WO1994023274A1

WO1994023274A1 - A compliant skirt for a drafting plotter

Info

Publication number: WO1994023274A1
Application number: PCT/US1994/003516
Authority: WO
Inventors: Robert M. Mcandrew; James R. Brandt; Nathan Hohle; Patrick T. Gilmore
Original assignee: Summagraphics Corporation
Priority date: 1993-03-31
Filing date: 1994-03-30
Publication date: 1994-10-13

Abstract

A compliant skirt (26) is provided for use with a large plotter (10) and media (14) being plotted upon. The skirt (26) is attached to a platen means (18) of the plotter (10) and complies or flexes in response to the moving media (14) and follows the media (14)as it moves over the skirt (26). Such compliance of the skirt helps maintain close contact between the media (14) and compliant skirt (26) which prevents separation of the media (14) from the skirt (26) and thereby eliminates formation of media waves. In addition, compliance of the skirt (26) absorbs energy associated with excess motion of the media (14) which reduces tugging loads.

Description

A COMPLIANT SKIRT FOR A DRAFTING PLOTTER FIELD OF THE INVENTION

This invention relates to drafting plotters and more particularly to a device that prevents media separation from a writing surface of the plotter as a result of disturbances 5 caused by the moving media.

BACKGROUND OF THE INVENTION

In a standard plotter the media path consists of a flat horizontal portion along the platen and a rigid extension that curves or extends from a horizontal to a vertical position. When the media moves during plotter operation, undesirable waves or bulges in the media 10 along the flat top surface or drive section of the platen are created for the following reasons. The first and primary reason is that initial difference in acceleration and velocity of the portion of the media at the drive section of the platen and that hanging below the platen causes the media to overrun itself, i.e., causes a bulge. Secondly, rapid media movement creates positive lift between the media and platen which causes the media to separate from the platen. 15 The wave or bulge creates two problems during plotting, namely pen disturbances and tracking errors. Pen disturbances occur because the wave interferes with the pen operation. The waves are large enough to lift the pen from the medium, strike the pen while it is in a pen- up position, and allow the pen to puncture and_. hold the media, dislodging the pen from its holder. Tracking errors occur because rapid variations in the size of the wave cause the drive 20 system to frequently load and unload. For instance, when the media wave releases, the slack in the media is removed and the media under the drive rollers pulls against the rollers. In addition, rapid changes in the media direction also cause pulling and tugging between the lower and upper sections of the media. The tugging and pulling weakens the media beneath the drive roller, and consequently the media may dislodge and/or tear. Both pen disturbances 25 and tracking errors are amplified with repetitive high acceleration and high velocity x-axis motions. In addition, the waves cause excess media movement which increases the noise of the plotter.

Another problem during plotter operation is that short vectors plotted near the center J of the medium exhibit distortion of the vector during short, high acceleration movements of

_* 30 the media. Specifically, for large media sizes holding the media at the extreme edges while the remaining media portion hangs unsupported creates noticeable deflection of the lines plotted in the middle of the sheet. Since the media is only driven at its edges parallel to the direction of motion the center of the media lags behind until constant velocity is reached. U.S. Patent No. ^' 4,734,987 to Cleveland attempts to provide a solution to these problems. This patent provides a moving node isolator that is a thin flexible cantilever beam consisting of three layered strips which extend away from the plotter. The patent states that the isolator reflects waves in the medium by (1) holding the medium away from the chassis, (2) forming a volume of air between the medium and the platen, and (3) thereby acting as a damper, pumping air in and out as the wave isolator reacts to the moving medium. The patent also states that the isolator also functions to reduce scalloping by isolating the free-hanging medium mass so that the load of the free-hanging medium on that portion under the pen is reduced. OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the invention disclosed to prevent media waves from interfering with the pen during plotter operation. Another object of the invention is to prevent separation between media and platen during media movement on a pen plotter.

Another object of the invention is to prevent pen skipping during media movement on the plotter.

Yet another object of the invention is to prevent pen dislodging during media movement on the plotter.

Another object of the invention is to prevent pen punctures in the media during media movement on the plotter.

It is another object of the invention to reduce the noise and motion in the free-hanging portion of the moving media. Another object of the present invention is to provide device which is capable of use with various types of media.

Another object of the present invention is to reduce the wear on the driving drums and their associated pinch-rollers during plotter operation.

