Microwave moisture measurement

A moisture content measuring system for sheet material which uses microwave radiation. Two pairs of microwave radiators and receivers are combined with surface and below surface temperature measuring sensors to furnish data to a computer which interprets the data and yields moisture readings. Each pair of microwave radiator and receiver straddles the sheet test sample and checks microwave transmission through the sample and reflected from it, but the two radiators are cross polarized so that signal interchange between them is avoided.

SUMMARY OF THE INVENTION 
This invention deals generally with the application of radiant energy to 
material inspection, and more specifically with the use of microwave 
radiation to measure the moisture content of wood and other dielectric 
materials as they move along a processing line. 
Microwave inspection of materials for moisture content has been suggested 
by U.S. Pat. No. 3,644,826 by Cornetet which accomplishes the task by 
evaluating the energy transmitted by the material and that scattered by 
it. However, such a system is subject to numerous problems, particularly 
those caused by changes in the test material's transmission 
characteristics due to temperature of the material and temperature 
differential through the material. Moreover, typical materials have 
localized variations, the effects of which cannot be recognized by any 
measurement system which depends on a single localized sensor. 
The present invention provides an accurate, high speed, continuous 
production line measurement system which overcomes the problems of a 
single sensor location, but also avoids the problem of cross-talk, the 
interchange of radiation between the radiator of one station and the 
receiver of another station. Furthermore, temperature sensors for both 
surface readings and below surface depths are included in the present 
invention to better interpret the microwave signal results with regard to 
temperature related phenomena. 
The present invention also uses a computer to interpret the great quantity 
of data generated and thus permits the system to operate both continuously 
and at high production speeds, which would be impossible if the operator 
were required to evaluate the data. 
The moisture content evaluator of the present invention is constructed with 
two microwave radiators located near each other past which the material to 
be tested moves while supported on rollers. These two radiators are cross 
polarized to each other and one irradiates the bottom surface of the test 
material while the other irradiates the top. 
Each radiator has associated with it two microwave receivers to capture the 
radiation transmitted through the test material and that reflected from 
it. One receiver structure for each radiator is directly opposite the 
radiator but located on the opposite side of the test material so that it 
receives only the radiation transmitted through the test material. The 
other receiver is located within the same waveguide structure as the 
radiator and therefore receives radiation reflected from the material. 
This arrangement places one radiator on each side of the material and a 
receiver for each radiator on each side of the material. Since the sets of 
radiators and receivers are relatively close in order to simultaneously 
test the same region of the test material and to be able to test material 
of narrow width, there is some danger that each receiver will receive not 
only its own radiator's signal, but also the signal from the other 
radiator. 
In order to overcome this problem without the need for two microwave 
sources of different frequencies the radiators are first located on 
opposite sides of the material and also the sets are oriented so that they 
are cross polarized to each other. Thus one radiator, for instance the one 
located under the test material, is oriented so that its H plane is 
parallel to the direction of motion of the test material and the other 
radiator is oriented so that its H plane is perpendicular to the test 
material's movement. The matching receiver structures, being in the same 
orientation as their respective radiators, are therefore cross polarized 
to the other radiator's signal and are little affected by it. Therefore, 
despite the proximity of the two irradiating systems and the fact that 
they operate at the same frequency, they are not affected by each other. 
Mere measurement of microwave transmission characteristics of test 
materials has, however, been found to be unreliable to properly measure 
moisture content. Since the microwave transmission characteristics and the 
moisture retention of most materials vary with temperature, it appears 
that material temperature is vital to evaluate the actual moisture 
present. To that end the present invention also reads material 
temperature, not at one location, but at three. 
The moisture measurement system uses independent sensors to measure the 
temperature of the material's top surface, the bottom surface and an 
interior point below surface depth. The data thus accumulated is fed, 
along with the information from the microwave receivers to a data 
reduction device, which in the preferred embodiment is a computer. 
The use of a computer permits the continuous accumulation of the several 
readings including six from the microwave system and three from the 
temperature sensors, along with location of the readings on the test 
material, to be interpreted into valid moisture content indiction on a 
moment-to-moment basis, even in a production line situation. 
The actual evaluation of moisture content is not done on the basis of some 
theoretical calculation, but rather by testing materials with known 
moisture content and recording that data for comparison to the results of 
unknown materials being tested. This comparison is also advantageously 
performed quickly by the computer. 
The apparatus of the present invention therefore supplies continuous 
moisture content reading in a production line environment with those 
reading accurately indicating the moisture present in a manner which 
directly compares to previous methods of measurement, despite the fact 
that the previous methods were essentially sampling methods testing only 
occasional pieces and furnishing results only after long delay.

