U.S. patent number RE35,607 [Application Number 08/185,696] was granted by the patent office on 1997-09-16 for distance measuring method and apparatus therefor.
This patent grant is currently assigned to NKK Corporation. Invention is credited to Yoshiyuki Kanao, Akio Nagamune, Koichi Tezuka.
United States Patent |
RE35,607 |
Nagamune , et al. |
September 16, 1997 |
Distance measuring method and apparatus therefor
Abstract
A distance measuring method and apparatus in which first and
second pseudo random signals which are the same in pattern but
slightly different in period are generated to obtain a correlation
output of the first and second pseudo random signals before
transmission thereof as a reference correlation output, and the
first pseudo random signal is directly transmitted toward a target
or alternatively a carrier wave is modulated by the first pseudo
random signal and transmitted toward the target. A correlation
output of the signal reflected and received from the target and the
second pseudo random signal is detected and the distance to the
target is measured from the time interval between the reference
correlation output and the received correlation output.
Alternatively, the modulated carrier wave reflected and received
from the target and the second pseudo random signal are subjected
to correlation processing to detect a correlative modulated carrier
wave and the correlative modulated carrier is subjected to
orthogonal detection by a reference carrier wave thereby obtaining
a target detection output. Then, the distance to the target is
measured from the time interval between the reference correlation
output and the target detection output.
Inventors: |
Nagamune; Akio (Tokyo,
JP), Tezuka; Koichi (Tokyo, JP), Kanao;
Yoshiyuki (Yokohama, JP) |
Assignee: |
NKK Corporation (Tokyo,
JP)
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Family
ID: |
26364392 |
Appl.
No.: |
08/185,696 |
Filed: |
January 24, 1994 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
307891 |
Feb 7, 1989 |
05075863 |
Dec 24, 1991 |
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Foreign Application Priority Data
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Feb 9, 1988 [JP] |
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63-26532 |
Oct 6, 1988 [JP] |
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63-250784 |
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Current U.S.
Class: |
702/158; 324/326;
342/145; 342/22; 367/99; 702/176 |
Current CPC
Class: |
B22D
2/003 (20130101); G01F 23/284 (20130101); G01S
13/288 (20130101); G01S 13/325 (20130101) |
Current International
Class: |
G01F
23/284 (20060101); G01S 13/00 (20060101); G01S
13/28 (20060101); G01S 13/32 (20060101); G01V
003/08 (); G01S 013/08 () |
Field of
Search: |
;324/326,327,329
;342/22,124,130,131,145 ;364/456,458,561
;367/40,99,100,118,125 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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52-31615 |
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Mar 1977 |
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JP |
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55-44916 |
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Nov 1980 |
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JP |
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61-57875 |
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Mar 1986 |
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JP |
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61-129858 |
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Aug 1986 |
|
JP |
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61-217516 |
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Sep 1986 |
|
JP |
|
63-021584 |
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Jan 1988 |
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JP |
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Other References
Patent Abstracts of Japan, Publ.No. JP60173486, dated Jun. 9, 1985.
Title: Survey Device For Underground Buried Body. .
"An Experimental Study of a Pulse Compression Radar Using Random
Series", Nishimoto, SANE 85-25, 1985. .
"Subsurface Radar Using Coded Pulse", by Suzuki et al, SANE 87-1,
1987..
|
Primary Examiner: Cosimano; Edward R.
Attorney, Agent or Firm: Meller; Michael N.
Claims
What is claimed is:
1. A distance measuring method comprising the steps of:
generating a first pseudo random signal having a clock frequency
f.sub.1 ;
transmitting said first pseudo random signal toward a target as an
electromagnetic wave signal;
receiving a signal reflected from said target and obtaining a
received signal;
generating a second pseudo random signal which is the same in
pattern as said first pseudo random signal but having a clock
frequency f.sub.2 which is slightly different from said clock
frequency f.sub.1 ;
multiplying said first pseudo random signal by said second pseudo
random signal to make a first product;
multiplying said received signal by said second pseudo random
signal to make a second product;
obtaining a first smoothed signal by passing said first product
through a first low-pass filter;
obtaining a second smoothed signal by passing said second product
through a second low-pass filter;
generating a first pulse when the value of said first smoothed
signal reaches a maximum;
generating a second pulse when the value of said second smoothed
signal reaches a maximum;
measuring the time interval between the point when said first pulse
is generated and the point when said second pulse is generated,
multiplying one half of said time interval by the propagation
velocity of said electromagnetic wave to form a first operation
value as the product of said multiplication, dividing the frequency
difference between said clock frequencies f.sub.1 and f.sub.2 by
said clock frequency f.sub.1 to form a second operation value as
the quotient of said division and multiplying said first operation
value by said second operation value to calculate the distance to
said target as the product of said multiplication.
2. A distance measuring apparatus comprising:
means for generating a first pseudo random signal having a clock
frequency f.sub.1 ;
means for transmitting an output of said first pseudo random signal
generating means as an electromagnetic wave signal toward a
target;
receiving means for receiving a reflected signal from said target
to obtain a received signal;
means for generating a second pseudo random signal which is the
same in pattern as said first pseudo random signal but having a
clock frequency f.sub.2 which is slightly different from said clock
frequency f.sub.1 ;
a first multiplier for multiplying the output of said first pseudo
random signal generating means by an output of said second pseudo
random signal generating means;
a second multiplier for multiplying an output of said receiving
means by the output of said second pseudo random signal generating
means;
a first low pass filter for smoothing the output of said first
multiplier and outputting a first smoothed signal;
a second low pass filter for smoothing the output of said second
multiplier and outputting a second smoothed signal;
first pulse generating means for generating a pulse when the first
smoothed signal from said first low pass filter reaches a
maximum;
second pulse generating means for generating a pulse when the
second smoothed signal obtained from said second low pass filter
reaches a maximum;
measuring and calculating means for measuring the time interval
between the point when said first pulse is generated and the point
when said second pulse is generated, multiplying one half of said
time interval by the propagation velocity of said electromagnetic
wave to form a first operation value as the product of said
multiplication, dividing the frequency difference between said
clock frequencies f.sub.1 and f.sub.2 by said clock frequency
f.sub.1 to form a second operation value as the quotient of said
division and multiplying said first operation value by said second
operation value to calculate the distance to said target as the
product of said multiplication.
3. A distance measuring apparatus according to claim 2 wherein:
said first pseudo random signal generating means comprises a first
clock generator having the clock frequency f.sub.1 and a first
clock-synchronizing type pseudo random signal generating means to
be driven by an output of said first clock generator;
said second pseudo random signal generating means comprises a
second clock generator having the clock frequency f.sub.2 and a
second clock-synchronizing type pseudo random signal generating
means to be driven by an output of said second clock generator;
said first clock-synchronizing type pseudo random signal generating
means has the same construction as that of said second
clock-synchronizing type pseudo random signal generating means;
and
the clock frequency f.sub.1 of said first clock generator is
slightly different from the clock frequency f.sub.2 of said second
clock generator.
4. A distance measuring method comprising the steps of:
generating a first pseudo random signal having a clock frequency
f.sub.1 ;
generating a second pseudo random signal which is the same in
pattern as said first pseudo random signal but having a clock
frequency f.sub.2 which is slightly different from said clock
frequency f.sub.1 ;
multiplying said first pseudo random signal by said second pseudo
random signal and outputting the correlated code signal as the
product of said multiplication;
generating a reference carrier wave;
modulating said reference carrier wave in coded phase by each code
of said first pseudo random signal;
transmitting said modulated carrier wave in coded phase toward the
target as an electromagnetic wave signal;
receiving a signal reflected from said target to obtain a received
signal;
multiplying said received signal by said second pseudo random
signal and outputting the phase correlated carrier wave as the
product of said multiplication;
respectively multiplying said phase correlated carrier wave by an
inphase component (I signal) and by a quadrature component (Q
signal) which are extracted from said reference carrier wave, their
phases being mutually orthogonal, and respectively outputting both
products, the real part and the imaginary part of the orthogonal
detected signal;
respectively smoothing said real part and imaginary part of said
orthogonal detected signal and calculating the signal absolute
value of a composed detection signal from said smoothed two
component signals;
passing said correlated coded signal through a low pass filter and
outputting the smoothed correlated coded signal;
measuring the time interval between point when said smoothed
correlated code signal value reaches a maximum and the point when
said signal absolute value of a composed detection signal reaches a
maximum, multiplying one half of said time interval by the
propagation velocity of said electromagnetic wave to form a first
operation value as the product of said multiplication, dividing the
frequency difference between said clock frequencies f.sub.1 and
f.sub.2 by said clock frequency f.sub.1 to form a second operation
value as the quotient of said division and multiplying said first
operation value by said second operation value to calculate the
distance to said target as the product of said multiplication.
5. A distance measuring apparatus comprising:
means for generating a first pseudo random signal having a clock
frequency f.sub.1 ;
means for generating a second pseudo random signal which is the
same in pattern as said first pseudo random signal but having a
clock frequency f.sub.2 which is slightly different from said clock
frequency f.sub.1 ;
a first multiplier for multiplying the output of said first pseudo
random signal generating means by the output of said second pseudo
random signal generating means and outputting the correlated code
signal as the product of said multiplication;
carrier wave generating means for generating a reference carrier
wave:
modulating means for modulating the output of said carrier wave
generating means in coded phase by each code signal of said first
pseudo random signal generating means;
transmitting means for transmitting the output of said modulating
means toward the target as an electromagnetic wave signal;
receiving means for receiving a reflected signal from said target
to obtain a received signal;
a second multiplier for multiplying an output of said receiving
means by the output of said second pseudo random signal generating
means and outputting the phase correlated carrier wave as the
product of said multiplication.[...]. .Iadd.;
orthogonal signal detecting means for respectively multiplying said
phase correlated carrier wave outputted from said second multiplier
by an inphase component (I signal) and by a quadrature component (Q
signal) which are extracted from said carrier wave generating
means, their phases being mutually orthogonal, and outputting
respective both products, the real part and the imaginary part of
the orthogonal detected signal;
signal absolute value calculating means for respectively smoothing
said real part and imaginary part of said orthogonal detected
signal and calculating the signal absolute vale of a composed
detection signal from said smoothed two component signals;
a first low pass filter for smoothing the correlated code signal
outputted from said first multiplier and outputting smoothed the
same signal; and
measuring and calculating means for measuring the time interval
between the point when the output of said first low pass filter
reaches to a maximum and the point when the output of said signal
absolute value calculating means reaches to a maximum, multiplying
one half of said time interval by the propagation velocity of said
electromagnetic wave to make a first operation value as the product
of said multiplication, dividing the frequency difference between
said clock frequencies f.sub.1 and f.sub.2 to make a second
operation value as the quotient of said division and.Iaddend .
multiplying said first operation value by said second operation
value to calculate the distance to said target as the product of
said multiplication.
