Abstract:
A treatment bed has two lifting motors connected in parallel mechanically in the height-adjustable lifter. In order to load the lifting motors evenly and avoid torsions in the lifter, a balancing circuit that measures the supply currents of the lifting motors is provided. If a difference is determined, then the respective current is briefly interrupted several times until the magnitudes of the currents approach one another.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to patient treatment beds and more particularly to a balancing circuit for balancing lift motor current during lifting and lowering of a patient treatment bed. 
       BACKGROUND OF THE INVENTION 
       [0002]    DE 10 2004 019 144 describes a treatment bed which has a height-adjustable base located on the mattress frame. With the aid of the height-adjustable base, the mattress frame can be lifted from the normal lowered bed height with the patient lying on it, to a higher level suitable for treatment, making it easier to treat a patient in need of care. 
         [0003]    For height adjustment, the treatment bed of DE 10 2004 019 144 has an electric motor which drives a threaded spindle via a worm gear. The threaded spindle extends between the foot of the base and its top, in order to extend the lifter of the base to the appropriate height. The drive is self-locking. The electric motor itself is a low-voltage DC motor. The supply voltage is about 24 V DC. 
         [0004]    Patients having less than a design-limited maximum body weight can be raised and lowered with beds such as the bed of DE 10 2004 019 144. The maximum body weight limit is a result of the limited lifting power of the electric motor that is used. The present invention allows a treatment bed that is able to raise and lower patients with a higher body weight. 
       OBJECTS AND SUMMARY OF THE INVENTION 
       [0005]    The treatment bed according to the invention has a height-adjustable base. Two electric motors, which work in parallel, are provided for the vertical adjustment of the base. Since these electric motors are self-locking due to the threaded spindle drive, torsions that damage the bed and the motors can occur if no countermeasures are taken. Moreover, because of the stiffness of the lifting mechanism of the base, small path differences of the electric motors are sufficient to cause such damage. 
         [0006]    In order to prevent this, a balancing circuit is provided for the treatment bed according to the invention. The balancing circuit measures the current input to the two electric motors, e.g., during the lifting operation. If the difference between the two currents exceeds a given amount, the current for the motor having the higher current consumption during the measurement is subsequently interrupted briefly for a constant, predetermined time. 
         [0007]    Thus it is ensured that the two motors draw about the same current, which ensures that both motors produce roughly the same force for raising the patient. In this way, torsional forces arising from one motor running ahead of the other motor are avoided. Otherwise, the motor that is further ahead is forced not only to lift the patient&#39;s weight, but also to work against the lagging motor. 
         [0008]    It is advantageous if the treatment bed is further improved in such a manner that the balancing circuit measures the current input not only during lifting, but also during lowering. During lowering the lagging motor typically exhibits a larger current draw because it is not supported. Therefore, it is advantageous in this situation that the power supply to that motor which shows the smaller power input be interrupted. 
         [0009]    However, in an embodiment of the invention balancing circuit is adaptive because in some cases the current relationships discussed above may be reversed and the foregoing current corrections would actually increase the problem. Thus, if the balancing circuit determines that the current difference is larger rather than smaller after the interruption of power, it will interrupt the power to the other motor and subsequently only perform the interruption of power for that motor. 
         [0010]    Since the measurement and adjustment process is executed continually, a steady state situation will develop after a relatively short time in which the two currents are practically the same. 
         [0011]    In order that the brief interruption of power does not hamper the operation or lead to unnecessary control swings, in an embodiment of the invention a tolerance window is defined for the differences of the motor currents. The power is switched off only if the difference goes outside the tolerance window. 
         [0012]    In a further embodiment of the invention, the tolerance window is a function of the magnitude of the current. The best values for use in any specific situation are easily determined empirically, as they depend on the precise construction and placement of the motors and the construction of the bed. 
         [0013]    Refinements of the invention in other respects are the subject matter of the dependent claims. 
         [0014]    When reading the description of the figures it will become clear to the person skilled in the art that a number of modifications originating from the respective conditions are possible. Further combinations are also conceivable, which cannot be presented in all permutations without unnecessarily increasing the length of the description. 
         [0015]    Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings, of which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a perspective view of a treatment bed in a bed position in accordance with an embodiment of the invention; 
           [0017]      FIG. 2  is a perspective view of a treatment bed in a rotated armchair position in accordance with an embodiment of the invention; 
           [0018]      FIG. 3  is a side partly exploded view of the structure of a treatment bed lifter according to an embodiment of the invention; 
           [0019]      FIG. 4  is a schematic diagram of a basic circuit for balancing the load distribution on the two lifting motors according to an embodiment of the invention; and 
           [0020]      FIG. 5  is a flow chart of a process for balancing the load distribution of the treatment bed during a lifting operation. 
