Multicomponent Force Transducer

Abstract

A piezotransducer device in which several piezoplates are located between force-transmitting members and are oriented with the force-sensitive axes depending on the type of force to be measured thereby.

Description

The present invention relates to piezotransducer units, and more particularly to a piezotransducer unit comprising a piezoelement mounted between two members adapted to transmit a force applied thereto to the piezoelement.

In the field of measurements, problems must often be solved in which more than one component of a force must be measured. Such forces may be compression, tension or shearing forces as well as moments. Known measuring arrangements for such multiple component measuring systems are generally constructed in accordance with the strain gauge strip method. Accordingly, the measuring device receiving the various forces must be mechanically worked in such a manner that individual components can be determined separately, thus necessitating a very complicated configuration. The reason for this is in particular that the various components must be prevented from affecting each other. Owing to the necessity that the measuring device must be divided mechanically into various stress components unintentionally a constructional form results which is difficult to produce and in particular a resilient structure which generally has totally different degrees of resilience in the various directions of the components and thereby a low and irregular natural frequency character. Moreover, the production of such multiple component measuring devices based on strain gauge strips is very expensive and not universally applicable. Admittedly, further force measuring methods are in existence, e.g. operating on an inductive or capacitative basis; these, however, are substantially never used for multicomponent force measurement.

The piezomeasuring technique provides better measuring conditions. Owing to the fact that piezocrystals can be used which are produced in various cut directions and which are suitable for the measurement of compression forces as well as shearing forces, simple stable constructions are obtained. Owing to the fact that in the piezomeasuring technique forces can be measured directly without the intervention of an elongation or other stress, this system is particularly well suited for the measurement of forces because piezocrystal cross sections can be used, whereby simultaneously a very high sensitivity, yet very great rigidity can be obtained. The ratio of rigidity to sensitivity obtainable thereby cannot be attained even approximately with any other system.

According to the invention, piezotransducer units are proposed comprising substantially two force-transmitting plates between which a plurality of piezoelectric plates are located which are mutually interchangeable and which can be assembled to form a transducer unit responsive to compression, shear or torque, dependent upon the orientation of the force sensitive axes. According to the invention, thus three basic elements are available in a simple manner, which differ from each other only by the direction of sensitivity of the respective crystal plates and which can be combined by mechanical series connection in such manner that any desirable multicomponent measuring value converter can be produced.

Several embodiments of the invention will be described below by way of example with reference to the accompanying drawings, in which:

FIG. 1 illustrates a cross section through a perforated disclike piezotransducer unit,

FIG. 2 is a section along line A--A in FIG. 1,

FIG. 3 illustrates a cross section of a different constructional arrangement of a disclike piezotransducer unit,

FIG. 4 is a section through the same piezotransducer unit along line B--B in FIG. 3,

FIG. 5 illustrates a piezocrystal from which two piezodiscs have been produced in different cut directions,

FIG. 6 illustrates a section through a piezodisc suitable for measurement of a compression force,

FIG. 7 illustrates a section through a piezodisc suitable for measurement of a shearing force,

FIG. 8 illustrates an embodiment suitable for the measurements of two force components, such as compression Z and moment M,

FIG. 9 illustrates an embodiment suitable for the measurement of two force components, such as shearing force X, and compression force Z,

FIG. 10 illustrates an embodiment suitable for the measurement of three force components and a moment,

FIG. 11 is a cross section through an embodiment of a piezotransducer unit suitable for the measurement of a moment, and

FIG. 12 is a cross section through an embodiment of a piezotransducer unit suitable for the measurement of a shearing force.

FIG. 1 illustrates a cross section through an embodiment of a piezomeasuring cell or transducer unit which is adapted for the measurement of rotary moments. The unit consists of annular force-transmitting plates 1 and 2 which are connected to each other by a thin tubular inner jacket 3 and a tubular outer jacket 4, exerting a mechanical bias or stress on the plates. The connection is effected preferably by annular welds 5, 6 and so on. Piezodiscs or crystals 7 are disposed in direct contact with the transmitting plate 1 by which positive charges produced under the effect of a rotary moment are transmitted directly to a casing 24 shown in FIG. 2 and thence to a threaded potion 8 of a connecting terminal. A ringlike electrode 9 is located on the other side of the crystals 7 and receives the corresponding negative charges which are transmitted to a central contact member 10 in the connecting terminal. A disclike plate 11 consisting of a highly insulating material, e.g. aluminum oxide, lies between the electrode 9 and the force-transmitting plate 2. However, other highly insulating and extremely rigid insulating materials are also known and may be used instead of aluminum oxide. The piezocrystals, the electrode and the insulating disc are centered by an inner insulating ring 12 and an outer insulating ring 13 in such a manner that contact with the walls at the inner and outer peripheries is avoided.

