Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to an apparatus for measuring the
thickness of a sheet and, more particularly, to an apparatus employing the
principle of mutual inductance to measure the thickness of a sheet of non-
magnetic material.
Caliper gauges, or more generally, apparatuses to measure the
thickness of a sheet, where typically the sheet is a material such as paper,
are well-known in the art (see for example United States Patent #2,665,333).
However, heretofore electromagnetic caliper gauges have been of the type
known as self inductance (see also for example United States Patent
#3,528,002). Self-inductance gauges, in general, comprise a coil of wire
wrapped about a U-shaped member of a magnetically susceptible material on one
side of the sheet to be measured. A current is passed through the coil
creating a magnetic field. On the other side of the sheet is a bar also of
a magnetically susceptible materialO Both the bar and the coil are main-
tained at a constant distance from the sheet, through the use of well-known
techniques, such as air bearings. Since the coil and the bar are maintained
at a constant distance from the sheet, the separation between the coil and
the bar is determined by the thickness of the sheet. As the thickness of
the sheet varies, the separation between the coil and the bar would also
vary. The measurement of the separation between the coil and the bar is based
on the principle of self-inductance.
The coil acts similar to an inductor. A capacitor is placed in
series with the coilO As is well-known from basic circuit theory, a capacitor
in series with an inductor would resonate at a frequency determined by the
factor 1/( ~i~). The coil, however, does not act similar to an inductor with
a constant value for its inductance. As the distance between the coil and
the bar changes, so does the inductance of the coil. Thus, the resonating
frequency of the capacitor in series with the coil is determined by the
inductance of the coil, which is determined by the separation between the
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coil and the ~.ar. The measure~e.nt o$ the resonati.n~ fre.~uency
would give a measurement fox the $eparation of th.e coil and the
bar. Therefore, the resonatin~ frequency of the circuit gives a
measure of the th~ckness of th.e s.h.eet. ~hile sel-inductance
caliper gauges are adequate for some applications, usin~ resonat-
ing frequency as a measurement of thicknes:s, th.e.y are inadequate
for measurement o~ sheets having large thicknes.s values.
The use of the amplitude of a ma~neti.c field as a
measurement of the thicknes.s o a sheet i$ disclosed in United
lQ States Patent #3,696,290. That patent, however, teaches the use
of a U-s.haped permanent magnet and a magneto resistor. The U-
shaped magnet suffers from the disadvantage that it is not axially
symmetrical and thus it is subject to alignment error. Further-
more, unlike an electromagnet whose amplitude can be varied, the
amplitude of a permanent magnet cannot be adjusted for varying
thicknesses of different sheets, as the gauge is being used.
A caliper gauge for measuring the thickness of a sheet
of a non-magnetic material comprises a transmitter to one side of
said sheet. The transmitter has a body of a magnetically sus-
Ceptible material, substantially cylindrical in shape, with anelectrical wire wound a plurality of times about said body. The
transmitter is held at a constant distance apart from the sheet,
such that the axis of the cylinder is substantially perpendicular
to the sheet. A receiver is to the other side of said sheet.
The receiver also comprises a body of a magnetically susceptible
material, substantially cylindrical in shape. An electrical wire
is wound a plurality of times about the receiver. The receiver
is ~ositioned such that the axis of the cylinder is subs.tantially
aligned with the axis of the transmitter.
In accordance with this invention there is provided an
apparatus for non-contacting measurement of the thickness of a
sheet of a non-magnetic material comprising: a first body of a
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magnetically susceptible material, substantially cylindrical in
shape, to one side of said sheet; said first body aligned such
that the axis of the cylinder is substantially perpendicular to
said sheet; a f~rst electrical wire wound a plurality of times
about said first body, capable of being energized to produce a
magnetic field; means for holding said first body at a constant
distance apart from said sheet; a second ~ody of a magnetically
susceptible material, substantially cylindrical in shape, having
a diameter less than the diameter of the first body, to other
side of said sheet; said second body positioned such that the axis
of the cylinder is substantially aligned with the axis of the
first body; a member of magnetically susceptible ~aterial, sub-
stantially disk shaped, attached to said second body with said
second body between said member and said sheet and with the center
of said member substantially aligned with the axis of said second
body and having a diameter larger than that of the second body;
a second electrical wire wound a plurality of times about said
second body, said second wire capable of detecting the amplitude
of said magnetic field, said field detected determinative of the
thickness of said sheet; and means for maintaining said second
body at a constant distance apart from said sheet.
