same electronics. 
     When used in flow environments which encourage the accumulation of surface coatings, debris or biogrowths, electrolytic means may be used to clean the acoustically active surfaces. This would consist in a sea water environment, for example, of a positive potential being applied to the flow sensing or nearby surfaces which had been platinum plated so as to cause a corresponding electric current to flow through the water and generate chlorine gas at those active surfaces. Nearby insulated electrodes or conductive surfaces with a corresponding negative potential complete the current path. Low currents of several milliamperes and less have been found effective in maintaining the surfaces of a small flow sensor of a few square inches in surface area clean in such environments. 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms applicable to transit-time ultrasonic flow sensors. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specifications and claims.