Ultrasonic Doppler flowmeter with correction for vibratory signals at zero-flow conditions

An ultrasonic Doppler flowmeter for measuring flowing fluid by reflection of sonic waves from particles in the fluid is provided with a discriminating circuit including a comparator amplifier connected to the received signal that produces a zero-flow output from the flowmeter when the signal received by the receiving means is produced by vibration of the reflecting particles in a zero-flow condition.

BACKGROUND OF THE INVENTION 
This invention relates to ultrasonic Doppler flowmeters for the measurement 
of flow of a fluid containing reflectors and more particularly flowmeters 
having the capability of distinguishing between energy reflected from 
flowing reflectors and energy reflected from vibrating reflectors under 
zero-flow condition. 
In ultrasonic Dopper flowmeters, an oscillator having a frequency f.sub.o 
is connected to apply this frequency to an ultrasonic transducer so that a 
beam of ultrasonic waves is propagated in a flowing fluid at an angle 
.theta. with respect to the direction of the flow. Part of the ultrasonic 
energy is reflected back to the transducer by air bubbles or particles in 
the fluid. If all of the reflectors were traveling at the same velocity as 
the fluid, the frequency of the reflected energy would be shifted from the 
transmitted frequency f.sub.o by an amount F.sub.d by virtue of the 
Doppler effect. The quantity F.sub.d is given by the well known equation 
EQU F.sub.d =2V.sub.f f.sub.o (cos .theta./C) (1) 
where V.sub.f is the fluid velocity and C is the acoustic velocity in the 
fluid. 
In practice, however, the received signal is not a single frequency but a 
broad spectrum of frequencies. This spectrum is produced because the 
particles do not all move at the same velocity, as each particle has a 
velocity which depends on its radial position in the pipe, and 
furthermore, the transmitted and received acoustic waves are not plane 
waves, but exhibit curved phase fronts. The resulting frequency spectrum 
is usually roughly Gaussian in shape with a half-power width equal to a 
mean frequency F.sub.d. 
Another problem that is usually encountered is that the reflectors in the 
fluid have imparted to them vibrational velocity components in addition to 
the desired flow velocity component. Such vibrations are usually caused by 
pumps and other vibration sources. This vibrational velocity component is 
particularly undesirable when the fluid flow is completely stopped as by 
means of a valve. Under these circumstances the vibration of the particles 
produces a Doppler shifted received signal that will provide an indication 
of flow at the output of the flowmeter, notwithstanding the fact that the 
flow has been reduced to zero by a valve. In general, the cumulative 
reflected vibratory signal from the reflectors in the fluid is 
substantially less in magnitude than the cumulative signal received from 
particles that are flowing, even though such flowing particles may include 
a vibrating component. The reason that the signal from vibrating particles 
is significantly smaller is that the received signal is due to the sum of 
the signals from each particle and in the vibrating mode, there is a 
strong tendency for the signals from the individual particles to cancel 
each other and to thus provide a signal at any frequency of magnitude 
significantly smaller than the magnitude from normal flowing particles in 
which the effect of the reflected signals from the flowing particles is 
additive. 
The receiving channel in ultrasonic Doppler flowmeters generally 
incorporates a zero-crossing detector as an input element in the 
frequency-to-voltage converter. In order to prevent the zero-crossing 
detector from being influenced by noise, it is common practice to have a 
built-in hysteresis in the zero-crossing detector. With hysteresis the 
zero-crossing detector indicates a crossing only when the signal exceeds 
the hysteresis threshold level in either a positive or a negative 
direction. 
In order to avoid a zero-flow signal caused by vibrating reflectors, some 
ultrasonic Doppler flowmeters are provided with a means to reduce the gain 
of the receiving channel so that the Doppler signal applied to the 
zero-crossing detector that is produced by the vibrating reflectors in 
zero-flow conditions is smaller than the hysteresis threshold level of the 
zero-crossing detector. To accomplish this, the user is instructed to 
produce a zero-flow condition and to gradually decrease the gain of the 
receiving channel until the indication from the flowmeter is zero. While 
this procedure eliminates the zero-flow error, it does introduce error 
into the flowmeter signal under normal flow conditions. It can be shown 
mathematically for any given rms level of a signal applied to a 
zero-crossing detector with hysteresis-threshold that the average 
frequency of the output will deviate from the input frequency inversely 
exponentially as the ratio of the hysteresis-threshold to the rms level of 
the input signal. Thus, using a gain adjustment of the receiving channel 
to reduce its gain so that there is a correct zero-flow signal introduces 
errors in the normal operation of the flowmeter by increasing the ratio. 
