A bang-bang servo system controls motors which provide drive for trucks on each end of a fabric support beam structure. A plurality of such truck/beam/fabric units comprise a retractable roof Separation of inner and outer truck guiding tracks/support platforms may be wide and somewhat variable. The radius of curvature of the inner and outer tracks may vary widely over the beam excursion path between open and closed roof conditions. Individual beam motion sequencing and stop position are commanded through a central control unit. These features allow great flexibility and adaptability in application of this invention.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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     REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISK APPENDIX 
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     BACKGROUND OF THE INVENTION 
     The advantages of fabric retractable roofs relative to solid element retractable roofs are: small mass to be moved, variability of applicable roof shapes, adaptability to existing facilities, and low cost (especially where snow/ice melting and tight weather seals are not an issue). Yet fabric retractable roofs (other than automobile convertible tops and shade awnings) are not commonly used. The reason is to be found in the complexity of the proposed systems, principally with respect to fabric handling and transport means. 
     The object of this invention is to provide a simple fabric transport means having no inherent fabric wear inducing characteristics. The simplicity of the preferred embodiments not only makes the invention applicable to covering a wide range of facilities, but in some instances offers portability (ease of dismantle/move/reassemble). 
     BRIEF SUMMARY OF THE INVENTION 
     The movable roof is comprised of a plurality of facility-spanning movable beams each supporting fabric sections. Transport is supplied by beam end support trucks running on substantially parallel track platforms. The first preferred embodiment of this invention is a “one sided” configuration with substantially horizontal support beams. The roof slope, required for rain or snow/ice melt runoff on one side, is based on each fabric section cut for slope along its center fold seam. If required, runoff catching gutter means can be attached to the platform support means beneath the lower fabric edge. 
     Each beam end support truck moves on rolling members, wheels or glide rollers, on a guide track. The truck drive means is an electric motor which either directly drives a wheel axle or, in the preferred embodiment (not relying on friction), drives a pinion gear linked to a rack paralleling the guide track. Clamp assemblies attached to each truck connect to rolling members in near contact with the platform underside. This is required to handle extraordinary situations, e.g. strong winds or system component failure where lift or twisting forces attempt to separate the truck from its track. For each truck a motor-stop signaling means is located on its associated platform resulting in motor turnoff/braking at the desired intermediate or “roof closed” positions. For widely separated platforms, spanning beams are comprised of truss structures, and fabric sections are comprised of overlapping horizontal strip segments. 
     A servo system controlling the motors at both beam ends consists of 
     1) An angle error signal sensor on either the inner or outer truck providing an electrical signal to control circuitry. 
     2) Control circuitry processing the error signal in order to provide error correcting drive to the lagging motor. 
     3) Electrical motors, necessary electrical cabling, cable handling means. 
     4) Raw power conversion circuitry, beam sequencing circuitry, and safety circuitry. Angle error is measured with respect to the perpendicular between the beam and track path tangent. 
     While continuous (linear or nonlinear) error sensing is possible, the resulting servo control circuitry and power control circuitry are complex and costly. The preferred embodiment is a bang-bang servo system employing angle error limit sensors and single phase AC gear-motors. Under bang-bang servo control, while accelerating and decelerating, the average velocity of each beam end is in proper ratio. That is, proportional to the local radius of curvature of the associated platform. Since at any instant either end of the beam may be leading the other, a means to accommodate small rotation of the beam-to-truck connection is required. This is accomplished via a turret means on each truck base. Further, due to associated non-perpendicularity of the beam and track path tangent, a means is required to accommodate small additional beam length variation. To accomplish this, one beam end is connected to its turret on a pin and bearings allowing only relative rotation of the beam about the pin in a vertical plane. The other beam end turret has an open channel member pinned to it also allowing relative vertical plane rotation motion only. The associated beam end passes through the open channel member and rides on low friction bearings therein providing freedom of translational motion with respect to the turret. Angle error limit sensors measure turret to truck base rotational motion. 
