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WO1991003712A1 - Measurement device - Google Patents

Measurement device Download PDF

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Publication number
WO1991003712A1
WO1991003712A1 PCT/GB1990/001375 GB9001375W WO9103712A1 WO 1991003712 A1 WO1991003712 A1 WO 1991003712A1 GB 9001375 W GB9001375 W GB 9001375W WO 9103712 A1 WO9103712 A1 WO 9103712A1
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WO
WIPO (PCT)
Prior art keywords
radiation
sensors
scale
grating
marks
Prior art date
Legal status (The legal status 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 status listed.)
Ceased
Application number
PCT/GB1990/001375
Other languages
French (fr)
Inventor
Mark Edward Pleydell
Richard Arthur John Hellen
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
HUMAN MACHINE INTERFACES Ltd
Original Assignee
HUMAN MACHINE INTERFACES Ltd
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
Priority claimed from GB898920082A external-priority patent/GB8920082D0/en
Priority claimed from GB909002771A external-priority patent/GB9002771D0/en
Application filed by HUMAN MACHINE INTERFACES Ltd filed Critical HUMAN MACHINE INTERFACES Ltd
Publication of WO1991003712A1 publication Critical patent/WO1991003712A1/en
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
    • G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
    • G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
    • G01D5/36—Forming the light into pulses
    • G01D5/38—Forming the light into pulses by diffraction gratings

Definitions

  • This invention relates to scale reading apparatus for use in measuring relative displacement of two members, one comprising a scale member having regularly spaced marks; the other member having an electromagnetic radiation source, at least two radiation-sensors, and a grating.
  • the invention preferably, although not exclusively, relates to apparatus, using near infra red or visible light radiation as the source.
  • phase shifted modulations in this way for the purpose of determining direction of movement and position interpolation between scale marks.
  • the method is an alternative to the one whereby phase shifted radiation modulations are derived by the cooperation between scale marks and several spatially separated coplanar gratings usually set out on a common glass substrate, each grating producing a different phase of modulated radiation.
  • Moire fringes used for position measurement are more usually formed between grating structures having a common pitch or between a first grating structure and a periodic radiation pattern of the same pitch produced (for example formed as a result of diffraction) by a second grating structure.
  • the scale marks are regarded as a grating structure in this context) .
  • the phase relationship of the radiation modulations falling on respective radiation-sensors is determined by the positions of the radiation-sensors in relation to the moire fringe pattern formed as a result of the aforesaid cooperation between grating and scale marks.
  • the present invention seeks to provide scale reading apparatus comprising an arrangement of radiation source, radiation-sensors and grating so configured to have good tolerance to angular misalignment between scale reader and scale member, particularly those angular misalignments known in the art as yaw and roll, in respect of the phase relationship of the radiation modulations falling on respective radiation-sensors.
  • a scale reading apparatus comprising a scale member having regularly spaced marks, a radiation source of electromagnetic radiation, at least two radiation-sensors, a grating having lines; the marks, when radiation is incident upon them producing a periodic radiation pattern, which cooperates with the grating to form moire fringes, the radiation-sensors being positioned to receive radiation from the radiation source, the radiation received by the radiation-sehsors being modulated by the moire fringes wherein the radiation modulations falling on the respective radiation-sensors occur in phase shifted relationship, characterised in that the alignment of the radiation-sensors, the alignment of grating lines and the alignment of the periodic radiation pattern are neither parallel nor perpendicular one to another.
  • scale reading apparatus comprising a scale member having regularly spaced marks a radiation source of electromagnetic radiation at least two radiation-sensors a grating having lines; the marks, when radiation is incident upon then, producing a periodic radiation pattern which cooperates with the grating to form moire fringes the radiation-sensors being positioned to receive radiation from the radiation source, the radiation received by the radiation-sensors being modulated by the moire fringes wherein the radiation modulations falling on the respective radiation-sensors occur in phase shifted relationship, characterised in that the alignment of the radiation-sensors is substantially perpendicular to the alignment of the bands in the periodic radiation pattern and the alignment of the grating lines is neither perpendicular nor parallel to the alignment of the radiation sensors or the alignment of the bands in the periodic radiation pattern.
  • scale reading apparatus comprising- a scale member having regularly spaced marks a radiation source at least two radiation-sensors a grating having lines positioned with its plane parallel with the plane of the marks the grating and marks cooperating to form moire fringes the radiation-sensors being positioned to receive radiation from the radiation source the radiation received by the radiation-sensors being modulated by the moire fringes wherein the radiation modulations falling on the respective radiation-sensors occur in phase shifted relationship, characterised in that the alignment of the radiation-sensors, the alignment of grating lines and the alignment of the scale marks is neither parallel nor perpendicular one to another.
  • scale reading apparatus comprising a scale member having regularly spaced marks a radiation source at least two radiation-sensors a grating having lines positioned with its plane parallel with the plane of the marks the grating and marks cooperating to form moire fringes the radiation-sensors being positioned to receive radiation from the radiation source the radiation received by the radiation-sensors being modulated by the moire fringes wherein the radiation modulations falling on the respective radiation-sensors occur in phase shifted relationship, characterised in that the alignment of the radiation-sensors is substantially perpendicular to the alignment of the scale marks and the alignment of the grating lines is neither perpendicular nor parallel to the alignment of the radiation sensors or the alignment of the scale marks.
  • the periodic radiation pattern referred to herein has grating-like structure, i.e. in the case of visible radiation, equally spaced alternately light and dark striped regions or bands, and the alignment of the periodic light pattern is defined by the direction of the stripes or bands in the pattern. In the case of other forms of electromagnetic radiation a similar banded structure is formed, due to the periodic variation in the amplitude of the radiation in the periodic radiation pattern. Alignments referred to herein define directions of straight lines lying parallel with said marks, periodic radiation pattern (i.e. parallel to the bands or stripes) or grating lines respectively, and also the direction of a straight line in which the radiation-sensors lie.
  • a scale reading apparatus comprising a scale member having regularly spaced marks, a radiation source, at least two radiation-sensors, a grating, having lines; the marks, when radiation is incident upon them, producing a periodic radiation pattern which cooperates with the grating to form moire fringes, the radiation-sensors being positioned to receive radiation from the radiation source, the radiation received by the radiation-sensors being modulated by the moire fringes wherein radiation modulations falling on the respective radiation-sensors, occur in phase shifted relationship, characterised in that the radiation-sensors and radiation source have a relative disposition such that the said phase shifted relationship varies with small variation in relative angular position from parallelism, between the grating and a plane containing the scale marks.
  • the radiation-sensors lie in a line which is, in plan view, either substantially perpendicular to the grating lines, this configuration conferring stability against relative yaw between the scale and grating members, or parallel to the grating lines but offset from the source in the X direction defined below.
  • This principle may also be used in other configurations of the radiation-sensors.
