GB2299407A - Strain transducers - Google Patents

Abstract

A strain transducer comprises one or more strain gauges 3 on a short cylindrical cap 2. The cap is pressed with an interference fit onto a solid cylindrical boss 4 which is securely attached, e.g. by stud welding, to the surface of a structure 1 that is to be monitored. A secure and positive attachment is thereby obtained without the need to drill the structure or employ penetrating welds, and the transducer can be removed and replaced for easy field service. As shown the cap 2 is housed within a cylindrical body 8 which is used to apply force to the cap to fit it on the boss and also for the attachment of a signal cable 6.

Description

"STRAIN TRANSDUCERS" This invention relates to strain transducers for the measurement of strain in a component by the use of at least one strain gauge.

Electrical strain gauges are able to provide an accurate and continuous response to the strains at selected points of a structure (this term being understood to include any load carrying component) to which they are fixed and transducers employing such gauges have become widely used for monitoring the state of strain and the load upon it.

In use, however, problems are met over fixing the transducers in place. Security and robustness of attachment are general needs, often in very arduous operating conditions, and because of this the simple method of attaching individual strain gauges by adhesive to the surface to be monitored has little place outside the laboratory. It has become conventional to attach the strain gauges to an intermediate support forming part of the transducer that is in turn attached to the structure.

Typically, the transducer is bolted onto the structure to be monitored but it cannot always be ensured that such means of attachment to the structure do not introduce measurement errors in the strain gauges themselves. The precautions needed to minimize such errors can make the transducer relativity bulky and complicated in construction and the transducer can usually only be sealed at the time of installation.

One method of minimizing errors is described in GB 2,5,624, where the transducer comprises a plug-like body that is inserted with an interference fit into a hole drilled in the surface of the structure to be monitored, so that a face of the transducer on which the strain gauges are mounted is effectively continuous with that surface. While the adoption of such a construction allows factory sealing and gives good results and small size, in many instances it is not possible to make a hole in the structure surface, e.g. because the hole can compromise the safety of the structure if added after design and manufacture of the structure. The hole can weaken the structure and promote fatigue cracking in certain operating conditions.Where the hole itself does not present a safety problem the interference fit between the transducer plug and the hole requires a precision difficult to obtain in the field.

A method of fixing a strain transducer which avoids these problems is described in GB 2,155,183 where the strain gauges are mounted on thin plate-like base that is fixed by stud welding to the surface of a structure that is to be monitored.

A secure and positive attachment is thereby obtained without the need to make a hole in the structure. This gives good results and the low energy welding technique does not compromise safety, however, the heat of welding is transmitted to the strain gauges and can cause damage or force the employment of expensive high temperature strain gauges in manufacture. A further problem, which is very restricting commercially, is the difficulty of field service replacement of the transducer in the event of failure or damage in situations where skilled personnel and/or stud welding equipment is not available.

According to one aspect of the invention, there is provided a method of securing a strain transducer to a small solid cylindrical boss securely fixed to the surface of a structure.

The transducer comprises a short circular tube closed at one end by a thin wall to form a cap-like member having at least one electrical strain gauge element mounted on the outer transverse surface of the wall. The cap is pressed on the boss which is an interference fit with the cap so that the strains in the boss, which correspond with precision to the strains in the structure, are faithfully transmitted to the strain gauge(s) mounted on the top of the cap.

With this technique a vendor can supply the cap and the boss with the tolerances of the mating diameters set in manufacture to control the degree of the interference fit so that only elastic deformation of the cap and boss occurs during installation. This permits installation, removal and replacement of the cap without affecting accuracy of the subsequent strain measurement. A tolerance system can be further arranged so that all bosses and caps are interchangeable whilst maintaining the interference fit required to ensure that only elastic deformation occurs on installation which allows easy replacement of a non-functioning cap in the field.

To enclose the strain gauge, and any necessary circuitry required to be adjacent an array of strain gauges the cap may form the bottom wall of an enclosed housing.

The housing may take a generally cylindrical form, to transmit force applied externally to the housing to the cap during installation and means for securing and sealing the cable carrying electrical input power and output signals.

By way of example an embodiment of the invention will be described in more detail with reference to the accompanying schematic drawings, in which: Figure 1 is an exploded view of the cap and boss according to the invention with the housing and cable not shown and; Figure 2 is a sectional side view of the transducer installed on a structure.