     The individual beam servo system allows a wide range of roof shapes, from roof sections involving sharp platform turns (low radius of curvature) to parallel platform sections. Exotic shapes such as winding platform path or arched platform path are possible. Certain platform transitions may require that the translational motion beam end and bearing channel have a round profile so that the beam may rotate somewhat about the beam axis relative to the turret. This is a requirement wherever significant loss of parallelism occurs between the platforms. Additionally, some applications may require that both beam ends be round where small beam rotation occurs relative to the trucks at both ends. In these instances collars on the round fixed beam end straddle the open channel preventing translation motion. The only overall requirement is that the variation in distance between platforms be small. Further simplification accrues from the fact that the bang-bang servo allows the same gear motor and rack-and-pinion to be used throughout. It is possible to configure paths where the two beam-end platforms are at different levels. The consequence of this, however, is that a component of beam/fabric weight can cause large forces/torques on truck clamps and attachments. This is an application design consideration. 
     In some applications fabric drape in the region of the platforms (during roof closing and opening) will contact physical obstructions in the facility if allowed. Shortening the drape by lessening the width of fabric sections is costly since additional beam assemblies are required. To avoid this a fabric raising means is required. In the preferred embodiment a vertical channel is connected to the beam where the fabric edge is to be raised. The channel length is somewhat greater than one quarter the fabric section width. Two lifting cables are used. One end of each cable is connected to the edge of the fabric at a distance of one quarter the fabric section width from the beams on each side. Each cable length is approximately three eighths the fabric section width. In one fabric lifting embodiment, a small motor with cable store pulley and ratchet, attached at the top of the vertical channel, is used to maintain cable tension. This motor is essentially run in blocked rotor mode between ratchet steps with sufficient torque to lift the fabric portion weight. In reverse, the ratchet is released and the fabric is lowered by virtue of the beam motion pulling against the cable. When the beam reaches the desired stop position on the platform the fabric lifting motor is de-energized. In the preferred passive (motor-less) embodiment, a small block-and-tackle means with a two-to-one lift ratio is attached at the top inside of each vertical channel. In each channel the associated lifting cable is threaded through the fixed top pulley, around the movable pulley and connected to the fixed block. A second set of cables are connected, one each, near the ends of the beams on each side of the fabric section. The length of each of these cables is somewhat larger than the associated fabric section width. For each of these cables a small block-and-tackle means with a four-to-one lift ratio is ganged to the two-to-one block-and-tackle attached at the top inside of the vertical channel. Each cable is threaded through a fixed top pulley, around a movable pulley, through the second fixed top pulley, around the second movable pulley and connected to the fixed block. The movable pulley block is connected to a weight sufficient to lift two times the sum of the weights of the fabric to be lifted plus the lifting cable and four times its beam connected cable weight. The movable pulley block and weight move inside the guiding vertical channel. Since the two block-and-tackle are ganged, the beam motion control cable mediates travel of the fabric lifting cable. For fabric lifting configurations fabric fold-enhancing seams, in addition to a normal center seam, are desirable. 
     Whether opening or closing a roof section, the associated beams are sequenced for motion by the central system controller circuitry in beam order 1 through N, where N is the beam closest to the non-moving end of the roof section. Partial closure is easily accomplished by stopping the motor drives at any point. By eliminating non-moving ends (i.e. beam N is the last beam in the section) selected portions of the open area may be covered. For each beam, a truck on one beam end contains a position sensor which detects position based on position indicators spaced along the platform. The central system controller communicates with the individual beam servo control circuits. Each servo control circuit may either be located with the central system controller or on its associated beam. Electrical cabling distribution means will vary according to location of control circuits. The central system controller will shut down all motor drives when any individual motor drive “over-current” condition is sensed or a manual shutdown is initiated. 
     A second preferred embodiment of this invention is an arched root or “two sided” configuration where roof slope is provided by arched beam structure. In one version of this embodiment truck-mounted short horizontal beams support an arched beam structure to which roof fabric is connected. These short support beams are inside the arch span. Truck and support beam operation is the same as with the one sided configuration. This two sided embodiment, however, requires large counterbalance weights suspended from the arch ends to prevent large applied torques to the truck clamps. In an alternate embodiment long support beams are located outside the arch upper span allowing the arch lower span, below the support beams, to supply the counterbalance weight. With either two sided embodiment, round beam ends are required at both ends to allow for small beam rotation. In addition use of fabric lifting means, previously described, is required with a vertical channel located at each arch peak so that the fabric peak is lifted during roof opening. As with the one sided configuration, need for lower edge fabric lifting is application dependent. 
