GB2050624A

GB2050624A - Strain transducers

Info

Publication number: GB2050624A
Application number: GB8015676A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-05-11
Filing date: 1980-05-12
Publication date: 1981-01-07
Also published as: GB2050624B

Abstract

A strain transducer comprises a cylindrical plug-like body (12) provided with a transverse diaphragm wall (16) on one face (18) of which is secured one or more strain gauges. The body is inserted with an interference fit in an opening (H) in a surface of the structure to be monitored so that the face (18) is at or adjacent said surface and is effectively continuous therewith. The wall thereby transmits forces that are experienced by said adjoining surface of the structure so that the strain gauges respond to the strains in the structure from these forces. The plug- like body can be formed as a projecting member dependent from a casing (2, 4, 8) in which electrical circuitry of the transducer is encapsulated, and the bottom face (2a) of the casing can form a peripheral extension of the plug-like body to be used to locate the diaphragm wall relative to the structure surface. <IMAGE>

Description

SPECIFICATION Strain transducers This invention relates to strain transducers for the measurement of strain in a structure by the use of at least one electrical strain gauge.

It is generally necessary for practical purposes, particularly if there is used a group of two or more strain gauges so disposed relative to each other as to avoid various sources of error in measurement, for the transducer to comprise a carrier on which the gauges are secured and which is in turn attached to the structure being monitored so as to be influenced by strain of the structure. The carrier may, for example, be a form of bridge that engages spaced points on the surface of the structure so that it is deformed by relative movement between those points. The strain gauges are not in direct contact with the structure therefore, and it can be difficult in many instances to ensure that their deformations reflect accurately the strains to be measured in the structure.

The problem is further complicated by the fact that the method of attachment to the structure should be one that needs little time or effort, and that can be carried out without much skill, if the transducer is to be employed on any large scale.

Moreover, strain transducers are often required to operate reliably for very long periods in adverse environmental conditions so that they must be protected from these conditions and must also be attached to the structure in such a way that they will remain securely fixed in place for an indefinite period.

According to one aspect of the present invention, there is provided a strain transducer comprising a carrier member in the form of a generally cylindrical plug-like member arranged to be inserted into an opening in the surface of a structure to be monitored with a peripheral outer face in closely fitting engagement with the periphery of said opening, said plug-like member comprising a face transverse to said peripheral outer face and on which is secured at least one strain gauge, said transverse face of the inserted member being disposed substantially parallel or co-planar with the direction of strain to be measured.

With such an arrangement, it is possible to fix the transducer in place very simply by boring in the structure a hole of the appropriate diameter to provide the required fit for the plug-like carrier member. The strains in the material of the structure around the hole are then transmitted directly to the plug-like carrier member as long as peripheral contact is maintained.

In general, there may be a minimum axial length of engagement necessary to facilitate the fitting of the plug-like member, and as the length of the member increases it may be more difficult to ensure that the strains of the structure are accurately reflected at the transverse face which carries the strain gauge or gauges. This problem can be avoided, however, by reducing the axial length of the member in a region carrying the gauge or gauges. The member can then comprise a diaphragm-like central transverse wall region that will respond in a substantially uniform way to compressive forces in its own plane, and on one face of which the strain gauges are mounted.

According to another aspect of the invention, there is therefore provided a strain transducer comprising a carrier member in the form of a cylindrical plug member that includes a tubular outer portion and inner transverse wall extending across the interior of said tubular outer portion substantially normal to the axis thereof and having at least one strain gauge mounted upon it, the thickness of said transverse wall being substantially less than the axial length of the tubular outer portion, the arrangement being such that the carrier member can be inserted into an opening in the surface of a structure to be monitored with the tubular outer portion having an interference fit with the peripheral wall of said opening at least in the region adjacent the transverse wall, and said transverse wall being disposed substantially parallel or co-planar with said surface of the structure.

It is possible to produce a strain transducer according to the invention in a relatively compact form that needs only a shallow hole to receive it, said hole providing the necessary fit for the pluglike carrier member, e.g. by means of a force drive or by a screw-thread engagement. The fit obtained should be sufficiently tight to ensure intimate contact between the outer periphery of the carrier member and the periphery of the hole, at least in the axial region containing the strain gauge or gauges. As already mentioned, it can help to ensure that strains of the surrounding region of the structure are accurately reflected by the strain gauges if they are mounted on a relatively thin transverse wall or disc-like portion that forms an inner region of the plug-like member.

It can also be arranged that the gauge or gauges and associated electrical circuitry of the transducer are suitably enclosed, and possibly encapsulated, to protect them from dirt or damage.

