GB2038483A - Measuring thickness capacitively - Google Patents

Abstract

A capacitance sensor 10 comprises a slotted conductive body 11 with an electrode 29 symmetrically disposed in the slot 13 and insulated from the body 11. The sensor 10 is used to sense the thickness of a dielectric body 40 by reciprocally traversing the body in engagement therewith while energized by an oscillator 61. An adjustable reference oscillator 70 is tuned to the frequency of the first- mentioned oscillator when disengaged from the body and its frequency is adjusted during disengagement periods to re-calibrate the apparatus. Comparison of the oscillator frequencies when the first- mentioned oscillator is engaged with the body gives an indication of the body thickness. <IMAGE>

Description

SPECIFICATION Method, apparatus and device for measuring the variations in thickness of elongated plastics films.

The invention relates to methods and apparatus for the use and control of machines adapted to film or cast films of dielectric material such as plastics which machines utilize adjustable filming, extruding and casting dies so as to fabricate elongated structures of uniform thickness or cross section.

The prior art with which the invention is concerned is exemplified by United State Patent 3,764 899 issued October 9, 1 973, for apparatus for measuring variations in thickness of elongated sample of thin plastic film, to Paul H.

Peterson and DuWayne M. Thon, and assigned to the present applicants. Further examples of similar devices and apparatus may be seen in United States Patents Nos. 2,562,575, 3,523,246, and 2,782,367.

The invention includes an improved method and apparatus for profiling the thickness of a plastics film, or the like, which enables that the measuring apparatus to be continuously recalibrated to maintain consistency in operation during successive cycles of operation and further includes an improved capacitive sensing element for determining thickness of a dielectric film from one surface only, and for reducing or eliminating the effect of ambient environmental temperature variations as are commonly found in connection with the operation of apparatus for producing the elongated plastics films.

Examples of the invention will now be described with reference to the accompanying drawings in which:~ Figure 1 is a side elevational view of a capacitive sensing device incorporating the principles of the invention.

Figure 2 is a top plan view of the device of Figure 1; Figure 3 is a sectional view taken along section line 3-3 on figure 2; Figure 4 is a sectional view taken along line 4-4 on Figure 1; Figure 5 is a schematic and diagrammatic representation of a typical embodiment of the method, apparatus and device of the invention; Figure 6 is a further embodiment of the device of the invention; and, Figure 6A is a partial fragmentary sectional view taken along section lines 6A-6A on Figure 6.

Referring now to Figures 1-4 of the drawings, a capacitive sensor 10 shown having a top portion 11 formed of suitable metallic substance such as aluminum, and having a cavity 12 and an upwardly opening slot 13 in a flat top surface 21.

Slot 13 is shown having parallel sides 14 and 15 and a plurality of passages 16, 17, 18 and 19 formed as downwardly facing openings along the lower surface of top 11. As may be seen on Figures 2 and 4 of the drawings, capacitive sensor 10 has a longitudinal extending center line plane indicated generally by reference character 20. It may be noted that the device is symmetrical about this center plane. Capacitive sensor 10 also has a bottom portion 23 formed of suitable metallic material, such as aluminum, and is provided with a pair of apertures 24 and 25. An elongated electrode 29, formed of a rod-like element preferably of material having a low or no temperature coefficient such as Invar, is shown disposed this longitudinal axis coincident with center line plane 20 and spaced equidistantly from sides 14 and 15 of slot 13 in top 11.Electrode 29 is shown supported on a pair of rod-like support members 30 and 31 that are supported and held toward their lower extremities in apertures 24 and 25, respectively, by masses of suitable insulating material 26 and 27 and further including terminals 34 and 33, respectively, extending downwardly of bottom element 23. Electrode 29 is suitably attached to the top ends of support members 30 and 31 and is preferably positioned with its top in or slightly above the plane of surface 21 on the upper side of top member 11.

It may thus be seen that capacitive sensor 10 is comprised of a metallic hollow housing that may be suitably grounded and a capacitance forming electrode disposed in a slot in its top surface for engagement with a stationary or moving film of plastics.

