Force Transducer Units With Multiple Sensing Elements

Abstract

A transducer unit in which the force or forces to be measured, are divided at a predetermined ratio by the use of at least one piezoelement and at least one idle element, of substantially similar thicknesses and characteristics, which are disposed between the force-transmitting supports.

Description

The present invention relates to transducer units, and is particularly concerned with a transducer unit suitable for measuring the magnitude of a force and comprising at least one force-responsive member, such as a piezoelectrical element, mounted tightly between two force-transmitting supports.

The measurement of dynamic forces plays an important role in the field of measurements. For example, in the case of large forces when only a fraction of the effective forces can be applied to the measuring element, or when it is necessary to measure individual component parts of such forces, measuring problems of particular difficulties arise. Complicated measuring problems of this kind cannot be solved by known measuring methods involving the strain gauge strip technique. Also, any other measuring techniques in which a deformation or a change of path is necessary for measuring a force cannot be considered for such complex systems. For solving such problems, only such measuring elements are suitable which measure a force directly, that is to say, which can produce a signal without prior involvement of an elongation or other stress. It is known that piezoelements are suitable for such purposes; these elements exist in the form of piezoelectric crystals and have been available recently also in the form of piezoresistive crystals, In the case of piezoelectric crystals, preferably quartz crystals, but also tourmaline crystals are used because of their high mechanical strength. However, piezoceramic materials can also be used for the measurement of dynamic forces.

In measurement problems of the kind referred to above, the problem may occur to divide the total force into a plurality of exactly definable individual forces and then perhaps to measure only individual components which have different force directions and which are selected from a plurality of differently directed individual forces. The invention provides means for solving this problem in a satisfactory manner.

The invention consists in a transducer unit with at least one piezoelement clamped between two force-transmitting supports and is characterized in that in addition to the piezoelement, at least one idle element is clamped between the force-transmitting supports, and that the surface parts of the idle element which are in contact with the support plates have the same spacing from each other as the corresponding parts of the piezoelement.

Some embodiments of the invention are described below by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a cross section through a transducer unit in which the path of the applied force is divided into three parts;

FIG. 2 is a cross section through a transducer unit in which the applied forces are divided into components of different force directions;

FIG. 3 is a cross section through a transducer unit in which the force flow is directed to a plurality of piezoelements and idle elements, and wherein the signals delivered by the individual piezoelements are guided away in groups by means of metallic layers which are deposited on an insulating plate;

FIG. 4 is a plan view of an insulating plate illustrated in FIG. 3, with conductive metallic layers deposited thereon;

FIG. 5 is a modification of FIG. 4;

FIG. 6 is a cross section through a transducer unit with an idle element which consists of a perforated metal plate;

FIG. 7 illustrates the idle element of FIG. 6 with piezomeasuring elements inserted therein;

FIG. 8 illustrates partly in section the upper part of a transducer unit shortly prior to assembly on a lower part; and

FIG. 9 illustrates partly in section the lower part of a transducer unit with parallel-connected piezoelements.

According to FIG. 1, a transducer unit or force-absorbing member consists of an upper force-transmitting support 1 and of a lower force-transmitting support 2 having faces 3 and 4, respectively, which are optically flat. A piezoelement 5 and two idle elements 6 and 7 are clamped between the faces 3 and 4 in rigid contact therewith. These parallel-connected elements 6 and 7 have been made optically flat simultaneously with the piezoactive element 5 in order that they all have exactly the same thickness; additionally, these idle elements may have similar properties in respect of resilience and coefficient of expansion. In this manner, a defined force division is obtained whereby the use of the transducer unit can be easily adjusted to various requirements. Thus, a large range of forces can be measured with the same piezoelement, depending upon the magnitude of the force division ratio. This is a procedure similar to the generally known procedure in electrical engineering; thus, in the measurement of electrical current, only part of the current is measured in a certain arrangement. The definable division of mechanical forces presumes very high precision in the construction of the absorbing members.

The piezoelement 5 of FIG. 1 may consist of two crystal plates or of one crystal plate and an isolating plate. An electrode 11 is clamped between these two plates and consists e.g. of a thin metal foil. This is connected to a connector terminal 9 by means of a conductor 8. The conductor 8 is embedded in a highly insulating material 10. In the present arrangement, the piezoelement 5 measures one-third of the sum of the individual forces P. Preferably the two force-transmitting supports 1 and 2 are subjected to resilient mechanical bias or prestressing by means of stressing screws. The gap between the idle elements and the piezomeasuring element is filled in a conventional manner with a highly insulating casting material which provides a perfect seal against external effects.