  • the geometry or relative disposition of the radiation-sensors and radiation source is arranged to favour different radiation paths between radiation source and radiation-sensors whereby periodic radiation patterns are formed at the grating in the neighbourhood of respective radiations-sensors; wherein relative shifts occur between respective periodic radiation patterns and the grating lines, with relative angular displacement, from parallelism, between the grating and the plane of the scale marks, about an axis which is substantially parallel with the axis, X; the axis X being parallel with the conventional roll axis of the scale reader.
  • suitable geometry different amounts of phase shift, including opposite phase shift, of the periodic radiation patterns are obtained in the neighbourhoods of different radiation-sensors.
  • the respective periodic radiation patterns cooperate with the grating lines to form moire fringes which modulate radiation falling on respective radiation-sensors in phase shifted relationship.
  • either the radiation may be reflected by the scale marks back to the grating forming a periodic radiation pattern which cooperates with the grating to form moire fringes thereby modulating radiation in the neighbourhood of the radiation-sensors, or the radiation may be transmitted through the scale between the scale marks thereby forming a periodic radiation pattern at an additional grating forming a periodic radiation pattern which cooperates with the additional grating to form moire fringes thereby modulating radiation in the neighbourhood of radiation-sensors positioned on the side of the additional grating opposite from the scale.
  • Figure 1 shows the general arrangement of scale member, grating, photo-sensors and light source in plan view.
  • Figure 2 shows an arrangement of photo-sensors and light source suitable for use with two photo-sensors.
  • Figure 3 shows an alternative arrangement of photo-sensors and light source for use with two photo-sensors.
  • Figure 4 shows a preferred embodiment of the invention viewed edge on to the grating.
  • Figure 5 shows an arrangement including three photo-sensors in which the alignment of the photo-sensors is defined by a slit aperture placed above three photo-sensitive regions.
  • Figure 6 shows a section through an arrangement adapted for use with a transmitting scale.
  • Figure 7 shows a graph of PM against angle A.
  • Figure 8 shows a general view of a further embodiment, wherein light paths passing from a point S, in a grating 12, are reflected by scale marks 11 at two points A and B in scale 10.
  • Figure 9 shows a plan view of a geometrical arrangement of light source and two photo-sensors.
  • Figure 10 shows a plan view of a geometrical arrangement of light source and three photo-sensors.
  • Figure 11 shows a schematic mechanism for effecting small variation in relative angular position between the grating and a plane containing the scale marks.
  • Figure 12 shows a plan view of an embodiment of the invention in which the geometrical arrangement of a light source and photo-sensors lies on a straight line.
  • Figure 13 shows loci of ray intercepts with the plane of the scale lines.
  • a plan view means a view along the direction of the Z-axis).
  • a scale member, 10, is shown having scale marks, 11, equally spaced at a pitch P, extending the full length of the scale.
  • a grating, 12, has lines, 12G, which lie at an angle.
  • A to the scale marks, 11, thereby producing moire fringes 15 (shown schematically).
  • the electromagnetic radiation source is provided by a light source, 13 (although other types of electromagnetic radiation such as near infrared radiation are contemplated for use as a source).
  • the photo-sensors, 14, have equal separation or pitch, L, and are aligned with their centres lying on a straight line 14D.
  • the line 14D makes an angle, B, with the normal, 12N, to the grating lines 12G such that the relative phase, PM, (expressed in degrees) between light modulations, produced by the moire fringes, 15, detected by any adjacent photo-sensor pair, is given substantially by the equation:
  • PM is usually required to be 90 degrees. It has been found that by introducing the angles A and B between the eleiuents as stated above, a position of stability in the phfase, PM, can be obtained with variations in the angle A. Phase stability implies an improved tolerance to variations in the angle A. In practice this provides a relaxation of the "yaw" angle setting tolerance of the scale reading apparatus in relation to the scale.
  • the light source, 13, and photo-sensors, 14, all situated on one side of a plane 12A containing the grating, 12, and with the scale member, 10, lying on the side of the grating 12 remote from both the light source, 13, and photo-sensors, 14, in order that light from the light source passes first through the grating to impinge upon the scale marks, 11, whereupon the light is diffracted by the scale marks, 11, and at the same time the light is reflected back towards the grating, forming a periodic light pattern IIP in the form of fringes, 16, in the plane, 12A, of the grating lines 12G, the light thereafter passing back through the grating to fall upon the photo-sensors 14.
  • the pitch of the fringes, IIP, 16, so formed is substantially equal to the pitch, P, of -li ⁇ the scale marks, 11.
  • the pitch of the grating lines in this case is also equal to the pitch, P, of the scale
  • Equation 4 differs from equation 1 as a consequence of the geometry of the preferred embodiment which uses the stand-off mode of operation.
  • Figures 2 and 3 show photo-sensor and light source geometries suitable when the phase between light modulations is detected using two photo-sensors, and especially when the light modulations are produced according to the preferred embodiment herein which uses the stand-off mode.
  • the phase difference, PM, between adjacent photo-sensors deviates from an optimal value of 90 degrees when the assembly of source, photo-sensors and grating taken as a whole is yawed with respect to the scale as shown in Figure - 7.
  • This figure shows the benefit of the current -12- invention, because the phase difference between adjacent photo-sensors decreases only slowly from its optimum value of 90 degrees of yaw of the above mentioned assembly about the optimum alignment.
  • a deviation from optimum alignment of this assembly of plus or minus 0.6 degrees results in the phase difference between adjacent photo-sensors decreasing from 90 degrees to 50 degrees, 26.
  • a more acceptable limit of plus or minus 0.2 degrees reduces this phase difference to 85 degrees, 27.
  • FIG. 5 Another embodiment of this invention shown in figure 5, provides a slit aperture, 17, through which the light passes before being incident upon the photo-sensors, 14, thereby defining an alignment of the photo-sensors, 14.
  • the presence of such a slit removes any need for accurate alignment of the photo-sensors; and the width of the slit may be chosen to suit a particular geometry of light source and photo-sensors in order to optimize performance.
  • the width of the slit effectively controls the width of the photo-sensors, whose width should be not more than about half the pitch of the moire fringes (15) in the region of phase stability if signal loss is to be avoided.
  • Figure 6 shows, in section, a further embodiment adapted for use with a transmitting scale, 10T, wherein the scale marks are defined by periodically arranged alternate transmitting, 11T, and opaque, UN, strips.
  • moire fringes are formed by the interaction between periodic structures, including periodic light patterns produced thereof of different pitches, there exist expressions equivalent to equations 1 to 5 above, wherein similar effects are obtained.
  • phase quadrature signals i.e. signals of sine/cosine relationship
  • four or more photo-sensors having substantially equal pitch, and lying in the a line 14D, are included within the scope of this invention.
  • Equations (1) and (4) described above can be shown to be special cases of a more general equation, given as:
  • phase difference, PM 360*2*L*SIN(A/Nl*SIN(A/ . (8)
  • N has a value between and including one and two.