     A third preferred embodiment of this invention combines features of the first and second embodiments. It provides a two sided roof slope from the peak by virtue of fabric cut such that only horizontal beams are required. Again, fabric lifting means at the roof peak near the horizontal beam center are required. 
     While it is envisioned that the majority of the applications of this invention will be in geographical regions and seasons where ice and snow are not a concern, it nevertheless must be addressed. Shielding most of the system components from ice and snow is either straightforward or not essential, however, this is not the case for the fabric cover. Two means are considered: one electro-thermal and the second chemo-thermal. The electrothermal embodiment uses resistance heating wire embedded in the fabric outer layer. Conventional AC power used for the system in general is used to power the resistance heating wiring. The chemo-thermal embodiment uses a deicing fluid. From a storage means the fluid is pumped, via tubing attached to the power cabling, to each beam along the length of which the fluid is sprayed onto the adjacent fabric. The fluid and melt are captured in a runoff gutter means and fed to a fluid separation means whence the fluid is returned to the storage tank. For facilities which are not weather sealed, use of heated air on the fabric underside can not be considered as an ice/snow melting solution. However, partial sealing (not hermetic) can be achieved with fabric panels attached using zippers or hook-and-loop fasteners. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  shows the principle components of a fabric retractable roof in a frontal aspect of its simplest “one-sided” embodiment with horizontal beams. 
         FIG. 2  shows the beam position angle error used by the servo control system. 
         FIG. 3  shows an inner platform truck with its principle components, in a frontal aspect with cutaway of the platform and attached fixed beam. 
         FIG. 4  shows an inner platform truck with its principle components, in a side aspect with cutaway of the platform and attached fixed beam. 
         FIG. 5A  shows an outer platform truck with its principle components, in a side aspect with cutaway of the platform and attached translatable beam. 
         FIG. 5B  shows a fixed beam with rounded end, allowing for beam rotation, whose collars prevent translation. 
         FIG. 6  shows a block diagram of the retractable roof control system elements. 
         FIG. 7A  shows a side view of a passive fabric lift means including cables and block-and-tackle guide channel. 
         FIG. 7B  shows a cutaway frontal view of the block-and-tackle and weight inside the guide channel. 
         FIG. 8  shows a frontal cutaway view of a “two-sided” retractable roof embodiment including sloped beams with internal platforms and counterbalance weights. 
         FIG. 9  shows a frontal cutaway view of a “two-sided” retractable roof embodiment including sloped beams with external platforms and horizontal beam supports. 
         FIG. 10  shows a frontal cutaway view of a “two-sided” retractable roof embodiment with horizontal beams and fabric cut slope. 
         FIG. 11  shows a frontal cutaway view of a retractable roof with deicing drainage and chemical recycling system. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In  FIG. 1  a “one sided” embodiment of a fabric retractable roof whereby drainage is from one side of the roof is shown with its principle components. Each beam  2  of a set of essentially horizontal beams, is supported at both ends by electric-motor driven trucks  1 . The material (wood, metal, plastic, etc.) and form (rectangular, I, T, channel, truss, etc.) of the beams are application dependant. The trucks ride on platforms, which for discussion purposes are called “inner”  4  and “outer”  5  normally based on platform radius of curvature, whose separation is essentially constant. Over the total length of a roof coverage area some platform sections may be parallel (infinite radius of curvature) or change their relative curvatures so that “inner” and “outer” becomes a label only. Fabric sections  3  are cut in width based on spacing between adjacent beams when the roof is closed. The cut is such that drainage be on one side and normally a center seam will guide folding. Attaching the fabric sections to the beam underside may be accomplished by one of numerous fabric retention methods. Fabric sections between beams need not be one-piece, but may consist of overlapping strips. These strips may be connected, if desired, using zippers or hook-and-loop fasteners. The fabric material may be any state-of-the-art type chosen for strength-to-weight ratio and water proof properties, e.g. Teflon-coated fiberglass. 