FIG. 2 illustrates a similar transducer unit, however, with the difference that the forces P have different magnitudes and engage in different directions. The transducer unit consists in this case also of an upper force-transmitting support 18 and of a lower force-transmitting support 12. Their mutually facing surfaces 13 and 14 are again optically flat. In place of the idle elements 6 and 7 of FIG. 1, piezoelements 16 and 17 are inserted, the directions of sensitivity of which have different axes. The element 15 which corresponds to the element 5 in FIG. 1 is force-sensitive along the Z axis, the element 16 is sensitive to a shearing force and is sensitive only along the Y axis, the element 17 is also sensitive to a shearing force and is sensitive along the X axis. The two elements 16 and 17 may alternatively be inserted in such manner that they are orientated in accordance with any other axis. It is important that they have exactly the same resilience and coefficient of expansion along the Z axis, but do not deliver a measuring signal in the Z direction, i.e., they behave like idle elements in the Z direction. Preferably the various elements 15, 16 and 17 are made optically flat simultaneously and thus have exactly the same thickness. The signals delivered by the individual elements are applied to terminals X, Y and Z in accordance with known methods. In this transducer unit also, each of the piezoelements may comprise two individual crystal plates, or one crystal plate and one insulating plate of the same dimension, which latter may consist of e.g. aluminum oxide. Again it is advantageous to subject the two force-transmitting supports 18 and 12 to relative mechanical bias or prestressing by means of stressing screws or other conventional means.

It lies, however, within the scope of the invention to dispose parallel-connected piezoelements and idle elements in such manner that piezoelements with differently directed sensitivity axes are disposed in the transducer unit at different locations and the signals are taken off individual elements or groups thereof, so that a locally effective force in the transducer unit can be detected and measured, only fractions of these force components being measured owing to the presence and the effect of the idle elements. It lies within the scope of the invention that the transducer unit may be shaped in any desirable manner. Depending upon the intended use thereof the transducer unit may be limited by flat or rounded faces. Alternatively it may be constructed in the general shape of a disc and the piezoelements may be disposed therein along a mean diameter; in this case the sensitivity axis of the individual piezoelements may be disposed in such manner that axial forces, in particular their distribution in relation to the axis of symmetry, as also torque moments, shearing forces along any desired axis and moments can be measured by means of the disclike transducer unit. The disclike configuration of a transducer unit according to the invention, moreover, leads to practical possibilities for inserting such units into machines as component parts thereof.

FIG. 3 illustrates in section a transducer unit of rectangular shape having an upper force-transmitting support 41 and a lower force-transmitting support 42, opposing faces 43 and 44 of which are optically flat. Between these faces, there is located an insulating plate 45 which projects from the transducer unit e.g. on one side thereof, forming a lug 46, from which a signal can be taken off. Conductive paths 47 are provided on the lug 46 by metal layers deposited thereon. Piezoelements 48 and idle elements 49 are disposed at locations determined by the conditions which prevail owing to the effect of the force intended to be applied to the unit.

An example for an arrangement of a plurality of elements is illustrated in FIG. 4 which shows an insulating plate 51 with a connecting lug 56. The insulating plate consists preferably of a ceramic material, e.g. aluminum oxide, and is ground optically flat on both sides. In accordance with the dimensions of the piezoelements, circular metallic layers 58 and 59 are deposited on the surface by any known method in such manner that they adhere securely to the insulating plate. The circular areas 58 are connected to a conductive path 60 and a connector 61 in accordance with a desired distribution. In contrast the circular idle elements 59 remain insulated. The metallic layers are provided at these locations only in order that the slightest difference in effective height between the piezoelements and the idle elements is avoided. Therefore, the signal is taken off in a manner similar to the manner used with printed circuits. In place of a highly insulating ceramic plate 51, alternatively an insulating foil may be used in certain cases on which the conductive layers shown can be deposited.

FIG. 5 illustrates a modification of FIGS. 3 and 4, wherein piezoelements with different sensitivity directions are used by way of example. Again circular disclike metallic layers 68, 69 and 70 are deposited on an insulating plate 51 in accordance with a desired distribution. The disc 68 is in engagement with a shearing force sensitive piezoelement which is orientated in the X direction; the disc 69 is engaged by a shearing force sensitive piezoelement which is orientated in the Y direction, and the disc 70 is engaged by a piezoelement, pressure sensitive in the Z direction which is parallel to the axis of the disc. The individual elements are connected in a distribution in accordance with the measuring problem to be solved and the forces occurring thereby. Depending upon their orientation, the individual piezoelements for a certain force direction operate as measuring elements or as idle elements. The connecting points for the X, Y and Z components are connected by means of conductive paths 62, 63 and 64. Obviously, additional idle elements may also be provided in such an arrangement for force splitting, such idle elements producing no signal at all. In place of circular elements, alternatively elements of different geometrical shapes, e.g. rings, rectangles and other configurations may be used.