In order to avoid the problems of flowmeter errors associated with 
reduction in gain of the receiving channel, Applicants have proposed a 
circuit arrangement using discriminating means to distinguish between a 
vibratory motion and a flow motion of the fluid particles. This 
discriminating means includes a comparator amplifier responsive to the 
amplified received Doppler signal. The threshold level of the comparator 
amplifier is adjusted under conditions of zero-flow when the Doppler 
signal from the reflectors in the fluid is due solely to a vibratory mode 
to cause the frequency-to-voltage converter to be disabled and produce a 
zero-flow output signal and to enable the frequency-to-voltage converter 
when the Doppler signal from the reflectors is caused by fluid flow. Such 
an arrangement avoids the problem of reducing the receiver gain to avoid 
zero-flow error. When the fluid begins to flow and the received signal 
exceeds the threshold level of the comparator amplifier, the ratio of the 
hysteresis to the rms value of the received signal is small to create a 
minimal error in the reading of the Doppler flowmeter. 
An object of this invention is to remove errors in ultrasonic Doppler 
flowmeters due to zero-flow vibrating particles without increasing errors 
during normal operation.

Referring to FIG. 1, there is shown a block diagram of an ultrasonic 
Doppler flowmeter including an oscillator 10 having an output frequency 
f.sub.o. The output frequency f.sub.o from the oscillator 10 is applied to 
an ultrasonic transducer 12 for producing a beam of ultrasonic energy 12a. 
The ultrasonic transducer 12 may be of any well known type, for example, 
as those manufactured from lead zirconate titanate. The ultrasonic 
transducer 12 is positioned with respect to a pipe or conduit such that 
the ultrasonic beam 12a is directed at an angle .theta. to the direction 
of flow in the pipe or conduit. Any particles or bubbles in the flowing 
liquid will reflect a portion of the energy back towards the vicinity of 
the transmitting transducer 12 as shown as a beam of ultrasonic energy 14a 
impinging upon an ultrasonic receiving transducer 14. The ultrasonic 
energy impinging upon the ultrasonic transducer 14 has a frequency that is 
displaced from the transmitted frequency f.sub.o by an amount dependent 
upon the rate at which the fluid and its particles and air bubbles is 
flowing in the conduit or pipe. 
The electrical signal from the transducer 14 is applied to the input of a 
tuned RF amplifier 16 that is tuned to have an optimum amplification in 
the range of frequencies which will include f.sub.o, the transmitted 
frequency, and include a band of frequencies at least as wide as the 
frequencies anticipated from the velocity of the fluid that is to be 
measured. Not only does the ultrasonic transducer 14 apply to the input of 
the RF amplifier 16, a frequency corresponding to the ultrasonic frequency 
included in the reflected sonic beam 14a, but it also includes the 
frequency f.sub.o. The signal having the frequency f.sub.o is generally 
obtained from coupling acoustic or electrical signals from the ultrasonic 
transducer 12 to the ultrasonic transducer 14. Thus, the output from the 
RF amplifier 16 includes not only the frequency of the reflected signal, 
but also includes a signal having the frequency f.sub.o of the transmitted 
signal. The output signal from the RF amplifier 16 is supplied to the 
input of an envelope detector 18. The envelope detector 18 produces at its 
output a signal having a frequency corresponding to the difference between 
the transmitted frequency f.sub.o and the received reflected frequencies. 
