Note: Descriptions are shown in the official language in which they were submitted.
108~305
This invention relates to apparatus for measuring the electrical
impedance of materials, and in particular relates to a contoured electrode,
a part of the apparatus for measuring the electrical impedance of material,
shaped to accommodate different types of sample material in order that the
electrical field radiated by the electrode passes through a sufficient
portion of material sample enhancing the accuracy of the impedance measure-
ment.
Optimizing the yield of various manufacturing processes often
requires very accurate monitoring of the moisture content of a given mater-
ial. For example, the milling of wheat is carried on most efficiently when
the wheat has a 15% moisture content. ~uring the pulping of wood chips,
the moisture content of the wood chips must be known in order to determine
the proper amounts of liquor necessary to be added for maximum delignif-
ication. The required accuracy for the measurement of moisture varies for
different materials and is shown below for some representative materials:
Typical Accuracy
MaterialMoisture Range of Moisture Reading
Wood Chips30%-70% +2%
Wheat 8%-15% +0.2%
Paper 5%-10% +0.5%
Plywood Veneer 3%-8% +0.5%
Potato Chips 12%-17% +0.5%
Lumber 30%-70% +3%
Tissue Paper 3%-10% +0.5%
The disclosures in the above-identified related patents should be
consulted to put the instant invention in perspectiveJ and for an in-depth
description of the component parts of a moisture detection system.
In order to make electrical impedance measurements of a sample mat-
erial with the degree of accuracy necessary to determine moisture content,
it has been found
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that it is desirable to construct the electrode to accommodate the different characteris-
tics of the material to be sampled. For example, particulate materials of relatively
uniform size and shape, such as grain or the like, are normally free flowing and are not
subject to blocking when flowing through an area confined by an impedance measuring
electrode. Other materials, such as corn fibers, potato chips and wood chips, are not
nearly so free flowing, and it has been found that additional handling systems are
generally desirable to pack the material and to transport it past the impedance measuring
electrode.
The flow problem associated with the material to be sampled is particularly
acute with wood chips, since, unlike grain, the size and shape variations in the chips tend
to cause "bridging" as they pass the impedance measuring electrode. Sticky wood chips,
such as those freshly cut during the early summer months, have an even greater than
normal tendancy to "bridge" or lodge in a sample box which also functions as an impedance
measuring electrode.
A more exhaustive discussion of the problems associated with wood chip
moisture measuring apparatus may be found in an article entitled IFactors Affecting
Automatic Wood Chip Moisture Measurement" by F. K. Preikschat, Ekhard Preikschat and
Daniel F. Pope, published in Paper Trade Journal dated July 29, 1974. This same article
also describes a sample box suited for use with wood chips including a clam shell door at
, 20 the lower end of the sample box.
In some manufacturing operations, such as pulp manufacturing, the bulk
density measurement of the wood chips can be as important as a determination of the
moisture content. Regardless of what type of digester is used in the pulping operation,
the wood chips often are initially measured on a volumetric basis which means that in
order to determine the total weight of chips entered into the digester the bulk density
must be known. If both the moisture content and the bulk density of the wood chips in the
material are known, these factors can then be used to determine the correct amount of
liquor that is needed for the digester process itself.
In the digester process, it turns out that the wood to liquor ratio is one of
the most important control parameters. The reason for this is that in order to obtain an
optimum delignification, i.e., a dissolution of the lignin bonding the wood fibers, a certain
, amount of liquor has to be added to a given amount of wood chips ~o obtain an optimum
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delignification with minimum destruction to the wood fiber itself. Typically, the moisture
content of the wood chips can vary over a 30% moisture range which is typically 35% to
65%. By the same token, the bulk density can also vary tremendously as determined by
the size distribution of the wood chips, wood species, the type of chips used, e.g., core
wood or sap wood, and the type of growth conditions encountered. Since there are so
many variables involved, once the wet weight bulk density is related to the moisture
content of the wood chips, it has been found that the bulk density may still vary over a
considerable range. For example, under Southern growth conditions, it has been found
that even though the moisture content does not vary by more than 4%, the buLc density
lO can still vary by more than +15%.
Furthermore, wood chips are not homogeneous and even those in the same
pile can differ widely in their physical makeup. Chips are produced from individual trees,
which themselves are trucked to a common point from a wide surrounding area. If the
moisture and bulk density of the chips contained in a storage pile were measured, both the
moisture and bulk density would vary in a random fashion. Most often these parameters
follow a normal distribution curve having an average deviation in moisture of +15% and an
average deviation in bulk density of about +~5%.
In the case of a continuous digester, wood chips and liquor are added in
predetermined amounts simultaneously and continuously so as to produce a pulp of a
20 certain quality. Here again, wood chips are added on a volumetric basis, as measured by
the revolutions of the star feeding valve. The star feeding valve has a certain number of
pockets, each with a given volume which makes it possible to readily calculate the
volumetric feeding rate. If the wet weight bulk density of the wood chips is known, the
volumetric feeding rate of the star valve can readily be translated into a wet weight flow
of chips. This, in conjunction with the known or measured moisture content, can be
translated into a bone dry weight flow of chips.
