Abstract:
A piezoelectric sensor for bending moment measurements has measuring elements separating at their axes and electrically connecting both halves with their opposing directions of polarization. Such a sensor, mounted with its axis on the neutral bending axis of a machine structure, thus measures concurrently both tension and compression of the machine on both sides of the axis thereof.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to Swiss Application No. 973/06 filed Jun. 14, 2006, and International Application Serial No. PCT/CH2007/000287 filed Jun. 11, 2007. 
       TECHNICAL FIELD 
       [0002]    The invention relates to a piezoelectric sensor for bending moment or torque measurements with at least one axis comprising at least two piezoelectric plate elements arranged in juxtaposition in a plane in accordance with the preambles of the independent claims. 
       BACKGROUND 
       [0003]    Sensors for bending moment or torque measurements are widely used in the industry, for example for the determination of the force of a force transmission arm of a wire welding contact plant, which is also referred to as wire bonding plant. 
         [0004]    In U.S. Pat. No. 6,425,514 a process is shown, in which a conventional force or pressure sensor is brought between the welding arm and the structure of a wire welding contact plant. Finally, the welding load at the tip of the welding arms may be computed by means of the measured forces. 
         [0005]    The disadvantage of such a device consists in the fact, that most preferable two of such sensors should be incorporated, in order to increase the measuring accuracy. However, due to reasons of economy this is done in the minority of cases. 
       BRIEF OBJECTS AND SUMMARY OF THE INVENTION 
       [0006]    The object of the present invention is to provide a sensor for bending moment or torque measurements which exhibits an increased sensitivity in comparison to a conventional sensor of this type at comparable production costs. 
         [0007]    The object is solved by the characteristics of the independent claims. 
         [0008]    The invention relates to the idea of metrological separating a measuring element in a piezoelectric sensor for bending moment or torque measurements in its axis and electrically connecting both halves with their opposing polarization directions. Thus, such a sensor, attached with its axis to the neutral bending axis of a machine structure, concurrently measures both tension and compression of the machine structure on both sides of the axis thereof. Then, a particular advantage exists if the measuring element is integrally formed and only the electrodes on the measuring element are separated from each other. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0009]    In the following, the invention is illustrated in more detail with respect to the drawings. 
           [0010]      FIG. 1   a  is a cross-sectional view of two plate elements, arranged in opposite orientation direction, with electrodes and the electrical connections thereof; 
           [0011]      FIG. 1   b  is a cross-sectional view of two plate elements, arranged in identical orientation direction, with electrodes and the electrical connections thereof; 
           [0012]      FIG. 1   c  is a cross-sectional view of two plate elements on the same plate, with electrodes and the electrical connections thereof. 
           [0013]      FIG. 1   d  is a cross-sectional view of a pair of two plate elements, arranged in opposite orientation direction, with electrodes and the electrical connections thereof; 
           [0014]      FIG. 1   e  is a cross-sectional view of a pair of two plate elements, arranged in identical orientation direction, with electrodes and the electrical connections thereof; 
           [0015]      FIG. 1   f  is a cross-sectional view of a pair of two plate elements each on the same plate, with electrodes and the electrical connections thereof; 
           [0016]      FIG. 2  is a top view of a sensor according to the present invention with two plate elements; 
           [0017]      FIG. 3  is a cross-sectional view of a machine structure in the form of a bond arm with its structure with possible positions for the assembly of a sensor according to the present invention; 
           [0018]      FIG. 4  is a front view of a machine structure in the form of a force transmission arm with its structure with possible positions for the assembly of a sensor according to the present invention; 
           [0019]      FIG. 5   a  is a top view of a sensor according to the present invention with four plate elements; 
           [0020]      FIG. 5   b  is a top view of another sensor according to the present invention with four plate elements; 
           [0021]      FIG. 6   a  is a top view of another sensor according to the present invention with two plate elements, which are sensitive to shear; 
           [0022]      FIG. 6   b  is a top view of another sensor according to the present invention with four plate elements, which are sensitive to shear. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0023]    In all Figures the designation and numbering is identical for the same components. 
         [0024]    The  FIGS. 1   a - f  each show a cross section of at least two plate elements  3   a ,  3   b  of a piezoelectric sensor  1  for bending moment or torque measurements. The double arrows in the  FIGS. 1  designate a possible force or tension effect on the plate elements which is to be measured. Plate elements  3   a ,  3   b  are meant to be parts of plates. These plate elements are juxtaposed in a plane in a laminar manner. The orientation direction P is meant to be the polarization direction of a plate element under the application of force. This direction is denoted by a simple arrow and P. 
