GB2242518A - Strain gauge - Google Patents

Abstract

Strain is measured at a surface (41) utilising a diffraction grating (42) secured to or formed directly on the surface (41). Light from a laser source (40) is directed onto the grating (42). Sensors (46, 48) measure the angle through which light is diffracted at the grating (42) to provide an indication of the strain at the surface (41). The grating may be formed by cutting using an excimer laser. <IMAGE>

Description

STRAIN GAUGE The present invention relates to an improved strain gauge, in particular, to a strain gauge using laser optical techniques.

Hitherto, the most commonly used transducer for strain measurement has been the bonded electrical resistance strain gauge.

Various forms of this device are known but all tend to suffer from a number of disadvantages. For example, specialised surface preparation techniques are needed to produce a gauge which provides accurate results. These may require considerable expertise to put into practice. Furthermore, once a bonded strain gauge has been attached to the surface on which strain is to be measured, problems arise due to temperature variations, humidity changes, 'creep' in the cement used for bonding and thermal noise. The results produced by these gauges are also adversely affected when they are used on hot surfaces or in other hostile environments and, also, when the strains present vary dynamically; the gauges can only produce an averaged indication of strain.

Because a number of new materials have been developed for use in aerospace applications, for example, carbon fibre reinforced plastic, it has become increasingly important that it be possible to predict the effects of fatigue in materials and to provide a strain gauge capable of providing accurate results in high intensity acoustic conditions or at high temperatures, where electrical resistance gauges cannot function.

In accordance with the invention, there is provided a method of measuring strain at a surface in which a diffraction grating is provided on the surface and light from a laser source directed onto the grating; the angle through which light is diffracted at the grating being detected to provide an indication of strain at the surface.

A strain gauge in accordance with the invention will now be described in detail, by way of example, with reference to the drawings, in which: Figure 1 shows schematically the plane wave construction of a diffraction grating; Figure 2 illustrates the change in diffraction angle e as the diffraction grating of Figure 1 is subject to strain; Figure 3 shows a reflection phase diffraction grating; and Figure 4 is a schematic diagram of a laser strain gauge in accordance with the invention.

We have appreciated that diffraction grating techniques may be used as the basis for strain measurement. As shown in Figure 1, a simple diffraction grating 10 is produced by interfering two plane wave laser beams 12 and 14. The result is a set of straight parallel interference fringes 11 whose spacing is determined by the angle e between the beams 12 and 14 used to make the grating 10.

Reconstructing the developed hologram grating 10 with an undiverged beam 16 will result in the first order beam 16' being diffracted through the same angle e as shown in Figure 2. If the grating 10 is subject to strain, the spacing between the fringes 11 will change and the angle through which the beam 16 is diffracted will change correspondingly. If a photodiode array is used to detect the diffracted beam 16', a very precise determination of this angular shift is possible. Consequently, the strain applied to the grating 10 can be measured to a high degree of accuracy.

It is apparent from this that a strain gauge capable of operating in a wide range of conditions could be produced by using a diffraction grating suitably secured to the surface of the object under strain. In practical applications, it is, however, necessary that a reflection phase grating be used as shown in Figure 3 as it is not possible in most cases to measure light transmitted at the surface subject to strain. The reflection phase grating is formed by etching onto a surface a number of closely-spaced parallel grooves, typically with separations of a few micrometres.

Figure 4 shows schematically a laser strain gauge ertbodying this principle. light from a laser light source 40 is transmitted onto a reflective phase grating 42 secured to a surface 41 under strain through a convex Fourier transform lens 43 which measures the spatial frequency of the grating. Two beam position sensing devices 46 and 48 are incorporated into the device to detect the two first order diffracted beams. Two sensing devices 46 and 48 are used in order to measure the differential angular movement of the beams and hence to provide an accurate indication of strain. If only one sensing device were used, the results obtained would be adversely affected by tilting of the surface 41 and the grating 42 secured to it.When the diffracted orders are captured by the lens aperture, the Fourier transform lens 43 also makes the strain gauge substantially immune to variations in the distance between the sensing devices and the grating.

Initially, tests were conducted using a transmission grating rather than a reflective phase grating. The gratings were formed holographically on photographic film with, typically 1000 lines per mm.

Sample gratings were stretched and their extension measured with a dial gauge and the strain values also measured optically.

