WO1979000320A1 - Procede et appareil pour determiner des contraintes planaires dans une surface - Google Patents

Procede et appareil pour determiner des contraintes planaires dans une surface Download PDF

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Publication number
WO1979000320A1
WO1979000320A1 PCT/GB1978/000043 GB7800043W WO7900320A1 WO 1979000320 A1 WO1979000320 A1 WO 1979000320A1 GB 7800043 W GB7800043 W GB 7800043W WO 7900320 A1 WO7900320 A1 WO 7900320A1
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WO
WIPO (PCT)
Prior art keywords
beams
plane
combined
pattern
combining
Prior art date
Application number
PCT/GB1978/000043
Other languages
English (en)
Inventor
J Mckelvie
C Walker
Original Assignee
Nat Res Dev
J Mckelvie
C Walker
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Nat Res Dev, J Mckelvie, C Walker filed Critical Nat Res Dev
Priority to DE19782857203 priority Critical patent/DE2857203A1/de
Priority to JP50008578A priority patent/JPS54500096A/ja
Publication of WO1979000320A1 publication Critical patent/WO1979000320A1/fr

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B11/00—Measuring arrangements characterised by the use of optical techniques
    • G01B11/16—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge
    • G01B11/165—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge by means of a grating deformed by the object
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B2290/00—Aspects of interferometers not specifically covered by any group under G01B9/02
    • G01B2290/30—Grating as beam-splitter

