Abstract:
A load cell includes a cylindrical ring with one end closed by a membrane configured to receive a load or force to be measured. A sensor carrier plate is arranged in a cavity formed by the ring and the membrane. The sensor carrier plate is coupled to the membrane to undergo a displacement upon deflection of the membrane. The sensor carrier plate has an inner and an outer portion and carries in the outer portion at least one sensor adapted for sensing the displacement and generating a signal based on the load or force applied to the membrane. The sensor carrier plate is movably coupled to the membrane at a location between the inner portion and the outer portion. At least one further sensor is arranged in the inner portion generating a signal changing with opposite sign with respect to the signal of the sensor.

Description:
PRIORITY CLAIM 
       [0001]    This is a U.S. national stage of application No. PCT/EP2008/067455, filed on 12 Dec. 2008. Priority is claimed on Denmark Application No.: 200701787, filed 14 Dec. 2007, the content of both applications are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a load cell comprising a cylindrical ring which at one end is closed by a membrane for receiving a load or force to be measured, and further comprising a sensor carrier plate arranged in a cavity formed by the ring and the membrane, the sensor carrier plate being coupled to the membrane to undergo a displacement upon deflection of the membrane and said sensor carrier plate having an inner and an outer portion and carrying in the outer portion at least one sensor adapted for sensing the displacement and generating a signal due to the load or force applied to the membrane. 
         [0004]    2. Description of the Prior Art 
         [0005]    A load cell is known from U.S. Pat. No. 4,089,036. If the known load cell is heated or cooled with a fast rate or with a temperature gradient across the load cell body a temporary shift of its measuring characteristic is experienced. 
       SUMMARY OF THE INVENTION 
       [0006]    An object of the invention is to provide a load cell of the aforementioned precision type, fitted with a sensor that automatically compensates fast changing temperatures and temperature gradients. 
         [0007]    According to the invention this object is achieved by a load cell of the above-mentioned type, in which the sensor carrier plate is movably coupled to the membrane at locations between the inner portion and the outer portion and that at least one further sensor is arranged in the inner portion, the sensor generating a signal changing with opposite sign with respect to the signal of the sensor. 
         [0008]    As the signal of the at least one sensor and at least one further sensor change with opposite signs, the sensors constitute a differential sensor means which allows load or force to be measured with high precision even in environments where the temperature changes fast and where the load cell experiences a high temperature gradient. The use of more than one sensor and one further sensor gives the possibility to average temperature effects. 
         [0009]    In a preferred embodiment of the load cell according to the invention, a second membrane is arranged in the cavity, the second membrane being coupled at its periphery to the ring or to the periphery of the membrane and further coupled in its center portion to a center portion of the load receiving membrane, the sensor carrier plate is arranged between the load receiving membrane and the second membrane and is supported by the second membrane. The advantage obtained by this embodiment is the possibility to create signals of opposite signs whereby a high sensitivity to the measured load or force and a high degree of automatic compensation to fast temperature changes or high temperature gradients is obtained. 
         [0010]    The second membrane may be directly suspended on the center of the main membrane. The advantage obtained by this embodiment of the invention is the possibility to create signals of opposite signs whereby a high sensitivity to the measured load or force and a high degree of automatic compensation to fast temperature changes or high temperature gradients is obtained. A further advantage is the decreased sensitivity to off center forces. 
         [0011]    The sensor and the further sensor may be adapted to sense the distance to the load receiving membrane. Preferably, the sensor carrier plate carries additionally at least one outer second sensor and inner second further sensor adapted to sense the distance to the second membrane. 
         [0012]    In another preferred embodiment of the load cell according to the invention, a third membrane is arranged in the cavity, the third membrane being coupled at its periphery to the ring or to the periphery of the membrane and further coupled in its center portion to a center portion of the membrane, and that the sensor carrier plate is arranged below and suspended on the third membrane. In this embodiment, the sensor and the further sensor are adapted to sense the distance to the third membrane. The advantage obtained by this embodiment of the invention is the possibility to create signals of opposite signs whereby a high sensitivity to the measured load or force and a high degree of automatic compensation to fast temperature changes or high temperature gradients is obtained. A further advantage by this embodiment of the invention lies in that a shock load does not introduce tension forces in the support between the sensor carrier plate and the second membrane. 