6. A distance measuring apparatus according to claim 5, wherein
said orthogonal signal detecting means comprises:
a first distributor for extracting a part of the output of said
carrier wave generating means;
a hybrid coupler for receiving an output of said first distributor
and converting the same to generate said I signal and Q signal
wherein their phases are mutually orthogonal;
a second distributor for dividing said phase correlated carrier
wave outputted from said second multiplier into an R.sub.1 signal
and an R.sub.2 signal;
a third multiplier for multiplying the I signal outputted from said
hybrid coupler by the R.sub.1 signal outputted from said second
distributor; and
a fourth multiplier for multiplying the Q signal outputted from
said hybrid coupler by the R.sub.2 signal outputted from said
second distributor.
7. A distance measuring apparatus according to claim .[.6.].
.Iadd.5.Iaddend., wherein said signal absolute value calculating
means comprises:
a second low-pass filter and a third low-pass filter for
respectively receiving the the real part and the imaginary part of
the orthogonal detected signal outputted from said orthogonal
signal detecting means and subjecting them to band limitation of
the same frequency range as said first low-pass filter;
first and second squaring devices for respectively receiving the
output of each of said second and third low-pass filters and
separately performing a squaring operation thereon; and
an adder for adding the outputs of said first and second squaring
devices.
8. A distance measuring apparatus according to claim 7, wherein
said distance measuring apparatus is arranged on an upper part of
either one of a melting reduction furnace, converter and blast
furnace to measure any one of a slag level, molten steel level and
charged raw material level.
9. A distance measuring apparatus according to claim 6, wherein
said distance measuring apparatus is arranged on an upper part of
either one of a melting reduction furnace, converter and blast
furnace to measure any one, of a slag level, molten steel level and
charged raw material level.
10. A distance measuring apparatus according to claim 5, wherein
said .Iadd.orthogonal signal detecting means comprises:
a first distributor for extracting a part of the output of said
carrier wave generating means;
a hybrid carrier for receiving the output of said first distributor
and converting the same to generate said I signal and signal
wherein their phases are mutually orthogonal;
a second distributor for dividing said phase correlated carrier
wave outputted from said second multiplier into two signals,
R.sub.l signal and R.sub.2 signal;
a third multiplier for multiplying the I signal outputted from said
hybrid coupler by the R.sub.1 signal outputted from said second
distributor; and
a fourth multiplier for multiplying the Q signal outputted from
said hybrid coupler by the R.sub.2 signal outputted from said
second distributor; and further
wherein said .Iaddend.signal absolute value calculating means
comprises:
a second low pass filter and a third low-pass filter for
respectively receiving the real part and the imaginary part of the
orthogonal detected .Iadd.signal .Iaddend.outputted from said
orthogonal signal detecting means and subjecting them to band
limitation of the same frequency range as said first low-pass
filter;
first and second squaring devices for respectively receiving the
output of each of said second and third low-pass filters and
separately performing a squaring operation thereon; and
an adder for adding the outputs of said first and second squaring
devices.
11. A distance measuring apparatus according to claim 10, wherein
said distance measuring apparatus is arranged on an upper part of
either one of a melting reduction furnace, converter and blast
furnace to measure any one of a slag level, molten steel level and
charged raw material level.
12. A distance measuring apparatus according to claim 5, wherein
said distance measuring apparatus is arranged on an upper part of
either one of a melting reduction furnace, converter and blast
furnace to measure any one of a slag level, molten steel level and
charged raw material level.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for
measuring the distance to a target to be detected in the ground,
water, snow or the like, the slag level or molten steel level in a
melting reduction furnace, converter or the like and the distance
to an ordinary target to be detected.
2. Description of the Prior Art
Methods heretofore known or conceived for measuring the distance to
a target to be detected in the ground or water in a noncontact
manner by using, for example, an electromagnetic wave include one
which transmits a monopulse of the order of several ns(10.sup.-9
seconds) as shown, for example, in Japanese Patent Publication No.
55-44916 or another which transmits a pseudo random signal instead
of a pulse signal as shown for example, in a publication
"Subsurface Radar Using Code Pulse" (Suzuki et al. Institute of
Electronics, Information and Communication Engineers, Technical
Report SANE 87-1, 1987).
In the method of transmitting a pseudo random signal, such a pseudo
random signal as an M-sequence (maximal length sequence) signal or
Barker code signal tending to easily produce an autocorrelation
output is generated with a given PRF (pulse repetition frequency)
so that after its power amplification, the generated signal is
transmitted as an electromagnetic wave into the ground or water
through a transmitting antenna. The reflected wave from the target
to be detected is received through a receiving antenna so that
after the conversion from the high-speed received signal to a
low-speed received signal by a sampling device, the received output
is subjected to pulse compression by a correlator and a detected
signal increased in signal amplitude is generated. The time
interval between the transmission of the pseudo random signal and
the generation of the detected signal from the correlator is equal
to the propagation time of the electromagnetic wave going back and
forth the distance between the transmitting and receiving antennas
and the target to be detected as in the case of the ordinary radar
and therefore the distance to the target to be detected can be
calculated. Generally, the electromagnetic wave is attenuated more
under the ground than in the space and its phase is also changed.
Thus, the frequency of the electromagnetic wave used is selected on
the basis of the properties of the earth (e.g., the wet soil or dry
soil), the detecting distance, etc.
The above-described conventional method and apparatus of the type
designed to measure the time between the transmission of a pseudo
random signal and the generation of a detected signal from the
correlator has the disadvantage of requiring a sampling device in
the apparatus, thereby increasing the number of component parts,
the scale of the apparatus and the cost. In addition, the
correlator requires a tapped delay line as its component part, with
the resulting disadvantage of causing a measuring error by a
waveform distortion caused by the passing of the received signal
through the delay line.
On the other hand, a method of forming a correlator by subjecting
the received signal to A-D conversion and performing a digital
signal processing has not been put in practice as an economical
apparatus due to the limited response speeds of circuit elements
and the difficulty of the real time processing.
Also, the conventional level measuring methods for the slag level,
molten steel level, etc., in melting reduction furnaces,
converters, etc., and position measuring methods for targets to be
detected have been divided roughly into two types, i.e., contact
type and non-contact type.
The contact-type methods include an electric conduction detection
type and an apparatus of this type has been devised as shown, for
example, in Japanese Laid-Open Utility Model No. 61-129858 in which
at least two electrodes are moved up and down from the top of a
furnace and a voltage is applied between the electrodes, thereby
detecting the presence of slag by an electric conduction between
the electrodes and measuring the slag level by the position of the
electrodes.
Also, for performing a temperature distribution measuring method,
which is one of the contact type methods, an apparatus has been
proposed in which, as shown, for example, in Japanese Laid-Open
Patent No. 61-217516, a large number of temperature sensors are
embedded at suitable intervals in the lance of a converter and the
temperature distribution in the furnace is continuously measured by
the temperature sensors, thereby measuring the slag level on the
basis of the characteristics of the temperature distribution.
The non-contact type methods include a microwave FMCW (frequency
modulated continuous wave) method which has been proposed, for
example in Japanese Laid-Open Patent No. 63-21584 or 61-57875 in
which a continuous microwave having a frequency of about 10 GHz is
frequency modulated and transmitted from an antenna toward a
surface to be measured, whereby the beat frequency resulting from
the mixing of the transmitting signal and the reflected wave from
the surface to be measured is counted and the level of the surface
is measured. This method measures the distance from the antenna to
the target to be measured on the ground that the required
propagation time for the microwave to go back and forth the
distance between the antenna and the target to be measured
corresponds to the beat frequency.
Also, as a microwave pulse modulation type, there has been known a
method in which, as in the case of the ordinary radar to detect a
flying target, a microwave having a frequency of about 10 to 20 GHz
is pulse modulated and transmitted to measure the distance to a
target to be detected on the basis of the fact that the required
wave propagation time until the reception of the reflected wave
from the target to be detected is proportional to the distance to
the target.
In addition, as shown, for example, in a publication "An
Experimental Study of a Pulse Compression Radar Using Random
Series" (Nishimoto et al. The Institute of Electronics, Information
and Communication Engineers, Technical Report SANE 85-25, September
1985), a measuring method employing a radar has been known in which
a carrier wave having a frequency of 1 to several tens GHz is
modulated by a pseudo random signal, e.g., a maximal length
sequence signal, and transmitted to a target to receive the
reflected wave from the target, and an optimal matched filter
combining a tapped delay line and a weighted adder is used in a
demodulation system effect a pulse compression and thereby to
improve the resolution and sensitivity.
