Opto-electronic scale reading apparatus

Abstract: An opto-electronic scale reading apparatus includes a reflective scale (410) relative to which a readhead is movable. The readhead includes a light source in the form of the emission end of an optical fibre (300), which directs light via collimating lens (420), beam splitter (310), and deflecting optics (312,314) onto the scale. The light reflected from the scale passes through an index grating (416) to generate a series of spatial fringes in the plane of an analyser (418). The analyser comprises a series of elongate light transmitting colour filters, extending substantially parallel to the fringes and arranged in a repeating pattern (e.g. red, blue, green, red, blue .....); like-coloured elements are separated by a distance corresponding to an integer multiple of the fringe pitch. The array (418) encodes the fringes into a plurality of phase-shifted signals, which pass through the beam splitter (310), lens (420) and back along fibre (300). A plurality of corresponding colour filters, provided at the other end of the fibre (300) deflect the relevant colour light onto individual photodetectors thereby to produce a plurality of cyclically modulating phase-shifted electrical signals, from which the relative displacement of the readhead and scale may be determined. Where the fibre is of plastics material the light is preferably in the infra-red spectrum to produce attenuation.

Description: The present invention relates to an opto-electronic scale reading apparatus, used on a coordinate positioning machine such as a machine tool for example, to measure the relative displacement of two members. It is known to construct an opto-electronic scale reading apparatus in accordance with the teaching of GB 1504691, incorporating a readhead having index and analyser gratings. A light source within the readhead projects a beam of light through the index grating and onto the scale, which interacts with the incident light to generate a series of spatial fringes in the plane of the analyser grating. Relative movement between the scale and the readhead causes corresponding relative movement between the fringes and the analyser grating, producing a light intensity modulation when the moving fringes are viewed from the side of the analyser grating remote from the scale as a result of the shuttering action of the analyser on the fringes. A plurality of phase-shifted light intensity modulations may be generated by providing a corresponding plurality of analyser gratings having lines shifted relative to each other by a fraction of the grating pitch corresponding to the required phase angle. For example, three analyser gratings each offset relative to each other by one third of the pitch will result in three sets of light intensity modulations having phase shifts of 120° (as shown in EP 272297). An alternative method of generating a plurality of phase-shifted electrical signals is to provide a structured photodetector, known as an electrograting, in place of the (or each) analyser grating. The electrograting comprises an array of elongate photosensitive elements, extending in a direction parallel to the lines of the scale, which are grouped into a plurality of sets (e.g. A,B,C); the number of sets of photosensitive elements corresponding to the number of phase-shifted signals required. The elements are provided in a repeating pattern (ABCABC...) with elements of each set being electrically connected in common. The spacing between any two elements in the same set is equal to an integer multiple of the pitch of the interference fringes generated by the readhead, and thus all elements in said given set are exposed to approximately the same intensity of light. Relative movement between the scale and the readhead causes the fringes to move across the surface of the electrograting, resulting in a series of phase-shifted modulating electrical signals from the sets of elements in the electrograting. Such an electrograting is known from EP 543,513 and has an advantage in that the light contributing to the generation of each of the plurality of phase-shifted signals originates from substantially the same region of the scale. This makes the apparatus relatively insensitive to contamination of the scale due to dirt or scratching, for example. Another method of generating a plurality of phase-shifted light intensity modulations is disclosed in WO90/13006, wherein an auxiliary grating is provided between the scale and the analyser grating, positioned with its lines generally perpendicular to the lines of the analyser grating but slightly skewed. Each primary diffraction order is split into a set of primary diffraction orders corresponding to second orders of diffraction, whereby each set of primary orders produces a light modulation having a phase shift relative to the others and each light modulation is detected by a respective light sensitive transducer. The use of colour modulation in optical displacement sensors is described in GB 2202937 which comprises first and second gratings, a polychromatic light source illuminating the gratings and a detector. At least one grating