A further object of the present invention is to reduce media distortion in the middle of the media. The above objects are achieved in accordance with the invention by a compliant skirt mounted to a platen or chassis of a plotter which complies or flexes in response to the moving media and follows the media as it moves over the skirt. Such compliance of the skirt helps maintain close contact between media and compliant skirt which prevents separation of the media from the platen and thereby eliminates formation of media waves. In addition, compliance of the skirt absorbs energy associated with excess motion of the media and thereby reduces tugging loads on the media drive system.

In particular, the compliant skirt runs the length of the platen and is mounted one to the front and one to the rear ends of the platen. The skirt comprises at least one flexible sheet of film that has one end mounted to the platen edge and the other end mounted either to the platen or to other mounting means of the plotter. The sheet of the skirt is curved to form a cavity or loop that is, open at the ends. The loop acts as a media damper, pumping air in and out as the unsupported hanging section of the medium sways back and forth. As a result there is reduced swaying and a reduction of excess motion and tugging loads. In the preferred embodiment of the present invention, the skirt comprises three sheets of flexible film. Each sheet has the same length as the paper, but the different sheets have different widths in the direction of paper movement. The skirt provides variable compliance so that it is effective for use with any media specified for pen plotters.

In accordance with the invention, a plotter operable with a sheet medium to be plotted on, the plotter including a chassis, a platen means for supporting the sheet during plotting, and drive means for moving the sheet to traverse the platen means and then moving the sheet outwardly and downwardly from the platen, the improvement of a compliant skirt for supporting the sheet exiting the platen means, the skirt comprising an upper portion mounted to the plotter adjacent the platen means, a lower portion spaced from the upper portion and also mounted to the plotter, and an intermediate supporting portion between said upper and lower portions, said intermediate portion extending outwardly and downwardly from the platen means for flexibly and resiliently supporting the portion of said sheet draped thereover. In accordance with another aspect of the invention, a plotter operable with a sheet to be plotted on, the plotter including a chassis, a platen means for supporting the sheet during plotting, and drive means for moving the sheet to traverse the platen means and then moving the sheet outwardly and downwardly from the platen, the improvement of a compliant skirt for supporting the sheet exiting the platen means, the skirt comprising an upper portion closely adjacent and having similar elevation as the platen means, a lower portion spaced from the upper portion, an intermediate supporting portion between said upper and lower portions, said intermediate portion extending outwardly and downwardly from the platen means for flexibly and resiliently supporting the portion of said sheet draped thereover, and means for mounting said skirt to said plotter.

The above and other objects, aspects, features and advantages of the invention will be more readily apparent from the description of the preferred embodiment thereof taken in conjunction with the accompanying drawings and appended claims. BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated by way of example and not limitation in the figures of the accompanying drawings in which like references denote like or corresponding parts, and in which: Fig. 1 is perspective view of a large drafting plotter employing the compliant skirt of the present invention;

Fig. 2 is a cross-sectional view of the rear of the plotter shown in Fig. 1 taken along line 2-2 incorporating the compliant skirt according to the preferred embodiment of the present invention; Fig. 2a is a cross-sectional view of the compliant skirt shown in Fig. 2 removed from the plotter;

Fig. 2b is a cross-sectional view showing the flat individual unattached and unbonded sheets of the compliant skirt shown in Fig. 2a;

Fig. 3 is a cross-sectional view of the rear of the plotter shown in Fig. 1 taken along line 3-3 incorporating the compliant skirt according to the preferred embodiment of the present invention;

Fig. 4 is a cross-sectional view of the rear of the plotter incorporating the compliant skirt according to another embodiment of the present invention;

Figs. 5-7 are schematic views of the embodiment shown in Fig. 4, flexing in response to the moving medium draped thereover; Fig. 8 is a schematic cross-sectional view of the plotter of Fig. 1 incorporating the skirt according to another embodiment of the present invention, flexing in response to the moving medium draped thereover; and

Fig. 9 is a cross-sectional view of the skirt of Fig. 1 illustrating the embodiment shown in Fig. 8 in a static equilibrium state.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to Fig. 1, a large format plotter with a drafting sheet medium 14 is shown incorporating the present invention. Medium 14 may be a sheet of paper, vellum or mylar or any medium specified for a large plotter. Also medium 14 can be a continuous roll supply. For such large plotters, the sheet can be as large as 36 inches by 48 inches and can weigh up to 6 ounces. The plotter 10 includes elements such as a carriage 12 for supporting a pen, a motor (not shown) for driving the carriage, a pen changer 13 for storing a quantity of pens for plotting, means (not shown) for moving the pen changer to select the pen from the pen changer 13, and a microprocessor (not shown) for controlling these elements. These elements are known to those skilled in the art and are not discussed further herein.

