Signal processing system for flow velocity in laser doppler velocimeter

A period of a correlation signal from a laser Doppler velocimeter is calculated from crossing points at which a correlation function crosses over a threshold level by clipping and digitizing a Doppler burst signal, thereby enabling an accurate and real time measurement to be made of a flow velocity, even in low SNR Doppler signals. Smoothing the waveform of a correlation signal further improves measurement accuracy. When the threshold level is set to be the 1/2 value of the autocorrelation function at .tau.=0, a crossing point is precisely detected even in an uneven correlation function or small amplitude of the crosscorrelation function, thus further increasing measurement accuracy.

BACKGROUND OF THE INVENTION 
(a) Field of the Invention 
This invention relates to a flow velocity measurement processor for a laser 
Doppler velocimeter which utilizes the interference of a laser beam to 
measure, for example, a flow velocity. More particularly, this invention 
relates to a signal processing system for Doppler burst signals. 
(b) Description of the Prior Art 
Generally, in laser Doppler velocimeters, a laser beam is divided at a beam 
splitter into two beams and focused with a lens. A particle, traveling 
through interference fringes at the measuring point, causes light 
scattering. The scattered light, by way of the focusing apparatus, is 
received at a photomultiplier wherein the scattered light is transformed 
into a Doppler burst signal. Then, to obtain a flow velocity of the 
particle, the Doppler burst signal is processed by a signal processing 
system. 
Conventionally, counters or trackers are used for this signal processing 
system. Counters measure the number of periods of an input frequency with 
a built-in clock and then calculate its reciprocal number to obtain a 
Doppler frequency. On the other hand, trackers use a 
Phase-Locked-Loop(PLL) by which an input frequency and a frequency of a 
built-in voltage controlled oscillator(VCO) are kept at a constant level. 
Thus, a Doppler frequency is obtained from this tracking frequency. As the 
Doppler frequency is proportional to the velocity of the particle, both 
counters and trackers calculate the flow velocity from the Doppler 
frequency. 
The above-mentioned Doppler burst signal consists of various kinds of noise 
such as shot noise or white noise. This means that the laser Doppler 
velocimeters employing counters or trackers for the signal processing 
system can not measure the flow velocity from the Doppler signal with low 
signal-to-noise ratio(SNR). In other words, there is a limitation of the 
measurable range for this type of laser Doppler velocimeter. Furthermore, 
both counters and trackers have another disadvantage; in counters, time 
dependent data required to obtain vortex scales in a turbulent flow are 
not available, while the analog processing employed in trackers makes its 
application for measurements in a high turbulent flow with low particle 
density impossible. 
SUMMARY OF THE INVENTION 
The principle object of this invention is to realize an accurate and real 
time measurement of the flow velocity from a Doppler burst signal even 
with low SNR by digitizing the Doppler burst signal to obtain a 
correlation function and then by calculating the period of the correlation 
function. 
To attain the above-mentioned object, this invention comprises a series of 
component means as shown in FIG. 1. Means provided in this invention are: 
clipping means for receiving a Doppler burst signal from the 
photomultiplier of a laser Doppler velocimeter and digitizing the Doppler 
burst signal by clipping it at a fixed level to output a pulse signal; 
sampling means for receiving the pulse signal from the clipping means and 
sampling the pulse signal with a pre-set clock signal to output a 
digitized pulse signal; digital correlation function calculating means for 
receiving the digitized pulse signal from the sampling means and 
calculating the correlation function of the digitized pulse signals to 
output a correlation signal; period calculating means for receiving the 
correlation signal from the digital correlation function calculating means 
and by detecting the points at which the correlation signal crosses over a 
pre-set threshold level, and by using the triangular wave of the 
autocorrelation functions the period and frequency of the Doppler signals 
is calculated based on the crossing points; and flow velocity calculating 
means for calculating the flow velocity of a measuring object from the 
frequency calculated at the period calculating means. 