is provided with coloured markings such that relative movement of first and second gratings causes a variation in the colour of the light reaching the detector. The present invention provides an alternative readhead. According to the present invention an opto-electronic scale reading apparatus includes a scale having a series of spaced-apart lines extending in a first direction, and spaced apart in a second direction, and a readhead, the scale and readhead being movable relative to each other in the second direction, the readhead including means for projecting incident light onto the scale, and generating, at an analyser, a periodic light pattern, and upon relative movement of the scale and readhead, a light intensity modulation, wherein the readhead further comprises phase encoding means to enable the generation of a plurality of phase-shifted electrical signals from said light intensity modulation, characterised in that said phase encoding means comprises an array having a plurality of elongate spectrally encoding elements, extending substantially in said first direction, said elements being divided into a plurality of sets, each set of elements having a different spectral encoding characteristic, with elements of a given set having the same spectral encoding characteristic, and wherein said elements are provided in said array in a repeating pattern. In one embodiment of the present invention the phase encoding light transmitting array is provided in place of the analyser, and the spectral encoding capability is provided by colour encoding of polychromatic light in said periodic light pattern. A plurality of light filters may then be provided downbeam of said analyser, in order to divert different light colours onto different photodetectors, thereby to generate said electrical modulating phase-shifted signals. Embodiments of the invention will now be described, by way of example, and with reference to the accompanying drawings, in which: Fig 1 is a schematic illustration of a first embodiment of the present invention; Fig 2 is a detail of Fig 1; Fig 3 is a modification of the detail of Fig 2; Fig 4 is a schematic illustration of a second embodiment of the present invention; Fig 5 is a schematic illustration of a third embodiment of the present invention; and Fig 6 is a schematic illustration of a fourth embodiment of the present invention. Referring now to Figs 1 and 2, an opto-electronic scale reading apparatus includes a reflective scale 10, defined by a series of substantially parallel, elongate nonreflective lines 10A. The scale 10 extends in an xy plane, with the lines 10A being spaced apart in the y direction and extending in the x direction. A readhead offset from the scale in the z direction, is movable relative to the scale 10 in the y direction, and includes a polychromatic diffuse light source 14, which projects a beam of light onto the scale 10 via an index grating 16. The scale interacts with the beam of incident light to generate a periodic light pattern (e.g. in the form of interference fringes) in the plane of an analyser 18, which lies adjacent the index grating 16. The interference fringes extend substantially parallel to the lines 10A of the scale, are spaced apart substantially in the y direction and their intensity modulates substantially sinusoidally with displacement in the y direction. The spacing in the z direction between index grating 16 and scale 10, and scale 10 and analyser 18, together with the relative pitch of index grating 16 and scale 10 which are required in order to generate the light pattern in the plane of analyser 18 are well known per se, and are described, for example, in GB 1504691 (NRDC). Light forming the periodic light pattern is incident upon, and passes through the analyser 18, which performs a spectral encoding function to enable the generation of a plurality of phase-shifted electrical signals. In the present example, the spectral encoding is performed by chromatic encoding of the light into three separate colours, with the light intensity of each of the colours modulating in phase-shifted relationship upon relative movement of the scale and readhead. Light passing through the analyser 18 is compressed into a relatively dense beam by means of convex and concave lenses, whereupon light from each of the three colour coded bands is decoded by three dichroic filters 30,32,34 and directed by means of these filters onto three corresponding photodetectors 40,42,44. The phase-shifted modulating outputs of the photodetectors 40,42,44 are combined in a manner known per se, in order to generate a pair of sinusoidally varying signals having a quadrature relationship which may be used to determine both the magnitude and direction of relative movement of the scale 10 and readhead. Referring now to Fig 2, the analyser 18 and its colour-encoding interaction with the interference fringes incident thereon will now be described. The analyser 18 comprises a light transmitting substrate 50 having a plurality of elongate light transmitting spectral encoding elements 52 provided thereon. The elements 52 extend substantially parallel to the lines 10A of the