US 3566163 A
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United States Patent 72] Inventors Hans Fischer Zollikerberg; Hans C. Sonderegger, Nel'tenbach; Gelli Spescha, Winterthur, Switzerland  Appl. No. 756,173
 Filed Aug. 29, 1968  Patented Feb. 23, 1971  Assignee Kistler Instrumente A.G. Winterthur, Switzerland  Priority Sept. 5, 1967 [3 3 Switzerland [5 4] MULTIPLE-COMPONENT PIEZO MEASURING CELLS 9 Claims, 9 Drawing Figs.  US. Cl 310/8.3, 310/8.4, 310/8.6, 310/8.7, 310/9.l, 310/9.4  Int. Cl H04r 17/00  Field olSearch 310/8.2, 8.3, 8.4, 8.5, 8.6, 8.7, 9.1, 9.5, 9.6, 9.7, 9.8; 340/ 10; 73/74 Primary Examiner-Milton O. l-lirshfield Assistant Examiner-Mark O. Budd Attorney-Craig, Antonelli, Stewart & Hill ABSTRACT: A multiple-component piezomeasuring cell in which an arrangement of crystal elements sensi tiv e to Both shear and compressional forces is provided as a self-contained integral assembly whereby individual outputs exist for each of the forces to be measured and the assembly is under high mechanical prestress in the assembled condition.
PATENTED FEB23 m SHEET 1 [IF 3 FIG. I
INVENTORS HANS FISCHER ms 0. SONDEREGGER H0 3 GELLI SPESCHA ATTORNEYS PATENTED FEBZB'IBYI SHEET 2 0F 3 INVENTORS HANS FISCHER HANS C. SONDEREGGER GELLI SPESCHA' ATTORNEY 5 PATENTED E823 1971 3563153 SHEET 3 BF 3 v INVENTORS HANS FISCHER HANS c. SONDEREGGER BY I 'GELLI SPESCHA ATTORNEYS MULTIPLE-COMPONENT PIEZO MEASURING CELLS The present invention relates to improvements in piezoelectric transducers, and more particularly to multicomponent piezotransducers for measuring forces, accelerations and moments.
Problems must often be solved in measurement techniques which require the simultaneous measurement of forces that act on an object in different directions. Such measurement problems occur in shock wave passages where, for purposes of determining the action of very steep pressure fronts, special models are used and several components have to be measured. Also in various other fields of research, measuring devices are used which permit the simultaneous measurement in several components of the reaction forces, moments and accelerations.
Various proposals have been made for such applications in the stress line technique. For this purpose, it is important to construct measurement bodies in which the forces of the individual components can be measured separately since, as known, it is not possible with stress lines to measure out and evaluate several directions of interaction on one measurement body. Because of the necessity of a completely satisfactory separation of the strain arms of the measurement body in the desired operative directions, i.e., in the desired directions of interaction, the measurement body becomes very complicated mechanically and additionally entails the disadvantage that it possesses a stiffness which is irregular or nonuniform in the various directions and generally relatively low. In particular, the low rigidity involves serious disadvantages in various applications since the natural frequency of such measuring devices is very low. The stress line technique could therefore offer no substantial advantages and make no significant progress in this field.
The piezomeasurement technique, however, provides the possibility of being able to determine forces in different directions of interaction in one and the same measurement body by the use of differently cut crystals. Very simple measurement bodies of great rigidity in every desired measuring direction are obtained thereby.
The underlying concept of the present invention is concerned with those piezoelements which form an inherently closed unit, i.e., a self-contained unit, and which can be assembled in any desired measuring system. Such multiple component piezomeasuring cells can be used both for force measurements and also for acceleration measurements. Depending on the particular arrangement, it is possible to measure in one cell only forces or forces and accelerations or also moments and forces and finally also moments, forces and accelerations with the same multiple component piezomeasuring cell.
The mean feature of the present invention is the arrangement of various piezoelements in plate form, which have piezoelectric properties both as regards shear forces and also normal pressures. Quartz crystals are used advantageously for the purposes of the present invention; however, any other known piezocrystals can be used, whose axes are correspondingly polarized. It is also possible to construct the multiple component piezomeasurement cells according to the present invention by the combination of quartz and other piezocrystals.