In the above arrangement, for example, in differential types of laser 
Doppler velocimeters, two laser beams are focused at a measurement volume 
to form an interference fringe. When a measuring object, for instance, a 
particle, passes through the interference fringe, light scattering is 
induced. The scattering light is collected at the photomultiplier. A 
Doppler burst signal is obtained from the scattering light. Then, the 
Doppler burst signal is clipped at a fixed level and digitized to produce 
a pulse signal at the clipping means. The pulse signal is next transmitted 
to the sampling means wherein the pulse signal is sampled in relation to a 
pre-set clock signal to output a digitized pulse signal. This digitized 
pulse signal is sent to the digital correlation function calculating means 
wherein the correlation function of the digitized pulse signal or 
autocorrelation function is calculated to output a correlation signal 
having a triangular wave. 
The correlation signal or a smoothed correlation signal is received at the 
period calculating means. Here, the crossing points are detected when the 
correlation function signal crosses over the pre-set threshold level to 
calculate the period of the correlation signal. Finally, as this period of 
the correlation signal corresponds to that of the Doppler burst signal, 
the flow velocity calculating means calculates the flow velocity of the 
particle from the frequency calculated at the period calculating means. 
The procedure mentioned above makes possible an accurate flow velocity 
measurement even for low SNR Doppler signals and hence significantly 
improves measurement accuracy as compared with conventional methods. A 
wide range of velocity measurement is achieved with this method. 
Compared with the conventional counters and trackers, this invention 
achieves an accurate real-time measurement even within the high turbulent 
flow of low particle density. 
In a preferred embodiment, the autocorrelation function signal obtained 
from the digital correlation function calculating means is smoothed at a 
waveform smoothing means, then transmitted to the period calculating means 
to provide the frequency. The wave smoothing means is effective in 
reducing the error caused by the miscounting of uneven signals.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A description is made below of a preferred embodiment of the present 
invention, with reference to the accompanying drawings. 
Referring now particularly to FIG. 2 of the drawings, reference numeral 1 
designates a laser Doppler velocimeter used for measuring a flow velocity, 
e.g. in internal combustion engines or gas-turbine combustors using the 
coherence characteristic of laser beam. 
The laser Doppler velocimeter 1 is composed of a differential-type optical 
system 2 for incidenting and receiving light such as laser beams and 
scattering light, and a signal processing system 3 for processing a 
Doppler burst signal obtained from the optical system 2. The optical 
system 2, adopting the backscatter mode, is composed of a laser beam 
focusing means 4 and a scattering light receiving means 5. Laser beam 
focusing means 4 includes a laser light source 42, a half-wave plate 43 
for adjusting polarization direction of laser beam 41, a beam splitter 44 
for splitting one laser beam adjusted by the half-wave plate 43 into two 
parallel beams 41a, 41a, and a focusing lens 45 for focusing beam 41a at a 
measuring point O. In this arrangement, two beams 41a, 41a interfere with 
each other and create interference fringes at the crossing point O. The 
passing of a particle traveling in air or liquid through the interference 
fringes causes scattering light 51. 
The scattering light receiving means 5 for receiving and photo-electrically 
converting the scattering light 51 includes a focusing lens 52 for 
focusing the scattering light 51, a reflector 53 for adjusting the 
propagation direction of the scattering light 51 focused through the 
focusing lens 52, and a photomultiplier 54 for converting the scattering 
light 51 into an electric signal to output a Doppler burst signal. The 
frequency of this Doppler burst signal of the scattering light is 
proportional to the velocity of the particle. 
The signal processing system 3, as shown in the waveform transformation 
diagram of FIG. 4, calculates a particle velocity based on the frequency 
of the Doppler burst signal. The signal processing system 3 first sends 
the Doppler burst signal obtained from the photomultiplier 54 to a 
bandpass filter 31 wherein only the Doppler burst signal of the fixed 
frequency band is extracted to eliminate noise. Then the signal processing 
system 3 outputs the Doppler burst signal shown in FIG. 4 (a) to a 
clipping circuit 32 of the clipping means. The clipping circuit 32 begins 
its signal processing when the pedestal or envelope of the Doppler burst 
signal crosses over a pre-set threshold level L.sub.1. Setting the 
threshold level higher than zero leads to an elimination of noise. The 
Doppler burst signal is digitized by the clipping circuit 32 and the 
digitized signal is sent to the sampling circuit 33 of the sampling means. 