A more detailed discussion of pulp processing methods and the effects of
variations in moisture content and bulk density may be found in an article entitled
"Comparison of Pulping Methods" by Burton E. Helberg et al., published in TAPPI, Vol. 59,
No. 5, May 1976.
Ln some manufacturing, a moisture control test may be used to ensure
quality of an end product by measuring the moisture content to determine if it is within
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predetermined limits. For example, it is important that facial tissue when shipped from
the manufacturer have a moisture content which does not exceed predetermined limits.
At present, the moisture content of facial tissue is normally determined on a spot check
basis. However, it would be desirable if all rolls or stacks of tissue paper could be
checked for moisture content on a continuous basis prior to packaging and shipping
without adding significantly to production cost.
Still another application where it is desirable to accurately measure
moisture content is in the drying of lumber. One method of drying lumber involves
positioning the rough cut wood in a closed heated structure, such as a kiln or the like, for
10 a period of from 1 to 5 days. The drying rate is not a constant but depends on a number of
variables such as the type of lumber, initial moisture content, temperature within the kiln,
etc. Presently there is no reliable way to measure the drying rate or to determine the
point during the drying cycle at which a desired amount of moisture has been removed
from the wood. A more precise and accurate measurement of moisture content during
; this drying cycle is important for sever~l reasons. First, the cost of energy and heating in
,
recent years has increased dramatically and it has become economically imperative that
the drying time be kept to a minimum. Second, when wood is overdried it becomes very
difficult to work with wood working machines, such as lathes and planers, further
increasing lumber production costs. Under current practice, if lumber has been overdried,
it is often rewetted in order to raise its moisture content to approximately 10%.
One known method of ascertaining dryness presently used by kiln operators
involves manual spot checking of the moisture content of individual pieces of lumber in
the kiln. This procedure involves the use of a two-prong resistance gauge which is pressed
against the surface of a piece of lumber to determine surface resistance. This resistance
reading is then related to moisture content via a chart or formula. ~naccuracies are
inhere~nt in this method since the resistance reading on the gauge can vary depending on
the amount of contact pressure, the temperature of the wood, the surface properties of
the wood, and its position in the stack, in addition to the moisture content of the lumber
itself.
The present invention discloses apparatus for determining moisture content
- of a variety of samples of materials by accurately measuring the electrical impedance of
the material sample. The present invention is particularly concerned with shaped sensing
:
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electrodes which are sized and contoured to accommodate the varying character-
istics of the material being sampled so that the electrical field radiated
by the electrode passes through a substantial portion of the material sample
enhancing the accuracy of the impedance measurement.
According to the invention there is provided an electrode which is
part of an apparatus for measuring the impedance of a sample of material,
said electrode being formed as a sample box for receiving a sample of .
material through which an electrical field is radiated, comprising: front,
rear and a pair of opposed, flat side walls of conductive material, together
forming a rectangular container in which material to be tested is deposited;
an inlet means situated at one end of said sample box, including an opening
which is si~ed to receive a sample of material therethrough; an outlet means
situated at the other end of said sample box, adapted to exhaust the sample
of material after its electrical impedance has been measured; and a flat
electrode plate of conductive material mounted within said sample box and
situated with its surfaces substantially parallel with and substantially
equidistant from said opposed, flat side walls, said electrode plate being
electrically insulated from the walls forming said sample box.
According to another aspect of the invention there is provided a
measuring apparatus for measuring the characteristics of a sample of material,
comprising: a sample box having a substantially constant volume and includ-
ing: front, rear and a pair of opposed, flat side walls of conductive
material, together forming a rectangular container in which material to be
tested is deposited; an inlet means situated at one end of said sample box,
including an opening which is sized to receive a sample of material there- .. -
thTough; an outlet means situated at the other end of said sample box,
adapted to exhaust the sample of material after its characteristics have
been measured; and, a flat electrode plate of conductive material mounted
within said sample box and situated with its surfaces substantially parallel
With and substantially equidistant from said opposed, flat side walls, said
electrode plate being electrically insulated from the walls forming said
sample box; bulk density means connected to said sample box for determining
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the bulk density of said sample of material by measuring the weight of said
sample of material within said sample box~ and including a readout means
providing a visual indication of the weight of said sample of material within
said sample box; and, moisture measuring means interconnected with said flat
electrode plate in said sample box and with said bulk density meansl and
having an output signal which is proportional to the moisture content of
said sample of material and which is compensated for variations in bulk
density of said sample of material.
In an embodiment of the instant invention particularly suited for
measuring the impedance of a sample of wood chips in order to determine its
moisture content, the electrode is shaped as a novel sample box in which the
wood chips are deposited. A unique dumping system eliminates any tendency
of the wood chips to hang up or bridge against the side walls of the sample
box. A mechanically integrated system incorporating a particularly shaped
outer ground electrode and an active central electrode plate also includes
a means for measuring both the temperature and weight of the sample of
material so that the output of a sensing circuit connected to the electrode
can be corrected to reflect both temperature and w0ight variations.
According to yet another aspect of the instant invention, a bulk
density circuit and a temperature circuit for a sample box are each provided
with a direct readout device so that the bulk density and temperature of
the sample of material within the constant volume defined by the sample box
can be visually observed and utilized in the processing of the sample of
material.
