Abstract:
A force transducer, in particular a load cell, includes a spring body that deforms when loaded with a force or load to be measured. Two support parts, which are separated by a gap, are moved out of a position of rest. A capacitive displacement detector is used to detect the relative movement of the support parts, where the capacitor includes two electrode combs that are each held on one of the support parts and includes a multiplicity of electrode fingers. The electrode combs are configured designed and mounted on the two support parts such that the electrode fingers of the one electrode comb pass into the finger interspaces of the other electrode comb when the spring body is loaded so that the force transducer is resistant to overloading.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This is a U.S. national stage of application No. PCT/EP2012/058735 filed 11 May 2012. Priority is claimed on Germany Application No. 10 2011 076 008.3 filed 17 May 2011, the content of which is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a force transducer, in particular a load cell, comprising a spring body, which deforms upon loading with a force or load to be measured and includes two carrier parts separated by a gap, where the carrier parts are moved out of a rest position upon loading the spring body, and comprising a capacitive displacement transducer for detecting the relative movement of the carrier parts, which consists of two electrode combs held respectively on one of the carrier parts and each including a multiplicity of electrode fingers, wherein the electrode combs are held on mutually parallel planar mounting surfaces of the two carrier parts in alignment parallel to one another and to the mounting surfaces, and the electrode fingers of one of the electrode combs move in the finger interspaces of the other electrode comb upon loading the spring body. 
     Such a Force transducers or load cells are known from FIGS. 8 to 10 of EP 0 534 270 A1, where the electrode combs are held on the carrier parts such that, with the spring body not being loaded, the electrode fingers of one electrode comb are respectively positioned asymmetrically-eccentrically in the finger interspaces of the respective other electrode comb. The electrode fingers produce a parallel connection of a plurality of identical capacitors and are adjusted such that a small and a significantly larger electrode spacing each succeed one another. The small spacings thus determine the capacitance of the displacement transducer. Upon loading the spring body, the distances between the electrode fingers and thus the capacitance of the displacement transducer vary. The electrode structures can be formed by anisotropically etching silicon, where the small electrode spacings can be, e.g., 20 μm. 
     The movement margin of the electrode combs is limited by the width of the finger interspaces and the finger width, such that the displacement transducer is destroyed upon overloading the force transducer or the load cell. 
     GB 2 076 970 A discloses a capacitive displacement transducer which consists of two electrode combs and in which the electrode combs are moved with the tips of the electrode fingers—the fingers here are actually coaxial cylinders having different diameters—toward one another, where the electrode fingers of one electrode comb respectively engage into the finger interspaces of the respective other electrode comb to an increasing or decreasing extent depending on the direction of movement. Here, there is also the risk of destruction of the displacement transducer in the case of an overload. In the case of the conventional displacement transducer, a realization of very small electrode structures, e.g., using Micro-Electro-Mechanical Systems (MEMS) technology, would lead to considerable problems in the adjustment of the electrode combs and is therefore virtually impossible. 
     WO 2007/086489 A1 discloses a pressure sensor using MEMS technology comprising a capacitive displacement transducer which, with regard to its basic construction and its manner of operation, corresponds to the conventional displacement transducer of GB 2 076 970 A. Here, there is once again a high risk of destruction of the displacement transducer upon overloading the pressure sensor. 
     US 2007/0284964 A1 discloses an electrostatically operating MEMS actuator in which a movement is produced by electrostatic forces between a movably mounted electrode comb and a stationary electrode comb. The electrode combs are formed and arranged such that they have a parallel offset with respect to one another, and that the electrode fingers of the movably mounted electrode comb dip into the finger insterspaces of the stationary electrode comb in a direction perpendicular to the finger length. US 2007/0284964 A1 also discloses, by way of example, how two electrode combs of this type can be produced from a substrate. 
