PATENT ABSTRACT
A direct data and power interface is provided to the patient support platform of a therapeutic bed that allows for a complete rotation of the patient support platform in either direction. In one embodiment, a data and/or power cable is housed within a chain-like cable carrier that is disposed within an annular channel attached to the patient support platform. In another embodiment, a flexible ribbon cable is disposed within the annular channel. The cable carrier or ribbon cable is long enough to allow a full 360 degrees of rotation of the patient support platform in either direction from 0 degrees supine flat while maintaining a direct data or power connection. To ensure that data and power connection is not articulated beyond its physical limit as a result of manually rotating the bed in the emergency backup mode, a mechanical stop is provided to limit rotation of the patient support platform to about 730 degrees. Sensors are provided to detect activation of the mechanical stop.

PATENT DESCRIPTION
RELATED APPLICATION INFORMATION  
       [0001]     This application is a divisional of patent application Ser. No. 09/821,552 filed Mar. 29, 2001, entitled “PRONE POSITIONING THERAPEUTIC BED.” This application also claims priority for commonly disclosed subject matter to patent application Ser. No. 09/884,749 filed Jun. 19, 2001, similarly entitled “PRONE POSITIONING THERAPEUTIC BED,” which is a continuation-in-part of Ser. No. 09/821,552. This application also claims priority for commonly disclosed subject matter to PCT/IE02/00085, filed Jun. 26, 2002, entitled “BED WITH POSITION CHANGE FACILITY,” which claims priority to Ireland Application No. S2001/0589, filed Jun. 26, 2001. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates generally to therapeutic beds, and more particularly to beds with a patient support platform operable to rotate about a longitudinal axis of the platform.  
         [0004]     2. Description of the Related Art  
         [0005]     One of the problems in the art of prone positioning therapeutic beds is to provide data and power connections to the bed for both the power and controller equipment that moves the bed and for the patient monitoring systems on the bed. To allow unrestricted rotation of the bed of WO 99/62454, for example, electrical power has been provided by wire brushes at the interface between the rotating part of the bed and the nonrotating part of the bed. However, due to vibration and other abrupt movements, such wire brushes cause problems of electrical intermittence, which can be detrimental to the therapy of the patient. A direct power or data carrier would be preferable to eliminate such intermittence, provided that the wired connection is capable of articulation during movement of the rotating part of the bed into the prone position, and provided that a mechanism is provided to prevent excessive rotation in any one direction.  
       SUMMARY OF THE INVENTION  
       [0006]     In U.S. patent application Ser. No. 09/821,552 filed Mar. 29, 2001, and Ser. No. 09/884,749 filed Jun. 19, 2001, the first of which is herein incorporated by reference, a prone positioning bed is disclosed that encompasses several distinct innovations. This divisional application is directed to a mechanism to provide a direct, wired connection to the patient support platform.  
         [0007]     A therapeutic bed in accordance with the present invention is provided comprising a base frame, a patient support platform rotatably mounted on the base frame for rotational movement about a longitudinal rotational axis of the patient support platform, and a drive system for rotating the patient support platform on the base frame. A direct, wired connection is provided to the patient support platform that allows for a complete rotation of the patient support platform in either direction. The necessary electrical wires are housed within a chain-like cable carrier that is disposed within an annular channel attached to the patient support platform. An annular cover is installed adjacent the annular channel to retain the cable carrier within the annular channel, but the annular cover is not attached to the annular channel. Rather, the annular cover is attached to the nonrotating part of the bed. One end of the cable carrier is attached to the annular channel, and the other end is attached to the annular cover. The length of the cable carrier is sufficient to allow a full 360 degree rotation of the patient support platform in either direction from 0 degrees supine flat while maintaining a direct electrical connection.  
         [0008]     More preferably, the direct, wired electrical connection to the patient support platform may be provided with a flat ribbon cable or flexible printed circuit board (PCB) cable in lieu of a chain-like cable carrier. The cable resides within an annular channel attached to the patient support platform, and an annular cover is fastened to a flange of the annular channel such that a gap exists between the annular channel and the annular cover around the outer periphery. One end of the cable is attached to the annular channel, which provides power and electrical signals to the rotating part of the bed, and the other end of the cable passes through the gap between the annular channel and the annular cover and is connected to the electrical apparatus on the nonrotating part of the bed. Like the cable carrier mentioned above, the cable has a length sufficient to allow a full rotation of the patient support platform in either direction while maintaining a direct electrical connection between the nonrotating and rotating parts of the bed. To ensure that the wired electrical connection is not articulated beyond its physical limit as a result of manually rotating the bed in the emergency backup mode, a mechanical stop is provided to limit rotation of the patient support platform to about 365 degrees. Sensors are provided to detect activation of the mechanical stop.  
