Abstract:
A hospital bed weighing system includes four carriage assemblies and a frame assembly. Each of the four carriage assemblies includes a wheel support member for receiving and supporting one of the four wheels of a typical hospital bed. Two caster wheels are mounted at opposite ends of each carriage assembly by a load cell. Inputs from the load cells is analyzed and displayed by a weight display unit which may be calibrated to weigh a bed occupant by subtracting out the weight of the supported bed. The four carriage assemblies are positioned relative to each other by an adjustable frame assembly which is designed to not transfer substantial loads between the carriage assemblies. Since the carriage assemblies are supported by caster wheels, a bed supported by the weighing system may be moved about substantially as if it were rolled upon its own wheels.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Patent Application No. 60/685,352 filed May 27, 2005. 

   FIELD OF THE INVENTION 
   This invention relates to an apparatus for weighing a patient when the patient is supported by a hospital bed. 
   BACKGROUND OF THE INVENTION 
   The prior art teaches hospital beds having integrated load transducer type weighing systems for providing patient weight data. Separate, stand alone hospital bed weighing devices for placement under each supporting wheel have also been proposed for use with hospital beds which lack an integrated weighing system. However, present stand alone bed weighing devices, once in place, prevent movement of the hospital bed. For various reason, there may exist a need, even an urgent need to move a hospital bed. What is needed is a stand alone weighing system which supports a bed and still allows rolling movement of the supported bed. 
   BRIEF DESCRIPTION OF THE INVENTION 
   In an embodiment of the present invention the aforementioned need is addressed by a portable weighing system including four carriage assemblies having load cells and a frame assembly. Each of the four carriage assemblies includes a wheel support member for receiving and supporting one of the four wheels of a typical hospital bed. Two caster wheels are mounted at opposite ends of each carriage assembly by load cells having load transducers for measuring the load supported by each caster wheel. The load cells provide inputs to a weight display unit which can be calibrated to cancel out the weight of the bed to facilitate measuring the weight of a patient occupying the bed. The four carriage assemblies are positioned relative to each other by the frame assembly. The frame assembly may be adjusted so that the positions of the four wheel support members corresponds to the relative positions of the four wheels supporting a hospital bed. The frame assembly is designed to position the carriage assemblies while not transferring substantial loads between them. Accordingly, the members of the frame assembly are connected to each other by design in a relatively loose and adjustable fashion. Since the carriage assemblies are supported by caster wheels, a bed supported by the weighing apparatus of the present invention may be moved about substantially as if it were rolled upon its own wheels. It is preferable that at least some of the wheels of the carriage assemblies have provisions for locking the wheel so that movement of the bed may be prevented. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of an embodiment of the weighing apparatus of the present invention supporting a hospital bed. 
       FIG. 2  is a perspective view of an embodiment of the weighing apparatus of the present invention. 
       FIG. 3  is a perspective view of an embodiment of the weighing apparatus of the present invention with one end of the apparatus partially disassembled. 
       FIG. 3A  is a side view of a three piece center strut assembly for the frame assembly of the weighing apparatus. 
       FIG. 4  is a magnified perspective view of a carriage assembly. 
       FIG. 4A  is a cross section view taken from plane A-A of  FIG. 4 . 
       FIG. 4B  is a detail view of a ramp assemblies attached to a wheel support member. 
       FIG. 4C  is a detail view of one end of a carriage assembly showing details of a shear load cell and a caster wheel. 
   

   DETAILED DESCRIPTION 
   Referring to the drawings,  FIG. 1  illustrate the portable weighing system  10  supporting a wheel supported hospital bed  5 . Generally, portable weighing system  10  includes carriage assemblies  20  and a frame assembly  100 . Frame assembly  100  further includes two opposite and end assemblies  110  and a longitudinal brace  102 . Each carriage assembly  20  includes two load cells which are electrically connected to weight display unit  200  which may, for example, be mounted on the base board of bed  5 . The collection, analysis and display of such load cell output data are steps that are well known in the art. In the alternative, the load cell signals may be collected by a signal conditioning device which converts load cell data to standard formats for use by other systems such as may display weight values for hospital staff at bedside or at a nursing station. Throughout this specification, components and assemblies which are identical will not be separately described or given separate reference numbers in the figures. The skilled reader should understand, for example, that carriage assemblies  20  are identical and include identical elements. End assemblies  110  are also identical and include identical elements. A useful feature of this invention is that standard parts and assemblies may be combined to fashion a weighing system  10 . 