         [0013]    Preferably, the sensor carrier plate is arranged and held between the second membrane and the third membrane. The advantage obtained by this embodiment is the possibility to create signals of opposite signs whereby a high sensitivity to the measured load or force and a high degree of automatic compensation to fast temperature changes or high temperature gradients is obtained. A further advantage by this embodiment lies in that eccentric load forces or side loads are compensated to a high degree. 
         [0014]    The second membrane and the third membrane may be arranged to form a housing containing the sensor carrier plate, with both, the second and third membranes coupled to the main membrane by the same means. The advantage obtained by this embodiment of the invention is the possibility to create signals of opposite signs whereby a high sensitivity to the measured load or force and a high degree of automatic compensation to fast temperature changes or high temperature gradients is obtained. Another advantage by this embodiment according to the invention lies in that eccentric load forces or side loads are compensated to a high degree. A further advantage of this embodiment is the possibility to preassemble and adjust the sensor unit comprised of the sensor carrier plate and the second and third membranes before mounting it in the load cell body. 
         [0015]    According to another embodiment of the invention, the sensor carrier plate is suspended directly on the load receiving membrane. The advantage obtained by this embodiment of the invention is a simple design. 
         [0016]    In a preferred embodiment of the load cell according to the invention, the above-mentioned sensors are of capacitive type. The advantage obtained by capacitive sensors is the very high sensitivity to the forces and loads applied to the load cell and the non-contacting measuring principle, which gives a high tolerance to shocks and overloads as only the elastic body is overloaded and not the sensor system. 
         [0017]    Alternatively, the sensor may be an inductive sensor. The advantage obtained by an inductive sensors is the possibility of functioning in extreme environments and the non-contacting measuring principle which gives a high tolerance to shocks and overloads as only the elastic body is overloaded and not the sensor system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    For further description of the invention, reference is made to the accompanying drawings, in which, by way of example: 
           [0019]      FIG. 1  is a sectional view of a load cell of prior art type; 
           [0020]      FIG. 2  is a sectional view of a first embodiment of the load cell according to the invention having a membrane for receiving the load or force to be measured, a sensor carrier plate with sensor and a second membrane supporting the sensor carrier plate; 
           [0021]      FIG. 3  is a plan view of an example of the sensor carrier plate of the embodiment of  FIG. 2 ; 
           [0022]      FIG. 4  is a plan view of an example of the second membrane of the embodiment of  FIG. 2 ; 
           [0023]      FIG. 5  is a sectional view of the load cell of  FIG. 2  when the membrane is deflected as a result of an applied load; 
           [0024]      FIG. 6  is a sectional view of a second embodiment of the load cell according to the invention with the sensor carrier plate arranged and hold between the second membrane and a third membrane; 
           [0025]      FIG. 7  is a sectional view of an embodiment including a modification of the load cell shown in  FIG. 6  where the second membrane and the third membrane are arranged to form a housing containing the sensor carrier plate; 
           [0026]      FIG. 8  is a sectional view of a third embodiment of the load cell according to the invention with the sensor carrier plate suspended on the third membrane; 
           [0027]      FIG. 9  is a sectional view of a fourth embodiment of the load cell according to the invention with the second membrane suspended on the load receiving membrane; 
           [0028]      FIG. 10  is a plan view of an example of the second membrane of the embodiment of  FIG. 9 ; 
           [0029]      FIG. 11  is a sectional view of a fifth embodiment of the load cell according to the invention with the sensor carrier plate suspended on the load receiving membrane; and 
           [0030]      FIG. 12  is a plan view of an example of the sensor carrier plate. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0031]      FIG. 1  shows a prior art load cell or a modification thereof which does not fall within the scope of the invention. An elastic body, which is normally fabricated in high strength stainless steel, includes a membrane  1  with a load button  2  for receiving the load or force to be measured and an outer cylindrical ring  3 . A bottom membrane  5  closes a cavity  6  formed by the membrane  1  and the cylindrical ring  3 . The membrane  1  comprises a cylindrical center part  4  protruding into the cavity  6 . In the cavity  6  is arranged a sensor carrier plate  7  which is fastened to the cylindrical center part  4  by a support  8 . On the upper surface of the sensor carrier plate  7 , which is made of insulating material, are mounted a sensor in the form of an outer circular electrode and a further sensor  10  in form of an inner circular electrode. The electrodes  9  and  10  constitute with the lower surface of the membrane  1  two measuring capacitances which change because of changing distance between the electrode  9  and the membrane  1  and between the electrode  10  and the membrane  1 , when the membrane  1  deflects as a result of a load applied to the load button  2 . 
