Flowmeter system with improved loop gain

A flowmeter system having circuitry defining a path for confining the flow of a fluid medium therethrough, first and second transducers disposed along said flow path for generating and receiving acoustic compression waves in the fluid medium between the transducers, a phase lock loop receiver/transmitter system including a voltage controlled oscillator for adjusting the frequency of the acoustic compression waves to maintain the compression wave length constant, a phase detector for measuring the phase difference of the received acoustic compression waves relative to that transmitted and for producing a sum signal proportional to the sum of the measured phase differences to vary the output of said voltage controlled oscillator, circuitry for producing a difference signal proportional to the difference of the measured phase differences representing the direction and magnitude of the flow of the fluid medium as well as changes in its composition and an active filter in the loop for increasing the loop gain of thesystem.

RELATED APPLICATIONS 
U.S. application for patent entitled "Flowmeter System with a Synchronous 
Clock for Generation of Timing Signals" by R. S. Loveland, filed even date 
herewith, Ser. No. 224,783; 
U.S. application for patent entitled "Flowmeter System With Ultrasonic 
Energy Improvement in Equillibration" by R. S. Loveland, filed even date 
herewith, Ser. No. 224,783; 
U.S. application for patent entitled "Flowmeter System With Improved 
Dynamic Range" by R. S. Loveland, filed even date herewith, Ser. No. 
224,725; and 
U.S. application for patent entitled "Flowmeter System With Digital Phase 
Shifter and Calibration" by R. S. Loveland, filed even date herewith, Ser. 
No. 224,723. 
BACKGROUND OF THE INVENTION 
This invention relates to acoustical flowmeter systems and is particularly 
directed to an improvement in the acoustical flowmeters of the type 
described and claimed in the U.S. Pat. No. 4,003,252 entitled "Acoustical 
Wave Flowmeter" by E. J. DeWath which issued Jan. 18, 1977 and the 
flowmeter system of the type described and claimed in the U.S. Pat. No. 
4,164,865 entitled "Acoustical Wave Flowmeter" by L. G. Hall and R. S. 
Loveland which issued Aug. 21, 1979. 
The invention of DeWath was directed to a flow meter having an unobstructed 
tubular wall thereby eliminating all impediments to the flow path of the 
fluid and eliminating all cavities in which debris might collect. The 
advantages of such a configuration is fully set forth in the DeWath 
patent. To measure flow of a selected fluid in the DeWath flowmeter, 
however, required a calibration for that particular fluid and required a 
recalibration if the flow of a different fluid was to be measured since 
the flowmeter was not responsive to changes in fluid species or densities. 
The Hall and Loveland invention improved the DeWath flowmeter by providing 
a flowmeter that measured flow accurately regardless of changes in fluid 
composition or temperature and by providing a flowmeter with a means for 
determining a change in velocity of sound of the fluid being measured. 
In order to accomplish this, the Hall and Loveland acoustical wave 
flowmeter system had two spaced apart crystal transducers in the wall of 
the flowmeter conduit (sometimes called a cavity) to produce ultrasonic 
acoustic compressions at selected frequencies in the fluid within the 
cavity. The transducers were alternately switched into a transmit and a 
receive mode to generate upstream and downstream transmitted and received 
signals with an automatic means to adjust the transmitted frequencies to 
compensate for changes in velocity of the acoustic compressions in the 
fluid caused by changes in fluid composition and temperature. The 
electronic circuitry involved in the Hall and Loveland flowmeter system 
include means for measuring and storing signals representing the phase 
difference between the transmitting transducer signal producing the 
acoustic compressions and the signal produced by the receiving transducer 
during each of two successive transmit/receive cycles. Circuit means were 
provided to determine the difference between the signals representing the 
two successive phase differences wherein the sign of the difference 
corresponds to the direction of the fluid flow and the magnitude of the 
difference corresponds to the rate of fluid flow through the flowmeter. 
Circuit means were also provided to add the two successive phase 
difference signals together to obtain a signal proportional to the 
velocity of sound in the fluid moving through the flowmeter. This latter 
signal indicated the change in composition of the fluid flowing through 
the meter. 
The Hall and Loveland flowmeter system had a phase lock loop in the 
receiver/transmitter system which included, among other circuit 
components, a phase detector, voltage controlled oscillator (VCO) and a 
loop filter. This loop filter was a passive filter of the RC type for 
filtering the error voltage signal applied to the VCO which would respond 
by changing the transmitted frequency of the transducer. The problem 
encountered with this system is that, once calibrated to operate at a 
certain fluid density, a change in fluid density, for example, would cause 
the VCO to operate at a different frequency which means that the phase 
detector has a constant phase error to create the voltage to drive the VCO 
to a new frequency and thus the range of the phase detector for measuring 
the magnitude of flow was thus limited. For example, if the offset, or 
voltage applied to the VCO, were to change one volt, this would mean that 
the output of the phase detector would be required to work at a greater 
phase error difference from that for which the system was calibrated. 