In detail, the digitized signal is started when the Doppler burst signal 
reaches the threshold level L.sub.1, and stopped when the Doppler burst 
signal reaches level 0. 
The sampling circuit 33 as shown in FIG. 3 consists of shift registers and 
is connected with a clock circuit 34 which outputs, for example, a clock 
signal of 500 MHz. The clock signal of 500 MHz is used to sample the pulse 
signal at a fixed period at the sampling circuit 33 to output the pulse 
signal shown in FIG. 4 (c) to a digital correlation function calculating 
circuit 35 (a summing network) of the digital correlation function 
calculating means. 
The digital correlation function calculating circuit 35 calculates the 
autocorrelation function R(.tau.) of the pulse signal and outputs a 
correlation signal of the triangular wave shown in FIG. 4 (d) to a 
waveform smoothing circuit 36 of the waveform smoothing means. The digital 
correlation function calculating circuit 35 as shown in FIG. 3 consists of 
a first shift register 35a and a second shift register 35b, and 
exclusive-NOR circuits 35c corresponding to the bit number of the shift 
registers lag time 35a and 35b. The shift registers 35a and 35b, for 
example, 64 bits in capacity, are connected to the shift registers of the 
sampling circuit 33 in series. The exclusive-NOR circuits 35c responds to 
the output signal from each bit of both registers 35a and 35b, and then 
outputs a logic signal to the waveform smoothing circuit 36. 
The waveform smoothing circuit 36, in processing a low SNR Doppler burst 
signal, smoothes an uneven correlation signal to reduce the miscounting 
error due to noise, and outputs a smoothed correlation signal to a period 
calculating circuit 37 of the period calculating means. Here, the SNR is 
defined as, 
EQU SNR=20.multidot.log (0.47.multidot.A/.alpha.) (1) 
where 
A: maximum amplitude of a Doppler burst signal 
0.47.multidot.A: effective value of a Doppler burst signal 
.alpha.: effective value of noise. 
The waveform smoothing circuit 36 uses the moving average method to smooth 
the uneven correlation signal, by averaging three consecutive correlation 
values R(t-1), R(t), R(t+1) of the original autocorrelation function R(t) 
as expressed in the equation; 
EQU R(t)={R(t-1)+R(t)+R(t+1)}/3 (2) 
The period calculating circuit 37 shown in FIG. 4 (d) calculates the period 
of the triangular waveform of the smoothed correlation signal from the 
crossing points when the smoothed correlation signal crosses over the 
pre-set threshold level L.sub.2, to output the calculated period to a flow 
velocity calculating circuit 38 of the flow velocity calculating means. 
More specifically, the period calculating circuit 37 calculates a peak 
position Pi from the mean value (M.sub.i +M.sub.i+1)/2 which is obtained 
by averaging two crossing points M.sub.i, M.sub.i+1 where the smoothed 
correlation signal and the threshold level L.sub.2 meet, and then 
calculates the period of the smoothed correlation signal by counting peak 
position Pi. The threshold level L.sub.2 of the period calculating circuit 
37 is set to be the half value of R(0) which indicates the value of the 
autocorrelation function at .tau.=0 (.tau.: delay time) and corresponds to 
the sample number N.sub.T of the correlation function calculation. Setting 
the threshold level L.sub.2 at the half value of R(0) is a powerful tool 
because the probability that the output from each bit of the shift 
registers 35a and 35b of the digital correlation function calculating 
circuit 35 becomes 0--0 or 1--1, and the probability the output becomes 
0--1 or 1--0 are both 1/2. That is, the autocorrelation function of a 
white noise has its value of R(0)/ 2(=N.sub.T /2). Consequently, the 
threshold level L.sub.2 is set to be R(0)/2. 
Furthermore, the period calculating circuit 37 is designed to determine the 
validation(VAL) of the calculated period. VAL is given by; 
EQU VAL=.vertline.1-(S.sub.2 /S.sub.1).vertline. and .vertline.1-(S.sub.n 
/S.sub.1).vertline. (3) 
where 
S.sub.1 : period calculated from the first triangular wave 
S.sub.2 : period calculated from the second triangular wave 
S.sub.n : period calculated from the last triangular wave. 
