US005438872A

United States Patent (19) 11 Patent Number: 5,438,872

Kobayashi et al. 45 Date of Patent: Aug. 8, 1995
54 MEASURING METHOD AND APPARATUS FOREIGN PATENT DOCUMENTS
USING A LAMB WAVE
60-001553 5/1985 Japan.
75) Inventors: Katsuyuki Kobayashi, Yokohama; 63-175712 11/1988 Japan.
Atsushi Tanaka, Kawasaki; Yuichiro 1525.455 1 1/1989 U.S.S.R. .
Yoshimura, Kamakura; Kiyoshi 1569534 6/1990 U.S.S.R. ................................ 73/597
Kaneko; Masaki Tokioka, both of 1631410 2/1991 U.S.S.R. ................................ 73/597
Yokohama, all of Japan Primary Examiner-Hezron E. Williams
Assistant Examiner-Rose M. Finley
73 Assignee: Canon Kabushiki Kaisha, Tokyo, Attorney, Agent, or Firm-Fitzpatrick, Cella, Harper &
Japan Scinto
(21) Appl. No.: 900,324 57 ABSTRACT
22 Filed: Jun. 18, 1992 An apparatus for measuring a thickness of plate material
includes a plate material whose thickness is to be mea
30 Foreign Application Priority Data sured, a vibration pen to generate a plate wave by ap
plying a vibration to the plate material, a vibration sen
Jun. 21, 1991 JP Japan .................................. 3-150046 sor which is disposed at an arbitrary position on the
Jun. 21, 1991 (JP) Japan .................................. 3-150.047 plate material and detects the plate wave which has
propagated on the plate material, an extracting circuit
51 Int. Cl. ........................ G01H 5/00; G01N 29/18 to extract different frequency components of the plate
52 U.S. Cl. ..................................... 73/597; 73/1201; wave detected by the vibration sensor, a timer to mea
73/602; 73/DIG. 1 sure arrival times to the vibration sensor of the signals
58) Field of Search ................. 73/579, 597, 643, 632, of the different frequency components extracted by the
73/865.3, 12.01, 12.08, 602,615, 599, 598, extracting circuit, and a controller to obtain a thickness
DIG. 1
of the plate material on the basis of the arrival times
56) References Cited measured by the timer and the frequency components
corresponding thereto. The extracting circuit is a band
U.S. PATENT DOCUMENTS pass filter.
4,429,575 2/1984 Akishika ............................... 73/598
5,035,144 7/1991 Aussel ................................... 73/602 4 Claims, 12 Drawing Sheets
1

CONTROLLER

ZZZZZZZZZZZZZZZZZZZZ
U.S. Patent Aug. 8, 1995 Sheet 1 of 12 5,438,872

FIG 1

SIGNAL
PROCESSOR

PRE-SAGE
AMPLFER

ZZZZZZZZZZZZZZZZZZZZ
U.S. Patent Aug. 8, 1995 Sheet 2 of 12 5,438,872

FIG 2

DRIVE
SIGNAL

7
ZZZZZZZZZZZZZZZZZZZZZZZ

FIG 3
DETECTED
SIGNAL
U.S. Patent Aug. 8, 1995 Sheet 3 of 12 5,438,872

s
U.S. Patent Aug. 8, 1995 Sheet 4 of 12 5,438,872

FIG 5
3500

3000

2500

2OOO

1 50 O

1000

F: WAVE FREQUENCY
d: PLATE THICKNESS
500

0 2 4. 6 8 10 12 14
Fxd (MHzkmm)
U.S. Patent Aug. 8, 1995 Sheet 5 of 12 5,438,872

O
O

O
3
N is
L
O
X
NO Z
X
e 5
N. C.
C On C1

t O
1.