         [0009]     It is an object of this invention to provide a prone positioning therapeutic bed having a direct, wired electrical connection between the rotating part of the bed and the nonrotating part of the bed.  
         [0010]     It is another object of this invention to mechanically limit rotation of the bed in either direction to one full 360 degree turn plus about 5 degrees, and to electrically detect when one full turn has been reached.  
         [0011]     Further objects and advantages of the present invention will be readily apparent to those skilled in the art from the following detailed description taken in conjunction with the annexed sheets of drawings, which illustrate a preferred embodiment of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a perspective view of a therapeutic bed in accordance with the present invention.  
         [0013]      FIG. 2  is a perspective view of the head portion of the therapeutic bed of  FIG. 1  looking toward the foot of the bed.  
         [0014]      FIG. 2A  is a perspective view of an alternative head restraint for the therapeutic bed of  FIG. 1 .  
         [0015]      FIG. 3  is a perspective view of the head portion of the therapeutic bed of  FIG. 1  looking toward the head of the bed.  
         [0016]      FIG. 3A  is an exploded perspective view of the clamping mechanism for the head restraints of the therapeutic bed of  FIG. 1 .  
         [0017]      FIG. 4  is a perspective view of a side rail of the therapeutic bed of  FIG. 1 .  
         [0018]      FIG. 4A  is a perspective view of the detent for the side rail of  FIG. 4 .  
         [0019]      FIG. 5  is a side elevational view of a strap connector for the side rail of  FIG. 4 .  
         [0020]      FIG. 6  is a rear elevational view of the strap connector of  FIG. 5 .  
         [0021]      FIG. 7  is a perspective view of the therapeutic bed of  FIG. 1  showing symmetric lateral support pads and leg adductors/abductors.  
         [0022]      FIG. 8  is a perspective view of the foot portion of the therapeutic bed of  FIG. 1  looking toward the foot of the bed.  
         [0023]      FIG. 9  is a front elevational view of a portion of  FIG. 8 .  
         [0024]      FIG. 10  is a front elevational view of the rotation limiter of the therapeutic bed of  FIG. 1  shown in a position of maximum negative rotation.  
         [0025]      FIG. 11  is a front elevational view of the rotation limiter of the therapeutic bed of  FIG. 1  shown in a position of maximum positive rotation.  
         [0026]      FIG. 12  is a perspective view of the foot portion of the therapeutic bed of  FIG. 1  looking toward the head of the bed.  
         [0027]      FIG. 13  is a rear elevational view of the therapeutic bed of  FIG. 1 .  
         [0028]      FIG. 14  is a perspective view of the quick release mechanism for the drive system of the therapeutic bed of  FIG. 1 .  
         [0029]      FIG. 15  is a perspective view looking up at a side rail folded under the patient support platform of the therapeutic bed of  FIG. 1 .  
         [0030]      FIG. 16  is a side elevational view of a side rail and cooperating tape switch on a therapeutic bed in accordance with the present invention.  
         [0031]      FIG. 17  is a cross-sectional view of the tape switch of  FIG. 16 .  
         [0032]      FIG. 18  is a rear elevational view of a flexible cable disposed within an annular channel of a therapeutic bed in accordance with the present invention.  
         [0033]      FIG. 19  is a cross-sectional view of the flexible cable and annular channel of  FIG. 18 .  
         [0034]      FIG. 20  is an enlarged cross-sectional view of the flexible cable of  FIG. 18 .  