       
    
    
       [0021]    While the invention is capable of various modifications and alternative constructions, certain illustrative embodiments thereof have 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 forms 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. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]      FIG. 1  shows a perspective view of a treatment bed  1  in the position allowing the patient to lie down, while  FIG. 2  shows treatment bed  1  in the seat or armchair position. 
         [0023]    Treatment bed  1  has a bed edge  2  with a head part  3  and a foot part  4  as well as, side panels  5  and  6 . As illustrated, side panel  5  facing the viewer is a distance away from the floor in the position for lying down, in which a gap exists between lower edge of the side panel  5  and the floor, making it possible for care-giving personnel to place the ends of their feet underneath the bed. Side panel  5  is movably mounted, and in the armchair position of the treatment bed  1 , as shown in  FIG. 2 , moves downward. The special mounting of side panel  5  is described in detail in, for example, DE 199 12 937 A1. 
         [0024]    Inside bed edge  2  is a bed lifter  7  as is recognizable in  FIG. 3 . Bed lifter  7  comprises a height-adjustable base  8 , on top of which a turning unit  9  with a vertical axis of rotation is mounted, an intermediate frame  10 , and a bed frame  11  on which a mattress  12  is situated. Bed frame  11  is rectangular in the plan view. 
         [0025]    Bed frame  11  is divided into a central section  13 , which is firmly connected to intermediate frame  10 , a back section  14 , which is articulated to central section  13 , a thigh section  15 , likewise articulated to central section  13 , and a lower leg section  16 . Lower leg section  16  is articulated to the end of thigh section  15  remote from central section  13 . The hinge axes around which sections  14 ,  15 ,  16  are movable relative to central section  13 , are horizontal. Finally, bed frame  11  comprises another foot section  17 , which is directly connected to base  8 . 
         [0026]    The central section  13  of bed frame  11  has two mutually parallel side beams  18 , separated from one another by a distance corresponding to the width of treatment bed  1 . Only one side beam  18  is visible in the illustrated side view. 
         [0027]    Back section  14  primarily comprises a beam  19  as well as an additional beam parallel to it (not shown). Beam  19  is hinged to beam  18 , while the additional beam (not shown) of back section  14  is connected to the additional beam (not shown) parallel to side beam  18 . The two beams  19  of back section  14  are connected by a cross beam (not shown) at the upper end at  20 . In addition, another cross brace  21  connects the two side beams  19  at the lower side. 
         [0028]    Thigh section  15  is also delimited by two side beams, of which one side beam  22  is shown. The other side beam is concealed by side beam  22 . The two side beams  22  are connected by a cross brace  23 . Cross brace  23  runs, for instance, roughly at the center of each side beam  22  on the lower surface. 
         [0029]    Finally, lower leg section  16  is also delimited by two side beams, of which one side beam  24  is shown in the figure. The two side beams  24  are connected at lower end  25  by a cross brace (not shown). In addition to this cross brace, the two side beams  24  are connected by a brace  26 , fixed to end  25  by two mutually parallel guide rails  27 . As shown, guide rails  27  run at an angle to side beam  24  in such a way that they converge in the direction of foot end  25 . The distance between the two guide rails  27  may be markedly smaller than the distance between the two side beams  24 . The guide rails  27  are offset relative to the two side beams  24  by roughly 20 cm. 
         [0030]    All side beams  18 ,  19 ,  22 , and  24  bear pins pointing to the center of the bed for connecting molded rubber parts to side beams  18 ,  19 ,  22 , and  24 , which anchor, in a known manner, spring strips that extend over the width of bed frame  11 . The hinges that connect respective adjacent side beams  18 ,  19 ,  22 ,  24  on each side of bed  1  are schematically represented at  29 ,  30  and  31 . 
         [0031]    Lower leg section  16  can be raised or lowered by an electric motor, not shown. The electric motor is coupled via a gear to a lever  32  and is situated in intermediate frame  10 . An additional electric motor supported in intermediate frame  10  is coupled to a lever  33  connected to cross brace  21 . In this way, back section  14  can be raised or lowered. 
