Microwave moisture sensor

A microwave moisture sensor comprising a first transmission and reception system comprising first transmission and reception horn antennas disposed in confronting relation with respect to each other and with a movable sheet like measurand disposed therebetween; and a second transmission and reception system comprising a second transmission horn antenna for re-emitting a microwave, as received by the first reception horn antenna against the measurand, and a second reception horn antenna for receiving the microwave emitted by the second transmission horn antenna and having passed through the measurand. The antennas disposed on one side of the measurand are spaced from each other by a distance of n.lambda..+-..lambda./4, wherein n is an integer and .lambda. is the wavelength of the microwave, in a direction substantially perpendicular to the plane of the measurand.

BACKGROUND OF THE INVENTION 
1. Field of Invention 
This invention relates to a microwave moisture sensor which operates on the 
principle that an emitted microwave is attenuated in rotary resonance with 
water molecules; and more particularly, to such a sensor comprising at 
least two transmission and reception systems, each comprising a 
transmission horn antenna and a reception horn antenna, with the antennas 
which are disposed on one side of a body being measured being spaced a 
predetermined distance in a direction perpendicular to the plane of the 
body. 
2. Description of the Prior Art 
Microwave moisture sensors are known in the art. One example of such a 
sensor is shown in FIG. 1. The illustrated sensor includes a transmission 
horn antenna 4 for emitting a microwave supplied from a microwave 
oscillator 2, through an isolator 1, toward a measurand 3 (that is, an 
object being measured), such as a sheet of paper, a reception horn antenna 
5 disposed in confronting relation to transmission horn antenna 4 with 
paper sheet 3 disposed therebetween, a transmission horn antenna 6 for 
re-emitting the microwave, as received by reception horn antenna 5, toward 
paper sheet 3, and a reception horn 7 disposed in confronting relation to 
transmission horn 6 with sheet 3 interposed therebetween. Horns 5 and 6 
may be interconnected by a coaxial cable (not numbered). The prior sensor 
also includes a detector (comprising, for example a Schottky diode) 8 
which is energized by a local oscillator, not shown, and which supplies a 
frequency signal having a frequency which is different from that of the 
signal from microwave oscillator 2. Detector 8 detects the microwave 
received by reception horn antenna 7. The sensor further comprises a 
signal processor 9 supplied with a signal from detector 8, and a preset 
signal indicative of the basis weight of paper sheet 3, and processes 
these supplied signals based on a predetermined calibration or working 
curve to generate a moisture percentage signal. Isolator 1, microwave 
oscillator 2, horn antennas 4,7 and detector 8 are normally housed in a 
first metal casing. Horn antennas 5,6 are normally housed in a second 
metal casing. 
As illustrated in FIG. 2, the first and second casings 10, and 11, 
respectively, are disposed in confronting relation on a centrally open 
frame 12 and jointly constitute a detector head 13. First and second 
casings 10,11, which may be of metal, synchronously scan or reciprocally 
travel over an interval or distance between limits L.sub.1 and L.sub.2, 
for detecting signals. The scanning direction extends substantially 
transversely of the moving direction of paper sheet 3, which moves in the 
direction of arrow A. 
While detector head 13 scans sheet 3, the transmission and reception horn 
antennas are kept at a constant distance l, as shown in FIG. 1, from sheet 
3, with the transmission and reception horn antennas 4 and 5, 6 and 7 
being equidistant from the travel of the paper sheet 3 as positioned in 
FIG. 1. 
Moisture measurement is effected by the microwave moisture sensor while the 
interval L.sub.1 -L.sub.2 is being scanned by detector head 13. At this 
time, a microwave emitted from transmission horn antenna 4 is propagated 
through a path from transmission horn antenna 4, to paper sheet 3, to 
reception horn antenna 5, via coaxial cable not numbered to transmission 
horn antenna 6, to paper sheet 3, to reception horn antenna 7, and is 
finally detected by detector 8. Signal processor 9 is fed with a signal, 
as detected by detector 8 and a preset signal, representative of the basis 
weight of paper sheet 3, and processes the supplied signals based on a 
calibration or working curve, thereby to generate a moisture percentage 
signal. 
A voltage V.sub.1 ( of a standing wave) in the propagation path can be 
derived on the basis of a wave motion from the following equation, 
provided there is no attenuation in the propagation path. 
EQU V.sub.1 =V.sup.+ e.sup.-j.beta.x +.GAMMA.V.sup.+ e.sup.j.beta.x ( 1) 
wherein V.sup.+ is the output voltage of the microwave oscillator, .GAMMA. 
is the reflectivity, .beta. is the phase constant, and x is the 
propagation path length from the microwave oscillator. The amplitude 
signal .vertline.V.sub.1 .vertline. in equation (1) is expressed by 
following equation (2), while the ratio (standing wave ratio) of the 
maximum value .vertline.V.sub.1max .vertline. to the minimum value 
.vertline.V.sub.1min .vertline. of amplitude signal .vertline.V.sub.1 
.vertline. is expressed by the following equation (3). The phase, 
amplitude and other parameters of tne voltage V.sub.1 vary with vibrations 
of paper sheet 3. 