         [0035]      FIG. 21  is a top view of a locking pin assembly for a therapeutic bed in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0036]     Referring to  FIGS. 1 and 2 , a therapeutic bed  10  in accordance with the present invention preferably comprises a ground engaging chassis  12  mounted on wheels  14 . A base frame  16  is mounted on chassis  12  with pivot linkages  18 . Rams  15 ,  17  housed within base frame  16  cooperate with pivot linkages  18  to form a lift system to raise and lower base frame  16  on chassis  12 . A patient support platform  20  having upright end rings  22 ,  24  is rotatably mounted on base frame  16  with rollers  26  such that patient support platform  20  may rotate about a longitudinal axis between a supine position and a prone position. Side support bars  28 ,  30  extend between end rings  22 ,  24 . At the head of bed  10 , a guide body  32  having a plurality of slots  34  for routing patient care lines (not shown) is slidably mounted on rails  36  with support rod  31 . Similarly, at the foot of bed  10 , a central opening  118  is provided for receiving a removable patient care line holder (not shown) having a plurality of circumferential slots for routing patient care lines. Central opening  118  is preferably of sufficient size to allow passing of patient connected devices, such as foley bags (not shown), through the central opening  118  without disconnecting such devices from the patient. For such purposes, central opening  118  is preferably as large as possible, provided that strength and configuration requirements of the bed are maintained. The foregoing basic structure and function of bed  10  is disclosed in greater detail in international application number PCT/IE99/00049 filed Jun. 3, 1999, which is incorporated herein by reference.  
         [0037]     Still referring to  FIG. 1 , bed  10  preferably comprises one or more folding side rails  62  pivotally mounted to patient support platform  20  to assist in securing a patient to support platform  20  before rotation into the prone position. As further described below in connection with  FIG. 15 , side rails  62  fold underneath platform  20  for easy access to a patient lying atop cushions  21   a ,  21   b ,  21   c  in the supine position. Bed  10  also preferably has a head rest  50  and a pair of head restraints  48 , which are described in more detail below in connection with  FIG. 3 .  
         [0038]     As shown in  FIG. 2 , end ring  22  at the head of bed  10  is split into two sections for improved access to a patient lying on bed  10 . Upper section  22   a  is removable from lower section  22   b . Upper section  22   a  has a pair of shafts  40  that are inserted into vertical stabilizer tubes  38  in the closed position. Likewise, tabs  46  on upper section  22   a  mate with tubular openings on lower section  22   b . Latches  44  secure upper section  22   a  to lower section  22   b  in the closed position. When latches  44  are unlatched, upper section  22   a  may be raised, pivoted about the vertical axis of one of the shafts  40 , and left in an open position supported by one of the shafts  40  in corresponding stabilizer tube  38 . Alternatively, upper section  22   a  may be removed entirely. In either case, upper section  22   a  may be moved out of the way for unobstructed access to the patient and manipulation of patient care lines. As an alternative to a split end ring, patient support platform  20  could be cantilevered from the base frame at one end of the bed, but such a configuration would be extremely heavy.  
         [0039]     Referring now to  FIGS. 3 and 3 A, head restraints  48  are slidably mounted to transverse support rails  58 ,  60  on guides  54  with mounting arms  52 . For the sake of clarity, only one head restraint  48  is shown in  FIGS. 2 and 3 . Each guide  54  has a clamp  56  that is manually operable by a handle  56   a  and serves to secure each guide  54  in a desired lateral position as further described below. Mounting arms  52  are slidably mounted in holes  56   h  of bosses  56   b  to provide vertical positioning of head restraints  48 . Handle  56   a  is attached to a drum  56   f  that is rotationally mounted to flanges  54   a  of guide  54  by shaft  56   g  which is disposed within hole  56   d  of drum  56   f . Drum  56   f  has a ramp  56   c  for engaging one of the flanges  54   a , and hole  56   d  is offset from the central axis of drum  56   f  to form a cam  56   e . Movement of handle  56   a  in the appropriate direction causes ramp  56   c  to engage one of the flanges  54   a  and thereby spread flanges  54   a  apart slightly, which causes one of the flanges  54   a  to frictionally engage mounting arm  52  and thereby fix the vertical position of head restraint  48 . Simultaneously, such rotation of handle  56   a  causes cam  56   e  to frictionally engage one of the transverse support rails  58 ,  60  and thereby fix the lateral position of head restraint  48 . Thus, clamps  56  simultaneously provide both lateral and vertical positioning of head restraints  48 , which have pads  48   a  for comfortably engaging the front and sides of the head of a patient whose head is resting on head rest  50 . Head rest  50  may be mounted to transverse support rails  58 ,  60  or to pad  21   a . Head restraints  48  thereby provide increased stability and comfort for a patient when bed  10  is rotated to the prone position.  