         [0032]    The two side beams  18  of central part  13  are rigidly connected to intermediate frame  10 . Intermediate frame  10  consists of square tubes welded together into a rectangular frame, of which only one square tube  34  is shown. The square tube parallel to it is concealed by square tube  34 . 
         [0033]    The rectangular frame is narrower than would correspond to the distance of side beams  18  from one another. A total of four arms  35  are welded to mutually parallel square tubes  34 , two of which are supported by each side beam  18 . Arms  35  run horizontal to, and at a right angle to, the longitudinal axis of treatment bed  1 . 
         [0034]    Turning unit  9  connects intermediate frame  10  to height-adjustable base  8 . It is composed of a ring  36  and a turntable  37  pivotably seated in ring  34 . Turntable  37  is bolted to intermediate frame  10  with bolts, not shown. An exemplary structure of turning unit  9  is described in DE 102 50 075 A1, incorporated herein by reference. 
         [0035]    By means of turning unit  9 , intermediate frame  10  together with bed frame  11  is pivotable about a vertical axis of rotation. The rotation is accomplished by means of an electric motor  38 , which is braced at one end on base  8  and at the other end on turntable  37 . 
         [0036]    Height-adjustable base  8  comprises an upper frame  39  as well as a lower frame  41 , both consisting of square tubes appropriately welded together, of which two mutually parallel square tubes form side beams  39   a  and  41   a . Upper frame  39  is supported on lower frame  41  by a total of four pairs of articulated levers  42  and  43 . Turning unit  9  is connected to upper frame  39 . 
         [0037]    The pairs of articulated levers  42 ,  43  are each situated next to a long side of base  8 , so that the corresponding pairs of articulated levers  42 ,  43  on the other long side are not recognizable in the side view of  FIG. 3 . 
         [0038]    The pair of articulated levers  42 ,  43  consists of an upper articulated lever  44  and a lower articulated lever  45 . Each articulated lever  42 ,  43  is articulated to upper and/or lower frame  39 ,  41  on the respective side of the bed by a hinge  46  having a horizontal axis. All axes of the hinges  46  are axially parallel to one another. The axes of hinges  46  are coaxial with the axes of the hinges of articulated levers  42 ,  43 , not shown. Hinges  47  connect the pairs of articulated levers  42 ,  43  to lower frame  41 . The axes of hinges  47  are parallel to the axes of hinges  46 . 
         [0039]    The two pairs of articulated levers  42 ,  43  on each side of base  8  are coupled to one another by an associated horizontal coupling strut  48 . Each coupling strut  48  is, as shown, connected in a hinge-like manner to knee joint  49  of each pair of articulated levers  42 ,  43 . 
         [0040]    Finally, a diagonally-running coupling strut  50  connects upper articulated lever arm  44  of the pair of articulated levers  42  to lower articulated lever arm  45  of the pair of articulated levers  43  on each side of base  8 . The articulated levers  45  on both sides of the bed at the foot end may be connected by a shaft, not shown. Similarly, the two lower articulated levers  45  may be connected at their top end as well. 
         [0041]    An electric lifting motor  51  which, like electric motors  33 ,  38 , may be implemented as a spindle motor, extends between upper frame  39  and lower frame  41 . It is articulated next to articulated lever  42  on a cross brace  52 , indicated by dashed lines, of lower frame  41 . Its other end is hinged onto a concealed cross brace of upper frame  39  next to articulated lever  43 . The motor lies between the two frames  39  and  41 , and is thus transverse to diagonal coupling strut  50 . Another lifting motor (not shown) is arranged parallel to the visible lifting motor  51  and is articulated in the same way. Both lifting motors operate in parallel kinematically and are arranged as closely together as possible. 
         [0042]    Articulated levers  42 ,  43  cooperate with horizontal coupling strut  48  and diagonal coupling strut  50  as a guide for the relative motion of the two frames  39  and  41 . 
         [0043]    The lifting mechanism of lifter  8  is itself very rigid. Because of the directly adjacent arrangement of the two lifting motors  51 , differential thrusts, and thus torsions, between the lifting motors can very easily occur, even if only small movement differences arise. A further difficulty is that the two lifting motors  51  are spindle motors, which by their nature are self-locking and are able to produce very large forces. 
         [0044]    Even if the lifting motors are initially aligned, it is practically impossible to prevent differences in running speeds from developing, due to the tolerances of the lifting motors. This leads in the course of the time to a difference in lift between the two lifting motors. 