2.
L

s
on

C
on
J n

N. 99
&on th N
es n wo i?
S. S. f
an a din Si asn a Con d.
&
Y4 cai

ADO.N di?00
U.S. Patent Aug. 8, 1995 Sheet 6 of 12 5,438,872

FIG 7
41 - I -
42

42-A

42-B

43-A

43-B

44 - A

ll-B

45 - A tg1

45-B
tg2
U.S. Patent Aug. 8, 1995 Sheet 7 of 12 5,438,872

FIG 8

1
OUTPUT DRIVE SIGNAL
TO DRIVER CCT 2
S2
START COUNTER 86

sc COUNTER 8 STOPPED
YES
S 4.
READ TIME
S5
OBTAIN (tg2 - tg)
S6
OBTAIN
(f1 - tg1 - f2-tg2)
ST
DETERMINE d
ACCORIDING TO
EQUATION (4)
S8
DISPLAY/OUTPUT d
END
U.S. Patent Aug. 8, 1995 Sheet 8 of 12 5,438,872

||

0/-/
6

TVNOIS80SEJ d YZO,
U.S. Patent Aug. 8, 1995 Sheet 9 of 12 5,438,872

N0D01?I,83T108]

0/
U.S. Patent Aug. 8, 1995 Sheet 10 of 12 5,438,872
U.S. Patent Aug. 8, 1995 Sheet 11 of 12 5,438,872
U.S. Patent Aug. 8, 1995 Sheet 12 of 12 5,438,872