         [0040]     If a particular patient requires only partial rotation for therapy such that patient support platform  20  need not be rotated beyond about, for example, 30 degrees in either direction, alternative head restraints  248  as shown in  FIG. 2A  may be mounted in clamps  56  using mounting arms  252  in like manner as head restraints  48 . Alternative head restraint  248  is designed to provide lateral support for the patient&#39;s head in instances when the patient will not be rotated into the prone position such that vertical restraint of the head is not required.  
         [0041]      FIGS. 4 and 15  illustrate a preferred structure and operation of folding side rails  62 . Preferably, four independently operable side rails  62  are pivotally mounted on each side of bed  10 . For each side rail  62 , main rail  66  is slidably mounted on shaft  80  with mounting cylinders  82 . Shaft  80  has a slot  80   a  for receiving guides such as set screws  83  installed in holes  82   a  of mounting cylinders  82 . Preferably, set screws  83  are not tightened against slot  80   a  but simply protrude into slot  80   a  to prevent side rail  62  from rotating with respect to shaft  80 . In that regard, set screws  83  could be replaced with unthreaded pins. When set screws  83  are loosened, side rail  62  is free to slide longitudinally along shaft  80  for proper positioning with respect to the patient. When set screws  83  are tightened, side rail  62  is fixed with respect to shaft  80 . Shaft  80  is rotatably mounted to side support bar  28 ,  30  with rail mounts  78 . Pivot link  68  is hinged to main rail  66  with hinge  72 , and cushion  64  is hinged to pivot link  68  with hinge  70 , which has a hinge plate  70   a  for attaching cushion  64 . Side rails  62  are thus capable of folding under patient support platform  20  as shown in  FIG. 15 , which is a view looking up from beneath patient support platform  20 . A strap  174  with one end secured around shaft  80  may be provided to retain cushion  64  in the folded under position with mating portions of a snap respectively provided on cushion  64  and strap  174 . A pair of straps  74  and an adjustable buckle  76  are provided to fasten each opposing pair of side rails  62  securely over the patient. One end of strap  74  is secured to side support bar  28  with a strap connector  88 , which is  15  slidably mounted in slot  28   a  of side support bar  28 . When strap  74  is properly secured with the appropriate tension using buckle  76 , tabs  160  on strap connector  88  are sandwiched between main rail  66  and side support bar  28 , which further helps to prevent longitudinal movement of side rail  62 . Side rails  62  thus serve to hold the patient securely in place as bed  10  is rotated into the prone position, and side rails  62  fold neatly out of the way for easy access to the patient in the supine position.  
         [0042]     As best illustrated in  FIG. 4A , an indexed disc  86  is preferably provided on one end of shaft  80  for cooperation with a pull knob  84  to form a detent that holds side rail  62  in one or more predetermined rotational positions. To that end, disc  86  preferably has one or more recesses  228  for receiving a pin  84   a  which is manually operated by pull knob  84 . Pull knob  84  is fixedly mounted to rail mount  78  with boss  230 . Preferably, pin  84   a  is biased into engagement with disc  86 . By engaging one of the recesses  228 , pin  84   a  prevents rotation of shaft  80  and thereby functions as a detent to hold side rail  62  in a predetermined rotational position. Side rail  62  may be moved to a different predetermined rotational position by pulling knob  84  sufficiently to disengage pin  84   a  from the given recess  228  so that shaft  80  is free to rotate. Preferably, one of the predetermined rotational positions of side rail  62  corresponds to the folded under position.  