The difference between these two frequencies is the Doppler frequency 
F.sub.d from the various particles included in the flowing medium. As the 
particles in the fluid flow at different velocities dependent upon the 
location of the particle with respect to the walls of the pipe or conduit, 
the frequency received includes a spectrum of frequencies. Rather than a 
single frequency, there is therefore produced at the output of the 
detector 18, a band of frequencies, roughly Gaussian in shape with a 
half-power width equal to a mean frequency F.sub.d as defined in Equation 
(1), which is applied to the input of an audio amplifier 20 and is, in 
turn, applied through a resistor 21 as an input to the non-inverting 
terminal of a comparator limiter amplifier 22 which, by virtue of a 
positive feedback connection through a resistor 23, has a built-in 
hysteresis-threshold. The limiter 22 produces at its output a 
substantially square wave which is created by the crossing of the output 
signal from audio amplifier 20 past the hysteresis-threshold of comparator 
limiter 22. The frequency of the substantially square wave produced by 
applying the band of frequencies to a zero-crossing detector having a 
hysteresis-threshold is defined by the following equation: 
##EQU1## 
wherein f' is the output frequency of the square wave signal, V.sub.HDC is 
the DC hysteresis-threshold level, and V.sub.s is the rms level of the 
signal from audio frequency amplifier 20. 
It will be noted from the above equation that the average frequency of the 
output from the limiter 22 deviates from the Doppler frequency in 
accordance with the ratio of the hysteresis-threshold to the rms voltage 
of the signal. Therefore in order to have the frequency output from the 
limiter 22 accurately follow the mean frequency of the band of Doppler 
frequencies appearing in the output of detector 18, it is important that 
the signal level be kept great with respect to the hysteresis-threshold. 
The square wave output signal from the comparator limiter 22 is applied to 
a frequency-to-voltage converter including a pair of D-type flip-flops 24 
and 26 and a low-pass filter 28. The D-type flip-flops 24 and 26 may 
preferably be of the type identified as 4013. Each of these flip-flops has 
independent data, set, reset, and clock inputs and an output. The logic 
level present at the data input D is transferred to the output Q during 
the positive-going transition of a pulse on the clock input CP. Setting or 
resetting is independent of the clock and is accomplished by a HIGH level 
on the set or reset line respectively. As shown, the output from the 
limiter 22 is applied to the clock input CP of flip-flop 24. 
Referring to D-type flip-flop 24, the data input D is connected to a 
positive voltage source +V, the set terminal SD is connected to ground, 
and the reset terminal MR is connected to the output Q of the flip-flop 
26. The output Q from flip-flop 24 is connected to the data input terminal 
D of flip-flop 26. 
While the operation of flip-flops 24 and 26 to produce an output signal at 
terminal Q of flip-flop 26 that varies in proportion to the Doppler 
frequency F.sub.d will be apparent to those skilled in the art, our 
application Ser. No. 953,739 filed Oct. 23, 1978, now U.S. Pat. No. 
4,183,245 is incorporated herein by reference to provide a detailed 
description of the operation of the flip-flops 24 and 26. 
It is common in industrial applications to have considerable vibration 
present in the pipe and fluid whose flow is being measured. Vibrations in 
a pipe clearly cause the fluid and the particles in the fluid also to 
vibrate. Thus, the fluid has some vibrational velocity components in 
addition to the desired axial velocity component. Since the Doppler 
flowmeter transducer is normally bonded to the pipe, it is subjected to 
the same vibration, i.e., it is moving in unison with the fluid and the 
particles, thus the secondary velocity components tend to cancel. While 
the cancellation is not complete, the relative magnitude of the vibration 
signal to the flow signal is normally small. Thus, during flow condition, 
the signal from transducer 14 may be considered as substantially entirely 
a flow signal. 
However, with the flow of the fluid stopped as by the closing of a valve, 
there will continue to exist from the transducer 14 a frequency signal 
that is relatively small in magnitude created by the vibratory mode of the 
reflectors in the fluid. 