Definitions

  • This invention relates to a method and apparatus for sensing in-plane deformation of a surface.
  • a method of sensing in-plane deformation of a surface on which there is a regular periodic pattern comprises deforming the surface so as to deform the pattern; illuminating the deformed pattern with a beam of coherent electromagnetic radiation so that the radiation is reflected as one zero order beam and a plurality of diffracted beams; and combining two of said beams, whereby the beams interfere in a manner related to the deformation along the direction of the intersection of said plane of the surface with the plane in which the two interfering beams lie.
  • the illuminating beam is collimated and is incident normally on the grating and two beams of the same diffracted order number and of opposite sign are combined.
  • the zero order beam can be combined with a diffracted beam, or two beams each in a different diffracted order may be combined.
  • the regular periodic patterns may be intrinsic to the surface, or may be a grating, preferably a phase grating such as a relief grating, attached to the surface, for example by a suitable adhesive, or formed in a material which itself adheres to the surface.
  • the relief pattern is such that all of the incident energy is diffracted into only a small number of diffracted orders.
  • Such a relief pattern may have a substantially sinusoidal transverse section and may be of pitch of between
  • the illuminating wave ⁇ length is not limited by the grating pitch, and for convenience visible light may be used.
  • the relief pattern may be periodic in that direction only.
  • measurement of strain in a single direction is not always adequate, and the direction of strain is not always predictable.
  • a method in which the pattern is periodic in two in-plane dimensions and at least three beams are combined as at least two pairs, one beam intersecting the plane defined by two other beams usually the pattern will be periodic in two orthogonal directions.
  • two pairs of beams may be combined, each pair lying in one of two planes relatively at right angles, or at another known angle, and both planes intersecting the deformed surface, preferably at right angles.
  • apparatus for sensing in-plane deformation of a surface on which there is a regular periodic pattern comprises means for illuminating the periodic pattern with-_a beam of coherent electromagnetic radiation; and receiving and combining means for receiving from the pattern at least two of a zero order beam and a plurality of diffracted beams, and for combining as at least one pair said at least two beams whereby the combined beams interfere.
  • the illuminating means will comprise a laser.
  • the or each combined pair of diffracted beams may be viewed by an observer for a qualitative assessment, or may be recorded, e.g. photographically, for subsequent quantitative assessment.
  • a measurement of strain can be made automatically and almost instantaneousl .
  • strain-sensing apparatus comprising receiving and combining means for receiving from a deformed periodic pattern illuminated by a beam of coherent electromagnetic radiation at least two of a zero order beam and a plurality of diffracted beams and for combining as at least one pair said at least two beams whereby the combined beams interfere; and sensing means to sense the average wavelength of the spatial variation transverse to the combined beam in intensity of radiation in each beam due to the interference.
  • the means to sense the average wavelength may comprise a linear array of photosensitive devices arranged transverse to each combined-pair beam, in the plane defined by the pair of beams before combination, the devices being small in comparison with the beam diameter.
  • the array may be an array of photodiodes or charge-coupled devices plus scanning means to repeatedly scan the array and provide an output signal depending on the intensity of the radiation on each diode or device.
  • the sensing means may be a vidicon device.
  • phase-modulation means arranged to cause a continuously varying phase relationship between the two beams in each combined pair, whereby a moving interference pattern is produced; and two photosensitive devices arranged to receive the pattern, the devices being spaced apart b a distance which is small compared with the spatial wavelength of the interference pattern and in a direction such that the devices are separated to some extent in the plane in which the two combined beams lie; and phase comparator means to sense the 'difference in phase of the output signals from the two photo- sensitive devices.
  • Figure 1 shows schematically apparatus for sensing strain in one dimension
  • Figure 2(a) is a section through part of a relief pattern grating suitable for use in the apparatus of Figure 1;
  • Figure 2(b) shows the diffracted beams when such a grating is illuminated normally
  • Figure 2(c) shows the position of the diffracted orders
  • Figure 3 illustrates a set of Moire fringes
  • Figure 4(a) shows schematically apparatus for sensing strain in three different coplanar directions
  • OMPI Figure 4(b) shows a relief grating for such an apparatus and the relative position of the diffracted orders
  • Figure 5 shows a typical interference fringe waveform viewed by an array of photodiodes
  • Figure 6 illustrates schematically a circuit for calculating strain in three in-plane directions
  • Figure 7 illustrates the use of a phase modulator in one diffracted beam
  • Figure 8 illustrates the effect of phase modulation
  • Figure 9 illustrates use of one phase modulator in the input beam of a polarised optical system
  • FIGS 10(a) and 10(b) show two possible arrangements of linear detector arrays.
  • Figure 10(c) shows a two dimensional matrix of photo- detectors.
  • FIG. 1 an object which is to be strained is represented by reference 10 and a relief grating 12 is attached to the surface * of the object by a suitable adhesive.
  • the grating 12 is illuminated by a normally-incident collimated beam of light l4 a few millimetres in diameter produced by a laser 15 having a power supply 17 and power source 19- For example a 2 milliwatt Hughes laser can be used.
