Dual frequency acoustic fluid flow method and apparatus

An acoustic fluid flow meter determines the flow velocity or volumetric flow from a measurement of the phase shift of acoustic signals propagated upstream and downstream along an acoustical path in the fluid. High resolution measurements of the phase difference of transmitted and received signals represent the fractional wavelength which exceeds the integral number of waves along the acoustic path. The low resolution determination of the integral and fractional number of waves for each direction is made by measuring the fractional phase shift at one frequency of the acoustic signal and again at a slightly different frequency of the acoustic signal. The difference of those phase shift measurements is proportional to the number of waves along the path for a given direction of propagation. That value is combined with high resolution fractional phase shift measurement to obtain a high resolution total wave number value for each direction of propagation. The difference of those two values then is proportional to the fluid flow.

This invention relates to a method and apparatus for measuring a fluid flow 
and particularly to such method and apparatus utilizing acoustic signals 
sensitive to the flow being measured. For one type of fuel control for 
automotive engines it is desirable to measure mass air flow and such 
measurements must be made with high resolution and with fast response to 
any changes in the air flow. In other applications it is desirable to 
measure the flow of air or other gases or the flow of liquids to obtain 
information such as the speed of the fluid and the volumetric rate of 
flow. 
In my copending patent application Method and Apparatus for Measuring Fluid 
Flow, Ser. No. 545,258, filed Oct. 25, 1983, some prior acoustic flow 
meters were described along with their drawbacks which primarily amounted 
to low measurement resolution and/or low response to fluid flow changes. 
According to the invention in that application upstream and downstream 
acoustic transducers adjacent a flow path are alternately energized by 
transmission signals having a long pulse train containing many wavelengths 
in the acoustic paths between the transducers to generate a received 
signal at the non-energized transducer, waiting for a delay period after 
initial signal transmission to allow the received signal to stabilize and 
then measuring the phase difference between the transmission signal and 
the received signal to determine the phase shifts for both upstream and 
downstream acoustic signal propagation, where the difference between the 
upstream and downstream phase shifts is a function of fluid flow through 
the passage. 
This scheme results in very accurate and fast flow measurements and the 
measured phase shift difference is proportional to the fluid flow velocity 
provided that the flow rate is small enough that the phase shift 
difference is less than one wavelength and does not change direction. The 
circuit operation is extended beyond these limits by providing a rollover 
circuit that indicates when such a limit has been passed and additional 
circuitry which deduces from the history of the system operation the true 
total phase shift difference. The utilization of such a flow meter would 
be enhanced and its range of application extended if the full amount of 
the phase shift could be measured directly, independently of the history 
of operation, and without sacrificing the high resolution and fast 
response. 
It is therefore an object of this invention to provide a method and 
apparatus for measuring the full phase shift attributable to fluid flow in 
an acoustic flow meter. 
The method of the invention is carried out by propagating acoustic signals 
upstream and downstream through a fluid flow along an acoustic path 
between a pair of transducers in response to transmission signals 
comprising wave trains of one frequency and generating corresponding 
received signals, and then repeating the signal propagation at a second 
frequency close to the first frequency, measuring the phase shift between 
the transmission and received signals with high resolution to determine 
the fractional portion of the number of waves between the transducers, 
calculating a low resolution value of the number of waves between the 
transducers for each direction of propagation by subtracting the low 
frequency phase shift from the high frequency phase shift for each 
direction, combining the low resolution value with the measured high 
resolution value for each propagation direction to obtain a high 
resolution value of the number of waves between transducers in each 
direction, and finding the difference of the number of waves in each 
direction which is proportional to fluid flow. 
The apparatus of the invention is carried out by providing upstream and 
downstream transducers defining an acoustic path through a fluid flow, a 
transducer energizing source for emitting transmission signals at two 
different closely spaced frequencies so that acoustic wave trains of both 
frequencies are propagated in both directions sequentially, a circuit for 
measuring the phase shift between transmitted and received signals, and a 
computer programmed to determine the difference of the phase shifts for 
the two frequencies in one direction and then the other direction and then 
to combine each of those differences with a phase shift measurement for 
the same direction, and finally to subtract one of the combined signals 
from the other to determine phase shift due to fluid flow.

The invention as described herein is applied to the measurement of 
volumetric airflow or mass airflow in the induction passage of an 
automotive engine to obtain the necessary data for engine fuel control, 
which data is required in real time so that any changes of airflow can 
result in immediate changes of fuel supply to meet rigorous control 
standards. The principles disclosed herein are not limited to such an 
application, however, since the fast response and very high resolution 
offered by this invention have much broader application and includes the 
flow measurement of gases other than air and the flow measurement of 
liquids as well. 