         [0032]    It is readily seen that the change of distance is higher for the outer electrode  9  than for the inner electrode  10  when the membrane  1  deflects. Therefore, the electrode  9  may be used as the measuring electrode with a high signal whereas the inner electrode  10  may function as a reference electrode which can compensate for a shift of position of the sensor carrier plate  7  relative to the membrane  1  if the signal of electrode  10  is subtracted from the signal of electrode  9 . 
         [0033]    If the sensor carrier plate  7  is mounted to the inner side of the cylindrical ring  3 , then the electrode  10  would be used as the measuring electrode and the electrode  9  would function as the reference electrode. 
         [0034]    A problem with the load cell shown in  FIG. 1  is that the sign of the signal change from the measuring electrode  9  and the sign of the signal change from the reference electrode  10  are the same and, as the reference signal is subtracted from the measuring signal, the useable signal is decreased. The decrease of the useable signal may be counteracted by a higher deflection of the membrane when loaded by the load or force to be measured, but a higher deflection results in a higher strain in the elastic material of the load cell and a higher nonlinearity. 
         [0035]    The problem of the same sign of the signal of the electrodes  9  and  10  is the reason why reference electrodes are not used in the load cell known from U.S. Pat. No. 4,089,036. 
         [0036]      FIG. 2  shows a first embodiment of the load cell according to the present invention where the elastic body, which is normally fabricated in high strength stainless steel, includes the membrane  1  with the load button  2  for receiving the load or force to be measured, the cylindrical ring  3  and the cylindrical center part  4 . The bottom membrane  5  closes the cavity  6  formed by the membrane  1  and the outer cylindrical ring  3  and cylindrical center part  4 . 
         [0037]    In the cavity  6 , the sensor carrier plate  7  is movably coupled to a second membrane  11  by a number of supports  12  in a region between the outer electrode  9  and the inner electrode  10 . The second membrane  11  is fastened to the inner surface of the ring  3  and to the surface of the center cylindrical part  4 . On the upper surface of the sensor carrier plate  7  are mounted the outer circular electrode  9  and the inner circular electrode  10 . On the lower surface of the sensor carrier plate  7  are mounted a second sensor in the form of a second outer circular electrode  13  and a second further sensor  14  in form of a second inner circular electrode  14 . 
         [0038]    The electrodes  9  and  10  constitute with the lower surface of the membrane  1  two measuring capacitances which change because of changing distance between the electrodes  9 ,  10  and the membrane  1  when the membrane  1  is deflected as a result of a load applied to the load button  2 . Likewise, the electrodes  13  and  14  constitute with the upper surface of the second membrane  11  two measuring capacitances which change because of changing distance between the electrodes  13 ,  14  and the membrane  11 , when the second membrane  11  is deflected as a result of a load applied to the load button  2 . 
         [0039]      FIG. 3  shows a top view of the sensor carrier plate  7  carrying the circular capacitance electrodes  9  and  10 . The electrodes  9  and  10  may be thick film electrodes applied to a ceramic sensor carrier plate  7  by screen printing. The correspondent set of further electrodes  13 ,  14  is applied to the other side of the sensor carrier plate  7 . The areas of the outer electrodes  9 ,  13  and inner electrodes  10 ,  14  and the positions of the electrodes  9 ,  10 ,  13 ,  14  are chosen to provide the best possible combination of linearity and temperature compensation for the load cells according to the invention. The supports  12  are also positioned to provide the best possible combination of linearity and temperature compensation. 