  • the grating 12 has a regular periodic structure in one dimension as shown in Figure 2(a), with a sinusoidally varying surface of pitch AB of about 1 or
  • the grating lines formed by the surface variations are therefore perpendicular to the plane of Figure 1; as shown in Figure 2(b) the incident light will be diffracted into only a few orders each of high intensity. As shown in Figure 2(c), the diffracted orders can be viewed as a line of dots of light parallel to the grating plane and perpendicular to the grating lines.
  • the second order diffracted beams of opposite sign l6, 18 are reflected by means of plane mirrors 20, 22 and 24 and a beam combiner 26 to form a combined beam 30. If the grating 12 is unstrained, the light in the combined beam 30 will be spatially constant, provided the mirrors are correctly aligned.
  • the combined beam will contain an interference pattern related to strain in that direction.
  • the arrangement in Figure 1 senses strain only in one dimension, such as the direction shown by arrow G, and represents the component in that direction of a complex strain.
  • the pattern in the combined beam 30 ⁇ although not a Moire pattern, has some of the properties of a set of Moire fringes. It can be displayed on a screen and viewed by eye, or recorded directly by photography, and the magnitude of the strain can be calculated.
  • Figure 4(a) illustrates a view equivalent to that along the laser beam 14 towards the grating (not shown) and shows the mirrors 22, ' 24 and beam combiner 26 arranged as in Figure 1.
  • Two other similarly arranged sets of mirrors 2, 3 ⁇ and 42, 44 each have an associated beam combined 36,' 46 as shown; each optical system can sense strain in a different direction in the plane of the relief grating.
  • two directions are orthogonal and the third makes an angle of 45 with the other two, although other relative angles can be used, provided they are substantial, e.g. more than 20 .
  • Figure 4(b) which is the two-dimensional equivalent of Figure 2(c)
  • the lines on grating 13 are seen to form a grid structure with the diffracted beams visible as a two-dimensional array of dots.
  • the combined beams from each optical system are in different directions, and can be received on a screen or recorded photographically e.g. at position 31 in Figure 1 and at the other two corresponding positions.
  • Each combined beam is in a different direction and contains information related to a strain along one in-plane direction so that calculation of the value of strain in each in- plane direction can be made simply and easily from each inter ⁇ ference pattern with no problem of superimposition of information from different, strain directions. Measurements can be made simultaneously in two or more directions. However, there is still the problem that, for an overall picture, the information on the measured strains must be combined if maximum strain and its direction are required * This may involve considerable mathematical computation.
  • each photodiode There may be 256 diodes in the array, and the voltage on each photodiode will depend on the incident light intensity. By sequential comparison of the voltage on each photodiode with a reference voltage by means of a voltage comparator, an output signal can be provided each time the base line 50 is crossed. By counting the number of photo- diodes between each output signal, an average wavelength of the intensity variation in the light pattern can be calculated.
  • a suitable apparatus is shown in Figure 6 in which three photodiode arrays 52A.
  • Each comparator is supplied with a reference voltage from a unit 57A, B, C.
  • the microprocessor can be programmed to calculate from the three input signals the maximum principal in-plane strain averaged over the field of view on the relief grating, the maximum shear strain, and the angle of the principal strain. The result is obtained almost instantaneously and can be in numerical form. Use by operators unskilled in the interpretation of interference fringes is therefore possible.
  • stresses rather than strains can be calculated by providing values of the Young's modulus and Poisson's ratio for the material which is being deformed.
  • the waveform is not the smooth sinusoid shown in Figure (whether of varying or invariant wavelength) but shows small-scale irregularities in its curvature; these may be caused, for example, by dirt in the optical system.
  • a working device may operate in many adverse industrial environments so that cleanliness is impossible, and a correction must be applied.
  • the microprocessor is operated so that from the output signals corresponding to each measuring direction an average wavelength is calculated, and the standard deviation of the distribution. Readings greater than one standard deviation from the average are discarded and the average wavelength is recalculated. It has been found that this conventional, statistical technique gives an accurate measurement of fringe spacing.
  • phase modulator is arranged to apply a continuous phase.- modulation to the beam 18 in a manner such as to change its wavelength.
  • the effect may be achieved using an electro- optic device, or a piezoelectric crystal, driven by a sawtooth waveform; or an acousto-optic device; or a moving diffraction grating; the change in wavelength willvthen be con inual rather than strictly continuous.
  • the effect of the change of wavelength in one component of the combined beam 33 is that the previously stationary inter ⁇ ference pattern now moves transversely to the beam, as indicated by the arrow in Figure 8. However, the background noise remains stationary.
  • the moving pattern can be viewed by two photodiodes 56, 58 separated by a distance $x which is small with respect to the wavelength ⁇ of the fringe pattern. Knowing£ x, a measurement by phase comparator 59 of the phase difference between the outputs of the two photodiodes 5 ,' 58 allows a calculation, of the average wavelength ⁇ of the pattern, which is unaffected by background noise.
  • the two photodiodes replace the diode array 52 in Figures 5 and 6 and this arrangement is alternative to use of a diode array plus calculation of standard deviation.
