Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~L13~
'
The present invention relates to an apparatus for
measuring moisture content of and weighing a sheet in which
the sheet is inserted into a cavity resonator and its
percentage moisture content is measured by detecting a
variable and reduced resonant voltage of microwave intro-
; duced into the cavity resonator. The weight of the sheet is
measured be detecting a variable resonant frequency of the
microwave introduced.
By convention, separate measuring systems are used
for measuring moisture content (or a percentage moisturecontent) and weight per unit area (or weiqht) of sheets as
references for indicating quality of the sheet. For example,
` to measure percentage moisture, infrared rays or microwaves
are used to irradiate a sheet and the amount of the reduced
wave is measured as electricity. To measure weight, rays
or X rays are used to irradiate the sheet and the amount of
,` the rays transmitted through the object is measured as
electricity.
As described above, the conventional measuring
apparatus of this type needs two independ~nt measuring
systems with different wave irradiating sources, so that the
measuring apparatus is bulky and expensive. Because of the
use of or X rays hazardous to a human body the conventional
measuring apparatus also needs a protective device for
protecting the human body. The use of two measuring systems
further makes it difficult to measure a paper making process
in on-line manner.
Thus the invention seeks to provide an apparatus
for measuring moisture content and weight of a sheet with a
microwave source commonly used for both measurements.
,. ~
~3~
According to the invention there is provided an
apparatus for measuring the moisture content and weight of a
sheet-like object comprising: (a) a single cavity resonator
having a pair of metal box sections with openings arranged
spaced from and facing each other thereby providing a gap
therebetween for insertion of said sheet-like object, (b)
means for generating microwave signals, (c) means for
coupling said microwave signals to one of said metal box
sections, (d) a microwave detector, (e) means for coupling
microwave signals from the other of said metal box sections
to the detector, said coupled microwave signals being
attenuated and shifted in frequency at a resonance condition
of said cavity resonsator when said sheet-like object is
inserted within said gap, (f) said de-tector generating
resonance signals upon detection of a resonance condition,
and (g) circuit means connected to said detector for receiv-
ing said resonance signals, said circuit means comprising:
(1) means for detecting the amount of attenuation of said
resonance signals for substantially determining the moisture
content of said sheet-like object, and (2) means for detec-
ting the resonance frequency of said resonance signals for
determining the weight of said sheet-like object.
With such an arrangement, a microwave generated
from a microwave oscillator may be used as a common irra-
diating source for both the measurements of moisture andweight of the sheet in the cavity resonator. Therefore,
both the measurements may be made by an identical measuring
system, without using rays or X rays hazardous -to a human
--2--
body. Accordingly, the measuring apparatus may be made
small. Further, such an arrangement enables a paper manu-
facturing process to be measured in on-line manner.
.The invention will be better understood upon
reading the following description in connection with the
accompanying drawings, in which:
Figure 1 shows a perspective view of a cavity
resonator according to the invention;
Figure 2 shows a block diagram of a measurement
system according to the invention;
Figure 3 shows a block diagram of the system shown
i~ Figure 2;
= ~3(~;3~
Figure 4 shows a block diagram of another embodi-
ment to be used in place of the measuring circuit and meters
shown in Figure 2;
Figure 5 shows characteristic curves of resonant
voltage values with respect to a resonant frequency when the
:' moisture content of a sheet changes;
Figure 6 shows a characteristic curve of the
resonant voltage with respect to the resonant frequency when
the weight of an object to be measured within the cavity
resonator changes under a fixed moisture content;
Figure 7, which is on the third sheet of the
;il drawings, shows characteristic curves of moisture content
with respect to an amount of reduced microwave obtained by
using a measuring apparatus according to the present inven-
tion; and
Figure 8 shows a characteristic curve of weight to
resonant frequency with respect to weight measured ~y
apparatus according to the invention.
Referring now to the drawings and particularly
Figures 1 and 2, a cavity resonator 11 a~cording to the
invention is comprised.of upper and lower metal box sections
12 and 13 for providing ~avities with openings 121 and 13
~` arranged facing each other with gap 15 intervening. In
> measurement, a sheet 14 to be measured is inserted into the
25 gap 15. A flange 122 provided~at the opening 121 oE the
upper section 12 and a-.flange 132 provided at the openin~
131 of the lower section 13 are capacitively coupled with
each other. One side of the upper section 12 is opened to
have a coupling hole 123 coupled with a wave guide 16. A
microwave oscillator 17 such as a VCO (voltage controlled
--3~
~L3~
oscillator) is mounted to the wave guide 16. The VCo 17 is
connected to a saw-tooth wave oscillator 18 whereby a saw-
tooth wave is applied to the oscillator 18 to oscillate a
microwave signal which in turn is guided through the wave
guide 16 to the cavity resonator 11. The lower section 13
is opened at one side to have a coupling hole 133 coupled
with a wave guide 19. Within the wav~ guide 19, a device to
detect a resonant output si~nal of the resonator 11, for
example, a diode detector 20, is provided. An output signal
from the detector 20 is applied -from a terminal a to a
measuring circuit 21. The measuring circuit 21 receives the
detected output from the detector 20 and produces at an
output terminal _ a resonant voltage dependent on the sheet
inserted into the gap-15 of the resonator 11. The resonant
voltage is then applied to a meter 22 where it is visually
indicated. The measuring circuit 21 also receives the saw-
tooth wave signal from the saw tooth wave oscillator 18 -to
produce a resonant frequency dependent on the sheet at an
output terminal _. The resonant frequency is then applied
to a meter 23 where it is visually indicated.