With the conventional level measuring methods above-mentioned or
the methods for measuring the distance to a target, if the methods
are of the contact type, they all tend to deteriorate the
durability of the portions which contact the slag or molten steel
in a furnace or tend to cause damages to those portions. In the
case of the electric detection type of conduction, there are
disadvantages that an erroneous signal is generated due to an
insulation failure of the electric insulating portion caused by the
dust or the molten steel splash in a furnace, that a continuous
measurement cannot be effected due to the detection by the vertical
movement of the electrodes and so on.
Also, in the case of the temperature distribution measuring method
of the contact type, the embedding of the temperature sensors in
the lance cooled with water deteriorates the response of the
temperature sensors due to the heat-transfer characteristic of the
lance. There is another disadvantage that when increasing the
number of temperature sensors for the purpose of improving the
measuring accuracy, there are many restrictions to the wiring of
the sensors from the spatial and temperature point of view.
In the case of the microwave FMCW method of the non-contact type,
since the inside space of the furnace is limited and the wave
reflectors such as the lance and the charging hole of the furnace
are present within the space, when a microwave is transmitted into
the furnace, undesired reflected waves, including multipath
reflected waves, are generated with the resulting disadvantage of
making it difficult to eliminate the undesired reflected signals
and thereby accurately measure only the reflected signal from the
intended target to be detected.
Also, in the case of the microwave pulse modulation method of the
non-contact type, there are disadvantages. For example, since
usually the signal reception is effected after the transmission of
the pulse modulated wave has been completed, considering the
propagation velocity of the microwave, it is necessary for a short
distance measuring radar to transmit a microwave having a
relatively large peak transmission power and modulated by a pulse
of a very short time width and to measure a small time required
until the reception of a received signal reflected from a short
distance target and it is difficult to technically realize these
operations. This makes the method unsuitable for use in short
distance measurements such as the level measurement in a furnace
and so on.
Further, in the case of the microwave pulse compression radar
method of the non-contact type, there are disadvantages that while,
after receipt the signal, the pulse width is compressed increase
the received power and thereby to improve the resolution and
sensitivity, the construction of the optimal matched filter
combining the tapped delay line and the weighted adder required in
the demodulation system is complicated. In particular, if a pseudo
random signal length is increased (e.g., 2.sup.5 to 2.sup.20) so as
to enhance the sensitivity, the apparatus is complicated in
construction and increased in size, thereby increasing the cost.
There is another disadvantage of requiring complicated operations
such as adjustments of the delay times among the taps and
adjustment for the correction of a waveform distortion during the
propagation in the delay line. Still another disadvantage is that
where the function of the matched filter is performed by a digital
signal processing, a high-speed A-D converter and a high-speed
computing unit are required, thereby similarly complicating the
construction of the apparatus, increasing its size and increasing
the cost.
SUMMARY OF THE INVENTION
It is a first object of the present invention to provide a distance
measuring method and apparatus which overcome the foregoing
deficiencies of the conventional methods and apparatus for
measuring the distance to a target to be detected in the ground or
water and also are capable of detecting the distance to such target
to be detected with a simple construction.
It is a second object of the invention to provide a distance
measuring method and apparatus which overcome the foregoing
deficiencies of the conventional methods and apparatus for
measuring the slag level or molten steel level in a blast furnace,
converter or the like and also are capable of accurately measuring
a level position such as the slag level or molten steel level or
the distance to a target to be detected from a short distance
continuously in a non-contact manner by the use of an inexpensive
apparatus without being affected by environmental conditions such
as the presence of dust.
To accomplish the first object, in accordance with the invention
there is thus provided a distance measuring method comprising the
steps of generating first and second pseudo random signals which
are the same in pattern but slightly different only in frequency,
branching the first pseudo random signal in two parts such that its
one part is transmitted toward a target to be detected and the
other part is multiplied by the second pseudo random signal to
calculate a first product value, calculating a second product value
of the received signal obtained by receiving the reflected signal
from the target and the second pseudo random signal, and measuring
the time difference between the time sequence pattern of the first
product value and the time sequence pattern of the second product
value, thereby measuring the distance to the target.
To accomplish the first object, in accordance with the invention
there is provided a distance measuring apparatus comprising first
and second pseudo random signal generating means for generating
first and second pseudo random signals which are the same in
pattern but slightly different only in frequency, transmitting
means for transmitting the output of the first pseudo random signal
generating means as a transmitted signal to a target to be
detected, receiving means for receiving a reflected signal from the
target to obtain a received signal, a first multiplier for
multiplying the outputs of the first and second pseudo random
signal generating means, a second multiplier for multiplying the
output of the receiving means and the output of the second pseudo
random signal generating means, and means for measuring the time
difference between the time sequence patterns of the outputs from
the first and second multipliers.
To accomplish the second object, in accordance with the invention
there is provided a distance measuring method comprising generating
a first pseudo random signal and a second pseudo random signal
which is the same in pattern but slightly different in frequency
from the first pseudo random signal, multiplying the first and
second pseudo random signals to generate a time sequence pattern of
a product value, modulating a carrier wave by the first pseudo
random signal and transmitting the modulated carrier wave toward a
target to be detected, multiplying a received signal consisting of
a received reflected signal from the target and the second pseudo
random signal, detecting the correlative modulated carrier wave to
generate a time sequence pattern of the detected signal, and
measuring the time difference between the product time sequence
pattern, and the detected signal time sequence pattern thereby
measuring the distance to the target.
To accomplish the second object, in accordance with the invention
there is provided a distance measuring apparatus comprising first
pseudo random signal generating means, second pseudo random signal
generating means for generating an output signal which is the same
in pattern but slightly different in frequency from the output
signal of the first pseudo random signal generating means, a first
multiplier for multiplying the output of the first pseudo random
signal generating means and the output of the second pseudo random
signal generating means, carrier wave generating means,
transmitting means for transmitting a signal obtained by modulating
the output signal of the carrier wave generating means by the
output of the first pseudo random signal generating means as a
transmitted signal to a target to be detected, receiving means for
receiving a reflected signal from the target to obtain a received
signal, a second multiplier for multiplying the output of the
receiving means and the output of the second pseudo random signal
generating means, detecting means for detecting the carrier wave
generated from the second multiplier, and means for measuring the
time difference between the time sequence pattern of the detected
signal generated from the detecting means and the time sequence
pattern of the product value generated from the first
multiplier.
In accordance with either of the first and second aspects of the
invention, by virtue of the fact that the measured time between a
detected signal from a target to be detected and a reference signal
is greatly expanded on a time base by the use of a method so
designed that a first pseudo random signal is directly transmitted
to a target to be detected or a carrier wave phase modulated by the
first pseudo random signal is transmitted to the target to be
detected and a received signal reflected from the target is
subjected to a correlation processing by a second pseudo random
signal which is the same in pattern and close in frequency to the
first pseudo random signal, the measurement of time is made
possible directly by use of a low-speed signal. As a result, the
need for the sampling device required in the conventional real time
high-speed signal processing is eliminated and the apparatus can be
constructed with low-speed circuit elements with the resulting
effect of reducing the size and cost of the apparatus.
Also, in the measurement of time proportional to the distance to be
measured, the measurement is made in terms of a time greatly
expanded as compared with the real time so that not only the
distance to a target can be measured accurately at a short
distance, but also the desired reflected signal from the intended
target and the undesired signals from outside the objective range
can be clearly discriminated and separated from each other on the
time base for the generation of detected signal. Thus, when
measuring the level within a furnace, even in such a measuring
environment as the limited space in the furnace where undesired
reflected waves tend to occur, there is the effect of eliminating
the undesired reflected waves and stably measuring the level.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a conventional underwater or
under-ground searching radar system which transmits a pseudo random
signal.
FIG. 2 is a block diagram of the correlator of FIG. 1.
FIG. 3 is a block diagram showing a first embodiment of the
invention.
FIG. 4 is a diagram showing the construction of the 7-bit maximal
length sequence signal generator of FIG. 3.
FIG. 5 is an output waveform diagram of the maximal length sequence
signal generator.
FIGS. 6A-6K show a plurality of signal wave-forms useful for
explaining the operation of the embodiment of FIG. 3.
FIG. 7 is a block diagram showing an embodiment of the clock
generator.
FIG. 8 is a block diagram of the image display system used with the
first embodiment.
FIG. 9 is an image display diagram of a detected signal according
to the first embodiment.
FIG. 10 is a block diagram showing a second embodiment of the
invention.
FIGS. 11A-11D a plurality of signal waveforms useful for explaining
the operation of the second embodiment.
FIG. 12 is a diagram showing the second embodiment incorporated in
an apparatus for measuring the slag level within a melting
reduction furnace.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
While the present invention and the conventional techniques have a
common feature with respect to the use of pseudo random signals,
the conventional techniques are methods of utilizing a single
pseudo random signal to measure the real propagation time of an
electromagnetic wave. On the contrary, the present invention is
directed to a method whereby, using two pseudo random signals which
are the same in pattern by slightly different in frequency from
each other, the correlation output between the two signals before
the transmission and after the reception is detected, thereby
considerably expanding the measured time as compared with the real
time. Therefore, the conventional technique will be explained first
with a view to clarifying the differences between the present
invention and the conventional techniques.
Referring to FIG. 1, there is illustrated a block diagram of a
conventional underground or underwater searching radar system which
transmits a pseudo random signal. In FIG. 1, reference numeral 107
designates a power amplifier, 108 a receiving amplifier, 109 a
transmitting antenna, 110 a receiving antenna, 116 a target in the
ground or in the water, 117 a maximal length sequence signal
generator, 118 a sampling device, 119 a correlator, and 120-1,
120-2 and 120-3 attenuators.
FIG. 2 is a block diagram of the correlator, wherein numeral 121
designates a tapped delay line, 122 a polarity converter, and 123
an adder.