In order to discriminate between signals received from vibrating reflectors 
under zero-flow conditions and signals received from reflectors during 
flow conditions, a discriminating circuit including a comparator amplifier 
32 has its inverting input terminal connected to the output from audio 
frequency amplifier 20 and its non-inverting input connected to the 
adjustable contact of a potentiometer 34. The resistance element of the 
potentiometer 34 is connected between a positive voltage source +V and 
ground or signal common. The comparator amplifier 32 is of the type having 
an open-collector output stage. This output stage is connected through a 
resistor 36 to the MR terminal of the flip-flop 26. The terminal MR of the 
flip-flop 26 is also connected to a common connection between a terminal 
of a resistor 38 and a terminal of a capacitor 40. The other terminal of 
the capacitor 40 is connected to ground or signal common and the other 
terminal of the resistor 38 is connected to a positive voltage supply +V. 
During operation, if the positive excursions of the input signal amplified 
and appearing at the output terminal of audio frequency amplifier 20 do 
not exceed the voltage level at the contact of potentiometer 34, the 
output stage of the comparator amplifier 32 remains open and the capacitor 
40 charges to the potential of the +V voltage source. Under these 
conditions the positive voltage appearing at the MR terminal of flip-flop 
26 disables the flip-flop 26 and there are no pulses produced at its 
terminal Q. There is thus produced at the terminal Q of the flip-flop 26 
an indication of zero-flow. When the signal appearing at the output of the 
audio frequency amplifier 20 exceeds the potential appearing at the 
contact of the potentiometer 34, the open-collector output stage of the 
comparator amplifier 32 is closed and the capacitor 40 is rapidly 
discharged through the resistor 36 to ground. This quickly lowers the 
potential applied to the MR terminal of the flip-flop 26 and permits the 
flip-flop 26 to operate to produce pulses at its terminal Q. If the output 
signal from the amplifier 20 drops below the potential appearing at the 
contact of the potentiometer 34, the output stage of the comparator 
amplifier 32 opens. The open condition in the output stage of the 
comparator amplifier 32 permits the capacitor 40 to be charged through the 
resistor 38 toward the potential of the voltage source +V. Because of the 
time delay required for the capacitor 40 to charge because of the 
magnitude of the resistor 38, the voltage appearing at the terminal MR of 
flip-flop 26 will increase exponentially. In practice, the resistance 
value is selected such that a time delay occurs before the flip-flop 26 is 
disabled. This delay prevents temporary low signal conditions from causing 
the output of the flowmeter to oscillate between a zero-flow indication 
and a flow indication due to the presence of noise or other signal 
disturbances. 
To adjust the equipment in use so that the discriminating network does, in 
fact, discriminate between signals from vibrating reflectors and signals 
from flowing reflectors, the flow is interrupted as by the closing of a 
valve with the flowmeter in operation, and the contact of the 
potentiometer 34 is adjusted to the grounded end of the potentiometer 
resistance to produce zero voltage applied to the input terminal of the 
comparator amplifier 32. Under these conditions there is no discrimination 
between the vibrating particles and flowing particles and an output signal 
will be produced at the output terminal Q of flip-flop 26 indicative of a 
flow condition, even though there is a closed valve preventing such flow. 
The operator then slowly adjusts the contact of the potentiometer 34 
relative to the potentiometer resistance to gradually increase the voltage 
applied to the terminal of the comparator amplifier 32. This adjustment is 
continued until the output appearing at the terminal Q of flip-flop 26 
indicates a zero-flow condition, i.e. when the flip-flop 26 is disabled by 
the voltage applied to its MR terminal. When the valve is then opened and 
a flow condition exists, the signal received from the flowing particles in 
the fluid will be of such magnitude that the signal appearing at the 
output of amplifier 20 will cause the output stage of amplifier 32 to be 
in a closed condition reducing the voltage applied to terminal MR of 
flip-flop 26 to a level that enables the flip-flop to produce an output 
signal indicative of the flow of the flowing particles in the fluid. 
While the invention has been described with respect to a particular 
embodiment, it is to be understood that other circuit arrangements could 
be used to accomplish the result of eliminating the signals due to 
vibrating particles in the fluid and other sources of vibrational signals 
such as vibrations of the ultrasonic transducers relative to the pipe or 
conduit and the like, and to provide for proper operation in the presence 
of flowing particles in the fluid without sacrifice of the gain in the 
receiving channel that can be mathematically shown to produce errors in 
the frequency of the square waves produced at the terminal Q of the 
flip-flop 26 and in the flow indication by the meter 30.