The invention will be described in more detail by way of example with reference to the accompanying diagrammatic drawings, wherein: Figs. 1 and 2 are axial sections of alternative forms of strain transducer according to the invention, Fig. 3 is a schematic section on the plane A-A in Figs. 1 and 2 showing one possible array of strain gauges, and Fig. 4 is a circuit diagram showing the connection of the strain gauges in the transducers of the preceding figures.

Referring more particularly to Fig. 1 of the drawings, the strain transducer has a casing that comprises a base carrier member 2, a cylindrical intermediate wall 4 secured by a screwthread connection 6 to the carrier member and a top cover 8 secured in turn by a screwthread connection 10 to the cylindrical wall 4.

Depending from planar bottom face 2a of the carrier member 2 is a tubular cylindrical extension 12 formed integrally therewith. In the top face of the carrier member there is a cylindrical recess 14 concentric with the extension 1 2 and having a similar diameter to the inner periphery of the extension 12, so that a disc-like transvarse wall 1 6 is formed closing the inner end of the tubular extension. The top face 1 8 of the wall is co-planar with the bottom face 2a of the main outer annular portion 20 cf the carrier member. The thickness of the transverse wall 1 6 is several times less than the axial depth of the cylindrical extension 12.The thickness of the wall 1 6 is similarly much less than the thickness of the surrounding annular portion 20 of the carrier member and its connection with the portion 20 is weakened by a peripheral undercut 24 that forms a continuation of the outer surface of the tubular extension to above the level of the bottom face 2a of the carrier member.

An array of 4 electrical strain gauges 30 is mounted on the top face 1 8 of the transverse wall 16. A platform 32 fixed on top of main annular portion 20 of the carrier member 2 carries further electrical components of a circuit in which the strain gauges are connected, and a sealing gland 34 is provided in the intermediate casing wall 4 for a cable 36 that supplies power to the circuit within the casing and also carries the measurement signal produced by the strain gauge array. The casing interior may be completely sealed from the exterior, with the electrical circuit comprising the strain gauges encapsulated for protection against damage.

As shown more clearly in Fig. 3, the strain gauges A, B, C, D are arranged in a 90 star pattern for the measurement of strain, in a direction parallel to one pair of the gauges B, D.

The circuit diagram in Fig. 4 shows the strain gauges connected in a measurement bridge with the gauges B, D, in opposite arms. An external power supply is provided through lines 40, 42 of the cable to the gauges and also to an amplifier 44 forming part of the circuit on the platform 32 within the casing. Strains sensed by the gauges causing imbalance of the bridge thus provides an input to the amplifier, in known manner, and results in a measurement signal being transmitted through output line 48.

To use the transducer, the structure to be monitored is prepared very simply by drilling and reaming a small hole H (Fig. 1), typically some 10 mm diameter and 10 mm deep. The position of the hole is of course chosen to avoid setting up any unduly large stress concentrations in the structure, which will normally present no difficulty as it will not be necessary for the transducer to be placed at the most highly stressed part of the structure. For example, the transducer may be placed in a column of a power press to monitor the loads at the press plates. The arrangement is particularly suited for the measurement of compressive strains, which would therefore help to minimise the effects of any stress concentrations in the structure resulting from its fitting.

The hole diameter is slightly less than the diameter of the tubular extension 12, to give a force fit therewith. The complete casing can therefore be placed over the hole and the tubular extension inserted plug-like by impact or by pressure, into the hole until the bottom face of the casing seats on the surface of the structure. The strain gauge array is aligned with one pair of gauges parallel to the expected direction of strain, as already mentioned, this giving maximum sensitivity from the bridge array. The transducer is now securely and permanently fixed in place, the extension effectively becoming a part of the structure, and it has been automatically ensured that the strain gauge array is located coplanar ir;cith the surface of the structure.

By so locating the array, the gauges are in a position at which the elastic forces in the structure are relatively large, and a correspondingly large response is produced to the strains sensed. It will be clear nevertheless that it can be equally easily ensured, with suitable modification, that the array is located at another desired level. Because of the force-fit engagement, stresses occurring in the surface are transmitted directly through the pluglike extension, and not only compression strains but also tension strains will be transmitted as long as the deformation is not so great as to remove the interference fit of tho extension in the hole.

Because of the relatively thin form of the transverse wall 16, this acts in the manner of a diaphragm and does not influence the much greater surrounding mass of the material of the structure itself. Nor does the tubular portion 12 have any material influence because, of course, it is relatively compliant to forces acting parallel to the surface of the structure.

It is to be noted that when the transducer is put in place, the compression forces to which it is subjected by its interference fit are symmetrical if the hole and the plug-like extension are both circular in cross-section. With the structure otherwise unstressed, therefore, the gauges are all subject to equal strains and there is no need to provide for zero point calibration before use.

It will be apparent from the foregoing description that the transducer can be an extremely robust and compact construction, produced as a factory-sealed unit, and be able to be put in place very easily and simply, without any special skill on a wide variety of structures. Once mounted the transducer is held in a way that permanently ensures the security of the gauges in all conditions.