The operation of the apparatus shown in FIGS. 1 and 2 will now be
described. Firstly, a trigger signal is applied at a given
repetition frequency to the maximal length sequence signal
generator 117. The maximal length sequence signal generator 117 is
used as a pseudo random signal generating means and its code has a
given periodicity. Each time a trigger signal is applied, the
maximal length sequence signal generator 117 generates a maximal
length sequence signal for one period. The output signal of the
maximal length sequence signal generator 117 is radiated as an
electromagnetic wave into the ground or water from the transmitting
antenna 109 through the attenuator 120-1, the power amplifier 107
and the attenuator 120-2. The radiated electromagnetic wave is
reflected from the target 116 to be detected in the ground or
water. The output signal from the receiving antenna 110 is applied
to the sampling device 118 through the receiving amplifier 108 and
the attenuator 120-3. The sampling device 118 has a function of
converting a high-speed signal to a low-speed signal. Assuming N
received signals of the same waveform are obtained in response to N
trigger signals, this received signal is divided in time by N into
signals x.sub.1, x.sub.2, . . . x.sub.N. Thus, only the signal
x.sub.1 is sampled from the first received signal and only the
signal x.sub.2 is sampled from the second received signal. Such
sampling operation is repeated so that a single received signal
x.sub.1, x.sub.2, . . . x.sub.n is reproduced from N received
signals. In this way, the sampling device 118 converts a high-speed
received signal to a low-speed received signal and its output is
supplied to the correlator 119. The correlator 119 has a function
of determining a correlation between the input signal and the
preliminarily stored maximal length sequence signal and FIG. 2
shows its detailed block diagram. The input signal is introduced
into the tapped delay line 121 and each signal corresponding to its
delay time is outputted from each of the taps. The output signals
from the taps of the tapped delay line 121 are applied to the
polarity converter 122. In the polarity converter 122, the mark "+"
indicates that no polarity conversion is effected and "-" indicates
that a polarity conversion is effected. The output signals are
applied in parallel to the adder 123 which in turn adds the
parallel input signals to obtain the summation. As a result, the
received signal is compressed in time but increased in amplitude
and it is then output from the correlator 119. In this technique,
the use of a pseudo random signal has the purpose of subjecting the
received signal to pulse compression and the measurement of
distance is made in terms of the propagation time required for the
electromagnetic wave to travel back and forth the distance between
the transmitting and receiving antennas and the target to be
detected, as in ordinary radar.
Next, the present invention will be described. A distance measuring
method according to the invention is a method comprising setting a
first pseudo random signal used for transmission and reception from
a target to be detected and a second pseudo random signal which is
used as a reference signal and is the same in pattern but slightly
different in frequency from the first pseudo random signal,
determining the time of detection of the maximum correlation value
between the second pseudo random signal and the transmitted first
pseudo random signal and the time of detection of the maximum
correlation value between the second pseudo random signal and the
received first pseudo random signal and measuring the interval of
time between both of the maximum correlation value detection times,
thereby measuring the time proportional to the distance to the
target to be detected. This measured time is in the form of a
greatly expanded time as compared with the real time required for
the propagation of the electromagnetic wave in the conventional
method, thereby making possible the measurement of time directly by
means of a low-speed signal.
A distance measuring apparatus according to the first embodiment of
the invention employs the above-mentioned measuring method so that
the sampling device heretofore required for high-speed signal
processing is no longer necessary and the apparatus is composed of
low-speed circuit elements. In addition, there is no need for the
tapped delay line which causes a measuring error due to a wave-form
distortion.
Also, the apparatus according to the first embodiment includes, for
the purpose of detecting the maximum correlation values between the
pseudo random signals, a pair of multipliers for detecting
correlation values, means for generating respective pulses when the
output of each of the pair of multipliers attains a maximum value,
and means for measuring the time interval between the generated two
pulses, thereby making it possible to make very accurate time
measurement.
In this embodiment, each of the two pseudo signal generating means
is composed of a clock synchronization-type pseudo random generator
and the two generators are identical in construction except that
their driving clock signal frequencies are slightly different from
each other. Therefore, the two pseudo random signal generating
means can be realized by means of common circuits.
The operation of the first embodiment can be formulated as
follows.
Assume that f.sub.1 represents the clock frequency of the first
pseudo random signal and f.sub.2 represents the clock frequency of
the second pseudo random signal and that the patterns of these
pseudo random signals are the same. Here, it is assumed that
f.sub.1 >f.sub.2.
If T.sub.B represents the period at which a reference signal
produced by determining a correlation between the transmitted first
pseudo random signal and the second pseudo random signal attains
its maximum value, the difference between the wave numbers of the
first and second pseudo random signals which are contained in the
period T.sub.B corresponds to the wave number N of one period.
In other words, T.sub.B .multidot.f.sub.1 =T.sub.B
.multidot.f.sub.2 +N
By simplifying the above equation, the period T.sub.B is given
as
In other words, the period T.sub.B at which the reference signal
attains the maximum value is increased with a decrease in the
difference between the two clock frequencies.
Then, if T.sub.D represents the time difference between the time
that the detected signal produced by determining the correlation
between the signal resulting from the first pseudo random signal
transmitted, reflected from a target to be detected and received
again after the expiration of a propagation time r and the second
pseudo random signal and the time that the reference signal attains
the maximum value, the wave number of the second pseudo random
signal generated during the time difference T.sub.D is smaller than
the wave number of the first pseudo random signal generated during
T.sub.D by the wave number of the first pseudo random signal
generated during the time .tau. and therefore the following
equation holds:
Simplifying the above equation, the time difference T.sub.D is as
follows:
In other words, the propagation time .tau. is expanded in time by a
factor of f.sub.1 /(f.sub.1 -f.sub.2) or it is measured in terms of
the low-speed T.sub.D. Due to this expansion of the measured time,
the distance measuring method and apparatus according to the
invention can be considered essentially suited for short distance
measuring purposes.
If v represents the propagation velocity and x represents the
distance to a target to be detected, the propagation time .tau. is
given by the following:
and therefore the following equation (3) is obtained from equation
(2): ##EQU1##
By measuring the time difference T.sub.D, it is possible to measure
the distance x from equation (3).
Referring now to the block diagram of FIG. 3 showing the first
embodiment of the invention, numerals 101 and 102 designate clock
generators, 103 and 104 maximal length sequence signal generators,
105 and 106 multipliers, 107 a power amplifier, 108 a receiving
amplifier, 109 a transmitting antenna, 110 a receiving antenna, 111
and 112 low-pass filters, 113 and 114 maximum value detectors, 115
a time-interval meter, and 116 a target to be detected.
Also, first pseudo random signal generating means is formed by the
clock generator 101 and the maximal length sequence signal
generator 103.
The second pseudo random signal generating means is formed by the
clock generator 102 and the maximal length sequence signal
generator 104.
The power amplifier 107 and the transmitting antenna 109 form means
for transmitting the output of the first pseudo random signal
generating means to the target 116.
The receiving antenna 110 and the receiving amplifier 108 form
means for receiving the reflected signal from the target 116 to
obtain the received signal.
The time difference measuring means is formed by the low-pass
filters 111 and 112, the maximum value detectors 113 and 114 and
the time-interval meter 115.
Referring to FIG. 4 showing the construction of a 7-bit maximal
length sequence signal generator, numeral 124 designates a shift
register of a 7-stage configuration, and 125 an exclusive-OR
circuit.
FIG. 5 is a diagram at the output waveform of the maximal length
sequence signal generator.
.[.FIG. 6 shows.]. .Iadd.FIG. 6(a) to FIG. 6(k) show .Iaddend.a
plurality of waveforms useful for explaining the operation of the
embodiment of FIG. 3.
The operation of the embodiment of FIG. 3 will now be described
with reference to FIGS. 4 to 6. The clock generator 101 generates a
clock signal of a frequency f.sub.1 and the clock generator 102
generates a clock signal of a frequency fhd 2. One feature of this
invention resides in that the difference between frequency f.sub.1
of the first clock signal and frequency f.sub.2 of the second clock
signal is very small. A description will now be made of a case in
which f.sub.1 =100.004 MHz, f.sub.2 =99.996 MHz and the difference
f.sub.1 -f.sub.2 =8 KHz. The clock signals of frequency f.sub.1
generated from the clock generator 101 are applied as the
synchronizing signals to generate the maximal length sequence
signal to the maximal length sequence signal generator 103, and the
clock signals of the frequency f.sub.2 generated from the clock
generator 102 are similarly applied to the maximal length sequence
signal generator 104. The maximal length sequence signal generators
103 and 104 are used as one form of the pseudo random signal
generating means and Baker code generators may, for example, be
used in place of the maximal sequence length signal generators. In
the case of this embodiment, a 7-bit maximal length sequence code
is used and its structure is shown in FIG. 4.
In other words, the shift register 124 composed of seven flip-flops
synchronized with clock signals is provided so that the output
signals of the sixth-stage and seventh-stage flip-flops are applied
to the first-stage flip-flop through the exclusive-OR circuit 125
and the clock signals which are not shown are supplied to the
respective flip-flop stages, thereby generating an output signal
from the seventh-stage flip-flop to generate a maximal length
sequence code synchronized with the clock signals. The thus
generated maximal length sequence code is a periodic recurrent code
composed of a combination of bits "1" and "0" or "+" and (-), and
in this embodiment the bits "1" and "0" respectively generate
positive-voltage (+E) and negative-voltage (-E) signals as shown in
FIG. 5. As regards the period of the maximal length sequence signal
which is generated recurrently, since the signal comprises 7 bits,
one period is completed upon the generation of N=2.sup.7 -1=127
signals. Then, the same signals as the preceding period are
generated starting at the next or 128th signal and this period
recurs repeatedly. Generally, while the maximal length sequence
signal is a random signal when considered partially, it is used as
a signal utilizing the autocorrelation function and the signal is
used in a pulse compression radar in the description of the
conventional apparatus.