The rear and front portions of the plotter 10 are shown in detail in Figs. 2 and 3 respectively. Plotter 10 includes a chassis 20 and support means 22 mounted to the chassis for providing continuous support to the medium 14 draped thereover. The support means 20 comprises a platen means ? 8, a compliant skirt 26, and a means 24 for mounting the skirt 26 to the platen means 18. Figs. 2 and 3 show medium 14 and skirt 26 in a static equilibrium state.

The platen means 18 includes a principal platen 30 mounted on the chassis 20 for supporting the medium 14 directly under the pen and a platen extension 34. In the preferred embodiment the platen extension 34 is mounted in a recess 38 in the chassis 20 just below the edge of principal platen 30. Platen extension 34 runs the length of platen 30. The platen extension 34 extends away from the chassis 20 and generally parallel to the platen 30 and provides additional support to medium 14 over a larger portion of medium 14. In the preferred embodiment the platen extension 34 is in the rear of the plotter 10. However, the location of the platen extension 34 (front or rear) is not critical. Platen extension 34 has a raised section 42 on the top thereof to better provide continuous support to medium 14 as it moves from platen 30 to platen extension 34. The raised section 42 are ribs, spaced 4 to 6 inches along the width of the platen extension 34. Raised section 42 extends generally the width of platen extension 34, leaving only a short portion at each end thereof.

The mounting means 24 for mounting the skirt 26 to plotter 10 comprises clamps 56a and 56b securing the compliant skirt 26 thereto, a bottom plate 54 mounted to the chassis 20, and an adapter 48 mounted to the edge 34a of the extension 34 and the bottom plate 54. The specific details relating to mounting compliant skirt 26 and adapter 48 to platen extension 34 will be discussed below.

In the preferred embodiment adapter 48 is generally "L" shaped. It has a ledge 48a, long portion 48b, short portion 48c, and a brace portion 48d. Long portion 48b extends downwardly from the bottom end edge 34a of extension 34 at an angle of approximately 90 degrees with respect to platen 30 and platen extension 34. Long portion 48b extends just below the plane of the bottom plate 54. The short portion 48c extends outwardly from the end of long section 48b at an angle of approximately 90 degrees. Brace portion 48d extends upwardly from the end of short portion 48c at an angle just slightly less than 90 degrees. Bottom plate 54 is mounted to the bottom of chassis 20 by screw 58 and mounted to long portion 48b of adapter 48 by screw 52 to stabilize adapter 48 when medium 14 moves over the compliant skirt 26 mounted thereto. Adapter 48 however may have many shapes and angle measures to properly mount the skirt 26 to the plotter structure.

Compliant skirt 26 comprises an upper portion 28 mounted to the chassis 20 adjacent the platen means 18, a lower portion 32 spaced from the upper portion 28 and also mounted to the chassis 20, and an intermediate supporting portion 36 between the upper and lower portions 28,32. The intermediate portion 36 extends outwardly and downwardly from the platen 30 for flexibly and resiliently supporting the portion of medium 14 draped thereover.

The skirt 26 comprises at least one sheet of flexible film. In the preferred embodiment shown in Figs. 2 and 3, the skirt consists of three sheets of film 44a, 44b and 44c. The outermost surface of the sheet which contacts medium 14 must be wear and electrostatic charge resistant as well as exhibit low friction. In the preferred embodiment the sheets of film comprise Fluorglas material (made by Fluorglas Company) because it exhibits these characteristics.