FIGURE 1 is a sectional elevation of a first embodiment of a
shaped electrode usable with free flowing material, such as grain, and
including a schematic diaBrmm of ~n impedance sensing circuit.
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FIGURE 2 is a sectional view of the shaped electrode taken along line 2-2 in
FIGURE L
FIGURE 3 is a second embodiment of a shaped electrode particularly suited
for measuring the electrical impedance of stacked paper or the like.
FIGURE 4 is a schematic diagram of a sensing circuit usable with the
electrode embodiment of FIGURE 3.
FIGURE 5 is a side elevational view of an embodiment of a shaped electrode
according to the instant invention particularly suited for measuring the electrical
impedance of bulk materials, such as wood chips.
FIGURE 6 is a side elevational view of the electrode of FIGURE 5 but with
the sample box shown in an open position.
FIGURE 7 is a perspective view of the electrode of FIGURE 6.
FIGURE 8 is a perspective view of a fourth embodiment of a shaped
electrode particularly suited for measuring the electrical impedance of stacked lumber in
a kiln.
Referring initially to FIGURE 1, a first embodiment of an electrode 10 is
illustrated as part of an apparatus for measuring the impedance of a sample of material
and, in turn, relating this impedance to a characteristic of the material, such as its
moisture content. In addition to electrode 10, the system includes a sensing circuit 12, a
ao bulk density circuit 14 responsive to weight variations and a temperature circuit 16. A
signal generator 18 is colmected to sensing circuit 12 by lead 20 and supplies a stabilized
reference signal of a cerhin frequency and phase. Sensing circuit 12 is also connected to
the active portion of electrode 10 by lead 22 and provides a high frequency electrical
signal which creates a high frequency electrical field in the material whose impedance is
to be measured
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Although described in greater detail in the above-identified U.S. patents, the
output signal from sensing circuit 12 is directly proportional to the moisture content of the
sampled material when properly corrected for variations in temperature and bulk density,
if necessary. Thus, the output of sensing circuit 12 is connected via lead 24 and amplifier
26 to operational transconductance amplifier 28. Amplifier 28 includes a control input
lead 25 which varies the amplification factor of the amplifier. A load cell 27 provides a
continuous output signal which is proportional to the weight of the fixed volume of
materials situated within electrode 10. The output signal from load cell 27 is connected to
bulk density circuit 14 In preferred form bulk density circuit 14 comprises an amplifier 29
and an adjustable voltage divider 3L In operation, amplifier 28 multiples the output signal
from sensing circuit 12 by the output signal from bulk density circuit 14 in amplifier 28 so
that the signal at the output of amplifier 28 is corrected for bulk density variation of the
sample material by bulk density circuit 14~
In a similar manner, the moisture measuring system for grain also includes a
temperature circuit to compensate for temperature variation in the sample materiaL In
preferred form, temperature circuit 16 comprises an amplifier 33 which is connected to a
variable voltage divider 34. An operational transconductance amplifier 38 includes a
control input lead 39 which varies the amplification factor of amplifier 38. A
temperature sensor 32 is connected to the input of amplifier 33 and provides a continuous
; ~ 20 output signal which is proportional to the temperature of the sample material situated
within the sample box~ In operation, amplifier 38 multiplies the output signal from
operational amplifier 28 by the output signal from temperature circuit 16 to compensate
for variation in the temperature of the sample materiaL
An important aspect of the present invention is that static bulk density and
temperature of the sample material can be directly read and used, if required, in a
;1 manufacturing process A display device 41, preferably calibrated in bulk density per
cubic foot, is connected to the output of amplifier 29 of bulk density circuit 14. A
temperature display device 43, preferably calibrated in ~C or VF, is connected to the
output of amplifier 33 of temperature circuit 16. This allows these parameters to be read
directly when the sample material is being utilized in a manufacturing process
After the output signal of sensing circuit 12 is corrected for variations in
temperature and bulk density, the combined signal is finally rectified in rectifier 40 and
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~080305
the resulting DC output signal is read on one or more of several output devices such as
meter 42, recorder 44, analog control circuit 46 or A/D converter 48, the latter two being
capable of providing an input to other devices. It should again be understood that a more
extensive and detailed discussion of the actual operation of the hereabove described
electrical circuits is provided in the hereabove-identified U.S. patent disclosures.