The whole crystal unit can be easily conveyed from one assembly station to another during the manufacturing process owing to the presence of these centering rings.

FIG. 2 illustrates the piezotransducer unit in section along the line A--A in FIG. 1. The outer tubelike wall 4 which is as thin as possible as well as an inner wall 3, which as also as thin as possible, and the outer and inner isolation and centering rings 13 and 12 are shown in section. The push or shear sensitive axes of the piezodiscs 7 are indicated by respective arrows.

These axes are placed during the assembly in exactly tangential directions to a circle of mean diameter D. In this manner, each piezodisc 7 is subjected to and stressed by a shearing force when a rotary moment is applied to the force-transmitting plates 1 and 2 whereby the discs deliver a corresponding charge to the electrode 9 and thus to the central contact member 10 of the connecting terminal.

FIG. 3 illustrates a simpler construction of a piezotransducer unit or measuring element in which the space between two force-transmitting plates 31 and 32 and the inner parts is filled with an epoxy resin 33. The transmitting plates 31 and 32 consist of an insulating material, e.g. aluminum oxide. A ring plate 34 of metal has a lug 35 to which a metal screen 36 of a connecting cable 37 is attached such as by soldering. An inner conductor 38 of the cable is connected to a disclike electrode 39. Piezodiscs 40 are located between the ring plate 34 and the electrode 39.

FIG. 4 illustrates the piezomeasuring element in section along the line B--B in FIG. 3. The push or shear sensitive axes of the individual piezocrystal disc 40 are indicated again by arrows. During assembly the measuring elements are deposited in such a manner that all the axes are placed exactly parallel to an X-axis whereupon the disclike electrode 39 and the transmitting plate 31 and deposited on the assembled discs 40. Thereafter, the whole measuring element is subjected to a vacuum while it is located in a special mold, and is impregnated with a highly insulating epoxy resin. The connector portion 49 is thereafter embedded in a silicone rubber 50. In this manner, a piezomeasuring element can be produced with simple means which is sensitive to push or shear along the axis X. Owing to physical properties of the piezocrystals forces along the Z and Y axes have no signal-producing effect.

It is obvious that the two constructional forms described with reference to FIGS. 1, 2 and FIGS. 3, 4 can be assembled at choice for the measurement of moments or shearing forces. Also a second series of similarly directed piezodiscs, such as discs 7, can be substituted without difficulty for the insulating ring 11. In this manner, twice the signal can be taken off electrode 9 between the two series of crystal discs. This arrangement is used very much and avoids the need for insulating discs. The two piezotransducer units according to the invention differ substantially only by the orientation of the axes of the individual crystal discs, as may be seen clearly also from FIGS. 2 and 4.

FIG. 5 illustrates, by way of example, a natural quartz crystal 51 in which the known axes X, Y and Z are shown. For producing a disc which is sensitive to pressure P, such as shown in FIG. 6 a piezodisc 52 must be cut from the crystal in the plane Y, Z. The force must be applied parallel to the axis X and the electrical charges are produced on the upper and lower disc surfaces.

In order to produce a piezodisc which is responsive to a push or shearing force, such as shown in FIG. 7 a disc 53 must be cut from the crystal 51 in the plane X, Z. The disc 53 (FIG. 5) is then sensitive to push or shearing forces P in the direction of the axis X, as shown in FIG. 7. COrresponding charges are delivered at the circular upper and lower limiting surfaces.

The discs 52 and 53 are insensitive for forces in the Y and Z directions. Similar force orientations can be obtained also with other crystals, and it is also possible to obtain such effects also with piezoceramic discs. Furthermore, semiconductor crystals with similar sensitivity can be produced which have piezoresistive properties.