FIG. 13

S11
OUTPUT DRIVE SIGNAL
21 T0 DRIVER CCT 12
S12
START COUNTER 183

se COUNTER 183 STOPPED

YES
READ TIME (tg1)
S13

S14

se SIGNAL 22 ROSE
YES
S15

S16
START COUNTER 183

COUNTER 18 STOPPED
YES
S8
READ TIME (tg2)
S19
DETERMINE d
S20
DISPLAY/PRINT d
END
 5,438,872
1. 2
and returned is reduced, so that when a thickness of thin
MEASURING METHOD AND APPARATUS USING plate is measured, it is difficult to accurately measure
A LAMIB WAVE such a return time. To solve the above problem, there is
used a method of increasing a resolution by raising a
BACKGROUND OF THE INVENTION 5 frequency of the ultrasonic wave signal which enters
1. Field of the Invention the plate or the like. However, any of those methods has
The invention relates to measuring method and an limits and the costs of the apparatus itself are also high.
apparatus for measuring a thickness of a plate material SUMMARY OF THE INVENTION
such as plate glass, iron plate, aluminum plate, or the 10
like. The invention is made in consideration of the above
2. Description of the Related Background Art conventional examples, and it is an object of the inven
Hitherto, as an apparatus for measuring a thickness of tion to provide a measuring method and an apparatus
plate material, there are known a simple measuring which can easily measure a thickness of plate material
apparatus for actually measuring a thickness by using a 15 by low costs.
micrometer, a caliper, or the like, a measuring apparatus Another object of the invention is to provide a
for optically measuring a thickness by using a laser method and an apparatus in which a lamb wave vibra
beam or the like, an apparatus for measuring a thickness tion applied to a plate material is detected, different
by using an ultrasonic wave or the like, and the like. frequency components of the vibration are extracted,
The above conventional measuring apparatuses, and a thickness of plate material is measured from an
however, have the following drawbacks. 20
arrival time to a sensor of the signal of each frequency
First, in the apparatus for measuring by using a mi component.
crometer, a caliper, or the like, only the portion around Still another object of the invention is to provide a
the plate as an object to be measured can be accurately method and an apparatus in which lamb wave vibra
measured and, for instance, in case of a plate material tions of different frequencies which have time sequen
having a wide area, the measurement of the plate thick 25 tially applied to a plate material are detected and a
ness of the central portion cannot help becoming the thickness of plate material is measured from the arrival
relative measurement. In other words, in such a mea time to a sensor of the signal of each frequency compo
surement by the micrometer or the like, a reference nent.
surface is first specified, a plate material as an object to
be measured is put on the reference surface, and a dis 30 BRIEF DESCRIPTION OF THE DRAWINGS
tance between the upper surface of the plate and the FIG. 1 is a block diagram showing a schematic con
reference surface is measured, thereby measuring a struction of a plate thickness measuring apparatus ac
thickness of the plate material. In such a case, when the cording to the first embodiment;
plate material as a measuring object is distorted or FIG. 2 is a diagram showing an example of a driving
warped, a gap occurs between the lower surface of the 35
waveform of a vibrator in the first embodiment;
plate material and the reference surface in dependence FIG.3 is a diagram showing an example of a detected
on the location of the plate material. When such a gap
occurs, the measured plate thickness differs from the signal waveform of a vibration sensor in the first em
actual plate thickness, so that the measurement may not bodiment;
be accurate. FIG. 4 is a diagram showing an example of a signal
On the other hand, in the case of the optical measur spectrum of a detection signal detected by the vibration
ing apparatus using the laser beam or the like, the appa sensor in the first embodiment;
ratus itself is expensive and its handling is also compli FIG. 5 is a diagram for explaining general character
cated. In the case of the apparatus for calculating the istics of a plate wave;
plate thickness by a path difference of the reflected 45 FIG. 6 is a diagram showing the relation between the
lights between the upper surface and the lower surface center frequency of a band pass filter and the group
of the plate by using a laser beam, the measuring object velocity of a plate wave in the first embodiment;
obviously must be a transparent plate, so that the appli FIG. 7 is a time chart showing signal processes in a
cation of such an apparatus is limited. On the other waveform detecting circuit in the first embodiment;
hand, there is a method whereby a laser beam is irradi 50 FIG. 8 is a diagram showing a plate thickness measur
ated to a plate which is not transparent from each of the ing process by a controller of a plate thickness measur
upper and lower surfaces and the plate thickness is ing apparatus according to the first embodiment;
measured on the basis of the reflected lights from the FIG. 9 is a block diagram showing a schematic con
upper and lower surfaces. Such a method, however, has struction of a plate thickness measuring apparatus ac
a problem such that the operations such as an adjust 55 cording to the second embodiment;