  • FIGS 8 to 13 show another embodiment of the optoelectronic scale reading apparatus according to the present invention.
  • a light source, 13, and two photo-sensors, 14, are shown situated on one side of a plane grating, 12, (for example of the Ronchi type).
  • the light from the source is incident upon the grating thereby forming a periodic pattern in light being transmitted by the grating.
  • the periodic light pattern thus formed interacts with scale marks, 11, in a manner known to produce reinforced diffraction fringes in a further plane.
  • the plane of the scale marks, 11, is nominally parallel with the plane of the grating lines, 12G.
  • the light, as well as being diffracted by the scale is shown to be reflected back towards the grating, 12, whereby the diffraction fringes are formed substantially in coincidence with the plane of the grating lines 12G.
  • the light rays RIO, R31 and R20, R41 may be regarded as central rays of two respective ray bundles passing between light source, 13, and two photo-sensors, 14, via the scale, 10. Rays Rll, R32 and R21, R42 may be similarly regarded.
  • the points C and D, at which the rays R31 and R41 intercept the plane of the grating marks, 12G, are considered to remain fixed in relation to the grating marks, 12G .
  • the position of the diffraction fringes, formed in the plane of the grating lines, 12G, in the neighbourhoods of the points C and D respectively, are fixed with respect to the points of interception of the respective rays, R31 and R41 with the plane of the grating lines, 12G.
  • Figure 13 gives an indication of the loci, 30X and 4OX, of the intercepts, E and F respectively, of the respective rays, Rll and R21, with the plane of the scale marks, 11, when the scale is tilted about an axis, 11X, relative to the grating, 12, either side of their position of parallelism.
  • the positions of fringes formed in the plane of the grating lines, 12G are fixed by the paths of rays RIO, R20, R31 and R41.
  • the displacement of the intercept point A to point E resulting from a small rotation of the scale plane about the axis 11X is accompanied by a change in the paths of rays RIO and R31 to ray paths Rll and R32 respectively and a corresponding displacement in the negative X direction of those fringes located in the neighbourhood of point C.
  • a concomitant displacement of ray intercept point B to point F in the plane of the scale marks, accompanied by a corresponding change in ray paths R20 and R41 to ray paths R21 and R42 respectively, resulting from the same rotation about axis llX is accompanied by a displacement of those fringes located in the neighbourhood of point D in the positive X direction.
  • the photo-sensors 14C and 14D in geometrical relationship with the light source, 13, and grating lines, 12G, define the two different light paths which produce distinct sets of fringes at two different locations in the plane of the grating lines; the two sets of fringes being displaced in opposite directions along the X direction, by rotating the scale plane about the axis 11X.
  • phase-quadrature signals In consequence, light received by photo-sensor. 14C, is modulated by moire fringes formed between one set of diffraction fringes and grating lines, at one location; whilst light received by photo-sensor, 14D, is modulated by moire fringes formed between a different set of diffraction fringes and grating lines at another location.
  • the phases of light modulations received by respective photo-sensors, 14C and 14D need not necessarily be the same and may be adjusted so that difference between the phases of the said respective light modulations (Ml and M2 in Figure 1) have a given value. This might for example be 90 degrees for the purpose of obtaining phase-quadrature signals.
  • the scale marks, 11, and the grating lines, 12G are substantially parallel in plan view. This condition affords a scale reader with maximum tolerance to rotation about the Z axis (conventionally the yaw axis of a scale reader).
  • the Figures 8, 9, 10, 11 and 12 may imply that the grating marks are aligned parallel to the scale marks, it is known in the art that the creation of moire fringe patterns suitable for the present invention requires a small angular difference between the grating and the scale, the angular difference being in relation to an axis substantially normal to the plane of the grating.
  • Figure 9 shows a plan view of the preferred embodiment in which the geometrical arrangement of light. source and photo-sensor is suitable for use with two photo-sensors.
  • the geometrical relationship between light source and photo-sensors is such that, when shown in plan view, the photo-sensors have differing off-sets from the axis of relative rotation between the grating and the scale members, the axis of rotation being substantially in the X-Y plane as shown in Figure 8.
  • Figure 10 shows in plan, another embodiment in which a geometrical arrangement, in accordance with this invention, of light source and three photo-sensors is suitable for producing phase separations between the light modulations detected by three photo-sensors.
  • the geometrical relationship between light source and photo-sensors is defined in plan by an isosceles triangle, 24, with the light source positioned at the apex, 19, of the triangle and with two photo-sensors positioned at either end of the base line, 18, of the triangle, with a third photo-sensor positioned substantially at the halfway point of the base line; the grating lines being substantially perpendicular to the base line of the triangle.
  • the rotation is about the X-axis, llX, no shift of the fringes is detected by the centre photo-sensor.
  • the two photo-sensors either side of the centre one detect opposite changes in phase of modulations, produced by opposite shift of the fringes, in the X direction, in the neighbourhoods of the two photo-sensors respectively, with relative scale rotation about the X- axis.
  • This provides equal phase separation between modulations detected by adjacent detector pairs.
  • a scheme involving three or more photo-sensors may be used to advantage.
  • Figure 12 shows a further embodiment in which two photo-sensors, 14, lie on either side of a light source, 13, all of which lie in a plane which is substantially perpendicular to both the grating lines, 12G, and the plane of the grating.
  • a phase separation in light modulations between the two photo-sensors is obtained by relative scale tilt about an axis which lies in an X-Y plane.
  • This geometrical arrangement provides good tolerance to relative rotation about the axis, 11X.
  • the photo-sensor 14C lies at a radial distance of 1.25 mm from the source, 13, in the direction of the X-axis, 11X
  • a second photo-sensor, 14D lies at a radial distance of 1.25 mm from the source, 13, in the direction of the Y-axis, 11Y
  • the spacing, H, between the grating and the scale i ⁇ 10 mm, and the angle of rotation about the Y-axis, 11Y is typically 5 degrees (from parallelism between grating and scale) a relative displacement in the X direction between the fringes in the neighbourhood of the photo-sensors of approximately 5 microns is produced.
  • the grating pitch is 20 microns, such a shift of the fringes in the X direction produces a difference in phase in the light modulations, detected between the two photo-sensors, of 90 degrees.
  • the above example shows a major benefit resulting from the present invention in that the assembly containing the photo-sensors, grating and source may be significantly reduced in size below existing scale reading apparatus designs.
  • FIG. 11 represents schematically, in section, a form of such a mean ⁇ , wherein an adjustment screws, 20, 21, screw 21 being displaced normal to the line joining the two screws 20, together with springs, 22, provide a spring tensioned three-point ⁇ upport for the scale reader.
  • Whil ⁇ t the invention herein describes differential fringe displacements in the X direction at two or more photo-sensor ⁇ obtained by reflection off scale marks, a similar effect can be obtained using a transmitting scale member wherein the angular displacements of transmitted rays are by virtue of refraction of the scale member, thi ⁇ member being a refractive medium.