The period calculating circuit 37 determines whether VAL satisfies the 
equation, 
EQU VAL&lt;.beta. (4) 
where, 
.beta.: fixed tolerance. 
Finally, the flow velocity calculating means 38 calculates the frequency f 
based on the following equation since the frequency of the smoothed 
correlation function corresponds to the frequency f of the Doppler burst 
signal. 
EQU f=(V/.lambda.).multidot.(k.sub.1 -k.sub.2) (5) 
where 
.lambda.: wavelength of laser beam 
k.sub.1 : unit vector of the moving direction of the first laser beam 
k.sub.2 : unit vector of the moving direction of the second laser beam. 
Although this preferred embodiment includes the waveform smoothing circuit 
36, when the noise level is low, the digital correlation signal can be 
sent directly to the period calculating means 37 from the correlation 
function calculating circuit 35, bypassing the waveform smoothing circuit 
36. 
Referring now to the actual results of experiments performed, the principle 
of flow velocity measurement through the laser Doppler velocimeter 1 is 
discussed. 
Laser beam 41 from the laser light source 42 first passes the half-wave 
plate 43 wherein the polarizing direction of the laser beam is adjusted. 
Laser beam 41 is split at the beam splitter 44 into two parallel beams 41a 
and 41a. Then the laser beams 41a and 41a are focused at the measuring 
point O by the focusing lens 45. When a particle passes through the 
measuring point O, the scattering light 51 appears. The scattering light 
51 is collected at the photomultiplier 54 by the focusing lenses 45, and 
52 and the reflector 53. Then, the photomultiplier 54 converts the 
scattering light and outputs the Doppler burst signal. 
The following descriptions concern the processing of the Doppler burst 
signal by referring to the waveform in FIG. 5. 
Generally, the Doppler burst signal consists of a noise. When the pedestal 
component is totally eliminated, the Doppler burst signal is; 
EQU B(t)=A.multidot.exp [-{(2.sqroot.2.multidot.t)/W.multidot.Nf}.sup.2 
].multidot.cos {(2.pi..multidot.t/W)+.phi.} (6) 
where 
B(t): Doppler burst signal 
t: the time made dimensionless by the sampling period 
A: constant representing amplitude 
W: the ratio of the sampling frequency to the Doppler frequency 
(hereinafter "input frequency ratio") 
Nf: fringe number 
.phi.: phase delay. 
FIG. 5 (a) shows the Doppler burst signal obtained by adding white noise 
generated artificially into the equation (6). Each waveform in FIG. 5 
shows the result of an experiment under the conditions of, SNR=-3 dB, 
W=20, Nf=12, A=1.0, trigger level L.sub.1 =0.3, lag time N.sub.T =128, and 
N.sub..tau. (maximum delay time)=128. This Doppler signal is filtered at 
the bandpass filter 31. Using the pedestal component of the Doppler burst 
signal as a trigger, the clipping circuit 32 clips and digitizes the 
Doppler burst signal at a fixed level to output the pulse signal (Refer to 
FIG. 4 (b)). Then the sampling circuit 33 samples the pulse signal at a 
constant period in relation to the clock signal of the clock circuit 34, 
to output a fixed number of the pulse signal, as shown in FIG. 5 (b). At 
this time, together with other pulse signals, pulse signals consisting of 
noise are shown in FIG. 5 (b) with the mark Z. 
After this processing, the digital correlation function calculating circuit 
35 receives the pulse signal and calculates autocorrelation R(.tau.) of 
the digital pulse signal and then outputs the correlation signal as shown 
in FIG. 5 (c). Because this correlation signal of triangular wave is 
uneven due to noise, the waveform smoothing circuit 36 smoothes the 
correlation signal with the moving average method represented by the 
equation (2). In this way, the waveform smoothing circuit 36 outputs a 
smoothed autocorrelation function as shown in FIG. 5 (d). 