Skirt sheets 44a, 44b and 44c with the assistance of adapter 48 and bottom plate 58 (and clamps 56a, 56b and 56c discussed below) extend the width of the platen means 18. Sheets 44(a-c) are of three different widths arranged so that the widest is on top and the narrowest is on the bottom. As best shown in Fig. 2a and 2b, the three sheets are joined or bonded to one another by pressure sensitive adhesive along a first edge point A and are clamped by clamp 56b. Also sheets 44a and 44b are joined or bonded together by adhesive at point B and sheets 44b and 44c are bonded together at point C to provide desired stiffness of the sheets at the end thereof to effect optimal compliance or flexibility of the sheets. The free edges of the sheets are secured to one another by clamp 56a. To achieve this result, the sheets are curled in the form of a loop with the widest sheet 44a on the outside. The sheets are curled with their free edges aligned and then inserted into clamp 56b and crimped. The distances between point A and B and point A and C are 3 inches and 1.5 inches respectively. Clamp 56a with the edges of the sheets 44(a-c) held therein is secured to the bottom end edge 34a of platen extension 34 by ledge 48a of adapter 48 and a screw (not shown) inserted therethrough. Clamp 56b is secured to brace portion 48d of the adapter 48 by a screw (not shown) inserted therethrough. In this configuration narrowest sheet 44c forms a loop with adapter 48, sheet 44b forms a loop which encompasses sheet 44c, and the widest sheet 44a forms a loop which encompasses sheets 44b and 44c. Medium 14 is shown draped over the widest sheet 44a. In the preferred embodiment, the outer two sheets 44a,44b are 3 mil thick Flourglas and the inner sheet 44c is 5 mil thick Flourglas. Also sheets 44a, 44b and 44c are 11, 9.25 and 7.5 inches wide respectively.

Fig. 3 shows a detailed view of the front portion of plotter 10. When possible the same reference numbers have been used to denote like parts. The front of plotter 10 is similar to the rear of plotter 10 shown in Fig. 2 with minor differences as follows.

A clamp 56c holds the first edge of each sheet. This clamp 56c is different than clamp 56a used in the rear of plotter 10, in that it has an extended edge to provide additional support for the sheets 44(a-c) when medium 14 is moving thereover. Clamp 56c is secured in recess 68 in the chassis 20 by screws (not shown). In addition clamp 56b shown in Fig. 3 is secured to bottom plate 54 which is shaped like the mounting means structure shown in Fig. 2 except that the long portion 48b of adapter 48 is not present. ' Bottom plate 54 is mounted to the chassis 20 in the rear by screw 60. Bottom plate 54 has a first portion 54a extending generally downwardly, a second portion 54b extending outwardly from the first portion 54a, and a brace portion 54c extending generally upwardly from second portion 54b. The clamp 56b is secured to brace portion 54c by a screw (not shown). Also, there is no platen extension in the front of the plotter 10.

In other embodiments other structural variations can have the same effect as that described in the preferred embodiment. For example, the compliant skirt may comprise a number of sheets of film as desired (discussed below). The sheets may be bonded at many points along the surface of the sheets, and may be mounted to the plotter in many ways. For example, a single sheet may be clamped at its edges to form a loop and mounted to the platen extension, to the mounting means structure or directly to the platen or chassis. A second sheet may be clamped at its edges to form a second loop, mounted to the plotter adjacent the first loop.

Alternatively, the second sheet may be bonded directly to the first sheet to form a loop therewith or mounted to the platen extension, mounting means structure or mounted directly to the platen or chassis which encompasses the first loop. Also alternatively, one edge of the second sheet may be bonded to the first sheet and the other edge may be mounted to the platen, chassis or extension. The sheets of film may also have many different widths. The same possibilities exist using three sheets of film.

Referring to Fig. 4 there is shown another embodiment of the compliant skirt 26 and mounting means 24. Mounting means 24 comprises an adapter 72 which has three parts 72a, 72b, and 72c, and adapter 72 is mounted directly to the platen 30 by part 72a. Part 72b provides support to moving medium 14 (not shown). Part 72c extends below the platen 30. The compliant skirt 26 comprises two sheet of film 76a, 76b. The edges of the sheets are secured by clamps 80a, 80b, with clamp 80a inserted in recess 82 and secured therein by a screw (not shown). Clamp 80b is secured directly to the end of part 72b of adapter 72. Fig. 5 is a schematic view of the embodiment of the skirt 26 illustrated in Fig. 4. In this Fig. 5 medium 14 is shown moving to the right (shown by an arrow marked with reference number 2 in a circle) at high acceleration and velocity. Skirt 26 begins to move upwardly from ^' a static equilibrium position (1), forming a wave or bulge as identified by (la). The compliant skirt 26 follows medium 14 and flexes upwardly as identified by dotted lines (lb). The compliant skirt 26 follows medium 14 for one or more reasons. First, the skirt's 26 own resilience urges it to move upward against medium 14 whose weight had caused the skirt 26 to deflect downward. Second, a low pressure zone is created between medium 14 and said outer surface of the outer sheet 44a of the skirt 26. Third, static electricity builds as medium 14 moves against the surface of the outer sheet 44a of the skirt 26. As shown at reference number (3) the skirt 26 shown in dotted line generally complies to and supports the lower side of medium 14 in the configuration taken by medium 14 as it moves upward.