   As can be seen in  FIG. 1 , hospital bed  5  is supported by wheels  5 A. Wheels  5 A are each supported by a carriage assembly  20 . Carriage assembly  20  is shown in greater detail in  FIG. 4 . As can be seen in  FIG. 4  carriage assembly  20  includes a wheel support member  22 , a longitudinal support member  24 , a transverse strut  26 , load cells  28 A and  28 B, caster wheels  30 A and  30 B and two support braces  32 A and  32 B. Wheel support member  22  is preferably fashioned from a channel which provides a track for receiving the wheel of a hospital bed as shown in  FIG. 1 . In this embodiment wheel support member  22  is shown as an elongated channel. However, any suitable member capable of receiving and supporting a hospital bed wheel may be used as a wheel support member. 
     FIG. 4B  illustrates an optional wheel ramp  22 A which may be mounted to one or both ends of wheel support member  22  to provide a ramp for receiving wheels  5 A of hospital bed  5 . Optional wheel ramp  22 A rotates between a first retracted position shown with phantom lines and a second extended position shown with solid lines in  FIG. 4B . Corresponding pin holes  22 A 1  and  22 A 2  in wheel support member  22  and wheel ramp  22 A receive pin  22 A 3  to lock wheel ramp  22 A in the first retracted position. When in the first retracted position, wheel ramp  22 A blocks wheel  5 A from rolling off of wheel support member  22 . When in the second extended position, wheel ramp  22 A receives wheel  5 A for rolling on to wheel support member  22 . 
   Load cells  28 A and  28 B are mounted at opposite ends of longitudinal support member  24 . As can be seen in  FIG. 4 , load cells  28 A and  28 B communicate between caster wheels  30 A and  30 B and longitudinal support member  24  and thus provide the only load path between caster wheels  30 A and  30 B and longitudinal support member  24 . 
   As shown in  FIG. 4A , wheel support member  22  and longitudinal support member  24  are fixed together at opposite ends by support braces  32 A and  32 B and at the center by transverse strut  26 . Transverse strut  26  also provides a means for connecting carriage assembly  20  to frame assembly  100 . Transverse strut  26  is preferably fashioned from tubular steel and extends away from wheel support member  22 . As noted above, wheel support member  22  is preferably fashioned from a steel channel. Longitudinal support member  24 , and support braces  32 A and  32 B are preferably fashioned from tubular steel. The various structural elements described above may be welded or fastened together as shown. 
   The purpose of frame assembly  100  is to locate or position wheel support members  22  of carriage assemblies  20  in positions that correspond to the positions of wheels  5 A of hospital bed  5 . The assembly of frame assembly  100  and carriage assemblies  20  can be best understood with reference to  FIG. 3 . Frame assembly  100  includes a center strut  102  and two opposite and preferably identical end assemblies  110 . Center strut  102  includes a central raised portion  102 B for allowing passage of the legs of standard hospital carts and tables and two end portions  102 A for engaging end assemblies  110 .  FIG. 3  shows center strut  102  as a single piece element. However, center strut  102  may also be fashioned from interconnecting tubular steel members as shown in  FIG. 3A . In  FIG. 3A , center strut  102  is replaced by a three piece center strut assembly  104  having two end pieces  104  and a center portion  108 . Each end assembly  110  includes one longitudinal member  112  for receiving one end of center strut  102  and two transverse members  114 A and  114 B for receiving the respective ends of transverse struts  26  of opposite carriage assemblies  20 . Each transverse strut  26  includes a plurality of pin holes  26 A while each of transverse members  114 A and  114 B includes a single corresponding set of pin holes for receiving a locking pin  115 . The skilled reader will note that the moment arm of transverse strut  26  of each carriage assembly  20  is relatively long so that the moment associated with the offset of wheel support member  22  from longitudinal member  24  of carriage assembly  20  results in a relatively small reactive force at the joint where transverse strut  26  joins a transverse member  114 A or  114 B. Transverse struts  26  of carriage assemblies  20  may be adjustably mounted to each end assembly  110  to accommodate hospital beds of varying widths. The pin joints made by inserting locking pins  115  into corresponding holes as shown in  FIG. 3  are preferably relatively loose but secure from falling apart during use. Frame assembly  100  preferably assembles as a loose fitting arrangement which generally does not transfer significant loads between carriage assemblies  20 . Accordingly, opposite carriages  20  on either side of an end assembly  110  may be capable of rotating about a transverse axis to a significant degree. This allows each of the four caster wheels  30 B associated with each end assembly  110  to make firm contact. This loose fitting arrangement enables the same independent, even contact for caster wheels  30 B associated with the opposite end assembly  110 . 
