PATENT ABSTRACT
The invention provides a hospital bed with two driving motors that are used in the vertically adjustable jack in order to increase the hoisting power of the bed so as to be able to lift heavy patients. The two driving motors are mounted so as to mechanically operate in parallel. In order to obtain a symmetric load distribution among the two motors, the motors are electrically connected in series and are supplied from a joint voltage source as a serial connection.

PATENT DESCRIPTION
FIELD OF THE INVENTION 
       [0001]    This invention relates to adjustable hospital beds. 
       BACKGROUND OF THE INVENTION 
       [0002]    In DE 10 2004 019 144, a hospital bed is described that has a height-adjustable lifter arranged on the mattress frame. With the help of the height-adjustable lifter, the mattress frame with the patient lying on top can be brought from the normal bed height to a care height that makes it easier for personnel to treat the patient in need of care. 
         [0003]    For adjusting the height, the known bed has an electric motor that drives a threaded spindle via a worm gear. The threaded spindle extends between the lifter foot and the lifter head, in order to extend the lifter accordingly in height. The drive is self-locking. The electric motor is a low-voltage DC motor. The power-supply voltage is approximately 24 VDC. 
         [0004]    With the known bed, patients that weigh up to a structurally predetermined maximum body weight can be raised and lowered. The structural limit is essentially defined by the lifting force of the electric motor that is used. 
       OBJECTS AND SUMMARY OF THE INVENTION 
       [0005]    In view of the foregoing, a general object of the invention is to provide a hospital bed that can raise and lower patients having a relatively larger body weight. 
         [0006]    The hospital bed according to the invention has a height-adjustable lifter. For adjusting the height of the base, two electric motors are provided which work kinematically in parallel. Because these electric motors are self-locking due to the threaded spindle drive, if no countermeasures are taken, twisting can occur that can damage the bed and the motors. Due to the stiffness of the lever mechanism of the base, small path differences in the electric motors are sufficient to cause such damage. 
         [0007]    To prevent this, the two electric motors in the hospital bed according to the invention are electrically connected in series. This circuit-related arrangement allows the mechanical load of the two motors to automatically be made symmetrical. The motor having the greater mechanical load exhibits a reduction in rotational speed, even if it is small. This reduces the counter-EMF in this motor, an as a result the drive voltage available for the other motor becomes higher. This higher voltage allows the rotational speed of the less strongly loaded motor to increase accordingly, until both motors are loaded equally. 
         [0008]    Experience has shown that this symmetry is paradoxically also possible when the two motors are coupled very rigidly to each other and the mechanical work is output via threaded spindle gear mechanisms that are actually self-locking in practice. 
         [0009]    The symmetry on the electrical side ensures that both motors deliver approximately the same mechanical output, which is equivalent to both motors generating approximately the same force for lifting the patient. This method prevents, in particular, continued twisting, which can occur when one motor leads the other motor. The leading motor would always be forced to carry a predominant portion of the patient weight. 
         [0010]    When reading the description of the preferred embodiment, those skilled in the art will appreciate that a series of modifications can be made based on specific circumstances. In addition, other combinations are conceivable. All conceivable embodiments cannot be described herein without unnecessarily increasing the length of the description of the preferred embodiments. 
         [0011]    Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings, in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a perspective view of an illustrative hospital bed according to the present invention in a bed position. 
           [0013]      FIG. 2  is a perspective view of the hospital bed of  FIG. 1  in a chair position. 
           [0014]      FIG. 3  is a partially exploded side view of the lifter of the hospital bed according to the invention. 
           [0015]      FIG. 4  is a schematic top view of the sub-frame of the lifter of  FIG. 3 . 
           [0016]      FIG. 5  is a schematic circuit diagram of the basic circuit for making the load distribution in the two lifter motors symmetric. 
       
    
    
       [0017]    While the invention is susceptible of various modifications and alternative constructions, a certain illustrative embodiment thereof has been shown in the drawings and will be described below in detail. It should be understood, however, that there is no intention to limit the invention to the specific form disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention. 
       DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    Referring now to the drawings, an illustrative hospital bed  1  is shown in the reclining position in  FIG. 1  and in the sitting or chair position in  FIG. 2 . The hospital bed  1  has a bed frame  2  with a head part  3 , a foot part  4 , and side walls  5  and  6 . As shown, the side wall  5  facing a viewer of  FIGS. 1 and 2  is located in the reclining position at a distance from the floor, producing a gap between the bottom edge of the side wall  5  and the floor. This gap allows the caregiver to place the tips of his or her feet under the bed. The side wall  5  is supported so that it can move and can be guided into a shifted-down position in the chair position of the hospital bed  1 , as can be seen in  FIG. 2 . The special mounting of the side wall  5  is explained in detail, for example, in DE 199 12 937 A1. 