  • the scale marks together act to diffract light, thereby giving rise to so called diffraction fringes.
  • the scale marks may take the form of a castellated relief or alternating strips of either reflecting or refracting optical material (as the case may be) rendering an effective optical phase delay between adjacent strips of 180 degrees whereby it is known to ⁇ ignificantly improve the diffraction efficiency of such a scale form.
  • Whil ⁇ t for the most part the description of the present invention is for a device capable of measuring linear displacement of the scale, 10, in the X- direction relative to the grating, 12, it is also within the scope of the invention to be capable of measuring rotation about the X-axis, llX, of the scale relative to the grating.

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Abstract

According to a first aspect of the present invention there is provided an apparatus comprising an elongate scale member (10) having scale marks (11) equally spaced at a pitch (P) extending the full length of the scale. The grating (12) has lines (12G) which lie at an angle (A) to the scale marks (11) thereby producing moire fringes (15) when electromagnetic radiation is incident upon the grating and scale. Radiation-sensors (14) have equal separation or pitch (L) and are aligned with their centres lying on the straight line (14D). The line (14D) makes an angle (B) with the normal (12N) to the grating lines (12G) such that the relative phase between the radiation modulations (PM) produced by the moire fringes (15) detected by any adjacent pair of radiation-sensors, has a position of stability with variations in the angle (A). For any given value of phase (PM) the position of phase stability being given substantially by the equation PM = 360*2*L*sin(A/2)*sin(A/2)/P. According to a second aspect of the present invention relative shift occurs between respective periodic radiation patterns and the grating lines (12G) with relative angular displacement from parallelism, between the grating (12) and the plane of the scale marks (11).

Description

MEASUREMENT DEVICE
This invention relates to scale reading apparatus for use in measuring relative displacement of two members, one comprising a scale member having regularly spaced marks; the other member having an electromagnetic radiation source, at least two radiation-sensors, and a grating. The invention preferably, although not exclusively, relates to apparatus, using near infra red or visible light radiation as the source.
It is known, in the art, to derive phase shifted modulations in this way for the purpose of determining direction of movement and position interpolation between scale marks. It is also known that the method is an alternative to the one whereby phase shifted radiation modulations are derived by the cooperation between scale marks and several spatially separated coplanar gratings usually set out on a common glass substrate, each grating producing a different phase of modulated radiation. Moire fringes used for position measurement are more usually formed between grating structures having a common pitch or between a first grating structure and a periodic radiation pattern of the same pitch produced (for example formed as a result of diffraction) by a second grating structure. (The scale marks are regarded as a grating structure in this context) . Whilst this invention is concerned, for the most part, with moire fringes so formed, it is also within the scope of the invention to include moire fringes formed by the interaction between periodic structures, including periodic radiation patterns produced thereof, of different pitches in order to obtain a similar effect. It is known for example that suitable diffraction fringes are formed in accordance with the invention described herein when the pitch of the scale markε is half that of the pitch of the grating lines.
The phase relationship of the radiation modulations falling on respective radiation-sensors is determined by the positions of the radiation-sensors in relation to the moire fringe pattern formed as a result of the aforesaid cooperation between grating and scale marks.
Previously, radiation-sensors have been positioned so that they are aligned across the moire fringes, that iε, lying in a line which is substantially perpendicular to the line of the moire fringes thereby receiving different phases of modulation of the radiation (as known in the art and described in a book entitled "Diffraction Gratings" by M. C. Hutley). This would normally involve aligning radiation-sensors with the grating lines. Though the previous arrangement has the appeal of simplicity of design, the phase relationship of radiation modulations falling on respective radiation sensors varies rapidly with small misalignments in yaw angle between the scale marks, or the periodic radiation pattern formed thereby, and the lines on the grating. According to the present invention, it is found that by aligning the radiation-sensors at an angle other than with the grating lines, considerable improvement in tolerance to this misalignment is obtained.
The present invention seeks to provide scale reading apparatus comprising an arrangement of radiation source, radiation-sensors and grating so configured to have good tolerance to angular misalignment between scale reader and scale member, particularly those angular misalignments known in the art as yaw and roll, in respect of the phase relationship of the radiation modulations falling on respective radiation-sensors.
According to one aspect of the present invention there is provided a scale reading apparatus comprising a scale member having regularly spaced marks, a radiation source of electromagnetic radiation, at least two radiation-sensors, a grating having lines; the marks, when radiation is incident upon them producing a periodic radiation pattern, which cooperates with the grating to form moire fringes, the radiation-sensors being positioned to receive radiation from the radiation source, the radiation received by the radiation-sehsors being modulated by the moire fringes wherein the radiation modulations falling on the respective radiation-sensors occur in phase shifted relationship, characterised in that the alignment of the radiation-sensors, the alignment of grating lines and the alignment of the periodic radiation pattern are neither parallel nor perpendicular one to another.
According to a second aspect of the present invention there is provided scale reading apparatus comprising a scale member having regularly spaced marks a radiation source of electromagnetic radiation at least two radiation-sensors a grating having lines; the marks, when radiation is incident upon then, producing a periodic radiation pattern which cooperates with the grating to form moire fringes the radiation-sensors being positioned to receive radiation from the radiation source, the radiation received by the radiation-sensors being modulated by the moire fringes wherein the radiation modulations falling on the respective radiation-sensors occur in phase shifted relationship, characterised in that the alignment of the radiation-sensors is substantially perpendicular to the alignment of the bands in the periodic radiation pattern and the alignment of the grating lines is neither perpendicular nor parallel to the alignment of the radiation sensors or the alignment of the bands in the periodic radiation pattern. According to a third aspect of the present invention there is provided scale reading apparatus comprising- a scale member having regularly spaced marks a radiation source at least two radiation-sensors a grating having lines positioned with its plane parallel with the plane of the marks the grating and marks cooperating to form moire fringes the radiation-sensors being positioned to receive radiation from the radiation source the radiation received by the radiation-sensors being modulated by the moire fringes wherein the radiation modulations falling on the respective radiation-sensors occur in phase shifted relationship, characterised in that the alignment of the radiation-sensors, the alignment of grating lines and the alignment of the scale marks is neither parallel nor perpendicular one to another.
