GB1576994A - Apparatus for measuring the thickness of a sheet of non-magnetic material - Google Patents

Description

(54) APPARATUS FOR MEASURING THE THICKNESS OF A SHEET OF NON-MAGNETIC MATERIAL (71) We, MEASUREX CORPORA TION, a Corporation organised and existing under the laws of the State of California, United States of America, of One Results Way, Cupertino, State of'California, 95014, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: 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 U.S. Patent No. 2,665,333).

However, heretofore electromagnetic caliper gauges have generally been of the type known as self-inductance (see also for example U.S. Patent No. 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 material. Both the bar and the coil are maintained 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 biased on the principle of self-inductance.

The coil acts similar to an inductor. A capacitor is placed in series with the coil. 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 /( yrCt-). 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 coil and the bar. The measurement of the resonating frequency would give a measurement for the separation of the coil and the bar. Therefore, the resonating frequency of the circuit gives a measure of the thickness of the sheet.While selfinductance caliper gauges are adequate for some applications, using resonating frequency as a measurement of thickness, they are inadequate for measurement of sheets having large thickness values.

The use of the amplitude of a magnetic field as a measurement of the thickness of a sheet is disclosed in U.S. Patent No.

3,-696,290. That patent, however, teaches the use of a u-shaped 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. Furthermore, unlike an electromagnet whose amplitude can be varied, the amplitude of a permanent magnet cannot be adjusted for varying thickness of different sheets, as the gauge is being used.

British Patent Specification No. 843,624 discloses a thickness measuring instrument comprising the combination of a transmitter and receiver for placing on either side on non-magnetic material the thickness of which is to be measured, and an indicator, the transmitter and receiver each comprising a magnetic core on which is mounted a winding, the winding of the transmitter when energised by an alternating current causing the transmitter to generate an external alternating magnetic field which when the receiver is aligned therewith induces in the receiver winding a signal dependent upon the spacing between the transmitter and receiver, and hence the thickness of the material, the signal being applied to the indicator which is calibrated to indicate thickness for a given level of energisation of the transmitter.

No guidance is given in this specification as to the optimum shapes for the magnetic cores.

The transmitter and receiver are to be placed in contact with the material whose thickness is to be determined.

According to the present invention there is provided an apparatus for non-contacting measurement of the thickness of a sheet of non-magnetic material which apparatus comprises a first substantially cylindrical body of a magnetically susceptible material arranged such that it can be disposed in use, to one side of said sheet with its axis substantially perpendicular to said sheet; a first electrical wire wound a plurality of times about said first body; means for holding said first body at a constant distance apart from said sheet; a second substantially cylindrical body of a magnetically susceptible material arranged such that it can be disposed, in use, to the other side of said sheet with its axis substantially aligned with the axis of the first body; a substantially disk shaped member of magnetically susceptible material substantially co-axial with and attached to the end of said second body remote from said first bodyand having a diameter greater than that of the second body; and a second electrical wire wound a plurality of times about the said cylindrical portion of the said second body; means for maintaining said second body at a constant distance apart from said sheet.

Specific embodiments of an apparatus for non-contacting thickness measurement of a sheet of non-magnetic material will now be described by way of example with reference to the accompanying drawings in which: FIGURE 1 is a cross-sectional view of a caliper gauge of the prior art; FIGURE 2 is a graph of distance versus frequency for the caliper gauge shown in Figure 1; FIGURE 3 is a cross-section view of an embodiment of a caliper gauge employing some but not all of the essential features of the invention; FIGURE 4 is a graph of distance versus amplitude for the caliper gauge shown in Figure 3; FIGURE 5 is a cross-sectional view of another embodiment of a caliper gauge of the present invention; FIGURE 6A is a schematic view of the operation of the caliper gauge shown in Figure 3; and FIGURE 6B is a schematic view of the operation of the caliper gauge shown in 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, etc.

The thickness gauge 10 comprises two parts, a first part 10a and a second part 10b. The first part 10a is positioned to one side of the sheet 12 with the second part 10b positioned to other side of the sheet 12. The first part 10a 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 10b is a bar member 17 also of a magnetically susceptible material. In the operation of the thickness gauge 10, both the first part 10a and the' second part 10b are maintained at a constant distance apart from the sheet 12. The first part 1 0a is maintained at a constant distance a from the sheet 12 by well known techno ques, such as air bearings (not shown).The second part 10b is also held at a constant distance b from the sheet 12 by well known techniques. The total separation between the first part 10a and the second part 10b is the sum of the distance a, b and the thickness t of the sheet 12. In the operation of thickness gauge 10, a capacitor (not shown) is connected in series with the wire 16. The wire 16 wound about u-shaped member 14 acts as an inductor. As is well known, an inductor and a capacitor would resonate at a frequency determined by 1/ V71TC), where L is the inductance and C is the capacitance. In the.

thickness gauge 10 of the prior art, the induc -tance 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 10a 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 art.

Referring to FIGURE 3, there is shown a cross-sectional view of a caliper gauge 20 of the present invention, measuring the thick- ness 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 20bow positioned to other side of the sheet 22. The transmitter 20a comprises a first member 24, substantially cylindrical in shape, of a magnetically susceptible material, such as iron.

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 magnetically 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 fluid, such as pressurized air, 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 first 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 a 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 2-2, 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 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 thickness gauge 10 is large (such as from D1 to D2, where D1 and D2 are the same for FIGURES 2 and 4), it is seen that the incremental change in signal (i.e. AF and hA) is also small. However, though the incremental change in AA for the caliper gauge 20 is small, it is seen that the proportional change in the total signal, i.e. AA/A, is large compared to the proportional change in the total signal (AF/F) of the thickness gauge 10.The larger proportional change df the total signal (hA/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 magnetically 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 magnetic 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 dashdot path. 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 20a.