     DE 32 18 577 A1 discloses a force transducer comprising a spring body, which has two carrier parts separated by a gap, the carrier parts being moved out of a rest position upon loading the spring body. A capacitive displacement transducer serving for detecting the relative movement of the carrier parts has two electrode surfaces, which are arranged on mutually opposite parallel mounting surfaces of the two carrier parts parallel to one another and to the mounting surfaces and move toward one another vertically with respect to their plane upon loading of the spring body. The capacitance is relatively low, and here there is also the risk of destruction of the force transducer upon overloading. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an overload-protected force transducer or such a load cell comprising a capacitive displacement transducer. 
     This and other objects and advantages are achieved in according with the invention by virtue of the fact that, in the force transducer or load cell of the type mentioned in the introduction, the mounting surfaces holding the electrode combs are formed on the carrier parts such that they are moved vertically with respect to their plane upon the loading of the spring body, and that the electrode fingers of the electrode combs each have a different parallel offset with respect to the mounting surfaces, such that, upon loading the spring body, the electrode fingers of one electrode comb dip into the finger interspaces of the other electrode comb. 
     On account of the parallel offset of the electrode combs with respect to one another, the capacitance of the displacement transducer with the unloaded spring body is only low and rises with increasing loading of the spring body, because then the electrode fingers respectively of one electrode comb increasingly engage into the finger interspaces of the other electrode comb. An overloading or destruction of the displacement transducer is precluded because the electrode fingers move only in the direction in which the finger interspaces are open toward both sides. 
     Preferably, the difference in the parallel offset of the electrode fingers of the electrode combs with respect to their mounting surfaces is less than or equal to the height/thickness of the electrode fingers, such that an overlap of the electrode fingers is already present with the spring body being unloaded and the capacitance of the displacement transducer increases largely linearly upon increasing loading. 
     In order to make it possible to realize very small electrode structures using MEMS technology without the problems associated therewith in the adjustment of the electrode combs, the displacement transducer is preferably produced as one piece and mounted as such on the two carrier parts. In this regard, for the case where an overloading of the force transducer is precluded from the outset or is prevented by additional measures for limiting the deformation of the spring body or in the form of a load stop, the displacement transducer can consist of the electrode combs and at least one flexible, e.g., thin, web connecting them and can be formed integrally in this case. Alternatively, the web can be formed as a fracture web which, after the integral displacement transducer has been mounted on the carrier parts, is removed with complete separation of the two electrode combs. Therefore, the electrode combs preferably each consist of a fragment of a near finished product integral design of the displacement transducer. 
     In one advantageous embodiment of the force transducer in accordance with the invention or of the load cell in accordance with the invention, the electrode combs are held on a flexible carrier film and are fitted with the flexible carrier film on the carrier parts of the spring body. The abovementioned separation of the electrode combs can then already be effected, if appropriate, before the displacement transducer is mounted on the spring body. The carrier film is preferably formed as a film printed circuit board, which is electrically contact-connected to the electrode combs and serves for connecting the displacement transducer to an evaluation device. Furthermore, the carrier film can particularly and advantageously form part of a flexible enclosure accommodating the electrode combs and protecting them against environmental influences from the surroundings of the location of use of the force transducer or of the load cell. For this purpose, for example, a cover film can be applied on the carrier film that covers the electrode combs and is tightly connected to the carrier film at the edges. 
     In accordance with a further advantageous embodiment of the force transducer in accordance with the invention or of the load cell in accordance with the invention, a further structurally identical displacement transducer parallel to the displacement transducer is held on the carrier parts, and an evaluation device is provided, which combines the signals of both displacement transducers to form a summation and/or difference signal. The further displacement transducer can be held on the same side of the carrier parts as the one displacement transducer alongside the displacement transducer or on the side of the carrier parts which faces away therefrom. The formation of the summation signal from the signals of the two displacement transducers corresponds to the doubling of the capacitance of an individual displacement transducer and therefore contributes to increasing the measurement sensitivity. At the same time, the difference signal from the signals of the two structurally identical displacement transducers must be zero or at least load-independent in the case of an offset, such that, based on the difference signal, the force transducer or the load cell can be continuously monitored for a possible malfunction. The formation of the difference signal is equivalent to monitoring the signals of the two displacement transducers for correspondence or corresponding behavior and should therefore be equated therewith. 