Thus, with less phase error to work with, the flow measurement range is 
decreased, making the system more sensitive to changes in fluid flow or 
density which could cause the entire system to go out of range or into an 
out-of-lock mode. 
This invention improves the prior system by requiring only a very small 
phase error to be detected by the phase detector in order to change the 
error signal applied to the VCO by a large amount thus improving the loop 
gain of the system. Accordingly, it is a primary object of this invention 
is to improve the loop gain of a phase lock loop circuit in a flow meter 
system. 
SUMMARY OF THE INVENTION 
The flowmeter system which meets the foregoing object comprises means 
defining a path for confining the flow of a fluid medium therethrough, 
first and second transducers disposed along said flow path for generating 
and receiving acoustic compression waves in the fluid medium between the 
transducers, circuit of the phase lock loop type having means for 
automatically adjusting the frequency of the acoustic compression waves to 
maintain the compression wave length constant in the fluid medium, means 
for measuring the phase difference of the acoustic compression waves 
transmitted upstream and downstream relative to that received and for 
producing a sum and difference signals dependent upon the difference 
between the transmit and receive two phases and transmitting said signal 
to said means for automatically adjusting the frequency of the acoustic 
compression waves, means for generating signals representing the direction 
and magnitude of the flow of the fluid medium as well as changes in the 
velocity of sound in the fluid medium, and an active filter including an 
operational amplifier whose output is connected to the input of the means 
for adjusting the frequency of the acoustic compression waves is that the 
gain of the phase lock loop so increased by the gain of the active filter 
thus reducing the need for large changes in phase difference to be 
detected by the phase detector before a change is augmented by the means 
for adjusting the frequency of the acoustic compression waves. 
Other objects and advantages of this invention will become apparent to 
those skilled in the art after a study of the drawings and detailed 
description hereinafter.

DETAILED DESCRIPTION 
FIG. 1 illustrates the flowmeter system of the present invention which 
includes a transducer assembly 10, shown in longitudinal section, which 
comprises a substantially cylindrical body having a central cylindrical 
opening, or bore 12, through which a fluid medium flows in both 
directions, as indicated by the arrows 14. 
The transducer assembly is made generally in accordance with the 
description in the U.S. patent to DeWath, supra, and is provided with 
spaced apart cylindrical crystal transducers whose inner diameters are 
substantially coextensive with the cylindrical bore 12 so that the wall is 
substantially uniform with no obstructions or cavities to provide a place 
for particulate matter to collect or to provide an impediment for the flow 
of fluid therethrough. The purpose of the transducers is described in the 
DeWath patent and in the Hall and Loveland patent, supra. 
While the Hall and Loveland patent also showed and described, in great 
detail, control circuitry for operating the crystal transducers to 
accomplish the desired results, for the purpose of this invention, this 
circuitry has been simplified into block diagrams and reference can be 
made to this patent if more detailed information on the operation of the 
circuit is thought necessary. 
As can be seen in FIG. 1, the two ultrasonic crystal transducers, 
represented by crystals 16 and 18, also identified as CR.sub.D and 
CR.sub.U, are alternately each connected to the transmission control 
circuitry via a switching mechanism 20. When one transducer is connected 
to the transmission circuitry via switching mechanism 20, the other 
transducer is in the receive mode the output of which in turn is connected 
via a second switching mechanism 26 to a phase detector 28, a signal 
integrator 30 and two sample-and-hold circuits 32 and 34, identified as 
upstream and downstream. The outputs of these two sample-and-hold circuits 
are connected to two operational amplifiers, one identified as a summing 
amplifier 36 and the other identified as a difference amplifier 38. The 
output of the summing amplifier 36 will indicate the velocity of sound and 
the output of the difference amplifier will indicate the magnitude and 
direction of the measured fluid flow. The output of the summing amplifier 
is connected to a loop filter 40 and to a voltage controlled oscillator 42 
(VCO) which is connected back to the phase detector 28 and to a phase 
shifter and square-wave-to-sine wave converter 44. The phase shifter and 
converter 44 output is connected back to the first switching mechanism 20. 