         [0025]    The p 1 ate elements  3   a ,  3   b  consist of piezoelectric material with longitudinal effect. One axis  2  of the sensor proceeds between said plate elements  3   a ,  3   b . The field of application of such a sensor  1  is assigned for bending moment or torque measurements with a neutral bending axis  9  ( FIG. 3 ) on the axis  2  of sensor  1 . Therefore, the moment to be measured would result from a force which acts corresponding to the double arrows shown. Such a moment would cause a compression in plate element  3   a  and an elongation in plate element  3   b . The plate elements  3   a ,  3   b  each exhibit electrodes  4 ,  4 ′ on both sides, which collect the charge developed there in a measurement. 
         [0026]    The electrode  4  or  4 ′ of the first plate element  3   a  is electrically connected with the electrode  4  or  4 ′ of the opposite polarization (+, −) of the second plate element  3   b . Such a connection can emerge in the sensor or in the evaluation device. 
         [0027]    Electrical connections  5  each connect the electrodes  4 ,  4 ′ with opposite polarizations (+, −) of the two plate elements  3   a ,  3   b , which are formed in a measurement. Thereby, in identical embodiments and symmetrical arrangements with respect to axis  2  of the two halves, the signal doubles itself. The electrodes  4 ,  4 ′ may be designed as metallizations on the piezoelectric plate elements or as independent components in the form of conductive plates. 
         [0028]    The plate elements  3   a ,  3   b  are attached in the sensor in a biased manner, so that pressure and tension can be measured. 
         [0029]    In  FIG. 1   a  the plate elements  3   a ,  3   b  are arranged in opposite orientation direction P, thus, each electrical connection  5  connects the electrodes  4  on the same side of the plate elements  3   a ,  3   b . In this example the electrodes  4 ,  4 ′ and the electrical connections  5  each may be commonly designed as a single conductive layer or plate. 
         [0030]    In  FIG. 1   b  a device is indicated, which in contrast to  FIG. 1   a  exhibits two plate elements  3   a ,  3   b  arranged in the same orientation direction P. Accordingly, the electrodes  4 ,  4 ′ are not connected with each other in a plane but in a diagonal manner, with opposite polarizations (+, −). This connection is not shown but is obvious only from the signs plus (+) and minus (−) at the ends of the electronic connections  5 . The ends with identical signs are electrically connected with each other in the sensor or in the evaluating device. 
         [0031]    The embodiment in  FIG. 1   c  largely corresponds to that of  FIG. 1   b , wherein the plate elements  3   a ,  3   b  are integrally designed in a plate  3 . This is possible without any difficulties, since the orientation directions P of the plate elements  3   a ,  3   b  proceed equally. Therefore, in this embodiment the piezoelectric plate may be used as a printed circuit board, on to which the electrodes are applied as metallized surfaces, similar to those of switch plates. The advantage of this arrangement consists in the fact that only one plate has to be processed and incorporated, which simplifies the handling. 
         [0032]    The  FIGS. 1   d ,  1   e  and  1   f  each represent a stack of plate elements  3   a ,  3   a ′ and  3   b ,  3   b ′ corresponding to the embodiments of the  FIGS. 1   a ,  1   b  and  1   c Analogously, three and more plate elements  3   a ″,  3   b ″ may also be arranged in a stacked manner. Preferably, the electrodes  4 ′ between the plate elements  3   a ,  3   a ′ abutting in the stack and the plate elements  3   b ,  3   b ′ are achieved by metallizations. Preferably, the metallizations interconnect adjacent plate elements. This connection facilitates the handling of individual plate elements. 
         [0033]    The electrodes  4  each are electronically connected with those of the layers after the next  4 ″ by electrical connections  5  in the sensor or in the evaluating device. Analogously, this also applies to more than two layers in the stack. 
         [0034]    In  FIG. 2 , a top view of a sensor  1  according to the present invention is shown corresponding to  FIG. 1   c  with a continuous plate  3  and two electrodes  4  with opposite polarizations (+, −), which appear in a measurement. The electrodes  4  are arranged on both sides of the axis  2 , which has to be on the neutral bending axis of a machine part in the integrated state. The corresponding opposite electrodes  4 ′ with opposite polarizations are arranged on the back side of the sensor. 
         [0035]    Preferably, an attachment device  6 , for example in the form of a recess  7  for feeding through a fixing screw or an equivalent is provided in the centre of the sensor. 