Referring additionally to FIGURE 2, a first embodiment of a contoured
electrode for an impedance measuring device, particularly adapted to be used with free
flowing particulate material, is illustrated. In order to make accurate impedance
measurements, the impedance measuring electrode must be sized and shaped both to
10 accommodate the varying properties of the material to be sampled and to create an
electrical field, in operation, which passes through almost all of the sample material in
the sample box. In preferred form, this embodiment includes an outer electrode which is
shaped as a box or container having a rectangular cross section. As illustrated, the box is
preferably positioned within a sampling tube 50 in which grain is continuously flowing in a
downward direction as indicated by arrows 52. The sample box comprises front wall 54,
rear wall 56 and side walls 58 and 6Q The sample box is supported in an upright position
by upper U-shaped arm 62 and lower U-shaped arm 64 which are themselves mounted on
the wall of sample tube 50 by means of a pair of upper support brackets 66 and lower
support brackets 67. As illustrated, each pair of support brackets 66 and 67 comprises
20 short pieces of resilient material such as piano wire mounted to extend between its
support arm and the sidle wall of tube 50. The outwardly extending ends of each pair of
arms 62 and 64 are connected to side walls 58 and 60 by means of a pair of mountings 68
and 69. Each of these mountings allows the sample box to flex or vibrate in a vertical
direction. As is best seen in PIGURE 2, each support arm 62 and 64 has a relatively
narrow top surface area, thereby reducing dust accumulation thereon which would possibly
falsify the weight reading obtained by load cell 27.
As is best seen in FIGURE 2, center electrode plate 70 is vertically disposed
within the sample box in parallel spaced relation to side walls 58 and 60. Upper rod 71 and
lower rod 72, each including an insulating bushing 73, support center electrode plate 70
30 along the center line of the sampling box.
It should be noted that the sample box is si~ed and shaped to cooperate with
the natural flow characteristics of particulate material, such as grain and still provide
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108V3~5
a spaced-apart pair of conductors which are positioned to radiate an electrical field into
substantially all of the sample material in the sample box. A top portion 74 of the sample
box is integrally connected with the front, rear and side walls and, at its upper end,
defines an opening 75 which is the inlet of the sample box. At the opposite end of the
sample box, a bottom portion 76 iS formed by an inwardly inclined wall, also integrally
connected with the sample box walls. Bottom portion 76 converges to an opening 77 at
the low end of the box which defines an outlet for the sample box. Outlet opening 77 in
bottom portion 76 iS slightly smaller in area than the inlet opening 75 at the top of the
sample box. As should be apparent, once the sample box is filled, it is maintained in a
constantly filled to overflowing condition, and at the same time, the contents of the
sample box are continuously changing so that repetitive impedance measurements reflect
the moisture variation in the sample stream. Excess sample material passes downwardly
over the outside walls of the sample box. To enhance this flow pattern, top portion 74iS
inclined at an angle which is steeper than the normal angle of repose of the sample
material funneled thereto. This assists in maintaining the sample box filled such that a
substantially constant volume of material with a uniform packing density related to the
drop height of the grain is obtained Accordingly, there is a continuous output signal from
load cell 27 related to the weight of the sample box, and this signal is proportional to the
bulk density of the material in the sample box.
Another feature of the electrode configuration is that a minor mechanical or
geometrical deviation in the sample box has a small effect on the electrical field radiated
through the sample material and, hence, minimal effect on the impedance measurement.
For example, while electrode plate 70 is mounted substantially along the center line of the
sample box, it should be understood that small lateral displacements will not cause a
significant change in the capacitance because the larger capacity on one side of the
electrode will be compensated by an accordingly smaller capacity on the opposite side.
Referring now to FIGURE 3, R second embodiment of R shaped electrode,
particularly well suited for use with stacked material such as tissue paper or the like, is
disclosed In this embodiment, a first electrode plate 79, comprising a flat planar
member, is positioned on edge so that its plane extends essentially in a vertical direction.
A second electrode plate 80 is laterally spaced from the first plate and is also disposed on
edge so that its surfaces extend along a second vertical plane which is substantially
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3-08030S
parallel to the plane of first electrode plate 79. The first and second electrode plates 79
and 80 are held in their spaced relationship by a U-shaped mounting bracket 81,
frabricated from an electrical insulating material, having a pair of downwardly extending
arms on which the electrode plates are mounted. The sample material is passed, such as
by a conveyor belt, through the area between the electrode plates so that its electrical
impedance can be measured by sensing circuit 82.
Rather than an active and grounded electrode as described in conjunction
with the first embodiment, in this second embodiment of a shaped electrode, both first
electrode plate 79 and second electrode plate 80 are electrically active or driven in the
sense that neither plate, in operation, is connected to ground. Referring now to FIGURE
4, one form of sensing circuit 82 is illustrated which is particularly adapted for use with
parallel plate electrode configurations. A sensing circuit 82, for reasons of convenience,
can be mounted on bracket 81 and is connected via leads 83 and 84 to first and second
electrode plates 79 and 80, respectively. An impedance including variable capacitor 86 is
connected to lead 84 so that any variation in capacitance between electrode plates 79 and
80, resulting from geometric variations in the plates or the influence of external objects,
can be equalized by adjustment of the capacitor. A signal generator (not shown) provides
a test signal to the sensing circuit input and transformer 85. In preferred form, sensing
circuit 82 is of a bridge design for use with an electrode assembly having two active
plates. Resistors 87 and 88 comprise one leg of the bridge and first electrode plate 79 is
connected via lead 83 between the resistors Resistors 89 and 90 comprise the other leg
of the bridge, with second electrode plate 80 connected therebetween via lead 84. The
bridge assembly itself is inherently stable and symmetric so that each of the electrodes is
separately driven to provide equal but oppositely phased field amplitudes in the area
between the electrode plates
The electrical signals to each leg of the bridge are of equal but opposite
polarity and the outputs from resistors 88 and 90 are connected via leads 91 and 92,
respectively, to the input of a comparator circuit 93. The output signal 93A from
comparator circuit 93 will then be directly proportional to the impedance of the sample
materiaL
In operation, sample material is positioned on the conveyor belt so that it
will pass between parallel electrode plates 79 and 80. When the sample material, such as
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a plurality of stacked sheets of tissue paper, has moved along a conveyor belt so that the
mid-point of the material to be sampled is approximately intermediate the electrode
plate, a reading from sensing circuit 82 is taken, preferably controlled by an automatic
position sensing device such as a photoelectric eye or the like. The electrical output
signal 93A from sensing circuit 82 is directly proportional to the impedance of the sample
material, and, in contrast to the moisture measuring system heretofore described for
grains, it is normally unnecessary to correct the output signal for temperature and weight
variations. Temperature correction is in most cases unnecessary since the ambient
temperature in the area where the paper is being processed is relatively constant and the
material sample temperature is stabilized at this ambient temperature. This allows a
constant temperature correction factor to be used, eliminating the necessity of individual
compensation for each sampling.