Also like the summing amplifier, the output of the difference amplifier 38 
is connected to the VCO 42 but through a multiplier 46 and a velocity of 
sound conditioning circuit 48. One output of the multiplier is the 
magnitude and direction of the fluid flow as stated above and the second 
output represents the relative velocity of sound. Shown connected by 
dotted lines are the first and second switching mechanisms 20 and 26 and 
two additional switching mechanisms 50 and 52 all under the control of a 
combinational logic and clock circuit 54. The circuit 54 alternates 
transmit and receive functions of the two crystal transducers 16 and 18, 
alternates the output of the upstream and downstream receivers 22 and 24, 
operates the integrator 30 between reset, integrate and hold functions 
and, finally, operates the upstream and downstream sample-and-hold 
circuits 32 and 34 through a sample, hold, and sample function. 
As shown in this Figure, the ultrasonic crystals 16 and 18 are alternately 
switched into either the transmit or receive mode by the combinational 
logic circuit. Thus, while one crystal is receiving, the other crystal is 
transmitting. 
For each transmit/receive cycle, the phase difference between the transmit 
signal and the received signal is detected by the phase detector 28. The 
average value is determined for each transmit/receive cycle by the 
integrator circuit 30 which goes through an integrate, hold and reset mode 
for each transmit/receive cycle. During each integrator hold period, the 
respective sample/hold circuit for the upstream phase and the downstream 
phase is ready to accept the new signal (sample mode) as data is available 
at the integrator output. The upstream and downstream sample/hold circuits 
are updated with new data at the end of each respective transmit/receive 
cycle and stores (holds) the information during the wait period. 
In the differential amplifier 38, the stored values are then subtracted 
with the output indicating the direction and magnitude of the fluid flow. 
In addition, the same stored values are added together in the summing 
amplifier to determine if a common mode change has occurred in the fluid 
medium. A common mode change is caused by a change in the velocity of the 
ultrasound which, in turn, may be due to either temperature or fluid 
species change. The result is that the sum of the upstream and downstream 
data, held by the respective sample-and-hold circuits, changes in a manner 
which causes an error voltage signal at the voltage controlled oscillator 
(VCO) 42 input to change the transmit frequency in a direction which 
returns the wave length of the ultra-sound frequency is its original value 
thereby keeping the wave length constant. 
The components of the control circuitry thus far described correspond to 
the control circuitry of the flowmeter system of the Hall and Loveland 
patent; it being understood that the foregoing is a simplification of the 
patented control circuitry. For example, the switching mechanism 20 in 
this disclosure is actually a combination of high speed transistorized 
switches comprised of transistors Q1 thru Q8 controlled from the clock 
source by pulses X,Y Q3 and Q3 applied to their respective inputs, 
switching mechanism 26 are transistors Q9 and Q10 with pulses A and B 
applied to their respective inputs operation of the logic and clock source 
but otherwise the block diagrams correspond to the patented circuitry, 
etc. Other switching mechanisms exist in the circuitry of the patent 
through the operation of the clock source but otherwise the block diagrams 
correspond to the patented circuitry. It is understood that the other 
switching mechanisms were shown here to illustrate the operation of the 
circuitry in the block diagram only. 
As hereinabove, stated, this invention improves the patented system by 
increasing the loop gain (gain is a function of components in the loop, 
eg, transducers, loop filter, integrator, VCO, etc), and this is 
accomplished by incorporating a new and improved loop filter into the 
flowmeter system. However, in order to understand the significant 
improvement in loop gain the prior art loop filter as used in the patented 
system will first be described. In connection with this, attention is now 
directed to FIGS. 2,3, and 4, where FIG. 2 is the prior art passive loop 
filter, FIG. 3 illustrates the phase detector input (transmitted and 
received) and output pulses, and FIG. 4 illustrates the improved active 
loop filter comprising this invention as part of the flowmeter system. 
As illustrated in FIG. 2, and as described in the Hall and Loveland patent, 
output pulses from the summing amplifier 36 are applied to the passive 
loop filter 40 which comprises a one megohm resistor R and a one 
micro-farad compacitor C connected in a conventional manner with the 
output therefrom applied directly to the input of the VCO 42. This filter, 
being passive, simply filters the input signal with no gain so that its 
output is simply a filtered voltage signal of essentially the same 
amplitude as the input pulse. 
To understand the need to improve loop gain, attention is now directed to 
FIG. 3 showing the timing pulses where line A represents the transmit 
pulses applied to one transducer generating the acoustic compression waves 
in the fluid medium and line B represents the received pulses received 
from either the upstream or downstream receiver and applied to the phase 
detector 28 under a no flow condition with a fluid for which the 
instrument has been calibrated. Thus, the received pulses are 90.degree. 
out of phase with the transmitted pulses under a calibrated ideal 
condition. Line C represents the output of the phase detector under such a 
condition. Note the pulses in line C are 1/2 the length of the pulses in 
lines A and B at twice the frequency of the received pulses. 