##EQU1## 
The conventional microwave moisture sensor is, however, disadvantageous, in 
that the measurement errors are increased since the detector also detects 
influences due to vibrations of paper sheet 3 which occur during its 
movement. 
The actual propagation path of the microwave includes, in addition to the 
illustrated propagation path, paths in which the microwave is reflected 
between confronting surfaces of casings 10,11, and between these 
confronting surfaces and sheet 3. Since such additional propagation paths 
vary due to vibrations of the travelling paper sheet 3, to thereby change 
the amount of microwaves falling on tne respective reception horn 
antennas, measurement errors are also increased. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the invention is to improve the prior art and to 
overcome the aforementioned and other disadvantages and deficiencies of 
the prior art. 
Another object is to provide a microwave moisture sensor which is less 
susceptible to influences due to vibrations of a sheet like material being 
measured, thereby to achieve high measurement accuracy. 
The foregoing and other objects are attained by the invention, which 
encompasses a microwave moisture sensor comprising a first transmission 
and reception system comprising first transmission horn antenna and first 
reception horn antenna, disposed in confronting relation to each other, 
with a sheet like measurand, such as a sheet of paper, disposed movably 
therebetween; and a second transmission and reception system comprising a 
second transmission horn antenna for reemitting a microwave as received by 
the first reception horn antenna, against the measurand, and a second 
reception horn antenna for receiving the microwave emitted by the second 
transmission horn antenna and having passed through the measurand. The 
antennas disposed on one side of the measurand are spaced from each other 
by a distance which is equal to n.lambda..+-..lambda./4, wherein n is an 
integer and .lambda. is the wavelength of the microwave, and in a 
direction substantially perpendicular to the plane of the measurand.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 3 schematically depicts a microwave moisture sensor embodying the 
invention. Simiiar or corresponding parts in FIG. 3 are denoted by similar 
or corresponding reference characters in FIG. 1 and will not be described 
hereat in detail for sake of clarity of description. 
The microwave moisture sensor comprises a first transmission and reception 
system 15 and second transmission and reception system 16 and other 
components already described in FIG. 1. The first system 15 comprises a 
transmission horn antenna 4 spaced a distance l.sub.1 from a paper sheet 3 
(which is the measurand) and a reception horn antenna 5 spaced a distance 
L from the transmission horn antenna 4, in confronting relation therewith. 
The second system 16 comprises a transmission horn antenna 6 for 
re-emitting a microwave, as received by reception horn 5 of the first 
system 16, and a reception horn antenna 7 spaced a distance l.sub.2 from 
paper sheet 3 and at a constant distance L from transmission horn 6 in 
confronting relation therewith. Reception horn antenna 7 and 5 is spaced, 
respectively, a distance l.sub.3 from transmission horn antenna 4 and 6 in 
a direction substantially perpendicular to the plane of paper sheet 3. 
Distances l.sub.1, l.sub.2, and l.sub.3 meet the following relationship. 
EQU l.sub.3 =l.sub.2 -l.sub.1 .div.n.lambda..+-..lambda./4 (4) 
wherein n is an integer and .lambda. is the wavelength of the microwave. 
As illustrated in FIG. 4, transmission and reception systems 15 and 16, are 
housed in casings 19,20, which may be of metal, having confronting 
surfaces 17,18 made of a wave absorber, such as comprising a mixture of 
epoxy paint and brass powder. The casings 19,20 are mounted in a centrally 
open frame, such as shown in FIG. 2, and jointly constitute a detector 
head 21, which is used for scanning sheet 3, for a predetermined distance, 
transversely across the paper sheet. 
FIG. 5 shows a cross section of casings 19,20, taken along the direction of 
travel (see arrow A in FIG. 4) of paper 3. As shown in FIG. 5, confronting 
surfaces 17 of casing 19, are inclined at angle .theta. with respect to 
the confronting surface 18 of casing 20. 
Moisture measurement is effected by the microwave moisture sensor while a 
predetermined distance over paper 3 is being scanned by detector head 21. 
At this time, a microwave emitted from transmission horn antenna 4, is 
propagated through a path from transmission horn antenna 4, to paper 3, to 
reception horn antenna 5, then via a coaxial cable, not numbered, to 
transmission horn antenna 6, to paper 3, to reception horn antenna 7, and 
is finally detected by detector 8. 
Since equation (4) is established for each transmission and reception 
system, reception horn antenna 7 can be regarded as substantially 
detecting the sum .vertline.V.sub.1 .vertline.+.vertline.V.sub.2 
.vertline., of the amplitude signal .vertline.V.sub.1 .vertline. defined 
by equation (2) and the amplitude signal .vertline.V.sub.2 .vertline. 
defined by equation (5), the signal .vertline.V.sub.2 .vertline. is 
.lambda./4 out of phase with signal .vertline.V.sub.1 .vertline.. 
##EQU2## 
Therefore, the ratio of the maximum value .vertline.V.sub.3max .vertline. 
to the minimum value .vertline.V.sub.3min .vertline. of a signal detected 
by detector 8 is given by equation (6). 