FIG. 6 illustrates a further embodiment in which the idle element is a perforated plate 75, the transducer unit further having an upper force-transmitting support 71 and a lower force-transmitting support 72, facing surfaces 73 and 74 of which again are optically flat, and individual piezoelements 76 are inserted in the idle plate 75. The measuring signal is taken off capacitatively through an insulating foil 78 by an electrode plate 77 having a connecting lug 79. The electrode plate 77 is isolated from the lower force transmitting support 72 by an insulating plate 80.

FIG. 7 illustrates once more in section the electrode plate and the idle plate of FIG. 6. The idle plate 75 is provided with recesses into which piezoelements 76 are inserted. It is preferable to join the plate and the piezoelements adhesively together and to grind both optically flat simultaneously. In this manner a force division ratio can be obtained which is equal to the area ratio. In this case also, care must be taken that a material is selected for the idle plate which is similar to the material of the piezoelements in respect of resilience and coefficient of expansion.

FIG. 8 illustrates the upper part of a transducer unit similar to the unit shown in FIG. 6, the part comprising an upper force-transmitting support 91, with an optically flat contact face 92. Circular disclike metallic layers 94 and conductive paths 95 are deposited, e.g. by vapor deposition of precious metals, on an insulating plate 93. They match elements 106 inserted in an idle plate 105 of the lower part of the transducer unit illustrated in FIG. 9. The piezoelements again are preferably adhesively fixed in the idle plate 105, and the two flat faces of this plate are ground optically flat simultaneously with the piezoelements. In order that the conductive paths 95 have no possibility to come into contact with metallic parts of the idle plate 105, the corresponding connections 107 and 108 are produced with sufficient width in the idle plate. Thereby short circuiting is prevented in a simple manner.

Thus the invention provides means for dividing forces which act from the outside on a unit receiving a measuring value, into exactly defined partial forces and under certain circumstances in the same receiving unit for dividing forces of any directions into components, as well as for determining the location, force direction and magnitude of complex forces which engage the measuring unit. The transducer units according to the invention can be produced with a completely rigid structure; the objects to be measured can therefore also be rigidly connected to the transducer units, leading to high natural frequencies of the whole measuring system.

In the examples illustrated, piezomeasuring elements are shown which are based on piezocrystals such as quartz or tourmaline. However, for purely dynamic force measuring problems, alternatively piezoelements comprising piezoeceramic materials may be used.

The invention may be performed also with the piezoresistive elements which are still being developed and which utilize crystals, the resistance values of which alters under the effect of a force. For such applications, the problems of applying and taking off the charges are somewhat more difficult. The elements, however, may be treated as equivalent because the crystals thereof like germanium and silicon show resilience properties and heat expansion properties similar to piezocrystals. Thus, the invention enables new measuring problems to be solved which heretofore had to be investigated one after the other in a completely unreliable and inaccurate manner with individual receiving units for measuring values. The possibility to solve complex phenomena in one experiment and with one transducer unit opens completely new aspects for the measuring technique.

Claims

We claim:

1. A force transducer unit comprising:

a pair of force transmitting members;

a plurality of piezoelements arranged mechanically in parallel and having their respective axes of sensitivity disposed in different directions; and

means for receiving a predetermined portion of the total force applied to said transducer including a plurality of idle elements having dimensions equivalent to the dimensions of said piezoelements and being loaded mechanically in parallel; and wherein

said plurality of piezoelements and said plurality of idle elements are arranged between said force-transmitting members.

2. A force transducer unit according to claim 1, further including an insulator having metallized conductor paths disposed adjacent thereto for transmitting signals from said piezoelements to connection terminals arranged on the outer periphery of said transducer, and wherein said insulator is located next to said elements and between said force-transmitting members.

3. A force transducer unit according to claim 1, wherein said idle elements consist of the same crystal material as said piezoelements.

4. A force transducer unit according to claim 1, wherein said idle elements consist of a material which has approximately the same resilience properties and coefficient of expansion as said piezoelement.

5. A force transducer unit according to claim 1, wherein said piezoelements and said idle elements are disposed within a spacer plate having the same stiffness as said elements and wherein the adjacent surfaces of said spacer plate and said elements are located in the same plane.

6. A force transducer unit according to claim 5, further including an insulator having metallized conductor strips printed thereon, upon which said spacer plate rests, and wherein said spacer plate further includes grooves therein for receiving said conductor strips while avoiding contact therewith.

7. A force transducer unit according to claim 6, wherein said piezoelements consist of piezoelectric crystals.

8. A force transducer unit according to claim 6, wherein said piezoelements consist of piezoresistant material.

9. A force transducer unit according to claim 1, wherein said axes of sensitivity are orthogonal to one another.

10. A force transducer unit according to claim 1, wherein said different axes of sensitivity correspond to orthogonally directed shear and compressional forces.