     Besides the force or load to be measured, disturbing forces or thermal expansions of the spring body that act on the spring body transversely with respect to the force or load to be measured can also influence the capacitance of the displacement transducer and, if appropriate, further displacement transducer. In order to eliminate these disturbing influences on the result of the force or load measurement, preferably at least one additional capacitive displacement transducer for detecting at least one movement component of the carrier parts that extends runs perpendicularly to the relative movement of the carrier parts that is to be detected by the one displacement transducer or by the one displacement transducer and the further displacement transducer is held on the carrier parts, and an evaluation device is provided, which is designed to correct the signal of the one displacement transducer or the summation and/or difference signal of the one displacement transducer and of the further displacement transducer with the signal of the additional displacement transducer. The additional capacitive displacement transducer can be arranged parallel to the one displacement transducer and likewise can consist of two electrode combs held respectively on one of the carrier parts, the electrode fingers of which electrode combs, however, with the spring body being unloaded, are positioned without a parallel offset with respect to one another but asymmetrically-eccentrically in the finger interspaces respectively of the other electrode comb. The compensation principle underlying this development of the force transducer in accordance with the invention or of the load cell in accordance with the invention is known per se from EP 0 776 467 B1. 
     Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to elucidate the invention further, reference is made below to the figures of the drawing, in which: 
         FIG. 1  shows a first exemplary embodiment of a load cell in accordance with the invention with an unloaded spring body and a capacitive displacement transducer; 
         FIG. 2  shows the exemplary embodiment of  FIG. 1  with a loaded spring body; 
         FIG. 3  shows a side view of a first exemplary embodiment of the capacitive displacement transducer; 
         FIG. 4  shows a side view of a further exemplary embodiment of the capacitive displacement transducer; 
         FIG. 5  shows a plan view of the displacement transducer of  FIG. 1 or 4 ; 
         FIG. 6  shows an exemplary embodiment of the load cell in accordance with the invention with a further or additional capacitive displacement transducer; 
         FIG. 7  shows an exemplary embodiment of the additional displacement transducer for detecting transverse forces; 
         FIG. 8  shows a further exemplary embodiment of the additional displacement transducer for detecting transverse forces or thermal expansions of the spring body; 
         FIG. 9  shows a side view of the displacement transducer fitted on a flexible carrier film and covered with a cover film; and 
         FIG. 10  shows a plan view of the displacement transducer fitted on the carrier film. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a greatly simplified illustration of a load cell comprising a spring body  1  in the form of a double bending beam composed of metal, which is fixedly mounted at one end  2  and contains a cutout  3  in the beam center. Regions having a reduced material cross section are provided on the top side and underside of the cutout  3 , the regions forming four bending locations  4 ,  5 ,  6  and  7  of the double bending beam. The material webs remaining between the bending locations  4  and  5 , and respectively  6  and  7 , form an upper guide link  8  and a lower guide link  9 , which both extend parallel to one another. Within the cutout  3 , protruding from the two ends  2  and  10  of the spring body  1 , two rigid beam-shaped carrier parts  11  and  12  extend toward one another and end at a distance from one another. Mounting surfaces  13  and  14 , which are parallel to one another and aligned with one another with the spring body  1  unloaded, are formed at the free ends of the two beam-shaped carrier parts  11  and  12 , on which mounting surfaces one of two electrode combs  15  and  16  of a capacitive displacement transducer  17  is respectively held. Alternatively, provision can be made for the mounting surfaces  13  and  14  to be aligned with the spring body  1  being prestressed, and for the displacement transducer  17  to be mounted on the mounting surfaces  13  and  14  in the prestressed state of the displacement transducer  17 . The construction of the displacement transducer  17  is explained in greater detail further below. 