DETAILED DESCRIPTION OF THE INVENTION 
The FIGURE depicts the preferred embodiment of moisture measurement 
apparatus 10 of the present invention in which sample sheet 12 moves in 
direction A over rollers 14 and through various measuring devices. 
Two microwave radiators 16 and 18 are located above and below sheet 12, 
respectively, and these radiators are cross polarized relative to each 
other to minimize the effect of each on the other. Radiator 16, above 
sheet 12, contains two power measuring devices, detector 20 which measures 
the power delivered to radiator 16, and detector 22 which measures the 
power reflected back into radiator 16 from sheet 12. Electrical signals 
representing these power levels are fed to computer 24 by cables 21 and 23 
attached to the detectors. 
Receiver 26 located across sheet 12 from radiator 16, intercepts the 
microwave power transmitted through sheet 12. This power level is 
converted into a proportional electrical signal by detector 28 and is fed 
to computer 24 by cable 29. Waveguide termination 30 is matched to the 
system so that no reflection occurs to cause any error. 
The original source for the microwave signal is a typical microwave 
generator (not shown) fed into adapter 32. Phase shifter 34 is then 
adjusted to maximize the power being delivered to sheet 16, as indicated 
by detector 20. 
The system of measurement associated with radiator 18 operates in the same 
manner. Microwave power is fed from the source (not shown) into adapter 
36, adjusted by phase shifter 38 and the forward power measured by 
detector 40, with the reflected power measured by detector 42, and sent to 
computer 24 by cables 41 and 43. 
On the other side of sheet 12, the microwave radiation transmitted through 
sheet 12 is intercepted by receiver 44, measured by detector 46 and sent 
to computer 24 by cable 47. The signal is absorbed in that arm by 
termination 48. 
The signal source in the preferred embodiment generates 20 milliwatts of 
power at 5.85 GHZ at each of two outputs. This power is transmitted to 
adapters 32 and 36 by cable 50 and 52 so that long runs of waveguide are 
not required and it enables convenient physical adjustment of the 
waveguide test apparatus. 
Three temperature measurements are taken by the preferred embodiment. 
Surface measurements are taken on the top surface by temperature 
measurement means 54 and sent to computer 24 by cable 55, while 
temperature measurement means 56 reads the bottom surface temperature and 
communicates with computer 24 by cable 57. In the preferred embodiment 
both surface temperature measurement devices are passive infrared 
thermopiles with cold junction cooling. 
Temperature measurements of the internal volume of sheet 12 are taken by 
scanner 58, which measures the temperature below the top surface and sends 
it to computer 24 by cable 59. 
It can readily be appreciated that the nine sources of signal previously 
enumerated generate a considerable amount of data, particularly if being 
used on a continuous production line. Computer 24 is therefore more than a 
mere convenience. It is rather a necessity to accommodate to continuous 
acquisition of a high volume of data. While individual laboratory samples 
could be tested by manually recording and comparing data, production 
speeds require the use of some means to acquire, record and compare the 
data at speeds which make manual recording unsatisfactory. A typical 
production speed for the preferred embodiment is 60 feet per minute and at 
that speed one sample period covers approximately one inch of running 
length. It would therefore be quite practical to produce readings at the 
rate of one per second. 
Computer 24 not only permits this radid acquisition of data, but also 
permits instantly computing the moisture level from a formula developed 
from the data from previous moisture content evaluations. These previous 
sets of data were secured with individual test samples for which microwave 
and temperature data were secured and then the moisture measured by 
conventional methods. Generally this test checks against previous methods 
to an accuracy of approximately one percent. It would also be practical, 
by use of the computer, to compare new data directly to the previous data 
and thus measure moisture content. 
The preferred embodiment shown in the FIGURE also includes material marking 
device 60. In the preferred embodiment this is cutting wheel 62 lowered 
onto sheet 12 by solenoid 64. This action is timed and initiated by 
computer 24 as the various measuring devices are activated. The marking is 
used essentially to indicate the exact area in which a test is being run, 
so that sample verification by other means can be accomplished if desired. 
This marking device need not be a cutting wheel and could instead be a dye 
marker or other such device in the appropriate application. 
It is to be understood that the form of this invention as shown is merely a 
preferred embodiment. Various changes may be made in the function and 
arrangement of parts; equivalent means may be substituted for those 
illustrated and described; and certain features may be used independently 
from others without departing from the spirit and scope of the invention 
as defined in the following claims. 
For example, other temperature measuring devices could be used.