However, in use, when the summing amplifier 36 indicates a change in 
density of the fluid in the cavity, an error signal is applied to the VCO 
42 so as to change the frequency of the transmit pulses applied to the 
transmitting transducer so that the wavelength of sound through the newly 
detected changed fluid remains the same. For example, as seen in line D of 
FIG. 3, considering the transmitted frequency which has changed due to a 
fluid density change, as a constant, the received pulses have moved in 
phase relative to the transmitted pulses, as for example 30.degree. on one 
side and 60.degree. on the other side from its original position of 
45.degree. on both sides. This means that when the detected phase offset 
is as shown in line D, the system can only respond to a further shift of 
30.degree. (due to a flow signal) in the direction which is already at 
60.degree. before the maximum limit of 90.degree. is reached before the 
system goes to an out-of-lock mode--an inoperative mode. Translating this 
into fluids being measured, for example, if the original instrument was 
calibrated to respond to a change of 6 liters per second in its original 
calibration, the system would only be capable of measuring 4 liters per 
second since 1/3 (60.degree.-45.degree.) of phase detector range has been 
used to change the VCO frequency. Thus, a change of flow of greater than 4 
liters per second in one direction would throw the system into an 
out-of-lock mode. 
What this means, is that, in the prior art, in order to have a change in 
phase error signal of one volt, wave form D would change its duty cycle. 
The duty cycle of wave form C is 50 percent and the signals entering the 
phase detector are exactly 90.degree. (when calibrated) out of phase, but 
if the fluid density is changed, then the duty cycle of the wave form must 
change. In changing the duty cycle of this wave form, however, the range 
in one direction is not the same as the range in the other. Thus, a sudden 
change of fluid in the wrong direction would cause the system to go into 
an inoperative mode. 
Explaining the operation of the flowmeter system in another way, and to 
thus inferentially explain the importance of this invention, attention is 
directed back to FIG. 1 where the upstream and downstream sample-and-hold 
circuits 32 and 34 have their outputs, respectively, identified as 
.phi..sub.U and .phi..sub.D, applied to the sum and difference amplifiers 
36 and 38. 
Turning now to FIG. 4, there is shown an active filter 40 which comprises a 
resistor R1 connected to the inverting input of an operational amplifier 
70 and also connected through a second resistor R2 to a capacitor C1 
which, in turn, is connected to the output of the operational amplifier. 
The noninverting input to the amplifier is grounded. The active filter is 
essentially an integrator, where C1 is the integrating component, R1 
defines the unity gain crossover frequency and R2 is used for loop 
stability. Thus, the voltage applied to this active filter 40 is 
multiplied several hundred thousand times according to, or equal to, the 
gain of the operational amplifier. It becomes apparent, then, that a 
minute change in density will cause a large error signal to be applied to 
the VCO to change the transmit frequency. Thus, the 50% duty cycle wave 
form remains virtually unchanged and the dynamic range of the system is 
substantially uneffected. 
How this increases the loop gain by a tremendous amount is explained 
further in connection with FIG. 5 which represents the open loop transfer 
function of the phase lock loop system--gain versus frequency. As can be 
seen, as frequency is increased, the 3 db roll off point is reached at 
about 0.16 hertz and it continues to roll off until it reaches about 16 
hertz. With the open loop gain of this system at about 100, the error 
voltage required to change the VCO frequency, is relatively high. This is 
true regardless of whether the phase detector is the one of the prior art 
or the new phase detector which is the subject matter of the currently 
filed U.S. patent application Ser. No. 224,725, supra. 
However, by replacing the passive filter of the prior art with the active 
loop filter, this loop gain increases by at least a hundred thousand, that 
is, the gain of the amplifier (shown by the dash line below 0.16 hertz 
point) times the gain of the loop provides a total open loop gain as a 
product of these two or 100,000.times.100. Thus, again the system 
maintains the 50 percent duty cycle wave form and the dynamic range of the 
system remains substantially unaffected by the change in flow or fluid 
density. 
It should be apparent from the foregoing that this invention may be 
incorporated into the circuitry of the Hall and Loveland patent, supra, to 
improve its performance, or may be incorporated in circuitry improved by 
the incorporation of any one or all of the inventions identified under 
RELATED APPLICATIONS. supra into a circuit to improve the performance of 
such circuitry. If the invention of the application Ser. No. 224,783 is 
not used, of course, line 56, shown herein, would be omitted.