         [0045]    In order to balance the lifting motors in operation such that each of the two lifting motors contributes about equally to the total lifting force, the balancing circuit represented as a schematic diagram in  FIG. 4  is provided. In  FIG. 4 , the two lifting motors operating in parallel mechanically are labeled A and B. Each lifting motor has an outer telescoping tube  52  as well as an inner telescoping tube  53  that can be set in rotation via a rotating threaded spindle  54 , drawn in dashes in  FIG. 4 , in order to displace inner lifting tube  53  axially in relation to outer telescoping tube  52 . An electric motor  55  mounted at one end of outer lifting tube  52  drives threaded spindle  54  via a worm gear. 
         [0046]    The lifting motor A has two power inputs  56  and  57 , via which power is supplied within the low voltage range of around 24-48 V in the illustrated embodiment. Lifting motor B has the same structure in principle, which is why the same reference symbols are used there to designate the mechanical components. Lifting motor B is supplied with power via power supply inputs  58  and  59 . The two power supply inputs  56  and  58  are connected in parallel and lead via a line  61  directly to a connecting terminal  62 . Terminal  57  leads to a controlled semiconductor switch  63  and from there to a current-sense resistor  64 , and via a line  65  to an additional power supply input  66 . 
         [0047]    The connection of power supply input  59  is similar. Power supply input  59  is connected via a controlled semiconductor switch  67 , from where the power connection leads via a current-sense resistor  68  to line  65 , and thus to power supply input  66 . The two semiconductor switches  63 ,  67  are controlled by a microprocessor/microcontroller  69 . The latter has two outputs  71  and  72 , which are connected to control inputs  73  and  74  of the two semiconductor switches  63  and  67 . 
         [0048]    In addition, the microprocessor  69  is connected at inputs  75 ,  76  in series with current-sense resistors  64  and  68  and at input  77  in parallel to current-sense resistors  64  and  68 . For this purpose, one input  77  is connected to line  65 , while input  76  is connected to the node between current-sense resistor  68  and semiconductor switch  67 . The input  75  is accordingly connected to current-sense resistor  64 . 
         [0049]    Behind the two inputs  75  and  76 , there are analog/digital converters in microprocessor  69 , which are able to convert the voltage measured at current-sense resistor  64  and  68  respectively into digital values that can be processed by the program in microprocessor  69 . 
         [0050]    The corresponding controlled output of a higher-order control circuit (not shown), with which, using a conventional manual push-button, the user can cause the two lifting motors A and B to run in the direction of lifting or lowering, depending on the actuation, is connected to power supply inputs  62  and  66 . When the button is released, the power supply to inputs  62  and  66  is switched off and lifting motors A and B remain self-locked in their respective positions. The power supply of microprocessor  69  is not shown, since it is obvious to those skilled in the art and is not the subject matter of the invention. 
         [0051]    The mode of operation of the balancing circuit shown above will be explained in connection with the flow chart of  FIG. 5 . If the user would like to raise the treatment bed, he presses the appropriate command button on his manual control. A voltage is thereby supplied via the central control unit to the two power supply inputs  62  and  66 . For example, the positive pole is connected to power supply input  66  in this mode of operation, while the negative pole is connected to power supply input  62 . 
         [0052]    In the idle state of the circuit, with microprocessor  69  activated, it supplies electrical signals at its outputs  71  and  72 , which ensure that the two semiconductor switches  63  and  67 , implemented as power MOSFETs for example, are conducting. 
         [0053]    Thus a current begins to flow that runs from power supply input  66  via current-sense resistor  68  and conducting semiconductor switch  67  to lifting motor B, and from there to power terminal  62 . Another current flows from power supply input  66  via resistor  64  and semiconductor switch  63  to lifting motor A, and from there to power supply input  62 . 
         [0054]    The currents flowing to each of the lifting motors A and B are detected continuously by the microprocessor  69  individually with the aid of current-sense resistors  64  and  68 . 
         [0055]    In an execution block  80 , microprocessor  69  forms the difference of the currents I A  and I B  drawn by lifting motors A and B on the basis of the voltages that are detected at the two resistors  64  and  68 . In a decision block  81  it is then determined whether the magnitude of the current difference D is greater than a preset error value F. If this is not the case, the program of the microprocessor  69  returns, via a short waiting loop, if necessary, to the start of execution block  80   
         [0056]    Otherwise, if the magnitude of the difference D exceeds the preset limit value F, the program continues at decision block  82 . In decision block  82 , it is determined whether current I A  is larger than current I B . 