The maximal length sequence signal generators 103 and 104 are
composed of the identical circuits for generating the same 7-bit
maximal length sequence signals and the only difference is that the
frequencies f.sub.1 and f.sub.2 of the inputted clock signals are
slightly different from each other. In addition, a shift register
having a clock frequency of about 100 MHz can, for example, be
realized easily with ECL (emitter coupled logic) elements. Each of
the maximal length sequence signal generators 103 and 104
recurrently outputs a maximal length sequence signal M.sub.1 or
M.sub.2 composed of 127 voltages +E and -E for each period.
However, since the frequencies of the input clock signals differ
slightly from each other, the respective lengths of the periods
differ slightly between the maximal length sequence signals M.sub.1
and M.sub.2. The periods of the maximal length sequence signals
M.sub.1 and M.sub.2 can be determined as follows: the period of
M.sub.1 =127.times.1/100.004 MHz.apprxeq.1269.9492 ns and the
period of M.sub.2 =127.times.1/99.996 MHz.apprxeq.1270.0508 ns. In
other words, while the periods of the maximal length sequence
signals M.sub.1 and M.sub.2 are about 1270 ns (10.sup.-9 seconds),
there is the time difference of about 0.1 ns between the two
periods. Thus, if the maximal length sequence signals M.sub.1 and
M.sub.2 are recurrently generated so that the patterns of the two
signals M.sub.1 and M.sub.2 coincide at a certain time t.sub.a, a
deviation of 0.1 ns is caused between the two signals after the
expiration of the time of one period and a deviation of 10 ns is
caused between the two signals after the expiration of 100 periods.
Since the maximal length sequence signal generates 127 signals for
one period of 1270 ns and thus the duration time of one signal is
10 ns. Thus, the occurrence of a deviation of 10 ns between the
maximal length sequence signals M.sub.1 and M.sub.2 means the
occurrence of a deviation corresponding to one maximal length
sequence signal. These timings are shown in .[.FIG. 6.]. .Iadd.FIG.
6(a) to FIG. 6(k).Iaddend.. More specifically, FIG. 6(a) shows that
the output of the reference maximal length sequence signal
generator 104 for one period includes 127 signals and its period is
1270 ns and FIG. 6(b) shows the output M.sub.2 of the maximal
length sequence signal generator 104 is recurrently generated from
the -100th to 300th periods. Also, FIG. 6(c) shows that the output
M.sub.1 from the maximal length sequence signal generator 103 is
short by 0.1 ns for one period and 10 ns for 100 periods as
compared with the output M.sub.2 from the maximal length sequence
signal generator 104 and that at the time t.sub.a, the maximal
length sequence signals M.sub.1 and M.sub.2 come into
synchronization, thus causing the patterns of the two signals to
coincide. Also, after the patterns of the maximal length sequence
signals M.sub.1 and M.sub.2 have coincided at time t.sub.a,
deviation is again increased gradually so that the patterns of the
two signals coincide again at the expiration of a given time
T.sub.B. When the maximal length sequence signals M.sub.1 and
M.sub.2 come into synchronism so that the patterns of the two
signals coincide, the correlation output of the two signals becomes
maximum and the calculation of this correlation is performed by the
multiplier 105, which will be described later. Also, in the case of
this embodiment, the period T.sub.B during which the maximum
correlation output is obtained can be computed as T.sub.B =15.875
ms by substituting the wave number N=127 for one period of the
maximal length sequence signals, the frequency f.sub.1 =100.004 MHz
and f.sub.2 =99.996 MHz into equation (1).
The maximal length sequence signals M.sub.1 and M.sub.2
respectively generated from the maximal length sequence signal
generators 103 and 104 are each branched in two so that one signal
is applied to the multiplier 105. The multipliers 105 and 106 are
each composed, for example, of a wide-band double balanced mixer
(DBM) and the multiplication for determining the correlation output
of the two maximal length sequence signals is performed. The
maximal length sequence signal comprises positive or negative
voltage signals as mentioned previously so that the multiplication
result of the same signs results in a positive voltage and the
multiplication result of the different signs results in a negative
voltage, thereby generating positive or negative voltage signals at
the output of the multipliers 105 and 106, respectively. Thus, the
output signal of the multiplier 105 consists of a dc positive
voltage or a pulse of positive voltage at around the time t.sub.a
at which the patterns of the maximal length sequence signals
M.sub.1 and M.sub.2 conicide. However, the periods of the maximal
length sequence signals M.sub.1 and M.sub.2 differ slightly so that
a deviation of 0.1 ns occurs between the two signals each time one
period expires. Thus, at the expiration of 100 periods from the
time t.sub.a, there is a deviation of 10 ns or a deviation
corresponding to one signal interval between the maximal length
sequence signals M.sub.1 and M.sub.2. In this condition, there is
no longer any correlation between the signals and a train of
positive and negative pulse signals is randomly generated at the
output of the multiplier 105. This output waveform of the
multiplier 105 is shown in FIG. 6(e). The output signal of the
multiplier 105 is supplied to the low-pass filter 111 which in turn
converts the signal to a DC voltage. Each of the low-pass filters
111 and 112 has a cutoff frequency f.sub.c and it serves the
function of attenuating the input components which are higher
frequency components than the cutoff frequency f.sub.c and
smoothing the input signal. The output signal of the low-pass
filter 111 attains the maximum value at the time t.sub.a at which
the patterns of the maximal length sequence signals M.sub.1 and
M.sub.2 coincide and it attains the minimum value at the time at
which the maximal length sequence signal M.sub.2 is shifted from
the time t.sub.a by about 100 periods, that is, at times of t.sub.a
.+-.127 .mu.s. Then, the output signal of the low-pass filter 111
takes the form of a triangular voltage signal which linearly
decreases from the maximum value or the apex to the minimum value
on both sides. This output waveform of the low-pass filter 111 is
shown in of FIG. 6(f). Also, as mentioned previously, this
triangular voltage signal is generated at the period T.sub.B
=15.875 ms at which period the two maximal length sequence signals
come into synchronization. The output signal from the low-pass
filter 111 is applied to the maximum value detector 113. Each of
the maximum value detectors 113 and 114 has a function of detecting
the maximum value of the triangular voltage signal applied from the
low-pass filter 113 or 114 or the voltage at the apex of the
triangle and generating a single pulse signal at the time of
detection of the maximum voltage value. The method of detecting the
time of generation of the maximum voltage may for example be one
comprising the steps of providing an A/D converter and a digital
data comparator, successively converting an input triangular analog
signal by high-speed sampling signals, comparing at all times the
digital data obtained by the preceding sampling signal and the
digital data obtained by the current sampling signal as to relative
magnitude using the digital data comparator and detecting the time
of change of the input signal from the increase to the decrease
with respect to time. The same function can be realized by
successively comparing the similarly sampled analog signals. Where
there is the danger of small peaks appearing due to noise or the
like, it suffices to establish a threshold value so that the
detection of the peak value is performed only on those signals
exceeding the threshold value. The maximum value detector 113
supplies a pulse output as a start signal for time measurement to
the time-interval meter 115 at the maximum value detecting time
t.sub.a of the input signal. When the start signal for time
measurement or a reference time is applied from the maximum value
detector 113, the time-interval meter 115 starts the measurement of
time. This condition is shown in FIGS. 6(i) and 6(k). Of the
maximal length sequence signal M.sub.1 generated from the maximal
length sequence signal generator 103 and then branched off in two,
the other maximal length sequence signal M.sub.1 is applied to the
power amplifier 107 so that the output power is amplified, for
example, to about 20 mW. The maximal length sequence output signal
from the power amplifier 107 is supplied to the transmitting
antenna 109. The transmitting antenna 109 radiates the
electromagnetic wave of the maximal length sequence signal into a
propagation medium. The radiated electromagnetic wave is reflected
by the target 116 whose conductivity or dielectric constant is
different from the value of the propagation medium and it is then
detected by the receiving antenna 110. The reflected signal thus
detected by the receiving antenna 110 is applied to the receiving
amplifier 108 so that the amplification and waveform reshaping of
the signal are performed. The output signal M.sub.1 ' of the
receiving amplifier 108 is the same as the signal delayed by the
propagation time of the maximal length sequence signal M.sub.1
radiated as an electromagnetic wave from the transmitting antenna
109, going back and forth the distance to the target 116 and then
reaching the receiving antenna 110.
Strictly speaking, while fixed delay times are involved in the
power amplifier 107, the signal amplifier 108, etc., these fixed
delay times can be eliminated from the measurement point of view
by, for example eliminating them at the stage of the processing for
measurement or by supplying the output signal M.sub.1 from the
maximal length sequence signal generator 103 through a delay
circuit having an equal delay time. In this way, the maximal length
sequence signal M.sub.1 ' having a delay time proportional to the
distance from the transmitting and receiving antennas 109 and 110
to the target 116, is generated from the receiving amplifier 108
and supplied to one input of the multiplier 106. Assume that the
target 116 is present in the air at a distance of 3 meters from the
transmitting and receiving antennas 109 and 110, respectively.
Since the electromagnetic wave required 20 ns to propagate in the
air and go back and forth the distance of 3 meters, the maximal
length sequence signal M.sub.1 ' generated from the receiving
amplifier 108 is delayed by 20 ns from the maximal length sequence
signal M.sub.1 generated from the maximal length sequence signal
generator 103. This condition is shown in FIG. 6(d). Also, of the
maximal length sequence signal M.sub.2 generated from the maximal
length sequence signal generator 104 and branched off in two, the
other maximal length sequence signal M.sub.2 is supplied to the
other input of the multiplier 106. In like manner as in the
multiplier 105, the multiplication of the maximal length sequence
signals M.sub.1 ' and M.sub.2 is performed.[...]. .Iadd.in the
multiplier 106.