Figure 1 is a cross-sectional Yiew of the caliper gauge
of the prior art.
Figure 2 is a graph of distance versus frequency for
the caliper gauge of the prior art.
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Figure 3 is a cross~sectional view of an embodiment of the caliper
gauge of the present invention.
Figure 4 is a graph of distance versus amplitude fQr the caliper
gauge of the present invention.
Figure 5 is a cross~sectional view of another embodiment of the
caliper gauge of the present invention.
Figure 6A is a schematic view of the operation of the caliper gauge
of Figure 3.
Figure 6B is a schematic view of the operation of the caliper gauge
of Figure 5.
Referring to Figure 1, there is shown a cross-sectional view of the
thickness gauge 10 of the prior art, measuring the thickness t of a sheet 12.
Typically the sheet 12 is a material such as paper, plastics, rubber, etcO
The thickness gauge 10 comprises two parts, a first part lOa and a second
part lOb. The first part lOa is positioned to one side of the sheet 12 with
the second part lOb positioned to other side of the sheet 12. The first
part lOa comprises a U-shaped member 14 of a magnetically susceptible material,
such as iron. Wound around the U-shaped member 14 is a wire 16. The second
part lOb is a bar member 17 also of a magnetically susceptible material, In
the operation of the thickness gauge 10, both the first part lOa and the
second part lOb are maintained at a constant distance apart from the sheet
120 The first part lOa is maintained at a constant distance a from the
she~t 12 by well known tec~miquss, such as air bearings ¦not shown). The
second part lOb is also held at a constant distance b from the sheet 12 by
well known techniques. The total separation between the first part lOa and
the second part lOb is the sum of the distances a, b and the thickness t of
the sheet 120 In the operation of the thickness gauge 10, a capacitor (not
shown) is connected in series with the wire 160 The wire 16 wound about
~-shaped member 14 acts as an inductor. As is well known, an inductor and a
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capacitor would resonate at a frequency determined by 1/( ~ ~, where L is
the inductance and C is the capacitance. In the thickness gauge 10 of the
prior art, the inductance of the wire 16 wound around the U-shaped member 14
is determined by the total distance (i.e. a + t + b) between the U-shaped
member 14 and the bar 17. As the distance between the first part lOa and
the second part lOb increased, so would the resonating frequency.
Figure 2 is a graph of the typical response of distance versus
resonating frequency of the thickness gauge 10 of the prior artO
Referring to Figure 3, there is shown a cross-sectional view of a
caliper gauge 20 of the present invention, measuring the thickness t of a
sheet 22. Typically, the sheet 22 is a material, such as paper, plastics,
rubber, etc. The caliper gauge 20 comprises two parts, a transmitter 20a
and a receiver 20b. The transmitter 20a is positioned to one side of the
sheet 22 with the receiver 20b positioned to other side of the sheet 220
The transmitter 20a comprises a first member 24, substantially cylindrical
in shape, of a magnetically susceptible material, such as ironO Wound around
the first member 24 is a first wire 26. The transmitter 20a is positioned
such that the axis of first member 24 is substantially perpendicular to the
sheet 22. The receiver 20b comprises a second member 28, substantially
cylindrical in shape, also of a magnetica'ly susceptible material. Wound
around the second member 28 is a second wire 30. The member 20b is positioned
such that the axis of the second member 28 is substantially aligned with the
axis of the first member 24.