     Control of truck motors for each beam so that the beam ends stay essentially perpendicular to the path-tangent is a key element of this invention. The method is based on measuring the beam deviation angle from perpendicular as shown in  FIG. 2 . Here, the inner platform  4  has a radius  8  and the outer platform  5  has a radius  9 . When small error angle limits,  6  and  7  (exaggerated here but typically under one degree), are detected between truck  1  on the inner platform and the corresponding outer platform truck, a bang-bang servo control system causes the leading truck motor to de-energize. For the outer platform truck, if the beam motion is right-to-left then 6 represents a lead angle error limit while for left-to-right motion  7  represents lead angle error limit. While small time-scale accelerations and decelerations are occurring the average velocity for the beam ends will be correct. 
     An example truck embodiment associated with an inner platform is shown in  FIG. 3 . The beam end  2  is connected to the top of the truck on a pin-and bearing assembly  15  which can rotate in a vertical plane. This allows for small unintentional or intentional non-coplaner alignment of the inner and outer platforms. Large non-coplaner alignment of the platforms is not recommended since unbalanced side forces are introduced on the pin and hence truck. The beam attachment assembly rests on a rotatable turret bearing  14  which has the angle error detector unit  16  attached to its side. The detector will typically consist of switches (electromechanical, electro-optical, etc.) while the associated error limit sending unit is fixed mounted to the top of the truck base  10  beneath the detector. The truck base contains a drive motor and linkage. An AC gear motor is recommended for system simplicity. The preferred linkage is a pinion gear  11  with associated rack assembly (shown in  FIG. 4  as  18 ) attached to the platform. In addition, the preferred roller/guide-path embodiment is glide-rollers  13  in a track  12 .  FIG. 4  also shows a beam position detector unit  19  attached to the bottom of the truck base while the associated position sending unit is fixed mounted on the top of the platform. Since high winds or system failures could cause sudden lifting of twisting forces on the truck, clamping means  17  are necessary. 
     The result of small beam angle error is a requirement for beam length somewhat greater than the platform separation related perpendicular distance. Further, since this error varies continually then dynamically variable beam length must be accommodated. The preferred solution is shown in the outer platform truck assembly of  FIG. 5A . Here the beam  2  has a round end  21  which passes through a collar channel and bearing  20  allowing the beam length to adapt to the angle error. The round beam end and collar has the additional role of allowing rotation of the beam end about its axis, necessary during unintentional or intentional transition in platform levels. The collar channel and bearing assembly is connected to a pin and bearing assembly  15  similar to the inner platform truck. These complement each other allowing small beam motion in a vertical plane. The remainder of the outer platform truck is the same as the inner platform truck, however angle error detection and beam position detection are not necessary. In certain applications where torsional forces on the entire beam can occur, such as the “two-sided” roof embodiments discussed later, it is necessary that both beam ends be round and rotatable. However, since the inner platform beam end should not be translatable, collars  45  shown in  FIG. 5B  are required to be fixed to the round end  21  and straddle the collar channel and bearing  20 . This prevents beam translation while permitting beam rotation at the inner platform truck. 
     Regarding the retractable roof as a system broadens the scope of system components to include control functions. These are summarized in the block diagram of  FIG. 6 . The System Control Unit incorporates circuitry performing system operator interface for manually selected coverage area, manual startup, status display, manual overrides, automatic beam sequencing on startup of roof closure or opening and emergency shutdown based on motor overload. The System Control Unit is connected to each beam Servo Control Unit which receives functional commands, handles beam position and angle error sensor signals for motor control, and feeds back status data. Physically, the Servo Control Units can be co-located with the System Control Unit or located on the individual beam trucks. This decision is application dependent. Flexible electrical cable means connect on-beam to off-beam circuitry and between beams if required. In  FIG. 6  it is assumed that sensors are located on the inner platform truck which is consistent with earlier assumptions. 