         [0043]     Referring now to  FIGS. 5 and 6 , each strap connector  88  comprises a tension-sensitive mechanism that provides both visual and electrical indications of whether strap  74  is properly secured over the patient. The following description describes the attachment of a strap connector  88  to side support bar  28 . It will be understood that strap connectors  88  may be similarly attached to side support bar  30 . Each strap connector  88  comprises a tension plate  90  that partially resides within a housing  96 . A cover plate  176  is attached to housing  96  by fasteners  182  inserted into holes  96   a . Tabs  160  extend from housing  96 , and studs  178  protrude from tabs  160  as shown. Discs  180  are mounted to studs  178  with screws  183 . Slots  28   b  on the inner side of support bar  28  provide access for installation of screws  183 . Studs  178  are adapted to slide in slots  28   a  of side support bar  28 , and discs  180  serve to retain strap connector  88  on side support bar  28 . Tension plate  90  has a slot  92  to which strap  74  is attached and a central cut-out  93  that forms a land  100 . Inverted U-shaped channels  102  protrude from the back of housing  96  into central cut-out  93  of tension plate  90 . Land  100  of tension plate  90  cooperates with channels  102  of housing  96  to capture springs  98  which tend to force tension plate  90  downward toward lower edge  95  of housing  96  such that switch  104  is disengaged when strap  74  is slack. Switch  104  is connected to an electrical monitoring and control system (not shown) in a customary manner. When strap  74  is buckled and tightened sufficiently, the tension in strap  74  overcomes the biasing force of springs  98 , and tension plate  90  moves upward to engage switch  104 , which sends a signal to the electrical monitoring and control system indicating that strap  74  is properly tensioned. Preferably, the electrical monitoring and control system is programmed such that bed  10  cannot rotate until each strap  74  is properly tensioned to ensure that the patient will be safely secured in bed  10  as it rotates to the prone position. Additionally, tension plate  90  preferably has a tension indicator line  94  that becomes visible outside housing  96  when strap  74  is properly tensioned.  
         [0044]     More preferably, as illustrated in  FIG. 16 , instead of utilizing tension-sensitive strap connectors  88 , a pressure-sensitive tape switch  234  may be installed to side support bars  28 ,  30  adjacent each side rail  62 . Tape switch  234  is preferably of the type commonly available from the Tape Switch company. Strap  74  is attached to a crossbar  240  that spans main rails  66 . When strap  74  is properly tensioned, main rails  66  depress tape switch  234 , which sends a signal through electrical leads  238  to the monitoring and control system indicating that side rail  62  is properly secured over the patient. Preferably, the monitoring and control system s programmed such that the patient support platform  20  is not allowed to rotate into the prone position unless all side rails  62  have been properly secured as indicated by tape switches  234 . To help calibrate each tape switch  234 , a pad  236  may be attached to side support bars  28 ,  30  below the tape switch  234  adjacent each side rail  62 . Pads  236  are made of a compressible material, such as rubber, having a suitable hardness and thickness so that, as strap  74  is buckled, main rails  66  will first compress pads  236  and then depress tape switch  234  when strap  74  is buckled to the appropriate tension.  
         [0045]      FIG. 17  illustrates a preferred embodiment of tape switch  234 . A mounting bracket  242 , which is preferably made of extruded aluminum, houses two conductive strips  250  and  246  that are separated at their upper and lower edges by insulator strips  248 . Conductive strip  250  is a planar conductor oriented in a vertical plane as shown. Conductive strip  246  is installed under a preload such that it is bowed away from conductive strip  250  in its undisturbed position. Conductive strips  250 ,  246  and insulator strips  248  are enclosed within a plastic shroud  244 . When main rails  66  engage tape switch  234  with sufficient pressure, conductive strip  246  is displaced to the position shown at  246   a , which completes the circuit with conductive strip  250  and sends a signal through leads  238  indicating that the strap  74  is properly secured.  
         [0046]     As shown in  FIG. 7 , bed  10  preferably comprises a pair of lateral support pads  116  for holding a patient in place laterally. Lateral support pads  116  are connected to mounts  108 , which are slidably mounted on transverse support rails  106  that span the gap between side support bars  28 ,  30 . Mounts  108  are also threadably engaged with a threaded rod  112 , the ends of which are mounted in side support bars  28 ,  30  with bearings  110 . Mounts  108  are symmetrically spaced from the longitudinal centerline of bed  10 . Preferably, another bearing  111  supports the  15  middle portion of rod  112 , and a manually operable handle  114  is provided on at least one end of rod  112 . With respect to element  114 , the term “handle” as used herein is intended to mean any manually graspable item that may be used to impart rotation to rod  112 . Alternatively, rod  112  may be motor driven. One side  112   a  of rod  112  has right-hand threads, and the other side  112   b  has left-hand threads. By rotating handle  114  in the appropriate direction, lateral support pads  116  are symmetrically moved toward or away from the patient, as desired. Due to the symmetrical spacing of mounts  108  and the mirror image threading  112   a ,  112   b  of rod  112 , lateral support pads  116  provide for automatic centering of the patient on bed  10 , which enhances rotational stability. Similarly, leg adductors/abductors  184  having straps  186  for securing a patient&#39;s legs may be mounted to mounts  108  in like manner as lateral support pads  116 . The term “patient support accessory” is used herein to mean any such auxiliary equipment, including but not limited to lateral support pads and leg adductors/abductors, that is attachable to mounts  108  for the purpose of providing symmetric lateral support to a patient on bed  10 .  