Referring to FIG. 1, a flow passage 10 contains in its wall a pair of 
electroacoustic transducers referred to as an upstream transducer A and a 
downstream transducer B. The terms "upstream" and "downstream" are 
relative to the arrow 11 indicating the usual flow direction, however, the 
instrument operates well for flow in either direction. The transducers are 
angularly positioned within the wall of the passage 10 so that an acoustic 
wave train 12 emitted from either transducer will, after reflection from a 
wall region 14, be transmitted to the other transducer. It is not 
essential that the reflection technique be employed, rather the transducer 
B, for example, may be located at the wall region 14 so that only a single 
pass of the wave train 12 occurs across the passage. It is important, 
however, that the frequency of the acoustic signal be such that many 
pulses or wavelengths occur along the acoustic path between the 
transducers. For example, the passage and transducer location may be so 
designed that there are nominally 16 wavelengths of the operating acoustic 
wavetrain between the transducers and the meter can be calibrated at a 
given flow rate (preferably zero flow) and temperature so that the 
transducer separation is exactly 16 wavelengths. Then any changes of flow 
rate will alter the wavelength so that the effective acoustic path changes 
by a fractional wavelength for small flow rates and by one or more 
wavelengths for higher flow rates. The circuit described herein, in 
effect, measures such wavelength changes to determine flow rate. An 
ultrasonic driver and analyzer circuit 16 is coupled to the transducers A 
and B to provide transmission signals for energizing the transducers 
alternately and to receive the output signals from the transducer which is 
receiving the acoustic energy. 
Piezoelectric crystals are used as transducers and since each one acts as 
transmitter and receiver, they should have the same characteristics. To 
optimize the efficiency of the transducers, they are chosen with a 
resonant frequency near the operating frequency. If the resonant frequency 
is at the operating frequency, the crystals are at their minimum impedance 
and acoustic signal transmission will be optimized but acoustic signal 
reception will be minimal. By choosing an operating point slightly spaced 
in either direction from the resonant point, the crystals will have 
moderate impedance so that both transmission and reception are good. For 
example, a crystal having a resonant point at 39 kHz and an antiresonant 
point at 42 kHz with impedances of 500 ohms and 22,000 ohms, respectively, 
is preferably operated at about 43 kHz with 7,000 ohms impedance or at 
about 37 kHz at the same impedance. Operating in this manner, a 
transmitting signal of 10 volts peak-to-peak results in a received signal 
of 0.5 volts peak-to-peak. Any acoustic noise at frequencies far from the 
crystal resonance is effectively filtered since the crystal is essentially 
nonresponsive to such frequencies. In the application to automotive 
engines, there is relatively little noise generated in the region of 35 to 
40 kHz but there is much noise below 20 kHz which is not sensed by the 
receiver. 
The circuit 16 is shown in detail in FIG. 2 and, as will be described, is 
capable of not only driving the transducers and receiving signals from 
them but also analyzing the signals to determine the phase of the received 
signal relative to the transmission signal and to provide output signals 
relaying the phase information to a utilizing circuit 18 which preferably 
is in the form of a microcomputer programmed to derive from the phase 
information the desired fluid flow outputs. In the case of engine fuel 
control, the circuit 18 is an engine control module which is a 
microcomputer having many inputs other than airflow information for 
controlling engine operation, particularly the supply of fuel thereto. 
Since mass airflow is a principle element in the engine control algorithm, 
the air temperature and pressure are important inputs to the 
microcomputer. The phase information from the driver and analyzer circuit 
16 contains the temperature information as well as the flow information. A 
separate pressure sensor 20 supplies pressure information to the 
microcomputer 18. 
Referring to FIG. 2, a clock 50 provides an output signal preferably at 10 
MHz which is connected to the input of a divider circuit 52. The divisor 
of the divider circuit is variable in response to a signal on an input 
line 54 so that either of two frequencies f.sub.H or f.sub.L is output 
from the divider on line 56. The divider output then is the source of dual 
operating frequencies. The operating frequency is input to a cycle counter 
57 which has several outputs having prescribed periods and prescribed 
relationships for controlling operations of various aspects of the 
circuit. A direction output on line 58 is connected to an analog switch 60 
to control which transucer A or B is energized with a transmission signal 
thereby controlling the direction of acoustic signal propogation through 
the air passage. In this specification, the direction A is used with 
reference to upstream propagation as measured by the acoustic signals 
received at the upstream transducer A, and direction B refers to 
downstream propagation. It is preferred that the direction signal change 
state every 256 clock pulses. Another cycle counter output is count 
duration on line 62 which preferably changes state approximately in the 
middle and at the end of the direction pulse and lasts for at least 128 
pulses. A mode output on line 54 begins when the upstream direction begins 
and extends for two direction periods or 512 pulses. This mode signal has 
the effect of toggling the divider circuit to change its output between 
the high and low operating frequency each time the upstream propagation 
begins. A ringdown duration pulse on line 64 occurs at each change of the 
direction signal and extends for about 16 pulses. 