Referring to Figure 5 of the drawings, apparatus for performing the method is shown in operative relationship to a typical plastics filming machine 41 which produces a continuous plastics film 40 by extruding liquid plastics material through a filming die 42 having an adjustable slit 43. In a typical installation, the plastics film is extruded in the direction shown in the arrow to the right of the drawing and is accumulated on a roller-type accumulator (not shown) after it has passed over roller 56. In the space between the filming apparatus 41 and roller 56, the film may be withdrawn to a point whereat it is stabilized and thus may be measured to determine that the proper thickness profile has been achieved.

In Figure 5, a capacitive sensor 10 is shown having its slot parallel to the direction of motion of the plastics film 40 and is suitably mounted on top of a mounting means therefor, 55, which may include manual means for adjusting sensor 10 upwardly and downwardly and may also include motor means for lowenng sensor 10 out of contact with the moving film of plastics 40 during portions of a traversing cycle of operation of sensor 10 in a direction perpendicular to the line of travel of plastics film 40.Mounting means 55 is shown disposed on the end of a piston rod 45 that is reciprocated back and forth through the operation of a double acting hydraulic cylinder 44 that is in turn controlled by a hydraulic supply 46 connected to cylinder 44 through tubing 47 and 48 and is in turn controlled from signal derived from adjustable limit switches 51 and 53 connected to terminals 49 and 50 through conductors 53 and 54, respectively, to provide continuous reciprocation of mounting 55 and sensor 10 when energized from a suitable source of power (not shown). Fragmentary views of sensor 10 are indicated by reference characters 1 OA and 1 OB to illustrate the position of sensor 10 during a cycle of operation in a traverse across plastics film 40.

A signal processor 60 comprised to like or similar element as is shown in United States Patent 3,764,899, is used to provide a source of reference signal of a frequency in the range of 106 Hertz is shown as oscillator 70 and a further oscillator incorporating the capacitance of sensor 10 is indicated by reference character 61. The frequencies of the oscillators are compared and any difference is present at the output of a discriminator 80. The signal appearing at the output of a discriminator 80 may be applied to a digital indicator 106 and/or to a chart recorder indicated generally by reference character 111.

Oscillators 61 and 70 are intended to have a predetermined constant relationship with respect to their frequencies of oscillation. Oscillation 70 is shown adapted to be suitable controlled in frequency by adjustable capacitors 75 and 76 connected between ground and input terminal 73 through conductors 74 and 79, respecively.

Adjustable capacitor 75 is shown connected through suitable driving means to dial 77 and is intended to be manually operated. On the other hand, adjustable capacitor 76 is shown connected through suitable driving means 103 to a servo motor 100 having input terminals 101 and 102.

Input terminals 101 and 102 are connected to output terminals 85 and 86 on discriminator 80 through conductors 104 and 105 and switch 96.

Switch 96 is adapted to connect the input terminals on servo 100 to the output of discriminator 80 at each end limit of a cycle of operation of sensor 10 and is connected to limit switches 51 and 53 through conductors 104 and 105, respectively, such that switch 96 is operative to connect the output of discriminator 80 to control servo 100 during the times that sensor 10 is out of contact with plastics film 40 either through attaining a position on either side extremity of film 40, or upon lowering out of contact with film 40 through motor means associated with mounting 55 that may, for example, be actuated by limit switches 51 and 53 to lower sensor 10 before reaching the edge extremities of plastics film 40.Although there are many ways of accomplishing this function, one example would be to use pneumatic air motors and valves operated by mechanical limits or through operation of electro-pneumatic valves controlled by limit switches 51 and 53.

Oscillator 61 is shown having a pair of input terminals 62 and 63 connected to a capacitive sensor electrode terminal 33 or 34 through conductor 64, terminal 65 and conductor 66 and to ground, respectively. Output terminal 68 of oscillator 61 is shown connected to input terminal 83 on discriminator 80 through conductor 69 and input terminal 84 thereon is shown connected to output terminal 71 on oscillator 70 through conductor 72. Discriminator 80 has a further terminal 81 connected to ground through conductor 82. The output terminals of discriminator 80 are connected to the input terminals on a level control calibrating potentiometer 90 through conductors 94 and 95.