The features of the present invention will be further explained with reference to the accompanying drawings, wherein:
FIG. 1 is an axial cross-sectional view through one embodiment of a multiple component measuring-cell according to the present invention, taken along line H of FIG. 2;
FIG. 2 is a transverse cross-sectional view through the multiple component measuring cell of FIG. 1, taken along line II-II OF FIG. 1;
FIG. 3 is a somewhat schematic, exploded view, in perspective, of the arrangement of the pairs of crystals together with the output electrodes thereof as used in FIGS. 1 and 2;
FIG. 4 is an axial, longitudinal cross-sectional view through a multiple component piezomeasuring cell with acceleration compensation and of in-line construction according to the present invention;
FIG. 5 is an axial longitudinal cross-sectional view through a modified embodiment of a measuring cell according to the present invention for moment and multiple component force measurements; I
FIG. 6 is an axial cross-sectional view through a measuring cell according to the present invention for acceleration measurements in several components;
FIG. 7 is an axial cross-sectional view through a further embodiment of a measuring cell according to the present invention for acceleration measurements;
FIG. 8 is an axial cross-sectional view through a measuring cell according to the present invention and corresponding to FIG. 1, for acceleration measurements; and
FIG. 9 is an axial cross-sectional view through a still further modified embodiment of a measuring cell according to FIG. 1, built into a casing for acceleration measurement.
The constructional details of multiple component piezomeasuring cells will be explained at first by reference to an embodiment for three components, whose axes extend perpendicularly to one another. As already mentioned above, similar constructions are possible with two or more measurement components. This compact constructional form is particularly suitable for investigations on machine elements.
Referring now to the drawing wherein like reference numerals are used throughout the various views to designate like parts and more particularly to FIGS. 1 and 2, the measurement cell illustrated in these FIGS. includes a housing or casing 1 which consists of a circular, rectangular or square plate provided with a through-bore. Connecting sockets 2 are fitted into the casing l on one side thereof; however, the connecting sockets 2 may also be arranged differently instead of being all disposed on one side, as shown. The piezoelement 3 consists of a multiple player crystal arrangement and lies between two force-transmitting discs 4 and 5 which are connected by means of resilient flanges 6 with the casing 1. The construction of these flanges 6 is of great importance for the functioning of the multiple component piezomeasuring cell of flat o'r plate-type construction. The flanges 6 have to connect the force-transmitting discs 4 and 5 with the casing 1 elastically both in the axial direction as also in the plane of the discs. However, these flanges 6 must also permit a constant mechanical prestressing to be exerted on the piezoelement 3 in order to avoid any faults in the spring action. To seal off the piezoelement form the side of the bore, a thin-walled sleeve 8 is connected with resilient flanges 9 of the force-transmitting discs 4 and 5. Also, these connections are made that the sleeve 8 is under a constant prestress.
This prestress is effected in the completely assembled cell by a special welding operation as the final operation. In place of a welding operation, crimping is also possible. For centering the piezoelement 3 during the assembly operation, a centering ring 7 which is provided with an opening or aperture for the electrodes 10, is inserted into the casing l. The centering ring 7, in turn, is constructed resiliently as compared to the piezo piezoelement 3 by means of grooves formed therein and consists of a highly insulating material of any known type. During assembly,the procedure is such that the upper force-transmitting disc 4 is only inserted, after the connection of the electrode lugs 10 with thecontact pins 11 has been made, whereupon the upper disc 4 is connected under prestress with the casing 1. The contact pins 11 are mounted in highly insulating guides 12 which, in turn, are fixed in and sealed to the connecting sockets 2. The connecting sockets 2 are so arranged that commercially available contact plugs can be connected thereto.