According to a fourth aspect of the present invention there is provided scale reading apparatus comprising a scale member having regularly spaced marks a radiation source at least two radiation-sensors a grating having lines positioned with its plane parallel with the plane of the marks the grating and marks cooperating to form moire fringes the radiation-sensors being positioned to receive radiation from the radiation source the radiation received by the radiation-sensors being modulated by the moire fringes wherein the radiation modulations falling on the respective radiation-sensors occur in phase shifted relationship, characterised in that the alignment of the radiation-sensors is substantially perpendicular to the alignment of the scale marks and the alignment of the grating lines is neither perpendicular nor parallel to the alignment of the radiation sensors or the alignment of the scale marks. The periodic radiation pattern referred to herein has grating-like structure, i.e. in the case of visible radiation, equally spaced alternately light and dark striped regions or bands, and the alignment of the periodic light pattern is defined by the direction of the stripes or bands in the pattern. In the case of other forms of electromagnetic radiation a similar banded structure is formed, due to the periodic variation in the amplitude of the radiation in the periodic radiation pattern. Alignments referred to herein define directions of straight lines lying parallel with said marks, periodic radiation pattern (i.e. parallel to the bands or stripes) or grating lines respectively, and also the direction of a straight line in which the radiation-sensors lie. According to a fifth aspect of the present invention there is provided a scale reading apparatus comprising a scale member having regularly spaced marks, a radiation source, at least two radiation-sensors, a grating, having lines; the marks, when radiation is incident upon them, producing a periodic radiation pattern which cooperates with the grating to form moire fringes, the radiation-sensors being positioned to receive radiation from the radiation source, the radiation received by the radiation-sensors being modulated by the moire fringes wherein radiation modulations falling on the respective radiation-sensors, occur in phase shifted relationship, characterised in that the radiation-sensors and radiation source have a relative disposition such that the said phase shifted relationship varies with small variation in relative angular position from parallelism, between the grating and a plane containing the scale marks.
According to the fifth aspect of the present invention, the radiation-sensors lie in a line which is, in plan view, either substantially perpendicular to the grating lines, this configuration conferring stability against relative yaw between the scale and grating members, or parallel to the grating lines but offset from the source in the X direction defined below. This principle may also be used in other configurations of the radiation-sensors. The geometry or relative disposition of the radiation-sensors and radiation source is arranged to favour different radiation paths between radiation source and radiation-sensors whereby periodic radiation patterns are formed at the grating in the neighbourhood of respective radiations-sensors; wherein relative shifts occur between respective periodic radiation patterns and the grating lines, with relative angular displacement, from parallelism, between the grating and the plane of the scale marks, about an axis which is substantially parallel with the axis, X; the axis X being parallel with the conventional roll axis of the scale reader. With suitable geometry, different amounts of phase shift, including opposite phase shift, of the periodic radiation patterns are obtained in the neighbourhoods of different radiation-sensors. The respective periodic radiation patterns cooperate with the grating lines to form moire fringes which modulate radiation falling on respective radiation-sensors in phase shifted relationship.
Thus, according to the fifth aspect of the present invention, either the radiation may be reflected by the scale marks back to the grating forming a periodic radiation pattern which cooperates with the grating to form moire fringes thereby modulating radiation in the neighbourhood of the radiation-sensors, or the radiation may be transmitted through the scale between the scale marks thereby forming a periodic radiation pattern at an additional grating forming a periodic radiation pattern which cooperates with the additional grating to form moire fringes thereby modulating radiation in the neighbourhood of radiation-sensors positioned on the side of the additional grating opposite from the scale.
For a better understanding of the present invention and to show how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which:
Figure 1 shows the general arrangement of scale member, grating, photo-sensors and light source in plan view.
Figure 2 shows an arrangement of photo-sensors and light source suitable for use with two photo-sensors.
Figure 3 shows an alternative arrangement of photo-sensors and light source for use with two photo-sensors.
Figure 4 shows a preferred embodiment of the invention viewed edge on to the grating.
Figure 5 shows an arrangement including three photo-sensors in which the alignment of the photo-sensors is defined by a slit aperture placed above three photo-sensitive regions.
Figure 6 shows a section through an arrangement adapted for use with a transmitting scale.
Figure 7 shows a graph of PM against angle A. Figure 8 shows a general view of a further embodiment, wherein light paths passing from a point S, in a grating 12, are reflected by scale marks 11 at two points A and B in scale 10.
Figure 9 shows a plan view of a geometrical arrangement of light source and two photo-sensors.
Figure 10 shows a plan view of a geometrical arrangement of light source and three photo-sensors.
Figure 11 shows a schematic mechanism for effecting small variation in relative angular position between the grating and a plane containing the scale marks. Figure 12 shows a plan view of an embodiment of the invention in which the geometrical arrangement of a light source and photo-sensors lies on a straight line. Figure 13 shows loci of ray intercepts with the plane of the scale lines.
(In this specification, a plan view means a view along the direction of the Z-axis).
Referring to Figure 1, a scale member, 10, is shown having scale marks, 11, equally spaced at a pitch P, extending the full length of the scale. A grating, 12, has lines, 12G, which lie at an angle. A, to the scale marks, 11, thereby producing moire fringes 15 (shown schematically). In this embodiment, the electromagnetic radiation source is provided by a light source, 13 (although other types of electromagnetic radiation such as near infrared radiation are contemplated for use as a source). The photo-sensors, 14, have equal separation or pitch, L, and are aligned with their centres lying on a straight line 14D. The line 14D makes an angle, B, with the normal, 12N, to the grating lines 12G such that the relative phase, PM, (expressed in degrees) between light modulations, produced by the moire fringes, 15, detected by any adjacent photo-sensor pair, is given substantially by the equation:
PM = 360*2*L*SIN.B-A/2.*SIN.A/2) (1)
P
In scale reading apparatus of the moire fringe type, PM is usually required to be 90 degrees. It has been found that by introducing the angles A and B between the eleiuents as stated above, a position of stability in the phfase, PM, can be obtained with variations in the angle A. Phase stability implies an improved tolerance to variations in the angle A. In practice this provides a relaxation of the "yaw" angle setting tolerance of the scale reading apparatus in relation to the scale.
The position of phase stability with respect to variations in the yaw angle. A, is described mathematically by differentiating PM in equation (1) with respect to A and equating this to zero. This corresponds to a maximum in the relative phase, PM, between light modulations detected by adjacent pairs of photo-sensors and results in the expression:
A = B (2) The relative phase, PM, between the light modulations at any two adjacent photo-sensors when the apparatus is configured to meet the conditions for phase stability with respect to yaw is therefore found by substituting this result in equation (1) whence: PM = 360*2*L*SIN(A/2.*SIN(A/2 ) (3)
P
It will be seen by reference to figure 1 that as A is equal to B, then the alignment 14D of the photo-sensors is exactly perpendicular to the scale marks 11. For example, in scale reading apparatus according to this invention whose photo-sensors are pitched at 10 mm. (L=10 mm. ) which reads a scale having marks pitched at 20 microns, (P=0.02 mm.) a maximum stability of the phase, PM, occurs with PM chosen to be close to 90 degrees when both A and B are 1.8 degrees. B is an angle which is defined by components which are fixed within the apparatus. In this case the range of angles. A, over which the phase may be acceptably stable is found to be about 1.5 degrees. This is a significant advantage over previous scale reading devices of the moire type referred to above where a corresponding acceptable variation in angle A is about 0.024 degrees for the same values of L, P and A used above. In some circumstances it is advantageous to design a scale reader having a maximum relative phase difference, PM, given by equations 3 or 5, somewhat in excess of 90 degrees (e.g. 120 degrees). This requires correspondingly larger values. A' and B', of the angles A and B. In this case a relative phase difference, PM, of exactly 90 degrees can be obtained when the angle between the scale marks and the grating lines. A', is made slightly less than the value of A given by equations 3 or 5. When such adjustments are made an improvement in signal level is obtained at the expense of some loss of phase stability.