     As shown in  FIG. 2 , when a weight force  18  is applied to the free end  10  of the double bending beam, the spring body  1  flexes downward, where the free end  10  together with the carrier part  11  proceeding therefrom and the electrode comb  15  held on the carrier part are deflected downward. The capacitive displacement transducer  17  detects the relative movement of the carrier parts  11  and  12 , the relative movement being proportional to the weight force  18 , by way of the changing capacitance between the electrode combs  15  and  16 . 
       FIG. 3  shows the displacement transducer  17  with the two electrode combs  15  and  16  in side view.  FIG. 5  shows the same displacement transducer in plan view. The electrode combs  15  and  16  each have a multiplicity of electrode fingers  19  and  20  that extend parallel to the mounting surfaces  13  and  14  with a respectively different parallel offset in relation to the mounting surfaces  13  and  14 . The resulting parallel offset between the electrode fingers  19  and  20  of the different electrode combs  15  and  16  corresponds approximately to the thickness or height D of the electrode fingers  19  and  20 . Furthermore, the two electrode combs  15  and  16  are arranged staggered in an offset manner, such that the electrode fingers of the electrode comb  15  are situated exactly above the finger interspaces of the electrode comb  16  and the electrode fingers  20  of the electrode comb  16  are situated exactly below the finger interspaces of the electrode comb  15 . 
     As shown in  FIG. 5 , the two electrode combs  15  and  16  are connected to one another via webs  21  and  22 . The webs  21  and  22  are removed only after the application of the displacement transducer  17  with the two electrode combs  15  and  16  on the mounting surfaces  13  and  14  of the carrier parts  11  and  12  or are left given sufficient flexibility of the webs  21  and  22 . 
     Upon loading of the spring body  1  with the weight force  18  (see  FIG. 2 ), the electrode fingers  19  of the electrode comb  15  dip into the finger interspaces of the electrode comb  16 , with the result that the capacitance value of the displacement transducer  17  increases. 
     In order to realize this electrode structure, the displacement transducer  17  is produced using MEMS technology from a silicon or Silicon on Insulator (SOT) substrate by etching on both sides from the top and bottom. 
       FIG. 4  shows a further exemplary embodiment of the displacement transducer  17  in side view, in which the parallel offset between the electrode fingers  19  and  20  of the different electrode combs  15  and  16  is less than the thickness or height D of the electrode fingers  19  and  20 , such that with a non-loaded spring body  1  (see  FIG. 1 ) the electrode fingers  19  of the electrode comb  15  partly dip into the finger interspaces of the electrode comb  16 . Upon loading of the spring body  1  with the weight force  18  (see  FIG. 2 ), the electrode fingers  19  of the electrode comb  15  dip further into the finger interspaces of the electrode comb  16 , with the result that the capacitance value of the displacement transducer  17  increases. By comparison with the exemplary embodiment of  FIG. 3 , in this case the increase in capacitance proceeds more linearly depending on the weight force  18 . The electrode structure can be produced for example by etching from a Double Silicon on Insulator (DSOI) substrate. The plan view of this exemplary embodiment of the displacement transducer  17  is the same as in the case of the example of  FIG. 3  and is illustrated in  FIG. 5 . 
       FIG. 6  shows an embodiment of the load cell illustrated in  FIG. 1 , where, apart from the displacement transducer  17 , a structurally identical further displacement transducer  23  parallel thereto is held on the carrier parts  11  and  12 . This further displacement transducer  23  can be fitted with its electrode combs  24  and  25  directly alongside the displacement transducer  17  on the mounting surfaces  13  and  14  or, as in the case of the embodiment illustrated, can be mounted on the underside of the carrier parts  11  and  12  facing away from the displacement transducer  17 . The two displacement transducers  17  and  23  therefore generate signals that are identical (or inverse, i.e., oppositely directed, if one of the two displacement transducers is mounted oppositely) and are added and subtracted from one another in an evaluation device  26 . The summation signal (or difference signal of the inverse signals) is a measure of the weight force  18  to be measured, while the difference signal (or summation signal of the inverse signals) is equal to or approximately zero in the case of fault-free operation of the load cell and indicates a fault in the displacement transducers  17  and  23  in the event of deviation from zero. 