The arrangement of the crystal packets is further explained by reference to FIG. 3, again for a measuring cell according to FIGS. 1 and 2. The correspondingly cut crystal discs are used for each direction of sensitivity. In this way, the respective takeoff electrodes can in each case be disposed between two discs. The discs 21 and 22 are cut from the crystal in such a manner that under lateral or shear stress in the plane of the discs, the outer surface of one disc emits positive charges and the other negative charges. The axis 30 of the maximum shear sensitivity is marked on the two discs 21, 22 and the axes are superimposed one on the other during assembly. The pair of discs 23 and 24 possess the normal piezosection or cut so that during stressing normal to the plane of the' discs, positive charges are produced on one outer face or surface and negative charges on the other outer face or surface'of the two discs. The takeoff electrode 27 is again arranged between the pair of discs. The pair of discs 25 and 26 is cut in the same way as discs 21 and 22 but the axis 31 of the greatest shear sensitivity is offset to the axis by 90. All the outer disc surfaces of the three pairs of crystals are connected to a common electrode insulating disc or foil must then be inserted. In this way, however, the output signal for each unit is reduced to one-half but the height of the cell can be reduced.
It will be seen from the foregoing that for the same constructional arrangement, various directions of the sensitivity axes are possible.
FIG. 4 shows a multiple component piezomeasuring cell of rod-type construction. In this case the problems of the elasticity of the casing are much easier to solve than in the case of the plate-type constructions. The cell consists of a casing 40 which includes a force input plate 41 with a mounting bore 42 and passes over into an elongated, longitudinally resilient tube 39 having a welding flange 43. The piezoelement 44, again consisting of a multiple player piezodisc arrangement, is arranged directly behind the force input plate 41. A resilient ring 45 is provided for centering the piezoelement 44, which ring is cut away segmentally at the point 46 in order that the electrodes 47 can be connected. The electrode connections are led out of the measuring cell in a multicore insulated cable 48 which extends through a sealing plug 49.
If it is desired, the piezoelement 44 may be followed by an additional mass 51 and by piezocompensating element 52 to provide for acceleration compensation. In many applications, however, this is not necessary so that the piezoelement may be fitted for such applications directly onto the tubular member 53. During the assembly, the casing 40 is held at the flange part 43 and placed under high mechanical prestress. Thereupon, the flange part 43 is joined to the tubular member 53 by means of spot welding points 55. To obtain a complete sealing, the liplike ends of the flanges 43 and 54 are joined together at 56 by welding.
The multiple-component piezomeasuring cell can be fitted into suitable mounting flanges 57 for incorporation in measurement systems. For investigations and tests on shock wave passages, any desired model 58 can be mounted on the force input plate 41. In the case of a built-in acceleration compensation, it is advantageous to make the mass 51 of a heavy metal, for example, of tungsten or of a lead alloy. The compensation plate 52 is polai'ized'oppositely to the plate 50. As a result thereof, a signal produced by the acceleration in the Z axis of the pressure sensitive plate arrangement is compensated for by the oppositely polarized plate 52. The magnitude of the compensation can be adjusted to suit the requirements by the number of the piezoplates 50 and 52 and by the size of the' 7 mass 51. It is assured by the construction of the casing 40 as a resilient tube that all forces acting on the force input plate 41 can be transmitted to the piezoelement without hindrance. The very compact arrangement of the piezoelement 44 in the casing 40 permits the accommodation of the shear plates, sensitive in the X- and Y-axes, exactly in the center of gravity 59 of the test model 58 whereby considerable simplifications in the evaluation of the measurement results are realized for certain types of measurements.