It should be noted that the equations 1, 2, and 3 described above are appropriate for the apparatus configuration wherein the plane of the grating marks is in direct contact with the plane of the scale marks; this is defined here as the contact mode. An alternative apparatus configuration is one wherein there is a separation or "stand-off" between the plane of the grating marks and the plane of the scale marks; this is defined here as the stand-off mode. In a preferred embodiment of the invention which uses the stand-off mode of operation, shown viewed from the side in figure 4, the light source, 13, and photo-sensors, 14, all situated on one side of a plane 12A containing the grating, 12, and with the scale member, 10, lying on the side of the grating 12 remote from both the light source, 13, and photo-sensors, 14, in order that light from the light source passes first through the grating to impinge upon the scale marks, 11, whereupon the light is diffracted by the scale marks, 11, and at the same time the light is reflected back towards the grating, forming a periodic light pattern IIP in the form of fringes, 16, in the plane, 12A, of the grating lines 12G, the light thereafter passing back through the grating to fall upon the photo-sensors 14. The pitch of the fringes, IIP, 16, so formed is substantially equal to the pitch, P, of -li¬ the scale marks, 11. The pitch of the grating lines in this case is also equal to the pitch, P, of the scale marks.
In the preferred embodiment the angle B, the angle A, the pitch of the scale marks P, and the pitch of the photo-sensors, L, are related to the relative phase between light modulations, PM, produced by the moire fringes, 15, detected by any adjacent photo-sensor pair, substantially by the equation: PM = 360*2*L*SIN.B-A)*SIN.A1 (4)
P
The apparatus configuration conferring phase stability with respect to yaw in this case is found when B is equal to twice A; whence the phase difference, PM, between the light modulations at any two adjacent photo-sensor pair is given by the equation:
PM = 360*2*L*SINfAΪ*SIN.AΪ (5)
P By rearranging this equation, A can be found in terms of the known parameters: PM, L, and P. B is then obtained by doubling A. Equation 4 differs from equation 1 as a consequence of the geometry of the preferred embodiment which uses the stand-off mode of operation.
Figures 2 and 3 show photo-sensor and light source geometries suitable when the phase between light modulations is detected using two photo-sensors, and especially when the light modulations are produced according to the preferred embodiment herein which uses the stand-off mode.
In this preferred embodiment, the phase difference, PM, between adjacent photo-sensors deviates from an optimal value of 90 degrees when the assembly of source, photo-sensors and grating taken as a whole is yawed with respect to the scale as shown in Figure - 7. This figure shows the benefit of the current -12- invention, because the phase difference between adjacent photo-sensors decreases only slowly from its optimum value of 90 degrees of yaw of the above mentioned assembly about the optimum alignment. A deviation from optimum alignment of this assembly of plus or minus 0.6 degrees results in the phase difference between adjacent photo-sensors decreasing from 90 degrees to 50 degrees, 26. A more acceptable limit of plus or minus 0.2 degrees reduces this phase difference to 85 degrees, 27.
Another embodiment of this invention shown in figure 5, provides a slit aperture, 17, through which the light passes before being incident upon the photo-sensors, 14, thereby defining an alignment of the photo-sensors, 14. The presence of such a slit removes any need for accurate alignment of the photo-sensors; and the width of the slit may be chosen to suit a particular geometry of light source and photo-sensors in order to optimize performance. The width of the slit effectively controls the width of the photo-sensors, whose width should be not more than about half the pitch of the moire fringes (15) in the region of phase stability if signal loss is to be avoided. Figure 6 shows, in section, a further embodiment adapted for use with a transmitting scale, 10T, wherein the scale marks are defined by periodically arranged alternate transmitting, 11T, and opaque, UN, strips. When moire fringes are formed by the interaction between periodic structures, including periodic light patterns produced thereof of different pitches, there exist expressions equivalent to equations 1 to 5 above, wherein similar effects are obtained.
It is known in the art that phase quadrature signals i.e. signals of sine/cosine relationship, are conveniently obtained when four or more photo-sensors are employed in scale reading devices. It is intended that, in accordance with the present invention, four or more photo-sensors, having substantially equal pitch, and lying in the a line 14D, are included within the scope of this invention.
Equations (1) and (4) described above can be shown to be special cases of a more general equation, given as:
PM = 360*2*L*SIN(B-A/N)*SIN.A/N. (6) P where the apparatus geometry can be arranged so that N has a value between and including two down to and including one.
The position of phase stability with respect to yaw in this case is found when:
B = 2^A (7)
N
The resulting phase difference, PM, between any two adjacent pair of photo-sensors is given by: PM = 360*2*L*SIN(A/Nl*SIN(A/ . (8)
P where N has a value between and including one and two.
Figures 8 to 13 show another embodiment of the optoelectronic scale reading apparatus according to the present invention.
Referring to Figure 8, a light source, 13, and two photo-sensors, 14, are shown situated on one side of a plane grating, 12, (for example of the Ronchi type). The light from the source is incident upon the grating thereby forming a periodic pattern in light being transmitted by the grating. The periodic light pattern thus formed, interacts with scale marks, 11, in a manner known to produce reinforced diffraction fringes in a further plane. The plane of the scale marks, 11, is nominally parallel with the plane of the grating lines, 12G. In Figure 8, the light, as well as being diffracted by the scale is shown to be reflected back towards the grating, 12, whereby the diffraction fringes are formed substantially in coincidence with the plane of the grating lines 12G.
More specifically, light rays RIO and R20 departing from a point S in the plane of the grating lines, 12G, are reflected by the scale at the points A and B respectively and continue as light rays R31 and R41 respectively whereafter they contribute to the formation of diffraction fringes at points C and D respectively in the plane of the grating lines, 12G. For the sake of clarity of explanation, the different diffracted light paths are not shown. The light rays RIO, R31 and R20, R41 may be regarded as central rays of two respective ray bundles passing between light source, 13, and two photo-sensors, 14, via the scale, 10. Rays Rll, R32 and R21, R42 may be similarly regarded.
For purposes of explanation, the points C and D, at which the rays R31 and R41 intercept the plane of the grating marks, 12G, are considered to remain fixed in relation to the grating marks, 12G . In consequence, it will be seen that the position of the diffraction fringes, formed in the plane of the grating lines, 12G, in the neighbourhoods of the points C and D respectively, are fixed with respect to the points of interception of the respective rays, R31 and R41 with the plane of the grating lines, 12G.