Thus, the output signal of the multiplier 106 consists of a dc
positive voltage or a pulse of positive voltage at around the time
t.sub.b at which the patterns of the maximal length sequence
signals M.sub.1 ' and M.sub.2 coincide. However, the periods of the
maximal length sequence signals M.sub.1 ' and M.sub.2 differ
slightly so that a deviation of 0.1 ns occurs between the two
signals each time one period expires. Thus, at the expiration of
100 periods from the time t.sub.b, there is a deviation of 10 ns or
a deviation corresponding to one signal interval between the
maximal length sequence signals M.sub.1 ' and M.sub.2. In this
condition, there is no longer any correlation between the signals
and a train of positive and negative pulse signals is randomly
generated at the output of the multiplier 106. This output waveform
of the multiplier 106 is shown in FIG. 6(q). .Iaddend.The
multiplier 106 supplies the multiplication result of the maximal
length sequence signals M.sub.1 ' and M.sub.2 to the low-pass
filter 112. The low-pass filter 112 generates a triangular voltage
signal whose apex corresponds to the time at which the patterns of
the two signals M.sub.1 ' and M.sub.1 coincide and this voltage
signal is supplied to the maximum value detector 114. .Iadd.The
output waveform of the low-pass filter 112 is shown in of FIG.
6(h). .Iaddend.The foregoing operation is identical with the
operation described in connection with the multiplier 105 and the
low-pass filter 111. The only difference is the time instant at
which the patterns of the maximal length sequence signals M.sub.1 '
and M.sub.2 coincide. Since the maximal length sequence signal
M.sub.1 ' is delayed by 20 ns from the maximal length sequence
signal M.sub.1 and since the period of the maximal length sequence
signal M.sub.1 ' is shorter, the patterns of the two signals
M.sub.1 ' and M.sub.1 coincide at a time t.sub.b which is delayed
from the time t.sub.a by 200 periods of the maximal length sequence
signal M.sub.2. Since one period of the maximal length sequence
signal M.sub.2 is 1.27 .mu.s, 200 periods amounts to 1.27
.mu.s.times.200=254 .mu.s and the time t.sub.b lags the time
t.sub.a by 254 .mu.s. The maximum value detector 114 generates a
pulse output when detecting the maximum value of the applied
triangular voltage and of course this pulse output is generated at
the time t.sub.b. .Iadd.The output waveform of the maximum value
detector 114 is shown in of FIG. 6(j). .Iaddend.The operation of
maximum value detector 114 is the same as that of maximum value
detector 113 and at this time the generated pulse output is
supplied as a stop signal for time measurement to the time-interval
meter 115. The time-interval meter 115 measures the time interval
between the time t.sub.a at which the start signal for time
measurement is applied and the time t.sub.b at which the stop
signal for time measurement is applied. In this embodiment, 254
.mu.s is obtained as the result of the measurement. The time
measuring method may, for example, be an ordinary method of
providing a time gate from the measurement starting time to the
stop time and counting the number of clock signals during the time
gate. The time measured by the time interval meter 115 is
proportional to the distance from the transmitting and receiving
antennas of the present apparatus to the target. In other words,
254 .mu.s corresponds to a distance of 3 meters and 2540 .mu.s
corresponds to a distance of 30 meters. Thus, by measuring such
time, it is possible to measure the distance to a target to be
detected. In addition, the present invention greatly differs from
the ordinary radar system in that the time proportional to the
distance is considerably expanded. In other words, to measure a
distance of 3 meters by ordinary radar is to measure a time span of
20 ns (20.times.10.sup.-9 seconds). In accordance with the
invention, however, to measure a distance of 3 meters is to measure
a time span of 254 .mu.s (254.times.10.sup.-6 seconds). The
expansion ratio of the measured time is computed by substituting
the frequencies f.sub.1 =100.004 MHz and f.sub.2 =99.996 MHz into
equation (2) to obtain the following:
In other words, the time is expanded by a factor of 12,500 on the
time base and it is only necessary to measure a very low speed
signal. Therefore, the radar system according to the invention has
great features in that the accuracy of short distance measurement
is improved and the apparatus can be easily constructed with
inexpensive low-speed element. The time measurement by the
time-interval meter 115 is performed in response to the application
of each time measurement start signal of a period of 15.875 ms.
Thus, if the target is moved, a change in the distance from the
transmitting and receiving antennas to the target can be detected
at intervals of 15.875 ms. Also, with the time measurement
according to the embodiment, 15.875 ms corresponds to a distance of
about 188 meters. While the maximum detecting distance of 188
meters is sufficient in such applications as the ordinary
underground search, by suitably selecting the clock frequencies
f.sub.1 and f.sub.2, it is possible to change the expansion ratio
on the time base and the maximum detecting distance.
FIG. 7 shows a block diagram for an embodiment of the clock
generator wherein numeral 126 designates a crystal oscillator
having a frequency of 3 MHz, 127-1, 127-2 and 127-3 designate
mixers each adapted to mix two signals of frequencies f.sub.A and
f.sub.B to generate a signal of a sum frequency f.sub.A +f.sub.B
and a signal of a difference frequency f.sub.A -f.sub.B, 128-1
designates an oscillator having a frequency of 4 KHz, 128-2
designates an oscillator having a frequency of 97 MHz, and 129-1,
129-2, 129-3 and 129-4 designate band-pass filters respectively
having selected pass frequencies of 3.004 MHz, 2.996 MHz, 100.004
MHz and 99.996 MHz.
The operation of the clock generator of FIG. 7 will now be
described. The crystal oscillator 126 generates a 3 MHz signal and
the oscillator 128-1 generates a 4 KHz signal. These signals are
mixed by the mixer 127-1 comprising for example, a balanced
modulator, thereby generating two signals of 3.004 MHz and 2.996
MHz, respectively. Of the output signals from the mixer 127-1, the
3.004 MHz signal is supplied to the mixer 127-2 through the
band-pass filter 129-1 and the 2.996 MHz signal is supplied to the
mixer 127-3 via the band-pass filter 129-2. The mixer 127-2 mixes
the 3.004 MHz signal and the 97 MHz signal supplied from the
oscillator 128-2 to generate their sum and difference signals, of
which the sum signal or 100.004 MHz signal is passed through the
band-pass filter 129-3 and generated as a clock frequency f.sub.1.
Similarly, the mixer 127-3 mixes the 2.996 MHz signal and the 97
MHz signal supplied from the oscillator 128-2 to generate its
outputs, of which the sum signal or 99.996 MHz signal is passed
through the band-pass filter 129-4 and generated as a clock
frequency f.sub.2. By virtue of this construction, the difference
between the clock frequencies f.sub.1 and f.sub.2 is accurately
maintained at 8 KHZ. In accordance with the invention, the two
clock frequencies f.sub.1 and f.sub.2 are used to generate two
pseudo random signals and the measurement is made by utilizing the
difference in period between the pseudo random signals. Therefore,
it is important to accurately maintain the difference between the
clock frequencies for the improvement of the measuring accuracy. As
a result, a clock signal generator capable of maintaining this
frequency difference constant may be constructed using a PLL
(phase-locked loop).
FIG. 8 shows a block diagram of an image display apparatus for
displaying a detected signal according to the invention in the form
of an image. In FIG. 8, numerals 105, 106, 111 and 112 designate
the same component parts as shown in FIG. .[.1.]. .Iadd.3.Iaddend..
Numeral 130 designates the image display apparatus incorporating an
image converter 131 and a display unit 132. The image converter 131
uses the output signal from the low-pass filter 111 as a reference
signal for distance measurement and the output signal from the
low-pass filter 112 as a detected signal. Using the time interval
between the reference signal and the detected signal as the
position information of the corresponding distance, the detected
signal is displayed as a dark or light image signal in accordance
with the received intensity. If the transmitting and receiving
antennas are moved, in response to the distance moved the scanning
starting position is moved on the CRT display. In this respect, the
apparatus features that the detected signal is sufficiently low in
speed and it can be directly supplied to the image converter
without passing through any sampling device as in the conventional
case.
FIG. 9 shows an example of the display by image according to the
invention of a detected signal of an underground object, e.g., a
plastic pipe at a depth of 3 meters. In FIG. 9, the abscissa
represents the moving distance of the transmitting and receiving
antennas moved in a direction crossing the pipe and the ordinate
represents the detecting distance. Also, the intensity of a
detected signal is displayed in terms of the degree of light or
shade. The upper part of a semi-circular waveform display opening
downward indicates the detected pipe. This image of the
semi-circular shape is due to the insufficient directivities of the
transmitting and receiving antennas and this gives rise to no
problem from the practical pipe detection point of view. Also,
where the reflected wave from the ground surface is strong and the
reflected wave from the target to be detected is weak, the method
of separately moving the transmitting antenna and the receiving
antenna to reduce the reflection from the ground surface is
effective. The present invention is also available for underground
survey such as geo-tomagraphy using bore-hole antennas which
operate in two holes each and detect transmitted electromagnetic
waves through the ground in addition to the reflected waves in the
ground.
While the embodiment of the invention as applied to the underground
or underwater searching radar has been described, the present
invention is also applicable to distance measurement by a TDR (time
domain reflector). The TDR is generally used for the purpose of
detecting the faulty location of an electric wire or the like and
an electric pulse in the form of a monopulse or step-formed pulse
is input to one end of an electric wire. Thus, the electric pulse
propagates and travels through the line so that it is returned to
the signal input end by being reflected from the location of the
changed characteristic impedance due to, for example, the
disconnection or the short-circuiting of the line. The location of
the changed characteristic impedance is detected in accordance with
the time interval between the time of application of the electric
pulse and the time of detection of the reflected signal and the
propagation velocity of the entered electric signal in the line.
Also, the TDR can be used for the detection of a fault point in an
optical fiber by the use of a light pulse according to the same
principle.
By applying the pseudo random signals according to the invention
instead of applying the electric pulse signal to the TDP, the
changed point of the characteristic impedance can be similarly
detected in accordance with the time interval between the detection
time of the start signal for time measurement and the time that the
maximum correlation output between the reflected pseudo random
signal and the reference pseudo random signal is obtained and the
propagation velocity of the entered electric signal in the line.