scale, and substantially perpendicular to the y direction of relative movement of the scale 10 and readhead. In the present example, the elements 52 are red, blue and green (RBG) colour filters, provided across the substrate 50 in a repeating pattern, and with the spacing between the centre of correspondingly coloured elements being equal to an integer multiple of the pitch of the interference fringes. In the present example the spacing between adjacent elements of the same colour is a single pitch P, as can be seen in Fig 2. A consequence of this geometry of the analyser 18 is that the light intensity of interference fringes incident upon each colour filter of a given colour will be substantially the same for any given relative displacement of the scale 10 and readhead. Thus, as the interference fringes move relative to the analyser 18 each of the sets of colour filters 52 RBG will transmit a substantially sinusoidal light intensity modulation, with the red, blue and green light intensity modulations having a phase separation of substantially 120°. NB. Additional sets of phase-shifted signals may be obtained by adding further sets of colour filters, such as yellow, for example. Different phase angles between adjacent colour filters may be obtained by shifting the spacing of the centre of each of the filters relative to each other with respect to the pitch of the interference fringes (e.g. a centre-to-centre spacing corresponding to one quarter of the pitch corresponds to a phase angle of 90°). The resolution of the encoding array increases with decreasing width of the elements in relation to the fringe pitch because the variation in the intensity of light incident upon an individual element of the array across the width of the element decreases with decreasing width of the element. Referring to Fig 3, a further example of encoding array can be seen in which only approximately half of the array surface is light transmitting; the areas between the individual elements 52' are made non-light transmitting, and the width of the elements is reduced relative to the fringe pitch. As a result, the intensity of light passing through the blue element 52B' varies only between I 11 and I 12 , as compared with a variation of I 1 to I 2 in the array of Fig 2. The result reducing the width in this way is therefore sharper output signals of lower amplitude. Referring now to Fig 4, in an alternative, but equivalent embodiment of the present invention, a readhead includes a light source 114 which projects a beam of light through an index grating, provided by a light transmissive spectral encoding array 118, and onto a scale 110. The light reflected from the scale 110 interacts therewith to form a series of colour coded interference fringes at an analyser, provided by grating 116. The resultant light intensity modulation, which occurs upon relative movement of the scale 110 and readhead is compressed by lenses and decoded by dichroic filters 130,132,134 onto photodetectors 140,142,144. In this embodiment, light transmissive array 118 may be said to act in a manner similar to a conventional grating, in that the blue and green elements of the array are opaque to red light emitted from source 114, the red and blue elements 52' of the array are opaque to green light emitted from the source 114, and so on. In a further embodiment shown in Fig 5, both the index and analyser of the readhead are provided by a light transmissive colour encoding array 218. Other parts of the apparatus having corresponding reference numerals function in the same way as described previously. In a further improvement of the illustrated apparatus, the light sources, imaging lenses, dichroic filters, and photodetectors may be provided remote from the readhead, with the incoming and outgoing light being transmitted to and from the readhead along one or more optical fibres. Referring now to Fig 6, an opto-electronic scale reading apparatus has a scale 410 with lines 410A, illuminated by a light source in the form of the emission end of an optical fibre 300, a beam splitter 310 and deflecting optics 312,314 project light onto the scale 410. The light, which is polychromatic light in the infra-red spectrum to reduce attenuation in the fibre (which may be of glass or plastics material), is reflected off the scale 410 and passes through an index grating 416. The interaction of light reflected from the scale 410 with the index grating generates interference fringes in the plane of analyser 418, in the form of a spectrally encoding array of the type described earlier. This optical mechanism is described in more detail in EP 207121. Light passing through the analyser is compressed by a lens 420 and the part of the light passing through the beam splitter 310 is transmitted into the end of fibre 300 and along the fibre to the decoding optics and photodetectors. In a modification, separate fibres are provided for incoming and outgoing light, enabling the removal of the beam splitter and deflecting optics from the readhead. Thus, in one embodiment of the invention the index is provided by a grating and the analyser by a spectrally encoding light transmitting