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In this paper, a time-delay estimation method using triple correlation with wavelet
transform was proposed. A measurement system of sound travel-time in stored grain
based on the virtual instrument was built. Sound travel-time in soybeans was
measured by the proposed method, cross-correlation method, and cross-correlation
with wavelet transform method. The stability and accuracy of three methods were
compared by experimental data.
2 Theory of Sound Travel-Time Measurement in Stored Grain
If sound travel time t from location A to location B is measured, average temperature
T of the sound propagation path is calculated by follow [3]
2
l
(1)
T
=
.
⎜ ⎟
tZ
⎝ ⎠
where l is the distance from A to B; Z is acoustic constant confirmed by composing of
gas, especially 20.045 for air. For making the equation (1) available in stored grain, a
parameter
called grain porosity influence factor is introduced in the paper. And the
model of acoustic temperature measurement in stored grain could be
λ
2
t λ
l
T
=
.
(2)
⎜ ⎟
⎝ ⎠
is affected by sound frequency, composition of gas and the average spacing
between the grain kernels. As sound frequency and composition of gas are fixed,
λ
principally depend on average spacing between the grain kernels. Therefore, the value
of
λ
could be variety in different spatial locations of the grain bulk, and it need to
calibrate in practice.
Supposed, t 1 and t 2 are sound travel-time between location A and B measured
respectively at temperatures of T 1 and T 2 in stored grain. With equation (2), we have
λ
t
T
2
=
1
.
(3)
t
T
1
2
Use the equation (3), the stability and accuracy of sound travel-time
measurement could be evaluated easily without calibration of
λ
.
3 The System of Sound Travel-Time Measurement
The system shown in Figure 1 includes that, a cubic tank (2m×0.6m×0.6m) filled with
soybeans; a computer installed LabVIEW, and the computer generates a 200~1500Hz
chirp signal which emitted by speaker as the sound signal; a fixture made to hold a
speaker and three microphones with preamplifiers (MIC1~MIC3); a signal conditioner
used to supply power for microphones and its preamplifiers; a data acquisition module
used to collect signal from MIC1~MIC3 simultaneously, its sampling rate was 100kHz,
each channel of module collected 30000 samples of signal.
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