FIG. 8 illustrates the use of two piezomeasuring elements in an arrangement wherein machining experiments are to be carried out on a test piece 81 be means of a drilling or milling tool 82. In this case, the rotary moment M and the feed force in the Z direction are to be measured. For M pf an arrangement a piezotransducer unit 83, including transmitting plates 83a and 83b for measuring torque according to FIGS. 1 and 2 is assembled together with a compression force-measuring transducer unit 84 of generally similar construction and shown in block form, the two transducer units being clamped between the test piece 81 and a support 86 by means of a stressing screw 85. An output signal caused by the prestressing can be reduced to zero by any known means. In contrast the feed force of the drill in the Z direction, as well as the reaction moment M of the drill 82 can be registered completely independently of each other with extremely high resolution. Owing to the large cross section of the crystals, the whole measuring system becomes extremely rigid, whereby also force fluctuations and moment variations of very high frequency can be measured.

FIG. 9 illustrates a further example for use in an arrangement wherein milling or grinding tests are to be carried out on a testpiece 91, and wherein forces in the Z and Y directions are to be measured. For this purpose a piezotransducer unit 94 for measuring pressure and a piezotransducer unit 93 for measuring a shearing force according to FIGS. 3 and 4 is used in a similar manner, and the units are clamped again between the testpiece 91 and a support 96 by means of a clamping or stressing screw. The force-transmitting plates 93a and 93b of unit 93 are shown for clarity.

FIG. 10 illustrates, by way of example, a test arrangement wherein particles 102 impinging upon a testpiece 101 produce corresponding reaction forces; the components of these forces are to be measured in all three directions X, Y and Z, and additionally also the moment M is to be determined. By mechanically connecting in series individual piezomeasuring elements 104, whose transmitting plates 104a and 104b are shown for measuring force along the Z axis, 105 for measuring shearing force along the Y axis, 106 for measuring shearing force along the X axis and 107 for measuring moment, the complicated measuring problem becomes a very simple matter. In this case the whole measuring arrangement becomes a rigid high frequency device owing to the prestress bias provided by a screw 108.

FIG. 11 illustrates in cross section a further embodiment of a piezomeasuring cell or transducer unit for the measurement of rotary moments or torque. A base plate 110 provided with two tubelike wall members 112 and 113 has a U-shaped cross section. A force-transmitting plate 114 is connected to two thin tubular ring members 115 and 116 which in turn are connected by rings welds 117 and 118 to the wall members 112 and 113 constituting the limbs of the U-shaped cross section of the baseplate 110. This fold construction is effected in such manner that the transmitting plate 114 is pressed with bias against a measuring arrangement and the base plate 110. The measuring arrangement comprises an electrode plate 120, an insulating plate 121, and a plate 119 with piezodiscs which may be embedded in a layer of, e.g. an epoxy resin.

The electrode plate 120 is connected to a connector 123 by means of a plug contact 122. In place of the insulating plate 121, alternatively a plate with piezodiscs may be used. In this case the plate may be provided with piezodiscs the sensitivity axes of which have different directions form the axes of the piezodiscs of the plate 119. The crystal arrangement is centered and detained by two insulating rings 124 and 125. However, the crystal discs may be cast in an epoxy resin disc as illustrated in FIGS. 3 and 4. Owing to the resilient connection between the transmitting plate 114 and the baseplate 110, the sensitivity of the measuring arrangement for rotary moment is altered only very little, because the stiffness thereof is considerably higher.

A further possible embodiment of a peizomeasuring cell which can be used for measuring rotary moments as well as shearing forces and pressure forces is illustrated in FIG. 12. In contrast to FIG. 11 a transmitting plate 131 is connected for resilient yield along two axes X and Y to a U-shaped base plate 130. In this case, the ends of the thin walls constituting the limbs of the U-shaped cross section of the base plate 130 are provided with annular enlargements 132 and 133 to which thin walled rings 135 and 136 attached to the transmitting plate 131 are welded under stress in the X direction. Thereby, annular gaps 134 and 137 are produced which afford resilience to the transmitting plate 131 in the Y direction. The measuring arrangement is substantially the same as described with reference to FIG. 11.

Simple embodiments, however, may be obtained also when the individual piezotransducer units or measuring elements are constructed without central opening. The problem of prestressing or bias must then be solved by an externally applied force, e.g. a sleeve. Obviously, such embodiments and applications fall also within the scope of the invention.