A constant correction factor could also be introduced into the electrical
output signal to compensate for bulk density. For example, a given sample material, such
as a stack of tissue paper, has a size and bulk which is essentially a constant, and hence,
its bulk density would be constant so that the output signal 93A from sensing circuit 82
can be initially set for a known bulk density dependence, so long as the type and size of
the material sampled does not deviate significantly.
Turning now to the third embodiment of a shaped electrode according to
the instant invention, this embodiment is well-suited for use with a system for measuring
the moisture content of irregularly shaped sample material such as wood chips. As
indicated herebefore, impedance measuring devices which are particularly adapted for
ascertaining the moisture content of wood chips normally require an electrode geometry
which differs from that used with particulate materials such as grains. For example, wood
chips, unlike grains, are not free flowing and have a tendency to 'bridge" or become
wedged together while flowing, and this tendency is more acute if the ships are required
to flow through a constrained passageway. For this reason, it has been found that it is
desirable that the sampling be done on a discrete or noncontinuous basis, such as by
consecutively filling a sample box, ascertaining the necessary characteristics of the
, 30 sample after each filling, and then discharging or exhausting the materiaL Also, it has
been found that the discharge action should be very positive to break down any tendency
of the wood chips to bridge or hold together during the sampling procedure so that each
discrete sample will be
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completely exhausted from the sample box before ~he next measuring operation is started.
Finally, the component parts of the measuring system which interact with the material
sample, such as the load cell, temperature sensor, suspension system, etc., should be built
into a basic support frame so that the entire system is integrally combined into a single
unit, except possibly for some of the electrical circuitry, to simplify the overall
mechanical instaUation of a wood chip moisture measuring system.
Now referring to FIGURES 5-7, the third embodiment of the electrode is
shown to include a grounded portion which is shaped as a box or container having a
rectangular cross section so that, if desired, it may be positioned in a stream of sample
10 material in the same manner as the sample box of FIGURE 1. This sample box comprises
front wall 94, side walls 95 and 96, and backing plate 97, together forming the closed walls
of a rectangular container. Front wall 94 and side walls 95 and 96 are preferably
integrally connected and pivotally mounted near the upper end of backing plate 97 by a
pair of pin joints 98. The sample box is supported in an upright position by a pair of upper
brackets 100 and lower brackets 102 which are themselves mounted on a vertical wall or
other rigid structure adjacent the flow path of the sample material. A main support
frame (not shown) attached behind backing plate 97 is fixedly mounted to upper and lower
support brackets 100 and 102 forming a laterally extending framework for supporting
backing plate 97 to which is mounted a portion of the electrical circuit components of the
20 moisture measuring system, these components being similar to those described in
conjunction with the first embodiment.
As is seen in FIGURE 5, a load cell 106, part of the bulk density circuit, is
directly built into the basic suspension frame of the sample box to simplify ~he overall
mechanical instaUation. In preferred form, load cell 106 is fixedly attached by a bracket
108 to backing plate 97 of the sample box. Load cell 106 is positioned so as to bear against
a bar 110 which is rigidly mounted on and extends laterally between the lower mounting
brackets 102. Backing plate 97 is held in a vertical position by the main support frame and
a set of upper and lower springs (not shown) mounted to extend lateraUy between the
support brackets. The steel springs are sufficiently flexible to allow unimpeded vertical
30 motion of backing plate 97 with respect to the main support frame, so that substantiaUy
all of the weight of the sample box is supported by load cell 106 as it bears against bracket
110.
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An active center electrode 112 is vertically disposed within the sample box in
parallel, spaced relationship to side walls 95 and 96 so that a uniform electrical field is
created during operation of the impedance measuring apparatus which passes through
substantially all of the material sample. Upper rod 114 and lower rod 116, together with an
insulating bushing (not shown), support center electrode plate 112 along the approximate
center line of the sample box. A top portion 117 of the sample box is preferably integrally
formed with front wall 94 and side walls 95 and 96 such that it extends inwardly creating
an inclined wall which, at its upward end, defines an opening 118, the inlet to the sample
box.