         [0047]      FIGS. 8 through 13  illustrate an apparatus at the foot of bed  10  for supplying a direct electrical connection between non-rotating base frame  16  and rotating patient support platform  20 . As best shown in  FIGS. 8 and 13 , end ring  24 , which is fastened to rotating patient support platform  20 , is also connected to an annular channel  126  that serves as a housing for a cable carrier  148 . Cable carrier  148  carries an electrical cable (not shown) comprising power, ground, and signal wires as is customary in the art. Channel  126 , which preferably has a C-shaped cross-section, may be attached to end ring  24  by way of support bars  192 . Because channel  126  is attached to end ring  24 , channel  126  rotates with patient support platform  20 . As shown in  FIGS. 12 and 13 , an annular cover  198  is connected to upright foot frame  144 , which extends upward from base frame  16 . Cover  198  is preferably mounted on a ring  196  with fasteners  200 , and ring  196  is preferably mounted to support bars  194  that extend from stiffeners  144   a  of foot frame  144 . Cover  198 , which is preferably made of metal to shield cable carrier  148  from radio frequency signals external of bed  10 , is positioned longitudinally adjacent channel  126  to retain cable carrier  148  within channel  126 , but cover  198  is not connected to channel  126 . Thus, channel  126  is free to rotate with end ring  24 , but cover  198  is stationary. One end  150  of cable carrier  148  is attached to channel  126 , and the other end  152  of cable carrier  148  is attached to cover  198 . The length of cable carrier  148  is preferably sufficient to allow patient support platform  20  to rotate a little more than 360 degrees in either direction. This arrangement provides a direct, wire-based electrical connection to the rotating part of bed  10  while still allowing a complete rotation of patient support platform  20  in either direction.  
         [0048]     More preferably, as shown in  FIG. 18 , instead of cable carrier  148 , a flexible cable  252  may be used to supply a direct electrical connection between non-rotating base frame  16  and rotating patient support platform  20 .  FIG. 18  is a view of a preferred embodiment in the same direction as  FIG. 13 , but  FIG. 18  shows only flexible cable  252  and its channel  260  and cover  264  for the sake of clarity. Like channel  126  described above, channel  260  is basically C-shaped in cross-section as shown in  FIG. 19 . However, channel  260  has an inner flange  258  to which cover  264  is attached, preferably with fasteners  262 . Flexible cable  252  resides generally within channel  260 . A gap  266  exists between channel  260  and cover  264  through which one end of flexible cable  252  may pass for attachment to non-rotating base frame  16  (not shown) at connection  256 . The other end  254  of flexible cable  252  is attached to channel  260 , which is attached to rotating patient support platform  20 . Like cover  198  above, cover  264  is preferably made of metal to shield flexible cable  252  from radio frequency signals external of bed  10 . As shown in  FIG. 20 , flexible cable  252  comprises a plurality of flexible conductive strips  268  surrounded by a flexible insulator  270 . Conductive strips  268  carry signals or ground connections, as desired, and multiple flexible cables  252  may be used if necessary, depending on the number of signals required. Like cable carrier  148  above, flexible cable  252  is preferably long enough to allow patient support platform  20  to rotate a little more than 360 degrees in either direction.  