After receiving the smoothed correlation signal from the waveform smoothing 
circuit 36, the period calculating circuit 37 calculates the period of the 
correlation signal by detecting a crossing point M.sub.i on the threshold 
level when the threshold level L.sub.2 is set to be the half value of the 
autocorrelation function R(0), and by calculating and counting peak 
position Pi of the correlation signal. Then, the period calculating 
circuit 37 determines VAL of the correlation signal by using equation (4). 
When VAL satisfies the equation (4), the flow velocity calculating circuit 
38 calculates a particle velocity V by using equation (5) based on the 
frequency obtained at the period calculating circuit 37. 
FIGS. 6 (a), (b), (c), (d), (e) represent numerical data acquired by the 
laser Doppler velocimeter 1. In FIG. 6 (a), (b), (c), (d), (e), the input 
frequency ratio W was respectively set at 5, 10, 20, 30, and 40. The 
horizontal axis represents SN ratio and the vertical axis represents the 
accuracy and VAL(%). This accuracy is expressed by the value of error of 
output against input, represented by the equation 1-(W'/W), where W' in 
the equation indicates the ratio of the sampling frequency to the Doppler 
frequency calculated by the period calculating circuit 37 (hereinafter 
"measured frequency ratio"). In FIG. 6, white circles show the original 
data, and black circles show the smoothed data. 
As obviously seen in FIG. 6, measurement error of less than 3% and VAL of 
over 80% are achieved within the range of input frequency ratio W of 10 to 
20 and SNR of more than -6 dB. Considering the fact that measurement error 
is usually 16% (not shown in the figure) at 0 dB of SNR in conventional 
methods, this demonstrates the significant improvement of accuracy 
achieved by this invention. 
FIGS. 7-(a), (b) are histograms indicating the number of validations, 
obtained out of 100 experimental data, which satisfied equation (4) under 
the condition of SNR=-6 dB, W=20, Nf=12. In FIG. 7, (a) shows validations 
when original data were used and (b) when smoothing technique was adopted. 
Average measured frequency ratio W' and standard deviation .sigma. are, 
19.31, 1.74 in (a) and 19.91, 0.65 in (b) respectively. FIG. 7 verifies 
that the waveform smoothing through the waveform smoothing circuit 36 
results in further improvement of the measurement accuracy. 
FIG. 8 shows error estimation and VAL at different SNR when the waveform 
smoothing technique was adopted. It is clear from FIG. 8 that over 65% of 
VAL is achieved to process the Doppler burst signal with SNR of more than 
-6 dB when the input frequency ratio W is within the range of 5 to 40. In 
other words, with a clock signal of 500 MH.sub.z, a Doppler burst signal 
of 12.5 to 100 MH.sub.z can be processed. Furthermore, over 45% of VAL is 
achieved to process the Doppler burst signal of -10 dB with an input 
frequency ratio W of 10 to 20. This means that, with a clock signal of 500 
MH.sub.z, a Doppler burst signal of 25 MH.sub.z to 50 MH.sub.z can be 
processed. 
FIG. 9 shows the relationship between standard deviation .sigma. and SNR, 
compared with the conventional prior art methods. White circles represent 
values for this invention and black circles represent values for the 
conventional counter method. FIG. 9 shows that this invention has lowered 
a standard deviation and considerably improved measurement accuracy. 
Thus, this invention first digitizes a Doppler burst signal for calculating 
a digital correlation and then calculates a period of the triangular 
waveform of the autocorrelation function from crossing points at which the 
correlation function and the threshold level L.sub.2 meet. This has led to 
considerably improved measurement accuracy over the conventional methods 
because even the flow velocity of low SNR Doppler burst signal can be 
measured precisely by this invention. Another advantage is the wider range 
of flow velocity measurement due to the increased dynamic range in this 
invention. 
Compared with the conventional counters and trackers, this invention makes 
possible a real time measurement and highly accurate velocity measurement 
even when encountering turbulent flow with low particle density. 
Moreover, the waveform smoothing technique applied to the correlation 
signal realizes an even more accurate calculation of the frequency, 
thereby increasing the overall accuracy of the measurement. 
It should further be noted that as the period calculating means 37 sets the 
threshold level L.sub.2 at 1/2 of the value of autocorrelation R(0), 
crossing points can be reliably detected, thus improving measurement 
accuracy. 