This method has a feature that even if any noise enters the
reflected wave, the correlator is not caused to operate erroneously
due to the noise, thereby ensuring stable measurement.
The distance measuring method and apparatus according to the second
embodiment differ principally from the distance measuring method
and apparatus according to the first embodiment in that instead of
directly transmitting a first pseudo random signal, a spectrum
spread signal obtained by phase modulating a carrier wave (e.g., an
X-band wave having a frequency of about 10 GHz) by the first pseudo
random signal is transmitted to a target to be detected and that
after the processing of the correlation between the received wave
and a second pseudo random signal, the received wave is subjected
to coherent detection by coherent detecting means. The method of
employing such a carrier wave has the purpose of meeting the object
of the second embodiment, that is, to adapt it to a measuring
method in which an electromagnetic wave is propagated in the air to
measure the slag level or molten metal level in a melting reduction
furnace, converter or the like.
In accordance with the distance measuring method and apparatus of a
second embodiment, the first pseudo random signal and a second
pseudo random signal which is identical in pattern but slightly
different in frequency from the first pseudo random signal are
respectively generated from first and second pseudo random signal
generating means and, after the phase modulation of a carrier wave
by the first pseudo random signal, the resulting spectrum spread
signal is transmitted by transmitting means toward a target to be
detected. A second multiplier performs the multiplication of a
received signal obtained by receiving the reflected wave from the
target by receiving means and the second pseudo random signal. When
the received signal phase modulated by the first pseudo random
signal and the second pseudo random signal are in phase the
multiplication result obtained as an output of the second
multiplier is the in-phase carrier wave and it is subjected to
synchronization detection by the following coherent detecting
means. This detected output is generated as a pulse-like target
detection signal through detection signal generating means
comprising a pair of low-pass filters, a pair of squaring devices
and an adder.
However, while the first and second pseudo random signals are codes
of the same pattern, the signal generating means are slightly
different in frequency with the result that starting at the time
that the two signals are in phase (that is, the correlation output
between the two signals reaches a maximum), the two signals go out
of phase with the passage of time so that there is no longer any
correlation between the two pseudo random signals when the phase
deviation becomes greater than one code. In this condition, the
phase of the carrier wave obtained as the result of the
multiplication of the received signal and the second pseudo random
signal becomes random and the frequency band of the carrier wave is
limited by the low-pass filter through which it passes after the
synchronization detection by the coherent detecting means. Thus, no
target detection signal is generated.
Then, as time passes further so that the phase deviation between
the first and second pseudo random signals amounts to just one
period of one of the pseudo random signals, the in-phase condition
is again attained and the correlation output of the two signals
reaches a maximum, thereby generating again a pulse-like target
detection signal through the coherent detecting means and the
detection signal generating means. As a result, this phenomenon is
repeated at intervals of a given time and a periodic pulse-like
signal is generated as a detected target signal.
On the other hand, it is necessary to establish a reference time
for measuring the time interval between the reference time and the
time of detection of a target detection signal from the received
signal. This time reference signal is generated as a pulse-like
signal of the same period as the target detection signal by
directly multiplying the first and second pseudo random signals by
a first multiplier and extracting the result of the multiplication
or the time sequence pattern through a low-pass filter.
Therefore, since the time interval between the time of generation
of the time reference signal and the time of generation of the
target detection signal derived from the received signal represents
an expanded value of the propagation time of the electromagnetic
wave to go back and forth between the transmitting and receiving
antennas and the target, the time interval between the two signals
is converted into the distance between the transmitting and
receiving antennas and the target.
The theoretic expressions of the first embodiment can be used as
such as the theoretic expressions relating to the operating times
of the second embodiment. The reason is that the second embodiment
operates in time at the same times as the first embodiment
excepting that in the second embodiment the signal of the modulated
carrier wave is transmitted and the received signal is subjected to
coherent detection after the correlation calculation. In other
words, it is possible to apply equation (1) to a period T.sub.B at
which a reference signal is generated, equation (2) to a measured
time T.sub.D and equation (3) to the calculation of the distance x
to a target to be detected.
In the distance measuring apparatus according to the second
embodiment, the coherent detecting means of the carrier wave
performs the operation of extracting a part of the output of
transmitting carrier wave generating means by a first distributor,
converting the extracted output into an in-phase component I and a
quadrature component Q, dividing the carrier wave generated from
the second multiplier into signals R.sub.1 and R.sub.2 through a
second distributor, and generating as orthogonal detection signals
a product I.multidot.R.sub.1 by a third multiplier and a product
Q.multidot.R.sub.2 by a fourth multiplier.
In the distance measuring apparatus according to the second
embodiment, a time difference measuring means for the time sequence
pattern of the product by the first multiplier and the time
sequence patterns of the orthogonal detection signals is designed
so that a time-interval timer measures the interval of time between
the time of generation of the maximum value of a pulse-like
reference signal obtained by subjecting the output of the first
multiplier to band limitation by the first low-pass filter and the
time of generation of the maximum value of a pulse-like detection
signal obtained by subjecting the products I.multidot.R.sub.1 and
Q.multidot.R.sub.2 of the third and fourth multipliers to band
limitation by the second and third low-pass filters, squaring the
resulting signals by a pair of squaring devices and obtaining the
sum of the resulting squared values by an adder.
The distance measuring apparatus according to the second embodiment
is used in such a manner that after the apparatus has been arranged
on a melting reduction furnace, converter or blast furnace, the
transmitting and receiving antennas are inserted into the furnace
through waveguides and the transmission and reception of an
electromagnetic wave are effected, thereby effecting the
measurement of such level as the slag level, molten steel level or
charged raw material level.
Referring to the block diagram of FIG. 10 showing an example of the
distance measuring apparatus according to the second embodiment,
numerals 201 and 202 designate clock generators, 203 and 204 pseudo
random signal generators, and 205 to 209 multipliers each composed,
for example, of a double balanced mixer. Numerals 210 to 212
designate low-pass filters, 213 and 214 distributors, 215 and 216
squaring devices, 217 an adder, 218 a time-interval meter, 219 a
carrier oscillator, 220 a hybrid coupler, 221 a transmitter, 222 a
receiver, 223 a transmitting antenna, 224 a receiving antenna, and
225 a target.
FIG. 11 shows a plurality of signal waveforms useful for explaining
the operation of the apparatus shown in FIG. 10.
The maximal length sequence signal generators 103 and 104 of FIG. 3
can be used as such for the pseudo random signal generators 203 and
204, respectively. Thus, in the description of FIG. 10, the pseudo
random signal generators 203 and 204 will be explained as adapted
to respectively generate 7-bit maximal length sequence signals
M.sub.1 and M.sub.2. However, in addition to the maximal length
sequence signals, Gold-sequence signals, JPL-sequence signals or
the like may be used as the pseudo random signals. While, like the
clock generators 101 and 102 of FIG. 3, the clock generators 201
and 202 each incorporate a crystal oscillator to generate clock
signals of a sufficiently stable frequency, their generated
frequencies are slightly different from each other. Thus, even in
the case of FIG. 10, it is assumed that the generated frequencies
f.sub.1 of the clock generator 201 is 100.004 MHz and the generated
frequency f.sub.2 of the clock generator 202 is 99.996 MHz and that
the frequency difference is f.sub.1 -f.sub.2 =8 KHz. The clock
signals f.sub.1 and f.sub.2 of the clock generators 201 and 202 are
respectively supplied to the pseudo random signal generators 203
and 204. While there is a slight difference in the length of one
period between the pseudo random signal generators 203 and 204 due
to the frequency difference between their driving clock signals,
they respectively generate maximal length sequence signals M.sub.1
and M.sub.2 of the same pattern. The output M.sub.1 of the pseudo
random signal generator 203 is applied to the multipliers 205 and
206 and the output M.sub.2 of the pseudo random signal generator
204 is applied to the multipliers 205 and 207.
The carrier oscillator 219 generates, for example, a microwave
having a frequency of about 10 GHz and its output signal is
distributed by the distributor 213 so as to be supplied to the
multiplier 206 and the hybrid coupler 220. The multiplier 206 is
composed, for example, of a double balanced mixer which multiplies
the carrier wave having a frequency of about 10 GHz applied from
the distributor 213 and the maximal length sequence signal M.sub.1
applied from the pseudo random signal generator 203 so that a
spectrum spread signal resulting from phase modulation of the
carrier wave is generated and supplied to the transmitter 221. The
transmitter 221 power amplifies the applied spectrum spread signal,
converts and radiates it as an electromagnetic wave to the target
225 through the transmitting antenna 223. It is to be noted that a
electromagnetic wave of frequency 10 GHz has a wavelength of 3 cm
in the air and it is sufficiently large as compared with the size
(diameter) of dust within a steelmaking furnace, for example,
thereby tending to be not easily affected by the dust, etc. Also,
the transmitting antenna 223 and the receiving antenna 224 are each
composed, for example, of a horn antenna and its directivity is
sharply confined so as to reduce as far as possible the reflected
power from other than the target to be measured. Then, the antenna
gain is selected, for example, to be about 20 dB for each of these
antennas. The electromagnetic wave radiated toward the target 225
from the transmitting antenna 223 is reflected by the target 225,
converted to an electric signal through the receiving antenna 224
and applied to the receiver 222. The timing of supplying the input
signal to the receiver 222 is of course delayed from the timing of
the radiation of the electromagnetic wave from the transmitting
antenna 233 by the time required by the electromagnetic wave to
propagate back and forth the distance to the target 225 and reach
the receiving antenna 224. The receiver 222 amplifies the input
signal and then supplies it to the multiplier 207.