         [0019]    Within the bed frame  2  there is a bed lifter  7 , as shown in  FIG. 3 . The bed lifter  7  includes a height-adjustable base  8  having a head on which a rotary hinge  9  with a vertical rotational axis is mounted, an intermediate frame  10 , and a reclining frame  11  on which a mattress  12  is arranged. The reclining frame  11  is rectangular when viewed from the top. 
         [0020]    The reclining frame  11  is divided into a central section  13  that is connected rigidly to the intermediate frame  10 , a rear section  14  that is hinged to the central section  13 , an upper-leg section  15  that is also hinged to the central section  13 , and a lower-leg section  16 . The lower-leg section  16  is hinged to the end of the upper-leg section  15  facing away from the central section  13 . The hinge axes, about which the sections  14 ,  15 ,  16  can move relative to the central section  13 , extend horizontally. Finally, the reclining frame  11  also includes a foot section  17  that is connected rigidly and directly to the base  8 . 
         [0021]    The central section  13  of the reclining frame  11  has two longitudinal beams  18  that run parallel to each other and are spaced apart from each other a distance corresponding to the width of the hospital bed  1 . Because  FIG. 3  is a side view, the visible longitudinal beam  18  hides the associated longitudinal beam of the central section  13  which lies behind it. 
         [0022]    The back section  14  is bounded by a beam  19  and another beam, which is parallel to the first beam and is not visible in  FIG. 3 . The beam  19  is hinged on the beam  18  while the hidden beam is connected to the longitudinal beam parallel to the longitudinal beam  18 . The two beams  19  of the back section  14  are connected to each other at the top end at  20  via a transverse beam. In addition, another transverse bar  21  connects the two longitudinal beams  19  at the bottom side. 
         [0023]    The upper-leg section  15  is also bounded by two longitudinal beams, of which only one longitudinal beam  22  can be seen. The other longitudinal beam is hidden by the longitudinal beam  22 . The two longitudinal beams  22  are connected by a cross bar  23 . The cross bar  26  runs approximately at the middle of each longitudinal beam  22  on the bottom side. 
         [0024]    Finally, the lower-leg section  16  is also bounded by two longitudinal beams, of which only the longitudinal beam  24  can be seen in  FIG. 3 . The two longitudinal beams  24  are connected to each other at the bottom end at  25  via a transverse bar. In addition to this transverse bar, the two longitudinal beams  24  are connected by a bar  26 , which is attached to two parallel guide rails  27  that reach up to the bottom end  25 . As shown, they run at an angle to the longitudinal beam  24 , so that they converge in the direction towards the foot end  25 . The separation distance of the two guide rails  27  is significantly smaller than the separation distance of the two longitudinal beams  24 . Relative to these beams, the guide rails  27  are offset by approximately 20 cm to the inside. 
         [0025]    All of the longitudinal beams  18 ,  19 ,  22 , and  24  carry pegs pointing toward the middle of the bed in order to connect rubber molded parts to the longitudinal beams  18 ,  19 ,  22 , and  24  The rubber molded parts, in a known way, anchor spring rods extending over the width of the reclining frame  11 . The hinges, which are connected on each side of the bed  1  to adjacent longitudinal beams  18 ,  19 ,  22 ,  24 , are shown schematically at  29 ,  30 , and  31 . 
         [0026]    The lower-leg section  16  can be raised or lowered by means of an electric motor. The electric motor is drivingly coupled with a lever  32  and is located in the intermediate frame  10 . Another electric motor  33  is supported in the intermediate frame  10  and leads to the transverse bar  21 . In this way, the back section  14  can be raised or lowered. 
         [0027]    The two longitudinal beams  18  of the central part  13  are connected rigidly to the intermediate frame  10 . The intermediate frame  10  is assembled from square tubes welded to each other to form a rectangular frame. Of these tubes, only one square tube  34  can be seen in  FIG. 3 . The parallel square tube is hidden by the square tube  34 . 
         [0028]    The rectangular frame is narrower than the distance of the longitudinal beams  18  from each other. In total, four extension arms  35 , of which two carry a longitudinal beam  18 , are welded to the parallel square tubes  34 . The extension arms  35  run horizontally and perpendicular to the longitudinal axis of the hospital bed  1 . 