This preferred embodiment is provided with single optical system 2, 
receiving scattering light 51 from one direction, to obtain a flow 
velocity through an autocorrelation function. However, it is readily 
apparent for those who are skilled in the art that, by receiving 
scattering light 51 from two different angles, a particle diameter and a 
flow velocity of a particle can be measured simultaneously in real time by 
obtaining phase difference from the crosscorrelation function. When the 
scattering light is collected at different angles, the phase delay between 
the Doppler signals appears, which has the characteristic of being 
proportional to the particle diameter at the measurement volume. In order 
to obtain the velocity and particle diameter simultaneously, the 
crosscorrelation of these two Doppler signals are digitized by using the 
same method described above. The output triangular waveforms of the 
crosscorrelation function provide the phase delay. The first peak of the 
correlation function is shifted proportionally to the phase delay, thus, 
obtaining the first peak position and calculating the period of the 
correlation signals. Thus, the simultaneous measurement of velocity and 
particle diameter can be realized. 
In practice, after processing the signals of the two scattered lights at 
different angles by the photomultiplier 54, the bandpass filter 31 and the 
clipping circuit 32, this embodiment inputs the pulse signal to each shift 
register 35a, 35b of the digital correlation function calculating circuit 
35 by means of the sampling circuit 33 to calculate the crosscorrelation 
function, as shown in the alternate long and short dashed lines of FIG. 3. 
By using this method, for example, the diameter of a particle of engine 
fuel can be measured in real time. 
In addition to one-point flow velocity measurement method employed in the 
preferred embodiment, another method of, for example, multiple point 
measurements detecting vortex pattern of the turbulent flow can be 
adopted. When applying this method, the time and spatial correlation in 
the flow is necessary for the measurement. This means that the velocity at 
multiple points in space should be measured simultaneously to obtain a 
spatial correlation and time-space correlation. Formerly, for multiple 
point measurements, a number of processors were required and a very large 
system had to be established to process the data to provide velocity and 
turbulence at each measuring point to obtain time-space correlation 
distributions. This invention is applicable for the multiple measurements 
wherein a compact and easily controllable system may be used. This is 
because, for instance, when measuring the flow velocity at 10 points 
simultaneously, as seen in FIG. 10, only one computer 39 including the 
correlation function calculating circuit 35 is required while 10 
respective circuits of the photomultiplier 54, the bandpass filter 31, the 
clipping circuit 32 and the sampling circuit 33 are necessary. As shown in 
FIG. 10, signals from 10 photomultipliers are filtered at each bandpass 
filter 31, and then digitized at each clipping circuit 32. Signals are 
next sent to each sampling circuit 33 where data of velocity and time are 
stored in memory. Information from each circuit is processed by one 
correlator and one computer 39 by operating the correlation function 
calculating circuit 35 with a multiplexer. In this way, only one 
correlation function calculating circuit 35 and one computer 39 are needed 
to operate multiple point velocity measurements. Thus the overall system 
furnishes compactness and reduced cost, and system control becomes simple. 
In this embodiment, if the IC is operated at 17 MH.sub.z, 256 bits of data 
can be processed at a data rate of 88.5 KH.sub.z. When 10 respective 
processors of the photomultiplier 54, the bandpass filter 31, the clipping 
circuit 32, the sampling circuit 33 are connected in parallel, the data 
rate for one series of processors becomes approximately 8 KH.sub.z, 
dividing 88.5 KH.sub.z over ten. This data rate of 8 KH.sub.z is 
sufficient for real time measurement because the actual data rate of the 
Doppler burst signals is less than 1 KH.sub.z. 
Instead of the backscatter mode employed in these embodiments discussed 
here, the forward scattering mode, or a reference light type, or a single 
beam type may be used alternatively for the optical system 2. 
It is also noted that the sampling circuit 33 and the digital correlation 
function calculating circuit 35 are not restricted to the shift registers. 
Likewise, the method of moving averages employed at the processing of the 
waveform smoothing circuit 36 is not restricted to the method of three 
correlation functions averaging. Other known techniques may be selected.