The device comprises the cap 2 on the top of which a strain gauge array 3 is firmly fixed so as to follow all the strains of the cap transmitted from the boss 4. The array may comprise preformed components secured to the surface or may take the form of a solid state circuit built up on the surface.

The cap forms the base of a housing comprising a cylindrical body 8 closed at one end with a rebate 11 at its inner rim to receive the cap flush with its bottom edge. The top of the cylindrical body has a recess 7 to engage a special tool (not shown) through which force is transmitted at the time of installation to the cap through the housing. The cap is pressed in place in this way either by impact or pressure on the housing top. A sealing gland 5 is provided in the housing wall for a cable 6 that supplies power to the circuit within and also carries the measurement signal produced by the strain gauge array. The housing may be be completely sealed from the exterior to protect the strain gauge circuit from environmental and mechanical damage.

Prior to installation of the transducer the boss is welded to the surface of the structure 1, in a position determined by the application, in the conventional manner using a studwelder. The boss has a conventional small central pip on its underside (not shown) to facilitate welding by a stud welder of the capacitor discharge type. During welding the discharge through the central pip and the resulting arc produces fusion between substantially the entire undersurface of the boss and the surface of the structure so that the boss is integrally bonded to the structure by the welding interface ie.

Of particular significance is the fact that the structure is not drilled or weakened in any way, while the boss is secured integrally to the structure surface by the welding fusion interface and the movements in the boss relate very accurately to the stresses at the surface of the structure.

To assist in locating the transducer cap on the boss, the top of the boss and the rim of the cap may be chamfered. To install the transducer it is located on the boss and pressed home either by light impact or pressure which can be applied with via a special tool engaged in the recess on top of the housing. An adhesive, typically of the anaerobic type, can be applied to the boss and cap before installation to prevent the possibility of fretting corrosion, to seal the interface 9 and to assist in retaining the transducer on the boss.

Depending on optional needs, the strains in the cap can be detected by a single strain gauge or by an array, and whichever is used, it can be provided in any of a variety of known forms, such as by thick film or thin film circuits, semi-conductors, or metal foil and wire. Typically, as shown more clearly in Figure 1, 4 strain gauges are arranged in 980 star pattern and wired as a Wheatstone bridge. With this arrangement it is to be noted that when the transducer is installed that the radial tension forces to which the cap is subjected by its interference fit with the boss are symmetrical if the cap and boss are both circular in cross section.With the structure otherwise unstressed, therefore, the strain gauges are all subjected to equal strains and there is no need to provide for zero point calibration after installation or as a result of stress relieving over time.

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 installed very easily and simply, without any special skill on a wide variety of structures. It will be understood that the described embodiments are capable of many modifications within the scope of the invention. The size of the transducer is only limited by practical sizes for the strain gauges, cable and any signal conditioning electronics mounted in the housing and can be as small as 7mm diameter and 5mm high. The device can be arranged for a wide variety of uses, e.g. to measure axle loadings of road vehicles, and can be employed to particular advantage on highly stressed components or in environments where an alternative connection between a monitoring device and a structure is likely to accelerate corrosion.

Claims (7)

1. A strain transducer comprising a short circular tube closed at one end by a thin wall to form a cap-like member having at least one electrical strain gauge secured to the transverse surface formed by the end wall, the inner diameter of said cap-like member is arranged to be an interference fit with a short cylindrical boss protruding from the surface of a structure to be monitored and on which boss said cap-like member is pressed to sense, via the strain gauge(s) secured to its transverse face, the strains in the boss transmitted from the structure.

2. A strain transducer according to claim 1 where the circular boss on which the cap-like member fits is a separate component stud welded to the monitoring surface.

3. A strain transducer according to claim 1 where the circular boss on which the cap-like member fits is formed on the structure by machining or other means and is an integral part of the structure.

4. A strain transducer according to claim 1 where the circular boss onto which the cap-like member fits is a separate component securely attached to the monitoring surface.

5. A strain transducer according to claim 1 where the cap-like member forms the bottom wall or part of the bottom wall of a housing containing electrical circuit elements of the strain transducer.

6. A strain transducer according to claim 1 where the elastic forces holding the cap-like member to the boss are augmented by an adhesive/sealant applied to both components at the time of installation.

7. A strain transducer constructed and arranged for use substantially as described herein.