At the lower end of the sample box, a hinge 119 is secured on backing plate
97 and mounts trap door 120 for pivotal movement between an open and closed position. In
the closed position, as illustrated in FIGURE 5, trap door 120 extends horizontally from
backing plate 97 closing the outlet of the sample box. As is seen in the drawings, front
wall 94 is tapered or inclined inwardly at its lower end, both to give the three wall
configuration forming the pivoting member additional rigidity and to reduce the size of
trap door 120. Additionally, this inward tapering reduces the outward extent of movement
of front wall 94 in the operation position (FIGURES 6 and 7) thereby reducing the forward
clearance required during the discharge of the sample material.
As indicated herebefore, a desirable feature of a discrete sampling system
20 for measuring moisture in wood chips is that the sample box have a positive action during
the exhaust cycle to break any bridging tendency. Accordingly, the sample box of the
instant invention pivots at its upper end to open outwardly thereby causing a sudden
relative movement between center electrode 112 and side walls 95 and 96. To cause this
pivotal action, one end of each oi a first pair oi linkage arms 121 is fixedly attaehed to
either side edge oi trap door 120. The opposite end oi each iirst linkage arm 121 is
pivotally attached to one end oi a pair of second linkage arms 122. As illustrated, the
outward end of each linkage arm 122 is pivotally secured to the forw~rd portion of each
side wall by a pair oi laterally extending pins 124. As is apparent by a comparison of
FIGURE 5, illustrating the sample box in its closed position, with FIGURES 6 and 7,
30 illustrating the box in its open position, movement of trap door 120 from its horizontal or
closed position to a vertical or open position causes, through linkage arms 121 and 122,
outward movement of the iront and side walls of the box thus dislodging the sample
material in the sample box.
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The mechanical force required to move the sample box between its open and
closed position is provided by an actuating means 126. In preferred form, actuating means
126 is mounted on backing plate 97 and includes a control rod 128 which is pivotally
attached at its outward end to door extension arm 129, the opposite end of which is fixedly
mounted on the underside of trap door 120. Movement of control rod 128 between its end
positions causes both pivotal movement of trap door 120 between its vertical and
horizontal orientation and outward movement of the lower portion of the sample box via
linkage arms 121 and 122. In preferred form, actuating means 126 is a pneumatically
actuated cylinder and piston but it should be understood that other types of actuating
10 means such as a hydraulic cylinder and piston or an electric solenoid or the like, could also
be used.
As is best seen in FIGURE 6, a temperature sensor 130, preferably a
thermistor, is mounted on backing plate 97 above trap door 120 so that it projects
outwardly for maximum contact with the sample material received within the sample box.
In the same manner as discussed herebefore with regard to the first embodiment,
temperature sensor 130 is electrically connected to a temperature compensating circuit
for correcting the output signal of the sensing circuit so that the sensing circuit output
signal is corrected for variations in the material sample temperature. The outwardly
projecting portion of the sensor is preferably triangularly contoured for maximum contact
20 with the wood chips. It is preferably fabricated from a material, such as phosphor bronze,
which has a high heat conductance coefficient thus minimizing the inaccuracies associated
with temperature compensation by minimizing hysteresis or time lag associated with the
transfer of heat flux from the material sample to the thermistor. Heat transfer is
particularly important because the wood chips may only contact the projecting triangular
surface of the temperature sensor at a limited number of points and the wood itself is a
poor conductor of heat.
In operation, the sample box is mounted on a vertical support so that wood
chips may be received into the inlet opening 118 in top wall 117. Wood chips for sampling
are often diverted from a main flow stream and may be brought to the sample box in any
30 of a number of ways including a conveyor belt, blown via air pressure through an air duct,
or merely shoveled by hand into the sample box inlet~ When the sample box is full, an
electrical impedance measurement is taken by a sensing circuit in the same manner as
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heretofore described in conjunction with the embodiment of FIGURE 1. With center
electrode plate 112 disposed between side walls 95 and 96, a uniform electrical field of
substantially constant strength passes through almost all of the wood chips in the sample
box. If necessary, the electrical output signal from the sensing circuit is weight
compensated by a bulk density circuit and temperature compensated by a temperature
circuit in the manner heretofore described. If disposed in a stream of wood chips such as
from a conveyor belt, this sampling operation and the cycling of the sample box may be
operated by an automatic control device such as a timing means or photoelectric cell
oriented to determine when the sample box is filled to overflowing. After the impedance
10 measurement is taken, actuating means 126 is cycled, thereby causing the front and side
wall portion of the sample box to pivot outwardly, and trap door 120 to open, to exhaust
the wood chips from the sample box. After a sufficient period of time for the wood chips
to drop clear of the sample box outlet has passed, the push rod of actuating means 126 is
returned to its opposite end position closing the sample box and completing the measuring
cycle.
