Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Mechanical measurement system
Reexamination Certificate
1998-12-04
2001-05-01
Hoff, Marc S. (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Mechanical measurement system
C702S045000, C073S861180, C073S861190, C073S861270
Reexamination Certificate
active
06226598
ABSTRACT:
The present invention relates to a method of measuring the propagation time of a sound signal in a fluid flow between a first transducer acting as a transmitter and a second transducer acting as a receiver and situated at a determined distance from the first transducer, the sound signal transmitted by the first transducer being constituted by at least one pulse transmitted at a determined sound frequency Fa, and the sound signal received by the second transducer comprising a series of characteristic oscillations of amplitude that increases initially over several periods, and then deceases over several following periods, the envelope of the characteristic oscillations being bell-shaped, the method consisting in sampling the received sound signal at a sampling frequency Fe, in digitizing the sampled received sound signal, and in seeking the first meaningful zero-crossing of the characteristic oscillations of the received sound signal by analyzing the sampled and digitized received sound signal.
It has been known for many years to measure the flow rate (or the volume) of a fluid flowing along a duct by using the propagation of sound signals transmitted between two sound transducers situated at points that are spaced apart in the flow direction of the fluid. In principle, a sound signal transmitted from the first transducer towards the second transducer is received by the second transducer and the propagation time Td of said sound signal is measured. Similarly, the propagation time Tu of a sound signal transmitted from the second transducer towards the first transducer is also measured on reception of said signal by said second transducer.
In a fluid meter, the flow rate can be obtained by combining a propagation time measurement for each of the two sound signals transmitted between the two points in opposite directions with a measurement of the sound phase shifts induced in each of the sound signals by the propagation of each of them in the flow. European patent application No. 0 426 309 describes an example of such a flow rate measuring system in which the received signal is sampled and converted into digital form, with sound phase shift being measured by performing synchronous detection on the digitized signal.
When measuring the speed of a gas flow in a gas meter that uses two ultrasound transducers, and when the propagation speed of the ultrasound wave depends on the nature of the gas, it is necessary to measure the travel time of the ultrasound wave between the instant at which it is transmitted and the instant at which it is received.
FIG. 2
shows the waveform of a rectangular pulse signal S
1
of width T transmitted at an instant T
O
by a first ultrasound transducer disposed in the flow of a fluid at a first point, and the waveform of the signal S
2
constituting the impulse response as received as an image thereof at an instant T
1
by a second ultrasound transducer located in the flow of the fluid at a second point that is distinct from the first point.
The sound signal S
2
received by the second transducer is constituted by a series of characteristic oscillations O
c
which increase in amplitude over several periods and then decrease, the envelope of the characteristic oscillations being bell-shaped. The characteristic oscillations O
c
of the signal S
2
are preceded and followed by interference oscillations O
p
of small amplitude. To determine the instant T
1
at which the characteristic oscillations begin, it is appropriate to identify the first meaningful zero-crossing of the characteristic oscillations O
c
of the received sound signal S
2
.
FIG. 3
is on a larger scale than FIG.
2
and shows an example of the sound signal S
2
received as an image of a rectangular pulse S
1
transmitted at a determined sound frequency Fa.
To determine the beginning of the characteristic oscillations O
c
using a known method, a threshold voltage V
s
is set relative to which the level of the received sound signal S
2
is compared, with the comparison being performed on a digitized signal obtained after the received analog signal has been sampled, at a sampling frequency Fe that, for example, is a multiple of the sound frequency Fa.
In this case, the instant T
2
is identified at which the amplitude of the received signal crosses the threshold voltage V
s
, and the instant of the preceding (or following) zero-crossing is identified, which instant is then considered as being the starting instant T
1
of the characteristic oscillations O
c
of the received sound signal S
2
.
That measurement method can lead to errors whenever the characteristics oscillations O
c
of the received sound signal S
2
might be amplified to a greater or lesser extent as a function of the nature of the gas. Thus,
FIG. 4
shows a curve S
21
which corresponds to the waveform of a received sound signal for nitrogen (N
2
) and a curve S
22
which corresponds to the waveform of a received sound signal for a mixture of carbon dioxide and methane (CO
2
/CH
4
). It can be seen that the curve S
21
crosses the threshold voltage V
s
at an instant T
4
which triggers identification of the preceding zero-crossing at an instant T
3
which is properly considered as marking the beginning of the characteristic oscillations O
c
. However, it can be observed that the curve S
22
which is in phase with the curve S
21
crosses the threshold voltage V
s
at an instant T
6
that is later than the instant T
4
and that is offset relative thereto by the value of one period T
R
in the received signal. The instant T
5
is then identified for the curve S
22
as being the zero-crossing point immediately preceding the threshold crossing at instant T
6
and it is then taken into consideration as marking the instant at which the characteristic oscillations O
c
of the curve S
22
begin. Unfortunately, as can be seen in
FIG. 4
, the curve S
22
has a negative lobe that merely comes close to the value of the threshold value V
s
without reaching or crossing said threshold.
Because of amplification or attenuation in the received signal S
2
varying as a function of the nature of the gas, the conventional method of measuring the time of the first zero-crossing in the characteristic oscillations of the received signal can give rise to an error of plus or minus one period that significantly decreases measurement accuracy.
The invention seeks to remedy the above-mentioned drawbacks and to make it possible to reduce the sensitivity of the method of measuring the zero-crossing time of a received sound signal relative to waveform variation in the signal and relative to any external disturbances that may be detected by a simple method of monitoring threshold crossings, and that can give rise to erroneous measurements.
According to the invention, these objects are achieved by a method of measuring the propagation time of a sound signal in a fluid flow between a first transducer acting as a transmitter and a second transducer acting as a receiver and situated at a determined distance from the first transducer, the sound signal transmitted by the first transducer being constituted by at least one pulse transmitted at a determined sound frequency Fa, and the sound signal received by the second transducer comprising a series of characteristic oscillations of amplitude that increases initially over several periods, and then deceases over several following periods, the envelope of the characteristic oscillations being bell-shaped, the method consisting in sampling the received sound signal at a sampling frequency Fe, in digitizing the sampled received sound signal, and in seeking the first meaningful zero-crossing of the characteristic oscillations of the received sound signal by analyzing the sampled and digitized received sound signal, the method being characterized in that in order to seek the first meaningful zero-crossing of the characteristic oscillations of the received sound signal, an ideal characteristic first period is initially defined for determining the first zero-crossing of characteristic oscillations of the received sound signa
De Vanssay Robert
Juillard Jérôme
Bui Bryan
Hoff Marc S.
Pojunas Leonard W.
Schlumberger Industries S.A.
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