  • phase modulator and one phase comparator must be supplied in each of the three optical systems shown in Figure 4, and the outputs from the two j-h ⁇ todibdes- associated with each system are connected to the respective comparator.
  • An alternative arrangement avoids the need for two or three phase modulators which may be bulky and/or expensive, and is shown in Figure 9- Only one phase modulator is used in conjunction with a polarised optical system. The general arrangement is similar to that in Figures 1 and 7. although one mirror is omitted for simplicity.
  • the modulated beam passes through a polarisation rotator 70 which rotates the plane of polarisation of the beam through 90 to a second plane of polarisation indicated by the dots on the axial path.
  • the modulated and rotated beam is reflected by a plane mirror 72 to a beam combiner fk which combines it with the part of the incident beam which passes through beam splitter 64.
  • the beam from the combiner fk is reflected from the relief grating 12 into two second order diffracted beams 16 and 18, which both contain components at two orthogonal planes of polarisation, one component being modulated.
  • the beam 18 is reflected from plane mirror 24 through a polariser 76 which allows passage of radiation only in' * .the second plane of polarisation, i.e.
  • the phase-modulated bea ⁇ the polarised radiation then passes through a polarisation rotator 78 which causes rotation through 90 into the first plane of polarisation.
  • the beam 16 is reflected by - ⁇ 4 - mirror 22 through a polariser 80 which allows passage of radiation only in the first plane of polarisation, i.e. which prevents passage of the modulated beam component.
  • the two transmitted feaa ⁇ isare combined by beam combiner 26 and are sensed by two closely-spaced photodiodes 56, 58 as before.
  • the combined beam is linearly polarised in the first plane of polarisation but contains both phase-modulated and unmodulated components from beams 18 and 16 respectively, so that a moving fringe system is provided as in the Figure 7 one-dimensional arrangement.
  • a typical beam diameter is a few millimetres, and the strain measured is averaged over the field of view (the photodiodes usually view an area smaller than the beam diameter).
  • 'She beam diameter may be about 50 millimetres, and strain variations within the field of view can be observed. It is an advantage of the arrangements using the larger beam area that buckling, of the grating out of the plane of measurement is only a second order effect. When an automatic system is used, the effect is still small, but can be compensated by measuring the position of the zero diffraction order, and applying the appropriate correction to the measured values.
  • a further modification involves the use of two beam pairs whose generating beams lie in planes which intersect at a substantial angle. Usually they will intersect at right angles.
  • a two-dimensional array of photosensitive devices may be used to sense the spatial variations of the:.interference fringes both parallel to the plane of two uncombined beams, and perpendicular to it.
  • Figures 10(a) and 10(b) show suitable detector arrays 82, 84 arranged to form a T shape or an L shape. A cross shape may also be used.
  • One such array is used in each combined beam, and the in-plane strain may be calculated from the four measurements. It is an advantage of this that the optical system is less complex.
  • a two-dimensional photosensitive device or a matrix 86 of discrete photosensitive devices 88 as shown in Figure 10(c) may be used to sense the spatial variation transverse to the combined beam, and at a number of points in the field of view.
  • the interference fringe spacings may be measured by selecting pairs of photosensitive devices and measuring the phase difference between the signal outputs, due to the interference fringes sweeping across.
  • pairs of photosensitive devices which are- -'arrayed along lines parallel to and perpendicular to the plane of the uncombined beams, the fringe spacings in these two directions can be gauged at a plurality of points in the field of view. From two such combined beam fringe pattern analyses, a complete strain contour pattern for the field of view can be constructed with suitable computation.
  • the illuminating beam in this modification may be 10 - 100 mm diameter.
  • the relief grating and viewing system need not be mounted rigidly with respect to each other. Vibrations out of the plane of the relief grating, and linear in-plane vibration, have only second-order effects on the interference fringes. In-plane rotation affects both components of each combined beam. Even if the fringes are not continuously visible, the trends of a changing situation can be followed.
  • the invention has been described with reference to a system which does not incorporate any lenses. If a very small field of view is required, for example when spatial resolution is important, a magnifying system can be provided to magnify the beam incident on the detector system.
  • the interfering beams will be of the same intensity; further, sensitivity is related to the angle between the diffracted beams, so a symmetrical system gives highest sensitivity.
  • the zero order beam can be combined with one or two or three diffracted beams. This minimises the physical size of the apparatus by halving the angle between the received diffracted beams, so that use in a restricted position, such as in a corner, may be facilitated, but there is the disadvantage that at least one additional beam splitter is required. • In the illustrations, the required beams are combined after reflection from small mirrors.
  • each mirror receives only one beam, either because the incident beam is narrow, or because the distance of the mirrors from the surface is sufficient for the diffracted beams in a wide-beam system to be separated.
  • a possible method of separating overlapping diffracted orders is to position a lens and an aperture stop between the beam combiner (reference 26 in Figure l) and the detecting system.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Length Measuring Devices By Optical Means (AREA)
  • Optical Transform (AREA)