As mentioned herebefore, an important aspect of the instant invention is
that the bulk density and temperature of a sample of material can be read directly via
display devices attached to bulk density circuit 14 and temperature circuit 16. It has been
found that knowing these parameters is particularly important in some manufacturing
20 operations such as pulp processing. For example, in a pulping operation the wood chips are
digested by the action of a cooking liquor which dissolves the lignin in the wood thereby
releasing the individual wood fibers. There are both batch and continuous types of
digesters available, and both require that the wood chips and liquor be added in a
predetermined ratio. With either the batch or continuous process, both the bulk density
and moisture content of the wood chips must be known to ascertain the proper amount of
liquor which must be added to optimize the wood chip-to-liquor ratio.
In the case of a continuous digester, wood chips and liquor are continuously
added to the digester. A quantity of wood chips are added basically on a volumetric basis
such as determined by a number of revolutions of a star feeding valve. As is known, such
30 a star feeding valve has a certain number of pockets, each with a given volume so that the
volumetric feed rate into the digester inlet can be readily calculated. Accordingly, if the
wet weight bulk density of the wood chips is known, as indicated by display device 41, the
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volumetric feed rate of the star valve can be readily translated into a wet weight flow
measurement of wood chips. This parameter, in conjunction with the moisture content
measurement of the wood chips, can be converted into a dry weight flow of wood chips. -
Finally, this calculation of the dry weight of wood chips is used to determine the amount
of liquor which must be added to the digester.
As pointed out above, the reading on display device 41 indicating the wet
weight bulk density is a primary factor for optimizing the operation of the digester. Bulk
deslsity is defined as the weight per unit of volume and in the case of wood chips depends
on a number of variables including wood species, size distribution of the chip particle, and
10 the amount of settling between the larger and smaller pieces. Wood chips are of
nonuniform size and a given volume of wood chips is to some degree compressible
depending on the extent that the normal handling of the chips has reduced the voids or air
spaces between chips. In the situation where a flow of chips is fed from a number of
different sources, each with a different bulk density and moisture content, there is no
practical way to accurately determine the weight of a specific portion of the chip flow as
the entire mass is transported by a bulk conveyor.
The inaccuracies inherently associated with the dynamic measurement of
the bulk density of such a moving chip stream has led to the conclusion that this
measurement is preferably ascertained by taking repetitive static measurements at
20 successive points in the chip stream under condltions which are consistent between
consecutive samples. With the instant invention, this is done by successively filling a
sample box, taking both bulk density measurement and moisture content measurement of
the contents of the sample box, and then exhausting the sample material from the sample
box. One method of ensuring that the measuring conditions of repetitive samplings are
uniform between consecutive samples is to drop the chips at a given rate, typically 12
inches, into the sample box so that the chip compaction factor will be the same for each
measurement.
In most pulping operations, because of the large volume of wood chips
required by the digester it is impractical to measure the bulk density and moisture content
30 of the entire chip ilow. Accordingly, the wood chips delivered to the sample box are only
a fraction of the total chip flow so it is important to ensure that the wood chips actually
deposited in the sample box reflect a cross section of the overall chip flow. Because of
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the inherent nonuniform size of the wood chips, a segregation often occurs during
transportation and handling in which the smaller or fine chips tend to settle to the bottom
of a chip flow. Accordingly, to obtain a representative sample from the chip flow, it is
necessary to extract wood chips from all layers of a cross section of the primary flow.
One method of ensuring a representative cross section is by installing a small sampling
conveyor belt at a point where the chip stream is dropped into a chute. By intersecting
the falling primary chip stream, the conveyor belt receives a representative sampling
from the various layers of the primary chip flow. The flow from this smaller, sampling
conveyor belt can then be used in the manner herebefore described to fill the sample box
so that a bulk density and moisture content measurement can be taken.
As indicated herebefore, the embodiments of the shaped electrode are
particularly sized to compensate for the varying characteristics of material particles to
be sampled and to radiate an electrical field through almost all of the material sample.
~or instance, a typical sample box forming the outer grounded portion of the electrode, as
illustrated in the first embodiment for measuring the electrical impedance of particulate
material, may have a volumetric capacity of approximately 24 cubic inches and a center
electrode plate situated so thut it is parallel to the side walls so that a uniform electrical
field is radiated. In its preferred vertical orientation during operation, the free flowing
properties of the grain are such that the partlculate material passes easily through the
sample box, eliminating the need for additional material handling systems. However, in
the case of wood chips, the sample box, as illustrated in the third embodiment according
to the instant invention, should have a capacity of at least a cubic feet to average out
variations in chip size and to provide sufficient room on either side of the center
electrode plate.
The sample box of the first and third embodiments discussed above forming
the grounded electrode may be constructed of stainless steel having a relatively low
thermal expansion coefficient of 10 X 10 6 per C., that is, a change of 7 X 10-4 for a
70 C. change in temperature. Capacitance is inversely proportional to the distance
between the active electrode plate and the outer housing of the sample box which is
grounded, and thus a sample box having a capacitance of 10 pF. is changed by less than 0.01
pF. for a 70 C. change in temperature. This change in capacitance may be ignored except
for cases where a very low moisture content is to be measured or where the material is
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10803~5
fluffy and has a dielectric coefficient very close to one. The first embodiment of the
sample box for particulate material has been adapted to compensate for this variation. As
is seen in FIGURE 1, the thermal drift is corrected for by mounting a bimetallic strip 140
on one of the support rods of the electrode to provide a small variable capacitance
between the bimetallic strip and grounded electrode 142 attached to the rear wall 56 of
the sample box. Thermal drift may then be offset by an opposing change of capacitance
due to the motion of the bimetallic strip. This assembly is protected by a dust cover 144.