         [0057]    If current I A  is larger than I B , this an indication that lifting motor A is contributing more to the lifting force than lifting motor B. It is assumed that the lifting force of the lifting motors is proportional to the current drawn, since the two lifting motors are otherwise dimensioned and constructed identically within component tolerances. 
         [0058]    If lifting motor A is drawing more current, this is an indication that it is leading the other lifting motor B, which at the same time implies a certain torsion in lifter  8 , which is generally undesirable. The program therefore executes instruction block  83 , which ensures that the current for lifting motor A is interrupted for a preset time t. For this purpose, microprocessor  69  supplies a signal at its output  71  that brings semiconductor switch  63  into the blocking state. The time t lies in the range between 0.01 sec. and 2 sec. The optimal value is to be determined empirically. Upon completion of the time t, the program continues at the input of an instruction block  80 . 
         [0059]    After the execution of decision block  81 , if it turns out after the check in decision block  82  that current I A  is smaller than current I B , the program branches to an instruction block  84 , which leads microprocessor  69  to interrupt current I B  for the duration of time t. For this purpose, microprocessor  69  supplies a signal at its output  82 , by which semiconductor switch  67  is blocked for the time t. After executing instruction block  84 , the program likewise returns to input instruction block  80 . 
         [0060]    The time t is selected such that, by repeated execution of instruction blocks  83  or  84 , currents I A  and I B  approximate one another. If after the interruption of power for one or the other lifting motor A or B, the power drawn reverses significantly, i.e. by more than the value F, time t may be deemed to be too long, 
         [0061]    On the other hand, in practice, currents I A  and I B  might not and need not ever precisely equal one another since there is a certain residual error even with a small setting of t, but any small remaining difference in the currents is harmless. Therefore, t should be selected in such a way that instruction blocks  83  and  84  are not constantly being executed one after the other because, for instance, the opposite error is present after execution of, for example, instruction block  83 , and the error difference current has now become greater than tolerance value F. 
         [0062]    The magnitude of t should also be matched to the duration of the program execution cycle, so that a balance between lifting motors A and B arises as quickly as possible. 
         [0063]    By briefly cutting off the lifting motor current by way of microprocessor  69 , the lifting motor is briefly stopped at the same time, so that the other lifting motor, still supplied with current, can catch up. 
         [0064]    After a finite number of program runs, a condition is reached in which both lifting motors A and B draw about the same current and thus produce approximately the same thrust force. At the same time, this also means that no torsion arises in the frame itself or that one lifting motor must drag the other motor. If the error again increases over time due to differences in rpm, it will be compensated automatically by the balancing circuit. 
         [0065]    In order to reach such balance as quickly as possible, in an embodiment of the invention both the magnitude of the preset permissible current difference F and the turn-off time t are dependent on the measured currents. 
         [0066]    If it is not desired for the control to intervene when lifting motors A and B are set in motion in the direction of lowering, the two semiconductor switches  63  and  67  can be shunted by diodes  86  and  87 , as indicated in broken lines. 
         [0067]    If the program is to be effective during lowering, however, diodes  86  and  87  are not used, but rather semiconductor switches  63  and  67 , which conduct current in both directions. If desired, this can also be achieved by MOSFETs connected back-to-back. Suitable circuitry measures for this purpose are known to those in the art and need not be described. As noted above, a control circuit for reversing the polarity of the current supplied to the electric motors may be located between the power source and the balancing circuit. 
         [0068]    Depending upon construction and conditions, it may be that balance can only be achieved in the lowering operation if the current is interrupted to the motor which is drawing less current, not that which is drawing more, as is the case for lifting. During the lowering operation, the force acting at lifting tube  53  supports the rotational motion of the armature; in other words, the lagging motor is the one that is most strongly loaded by the weight. During the lowering operation in this alternative embodiment of the invention, a program corresponding to that of  FIG. 5  is executed, with the &gt; sign used in decision block  82  replaced by a &lt; sign. 
         [0069]    Finally, it is conceivable that conditions may vary during the lowering operation. Thus, during the lowering operation the output of the two instruction blocks  83  and  84  may be monitored to determine whether the current difference measured in instruction block  80  became larger after they were executed. For this purpose, an appropriate additional query block can be inserted, which will then have the effect that the &gt; sign will be dynamically exchanged with the &lt;, or vice versa, in query block  82 . In this way, the arrangement becomes self-learning and briefly switches off the current to the lifting motors so that the currents in both lifting motors A and B become essentially equal in magnitude and are kept essentially equal.