         [0049]     To prevent excessive rotation of patient support platform  20  and the attendant damage that excessive rotation would cause to cable carrier  148  or flexible cable  252  and its enclosed electrical wires, a rotation limiter  128  is provided on the inner surface of upright foot frame  144  as shown in  FIGS. 8, 10 , and  11 . Rotation limiter  128  is pivotally mounted on frame  144  at point  162  and comprises contact nubs  128   a  and  128   b  for engaging a boss  134  that protrudes from frame  144 . Thus, rotation limiter  128  may pivot about point  162  between the two extreme positions illustrated in  FIGS. 10 and 11 . Rotation limiter  128  preferably has a pair of tabs  130 ,  132  that cooperate with sensors  140  and  142 , respectively, which are mounted in frame  144 . Sensors  140 ,  142  are preferably micro switches but may be any type of sensor that is suitable for detecting the presence of tabs  130 ,  132 . By respectively detecting the presence of tabs  130  and  132 , sensors  140  and  142  provide an indication of the direction in which patient support platform  20  has been rotated. A spring  136  is attached to rotation limiter  128  at over-center point  164  and to boss  134  at point  166 . Spring  136  keeps rotation limiter  128  in either of the two extreme positions until rotation limiter  128  is forced in the opposite direction by a stop pin  146 , as discussed below.  
         [0050]     Still referring to  FIGS. 8, 10 , and  11 , rotation limiter  128  has fillets  128   c ,  128   d  and flats  128   e ,  128   f  for engaging stop pin  146 , which is rigidly attached to crossbar  168 . When patient support platform  20  is in its initial supine position (i.e., the position corresponding to zero degrees of rotation and referred to herein as the “neutral supine position”), stop pin  146  is located at the top of its circuit between flats  128   e  and  128   f . As used herein to describe the rotation of end ring  24  and, necessarily, patient support platform  20 , “positive” rotation means rotation in the direction of arrow  170  as shown in  FIG. 8 , and “negative” rotation means rotation in the direction of arrow  172 . As end ring  24  is rotated in the positive direction, stop pin  146  engages flat  128   f  and forces rotation limiter  128  into the extreme position shown in  FIG. 11  under the action of spring  136 . End ring  24  may be rotated slightly more than 360 degrees in the positive direction until stop pin  146  engages fillet  128   c , at which point rotation limiter  128  prevents further positive rotation. End ring  24  may then be rotated in the negative direction to return to the neutral supine position. As end ring  24  approaches the neutral supine position, stop pin  146  will engage flat  128   e . Further rotation in the negative direction beyond the neutral supine position will force rotation limiter  128  into the extreme position shown in  FIG. 10  under the action of spring  136 . End ring  24  may be rotated slightly more than 360 degrees in the negative direction until stop pin  146  engages fillet  128   d , at which point rotation limiter  128  prevents further negative rotation. In this manner, stop pin  146  and rotation limiter  128  cooperate to limit the rotation of platform  20  so that the electrical wires in cable carrier  148  will not be ripped out of their mountings and the direct electrical connection will be preserved.  
         [0051]     Referring to  FIGS. 8, 9 ,  12 , and  13 , the foot of bed  10  preferably has a positioning ring  122  with a central opening  118  through which patient care lines may pass as discussed above. Positioning ring  122 , which is preferably fastened to support bars  192 , preferably has a plurality of circumferential holes  124  for cooperation with a longitudinal lock pin  120  to lock patient support platform  20  in one of several predetermined rotational positions. Lock pin  120 , which is mounted in upright frame  144 , is capable of limited longitudinal movement along its central axis to engage or disengage a hole  124  of positioning ring  122 , as desired. Preferably, lock pin  120  and positioning ring  122  include a twistable locking mechanism for preventing accidental disengagement of lock pin  120  from positioning ring  122 . For example, lock pin  120  may be provided with a protrusion such as nub  120   a  that fits through slot  124   a  of hole  124 . After pin  120  is pushed through hole  124  sufficiently for nub  120   a  to clear positioning ring  122 , handle  120   b  may be used to twist lock pin  120  such that nub  120   a  prevents retraction of pin  120 . Alternatively, lock pin  120  and positioning ring  122  may be respectively provided with cooperating parts of a conventional quarter-turn fastener or the like. Any such suitable device for preventing disengagement of lock pin  120  from positioning ring  122  by twisting lock pin  120  about its central axis is referred to herein as a twist lock.  