On the other hand, the maximal length sequence signals M.sub.1 and
M.sub.2 respectively applied from the pseudo random signal
generators 203 and 204 to the multiplier 205 are multiplied and the
time sequence signal of the resulting product is supplied to the
low-pass filter 210. FIG. 11(a) depicts the waveform showing the
input signal to the low-pass filter 210, e.g., the time sequence
signal or the product of the multiplier 205, and the waveform shows
that an output voltage of +E is generated continuously while the
two pseudo random signals are in phase and output voltages of +E
and -E are generated randomly while the two signals are out of
phase. Each of the low-pass filters 210 to 212 performs a frequency
band limiting operation and hence a sort of integrating function,
so that a pulse-like signal such as shown in FIG. 11(b) is
generated as an integrated signal of the processed correlation
value between the two signals while the two signals are in phase.
On the contrary, the output of the low-pass filter is reduced to
zero while the two signals are out of phase. Thus, a pulse-like
signal is generated periodically at the output of the low-pass
filter 210. This pulse-like signal is supplied as a time reference
signal to the time-interval meter 218. The period T.sub.B of this
reference signal can be computed from equation (1) as T.sub.B
=15.875 ms since, in this embodiment, the wave number of one period
of the maximal length sequence signals is N=127 and the frequencies
are f.sub.1 =100.004 MHz and f.sub.2 =99.996 MHz, the same as in
the case of FIG. 3. This reference signal and its period T.sub.B
are shown in FIG. 11(d).
Also, the received signal from the receiver 222 and the maximal
length sequence signal M.sub.2 from the pseudo random signal
generator 204 are applied to the multiplier 207 to multiply the two
signals. The output of the multiplier 207 is the in-phase carrier
signal while the received signal of the transmitting carrier wave
which was phase modulated by the first maximal length sequence
signal M.sub.1 and the second maximal length sequence signal
M.sub.2, are in phase whereas it is the carrier wave of a random
phase while the the received signal and the maximal length sequence
signal M.sub.2, are out of phase and the carrier wave is supplied
to the distributor 214. The distributor 214 distributes the input
signal in two so that its distributed outputs R.sub.1 and R.sub.2
are respectively supplied to the multipliers 208 and 209. The
hybrid coupler 220, which is supplied with a part of the
transmitting carrier wave from the distributor 213, generates a
signal I of the in-phase component (phase zero) and a signal Q of
the quadrature component (phase quadrature) with respect to the
input signal and the signals are respectively supplied to
multipliers 208 and 209. The multiplier 208 multiplies the signal I
applied from the hybrid coupler 220 (i.e., the signal which is in
phase with the output of the carrier oscillator 219) and the signal
R.sub.1 applied from the distributor 214 and similarly multiplier
209 multiplies the input signals Q (i.e., the signal having a
90-degree phase difference with the output of the carrier
oscillator 219) and R.sub.2, thereby respectively extracting the
in-phase component (I.multidot.R.sub.1) and the phase quadrature
component (Q.multidot.R.sub.2) of the received signal and
generating them as orthogonal detection signals. The orthogonal
detection signals I.multidot.R.sub.1 and Q.multidot.R.sub.2 are
respectively supplied to the low-pass filters 211 and 212
performing band limitation of the same frequency range as the
low-pass filter 210. Each of low-pass filters 211 and 212 performs
a frequency band limiting operation and hence an integrating
function and thus it integrates the processed correlation value of
the two signals. In other words, when the signal R.sub.1 applied to
the multiplier 208 from the multiplier 207 through the distributor
214 and the signal I applied to the multiplier 208 from the hybrid
coupler 220 are in phase and similarly when the signals R.sub.2 and
Q applied to the multiplier 209 are in phase, the output signals of
the multipliers 208 and 209 take the form of pulse signals of a
constant polarity (pulse signals of voltage +E) and the signals are
respectively integrated by the low-pass filters 211 and 212,
thereby generating large positive voltages at their outputs. On the
contrary, if the signals R.sub.1 and I are out of phase and if the
signals R.sub.2 and Q are out of phase, the output signals of the
multipliers 208 and 209 take the form of pulse signals of randomly
varying positive and negative polarities (i.e., pulse signals of
the voltages +E and -E) and these signals are respectively
integrated by the low-pass filters 211 and 212, thereby generating
no outputs. After having been integrated by the low-pass filters
211 and 212 in this way, the in-phase component signal and the
phase quadrature component signal are respectively supplied to the
squaring devices 215 and 216. Each of the squaring devices 215 and
216 squares the amplitude of the input signal and supplies the
resulting output signal to the adder 217. The adder 217 adds the
two input signals so that a pulse-like detected target signal such
as shown in FIG. 11(c) is generated and supplied to the
time-interval meter 218. Assume now that the maximum value of this
detected target signal occurs at a time t.sub.b. While the
described method comprising the steps of detecting the in-phase
component and the phase quadrature component of the transmitting
carrier wave from the signal obtained by a correlation process on
the received signal and the maximal length sequence signal M.sub.2,
squaring the orthogonal detection signals after the integration
thereof and generating a detected target signal as the sum of the
pair of squared values is more or less complicated in construction,
the method can produce a detected target signal of a high degree of
sensitivity. Also, due to the production of the correlation output
of the pseudo random signals such as maximal length sequence
signals, the effect of noise is reduced and the signals are
enhanced, thereby realizing a measuring system having a high
signal-to-noise (S/N) ratio. As regards the carrier detecting
method, there is of course a detecting method employing a crystal
which, although low in sensitivity, is simplified in construction
and this method may be used depending on the specification and
cost.
The time-interval meter 218 measures the time interval T.sub.D
between the time t.sub.a of generation of the maximum value of the
reference signal applied from the low-pass filter 210 and the time
t.sub.b of generation of the maximum value of the detected target
signal applied from the adder 217. For this purpose, the
time-interval meter 218 has a function of detecting the maximum
value generation times of the two input signals. For instance, the
input voltage value is successively sampled and held by clock
signals in such a manner that the sampled value by the current
clock signal and the sampled value by the preceding clock signal
are successively compared by a voltage comparator and the time of
transition from the positive-going to the negative-going condition
of the input signal is detected, thereby detecting the time of
generation of the maximum value of the input signal. The time
interval T.sub.D is shown as the interval of time between the time
t.sub.a of generation of the maximum value of the reference signal
shown in FIG. 11(d) and the time t.sub.b of generation of the
maximum value of the detected signal shown in FIG. 11(c). As shown
by equation (2), the time interval T.sub.D is obtained in an
expanded form in time as f.sub.1 /(f.sub.1 -f.sub.2) times the
propagation time .tau. actually required for the electromagnetic
wave to go back and forth the distance between the transmitting and
receiving antennas 223 and 224 and the target 225. In the case of
this embodiment, f.sub.1 =100.004 MHz and f.sub.2 =99.996 MHz so
that in accordance with equation (4), the measured time T.sub.D is
expanded in time to 12,500 times the wave propagation time .tau..
The measured time T.sub.D is obtained at intervals of the period
T.sub.B of the reference signal.
It will thus be seen that in accordance with this embodiment the
measured time is expanded very greatly and the distance to any
target can be measured accurately even if the distance is short.
Therefore, the embodiment is suitable as a short-distance level
measuring apparatus for measuring, for example, the slag level or
molten steel level in a furnace.
Therefore, by determining the distance of x meters from the
transmitting and receiving antennas 223 and 224 to the target 225
from equation (3), we obtain the following:
Referring now to FIG. 12, there is illustrated a schematic diagram
showing the second embodiment of the invention which is applied to
an apparatus for measuring the slag level in a melting reduction
furnace. In the figure numeral 240 designates a melting reduction
furnace, 241 a level measuring apparatus proper, 242 a transmitting
antenna, 243 a receiving antenna, 244 waveguides, and 245 slag. The
transmitting and receiving antennas 242 and 243, which are
installed inside the furnace, are composed of water-cooled horn
antennas and are connected through the waveguides 244 to the level
measuring apparatus 241 disposed at the top of the melting
reduction furnace 240. The level of the slag 245 within the furnace
is obtained by transmitting an electromagnetic wave from the level
measuring apparatus 241 through one of the waveguides 244 and the
transmitting antenna 242, receiving the electromagnetic wave
reflected from the surface of the slag 245 through the receiving
antenna 243 and the other of the waveguides 244 and then
substituting the previously mentioned measured time T.sub.D into
equation (5) for computation. Actually, the measured value of the
level measuring apparatus according to the present embodiment
showed a satisfactory coincidence with the measurement result
obtained by measuring the position of slag deposition on the
sublance.
In addition, in a measuring environment tending to cause any
undesired reflected wave, e.g., the limited space in a furnace, in
accordance with this embodiment the advantage of the greatly
expanded time for measuring the distance to the target is utilized
in such a manner that only the detected signal due to the desired
reflected wave is extracted through a time gate circuit and the
detected signals due to the undesired reflected waves are
eliminated, thereby stably measuring the desired level position or
distance.
While, in this disclosed embodiment, the transmitting and receiving
antennas comprise two separately arranged antennas, the invention
is not limited thereto and the antenna system may be designed so
that a single antenna is used as a common transmitting and
receiving antenna and a directional coupler or duplexer is added to
separate a transmitted signal from a received signal.
Further, while the present embodiment has been described as applied
for level measuring purposes, by suitably selecting clock
frequencies for generating two pseudo random signals, the
embodiment can be satisfactorily applied to the distance
measurement of ordinary targets, including long-distance ones, such
as, flying targets, ships and automobiles.
Still further, while in this embodiment the carrier wave comprises
a microwave of about 10 GHz by way of example, it is also possible
to use a millimeter wave, light, a sound wave, an ultrasonic wave
an acoustic wave as the carrier wave.
Still further, the apparatus may be additionally provided with a
clock to compute a change of the measured distance of a target in
unit time so as to measure the speed of the target.
* * * * *