         [0029]    The rotary hinge  9  connects the intermediate frame  10  to the height-adjustable base  8 . The rotary hinge  9  is assembled from a ring  36  and a center pivot plate  37  mounted so that it can rotate in the ring  34 . The center pivot plate  37  is screwed to the intermediate frame  10  via screws. The specific construction of the rotary hinge  9  is explained in DE 102 50 075 A1, which is incorporated herein by reference. 
         [0030]    By means of the rotary hinge  9 , the intermediate frame  10  can rotate relative to a vertical rotational axis together with the reclining frame  11 . The rotation is effected by an electric motor  38 , which has one end supported on the base  8  and on the other end supported on the center pivot plate  37 . 
         [0031]    The height-adjustable base  8  includes an upper frame  39 , and also a lower frame  41 , composed of square tubes appropriately welded to each other, of which two parallel square tubes form the longitudinal beams  39   a  and  41   a . The upper frame  39  is supported on the lower frame  41  by a total of four knee lever pairs  42  and  43 . The rotary hinge  9  is connected to the upper frame  39 . The knee lever pairs  42 ,  43  are each located next to one longitudinal side of the base  8 , so that the corresponding knee lever pairs  42 ,  43  cannot be seen on the other longitudinal side in the side view of  FIG. 3 . 
         [0032]    The knee lever pair  42 ,  43  is assembled from an upper knee lever arm  44  and a lower knee lever arm  45 . Each knee lever  42 ,  43  is connected in an articulated way, by a hinge  46  with a horizontal axis, to the upper or lower frame  39 ,  41 , on the associated side of the bed. All of the axes of the hinges  46  are parallel to each other. The axes of the hinges  46  are coaxial with the hinges of the not visible knee lever  42 ,  43 . 
         [0033]    Hinges  47  connect the knee lever pairs  42 ,  43  to the lower frame  41 . The axes of the hinges  47  are parallel to the axes of the hinges  46 , with the axes of the hinges  46 ,  47 , which correspond to each other on the two sides, being coaxial to each other. 
         [0034]    The two knee lever pairs  42 ,  43  on each side of the base  8  are coupled to each other by an associated horizontal coupling bar  48 . Each coupling bar  48  is connected in a hinge-like way to the knee joint  49  of each knee lever pair  42 ,  43 , as shown. 
         [0035]    Finally, on each side of the base  8 , a diagonal coupling bar  50  connects the upper knee lever arm  44  of the knee lever pair  42  to the lower knee lever arm  45  of the knee lever pair  43 . At least the knee levers  45  aligned with each other on the two sides of the bed at the foot end are connected to each other by a shaft, of which only the shaft  51  can be seen in  FIG. 4 . The same applies for the two lower knee levers  45  at the head end. 
         [0036]    As shown in  FIG. 4 , two spindle lifting drives  52  and  53  are provided for adjusting the height of the lifter  8 . The spindle lifting drive  52  includes a permanently excited electric motor  54   a  and a worm gear contained in a gear housing  55   a . An outer telescoping tube  56   a , in which an inner telescoping tube  57   a  is guided so that it can move longitudinally, connects to the gear housing  55   a  of the worm gear. The inner telescoping tube  57   a  is moved back and forth in a known way by a threaded spindle located in this tube. 
         [0037]    The inner telescoping tube  57   a  is anchored by a bracket  58  on a bottom transverse beam  59  so that it can pivot. The cross beam  59  extends next to the shaft  51 , i.e., next to the lower ends of the two knee lever arms  45 . 
         [0038]    Another connection yoke  61   a  is provided next to the gear housing  55   a  in extension of the inner telescope tube  57   a . This connection yoke  61   a  is anchored on a bracket  62  so that it can pivot. The bracket  62  sits on a cross beam  63  that extends between the longitudinal beams  39   a  of the upper lifter frame  9 . 
         [0039]    The spindle lifting drive  53  lies mechanically parallel to the spindle lifting drive  52  and is constructed from the same components. Accordingly, the construction and structural elements appearing there are provided with the same reference symbols and the added character b. The spindle lifting drive  53  extends between brackets  64  and  65  that are provided on the bottom cross beam  59  and the upper cross beam  63 , respectively. 
         [0040]    With reference to  FIG. 4 , it is not difficult to see that each irregularity in the travel of the two spindle lifting drives  52  and  53  leads to twisting in the lifter  8 , with the consequence that only the leading motor would receive the load during the lifting. In this respect, there is a large risk of overloading this motor mechanically and destroying its drive parts. 
