US 3572566 A
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United States Patent OSCI LLATOR DISCRIMINATOR D. C. OUTPUT 3,156,109 11/1964 Fieldgate ABSTRACT: A vacuum box having a capacity transducer located in a slot in one of the sidewalls. The transducer consists of a conductive mesh, a porous dielectric material and a flexible metallic strip which at one end is biased away from the sidewall and fixed to the wall at the other end. As a web loop enters the box, a pressure differential is created by the web loop between the atmosphere above the loop and that below the loop. The vacuum below the loop acts upon that portion of the metallic strip below the loop bringing it close to or in contact with the porous dielectric and the conductive mesh thereby changing the capacitance of the transducer as a function of the length of the loop. The capacitor forms a portion of the tuned element of an oscillator, the frequency of which varies linearly with changes in the position of the loop.
Patent ed March 30, 1971 3,572,566
PE/PFOR/JTED 0/ELEC77P/C LAYER l5 F/ LOOP 33 CONDUCT/l E ME .5 H [4 QSC I LLATOR DISCRIMINATOR 20 l VACUUM PUMP 0.0. OUTPUT Fl RoaERr M FRASER ROGER K. LEE
AGENT BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a web position sensor and more particularly to such a sensor employing a capacitance transducer.
2. Description of the Prior Art In web transportation systems, such as employed in film processing and reproducing apparatus, tape storage, processing and recording apparatus, and web treating apparatus, it is desirable and in many instances necessary to provide tensioning means at various points in the system, to insure high quality and uniformity of operation of the apparatus by way of providing precise tension control and/or control of the transport system; and assuring proper feed and alignment of the web at the processing or reproduction station.
Very often vacuum tensioning boxes are employed for this purpose. A vacuum tensioning box is an enclosure in which the web is looped. A vacuum is created in the box acting upon the loop to provide a fixed or variable length of web material in the box. The most obvious advantage of a vacuum technique over other tensioning means is that the web need not come into contact with any mechanical elements and thereby web length is controlled without endangering the surface thereof, that is without scratching or rubbing the surface and destroying the quality of the web, orremoving or distorting information which may be recorded or is about to be recorded on the surface. By applying'a tension to the web and providing a loop of material prior to an operating station, irregularities in the drive system which may sever the web or cause misalignment or bunching at the station are eliminated. Further signals indicative of the web loop length in a vacuum box or boxes may be used to adjust or control the drive means.
Apparatus is disclosed which utilizes a transducer which senses the position of a web loop in a tensioning box to produce loop length signals which may be employed to change the drive velocity or velocities in a predetermined manner.
Prior art methods to determine the positioning of a loop of web material in a vacuum or pneumatic tensioning box have utilized photoelectric cells or pressure transducers to determine the web loop position thereby providing error or positional signals for the web drive means. A high performance pneumatic web transport system in which photosensitive devices are used is limited since the physical size of the photo cell is limited necessitating amplification of the signal therefrom in the servo system loop at unreasonable levels, thereby causing distortion and a lack of linearity of the signal. Further, such devices possess nonlinear responses caused by laclr of uniformity of light reflections when the web loop passes the photo cell. Pressure transducers sense the pressure in the vacuum box, a precalculated indication of the loop position", Often times it is desirable to control the web length by SUMMARY OF THE INVENTION The present invention eliminates the problems of the prior art by pneumatically detecting the line of demarcation at the web loop, between the atmospheric pressure and pressure caused by the presence of the web loop. Operation of the inventive device is independent of pressure variations above threshold pressure values. The sensor responds linearly as a direct function of the loop position and quite independently of the pressure in the atmosphere or the pressure of the vacuum and independently of web tension. The sensor is a capacitor, the output of which varies linearly with respect to loop position, having a perforated conductive mesh placed over a slot in a tensioning box and a flexible conductive strip biased away from the mesh at one end and fixed at the other. Between the mesh and strip, the plates of the capacitor, is a permeable or porous dielectric material.
DESCRIPTION OF THE DRAWING FlG. 1 shows a schematic presentation of the capacitive transducer mounted on a vacuum tensioning box.