The divider output on line 56 is selectively connected through the analog 
switch 60 to the transducer A or B to effect acoustic signal transmission. 
The line 56 is also connected to the positive input of a comparator 66, 
the negative input being connected to ground so that the comparator issues 
a positive output pulse whenever the input goes from negative to positive 
thereby serving as a zero crossing detector. A second comparator 68 has 
its positive input connected through the analog switch to the transducer A 
or B which is receiving acoustic signals. The outputs of the comparators 
66 and 68 are connected to one-shots 69 and 70, respectively, which in 
turn have their outputs connected by lines 72 and 74 to the set and reset 
inputs of a flip-flop 76. The time between the rising edges of the input 
signals to the flip-flop determine the pulse width of the flip-flop output 
on line 78 which is proportional to the difference in phase between the 
transmission signal input to the comparator 66 and the received signal 
input to the comparator 68. The flip-flop output on line 78 is fed to a 
three-input AND gate 80. A second input of the AND gate is the count 
duration line 62 from the cycle counter and the third input is from the 10 
MHz clock 50. Thus, the AND gate output will be a series of pulses at 10 
MHz in frequency in bursts lasting for the pulse width of the flip-flop 76 
provided that the count duration signal is present. Thus, the number of 
pulses contained in each frequency burst from the AND gate is a measure of 
the phase difference between the transmission pulse and received pulse at 
the transducers. 
As shown in FIG. 3, when the transducer pulses are in phase or at zero 
shift, the number of pulses output from the AND gate at each burst is zero 
and as the phase shift increases, the number of pulses increases until a 
phase shift of 360.degree. is attained at which point a discontinuity 
occurs and the number of pulses drops to zero and again increases for 
phase shifts above 360.degree.. Similarly, if the phase shift is in the 
other direction, that is, becomes less than zero, the number of pulses 
jumps to a high value and decreases as the phase shift further decreases 
from the zero point. Thus the number of pulses is a measure of phase 
difference and is directly proportional to phase shift only between 
0.degree. and 360.degree.. A rollover circuit is used to indicate when 
such a discontinuity occurs and conditions the analyzing circuit to 
properly interpret the discontinuity. A flip-flop 84 has its set input 
connected to the line 72 which is triggered by the transmission signal and 
its reset input connected to the line 74 which is triggered by the 
received signal. A second flip-flop 86 has inputs connected to the lines 
72 and 74 so that it will toggle, that is, change state each time a pulse 
occurs on either input line. The outputs of the flip-flops 84 and 86 are 
connected to an exclusive OR gate 88. When the transmission and received 
signals appear alternately during a continuous train of pulses, the 
flip-flop 84 and 86 will change state at the same time so that, for 
example, if both flip-flops are turned on and off simultaneously, their 
outputs are in phase and the exclusive OR 88 will have a low output. If, 
however, two transmission pulses occur in sequence without an intervening 
received pulse, the toggle flip-flop 88 will change state but the 
flip-flop 84 will not change state so that the flip-flop outputs will be 
out of phase and the exclusive OR will be turned on to produce a high 
output. The exclusive OR output occurs on line 91 and is termed a "roll 
over flag". The roll over occurs when there is the discontinuity where the 
phase shift goes beyond zero or a multiple of 360.degree. so that the roll 
over flag is used to aid the circuit in recognizing that event. 
The first pulses from the AND gate 80 are fed to an up counter/shift 
register 90 having as control inputs, the direction signal on line 58, the 
mode signal on line 54 and the roll over flag from the OR gate 88. Under 
control of the direction signal, the up counter counts the pulses in the 
input signal and at the change of state of the direction signal which 
indicates that signal reading for one direction of propagation has 
terminated, the counter value is transferred to the shift register 
whereupon it is serially output in binary form to the microcomputer 18. 
That binary signal represents the measured phase shift in one direction. 