##EQU3## 
Equations (3) and (6) are indicative of variations of the detected signal 
at the time the path lines (i.e. path of travel) of paper sheet 3 is 
varied. Since 0&lt;.GAMMA.&lt;1, the relationship 
(1+.GAMMA.)/(1-.GAMMA.)&gt;.sqroot.1+.GAMMA..sup.2 exists. Comparison of the 
equations (3) and (6) indicates that with the inventive microwave moisture 
sensor, the detected signal has a small amplitude with respect to 
vibrations of paper sheet 3. That is to say, the inventive microwave 
moisture sensor has good path line characteristics. 
Where the antennas in the transmission and reception systems 16,15 meet the 
following relationship (7), the sensor can detect signals of good path 
line characteristics. 
EQU l.sub.1 -l.sub.2 .div.n.lambda..+-..lambda./4 (7) 
FIGS. 6A and 6B illustrate the relationship between the amplitude signals 
.vertline.V.sub.1 .vertline., .vertline.V.sub.2 .vertline. and the signals 
.vertline.V.sub.1 .vertline.+.vertline.V.sub.2 .vertline. detected by 
detector 8. FIG. 6(A) shows the relationship at the time the reflectivity 
.GAMMA. is 0.1. FIG. 6(B) shows the relationship at the time the 
reflectivity .GAMMA. is 0.2. FIGS. 6(A) and 6(B) indicate that the 
inventive microwave moisture sensor has good path line characteristics. 
Operation of the embodiment, and especially the multipaths taken by the 
microwave, is now explained with reference to FIG. 5. The microwave 
emitted from antenna 4 against paper 3 and passing through the above 
described path is main beam B.sub.M. Side beams B.sub.S emitted from 
antenna 4 are either absorbed by the wave absorbing confronting surfaces 
17,18; or are led out of casings 19,20, after going through repeated 
reflections between confronting surfaces 17,18 and paper 3, and also 
between the confronting surfaces 17,18, as shown by the broken line arrows 
in FIG. 5. 
The amounts of microwaves falling on antenna 5,7 are not affected by a 
spread around the transmitted column of main beam B.sub.M, which is 
coextensive with the open ends of the antennas, and a moisture signal g 
(MW) is rendered, irrespective of the size of the paper sheet being 
measured (thus resulting in a substantially reduced measurement area). 
Hence, signal characteristics, due to the sheet like paper, agree with the 
calibration or working curve determined by using paper of a brand to be 
measured which is cut to size. 
FIG. 7 illustrates paper position characteristic in the inventive sensor. 
The vertical axis is indicative of a moisture signal g (MW). The 
horizontal axis is indicative of the position of sheet 3 between the 
casings. A characteristic curve A.sub.1 is drawn by plotting moisture 
signals g (MW) when an end 3a of paper 3 is moved from position X.sub.0 to 
X.sub.1 to X.sub.2. Characteristic cuve A.sub.1 shows that when paper 
sheets having widths 2X.sub.0 X.sub.2 and 2X.sub.1 X.sub.2 are positioned 
with their centers aligned with the position X.sub.2, the moisture signal 
g(MW) remains unchanged regardless of the different paper widths. Stated 
in a different way, the microwaves which are repeatedly reflected between 
positions X.sub.0, X.sub.1, that is, the microwaves reflected between the 
confronting surfaces 17,18 and the paper sheet 3, and between the 
confronting surfaces 17,18, do not fall on reception horn antenna 5. 
Hence, the moisture signal is not affected by differences between sizes of 
measurands. 
A characteristic curve A.sub.2 is representative of a conventional 
microwave moisture sensor (the confronting surface of casing 20 is 
indicated by dot-and-dash line Y), 
FIG. 8 depicts another illustrative embodiment. Identical and corresponding 
components in FIG. 8 are denoted by identical and corresponding characters 
in FIGS. 1 and 3, and will not be described hereat in detail for sake of 
clarity of description. The sensor in FIG. 8 comprises two sets of 
transmission and reception systems 15,16, to provide a propagation path 
wherein a main microwave beam, supplied by oscillator 2, through isolator 
1, and emitted from leftmost transmission horn antenna 4 against paper 
sheet 3, passes through paper sheet, four (4) times, before the microwave 
is detected by detector 8. 
The detecting sensitivity of the sensor of FIG. 8 is thus increased because 
of the increase, i.e. doubling, of the two systems in the manner depicted. 
With the microwave moisture sensor of the invention, there are provided at 
least two transmission and reception systems, each comprised of 
transmission and reception horn antennas. The antennas are disposed on one 
side of a measurand (i.e. object being measured) at a prescribed distance 
from each other and in a direction which is perpendicular to the plane of 
the measurand. This arrangement reduces any adverse influence due to 
vibrations of the measurand and thereby results in an increased 
measurement accuracy. 
The confronting surfaces of the casing which constitute the detector head 
are wholly or partly inclined with respect to each other, and are covered 
with a wave absorber. This also reduces any adverse influence due to 
vibration of the measurand. 
The foregoing description is illustrative of the principles of the 
invention. Numerous modifications and extensions thereof would be apparent 
to the worker skilled in the art. All such modifications and extensions 
are to be considered to be within the spirit and scope of the invention.