Even further combinations can be devised for use of the individual measuring cells and are also part of the invention. Thus two or more individual cells or units may be disposed in a common housing and separate connections to the individual component arrangements may be provided. In the first place, quartz is a suitable material for a piezocrystal for the intended use. The invention, however, can be performed without difficulties with any other piezoelectrical materials. In place of individual piezocrystals, piezoresistive crystal discs, akin to semiconductors may also be mounted in the elements or units according to the invention. However, multiple core connection conditions arise then.

It will be clear that the invention permits any multiple force component measurements and moment measurements in test articles to be effected in a simple manner in that a rigid measuring structure can be obtained by a combination of individual piezomeasuring elements and by mechanically stressing them. The construction of the individual piezomeasuring elements or transducer units may be effected in accordance with uniform principles depending upon whether the element is to be used for the measurement of pressure forces, shearing forces or moments, in that the directions of the sensitivity axes of the piezocrystals are suitably aligned during assembly. Furthermore, individual crystals with a different shape, e.g. rectangular or trapezoidal shape, can be used in place of the circular disclike crystals. From the point of view of production, however, a circular disc in considerably simpler and cheaper. Also the manner of construction of the piezomeasuring element, whether it is constructed according to the principle in accordance with FIGS. 1, 3, 11 or FIG. 12, has no effect on the idea of the invention. It is also within the scope of the invention that only one of the two piezomeasuring cells illustrated in any of FIGS. 8 to 10 and having crystal arrangements according to FIG. 2 or FIG. 4 may be assembled in one measuring device. The piezocells can thus be utilized individually or in any combination in accordance with the requirements of the measurement to be effected.

Claims

We claim

1. A piezomeasuring device with a force-receiving body for measuring a plurality of forces impinging thereon having a plurality of separate transducer units mounted together, each transducer unit comprising:

a pair of force-transmitting members disposed about a common axis; and

a plurality of piezocrystals disposed between the force-transmitting members of said pair and being sensitive to forces in a single direction with respect to said axis;

wherein the crystals of each separate transducer unit are sensitive to forces in a direction different from the direction of sensitivity of the crystals of the other transducer units, whereby said measuring device provides a compact arrangement sensitive to forces in a plurality of directions.

2. A piezomeasuring device according to claim 1, wherein said pair of force-transmitting members of each transducer unit are annular plates, between which said crystals are arranged, said crystals being disc-shaped and disposed in a conductive casing contacting a connecting terminal located on the periphery of said unit, whereby charges produced under the effect of forces acting on said crystals may be conducted to said terminal to provide an electrical indication of said forces.

3. A piezomeasuring device according to claim 2, further including an outer cylindrical jacket surrounding the outer portion of said plates and an inner cylindrical jacket disposed adjacent the inner surface of said plates.

4. A piezomeasuring device according to claim 3, further including a pair of insulating rings disposed between said casing and said outer and inner jackets, respectively.

5. A piezomeasuring device according to claim 4, wherein said piezodiscs are held between said pair of force-transmitting plates by a ring-shaped electrode and a ring-shaped insulator disposed between said plates.

6. A piezomeasuring device according to claim 5, wherein one unit of said plurality of transducer units is sensitive only to axial compressional forces, while another unit is sensitive only to shear forces in a first direction.

7. A piezomeasuring device according to claim 6, wherein an additional transducer unit in said plurality of transducer units is sensitive only to rotary moments.

8. A piezomeasuring device according to claim 7, wherein another transducer unit in said plurality of transducer units is sensitive only to shear forces in a second direction different from said first direction.

9. A piezomeasuring device according to claim 8, wherein said force receiving body is a ring of U-shaped cross section in which each transducer unit is assembled with axial prestress, so that said force-transmitting members are resilient relative to said force receiving body.

10. A piezomeasuring device according to claim 1, wherein the individual piezocrystals are of approximately circular disclike form and have substantially the same dimensions independently of the crystal cut, and are inserted into retaining ring means operable to hold the sensitivity axes of the individual crystals in a fixed position until assembly.

11. A piezomeasuring device according to claim 10, wherein the force-transmitting members are constructed as discs with a central opening and are connected to each other under stress by thin-walled tubular members.

12. A piezomeasuring device according to claim 1, wherein said force-transmitting members are constructed as discs with a central opening and are connected to each other under stress by thin-walled tubular members.

13. A piezomeasuring device according to claim 1, wherein the force receiving body is a ring of U-shaped cross section in which the measuring units are assembled with axial prestress in such manner that a force-transmitting member is resilient relative to the force receiving body.