         [0036]      FIG. 3  represents a cross section of a machine structure  8  in the form of a force transmission arm  10  with a structure  11  with various possible positions for the assembly of a sensor according to the present invention  1 . The neutral bending axis  9  is the area in the machine structure, in which neither an elongation nor a compression arises in the course of appropriately applying stress to the machine structure. The sensor  1  according to the present invention may be attached at any location along this neutral bending axis  9 , wherein axis  2  of sensor  1  always has to be in this neutral bending axis  9 . Thus, a compression on one side of axis  2  has an opposite and identical effect to a tension on the other side thereof. By proper combination of the electrodes  4 ,  4 ′, in particular by the connection of the opposite polarizations of the electrodes  4 ,  4 ′ on both sides of the axis  2 , the signals are summed. In a symmetrical arrangement, this means a doubling. 
         [0037]    According to the present invention, as is also shown in  FIG. 3 , the sensor  1  can also be attached in a section in an arm of the machine structure, wherein the section specifically is made for this purpose. By the assembly in the centre of the sensor by an assembly screw said assembly axis becomes the neutral bending axis  9  under a load at the end of the arm. 
         [0038]      FIG. 4  shows the same arrangement of a machine structure as  FIG. 3 , however, in a front view. The structure  11 , adjacent to an actuator  13 , accommodates force transmission arm  10  by means of attachment  14 . The neutral bending axis  9  proceeds as indicated by the entire machine structure  8 . The sensor  1  according to the present invention can also be arranged laterally of the center, for example in one assembly point  12 , which proceeds on the neutral bending axis  9 . The other assembly point  12  should exhibit the same rigidity as sensor  1 , in order to generate no measurement errors. Otherwise, a second sensor can be attached in a second assembly point to increase the measurement accuracy. 
         [0039]    The advantage of this sensor according to the present invention is its simple installability without causing process interferences. Further, the sensitivity is increased by the division of the electrode. 
         [0040]    A further advantage of the present invention resides in the ability of evaluation. If the electrodes  4  of the individual plate elements  3   a ,  3   b  are not combined in sensor  1  by the electrical connections  5 , but are directed separately to the evaluating device or amplifier, additional physical variables may then be determined. On the one hand, the bending moment which would have emerged by the electrical connections  5  may be calculated by the summation of the measurement values. On the other hand, the force which affects the sensor may be determined by calculating the difference of the measurement values. This applies to an arrangement of the plate elements in accordance with  FIG. 1   a , in the examples according to  FIG. 1   b  or  1   c  the calculation of the difference results in the bending moment and the summation results in the force. 
         [0041]    An extension of the invention is shown in  FIG. 5   a  and  5   b . Analogously to the embodiment described, the sensor may also be designed with four instead of two plate elements  3   a ,  3   b ,  3   c ,  3   d , by insertion of a second axis  2 ′, which is vertically arranged to the first axis  2  and which divides the sensor into four parts. A sensor designed in such a manner permits flexures in both axes and in the combination thereof. For this purpose, the electrodes  4  have to be evaluated separately. The graduation of the electrodes may be selected, so that either equal orientation directions P are located on diagonal opposite sites, as shown in  FIG. 5   a , or next to each other, as shown in FIG.  5   b . Therefore, a sensor according to  FIG. 5  permits bending moments in two directions as well as a force measurement in one direction. 
         [0042]    A further extension of the present invention is shown in  FIG. 6   a . Instead of plate elements from piezoelectric material with longitudinal effect, a material with longitudinal shearing effect is used. Thus shear and torque may be measured simultaneously, if the signals of the plate elements are separately evaluated as described in  FIG. 5 . Here, similar to the type described in  FIG. 1   a  to  1   f  plate  3  can also be divided into plate elements  3   a ,  3   b  or the electrode  4  may be separated, and several plates may be used in a stacked manner. 
         [0043]      FIG. 6   b  shows a combination of shear sensitive plates in various directions, which may be evaluated analogously to the description in  FIG. 5 . 
         [0044]    Various planes with plate elements of various piezoelectric effects may be combined in a sensor, so that a variety of physical variables may be determined simultaneously. 
       REFERENCE SYMBOL LIST 
       [0000]    
       
           1  sensor 
           2   2 ′ axes 
           3  plate 
           3   a ,  3   b ,  3   c ,  3   d ,  3   a ′,  3   b ′ plate elements 
           4   4 ′ electrodes 
           5  electrical connections of the electrodes 
           6  attachment device 
           7  recess 
           8  machine structure 
           9  neutral bending axis 
           10  force transmission arm 
           11  structure 
           12  point of assembly 
           13  actuator 
           14  attachment