     In  FIG. 6 , the effective direction of the weight force  18  to be measured coincides with the y-direction of a coordinate system. Besides the weight force  18  to be measured, disturbing forces or thermal expansions of the spring body  1  that act on the spring body  1  transversely with respect to said weight force to be measured, i.e., in the x- and z-directions, can also influence the capacitance of the displacement transducer  17  and, if appropriate, further displacement transducer  23 . In order to eliminate these disturbing influences on the result of the weight force measurement, additional capacitive displacement transducers for detecting relative movements of the carrier parts  11  and  12  in the x- and z-directions can be arranged on the carrier parts  11  and  12 . 
       FIG. 7  shows one such additional capacitive displacement transducer  27  for detecting relative movements of the carrier parts  11  and  12  in the z-direction. This additional capacitive displacement transducer  27  likewise consists of two electrode combs  28  and  29  having electrode fingers  30  and  31  arranged in a staggered manner. In contrast to the displacement transducer  17 , however, there is no parallel offset in the direction of the finger thickness or height. As a result, the electrode fingers  30  and  31  of each of the electrode combs  28  and  29  permanently engage into the finger interspaces of the respective other electrode comb. This engagement is effected asymmetrically-eccentrically, however, such that a small and a significantly larger electrode spacing each succeed one another. The small spacings thus determine the capacitance of the additional displacement transducer  27  which, in the same way as described above for the further displacement transducer  23 , is mounted parallel to the displacement transducer  17  on the carrier parts  11  and  12 . Upon transverse loading of the spring body  1  in the z-direction, the distances between the electrode fingers  30  and  31  and thus the capacitance of the additional displacement transducer  27  vary. 
       FIG. 8  shows an example of an additional capacitive displacement transducer  32  for detecting relative movements of the carrier parts  11  and  12  in the x-direction. Relative movements of the carrier parts  11  and  12  in the x-direction in the case of the displacement transducer  17  bring about only small changes in capacitance. As a result, the additional displacement transducer  32  is realized as a simple plate capacitor having capacitor plates  33  and  34  formed using MEMS technology. In particular, the capacitor plates  33  and  34  can be formed directly on the displacement sensor  17  itself, i.e., the displacement transducer  17  and further and/or additional displacement transducers, such as  23 ,  27  and/or  32 , are produced from a substrate and fitted on the spring body  1 . In the evaluation unit  26  shown in  FIG. 6 , the signal of the displacement transducer  17  and, if appropriate, the summation and/or difference signal of the two displacement transducers  17  and  23  are corrected with the signals of the additional displacement transducers  27  and  32 . All the displacement transducers  17 ,  23 ,  27  and  32  react more or less to all movement components in the x-, y- and z-directions. As a result, they can be calibrated for each of these movement components. With three calibrated displacement transducers  17 ,  27  and  32 , three signals S i =a i •x+b i •y+c i •z, i=1, 2, 3, are obtained, which conform to three equations, such that the movement components in the x-, y- and z-directions can be determined from the signals S i . 
       FIGS. 9 and 10  show respectively in side view and plan view a further exemplary embodiment, in which the electrode combs  15  and  16  of the displacement transducer  17  are held on a flexible carrier film  35 , here e.g.: in the form of a film printed circuit board, and are mounted therewith, if appropriate with the interposition of carrier plates  36  and  37  composed of ceramic, on the carrier parts  11  and  12  of the spring body  1 . The ceramic carriers  36  and  37  serve as a fixed support during the production and mounting of the displacement transducer  17  and are connected to one another via fracture webs  38  and  39 , which are removed after the displacement transducer  17  has been mounted on the spring body  1 . The electrode combs  15  and  16  are mounted directly or via spacers  40  and  41  on the film printed circuit board  35  and are contact-connected thereto. Together with a cover film  42 , the film printed circuit board  35  forms a flexible enclosure  43  accommodating the electrode combs  15  and  16  and protecting them against environmental influences. 
     Thus, while there have shown, described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.