Using these samples, it proved possible using the optical methods to produce measurements sensitive down to a few u strain; a degree of sensitivity comparable to that of an electrical resistance strain gauge.

Using optical methods, the strain gauge sensitivity is ultimately dependent on the accuracy to which the angular displacement of the first order diffracted beams can be measured.

Using a photodiode array with a spatial resolution of 25 um at a distance of one metre from the grating, a minimum detectable angular displacement of the order of 10-3 degrees is possible. This corresponds to a minimum detectable strain of 0.3 u strain.

Current advanced position sensing devices such as the Hanmatsu 53931 are available with spatial resolution less than 1 um and such devices therefore offer a laser strain gauge constructed on the principle outlined above with sensitivity far superior to that of currently available electrical resistance gauges. The same position sensing devices also permit displacement to be measured over a range of up to 2.5 cm. Consequently, strain measurement using a gauge incorporating such a device is possible over a large dynamic range, say, around 90dB.

In order to be able to put the strain gauge using optical techniques into practice, it is, of course, also necessary that it be possible to produce suitable diffraction gratings to the required degree of accuracy. Hitherto, most cutting and drilling operations using lasers have utilised relatively long wavelengths which produce heat to cut, drill or weld materials. These tend as a result to be unsuitable for use in producing diffraction gratings because of the melting, flow, debris and other thermal effects produced.

It is, however, possible to produce accurately cut diffraction gratings using an excimer laser. Such lasers produce high energy short wavelength laser photons which are sufficiently energetic that they can break directly the chemical bonds which hold most molecular-based materials together. Thus, cutting and drilling with these lasers in such materials is characterised by an almost complete absence of thermal effects such as melting. The cutting takes place by photoablative decomposition which is observed only when using the extremely short wavelengths, short pulse durations and intense powers which are characteristic of the excimer laser.

We have appreciated that, using an excimer laser suitable phase, gratings can be 'ablated' directly onto a variety of engineering materials including modern ceramics capable of withstanding very high temperatures. Strain measurements using the optical techniques outlined above can be employed either by etching a suitable phase grating directly onto a component to be monitored, bonding a phase grating manufactured in a laser laboratory onto the surface of the component or tasking a laser-based system to the component of interest. The manner in which the phase grating is formed will depend on the environment in which the strain measurement is to be taken and the portability of the component.

Claims (14)

1. A method of. measuring strain at a surface in which a diffraction grating is provided on the surface and light from a laser source directed onto the grating; the angle through which light is diffracted at the grating being detected to provide an indication of strain at the surface.

2. A method according to claim 1 in which the diffraction grating is formed by cutting grooves in the grating material using a laser.

3. A method according to claim 2 in which an excimer laser is used to form the grooves of the diffraction grating.

4. A method according to any of claims 1 to 3 in which the grooves of the diffraction grating are formed directly on the surface of a component subject to the strain to be measured.

5. A method according to any of claims 1 to 3 in which the grooves of the diffraction grating are formed on a substrate which is bonded to the surface of a component subject to the strain to be measured.

6. A method according to any preceding claim in which the angles through which two diffracted beams of the same order are diffracted are detected to provide an indication of strain at the surface.

7. A method of measuring strain at a surface, the method being substantially as hereinbefore described.

8. Apparatus for measuring strain at a surface the apparatus including a diffraction grating for mounting on the surface; a laser light source arranged to direct light onto the diffraction grating; and means for detecting the angle through which light is diffracted at the grating to provide an indication of strain at the surface.

9. Apparatus according to claim 8 in which the diffraction grating is formed by cutting grooves in the grating material using a laser.

10. Apparatus according to claim 9 in which the grooves of the diffraction grating are cut using an excimer laser.

11. Apparatus according to any of claims 8 to 10 in which the grooves of the diffraction grating are formed directly on the surface of a component subject to the strain to be measured.

12. Apparatus according to any of claims 8 to 10 in which the grooves of the diffraction grating are formed on a substrate which is bonded to the surface of the component subject to the strain to be measured.

13. Apparatus according to any of claims 8 to 12 including means for detecting the angles through which two diffracted beams of the same order have been diffracted to provide an indication of strain at the surface.

14. Apparatus for measuring strain at a surface, the apparatus being substantially as hereinbefore described.