Calibrator 90 includes a manually adjustable dial 91 and a pair of output terminals 97 and 98.

Output terminals 97 and 98 are shown connected to digital indicator 106 (which may have a suitable analog to digital converter therein) through conductors 110 and 109 connected to input terminals 108 and 107, respectively. Output terminals 97 and 98 are also connected to the stylus positioning motor 115 on strip recorder 111 through conductors 118 and 119 connected to input terminals 116 and 117, respectively.

Strip chart recorder 111 is shown a moving strip of suitable recording paper responsive to a typical stylus positioned by motor 115 in accordance with the signals applied thereto for permanently recording the variations and position of the stylus. Strip chart 112 includes a plurality of parallel longitudinally extending lines which may be provided with appropriate scales depending upong the application and, under operating conditions, will provide a plurality of successive profiles of plastics film 40 as sensor 10 is successively moved from side to side. At the beginning and end of each cycle of traverse, a notch in the profile is indicated by zeros 11 3 for reasons set forth below.

Referring to Figure 6 and 6A of the drawings, a rotary die filming apparatus is shown indicated -generally by reference character 120 to include a rotating circular slit 121 through which liquid plastic is continuously extruded and drawn off as the die is rotated. A plastics bubble indicated by reference character 122 is maintained through the use of suitable air pressure interiorly of the apparatus and because the film is continuously rotating, a continuous profile may be extracted by the positioning of sensor 10 in the manner shown near the top of Figure 6 in an attitude indicated in Figure 6A with top surface 21 in capcitance sensing engagement with the film 122.In a typical rotating filming apparatus, a plurality of rollers 123 and 124 are utilized to compress the bubble into a two-layered film shown passing toward the right over roller 125 to a suitable accumulator.

Assuming that plastics filming apparatus 41 is in operation, the various hydraulic electrical pneumatic and electronic elements shown in Figure 5 are energized from suitable sources of energy (not shown). Sensor 10 is positioned out of contact with film 40. A sample of plastics film of the desired thickness is disposed on top surface 21 in contact therewith and therefore with the top of electrode 29. The frequency of oscillator 70 is adjusted by dial 77 so that stylus 115A of motor 11 5 is disposed at a suitable location transversely of the desired scale on strip chart 112 (it should be understood that the range of movement of stylus 11 5A may be suitably adjusted by the use of level control 90).It should further be noted that during this intial calibration, servo 100 is disabled or remains unconnected to the output of discriminator 80. The sample may then be removed and motor 11 5 will retract stylus 1 1 5A into its at rest position. At this time, there should be no output appearing at output terminals 85 and 86 of discriminator 80. Should the frequency of oscillator 70 change, for any reason, the connection of servo 100 to the output of discriminator 80 will cause adjustment of adjustable capacitor 76 to provide the desired frequency.

Having calibrated oscillator 70 to the desired frequency the remainder of the system may be energized and hydraulic cylinder 44 will be controlled to continuously reciprocate sensor 10 intermediate its extremes of travel as determined by limit switches 51 and 50. The top surface 21 of sensor 10 is adjusted so that during the transverse of plastics film 40 a continuous contact with the lower surface of film 40 is attained and this may cause a slight upward deformation of plastics film 40 but not such as to cause any permanent deformation or change in thickness of the film.

It may be noted that during the cyclical operation of the transverse movement of sensor 10, the method of operation of the overall apparatus in such that when sensor 10 reaches its extreme limits of operation at either side of its transverse motion, limit switches 51 and 53 are effective to either place sensor 10 transversely out of contact with the plastics film or may be utilized, in the manner suggested above, to cause retraction of sensor 10 downwardly into mounting means 55, and in either event, sensor 10 is out of contact with the plastics film and servo motor 100 is connected to the output of discriminator 80 through switch 96 to, if necessary, readjust the frequency of oscillator 70 to, in effect, continuously recalibrate oscillator 70 to maintain the desired mode of operation.