Consider now the plane of the scale, 10, being rotated by a small amount (one degree, say) about an axis which is parallel to the X-axis in Figure 8, relative to the plane of the grating, 12. For convenience, consider a relative angular displacement of the scale about an axis, 11X, containing the points A and B. Whilst in this embodiment, the rotation axis is shown parallel to the X axis, this is not a prerequisite for effective operation of the apparatus and the axis 11X may have other orientations in the X-Y plane.
In consequence of the said relative angular displacement the rays RIO and R20 which were incident at the scale marks at points A and B respectively, will no longer be reflected back into the photo-sensors along the optimum paths. Instead, new rays Rll and R21, incident scale marks at points E and F respectively, will be reflected along the new optimum paths R32, R42 to strike the grating lines 12G at points C and D respectively.
It can be shown that the new intercepts, E and F, of the rays Rll and R21 respectively, with the plane of the scale marks, 11, move along loci 3OX and 4OX respectively as the scale, 10, is rotated about an axis, 11X, and are displaced from their original positions A and B respectively with components not only along the Y direction but also with components, X30, X40, respectively, along the X direction. The magnitude and direction of such a component of displacement in the X direction, is dependent upon the geometrical relationship of the light source and photo-sensor with the grating lines. For example, if a light source and a photo-sensor lie in a plane parallel with the grating lines but substantially perpendicular to the plane of the grating, then no component of displacement is observed in the X direction. A small component of displacement, only, is observed in the X direction, when a light source and a photo-sensor lie in a plane which is substantially perpendicular to both the grating lines and the plane of the grating. This may be useful in circumstances where a good degree of insensitivity to rotation about the axis 11X is desirable. Generally, a useful amount of displacement in the X direction is obtained, with . rotation of the scale plane about the axis llX, when the projection on the grating plane of the line joining a light source to a photo-sensor is finite and is neither perpendicular to, nor parallel with the grating lines. Figure 13 gives an indication of the loci, 30X and 4OX, of the intercepts, E and F respectively, of the respective rays, Rll and R21, with the plane of the scale marks, 11, when the scale is tilted about an axis, 11X, relative to the grating, 12, either side of their position of parallelism.
As mentioned above, the positions of fringes formed in the plane of the grating lines, 12G, are fixed by the paths of rays RIO, R20, R31 and R41. The displacement of the intercept point A to point E resulting from a small rotation of the scale plane about the axis 11X is accompanied by a change in the paths of rays RIO and R31 to ray paths Rll and R32 respectively and a corresponding displacement in the negative X direction of those fringes located in the neighbourhood of point C. A concomitant displacement of ray intercept point B to point F in the plane of the scale marks, accompanied by a corresponding change in ray paths R20 and R41 to ray paths R21 and R42 respectively, resulting from the same rotation about axis llX is accompanied by a displacement of those fringes located in the neighbourhood of point D in the positive X direction.
The photo-sensors 14C and 14D, in geometrical relationship with the light source, 13, and grating lines, 12G, define the two different light paths which produce distinct sets of fringes at two different locations in the plane of the grating lines; the two sets of fringes being displaced in opposite directions along the X direction, by rotating the scale plane about the axis 11X.
In consequence, light received by photo-sensor. 14C, is modulated by moire fringes formed between one set of diffraction fringes and grating lines, at one location; whilst light received by photo-sensor, 14D, is modulated by moire fringes formed between a different set of diffraction fringes and grating lines at another location. The phases of light modulations received by respective photo-sensors, 14C and 14D, need not necessarily be the same and may be adjusted so that difference between the phases of the said respective light modulations (Ml and M2 in Figure 1) have a given value. This might for example be 90 degrees for the purpose of obtaining phase-quadrature signals.
In the preferred embodiment of the invention described herein, the scale marks, 11, and the grating lines, 12G, are substantially parallel in plan view. This condition affords a scale reader with maximum tolerance to rotation about the Z axis (conventionally the yaw axis of a scale reader). However, it should be noted that, although the Figures 8, 9, 10, 11 and 12 may imply that the grating marks are aligned parallel to the scale marks, it is known in the art that the creation of moire fringe patterns suitable for the present invention requires a small angular difference between the grating and the scale, the angular difference being in relation to an axis substantially normal to the plane of the grating.
By way of an example and referring to Figure 8, when L is 6 mm and W is 2mm, and spacing, H, between grating and scale is 10 mm, then with 0.5 degrees of rotation of the scale about the axis 11X, a relative displacement in the X direction between the fringes, in the neighbourhood of the photo-sensors, of approximately 5 microns is produced. If the grating pitch is 20 microns, such a shift of the fringes in the X direction produces a difference in phase in the light modulations, detected between the two photo-sensors, of 90 degrees.
Figure 9 shows a plan view of the preferred embodiment in which the geometrical arrangement of light. source and photo-sensor is suitable for use with two photo-sensors. The geometrical relationship between light source and photo-sensors is such that, when shown in plan view, the photo-sensors have differing off-sets from the axis of relative rotation between the grating and the scale members, the axis of rotation being substantially in the X-Y plane as shown in Figure 8.
Figure 10 shows in plan, another embodiment in which a geometrical arrangement, in accordance with this invention, of light source and three photo-sensors is suitable for producing phase separations between the light modulations detected by three photo-sensors. The geometrical relationship between light source and photo-sensors is defined in plan by an isosceles triangle, 24, with the light source positioned at the apex, 19, of the triangle and with two photo-sensors positioned at either end of the base line, 18, of the triangle, with a third photo-sensor positioned substantially at the halfway point of the base line; the grating lines being substantially perpendicular to the base line of the triangle. In this case where the rotation is about the X-axis, llX, no shift of the fringes is detected by the centre photo-sensor. The two photo-sensors either side of the centre one detect opposite changes in phase of modulations, produced by opposite shift of the fringes, in the X direction, in the neighbourhoods of the two photo-sensors respectively, with relative scale rotation about the X- axis. This provides equal phase separation between modulations detected by adjacent detector pairs. A scheme involving three or more photo-sensors may be used to advantage. Figure 12 shows a further embodiment in which two photo-sensors, 14, lie on either side of a light source, 13, all of which lie in a plane which is substantially perpendicular to both the grating lines, 12G, and the plane of the grating. A phase separation in light modulations between the two photo-sensors is obtained by relative scale tilt about an axis which lies in an X-Y plane. This geometrical arrangement provides good tolerance to relative rotation about the axis, 11X. By way of an example, if the photo-sensor 14C lies at a radial distance of 1.25 mm from the source, 13, in the direction of the X-axis, 11X, and if a second photo-sensor, 14D, lies at a radial distance of 1.25 mm from the source, 13, in the direction of the Y-axis, 11Y, and the spacing, H, between the grating and the scale iε 10 mm, and the angle of rotation about the Y-axis, 11Y, is typically 5 degrees (from parallelism between grating and scale) a relative displacement in the X direction between the fringes in the neighbourhood of the photo-sensors of approximately 5 microns is produced. If the grating pitch is 20 microns, such a shift of the fringes in the X direction produces a difference in phase in the light modulations, detected between the two photo-sensors, of 90 degrees.