ment of the optical axis and the like are complicated and FIG. 10 is a block diagram showing a construction of
the plate thickness cannot be easily measured. a driver circuit of a vibrator in the second embodiment;
According to the method of measuring a plate thick FIG. 11 is a diagram showing an example of a de
ness by using an ultrasonic wave, a time which is re tected signal waveform in each section of the driver
quired from a time point at which an acoustic wave is circuit shown in FIG. 10;
generated from a signal generating source and enters FIG. 12 is a time chart showing signal processes in a
the plate material as a measuring object to a time point waveform detecting circuit on the basis of the relation
at which the acoustic wave is reflected by the bottom between the driving signal waveforms of different fre
surface of the plate and is returned is measured, thereby quencies and the group velocities of the lamb wave
calculating the plate thickness. According to such a 65 corresponding thereto in the second embodiment; and
method, however, as the plate thickness decreases, a FIG. 13 is a diagram showing plate thickness measur
time difference which is caused until the irradiated ing processes by a controller of the plate thickness mea
ultrasonic wave signalis reflected by the bottom surface Suring apparatus according to the second embodiment.
 5,438,872
3 4
DETALED DESCRIPTION OF THE
as loads, the vibration of the vibrator 4 is dumped and
PREFERRED EMBODIMENTS the mechanical energy of a wide band as a whole is
generated from the tip of the vibration pen 4. Therefore,
First embodiment the electric signal which is detected and generated by
The preferred first embodiment of the invention will the vibration sensor 6 has a synthetic wave in which
be described in detail hereinbelow with reference to the various frequency components are mixed.
drawings. In FIG. 4, an axis of abscissa indicates a frequency
FIG. 1 is a block diagram showing a schematic con (kHz) and an axis of ordinate indicates again (3 dB). A
struction of a plate thickness measuring apparatus of the peak value is equal to -40.83 dBV at 250 kHz.
first embodiment. 10 FIG. 5 is a diagram showing general characteristics
In FIG. 1, reference numeral 1 denotes a controller of an elastic wave (plate wave) which propagates on the
for controlling the entire measuring apparatus and for plate material. Reference numeral 51 denotes V (phase
measuring a thickness of plate material 7 as an object to velocity) and 52 indicates Vg (group velocity).
be measured. Reference numeral 2 denotes a driver As will be obviously understood from FIG. 5, it is
circuit for receiving a driving signal from the controller 15 well known that when the quality of plate material is
1 and for driving a vibration output actuator 3 of a equal, the phase velocity V and group velocity Vg of
vibration pen 10. The actuator 3 has therein a vibrator 4 the lamb wave depend on the product of a thickness d of
comprising a piezoelectric transducer or the like, and an the plate material and a frequency F of the wave.
electric driving signal which is supplied from the driver As mentioned above, since the lamb wave which
circuit 2 is converted into a mechanical ultrasonic vibra 20 propagates on the plate material 7 is dumped, its lamb
tion by the vibrator 4. The ultrasonic vibration is trans wave signal has a frequency band wider than that of the
ferred to the plate material 7 as a measuring object electric driving signal (FIG. 2). Therefore, the signal
through a horn portion 5 having a sharp tip. Conse waveform which is detected by the vibration sensor 6 is
quently, the ultrasonic vibration which has propagated as shown in FIG. 4, that is, when comparing on the time
on the plate material 7 is detected by a vibration sensor 25 base, the head portion of the signal waveform is con
6 comprising a mechanical/electric converting element structed by a wave having a high propagating speed and
such as a piezoelectric element or the like. An output a relatively high frequency, the lamb wave of a low
signal of the vibration sensor 6 is supplied to a wave frequency gradually arrives at the vibration sensor 6
form detecting circuit 8 and converted into a detection and is converted into the electric signal with the elapse
signal which can be processed by the controller 1. The 30 of the time and is multiplexed to the wave of a high
controller 1 calculates a thickness of plate material 7 on frequency which has already reached and the resultant
the basis of the result of the process by the waveform multiplexed signal is generated.
detecting circuit 8. Reference numeral 9 denotes an Therefore, by paying attention to the difference be
output unit to display or print and generate the result of tween the frequencies of the lamb waves, the waveform
the above measurement. 35 of the detection signal detected by the vibration sensor
A method of obtaining the plate thickness will now 6 is processed through band pass filters having different
be described in detail hereinbelow. center frequencies, thereby obtaining a thickness of
FIG. 2 shows a vibrator driving waveform to drive plate material. Explanation will now be described in
the vibrator 4 in the vibration pen 3. detail hereinbelow.
As shown in FIG. 2, according to the embodiment, The same electric signal generated from the vibration
the vibrator driver circuit 2 receives a rectangular wave sensor 6 in case of a predetermined plate thickness is
pulse train having a predetermined length by the vibra transmitted and processed through band pass filters