Abstract

On deforme une surface presentant une trame periodique et reguliere, souvent une trame en relief (12). La contrainte planaire est detectee par l'illumination de la trame deformee avec un faisceau (12) de lumiere coherente de maniere telle que la lumiere soit reflechie sous la forme d'un faisceau d'ordre zero et d'une pluralite de faisceaux diffractes, et par la combinaison de deux des faisceaux (16, 18) pour qu'ils interferent de maniere a etre en rapport avec la contrainte le long de la direction d'intersection de la surface plane et du plan forme par les deux faisceaux interferents. La grandeur et la direction de la contrainte planaire dans une ou plusieurs directions et la grandeur et la direction du maximum de la compression ou de la tension planaire peuvent etre determinees automatiquement.
PCT/GB1978/000043 1977-11-25 1978-11-24 Procede et appareil pour determiner des contraintes planaires dans une surface WO1979000320A1 (fr)

Priority Applications (2)

Application Number Priority Date Filing Date Title
DE19782857203 DE2857203A1 (de) 1977-11-25 1978-11-24 Method and apparatus for sensing in-plane deformation of a surface
JP50008578A JPS54500096A (fr) 1977-11-25 1978-11-24

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB49170/77 1977-11-25
GB4917077 1977-11-25

Publications (1)

Publication Number Publication Date
WO1979000320A1 true WO1979000320A1 (fr) 1979-06-14

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PCT/GB1978/000043 WO1979000320A1 (fr) 1977-11-25 1978-11-24 Procede et appareil pour determiner des contraintes planaires dans une surface

Country Status (6)

Country Link
EP (1) EP0006945A1 (fr)
JP (1) JPS54500096A (fr)
CH (1) CH626992A5 (fr)
FR (1) FR2454602A1 (fr)
GB (1) GB2008791B (fr)
WO (1) WO1979000320A1 (fr)

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2490808A1 (fr) * 1980-09-22 1982-03-26 Philips Nv Interferometre et dispositif comportant cet interferometre
EP0060685A1 (fr) * 1981-03-11 1982-09-22 National Research Development Corporation Procédé de mesure de déformation
WO2000042382A1 (fr) * 1999-01-15 2000-07-20 N.V. Kema Procede et dispositif permettant de mesurer les deplacements dans un plan

Families Citing this family (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2504256A1 (fr) * 1981-04-16 1982-10-22 Euromask Procede et dispositif de mesure optique de deplacement et application aux photorepeteurs sur tranche
GB2242518B (en) * 1990-03-12 1994-03-30 Univ Southampton Strain gauge
CN103278268A (zh) * 2013-05-31 2013-09-04 哈尔滨工业大学 基于散斑干涉原理的应力测试装置及应力集中测试方法
US9459093B2 (en) * 2014-02-20 2016-10-04 Kabushiki Kaisha Toshiba Deflection measuring device and deflection measuring method

Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3822942A (en) * 1971-06-03 1974-07-09 Leitz Ernst Gmbh Method of testing a length, angle, path difference or speed by detecting interference and apparatus therefor

Patent Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3822942A (en) * 1971-06-03 1974-07-09 Leitz Ernst Gmbh Method of testing a length, angle, path difference or speed by detecting interference and apparatus therefor

Non-Patent Citations (3)

* Cited by examiner, † Cited by third party
Title
Instruments and Control Systems, Volume 44, No. 12, issued December 1971, Radnor R.S. STEELE et al., "Parallel Incidence for diffraction grating strain gages", see pages 64-66. *
Journal of Physics, E:Scientific Instruments, Volume 5, No. 9, issued September 1972, London. C.S. SCIAMARELLA "Use of gratings in strain analysis", pages 833-835, see pages 833-835. *
Proceedings of the Soc. of Photo-Optical Instrumentation Engineers. Laser Range Instrumentation. Seminar, 16-17 October 1967, El Paso B.D. KROEGER "Electro optical shaft angle encoder utilizing laser interferometry" see pages 93-97. *

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2490808A1 (fr) * 1980-09-22 1982-03-26 Philips Nv Interferometre et dispositif comportant cet interferometre
EP0060685A1 (fr) * 1981-03-11 1982-09-22 National Research Development Corporation Procédé de mesure de déformation
US4474466A (en) * 1981-03-11 1984-10-02 National Research Development Corporation Measurement of deformation
WO2000042382A1 (fr) * 1999-01-15 2000-07-20 N.V. Kema Procede et dispositif permettant de mesurer les deplacements dans un plan

Also Published As

Publication number Publication date
CH626992A5 (fr) 1981-12-15
JPS54500096A (fr) 1979-12-20
FR2454602B1 (fr) 1983-01-28
EP0006945A1 (fr) 1980-01-23
GB2008791B (en) 1982-04-28
FR2454602A1 (fr) 1980-11-14
GB2008791A (en) 1979-06-06

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