The de~ailed circuit diagram of the measuring
circuit 21 will be descrihed with reference to Figure 3. The
detected output signal from the detector 20 is fed in
through the terminal a to an amplifier 31. The output
signal of amplifier 31 is then applied to a known peak
voltage hold circuit 32 which holds a peak value of the
resonant voltage dependent on the sheet. The peak voltage
is then compared with a reference voltage 34 in a differen-
tial amplifier 33. The output signal from the differential
amplifier 33 emerges at a terminal c. The output signal
1~3(.~
from the amplifier 31 is applied to a differentiationcircuit 35 where it is differentiated to produce a differen-
tiated waveform signal which in turn is fed into a Schmidt
trigger circuit 36. Upon.receipt of the dif~erential
signal, the Sch~idt trigger circuit 36 procudes a rectangu-
lar wave signal. A saw-tooth wave signal fed in through the
terminal b is fed through an.amplifier 37 to a sample hold
circuit 38. Upon receipt of the rectanyular wave signal
derived from -the Schmidt trigger circuit 36, the sample hold
circuit 38 sweeps the saw-tooth wave signal to produce an
output signal proportional to the resonant frequency of the
signal of those detec.ted output signals. The output signal
from the sample hold circuit is applied through an amplifier
39 to an adder 40. A resonant voltage derived from the
differential amplifier 33 also applied to adder 40 and
compared with the output s.ignal proportional to the resonant
: fre~uency to compensate for.deviation of the moisture
content of the resonant frequency value at the resonant
voltage. The compensated value emerges from a terminal d.
In place of~the measuring circuit 21 and the
meters 22 and 23, an oscilloscope 41 may be used. In this
case, the output signal from the detector 20 is applied as a
vertical input signal to a terminal a of the oscilloscope
41. On the other hand, a saw-tooth wave signal ~rom the
saw-tooth wave oscillator 18 is supplied as a horizontal
input signal to a terminal b. As a result, a resonant curve
dependent on the object to be measured is depicted on the
display surface and from the curve depicted, the resonant
voltage and the resonant frequency may be measured.
i 30 By using the above mentioned measuring circuit 21
~13(~3~
~ or the oscilloscope 41, the sheet 14 is inserted into the
: gap 15 of the cavity resonator 11 and the saw-tooth wave
volta~e from the saw-tooth wave oscillator 18 is applied to
the VCO 17. Upon application thereto of the saw-tooth wave,
the VCO 17 produces a microwave signal frequency-modulated
in accordance with the.level of the applied voltage which in
turn is led to the cavity resonator 11 by way of the wave
guide 16. When the oscillating frequency introduced approaches
the resonant frequency of the cavity resonator 11, the
detected output is guided out from the detector 20 through
the wave guide 19.. In this case, if the sheet 1~ has a
given weight, and the resonant voltage value and the reso-
nant frequency.are L(V) and l~f/s), a resonant curve is
depicted as indicated by curve I in Figure 5. As the
moisture content of the object with the given weight in-
creases, the reduction of the microwave increases t SO that
the resonant voltage decreases from L(V) to L(M) and the
resonant curve changes to become a curve II. When the
moisture content further increases, the resonant voltage
decreases from M(V) to N(~) and the resonant curve becomes a
curve III.
The relationship between water content of the
sheet or object to be~measured and the reduction of the
microwave transmitted therethrouyh is as shown in Figure 6,
25 as a result of the measurementO As seen from the Figure,
the reduction substantially linearly changes with increase
of the moisture content. Therefore, by Measuring the
resonant voltage by the measuring circuit 21 or the oscillo-
scope 41 microwave and measuring the reduction of the
microwave, a percentage of moisture content proportional to
L3(~8(~
the reduction may be obtained.
When the sheet 14 contains water with a given
percent of moisture, the resonant voltage is M(V) and the
amount of a frequency deviation from the resonant frequency
detected when no object is inserted in the gap of the cavity
resonator is l(f/s), then the resonant curve obtained is
depicted as curve I in Figure 7. When the weight of a sheet
with given moisture content changes, the resonant frequency
de~reases to be m~f/s~, ( m) and the resonant curve changes
to become curve II. When ~he weight increases, a deviation
of the resonant frequency decreases from m(f/s), to n(f/s), --
and its resonant curve is depicted as curve III.
The experiment showed a relationship between the
weiyht and the resonant frequency detected as shown in
Figure 8. As shown, the~resonant fxequency substantially
linearly changes as the weight increases. Accordingly, by
measuring the deviation of the resonant frequency by the
measuring circui-t 21 or the oscilloscope 41, a weight
proportional to the-resonant frequency displacement may be
obtained.
Having described a specific embodiment of the
invention, it is obvious that modification and variation of
the invention is possible in light of the above teachings.