A further arrangement of a multiple-component piezomeasuring cell according to the present-invention is shown in FIG. 5. Also in this case a rod-type construction is involved which is particularly useful for aerodynamic tests and experiments. The casing 60 consists again of a force input plate 61 with mounting bolts 62 and of a tubular transfer member 63 having a flange portion 64. A torque-measuring device precedes the multiple-component force-measuring element 65. The torquemeasuring device consists of the carrier flange 66 and of the carrier rod 67 which is firmly and securely pressed into this carrier flange 66. The support discs 68 and 69, constructed as electrodes, are supported on the carrier rod 67 with a spacing a. These electrodes 68 and 69 are each electrically insulated from the carrier rod 67 by means of an insulating ring 70 consisting of conventional high-strength highly insulating material. On both sides of the support discs 68 and 69 are disposed shear-sensitive piezodiscs 71 and 7 2. The two illustrated piezosystems 71 and 72 can be so arranged and connected that their signals are of the same polarity when subjected to the action of a moment at the torque-engagement point 73 and are thus automatically added in se ries. It is, however, also possible to lead-out the signals from the electrodes 68 and 69 individually from the measurement cell for evaluation in the connected electronic circuitry" i I In the arrangement shown in FIG. 5, the torque or moment is measured in one plane. It is, however, also possible without any difficulty to measure moments in two or more planes. As an example for two planes, the support discs 68 and 69 may each consists of two discs placed one against the other by means of an intermediate insulating layer. Then each of the two associated piezodiscs 71, 72 would be adjusted for a desired axial direction. The piezodiscs 71 and 72 as also the support discs 68 and 69 are forced by correspondingly machined intermediate members 74, 75 and 76 against the carrier flange 66. This flange 66 itself is pressed against the main flange 77 by way of the multiple-component piezoelement 65 whereby the axial stress is applied according to known principles by means of a threaded bush 78.
It is assured by the construction of the body 60 as a resilient prestressed tube that the thin casing walls exert only a very small influence on the transmission of moments and forces. Owing to the large cross sections of the material of the piezodiscs, which are thus available, a rigid measuring system in each direction of measurement with a correspondingly high natural frequency is obtained. The measuring cell is mounted by means of an assembly thread 79 in any desired holder or mount. As in FIG. 4, the test model 80 is mounted on the measurement cell. Also, the measurement cell according to FIG. 5 can, if necessary, be provided with an acceleration compensation in the axial direction A-A.
A multiple-component piezomeasuring cell for measuring accelerations is illustrated in FIG. 6. The acceleration forces derived from the force action on the seismic mass can be resolved in the illustrated example into three components X, Y and 2. For this purpose, the described multiple layer piezoarrangement 91 is mounted on the base plate 92 of the casing 93. A piezodevice 94 is mounted above the seismic mass 90 which in the illustrated case, measures the X and Y components. In the element 91, the arrangement of the discs for the X and Y components is such that they lie nearest the mass 90. The piezoelements and the mass are subjected to a high mechanical prestress by means of the cap 96 and the transmission member so that lateral acceleration forces can be transmitted to the mass 90 merely by friction and by way of the piezoelements 91 and 94. The piezoelements 91 and 94 are again centered by elastic segmental rings 98 and 99, respectively. The electrode outputs of the two X and Y elements are connected in series and are connected to the output pins 100. In place of the two-component piezoelement 94, also a second three-component element can be used like the piezoelement 91 but located above the mass.
. however, only one multiple layer piezoarrangement is used to simplify the construction. The piezocell .110 is forced against the base plate 111 by the mass'1l'2 by means of the sleevelike prestressing spring 113. The prestressing spring or sleeve 113 is welded to the base plate 111 at the points 115 under mechanical prestress. A segmental interrupted ring 116 is.
again used for centering the piezoc'ell 110. In order that the mass can make fewer vibratory excursions, it is axially connected to the diaphragm 118 ing 117. The base plate 111 is fixedly, threadably con nectedto the casing 117 while the connector pins 114 are likewise fixedly mounted in the casing 117. The arrangement of the diaphragm 118 adversely affects or impairs the movability of the mass 112 in the X and Y axes so-that the sensitivity of the measuring cell is somewhat reduced in these two axes; It is, however, possible without any difficulty to omit this diaphragm, especially if relativelysmall lateral accelerations have to be measured.
in place of the arrangements of FIGS. 6 and 7, where the multiple layer piezocells'are fitted into special casings, accelerations can also be measured directly with multiple component measuring-cells according'to FIG. 1.-Such an arrangement is shown in FIG. 8 where .a mass 123 is mounted by means of a screw or bolt 121 on a multiple component piezomeasuring cell 122.
in H6. 9, a standard multiple'component measuring cell 130 is mounted by means of a centering pin 131 in a casing 132. The mass 133 is forced into the casing 132 under prestressing by means of the screw cover 134 and the central pressure point 135. In this'manner, complex acceleration problems can be solved with standard elements.