The direction signal on line 58 is also effective to insert a direction 
bit in the serially output signal to identify each signal as A or B 
direction. The mode signal on line 54 is the signal which controls the 
frequency output of the divider 52 and it is effective to insert in the 
serial output a bit indicating high or low frequency operation. Thus, the 
microcomputer receives phase shift information A.sub.H and A.sub.L for 
upstream and downstream propagation at high and low frequencies f.sub.H 
and F.sub.L respectively, and corresponding phase shift information 
B.sub.H and B.sub.L for downstream propagation. The roll over flag to the 
up counter/shift register 90 has the effect of setting the counter output 
to zero if roll over occurs during a given count period. This prevents the 
averaging of high and low count rates representing, say 359.degree. and 
1.degree. to obtain some spurious intermediate value by assuming a 
0.degree. value, which is a good approximation of the proper reading. 
When a transducer is vibrating during transmission mode and is then 
switched to a receiving mode the vibrations continue for a time and 
produce ringing signals which gradually decay thereby rendering the 
transducer ineffective as a receiver during the beginning of its receiving 
mode. The ringing signals can last for a long time. However, to dampen the 
ringing, a ring down logic circuit 94 is effective to connect a damping 
resistor 96 between the input of the comparator 66 and ground which in 
effect couples the damping resistor 96 across the transducer which is set 
for receiving. The ring down logic circuit 94 is controlled by the 
direction signal on line 58 and the ring down duration signal on line 64. 
The ring down duration is typically 16 cycles so that each time the 
acoustic signal direction changes, as noted by the direction signal on 
line 58, the ring down logic circuit is effective to place the resistor 96 
across the receiving transducer for a period lasting for 16 pulses. This 
assures that the ringing signal on the receiving transducer is quickly 
dissipated so that the ringing will not thereafter interfere with the 
sensing of the incoming acoustic pulses. 
The circuit as thus far described is better explained with reference to 
FIG. 4. FIG. 4 is a series of waveforms illustrating the voltages 
occurring at the transducers A and B as shown on axes a and b, 
respectively, or they indicate the logic state or the number stored in 
various parts of the circuit at a given moment of time as depicted on axes 
c and d. FIG. 4 will be described in terms of a preferred embodiment 
having the dual frequency transmission signal being provided in pulse 
trains of 256 pulses, but for purposes of clarity a fewer number of pulses 
in each train is depicted. This same type of compromise for the sake of 
drawing clarity is continued throughout FIG. 4. In other words, the time 
scale is not strictly true, but the sequence of events occurring on the 
various axes illustrates the proper sequence of events in the circuit. 
The square wave pulses at axis b illustrates the input voltage from the 
divider 52 to the transducer B, which occurs when the analog switch is in 
the condition illustrated in FIG. 2. The 256 pulse train extends from time 
t.sub.1 to t.sub.3 which covers a period of about 6.25 milliseconds. At 
time t.sub.3, the analog switch changes state and the transducer B is no 
longer fed from the clock 50, however, ringing in the transducer crystal 
creates large voltage signals 92 which gradually decay. After a new train 
of acoustic pulses from the other transducer arrives at the transducer B, 
small output signals 98 are produced at transducer B which gradually 
increase in magnitude as the sensor begins to resonate in harmony with the 
incoming signals. Thereafter, the sensor output stabilizes and continues 
at a fairly constant amplitude. Preferably, the transmission signal 
driving the transducer is about 10 volts peak-to-peak. The received signal 
100 reaches an amplitude of about 0.5 volts peak-to-peak and varies only 
in phase as caused by the effect of fluid flow changes on the acoustic 
signal propagation through the passage. Noise signals from extraneous 
sources can also cause some phase shifts in individual pulses. To avoid 
taking any measurements during the beginning of each receiving period when 
the ringing 92 and the received pulse build up 98 is occurring, the 
circuit is conditioned to ignore any pulses occurring during the first 128 
pulses of the transmission wave train and then readings are made of the 
received pulses during the remaining 128 pulses in the transmission wave. 
In FIG. 2, the count duration signal on line 58 has a low value during the 
first 128 pulses of each transmission period (i.e., between t.sub.1 and 
t.sub.2 and between t.sub.3 and t.sub.4) to assure that the AND gate 80 is 
disabled thereby inhibiting the reading of any data. The count duration 
signal changes to a high value at t.sub.2 and t.sub.4 to allow reading of 
the phase difference between the transmission pulses at one transducer and 
the received pulses from the output of the other transducer. For example 
the signals 92 and 98 on axis 6 are ignored between times t.sub.3 and 
t.sub.4 and phase comparisons between the received signal 100 on axis b 
and the transmitted signal f.sub.H on axis a are read between times 
t.sub.4 and t.sub.5. As previously described, the zero crossing point of 
the transmission and received pulses triggers the comparator 64 and 66 
which in turn control the one shots and the flip-flop 76 to enable the AND 
gate for a period proportional to the phase shift during which the 10 MHz 
clock pulses pass through the AND gate. It will thus be seen that as shown 
in axes a and b of FIG. 4, that the transducers A and B alternate as 
acoustic sources and receivers and that the upstream and downstream 
propagation velocities determine the relative phases of the transmission 
and received pulses. 