tor 4 at a predetermined period. The vibrator 4 vibrates (BPFs) of different center frequencies. The group ve
by the driving signal and the electric driving signal is locities V (thousand m/sec) in the respective cases are
converted into the mechanical energy. The vibration is 45 obtained from the relations between the arrival delay
transferred as an elastic wave vibration on the plate times of the waves and the distance between the vibra
material 7 and is detected by the vibration sensor 6 tion pen 4 and the vibration sensor 6. The results are
provided similarly on the plate material 7. The electric shown in FIG. 6.
energy supplied to the vibrator 4 by the driver circuit 2 As will be obviously understood from FIG. 6, as the
as mentioned above is converted into the mechanical SO center frequency of the BPF is high, the group velocity
energy by the vibrator 4 and is again converted into the V is high. As the center frequency of the BPF is low,
electric energy by the vibration sensor 6 through the the group velocity Vg is low. Therefore, even if the
plate material 7 and is generated. FIG. 3 shows an ex signals which are detected and generated by the vibra
ample of a signal waveform detected by the vibration tion sensor 6 are equal, by transmitting and processing
sensor 6. 55 the same signal through the band pass filters having
The signal which has been converted from the elec different center frequencies, the lamb wave of a special
tric signal to the mechanical energy and subsequently velocity corresponding to the center frequency can be
from the mechanical energy to the electric signal, detected.
namely, the signal which is detected by the vibration A construction of the wave form detecting circuit 8
sensor 6, doesn't exhibit a spectrum of a single fre to detect such group velocities and to measure the plate
quency as shown in FIG. 4. This is because each section thickness will now be described.
of the vibration pen 10 including the vibrator 4, horn In FIG. 1 of the embodiment, the measuring appara
portion 5, and the like has a peculiar vibrating fre tus uses a band pass filter 82 and a band pass filter 83
quency as compared with the frequency of the electric whose center frequency is different from that of the
driving signal supplied to the vibrator 4, so that a me 65 BPF 82 and processes the signal which is generated
chanical response doesn't have a single frequency and from the vibration sensor 6. After the output signal from
those portions respond in various vibrating mode. In the the vibration sensor 6 was amplified by a pre-stage
vibrator 4, since the horn portion 5 and the like function amplifier 81, it is supplied to the BPFs 82 and 83. It is
 5,438,872
5 6
now assumed that a group velocity and a frequency of 85. When each signal processing circuit detects the peak
the wave which are calculated by the signal waveform of the envelope, it generates a count stop signal to a
processed by using the BPF 82 are respectively set to counter 86.
Vg1 and f and that a group velocity and a frequency of The counter 86 is constructed by two counters. The
the wave which are calculated by the signal waveform 5 counter 86 starts the counting operation at a time when
processed by the BPF 83 are respectively set to V2 and the controller 1 generates a driving signal to the driver
f2. circuit 2 and starts the driving of the vibrator 4. The
It is now assumed that a group delay time which is counter stops the counting operation by peak detection
caused until the ultrasonic wave vibration which has
been generated from the actuator 3 and propagated on O signals from the signal processing circuits 84 and 85.
the plate material 7 reaches the vibration sensor 6 and is Therefore, by reading a count value of the counter 86
processed by the BPF 82 and is detected is set to T1 and by the controller 1, the time which is required from a
a group velocity which is detected by the BPF 83 is set time point at which the vibration is generated from the
to T2. When the group velocities are approximated by vibration pen 10 to a time point at which the vibration
FIG. 6, 15 sensor 6 detects the vibration can be obtained.
As mentioned above, on the basis of the peak signal of
the envelopes of the detection signal waveforms, the
waveform detecting circuit 8 supplies the group delay
where, times tign to the controller 1 as count values of the
n=1, 2 counter 86.
d: thickness of plate material 7 FIG. 8 is a flowchart showing plate thickness measur
o, G constants ing processes in the controller 1 according to the first
Now, assuming that a distance between the vibration embodiment. A control program to execute such pro
output actuator 3 and the vibration sensor 6 is set to L, cesses has been stored in an ROM (not shown) or the
L is obtained by using the group delay times (T) and 25 like in the controller 1.
group velocities (V) which are measured. In the plate thickness measuring processing routine,
L=Vg1'tgi=Vg2tg2 (2) the driving signal to drive the vibrator 4 is first supplied
to the driver circuit 2 in step S1. The counter 86 is also
From the relation of the equation (1), 30
activated in step S2 and the counting operation is
started. In step S3, the apparatus waits for the stop of
a(fixd)+ 6tgl=(a(f2xd)+T2 (3) the counting operation of the counter 86. When the
By solving the equation (3) with respect to the plate counter 86 finishes the counting operation, step S4 foll