array; in a further embodiment the index is provided by a spectrally encoding light transmitting array and the analyser is provided by a grating; in yet a further embodiment both the index and analyser are provided by spectrally encoding light transmitting array; and in yet a further embodiment, not described in detail, the scale (being either transmissive or reflective) is provided by a spectrally encoding array. If the light source is provided in the form of a plurality of coherent light beams of differing wavelengths, and the scale is of diffraction quality, then no index grating is required, and a periodic light pattern will be generated simply by the interaction of the light with the scale (a plurality of beams are required because coherent light has substantially only a single wavelength and therefore a single coherent beam cannot easily be spectrally encoded into a plurality of phases). Further configurations of readhead known per se which generate a periodic light pattern upon an analyser may also incorporate a spectral encoding light transmitting array in accordance with the present invention such as, for example, "contact-type" readheads, which operate primarily upon a simple shuttering principle. In this application the term "light" does not necessarily relate to electromagnetic radiation in the visible spectrum, and includes within its scope light in the ultra violet and infra red spectra. Further, the different features of the invention as described above are not necessarily limited to their association with the embodiments in connection with which they were described. Many aspects of the invention are generally applicable to other embodiments of the invention described herein.

Claims: Opto-electronic scale reading apparatus comprising a scale (10) having a series of spaced-apart lines (10A) extending in a first direction, and spaced apart in a second direction, and a readhead, the scale and readhead being movable relative to each other in the second direction, the readhead including: means for projecting incident light onto the scale, and generating, at an analyser (18), a periodic light pattern; relative movement of the scale and readhead thereby resulting in a light intensity modulation at said analyser; the readhead further comprising phase encoding means to enable the generation of a plurality of phase-shifted electrical signals from said light intensity modulation, characterised in that said phase encoding means comprising an array (50) having a plurality of elongate spectrally encoding elements (52), extending substantially in said first direction, said elements being divided into a plurality of sets, each set of elements having a different spectral encoding characteristic, with elements of a given set having the same spectral encoding characteristic, and wherein said elements are provided in said array in a repeating pattern. Apparatus according to claim 1 wherein said scale (10) is light transmissive. Apparatus according to claim 1 wherein said scale (10) is light reflective. Apparatus according to any one of the preceding claims wherein said projecting means comprises a light source and an index grating (416), and wherein light passes from said light source onto said scale (410), and subsequently interacts with said index grating to generate said periodic light pattern at said analyser (418). Apparatus according to any one of claims 1 to 3, wherein said projecting means comprises an index grating (16) and a light source (14), and wherein light passes from said light source onto said index grating, and subsequently interacts with said scale (10) to generate said periodic light pattern at said analyser (18). Apparatus according to any one of claims 1 to 3, wherein said projecting means comprises a plurality of coherent light sources and a diffraction quality scale, wherein said light passes from the light sources, onto the scale, to generate said periodic light pattern at said analyser. Apparatus according to claim 4 or claim 5 wherein said array is one of said scale (10), said index grating (16), and said analyser (18). Apparatus according to claim 6, wherein said array is one of said scale (10) and said analyser (18). Apparatus according to claim 7 or claim 8 further comprising a plurality of spectral filters (30,32,34) provided downbeam of said analyser (18), and a plurality of photodetectors (40,42,44) to decode said spectrally encoded light into said plurality of phase-shifted electrical signals. Apparatus according to claim 9 comprising an optical fibre for transmitting light from said analyser to said plurality of spectral filters. Apparatus according to claim 10 wherein said at least one light source is the emission end of an optical fibre (30). Apparatus according to any one of claims 9 to 11 wherein said at least one light source emits polychromatic light in the visible spectrum. Apparatus according to any one of claims 9 to 11 wherein said at least one light source emits polychromatic light in the infra-red spectrum.

Inventors: HENSHAW JAMES R, HENSHAW, JAMES R.
Applicants: RENISHAW PLC
Application year: 1995
Main classification: G01D5
IPC codes: G01D5