Accordingly, it is possible "by. means of the invention described herein to construct multiple component piezomeasuring cells which are built-up inla constructively simple manner and which are suitable both for the measurement of forces, moments, as well as accelerations in several components. The rigid construction in conjunction with the highly rigid constants of the materialsof the crystals provides comwhich, in turn,is connected by I means of a screw flange at the annular surface 119 to the caspact, universally applicable measuring cells with a high natural frequency.
While we have shown anddescribed several embodiments in accordance with the present invention, it is obvious that the same is not limited thereto but is susceptible of numerous changes and modifications as known to persons skilled in the art. Accordingly, we do not wish to be limited to the details shown and described herein but intend to cover all such changes and modifications thereof as are within thescope of those skilled in the art.
l. A multiple component piezomeasuring cell for measuring forces and accelerations which includes an arrangement of superimposed piezodiscs, at least one of said discs being fonned and positioned to be sensitive to compressional forces and atleast two of the other discs being formed and positioned to be sensitive to shear forces, characterized in that the measuring cell forms an integral, self-contained element in which forcetransrnitting disc means are connected to a casing of the cell by way of transversely and longitudinally resilient collar portions, and in which the piezoelement of multiple layer construction is mounted under axial prestress.
2. A multiple component piezbmeasuring cell according to claim 1, each consisting of three superimposed crystal arrangements, of which one is sensitive to pressure and the others are sensitive to shear, characterized in that the axes of the primary sensitivities of the individual arrangements are arranged in the X, Y andZ directions of a rectangular coordinate system, one of said directions being perpendicular to the discs, a continuous central assembly bore being provided which is formed by a tube resilient in various directions, and said tube being connected in a sealing manner with the forcetransmittin disc means.
3, A mul ple component piezomeasuring cell for measuring forces and accelerations which includes an arrangement of supcrimposed piezodiscs, with at least one of said discs being formed and positioned to be sensitive to compressional forces and at least two of the other discs being formed and positioned to be sensitive to shear forces, characterized in that the measuring cell comprises a cup-shaped body which extends with a tubular portion thereof over the piezoarrangement and is connected to a flange of a force transmitting member under high prestress, and in which the tubular portion of the body has sufficient elasticity so that practically all the applied forces are exerted on the piezoarrangement.
4. A multiple component piezomeasuring cell according to claim 3, characterized in that a seismic mass is provided in operative association with the multiple component element which acts on a compensating piezoelement, said compensating piezoelement being connected in opposition to the output of the Z axis of the multiple component element, said Z axis being parallel to the direction in which said discs are superimposed, whereby acceleration signals produced by the mass arranged in front of the multiple component elements can be compensated. I
5. A multiple component piezomeasuring cell according to claim 3, characterized in that the multiple component forcemeasuring element is preceded by a torque-measuring element including a carrier with a flange and two electrodes formed as support rings which are clamped between two shear sensitive piezodiscs under high prestress so that the support rings convert an externally applied torque into a pair of forces at spaced points.
6. A multiple component piezorneasuring cell for measuring acceleration forces according to claim 1, characterized in that a seismic mass is arranged in a casing under high mechanical prestress between 'two sets of multiple layer piezoelements whose corresponding sensitive axes are connected in series to corresponding output contacts.
7. A multiple component piezomeasuring cell for measuring acceleration forces according to claim 1, characterized in that a seismic mass is clamped by a prestressing sleeve and by way of a multiple layer piezoelement against a base plate which is assembled in a casing and provided with corresponding outputs, and diaphragm means for'avoiding idle swinging of the mass which is connected to the center portion of the mass and is clamped at the periphery to the casing.
8. A multiple component piezomeasuring cell for measuring acceleration forces according to claim 1, characterized in that a standard, self-contained measurement cell, which includes an arrangement of superimposed piezodiscs, and a mass are clamped to the object to be measured by means of a prestressing screw.
9. A multiple component piezomcasuring cell for measuring acceleration forces, which includes an arrangement of superimposed piezodiscs, at least one of said discs being formed and positioned to be sensitive to compressional forces, and at least two of the other discs beingformed and positioned to be sensitive to shear forces, characterized in that a seismic mass is fitted into a casing under high prestress between a forcetransmitting member and a standard measuring cell.