         [0041]    To synchronize the two spindle lifting drives, a circuit arrangement according to  FIG. 5  can be provided. The electric motors  54   a  and  54   b  and the two spindle lifting drives  52  and  53  involve permanently excited DC motors. It is supplied with power via two two-pin connection cables  66   a  and  66   b . Thus, the spindle lifting drive  52  includes two electrical connections  67   a  and  68   a  and the other spindle lifting drive  53  includes the electrical connections  67   b  and  68   b.    
         [0042]    The two electric motors  54   a  and  54   b  are electrically connected in series, i.e., the motor current input  68   a  is connected to the motor current input  67   b . The motor current input  67   a  is led to a moving contact of a pushbutton  69 , while the current input  68   b  is connected to a moving contact of another pushbutton  70 . Their stationary contacts  71  and  72  together contact a connection  73  of a voltage or current source  74 . 
         [0043]    The two pushbuttons  69  and  70  have additional working contacts  75  and  76 , which are also electrically connected to each other and which contact a second connection  77  of the voltage source  74 . 
         [0044]    The illustrated position is the home position of the two pushbuttons  69 ,  70  and leads to the result that the two electric motors  54   a  and  54   b  of the spindle lifting drives  52 ,  53  are not powered. 
         [0045]    When the pushbutton  69  is pressed, the current input  67   a  of the series circuit of the two electric motors  54   a  and  54   b  contacts the connection  77  of the DC voltage source  74 , while the other current connection  68   b  is connected to the connection  73 . Therefore, the two spindle lifting drives  52 ,  53  are set in motion, for example, in the direction for deploying or extending the lifter  8  in the same direction. 
         [0046]    When the pushbutton  69  is released and the other pushbutton  70  is pressed, the current input  68   b  is connected to the connection  77  in the voltage source  74 , while the current input  67   a  of the series circuit of the two spindle lifting drives  52  and  53  remains connected to the connection  77  of the voltage source  74 . In this way, the series circuit of the two spindle lifting drives  52 ,  53  can be set in motion, for example, in the direction for retracting the lifter  8 . Based on the circuit shown, the load distribution is automatically made symmetric between the two spindle lifting drives  52  and  53 . 
         [0047]    The following is an explanation of the operation of the circuit although the theory of how the circuit functions may not be completely understood. In the following explanation, it shall be assumed that, for example, when the button  69  is pressed to raise the lifter, the spindle lifting drive  52  is in a position in which it would lead the spindle lifting drive  53 . Such a situation can occur when the lifter  8  starts from the lower mechanical end position. The lead initially causes an increased load acceptance compared to the spindle lifting drive  53 . Because of the higher loading of the spindle lifting drive  52 , the rotational speed of the associated driving electric motor  54   a  is slightly lower than the rotational speed of the other electric motor  54   b . Therefore, the counter EMF generated by the armature of this motor  54   a  is smaller than the counter EMF of the armature of the electric motor  54   b . In this way, the current through the series circuit of the motors can increase. However, because the motor  54   a  is loaded more strongly, its rotational speed can increase less quickly than that of the less loaded motor  54   b . In this way, the less loaded electric motor  54   b  can catch up in terms of travel. The motor  54   b  is braked in acceleration only when its extended position overtakes that of the initially more heavily loaded electric motor  54   a.    
         [0048]    According to the elasticity against which the two spindle lifting drives  52  and  53  work, this compensation play repeats alternately until, after a few alternations, the load is distributed symmetrically between the two motors  54   a ,  54   b , and both generate equal force for extending the lifter  8 . 
         [0049]    Based on the centrifugal mass of the armature of the two electric motors  54   a  and  54   b , a certain overshoot occurs after reaching the equilibrium state, which causes the searching described above. The searching dies away, however, within a short time after a few overshoots. 
         [0050]    So that the desired target can be achieved, the motors  54   a ,  54   b  also must essentially match in terms of the significant structural details for the force generation and centrifugal masses, since both motors  54   a ,  54   b  should generate the same force. The direction of rotation, however, plays no role. Preferably, maximum loading is to be reached when the nominal voltage of each of the two spindle lifting drives  52 ,  53  is half as great as the output voltage of the current source  74 . 
         [0051]    The invention has been described in connection with a rotating hospital bed. It is understood, however, that the ability to rotate can also be eliminated and the invention can also be applied to a normal hospital bed. 
         [0052]    To increase the lifting capacity of a hospital bed and to be able to lift heavier patients, two drive motors are used in the height-adjustable lifter. The two drive motors are installed so that they work in parallel mechanically. To achieve a symmetric load distribution between the two motors, they are electrically connected in series and are powered from a common voltage source as a series circuit.