lows and the count value at that time is read. The count
thickness d, values which are read as mentioned above correspond
35
d=(6/a)x{(tg2-igi) / (fitga-ptg2)} (4) to the times which are required until the peak values of
the signals which have been transmitted through the
By respectively detecting the group delay times of BPFs 82 and 83 are detected and are expressed by til
the lamb waves of different velocities by using a princi and tig2, respectively.
ple such that the velocity of the wave (lamb wave) 40 In step S5, a difference (t2-ti) between those time
which propagates on the plate depends on the vibrating data is calculated. In step S6, the center frequencies (f,
frequency, the plate thickness can be easily measured. f2) of the band pass filters (82 and 83) are multiplied
In addition to the control of the measuring apparatus, with the elapsed times (tg1, t2) corresponding to them,
the controller 1 can also calculate the plate thickness of thereby obtaining the group velocities (fitgitg2) corre
the measuring object by executing the above calcula- 45 sponding to the frequencies. By substituting the times
tions and can display or print the result to the output and group velocities obtained as mentioned above into
unit 9 and can also generate the result to the external the equation (4), the thickness of plate material 7 as a
apparatus. measuring object is obtained. The processing routine
FIG. 7 is a timing chart showing timings of the signal advances to step S8 and the calculated plate thickness is
processes which are executed in the signal detecting 50 supplied to the output unit 9 and is displayed or printed.
circuit 8.
In the diagram, reference numeral 41 denotes a driv The invention can be applied to a system which is
ing waveform to drive the vibrator 4 in the vibration constructed by a plurality of apparatuses or to an appa
pen 10 and 42 indicates a detected signal waveform ratus comprising one apparatus. The invention can also
when the vibration which has propagated on the plate 55 be obviously applied to the case where the invention is
material 7 as a measuring object is detected by the vi accomplished by supplying a program to execute the
bration sensor 6. Reference numerals 42-A and 42-B processes specified by the invention to a system or an
denote signal waveforms after the detected signal wave apparatus.
forms were processed by the BPFs 82 and 83, respec According to the first embodiment as described
tively; 43-A and 43-B indicate envelopes of the detected above, since the distance between the actuator 3 and the
signal waveforms; and 44-A and 44-B represent differ vibration sensor 6 can be measured without calculating
entiated waveforms of the envelopes 43-A and 43-B, the plate thickness, there is no need to strictly set the
respectively. The envelopes are differentiated in order distance between them. Consequently, since the actua
to determine the delay times of the vibration waves tor 3 and vibration sensor 6 can be set to arbitrary posi
detected by the vibration sensor 6. The peak of each 65 tions of the measuring object, there is a generality and
envelope is detected by the zero-cross signal (corre there are also effects such that the construction of the
sponding to the peak of the envelope). The above pro apparatus is simple and can be realized by low costs.
cesses are executed by signal processing circuits 84 and Second embodiment
 5,438,872
7 8
The preferred second embodiment of the invention The oscillator 201 starts the oscillation by a signal 21
will now be described in detail hereinbelow with refer which is supplied from the controller 11. The signal 213
ence to the drawings. which is generated from the frequency divider circuit
FIG. 9 is a block diagram showing a schematic con 203 is sent to the controller 11. Thus, the controller 11
struction of a plate thickness measuring apparatus of the can detect the output timing of the signal from the
second embodiment. driver circuit 206 shown in the timing chart of FIG. 11.
In FIG. 9, reference numeral 11 denotes a controller In this manner, the start of the counting operation of a
to control the entire measuring apparatus and to mea counter 183 of the waveform detecting circuit 18,
sure a thickness of plate material 17 as an object to be which will be explained hereinlater, can be instructed.
measured. Reference numeral 12 denotes a driver cir 10 The vibration of the vibrator 14 generated as men
cuit for receiving a driving signal from the controller 11 tioned above is given to the plate material 17 via the
and for driving a vibration output actuator 13 of a vibra horn portion 15 and propagates as an elastic wave vibra
tion pen 20. The actuator 13 has therein a vibrator 14 tion on the plate material 17 and is detected by the
comprising a piezoelectric transducer or the like and the 15
vibration sensor 16. As mentioned above, the electric
electric driving signal which is supplied from the driver energy supplied to the vibrator 14 is converted into the
circuit 12 is converted into the mechanical ultrasonic mechanical energy by the vibrator 14 and is again con
vibration by the vibrator 14. The ultrasonic vibration is verted into the electric energy by the vibration sensor
transferred to the plate material 17 as a measuring ob 16 through the plate material 17 as a measuring object
ject through a horn portion 15 having a sharp tip. Due and is generated.
to this, the ultrasonic vibration which has propagated In the second embodiment, the arrival delay times of