   Geometrically, three points make a plane. This basic three point principle in the overall design of frame assembly  100  and carriage assemblies  20  and the way in which frame assembly  100  and carriage assemblies  20  are joined together. To insure that caster wheels  30 A and  30 B in each carriage assembly  20  and thus all four carriage assemblies  20  make uniform contact with the floor, the structure of each carriage assembly  20  with respect to caster wheels  30 A and  30 B creates a three point contact. Each caster wheel  30 A and  30 B provides two contact points and the third contact point is provide by the relatively loose joint between transverse strut  26  and whichever of transverse members  114 A or  114 B receives transverse strut  26 . Accordingly, scale frame  12  is relatively flexible with each caster wheel  30 A or  30 B and each carriage assembly  20  free to move independently. This is possible because the frame of hospital bed  5  is relatively strong and rigid. The frame of hospital bed  5  is strong and rigid for supporting a patient, whereas, scale frame  12  is generally flexible and thus particularly adapted for the purpose of weighing a patient supported by hospital bed  5 . 
   Load cells  28 A and  28 B of carriage assembly  20  are preferably identical and are preferably shear beam load cells. Shear beam load cells include strain gauge elements for measuring the magnitude of shear stress resulting from the applied load. The advantage of a shear beam load cell as opposed to a cantilever beam load cell is that a shear beam load cell will register generally the same amount of shear regardless of the position of a caster wheel  30 A or  30 B. Load cells  28 A and  28 B as well as caster wheels  30 A and  30 B are preferably symmetrical and identical.  FIG. 4C  illustrates shear load cell  28 B and caster wheel  30 B. Shear load cell  28 B further includes a load cell member  28 B 1  and a strain gauge  28 B 2  for measuring shear stress in load cell member  28 B 1 . Caster wheel  30 B includes a wheel  30 B 1 , an axle  30 B 2  and caster stem  30 B 3 . Caster stem  30 B 3  is received by a corresponding caster stem bore  28 B 3  in the distal end of load cell member  28 B 1 . Since shear load cells  28 A and  28 B and caster wheels  30 A and  30 B,  FIG. 4  should be understood as illustrating similar structures associated with shear load cell  28 A and caster wheel  30 A mounted to the opposite end of longitudinal member  24 . 
   By using shear beam load cells, errors due offset loading problems or eccentric loading are minimized. Those familiar with shear and moment diagrams will appreciate that bending stresses may vary according to the applied bending moment whereas a shear stress will remain relatively constant past the point where a force is applied. In order to exploit shear beam load cells for this purpose, the distance D 2  between caster stem  30 B 3  and strain gauge  28 B 2  should be greater than the horizontal distance D 1  between caster axle  30 B 2  and caster stem  30 B 3 . It is also important that the hospital bed wheel  5 A as shown in  FIG. 4  be disposed generally between opposite shear load cells  28 A and  28 B. 
   Data from load cells  28 A and  28 B of each carriage assembly  20  are communicated to a weight display unit  200 . In  FIG. 1 , weight display unit  200  is shown mounted to hospital bed  5 . Weight display unit  200  may be of a conventional type well known in the art and may placed or mounted in any convenient location. Weight display unit  200  is adapted to receive inputs from the eight load cells  28 A and  28 B of the four carriage assemblies  20  and add those inputs to determine a weight value which is displayed by weight display unit  200 . As is well known in the art, the purpose of a hospital bed weighing system is not to find the weight of a hospital bed. The purpose of such a system is to monitor the weight of a patient, indicated as a patient  2  in  FIG. 1 , occupying a bed. Accordingly, weight display unit  200  preferably includes a calibration function which allows an operator to tare out the weight of the bed in order to display the weight of the patient. 
   It is to be understood that while certain forms of this invention have been illustrated and described, it is not limited thereto, except in so far as such limitations are included in the following claims and allowable equivalents thereof.