Howeve-r, in the wood chip embodiment of FIGURES 5-7, it has been found that the
thermal expansion error is slight and no correction apparatus need be specifically attached
10 to the sample box.
An alternate but more expensive solution is to construct a sample box and
center plate electrode from a material such as Invar Steel which has a thermal expansion
coefficient of only 0.8 X 10 6 per C. This construction reduces thermal drift by a factor
of more than 10 compared to stainless steeL
Finally, turning to the fourth embodiment of the shaped electrode, this
embodiment is particularly well-suited for measuring the electrical impedance oî sample
material drying in a closed drying structure, such as a kiln or the like, to determine its
moisture content. As is seen in FIGURE 8, a kiln 150 typically comprises an outer housing
or wall defining a relatively large internal chamber 151 in which the sample material 152 is
20 disposed during the drying cycle. Sample material 152 is often green lumber which has
been rough cut and stacked on cart 154 so that it can be conveniently moved Cart 154,
with its stacked sample material thereon, can be pushed along a pair of rails 156 into
internal chamber 151 of kiln 150. An axially elongated center electrode 158, having a
length sized to extend approxiamtely the length of the stack of lumber, is inserted into
the approximate mid-point of the end face of the stack. In the same manner as heretofore
described in connection with the embodiment of FIGUR~ 1, a lead 160 is connected to a
sensing circuit (not shown) which may be either at a remote location or attached to kiln
150. In any event, it is desirable to thermally insulate the sensing circuit from any
fluctuating heat source such as the kiln, so that the electrical characteristics of the
30 components of the sensing circuit will not be changed by the periodic heat variationæ The
sensing circuit provides a high frequency electrical signal to electrode 158, creating a high
frequency electrical field which passes through the sample material, the impedance of
which is to be measured
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In this embodiment, the walls or housing of the kiln 150, together with metal
rails 156, act as a surrounding grounded electrode which cooperates with the center
electrode in measuring the impedance of the material sample. Most kilns have sufficient
metallic conductors in the walls and floors to act as an outer grounded electrode for
impedance measurement, the component parts of such an impedance measuring system
being as described both heretofore and in my prior related patents. A temperature sensor
162 is preferably provided and is connected via lead 164 to a temperature circuit (not
shown).
In operation, when cart 154 contianing the stacked lumber has been disposed
within the closed internal chamber of kiln 150 and active electrode 158 inserted into the
center of the stack, the drying cycle begins and the temperature in internal chamber 151 is
raised by turning on heater 166. The high frequency electrical signal applied to central
electrode 158 creates an electrical sensing field which passes through the stacked wood to
the outer metal conductors in the walls and floor of kiln 150. Because of the large
distance separating the inner and outer conductors, the exact location and external
geometry of the active electrode 158 is not important so long as it is located
approximately near the center of the stacked lumber. For the case where the active
electrode is a plain antenna rod, and the outer conductors are in the near vicinity of the
stack of wood, the lineal capacitance between the two "electrodes" i9 given by:
C = 7;4 e pF/ft, where
.
e i~ the dielectric constant (relative to air), b is the distance between center and outer
conductors, and a is the radius of the center conductor. For the typical case, e is between
1 and 2, and b/a is between 100 and 10,000, in which case the lineal capacitance will vary
over a range of 1.85 to 7.4 pF/ft., assuming the existance of a small air gap between the
lumber stack and the outer conductors. Where there is a sizeable air gap the capacitance
will be correspondingly reduced. Also, for an active conductor of small sized diameter,
the ratio of b/a can vary greatly without significantly affecting the measured
30 capacitance. For the case where the ratio of b/a is held fixed, the measured capacitance
(or impedance reading) can be directly calibrated in average percent moisture and used as
a set point control for terminating the kiln drying.
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1080305
In the same manner as heretofore described, the electrical output signal
from the sensing circuit is directly proportional to the moisture content of the sample
material when properly corrected for temperature and bulk density dependency. Bulk
density correction can be introduced as a constant factor since the amount of sample
material stacked on each cart 154 and the type of lumber are known. As the drying cycle
for the lumber begins, the elevated temperature in the kiln causes a moisture content
decrease within the material sampled. ~ turn, this changes the dielectric coefficient
between active electrode 158 and the passive grounded conductors of kiln 150, and this
change is reflected by a proportional deviation in the output signal of the sensing circuit
10 when corrected for temperature dependence. After the moisture content of the lumber
has been reduced to approximately 10%, the drying cycle is complete and the lumber can
then be removed from the kiln.
The invention may be embodied in other specific forms without departing
from the spirit or central characteristics thereof. The present embodiments are therefore
to be considered in all respects as illustrative and not restrictive, the scope of the
inveniton being indicated by the appended claims rather than by the foregoing description,
and all changes which come within the meaning and range of equivalency of the claims are
therefore to be embreced therein.
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