on the plate material 17 is detected by a vibration sensor the lamb wave of the first frequency (f) and the lamb
16 comprising a mechanical/electric converting ele wave of the second frequency (f2) different from the
ment such as a piezoelectric element and the like. An first frequency are measured, thereby calculating the
output signal of the vibration sensor 16 is supplied to a 25 plate thickness.
waveform detecting circuit 18 and is converted into a As mentioned above, the group delay times of the
detection signal which can be processed by the control waves of different velocities are respectively detected
ler 11. On the basis of the result processed by the wave by using a principle such that the velocities of the waves
form detecting circuit 18, the controller 11 calculates a (lamb waves) which propagate on the plate material 17
thickness of plate material 17. depend on the frequency, so that the thickness of plate
Reference numeral 19 denotes an output unit to dis material can be easily measured.
play or print and generate the result of the measure In addition to the control of the measuring apparatus,
ent. the controller 11 calculates the plate thickness by exe
A method of obtaining the plate thickness will now cuting the above-mentioned calculations and displays
be described in detail hereinbelow. 35 the result or generates the result to an external appara
FIG. 10 is a block diagram showing a practical con tuS.
struction of the vibrator driver circuit 12 to drive the FIG. 12 is a time chart of the signal processes which
vibrator 14 in the vibration pen 13. FIG. 11 is a timing are executed in the signal detecting circuit 18.
chart. In FIG. 12, reference numeral 41' denotes a driving
In FIG. 10, a clock signal generated from an oscilla waveform (214 in FIGS. 10 and 11) to drive the vibrator
tor (OSC) 201 is frequency divided by a frequency 14 of the vibration pen 20. Reference numeral 42 de
divider circuit 202 and converted into a frequency sig notes a detected signal waveform when the vibration
nal of a frequency f. (211 in the timing chart of FIG. which has propagated on the vibration propagating
11). After that, it is supplied to a switch circuit 204 to plate is detected by the vibration sensor 16. Reference
switch between frequency signals ft (211) and f (212). 45 numerals 43 (A) and 43 (B) denote envelopes of the
The clock signal f, generated from the OSC 201 is fre detected signal waveform 42 and 44 (A) and 44 (B)
quency divided by another frequency divider circuit indicate differentiated signals which are obtained by
203 and converted into a clock signal shown at 213 in differentiating the envelopes 43 (A) and 43' (B) in order
the timing chart of FIG. 11. The switch circuit 204 is to determine the delay times of the waves of those en
switched by the clock signal 213. The clock signals of 50 velopes. The signals 44 (A) and 44 (B) are used to
the frequencies f, and f are alternately selected and detect the zero-cross signals (corresponding to the
supplied to a shift register 205. The shift register 205 is peaks of the envelopes).
constructed so as to generate a pulse train of the number Due to this, the signal waveform detecting circuit 13
of a few periods (five periods in the case of the second generates group delay times ten for the driving signals to
embodiment) just after the clock signal 213 changed. 55 the controller 11 as signals indicative of the peak values
The frequency of such a pulse train 214 depends on the of the envelopes of the detected signal waveforms,
frequencies fi, and f. respectively. That is, the waveform detecting circuit 18
As shown in the timing chart of FIG. 11, conse has the counter 183 to start the counting operation
quently, clocks of frequencies f and f2 shown by the simultaneously with the timings at which the clock
clock signal 214 are alternately generated from the shift 60 signals of different frequencies shown at 41" in FIG. 13
register 205 just after a clock signal 213 changed. A are generated from the driver circuit 12. A signal pro
driver circuit 206 converts the clock signal 214 gener cessing circuit 182 receives the detection signal 42 of
ated from the shift register 205 into the electric signal the vibration sensor 16 for the clock signals of different
level which is optimum to drive the vibrator 14. In case frequencies and obtains the envelopes 43 (A) and 43
of the second embodiment, the clock signal 214 drives 65 (B) of the detection signals 42 and generates zero-cross
the vibrator 14 as a rectangular wave pulse train. How signals 44 (A) and 44 (B). When the zero-cross signals
ever, no problem occurs even when a size wave or the are received, the counter 182 finishes the counting oper
like is used as a waveform of such a driving signal. ation and generates the count value to the controller 11.
 5,438,872
9 10
FIG. 13 is a flowchart showing the plate thickness detecting means disposed at an arbitrary position on
measuring processes in the controller 11 of the embodi the plate material for detecting the lamb wave
ment. A control program to execute those processes has which has propagated on the plate material;
been stored in an ROM (not shown) of the controller 11 extracting means for extracting a plurality of lamb
or the like. wave components having different frequencies
In the plate thickness measuring processing routine, from the lamb wave detected by said detecting
means;