In the operation of the caliper gauge 20, the transmitter 20a is
maintained at a constant distance a from the sheet 22, while the receiver 20b
is held at a constant distance b from the sheet 22. In the embodiment shown
in Figure 3, this is accomplished by placing the transmitter 20a in a first
housing 32. The first housing 32 has an input port 34 and an output port 36,
comprising a plurality of tiny orifices. A fluid7 such as pressurized air,
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enters the first housing 32 through the input port 34. The air exits from
the first housing 32 via the output port 36. The fluid exits from the housing
32, under pressure, impinges the one side of the sheet 22. By directing a
constant flow of fluid from the first housing 32 impinging on the sheet 22,
the first housing 32, with the transmitter 20a in it, would be maintained at
a constant distance a from the sheet 22. Similarly, the receiver 20b is
placed in second housing 38. The second housing 38 has an input port 40 and
an output port 42~ comprising a plurality of tiny orifices. A fluid, such as
pressurized air, enters the second housing 38 through the input port 40. The
air exits from the second housing 38 via the output port 42, under pressure,
impinges the other side of the sheet 22. By directing a constant flow of
fluid from the second housing 38 impinging on the sheet 22, the second housing
38, with the receiver 20b in it, would be maintained at a constant distance b
from the sheet 22.
The total separation between the transmitter 20a and the receiver
20b is the sum of the distances a, b and the thickness t of the sheet 22. A
current is passed through the first wire 26 generating a magnetic field with
an amplitude from the transmitter 20a. The amplitude of the magnetic field
generated from the transmitter 20a is detected by the receiver 20b. The
intensity of the magnetic field or the amplitude received at the receiver 20b
is a function of the total separation between the transmitter 20a and the
receiver 20b. As the distance between the transmitter 20a and the receiver
20b increased, the amplitude of the magnetic field sensed at the receiver
20b would decrease. A plot of a typical distance versus amplitude is shown
in Figure 4.
One of the advantages of the caliper gauge 20 of the present
invention can be seen by comparing Figure 2 to Figure 4. For small changes
in distance where the separation between the caliper gauge 20 or the thick-
ness gauge 10 is large (such as from Dl to D2, where Dl and D2 are the same
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for Figures 2 and 4), it is seen that the incremental change in signal
(i.e ~ F and ~ A) is also smallO However, though the incremental change in
~ A for the caliper gauge 20 is small, it is seen that the proportional
change in the total signal, i.e. ~ A/A, is large compared to the proportional
change in the total signal ( ~ F/F) of the thickness gauge 10. The larger
proportional change of the total signal ( ~ A/A) of the caliper gauge 20 of
the present invention results in a larger signal to noise ratio resulting in
a more accurate measurement.
A second advantage of the caliper gauge 20 of the present invention
can be seen by referring back to Figure 1. It is well known that magnetic
flux flow along the path of least reluctance. One such path is the dotted
line; another is the dot-dash line. Where the path flows through magnetical-
ly susceptible material (such as through the bar 17) the reluctance is
virtually zero. The reluctance through air, however, is non-zero. If the
separation between the U-shaped member 14 and the bar 17 is large and length
of the path shown by the dotted line is short in comparison, then the magne-
tic flux would have a preference to flow along the dotted path. However, to
operate the thickness gauge 10 the magnetic flux must flow along the dash-dot
pathO Thus, where the distance to be measured is large, the physical
dimension of the U-shaped member 14 must also be large. In the caliper
gauge 20 of the present invention, the physical dimensions of the gauge 20
need not be increased to detect separation of large distances. Since the
caliper gauge 20 measures the thickness of the sheet through the detection of
the amplitude of the magnetic field, for measurement of thick sheets only the
amplitude of the field of the caliper gauge 20 needs to be increased. This
can be accomplished by simply increasing the amount of current flowing through
the electrical wire of the transmitter 20aO
Compared to the gauge disclosed in United States Patent~#3,696,290,
the caliper gauge 20 of the present invention offers the advantage that the
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amplitude of the magnetic field can be varied as the gauge 20 is used to
measure sheets having varying thickness values. Furthermore, the axial
symmetry of the gauge 20 offers the advantage of ease of alignment.
Referring to Figure 5thereis shown another caliper gauge of the
present invention, generally designated as 50. The caliper gauge 50 comprises
a transmitter 50a and a receiver 50b. The transmitter 50a is positioned to
one side of a sheet 52, with the receiver 50b positioned to other side of
of the sheet 52. The transmitter 50a comprises a first member 54~ substantial-
ly cylindrical in shape, of a magnetically susceptible material, such as iron.