Figs. 6 and 7 show the skirt 26 in a dynamic state when medium 14 changes directions rapidly. As a result of the rapid direction change medium 14 may swing back and forth as identified by (1) and (2) in Figs. 6 and 7. In Fig. 7 medium 14 compresses compliant skirt 26, expelling air out the ends of the loop or pillow, as indicated by (2). The skirt 26 complies somewhat to the new configuration of medium 14. Then the tendency of medium 14 to flap back outward is reduced because kinetic energy in moving medium 14 dissipates when the skirt 26 is compressed. As a result, excess motion in medium 14 is reduced as identified by (3). It is noted that the skirt 26 movement is more pronounced when the portion of moving medium 14 hanging from the platen 30 is longer or when medium 14 itself is longer.

In Fig. 8 medium 14 is shown to move left according to arrow marked (2) from a static equilibrium state; medium 14 in static state is shown by solid line (1) and in moving state by dashed line (la). Medium 14 has short, high acceleration movements. The skirt 26 compresses as shown in dashed line (3). The compliant skirt 26 allows the medium 14 to be displaced laterally to the left, and the driven portion 14a of medium 14 does not cause the hanging portion at reference number (4) to lift.

It is noted that in all embodiments the compliant skirt 26 at all times remains in substantial contact with the portion of medium 14 extending beyond and hanging from the platen 30 (platen means 18) that overlies the skirt 26. Figs. 5-7 show the compliant skirt comprising two looped sheets; Fig. 8 shows the skirt as a single sheet. These figures are schematic drawings to illustrate the basic structure and operation of this invention, and the skirt may vary in its number of layers in different embodiments depending on size, weight and other characteristics of the medium being supported and guided.

The details of one specific embodiment of the compliant skirt are shown in the attached appendix.

Referring to Fig. 9 there is shown a rear portion of plotter 10 incorporating the compliant skirt 26 according to another embodiment of the present invention. The compliant skirt 26 and medium 14 are shown in astatic equilibrium state. The compliant skirt 26 here comprises only one sheet of film 92. Otherwise, the compliant skirt 26 is the same as shown in Fig. 2.

The compliant skirt 26 described herein can be used not only on large format plotters but on many other paper handling machine such as printers, plotters, cutters and copiers that move media at high accelerations and high velocity and require predictable media behavior.

It should be noted that the skirt provides variable compliance which allows the skirt to be effective for many types of media.

Although the invention has been described with reference to preferred embodiments, it will be apparent to one skilled in the art that variations and modifications are contemplated within the spirit and scope of the invention. The drawings and description of the preferred embodiments are made by way of example rather than to limit the scope of the invention, and it is intended to cover within the spirit and scope of the invention all such changes and modifications.

Claims

WHAT IS CLAIMED IS:

1 . A plotter operable with a sheet medium to be plotted on, the plotter including a chassis, a platen means for supporting the sheet during plotting, and drive means for moving the sheet to traverse the platen means and then moving the sheet outwardly and downwardly from the platen, the improvement of a compliant skirt for supporting the sheet exiting the platen means, the skirt comprising: an upper portion mounted to the plotter adjacent the platen means, a lower portion spaced from the upper portion and also mounted to the plotter, and an intermediate supporting portion between said upper and lower portions, said intermediate portion extending outwardly and downwardly from the platen means for flexibly and resiliently supporting the portion of said sheet draped thereover.

2. A plotter according to claim 1 wherein said skirt's upper, intermediate and lower portions comprise a continuous first sheet, and said lower portion extends inwardly toward said chassis.

3. A plotter according to claim 2 wherein said sheet is bent forming in cross-section a loop.

4. A plotter according to claim 3 wherein said sheet is bent forming a smooth curve with said lower portion extending inward and upward defining a cavity between said intermediate and lower portions.

