Digital speed determination in ultrasonic flow measurements

Details Digital speed determination in ultrasonic flow measurements Digital speed determination in ultrasonic flow measurements

1998-07-06

2001-10-23

Patel, Harshad (Department: 2855)

Measuring and testing

Volume or rate of flow

By measuring vibrations or acoustic energy

C073S861270

Reexamination Certificate

active

06305233

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a device for measuring the flow speed of a moving fluid and which utilizes ultrasonics and digital speed determination.
DESCRIPTION OF THE RELATED ART
The measurement of the difference in propagation time for the upstream and downstream travel of an ultrasonic signal in a moving fluid is an established method used to measure the rate of flow of that fluid. The ultrasonic signal used for such purposes typically consists of a wave packet of sinusoidal shape with peaks of differing amplitudes. An example of such a wave packet is depicted in
FIG. 3
which shows an electronically received ultrasonic signal
40
. The problem of accurately determining the time of arrival of such a signal is generally considered to involve two specific criteria.
The first is the unique identification of a particular part of the received signal
40
that can be used as a timing reference. The part usually chosen is a point at which one cycle of the wave crosses the signal axis
42
, a so-called “zero crossing”. The identification of a particular zero crossing is conventionally done with reference to the magnitude of the largest peak
41
in the received signal
40
. This method has several difficulties. The first is that the size of the largest peak
41
can vary considerably, and is dependent on the conditions under which the signal is being transmitted. For example, if ultrasonic transducers based upon piezoelectric materials such as PVDF are being used, the size of the peak
41
can change by a factor of 30 as the fluid temperature changes from +60 to −20° C. Even more importantly, for a wave packet in a tube, the particular cycle of the received signal
40
where the largest peak
41
occurs often changes, and is dependent on conditions such as temperature and frequency. This is largely because the maximum of such a signal often, but not always, occurs where secondary acoustic modes make up a major part of the received signal
40
. Generally depicted at
43
in
FIG. 3
is an example of the influence of secondary and other high order acoustic modes These secondary modes are much more affected by temperature and frequency than is the plane wave (primary mode). The condition where two peaks within the wavepacket are identical in magnitude is one which the zero-crossing method finds particularly difficult to accommodate.
The second criterion is the identification of the arrival time of the identified zero-crossing, which is generally the zero-crossing
44
immediately after the largest peak
41
, is received at the signal detector with respect to the time scale being used. The accuracy of the timing of the arrival of the wave is usually limited to one clock pulse, this time interval therefore represents the uncertainty of the measurement.
International Patent Publication No. WO 93/00569 entitled “An Electronic Fluid Flow Meter” discloses an acoustic wave packet detection arrangement and associated flow measuring apparatus that embodies one solution to the above problems. Such an arrangement utilizes envelope detection and an arming method for zero-crossing detection. International Patent Publication Nos. WO 93/00570 and WO 94/20821 each disclose different methods for lessening the propagation of high order acoustic modes which, as discussed above, can contribute significantly to the received wave packet and thus cause timing errors. U.S. Pat. No. 5,206,836 discloses a digital arrangement for determining, based on linear regression about a single zero-crossing, the arrival of a wave packet.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a measurement device and method for ultrasonic timing which permits both the unique identification of a part of an ultrasonic signal and the timing of the arrival of that part of the signal at a detector.
In accordance with a first aspect of the present invention there is disclosed a method of detecting an arrival time at a transducer of an acoustic wave packet transmitted at a predetermined frequency, said method comprising the steps of:
(a) transmitting an acoustic wave packet at a predetermined frequency;
(b) detecting at a transducer the acoustic wave packet and producing an analogue signal output;
(c) converting, at a sampling frequency, the analogue signal output from said transducer into digital signal data having consequential spacing;
(d) determining a measurement portion of said digital signal data, said measurement portion corresponding to a response of said transducer to said acoustic wave packet;
(e) determining from said measurement portion a plurality of slopes S
i
for a corresponding plurality i, where i is an integer between zero and 20, of measurement segments of said response, each of said segments being about a signal level value V; and
(f) determining from at least selected ones of said slopes S
i
, an arrival time t of said acoustic wave packet.
Generally, the method comprises the further step, between steps (e) and (f) of:
(ea) determining, for selected ones of said segments, a corresponding measurement time A
i
indicative of a time at which said response intercepts said signal level value V;
wherein step (f) comprises determining said arrival time t from said corresponding measurement times A
i
.
Preferably, a further step is provided, between steps (e) and (ea) of:
(eaa) matching said slopes S
i
with a corresponding reference set of slopes P
j
to determine a measurement position of each of said segments in said measurement portion;
wherein step (ea) comprises determining said corresponding measurement times A
i
from the corresponding one of said measurement positions.
Generally, step (e) comprises allocating a set of points including an integer k number of points of said digital signal data to each said measurement segment i, each said measurement segment i being centered around two adjacent points, each of the two adjacent points having a value which lies on alternative sides of said signal level value V, fitting a straight line to each set of points, and then determining the slope S
i
of each said straight line. Advantageously, the straight line comprises a line joining the two adjacent points. Preferably the value of k is related to a ratio of the sampling frequency to said predetermined frequency at which said acoustic wave packet is transmitted, and to the consequential spacing of said digital signal data in said measurement portion. Typically, k is equal to one eighth of the number of points per cycle. Most preferably, the value of k is 10.
In one advantageous arrangement, each slope P
j
of said reference set of slopes is associated with a specific position parameter n, said specific position parameter n marking a position corresponding to a signal polarity transition within said wave packet, wherein each specific position parameter n, corresponds to an integral number of half-wave periods of said wave packet between a beginning of said wave packet corresponding to the arrival time of the wave packet and the corresponding signal polarity transition within said wave packet.
In a preferred implementation, step (f) comprises the following sub-steps:
(fa) assigning a weighting factor W
i
to each of said measurement times A
i
based upon the corresponding position parameter n;
(fb) selecting a number m of said measurement times A
i
for using in the determination of the arrival time t;
(fc) determining a mean half-wave period &tgr; from time intervals between adjacent ones of said number m of selected measurement times A
i
;
(fd) determining, for each of the number m of selected measurement times A
i
, an estimated arrival time t
i
of said wave packet at the transducer;
(fe) determining a sum, over the number m of measurement times A
i
, of a product of the weighting factor W
i
assigned to each of said measurement times and the corresponding estimated arrival time t
i
; and
(ff) determining the arrival time t of said acoustic wave packet by dividing the sum determined in step (fe) by a sum of each weighting factor W
i
assigned to each o

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