The modified transducer illustrated in Fig. 2 is similar in many respects to that already described, having in common with the first embodiment the gauge array shown in Fig. 3 and the circuit shown in Fig. 4, and the same reference numbers indicate corresponding parts. The base carrier member 2 now comprises modified form of tubular extension 56, comprising a screwthreaded portion 58 and a frusto-conical root portion 60 between the threaded portion and the main bottom face 2a of the carrier member.

This transducer is fixed in position in a hole H' that has been tapped at 64 to receive the screwthreaded portion 58 and that has a chamfer 66 at its outer edge with the same taper as the root portion 60. As the tubular extension is screwed into the hole, the root portion 60 is drawn into engagement with the chamfer 66 and the thread is tightened sufficiently to force them together such that a substantial radial compression is applied to the root portion analogously to the force fitting assembly of the first example. It is also possible in this case to use the bottom face 2a as a location plane for setting the position of the strain gauges relative to the adjacent surface of the structure.

To facilitate the mounting of the transducer, the casing can itself be formed with engagement means for a tool such as a spanner. Fig. 2 shows a pair of blind bores 68 in the top cover 70 of the casing for a C-spanner, and that can also be used to secure together the parts of the casing, all the screwthreads having the same hand. The manner of setting adjustment and operation of the mounted transducer will be the same as the first described example.

It will be understood that the described embodiments are capable of many modifications within the scope of the invention. The size of the transducer may be considerably reduced if the electrical circuitry is suitably miniaturised and, indeed, the transverse dimensions of the device need be no greater than that of the tubular portion engaging the hole in the structure. If needed, it would then be possible to arrange the transducer such that it is inserted flush with the surface of the structure in which it is mounted.

Claims

1. A strain transducer comprising a carrier member in the form of a generally cylindrical pluglike member arranged to be inserted into an opening in the surface of a structure to be monitored with a peripheral outer face in closely fitting engagement with the periphery of said opening, said plug-like member comprising a face transverse to said peripheral outer face and on which is secured at least one strain gauge, said transverse face of the inserted member being disposed substantially parallel or co-planar with the direction of strain to be measured.

2. A strain transducer according to claim 1 wherein the depth of said peripheral outer face is substantially greater than the thickness of a wall portion of the member forming said transverse face.

3. A strain transducer according to claim 2 wherein the transverse face is formed by a diaphragm-like wall portion integral with a tubular portion forming said peripheral outer face of the plug-like member.

4. A strain transducer according to any one of claims 1 to 3 wherein a transversely projecting element extends away from the peripheral outer face of the plug-like member and is arranged to provide a location for the plug-like member relative to said structure surface to determine said disposition of the transverse face.

5. A strain transducer according to claim 4 wherein said transversely projecting element forms the bottom wall of a casing containing electrical circuit elements of the strain transducer.

6. A strain transducer according to claim 4 or claim 5 wherein said transverse face is connected to the transversely projecting element through a portion having a relatively small wall thickness parallel to the plane of the transverse face.

7. A strain transducer according to claim 6 wherein the transversely projecting element is in the form of an annular extension projecting outwardly of said peripheral outer face of the pluglike member.

8. A strain transducer according to claim 7 wherein the transversely projecting element has a bottom face substantially coplanar with said transverse face of the plug-like member, there being an annular undercut formed around the junction of said bottom face with the peripheral outer face plug-like member whereby portion of the member forming said transverse face is connected to said transversely projecting element through a thin walled cylindrical portion of the plug-like member.

9. A strain transducer according to any one of the preceding claims wherein said peripheral outer face is of frusto-conical form at least adjacent to the plane of said transverse face.

10. A strain transducer according to any one of the preceding claims wherein said plug-like member is arranged to be axially force-fitted into its opening in the structure surface.

11. A strain transducer according to any one of claims 1 to 9 wherein said plug-like member has a screw thread formed on said peripheral outer face for engagement wtih the opening in the structure surface.

1 2. A strain transducer comprising a carrier member in the form of a cylindrical plug member that includes a tubular outer portion and inner transverse wall extending across the interior of said tubular outer portion substantially normal to the axis thereof and having at least one strain gauge mounted upon it, the thickness of said transverse wall being substantially less than the axial length of the tubular outer portion, the arrangement being such that the carrier member can be inserted into an opening in the surface of a structure to be monitored with the tubular outer portion having an interference fit with the peripheral wall of said opening at least in the region adjacent the transverse wall, and said transverse wall being disposed substantially parallel or co-planar with said surface of the structure.

1 3. A strain transducer constructed and arranged for use and operation substantially as described herein with reference to either of Figs. 1 and 2 in combination with Fig. 3 of the accompanying drawings.