Wound around the first member 54 is a first wire 56. The transmitter is
positioned such that the axis of first member 54 is substantially perpendicu-
lar to the sheet 52. The transmitter 50a is the same as the transmitter 20a
of Figure 3. The receiver 50b comprises a second member 58, substantially
cylindrical in shape, also of a magnetically susceptible material. Wound
around the second member 58 is a second wire 60. The second member 58 is
positioned such that its axis is substantially aligned with the axis of the
first member 54. The receiver 50b also comprises a disk shaped member 62,
of a magnetically susceptible material. The disk shaped member 62 is attached
to the second member 58, with the center of the disk 62 substantially aligned
with the axis of the second member 58. The second member 58 is between the
sheet 52 and the disk 62. Preferably, as will be explained hereafter, the
diameter of the disk 62 is approximately the same as the diameter of the first
member 54 and the diameter of the second member 58 is less than the diameter
of the disk 62. Except for the addition of the disk 62, the receiver 50b is
the same as the receiver 20b of Figur~ 3.
In the embodiment sho~n in Figure 5, there is also a first pair 64
of sensors 64a and 64b, each capable of detecting the amplitude of a magnetic
field. Each of the sensors 64a and 64b comprises a coil of electrical wire~
The second member 58 is positioned between the first pair 64 and forms a line
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with the first pair 64. A second pair 66 of sensors (not shown) also
capable of detecting the amplitude of a magnetic field (e.g. each is a coil
of electrical wire) is positioned such that the second member 58 is between
the second pair 66 and forms a line with the second pair 66. The line formed
by the second pair 66 is approximately perpendicular to the line formed by
the first pair 64. The first pair 64 and the second pair 66 are used for
alignment purpose, i.e. to ensure and to correct for any deviation of signal
caused by the lateral displacement of the first member 54 and the second
member 58 from one another. Similar to the embodiment shown in Figure 3,
the transmitter 50a and receiver 50b are placed in housings with fluid, such
as air, directed from the housings impinging on the sheet 52 to maintain the
transmitter 50a and the receiver 50b at a constant distance apart from the
sheet 52.
All of the advantages, previously discussed for the caliper gauge
20 as shown in Figure 3, are also present in the caliper gauge 50 as shown in
Figure 5, i.e. measure sheets with large thickness values, axial symmetry,
vary the strength of the magnetic field as sheets with different thickness
values are measured, etc. However, in addition, the advantage of the caliper
gauge 50 of Figure 5 is its greater sensitivity to measurement at small
distance. This is shown in Figures 6A and 6B. Figure 6A is a schematic
drawing of the ca]iper gauge 20 of Figure 3. Figure 6A shows a transmitter
24 and a receiver 280 The magnetic field lines are shown as dotted lines.
From Figure 6A it is seen that the receiver 28 intercepts only a portion of
the magnetic field lines. In Figure 6B it is seen that the receiver compris-
ing the member 58 and the disk 62 intercepts a greater portion of the magnetic
field lines emanating from the transmitter 54. The disk 62 aids the member
58 in intercepting a larger portion of the magnetic linesO Thus, a great
signal is produced for small distance measurement.
It is in theory possible to have the caliper gauge 20 perform as
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well as the caliper gauge 50 for small distance measurements. This can be
accomplished by increasing the diameter of the receiver 28 to as large as the
diameter of the transmitter 24. However, this would necessitate a large
receiver. Moreover9 for large distance measurements the diameter of the re-
ceiver 28 is almost inconsequential, i.eO a cylinder with a small diameter
would intercept almost as much magnetic field as a cylinder with a large
diameter. This is because the angle intercepted would be small. What is
accomplished by the addition of the disk 62 to the member 58 is to make the
receiver 50b sensitive to measurement at a wide range of distances - without
the need to make a receiver with a large cylinder. A receiver with a large
cylinder would be more massive than the receiver 50b of Figure 5. Since the
receiver 20b or 50b is supported on air bearings, the reduction in mass with-
out loss of sensitivity is significant.
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