The transmission signals from t.sub.1 through t.sub.5 are high frequency 
signals f.sub.H, whereas, during the period t.sub.5 through t.sub.9, the 
transmission signals are low frequency signals f.sub.L. Thus, the 
waveforms occurring during the low frequency mode are identical to those 
previously described in the high frequency mode and the frequencies are so 
close that the difference is not visually apparent. The axis C in FIG. 4 
shows the incrementing count of the upcounter shift register 90 during 
each read period so that at the end of each period, it accumulates the 
value of A.sub.H, B.sub.H, A.sub.L or B.sub.L, respectively. Following 
each such period as shown on axis d, that value is serially output in 
binary form carrying appropriate flag bits to denote the direction A or B 
and the frequency fH or f.sub.L. For example the value of A.sub.H is 
emitted beginning at time t.sub.3. 
The microcomputer 18 is programmed to determine from the phase shift 
information the number of waves between the transducers along the acoustic 
path for the upstream and the downstream propagation directions and then 
to determine the difference of those wave number values, which difference 
is directly proportional to the velocity of the fluid flow. For upstream 
propagation the measured values A.sub.H and A.sub.L provide high 
resolution values of the fractional portion of the phase shift for high 
and low frequency operation. The low frequency value will be used for 
purposes of the present description, however, either of the values might 
be used. Since the system is designed to have an acoustic path many 
wavelengths long, the number of waves in the acoustic path is generally 
some integral number plus a fractional number. The value A.sub.L (when 
multiplied by a constant K.sub.2) accurately represents the fractional 
value and in itself gives no information as to the integral number of 
waves. If, however, the operating frequency is slightly increased by some 
value, say X percent, then the measured fractional wave number will 
increase X percent times the number of waves along the acoustic path. 
Consequently, the difference between the high and low frequency phase 
shift measurements is proportional to the number of waves in the acoustic 
path. Mathematically the approximate number of waves A.sub.A is expressed 
as A.sub.A =K.sub.1 (A.sub.H -A.sub.L) where K.sub.1 is a constant. 
Similarly, the approximate number of waves Ba in the acoustic path for 
downstream propagation is B.sub.A =K.sub.1 (B.sub.H -B.sub.L). These 
values, however, are low resolution values and by themselves are 
inadequate for some purposes. Where accurate measurements are needed, 
these values of the total number of waves in the propagation path do, 
however, provide the integral number of waves in the path and can be 
combined with the high resolution fractional value K.sub.2 A.sub.L or 
K.sub.2 B.sub.L to derive a high resolution value or precise value A.sub.p 
or B.sub.p for the total number of wavelengths for each direction. In the 
preferred embodiment, the divisor of the divider 52 alternates between 268 
and 272 to provide operating frequencies of f.sub.H =37.313 kHz and 
f.sub.L =36.764 kHz. Using these frequencies, the expression K.sub.1 
(A.sub.H -A.sub.L) yields the total number of waves in the acoustic path 
within 1/16th of a wavelength. The high resolution fractional value 
K.sub.2 A.sub.L, however, is accurate to within 1/1000th of a wavelength. 
Thus, when these values are combined in the microcomputer, a high 
resolution value of the total number of waves is obtained. For example, if 
A.sub.A= 17.10 waves and K.sub.2 A.sub.L =0.026, then the high resolution 
total value is 17.026. As a further example, if the low resolution total 
value is A.sub.A =16.98 waves and the high resolution value K.sub.2 
A.sub.L =0.026, then the high resolution total value is 17.026 waves. The 
flowchart in FIG. 5 illustrates the computation process for determining 
the precise value of the difference in wave numbers from measured phase 
shifts. 
The difference in the upstream and downstream number of waves (A.sub.p 
-B.sub.p) is proportional to the fluid velocity and may be combined with 
an area flow determinant to yield volumetric flow or in the case of 
gaseous flow may be combined with temperature and pressure measurements to 
yield a mass flow rate. It will thus be seen that this invention provides 
an acoustic fluid flow measuring method and apparatus useful to obtain 
fluid flow information at a fast response time and high resolution and 
which is useful over a wide dynamic range of fluid flow.