As may now be apparent, the record generated on chart strip 1 12 may indicate the exact thickness of plastics film 40 or may serve to indicate variations of plastics film 40 with respect to successive transversely spaced portions thereof.

Further, it has been determined that the hollow interior of sensor 10 may be provided with suitable air passages as indicated on Figures 1-4 to provide for the free flow of air into and out of the hollow cavity or, may, under certain circumstances, be connected to a source of vacuum to tend to maintain a downwardly acting force on plastics film 40 to maintain it into secure close contact with top surface 21 and electrode 29 of sensor 10.

It has further been noted that the electrode and ,its mounting may be comprised of suitable material having a very low temperature coefficient, i.e. comparable with that of Invar, and it may also be desirable, under certain circumstances, to fabricate the hollow conductive body of sensor 10 of the same or similar material.

From a consideration of the above description of the elements and of the operation of the thickness profiling system and measuring device, it should now be clear that the method and apparatus for performing the method, will provide a unique and unobvious concept for determining the thickness of a film or body of dielectric material in which successive transverse measurements are made to provide signals proportional to the thickness of the material and, at appropriate, repetitive times during the cyclical transverse measuring cycle, a determination is effected to ensure that the measurements are consistently based upon a uniform reference datum. It may further be appreciated that the effect of environmental ambient temperature variations is minimized by the use of the unique and unobvious sensor as well as the application of the principles of the method and apparatus for performing the method as recited above.

Claims (9)

1. A method of measuring the thickness of a dielectric body which comprises: traversing a body of dielectric material with a capacitance sensor in engagement with said dielectric; utilizing said sensor as a frequency determining means in an oscillator: comparing the frequency of said oscillator with a source of reference frequency to provide a signal proportional to the thickness of said dielectric material; displaying said signal on a recording means including a reference index; periodically disengaging said sensor from said body of dielectric material and comparing the signal displayed with said reference index.

2. A capacitance sensor for determining the thickness of a plastics film from one or the other surface thereof, comprising in combination: an elongated hollow conductive housing having a major surface configured to engage one surface of a film of plastics and having an elongated aperture of uniform width to define parallel sides extending along said elongated major surface to provide an opening into the hollow interior of said conductive housing; a conductive electrode configured so as to provide equidistant spacing from the side and end extremities of said elongated aperture in said hollow conductive housing; conductive means mounting said conductive electrode in said elongated aperture, said last named means extending through another surface of said hollow conductive housing and including means insulating said conductive means from said housing; and a first terminal connected to said hollow housing, and a second terminal connected to said conductive means exteriorly of said hollow housing.

3. The device of claim 2, in which the major surface is planar.

4. The device of claim 2 or claim 3, in which the top of the conductive electrode and the major surface of the hollow housing are co-planar.

5. The device of any one of claims 2 to 4, in which the conductive electrode comprises material exhibiting a very low coefficient of thermal expansion.

6. Apparatus for continuously determining the thickness of a moving dielectric comprising: in combination, a capacitance sensor; mounting means therefor, said mounting means including means for cyclicly reciprocating said sensor transversely of the direction of travel of a moving dielectric body and for moving said sensor into and out of contact with one surface of said dielectric body; first oscillator means connected to said sensor so that the frequency of oscillation is determined by the capacitance of said sensor; second adjustable oscillator means; means for comparing the frequencies of said first and second oscillator means; means connected to said second oscillator means for adjusting the frequency of said oscillator means to that of said first oscillator means when said sensor is out of contact with said moving dielelctric body; and control means connected to said second oscillator means including means for comparing the outputs of said oscillator means when said sensor is out of contact with said moving dielectric body and means for adjusting the frequency of said second oscillator means whenever the frequency of said second oscillator means undergoes a change in frequency.

7. A method of measuring the thickness of a dielectric body as hereinbefore described with reference to the accompanying drawings.

8. A capacitance sensor for determining the thickness of a plastics film from one or the other surface thereof substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

9. Apparatus for continuously determining the thickness of a moving dielectric substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.