     As implied in  FIG. 1  a significant cable drape can occur when the retractable roof is being opened. The potential exists in certain applications for undesirable contact between fabric and objects inside the facility. To prevent this contact the fabric must be lifted. The preferred embodiment of the fabric lifting method is shown in  FIG. 7A  where, in this case, the lower edge of the fabric  3  is to be lifted. The fabric lift connection point  46  is approximately one-quarter the width of the fabric at the lift edge or line measured from the closest beam attachment point. A vertical channel  22  whose height is somewhat greater than one-quarter the width of the fabric to be lifted is attached to the beam at a position nearest to the fabric lift point. In this roof closed position a cable  24 , whose exposed length is approximately three-eighths the width of the fabric to be lifted, has one end attached to the fabric lift point and the other connected to a two-to-one block-and-tackle at the top  23  of the vertical channel. A second cable  25  has one end connected to the beam near the opposite end of the fabric section and the other end connected to a four-to-one block-and-tackle also at the top of the vertical channel.  FIG. 7B , a cutaway view I-I from  FIG. 7A , shows that the two block-and-tackle are ganged together. The cable  24  is fed around its fixed block  26  pulley, through the movable block  27  pulley and up to the fixed block. The cable  25  is fed around its first fixed block  29  pulley, through the first movable block  30  pulley, up and around the second fixed block pulley, through the second movable block pulley, and up to the fixed block. In the closed roof position shown, the movable block is at its top position. Based on the mechanical advantage of the two ganged block-and-tackle the lifting weight  31  must be somewhat in excess of twice the weight of the fabric to be lifted plus cable  24 , plus four times the weight of cable  25 . Because of the ganged configuration cable  25  will mediate the motion of the weight and hence the fabric. As shown in  FIG. 7A , identical lifting components are established at the adjacent beam. The result is that throughout opening and closing of the roof the cable drape stays nominally constant. To facilitate folding from the lift points, additional seams in the fabric are desirable. 
     Where roof symmetry and drainage from two sides is desired, there are three basic preferred “two sided” embodiments. The first is shown in the cross-section view of  FIG. 8 . Beams  2  provide the roof slope for drainage from the fabric  3 . Because of the roof peak, folding of the fabric in the open roof condition requires lifting of the peak fabric section  43 . This is accomplished using the lifting means previously described with vertical channel  22 , block-and-tackle  23 , and lifting cable  24  shown. Fabric section  43  overlaps the sections on each side below and is not attached to them. Additional use of lifting means near the fabric edges is application dependent. This beam structure configuration has a high center-of-gravity relative to the pivoting beam ends supported on the trucks, if not properly compensated. Lowering the center-of-gravity for stability is accomplished with suspended weights  44 . An alternate sloped beam roof is shown in the cross-section view of  FIG. 9 . In this second “two sided” embodiment the sloping beams  2  themselves provide the stable center-of-gravity based on horizontal support beams  2  connected at a relatively high point on the sloped beams. The third “two sided” embodiment is shown in the cross-section view of  FIG. 10 . In this embodiment the support beam  2  is horizontal so that drainage is based on the cut of the fabric  3 , as with the “one sided” embodiment. Here, as with all “two sided” embodiments, a roof peak fabric section  43  lifting means is required. 
     While it is expected that the vast majority of applications of this invention will not require operation where snow and ice conditions exist, there are means to enable such operation. One such embodiment is shown in  FIG. 11  where a “one sided” roof example is used. Here the ice/snow melting means is chemical in the form of circulating deicing solution. The solution, held in reservoir  38 , is sent via pump  39  through fixed pipe  40  and flexible pipe  41  to fluid distribution pipe  42  on the beam  2 . Flexible pipe  41  basically follows the same path as the electrical cables to beam  2 . Fluid distribution pipe  41  straddles the beam and supplies a continuous spray flow of deicing fluid to the fabric via holes in the side of the pipe. Collection of all runoff is made by gutters  33  and sent by pipes  34  to water/deicer separation processor  36 . Separation processing will used established means (evaporation, specific gravity, etc.) based on the specific deicing fluid. Separated deicer is sent via pump  37  back to reservoir  38 , while separated water is sent to the gravity drain  36 . An alternate snow/ice melting means uses electrical resistance wire embedded in the fabric. Wire heating power is the standard AC power used by the roof electrical system. Runoff, in this case, is water only. For the comfort and/or protection of people, animals and plants in the covered facility, means of forming non-hermetic closure between the roof fabric and support walls/columns  32  is possible using zippers or hook-and-loop fasteners. With such closure, internal facility air heating may be achieved providing a third means of roof snow/ice melting. Platform covering is provided by shield means  48 . 