         [0040]    The sensor carrier plate  7  is not necessarily produced of insulating material, but may be produced of any suitable dimensionally stable material applied with insulated layers or insulated parts to support the capacitive electrodes. 
         [0041]    The electrodes  9  and  10  may also be conductive rings bonded to an insulating sensor carrier plate  7 . Alternatively the electrodes  9  and  10  may be produced by printed circuit technology for low cost products. 
         [0042]      FIG. 4  shows a top view of the second membrane  11  with the supports  12 , outer mounting lugs  15  and inner mounting lugs  16 . The second membrane  11  may be mounted to the ring  3  and the center cylindrical part  4  by laser or spot welding. 
         [0043]      FIG. 5  shows the load cell of  FIG. 2  where the membrane  1  and the second membrane  11  are deflected as a result of a load applied to the load button  2 . As the second membrane  11  is coupled to the membrane  1  or ring  3  by means of the outer mounting lugs  15  and to the central part  4  of membrane  1  by means of the inner mounting lugs  16 , it is readily seen that the deflection of the two membranes  1  and  11  is essentially the same. By designing the stiffness of the flat portion of the membrane  11  relative to the stiffness of the mounting lugs  15  and  16 , a linearity and a hysteresis compensation may be obtained. The stiffness of the second membrane  11  may be tailored by the introduction of cut outs and by slitting the surface at different positions with some of the slits advantageously going to the edges of the flat surface. The supports  12  are designed to allow an angular shift between the surface of the second membrane  11  and the surface of the sensor carrier plate  7 , while at the same time keeping the distance between these two members constant at the positions of the supports  12 . It is readily seen that the sensor carrier plate  7  does not undergo a deformation, but moves an amount proportional to the deflection of the second membrane  11 , where the factor between the movement of the sensor carrier plate  7  and the deflection of the second membrane  11  is geometrically dependent of the position of the supports  12  on the second membrane  11  relative to the distances to the mounting lugs  15  and  16 . 
         [0044]    It is also readily seen that, through the deflection of the membrane  1 , the distance between the membrane  1  and the outer electrode  9  is increasing while the distance between the membrane  1  and the inner electrode  10  is decreased. Likewise, through the deflection of the second membrane  11 , the distance between the membrane  11  and the outer electrode  13  is decreased while the distance between the second membrane  11  and the inner electrode  14  is increased. 
         [0045]    The load cell shown in  FIGS. 2 to 5  has the very important advantage that a true differential measurement is obtained. A true differential measurement has the advantage of better linearity and better zero point stability than could be obtained by prior art. 
         [0046]    The dimension of the supports  12 , perpendicular to the surface of the sensor carrier plate  7 , which is the only parameter governing the mean distance between the second membrane  11  and the electrodes  13  and  14 , may be kept very small, and therefore a change of temperature will only have a minor influence on the distances and thus the measurement performed by the differential sensor means consisting of the electrodes  13 ,  14  and the second membrane  11  will be very stable. 
         [0047]    The other differential sensor means consisting of the electrodes  9 ,  10  and the membrane  1  does not exhibit the same stability because the distances are governed by the thermal expansion of the height of the sensor carrier plate  7 , the thermal expansion of the mounting lugs  15  and  16  and the thermal expansion of the outer cylindrical and the center cylindrical parts. 
         [0048]    The load cell shown in  FIG. 6  does not use the membrane  1  as a measuring membrane, but only as a load carrying membrane which is tailored to the load capacity of the load cell. The measurement is performed by two differential sensor means, one consisting of the electrodes  9 ,  10  and a third membrane  17  and the other consisting of the electrodes  13 ,  14  and the second membrane  11 . Both differential sensor means share in this embodiment the same sensor carrier plate  7 , and both exhibit a very high stability due to the differential measurement in combination with the stable supports  12 . A further advantage obtained by the load cell shown in  FIG. 6  lies in the fact that a tilting of the load receiving membrane  1  by eccentric or side forces applied to the load cell through the load button  2 , is only partially transferred to the second and third membranes  11 ,  17  and that the small amount of tilting transferred to the second and third membranes  11 ,  17  is compensated by said membranes  11 ,  17  acting differentially. 