5. A plotter according to claim 4 wherein said skirt comprises a second sheet generally similar to said first sheet, one sheet generally encompassing the other with a space defined between the intermediate portions of said sheets.

6. A plotter according to claim 5 wherein the upper portions of the two sheets are closely adjacent where they mount to the chassis, and the lower portions of the two sheets are closely adjacent where they mount to the chassis.

7. A plotter according to claim 5 wherein said skirt comprises three sheets, with the third encompassing the second similarly as the second encompasses the first.

8. A plotter according to claim 6 wherein the upper portions of said first and second sheets are secured to each other, and said lower parts of said first and second sheets are secured to each other.

9. A plotter according to claim 3 wherein said lower portion is secured to the plotter at a location outward and below the location where the upper portion is secured to the plotter.

10. A plotter according to claim 1 wherein said platen means comprises a principal platen for supporting said sheet during plotting and an extension platen extending transversely and generally horizontally from the principal platen.

11. A plotter operable with a sheet to be plotted on, the plotter including a chassis, a platen means for supporting the sheet during plotting, and drive means for moving the sheet to traverse the platen means and then moving the sheet outwardly and downwardly from the platen, the improvement of a compliant skirt for supporting the sheet exiting the platen means, the skirt comprising: an upper portion closely adjacent and having similar elevation as the platen means, a lower portion spaced from the upper portion, an intermediate supporting portion between said upper and lower portions, said intermediate portion extending outwardly and downwardly from the platen means for flexibly and resiliently supporting the portion of said sheet draped thereover, and means for mounting said skirt to said plotter.

12. A plotter according to claim 11 wherein said lower portion is an extension of said intermediate portion and extends inwardly and upwardly toward said chassis forming in cross section a loop.

13. A plotter according to claim 12 wherein said mounting means comprises a first connector for securing said upper portion to said platen means and a second connector for securing said lower portion to said chassis.

14. A plotter according to claim 13 wherein said platen means comprises a principal platen for supporting said sheet during plotting and an extension platen extending transversely and generally horizontally from the principal platen.

15. A plotter according to claim 1 wherein said intermediate portion of the skirt is formed of flexible and resilient material which deflects and stays generally close to the portion of said sheet that overlies said intermediate portion when said portion of the sheet moves downward, inward, upward, outward or moves in combinations or sequences of said directions.

16. A plotter according to claim 11 wherein said intermediate portion of the skirt is formed of flexible and resilient material which deflects and stays generally close to the portion of said sheet that overlies said intermediate portion when said portion of the sheet moves downward, inward, upward, outward or moves in combinations or sequences of said directions.

17. A plotter according to claim 1 wherein said intermediate portion of the skirt has a static equilibrium configuration defined by elevation and lateral extension of each part thereof relative to the platen, said intermediate portion being formed of flexible and resistant material which deflects downward and/or inward from said configuration due to correspondingly directed forces applied by said sheet and deflects upward and/or outwardly from said configuration conforming to similar movement of said sheet when said sheet moves thusly, the skirt complying to the sheet.

18. A plotter according to claim 11, wherein said intermediate portion of the skirt has a static equilibrium configuration defined by elevation and lateral extension of each part thereof relative to the platen, said intermediate portion being formed of flexible and resistant material which deflects downward and/or inward from said configuration due to correspondingly directed forces applied by said sheet and deflects upward and/or outwardly from said configuration conforming to similar movement of said sheet when said sheet moves thusly, the skirt complying to the sheet.

19. A plotter operable with a sheet medium to be plotted on, the plotter including a chassis, a platen means for supporting the sheet during plotting, and drive means for moving the sheet to traverse the platen means and then moving the sheet outwardly and downwardly from the platen, the improvement of a compliant skirt for minimizing separation of said medium from said platen means as said medium moves transversely across said platen means, the skirt comprising: an upper portion mounted to the plotter adjacent the platen means, a lower portion spaced from the upper portion and also mounted to the plotter, and an intermediate supporting portion between said upper and lower portions, said intermediate portion, the skirt being formed of flexible and resilient material which deflects and stays generally close to the portion of said sheet that overlies said intermediate portion when said portion of the sheet moves downward, inward, upward, outward or moves in combinations or sequences of said directions.

20. Apparatus according to claim 17 wherein said intermediate portion of the skirt moves to generally comply in configuration with said sheet overlying said intermediate portion of said skirt and thus reduces excess motion of said sheet.