     Various systems, methods and apparatuses are described herein including: 
     (A) A retractable fabric roof system based on: a plurality of facility spanning substantially horizontal beams of fixed span length each supporting fabric sections which are cut to provide precipitation runoff slope on one side, each beam end supported by a truck which moves in guide rail means on a platform and powered by an electric motor located in the truck base, said truck motion controlled by a servo system providing proper average velocity according to the truck's local path, turrets located atop each truck base allowing for small rotational deviations of the associated beam from its ideal path where small resulting linear deviation of span length is allowed via a round beam end slip-collar bearing assembly in one beam end turret, while small roll motion of each beam is allowed by having both beam ends round and in roll bearings, and small beam tilt is allowed by a tilt bearing on each turret. 
     There is also described herein (B) a central control circuitry providing: beam power sequence-enabling at roof open/close startup; beam motion direction switching for open/close; beam motor off/brake enabling at selected travel limit; motor overcurrent shut-down; manual over-rides; a bang-bang servo system to control average velocity of each beam in (A), beam angle error being measured with respect to the perpendicular to the path tangent, for each beam angle error limit detection being based on a switch activation at the turret/truck-base interface on one beam end, said switch activation causing motive power to the leading truck motor to be removed while maintaining motive power to the lagging truck motor, said motors being reversible single phase ac gear motors with power controlled through solid state devices, with selected beam travel limit switch activation occurring at the associated truck base/platform interface. 
     There is also described herein (C) a fabric lift means to prevent fabric drape during roof opening/closing resulting in interference with proper fabric folding due to self-interference or contacting objects in the spanned facility, based on a vertical channel assembly whose height is somewhat in excess of one quarter the associated fabric section width and which is attached to each beam near the fabric edge to be lifted, said channel containing a two-to-one block-and-tackle means attached to the channel top whereby one end of a lifting cable is threaded through the pulleys and connected to the fixed block while the other cable end is attached to the associated fabric edge at a distance from the beam at approximately one quarter the fabric width; said channel also containing a four-to-one block-and-tackle means whereby one end of a control cable is threaded through the pulleys and connected to the fixed block while the other cable end is attached to the associated adjacent beam end; the two block-and-tackle means being ganged together with a fabric and cable lifting weight suspended from the joint moveable block thereby allowing the beam connected cable to control the motion of the fabric lifting cable. 
     There is also described herein (D) a retractable fabric roof using the means of (A), (B), and (C) whose fabric supporting beams are arch shaped providing liquid runoff on two sides, said arched beam extending over and beyond the truck support platform on each side and being stabilized by counterbalancing weight symmetrically disposed on each side of the arch below the platform level, and whose roof peak fabric section uses the lifting means of (C). 
     There is also described herein (E) a retractable fabric roof using the means of (A), (B), and (C) whose fabric supporting beams are arch shaped providing liquid runoff on two sides, said arched beam connected to horizontal support beams connected high on the arch shaped beam sides resulting in a stable weight distribution, said horizontal support beams extending beyond the arched beam and connected to trucks in a manner similar to (A), and whose roof peak fabric section uses the lifting means of (C). 
     There is also described herein (F) a retractable fabric roof using the means of (A), (B), and (C) using horizontal support beams similar to (A) but whose two sided liquid runoff is allowed by the fabric cut on each side, and whose roof peak fabric section uses the lifting means of (C). 
     There is also described herein (G) a snow/ice melting capability in the form of either resistance heating wire embedded in the outer layer of fabric sections where heating power is provided from the standard ac system power, or a deicing fluid supply/recovery system consisting of: deicing fluid reservoir, pumps, plastic supply tubing flexibly connected to the moving beams with the electrical cables, deicer spray lines connected to the beam sides, plumbing to route all fluid from drain gutter means to deicer recovery means and separated deicer back to the reservoir.