         [0049]    In the load cell shown in  FIG. 7  the sensor carrier plate  7  is mounted by the supports  12  between the second membrane  11  and the third membrane  17 . This embodiment exhibits the same stability as the load cell according to  FIG. 6 , but has the added advantage that the membranes  11  and  17  form a housing containing the sensor carrier plate  7 . The thus obtained unit may be preassembled and tested before being mounted in the load cell cavity  6 . 
         [0050]    A further advantage of the load cell according to  FIG. 7  lies in the fact that a tilting of the main membrane  1  by eccentric or side forces applied to the load cell through the load button  2 , is only partially transferred to the second and third membranes  11 ,  17  and that the small amount of tilting transferred to the membranes  11 ,  17  is compensated by the membranes  11 ,  17  acting differentially. 
         [0051]    In  FIG. 8  an embodiment of the load cell according to the invention is shown with the sensor carrier plate  7  mounted or suspended on the lower side of the third membrane  17  which coupled to the main membrane  1 . The second membrane has been omitted. The advantage by this embodiment is low cost and the ability to withstand very high shock loads as the supports  12  only undergoes compression forces during the overload. The load button  2  is here shown as a removable member which is mounted in a recess in the center circular part  4  of the membrane  1 . The advantage lies in the possibility to adapt the load button  2  to the actual application and also in the possibility that the load button  2  may designed to transfer the load to be measured to the bottom of the recess and from there to the main membrane  1  and not directly to the surface of the main membrane  1 . Hereby stresses are not introduced to the surface of the main membrane  1 . 
         [0052]    The load cell shown in  FIG. 9  has basically the same characteristics as the load cell according to  FIG. 2 , but has the added advantage that the tilting of the load receiving membrane  1 , when off center loads and side loads are applied to the load button  2 , will not be transferred to the second membrane  18  because the coupling between the membranes  1  and  18  is performed by a member  19  which is stiff lengthwise but allows angular movements at both ends. The tilting of the membrane  1  will however be measured by the electrodes  9  and  10  and will give an error. This error may be avoided if a third membrane is introduced above the sensor carrier plate  7 , basically as shown in  FIG. 6  or  7 . 
         [0053]      FIG. 10  shows a top view of the second membrane  18  which, unlike the annular disks  11  and  17 , is a solid disk. 
         [0054]      FIG. 11  shows a load cell with the sensor carrier plate  7  suspended directly on the load receiving membrane  1  through the supports  12 . The advantage lies in the very simple construction. However, tilting of the main membrane  1  due to eccentric forces or side loads is not compensated and shock loads are transmitted directly to the sensor carrier plate  7  and are not damped by being transferred through a second membrane. 
         [0055]    The capacitor electrodes  9 ,  10 ,  13 ,  14  in all embodiments may be connected to a capacitance measuring circuit mounted in the cavity  6  and a cable may be brought out to the instrumentation through the wall of the outer cylindrical part. This circuit could, for example, be according to U.S. Pat. No. 4,737,706. 
         [0056]    Instead of the capacitor electrodes, inductive sensors in the form of small coils may be placed preferably at the same positions on the sensor carrier plate  7  as shown for the capacitor electrodes  9 ,  10 ,  13 ,  14 . 
         [0057]    Due to the fact that preferred embodiments of the invention has been illustrated and described herein it will be apparent to those skilled in the art that modifications and improvements may be made to forms herein specifically disclosed. For example, in the region where periphery of the load receiving membrane  1  is attached to or merges with the ring  4 , a weakening groove in the outer ring  4  may be provided which may have any depth or may be placed at any height in order to tailor the deflection of the membrane  1  to the load cell measuring capacity. 
         [0058]      FIG. 12  shows an example of the sensor carrier plate  7  where an outer electrode ring and inner electrode ring are divided into two or more, here three, segments of separate electrodes  9 ′,  9 ″,  9 ′″ and 10′, 10″,  10 ′″. Each segment may be measured separately by the measuring electronics. The advantage in this embodiment lies in the possibility of tailoring the characteristics of the electrode areas separately.