         [0052]     More preferably, as illustrated in  FIG. 21 , a lock pin  274  with a spring-loaded detent  278  and proximity switches  288 ,  290  may be mounted to frame  144  with a bracket  272 . Lock pin  274  has a central boss  292  with a peripheral groove  280  for cooperation with ball  282  of detent  278  in the neutral position shown in  FIG. 21 . In the neutral position, pin  274  is disengaged from hole  124  of locking ring  122 , and proximity switches  288 ,  290  preferably send “neutral” signals to the control system to electrically prevent rotation of patient support platform  20 . If handle  276  is used to push pin  274  into engagement with a hole  124  of locking ring  122 , ball  282  of detent  278  engages edge  284  of boss  292 , and proximity switch  288  senses edge  286  of boss  292  and sends a “locked” signal to the control system to electrically prevent rotation of patient support platform  20  in addition to the mechanical locking of pin  274  in locking ring  122 . If manual rotation of patient support platform  20  is desired, handle  276  may be used to pull pin  274  to its fully retracted position in which ball  282  of detent  278  engages edge  286  of boss  292 , and proximity switch  290  senses edge  284  of boss  292  and sends an “unlocked” signal to the control system to allow rotation of patient support platform  20 .  
         [0053]     As discussed in international application number PCT/IE99/00049, bed  10  preferably has a drive system essentially comprising a belt drive between patient support platform  20  and an associated electric motor  152  at the foot end of base frame  16 . The drive system may be of the type described in Patent Specification No. WO97/22323, which is incorporated herein by reference. As illustrated in  FIG. 14 , bed  10  preferably includes a quick release mechanism  156  installed on foot frame  144  to provide a means to quickly disengage patient support platform  20  from the belt drive system. Quick release  156  may be conveniently made from a tool and jig lever available from WDS Standard Parts, Richardshaw Road, Grangefield Industry Estate, Pudsey, Leeds, England LS286LE. Quick release  156  comprises a mounting tube  210  secured to foot frame  144 . A lever  222  is pinned to tube  210  at point  220 . A tab  218  extends from lever  222 , and a linkage  214  is pinned to tab  218  at point  216 . Linkage  214  is also pinned at point  212  to a shaft  208  that is slidably disposed within tube  210 . Shaft  208  extends through foot frame  144  toward belt  204  which is engaged with pulley  202  of the drive system. A roller  206  is attached to shaft  208  for engaging belt  204 . By rotating lever  222  in the direction of arrow  224 , roller  206  is forced into engagement with belt  204 , which provides sufficient tension in belt  204  to engage patient support platform  20  with the drive system. By rotating lever  222  in the direction of arrow  226 , roller  206  is retracted from belt  204 , which disengages patient support platform  20  from the drive system thereby allowing manual rotation of patient support platform  20 . This capability of quick disengagement of the drive system to allow manual rotation of patient support platform  20  is very useful in emergency situations, such as when a patient occupying bed  10  suddenly needs CPR. In such a circumstance, if patient support platform  20  is not in a supine position, a caregiver may quickly and easily disengage the drive system using quick release  156 , manually rotate patient support platform  20  to a supine position, and begin administering CPR or other emergency medical care.  
         [0054]     As disclosed in international application number PCT/IE99/00049, the rotational position of patient support platform  20 , which is governed by motor  152  of the aforementioned drive system, may be controlled through the use of a rotary opto encoder. Alternatively, the rotational position of patient support platform  20  may be controlled through the use of an angle sensor  232  (shown schematically in  FIG. 13 ) of the type disclosed in U.S. Pat. No. 5,611,096, which is incorporated herein by reference. As disclosed in the &#39;096 patent, angle sensor  232  comprises a first inclinometer (not shown) that is sensitive to its position with respect to the direction of gravity. By mounting angle sensor  232  to patient support platform  20  in the proper orientation, the output signal from angle sensor  232  may be calibrated to control the rotational position of patient support platform  20  in cooperation with motor  152 . Likewise, angle sensor  232  may include another properly oriented inclinometer (not shown) that may be used in association with rams  15  and  17  (see  FIG. 1 ) to control the Trendelenburg position of patient support platform  20 .  
         [0055]     Although the foregoing specific details describe a preferred embodiment of this invention, persons reasonably skilled in the art will recognize that various changes may be made in the details of the method and apparatus of this invention without departing from the spirit and scope of the invention as defined in the appended claims. Therefore, it should be understood that this invention is not to be limited to the specific details shown and described herein.