the driving signal 21 to drive the oscillator 201 of the measuring means for measuring a time difference
driver circuit 12 is first supplied to the driver circuit 12 between arrival times to said detecting means of
in step S11. The counter 183 is activated in step S12 and 10 the plurality of lamb wave components extracted
the counting operation of the counter 183 is started. In by said extracting means; and
step S13, the apparatus waits for the stop of the count calculating means for calculating the thickness of the
ing operation of the counter 183. When the counter 183 plate material based on the time difference mea
finishes the counting operation, step S14 follows and the sured by said measuring means and velocities of the
count value is read. The time t1 shown in FIG. 12 is 15 plurality of lamb wave components corresponding .
read as a count value. to the arrival times.
In the next step S15, a signal 22 from the driver cir 2. A measuring method of measuring the thickness of
cuit 12 is received and a leading timing of the signal 22 a plate material, comprising the steps of:
is detected. When the signal 22 rises, step S16 follows generating a lamb wave by applying a vibration to a
20 plate material;
and the counting operation by the counter 183 is started detecting a lamb wave which has propagated on the
because it indicates the output timing of the clock signal plate material and extracting a plurality of lamb
(f2) shown in FIG. 13. The apparatus waits for the stop wave components having different frequencies
of the counting operation of the counter 183 in step S17. from the detected lamb wave, using a detector
When the counter 183 stops the counting operation, 25 disposed at an arbitrary position on the plate mate
step S18 follows and the count value (t2 in FIG. 13) is rial;
read. measuring a time difference between arrival times to
In step S19, a difference (t2-tigl) between the above the detector of the plurality of lamb wave compo
time data is calculated and the predetermined frequen nents extracted in said detecting and extracting
cies (fi, f2) are multiplied with the elapsed times (t1/tc2) 30 step; and
corresponding to them, thereby obtaining the group calculating the thickness of the plate material based
on the measured time difference and velocities of
velocities (fl-tg1, f2t2) corresponding to those frequen the plurality of lamb wave components corre
cies. By substituting the times and group velocities ob sponding to the arrival times.
tained as mentioned above into the equation (4) men 35 3. A measuring apparatus for measuring the thickness
tioned above, the thickness of plate material 17 as a of a plate material, said apparatus comprising:
measuring object is obtained. In the next step S20, the vibration generating means for generating lamb
calculated plate thickness is sent to the output unit 19 waves by time sequentially applying vibrations of
and is displayed or printed. different frequencies to a plate material;
The invention can be applied to a system comprising detecting means, disposed at an arbitrary position on
a plurality of apparatuses or can also be applied to an the plate material, for detecting the lamb waves
apparatus comprising one apparatus. The invention can which have propagated on the plate material;
also be obviously applied to the case where the inven measuring means for measuring arrival times of the
tion is accomplished by supplying a program to execute 45 vibrations of the different frequencies of the lamb
waves detected by said detecting means; and
the processes specified by the invention to the system or calculating means for calculating the thickness of the
apparatus. plate material based on the arrival times measured
According to the second embodiment as described by said measuring means and the frequencies corre
above, since there is no need to previously decide the sponding to the arrival times.
distance between the vibration pen 20 and the vibration 50 4. A measuring method of measuring the thickness of
sensor 16, the pen 20 and sensor 16 can be put at arbi a plate material, comprising the steps of:
trary positions of the measuring object and the plate generating lamb waves by applying vibrations of
thickness can be measured. Moreover, an excellent ef different frequencies to a plate material;
fect such that the apparatus can be constructed by low 55 detecting the plate waves which have propagated on
costs is obtained. the lamb material and measuring arrival times cor
We claim:
responding to the vibrations of the different fre
quencies; and
1. A measuring apparatus for measuring the thickness calculating the thickness of the plate material based
of a plate material, said apparatus comprising: on the arrival times measured and the frequencies
vibration generating means for generating a lamb 60 of the vibrationsccorresponding to the arrival times.
wave by applying a vibration to a plate material; k k

65
 UNITED STATES PATENT AND TRADEMARK OFFICE
CERTIFICATE OF CORRECTION
PATENT NO. : 5,438,872
DATED August 8, 1995
NVENTOR(S) : Kobayashi et al.
It is certified that error appears in the above-indentified patent and that said Letters Patent is hereby
Corrected as shown below:
COLUMN 5:

Line 21, "B" should read --B:--.

COLUMN 9

Line 30, (t/t) should read ti, t2) .

COLUMN 10 :

Line 54, "plate" should read --lamb--.

Line 55, "lamb" should read --plate--.

Signed and Sealed this

Second Day of January, 1996

Attest: (a (e?em
BRUCE LEHMAN

Attesting Officer Commissioner of Patents and Trademarks