Compared to the gauge disclosed in U.S.

Patent No. 3,696,290, the caliper gauge 20 offers the advantage that the amplitude ofthe 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 advan tage of ease of alignment.

Referring to FIGURE 5. there is shown caliper gauge of the present invention, gen erally designated as 50. The caliper gauge 50 comprises a transmitter 50a and a receiver SOb. The transmitter 50a is positioned to one side of a sheet 52, with a receiver 50b positioned to other side of the sheet 52. The transmitter 50a comprises a first member 54, substantially cylindrical in shape,'of a mag netically 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 perpendicular 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 caliper gauge 50 is distinguished from that shown in Figure 3 in that the receiver SOb 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 Figure 3. The advantage conferred by the disk shaped member 62 will be explained hereafter.

In the embodiment shown 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 with the first pair 64. A second pair 66 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 the 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 FIG URE 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 FIG URE 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 caliper gauge 20 of FIGURE 3. FIGURE 6A shows a transmitter 24 and a receiver 28. The magnetic field lines are shown as dotted lines. From FIG URE 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 comprising 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 lines. Thus, a great signal is produced for small distance measurement.

It is in theory possible to have the caliper gauge 20 perform as well as the caliper gauge 50 for small distance measurements. This can be accomplised 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. Moreover, for large distance measurements the diameter of the receiver 28 is almost inconsequential, i.e.

a cylinder with a small diameter would inters cept 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 FIGURES.

Since the receiver 20b or 50b is supported on air bearings, the reduction in mass without loss of sensitivity is significant.

WHAT WE CLAIM IS: 1. An apparatus for non-contacting measurement of the thickness of a sheet of a non-magnetic material which apparatus comprises a first substantially cylindrical body of a magnetically susceptible material arranged such that it can be disposed, in use, to one side of said sheet with its axis substantially perpendicular to said sheet; a first electical wire wound a plurality of times about said first body; means for holding said first body at a constant distance apart from said sheet; a second substantially cylindrical body of a magnetically susceptible material arranged such that it can be disposed, in use, to the other side of said sheet with its axis substantially aligned with the axis of the first body; a substantially disk shaped member of magnetically susceptible material substantially co-axial with and attached to the end of said second body remote from said first body and having a diameter greater than that of

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. 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 perpendicular 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 caliper gauge 50 is distinguished from that shown in Figure 3 in that the receiver SOb 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 Figure 3. The advantage conferred by the disk shaped member 62 will be explained hereafter. In the embodiment shown 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 with the first pair 64. A second pair 66 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 the 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 FIG URE 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 FIG URE 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 caliper gauge 20 of FIGURE 3. FIGURE 6A shows a transmitter 24 and a receiver 28. The magnetic field lines are shown as dotted lines. From FIG URE 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 comprising 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 lines. Thus, a great signal is produced for small distance measurement. It is in theory possible to have the caliper gauge 20 perform as well as the caliper gauge 50 for small distance measurements. This can be accomplised 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. Moreover, for large distance measurements the diameter of the receiver 28 is almost inconsequential, i.e. a cylinder with a small diameter would inters cept 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 FIGURES. Since the receiver 20b or 50b is supported on air bearings, the reduction in mass without loss of sensitivity is significant. WHAT WE CLAIM IS:

1. An apparatus for non-contacting measurement of the thickness of a sheet of a non-magnetic material which apparatus comprises a first substantially cylindrical body of a magnetically susceptible material arranged such that it can be disposed, in use, to one side of said sheet with its axis substantially perpendicular to said sheet; a first electical wire wound a plurality of times about said first body; means for holding said first body at a constant distance apart from said sheet; a second substantially cylindrical body of a magnetically susceptible material arranged such that it can be disposed, in use, to the other side of said sheet with its axis substantially aligned with the axis of the first body; a substantially disk shaped member of magnetically susceptible material substantially co-axial with and attached to the end of said second body remote from said first body and having a diameter greater than that of

the second body; and a second electrical wire wound a plurality of times about the said cylindrical portion of the said second body; means for maintaining said second body at a constant distance apart from said sheet.

2. An apparatus as claimed in claim 1, wherein the diameter of said disk shaped member is substantially the same as the diameter of said first body.

3. An apparatus as claimed in claim 2, further comprising a first pair of sensors, each of said sensors being capable of detecting the amplitude of a magnetic field, and said second body being in between said first pair and forming a first line with said first pair; and a second pair of sensors, each of said sensors being capable of detecting the amplitude of a magnetic field, and said second body being in between said second pair and forming a second line with said second pair.

4. An apparatus as claimed in claim 3, wherein said second line is substantially at right angles to said first line.

5. An apparatus as claimed in any one of claims 1 to 4, further comprising a first housing, said first body being in said first housing.

6. An apparatus as claimed in claim 5, wherein said holding means is means for producing a constant flow of fluid directed, in use, from said first housing impinging on said sheet to maintain the housing at a constant distance from the sheet.

7. An apparatus as claimed in any one of claims 1 to 6, further comprising a second housing, said second body being in said second housing.

8. An apparatus as claimed in claim 7, wherein said maintaining means is means for producing a constant flow of fluid directed, in use, from said second housing impinging on said sheet to maintain the second housing at a constant distance from the sheet.

9. An apparatus for non-contacting measurement of the thickness of a sheet of non-magnetic material, substantially as hereinbefore described with reference to and as shown in Figure 5 of the accompanying drawings.