FIG. 2 is a side view of the capacitive transducer illustrating its various elements. DESCRIPTION OF THE EMBODI- MENTS Turning now to FIG. 1 which depicts the invention in a preferred embodiment, there is shown a vacuum or tensioning box 11 of rectangular cross section, slightly tapered, having an elongated aperture or slot 12 formed in one side wall 13 thereof of a height slightly less than the overall height of the vacuum box. Placed over the aperture is a fixed strip of perforated or porous dielectric material 15. A thin, flexible conductive strip 16 is fixed to the sidewall 13 of box 11 at one end and biased away from the side wallat the other end by a suiteble tensioning means such as spring 17 placed over the dielectric material 15. The strip 16 maybe made of a material such as copper foil or any other suitable, flexible, conductive material. A tuned oscillator circuit 2l is electrically connected between the fixed capacitorstrip 14 and the movable strip 16. The strip 16, dielectric l5 and perforated conductor 14, form a capacitor 18 which varies in value as a function of movement of the strip 16. The capacitive transducer 18 forms part of the tuned element of the oscillator 21;. The oscillator output is fed to a discriminator 22. i
A loop of web material 31 is placed in the vacuum box. The web material may be a photographic film, magnetic tape, or a like material which is being driven through a processor, reproduction apparatus or any type-of web processing or handling apparatus. A vacuum pump 20 is connected to a conduit 19 emanating from the bottom of the vacuum box thus evacuating the box and causing a tension to be applied to the web loop 31. By properly designing the box by empirical methods, a sharp pressure gradient will exist at the bottom 33 of the web loop 31, atmospheric pressure above the loop and substantial vacuum pressure below the loop. The vacuum created below the loop forces the movable, flexible capacitor plate 16 against the dielectric material 15 for that length of the material below the loop edge while the remaining length of the flexible capacitor plate 16 remains biased away from the fixed capacitive plate 14 by the spring tensioning or bias means 17. The capacitive transducer 18 forms the capacitive portion of the oscillator 21 such as a Colpitts Oscillator, the frequency of which varies directly as a function of capacitance. The discriminator circuit 22 to which the oscillator 21 output is coupled converts the varying frequency output of the oscillator to varying DC voltage.
FIG. 2 illustrates a side view of the variable capacitor 118. The film loop 33 is shown on the interior of container wall 13. On the exterior of container wall 13 is located conductive mesh 14 upon which there is placed the perforated dielectric layer 15 and finally the flexible capacitive plate.
By tapering the flexible strip 16, that is by decreasing the width of the strip from top to bottom as shown, the linearity of the change in capacitance with the change in film loop length can be controlled, or some desired nonlinearity can be introduced into the system. Thus, as a result of the chain of events described immediately above, the DC output voltage will vary directly as a function of the height of the web loop in the vacuum box.
it is obvious to one skilled in the art that the cross-sectional shape of the vacuum box is a function of the design of the system and well within the purview of one skilled in the art. The shape is dependent upon many factors such as the vacuum pressure desired, the character and dimensions of the web to be tensioned, and other similar factors.
Although the web position sensor has been described utilizing a variable capacitor, other electrical parameters could be sensed in a similar fashion. For instance, the resistance between the two conductive elements could be the parameter indicative of the web loop position, or alternatively a variable inductance might be used. Another possibility is the utilization of the vacuum box itself as one of the conductive elements in the position sensor. Also, the particular arrangement of a fixed member and a flexible member might be varied in a fashion to provide the same functional result. For instance, an arrangement might be designed in which both electrical conductors are flexible and each varies its position relative to the other.
Thus, a web loop sensing device has been fully and completely described which detects web loop position in a variable pressure system obviating the problems of prior art sensing systems of this nature.
1. Apparatus for measuring the position of a loop of web material in a vacuum tensioning container having the loop of web material variably positioned therein under the influence of a vacuum to maintain a relatively constant tension on the web, and comprising a variable capacitive sensing means positioned along the length of a wall of said container for producing a varying capacitance depending upon the relative position of the web loop in said container, and including:
a. a first, relatively inflexible, capacitive element means positioned along the length of said wall and having an interior side in communication with the interior of said container and an exterior side in communication with the exterior of said container, said first capacitive element means having at least one orifice formed therein along its length for allowing a pressure differential between the interior and exterior sides of said first capacitive element means to be transmitted therethrough;
b. a dielectric means positioned in proximity to said exterior side of said first capacitive element means, said dielectric means also having means for allowing a pressure differential to be transmitted through the dielectric means; and
c. a second, relatively flexible, capacitive element means,
positioned to have said dielectric means interposed between the second capacitive element means and said exterior side of said first capacitive element means, for responding to a pressure differential above and below the web loop in said container by flexing along its length depending upon the position of the web loop in said container to change the total capacitance of said variable capacitive sensing means, whereby said second capacitive element means is drawn into a position of close proximity with said first capacitive element means below the position of the web loop in said tensioning container by the pressure differential transmitted through said first capacitive element means and said dielectric means, and flexes away from said first capacitive element means at positions above said web loop.
2. Apparatus as set forth in claim 1 wherein said dielectric means includes a strip of dielectric material having at least one orifice formed therein along its length.
3. Apparatus as set forth in claim 2 wherein said wall has an elongated aperture formed therein along its length, said first capacitive element means is fixed to said wall over said aperture, and said second capacitive element means is fixed to said wall at one end of said aperture and is biased away from said wall at the other end of said aperture.
4. Apparatus as set forth in claim 3 wherein the width of said second capacitive element means is tapered along its length.
5. Apparatus as set forth in claim 1 wherein said wall has an elongated aperture formed therein along its length. said first capacitive element means is fixed to said wall over said aperture, and said second capacitive element means is fixed to said wall at one end of the aperture and is biased away from said wall at the other end of said aperture.
6. Apparatus as set forth in claim 1 wherein the width of said second capacitive element means is tapered along its length.