The above example shows a major benefit resulting from the present invention in that the assembly containing the photo-sensors, grating and source may be significantly reduced in size below existing scale reading apparatus designs.
Provided within the scope of this invention is means for effecting small variation in relative angular position between the grating and a plane containing the scale marks, from nominal parallelism, about an axis lying in an X-Y plane as shown in Figure 8, for the purpose of obtaining a desirable phase relationship of the light modulations detected by two or more photo-sensorε. Figure 11 represents schematically, in section, a form of such a meanε, wherein an adjustment screws, 20, 21, screw 21 being displaced normal to the line joining the two screws 20, together with springs, 22, provide a spring tensioned three-point εupport for the scale reader.
Whilεt the invention herein describes differential fringe displacements in the X direction at two or more photo-sensorε obtained by reflection off scale marks, a similar effect can be obtained using a transmitting scale member wherein the angular displacements of transmitted rays are by virtue of refraction of the scale member, thiε member being a refractive medium. In the preferred embodiment deεcribed herein, the scale marks together act to diffract light, thereby giving rise to so called diffraction fringes. The scale marks may take the form of a castellated relief or alternating strips of either reflecting or refracting optical material (as the case may be) rendering an effective optical phase delay between adjacent strips of 180 degrees whereby it is known to εignificantly improve the diffraction efficiency of such a scale form. Whilεt for the most part the description of the present invention is for a device capable of measuring linear displacement of the scale, 10, in the X- direction relative to the grating, 12, it is also within the scope of the invention to be capable of measuring rotation about the X-axis, llX, of the scale relative to the grating.

Claims

1. Scale reading apparatus comprising a scale member (10) having regularly spaced marks (11), a radiation source (13), at least two radiation-sensors (14), a grating (12) having lines (12G); the marks, when radiation is incident upon them producing a periodic radiation pattern (IIP), which cooperates with the grating to form moire fringes (15) , the radiation-senεors being positioned to receive radiation from the radiation source, the radiation received by the radiation-sensors being modulated by the moire fringes wherein the radiation modulations falling on the respective radiation-sensors occur in phase shifted relationship, characterised in that the alignment (14D) of the radiation-sensors, the alignment of grating lines (12G) and the alignment of the periodic radiation pattern are neither parallel nor perpendicular one to another.
2. Scale reading apparatus comprising a scale member (10) having regularly spaced marks (11), a radiation source (13) , at least two radiation-sensors (14), a grating (12) having lines (12G); the marks, when radiation is incident upon them, producing a periodic radiation pattern (IIP), which cooperates with the grating to form moire fringes (15), the radiation-sensors being positioned to receive radiation from the radiation source, the radiation received by the radiation-sensors being modulated by the moire fringes wherein the radiation modulations falling on the respective radiation-sensors occur in phase shifted relationship, characterised in that the alignment of the radiation-sensors (14D) is substantially perpendicular to the alignment of the bands in the periodic radiation pattern (IIP), and the alignment of the grating lines (12G) is neither perpendicular nor parallel to the alignment of the radiation sensors or the alignment of the bands in the periodic radiation pattern.
3. Scale reading apparatus comprising a scale member (10) having regularly spaced marks (11), a radiation source (13), at least two radiation-sensors (14), a grating (12) having lines (12G) positioned with its plane parallel with the plane of the marks (11), the grating (12) and markε (11) cooperating to form moire fringes (15) , the radiation-sensors (14) being positioned to receive radiation from the radiation source (13); the radiation received by the radiation-sensorε (14) being modulated by the moire fringes (15) wherein the radiation modulations falling on the respective radiation-εenεorε (14) occur in phase shifted relationship, characterised in that the alignment (14D) of the radiation-sensors (14), the alignment of grating lines (12G) and the alignment of the scale marks (11) is neither parallel nor perpendicular one to another.
4. Scale reading apparatus comprising a scale member (10) having regularly spaced marks (11), a radiation source (13) , at leaεt two radiation-sensors (14), a grating (12) having lines (12G) positioned with its plane parallel with the plane of the marks (11), the grating (12) and marks (11) cooperating to form moire fringes (15) , the radiation-sensors (14) being positioned to receive radiation from the radiation source (13); the radiation received by the radiation-sensors (14) being modulated by the moire fringes (15) wherein the radiation modulations falling on the respective radiation-sensors (14) occur in phase shifted relationship, characterised in that the alignment of the radiation-sensors (14D) is substantially perpendicular to the alignment of the scale marks (11), and the alignment of the grating lines (12G) is neither perpendicular nor parallel to the alignment of the radiation sensors or the alignment of the scale marks.
5. Scale reading apparatus according to Claims 1, 2, 3 and 4 having the radiation source (13) and radiation-sensors (14) all situated on one side of a plane containing the grating (12), and with the scale member (10) lying on the side of the grating (12) remote from both the radiation source (13) and radiation-sensors (14) , in order that radiation from the radiation source (13) passes first through the grating (12) to impinge upon the scale marks (11), whereupon the radiation is diffracted by the scale marks (11) and at the same time the radiation being reflected back towards the grating (12) and forming a periodic radiation pattern (IIP) in the form of diffraction fringes (16) in the plane of the grating lines (12G), the radiation thereafter passing back through the grating (12) to fall upon the radiation-sensors (14) .
6. Scale reading apparatus according to Claims 1, 2 and 5 wherein the phase relationship between the radiation modulations produced by the moire fringes (15) detected by an adjacent pair of radiation-sensors
(14) is related to the angular alignments of radiation-senεors (14), grating lines (12G), periodic radiation pattern (1IP) , and the pitch of the radiation-sensorε (14)and the pitch of the grating markε (11) substantially by the expression:
PM = 360*2*L*SINfB-A/2)*SIN.A/2 ) P wherein: PM is the relative phase between the radiation modulations produced by the moire fringes
(15) detected by an adjacent pair of radiation-sensors (14) ; L is the pitch of the radiation-sensors (14) ; P is the pitch of the scale marks (11); B is the angle between the normal (12N) to the grating lines (12) and the alignment (14D) of the radiation-sensors (14); A is the angle between the direction of the stripes of the periodic radiation pattern (IIP) and the grating lines (12G).
7. Scale reading apparatus according to Claims 3, 4 and 5 wherein the phase relationship between the radiation modulations produced by the moire fringes (15) detected by an adjacent pair of radiation-sensors
(14) is related to the angular alignments of radiation-sensorε (14), grating lineε (12G), εcale markε (11), and the pitch of the radiation-sensors (14) and the pitch of the grating marks (11) substantially by the expression:
PM = 360*2*L*SIN(B-A/2 )*SIN(A/2. P wherein: PM is the relative phase between the radiation modulations produced by the moire fringes
(15) detected by an adjacent pair of radiation-sensors (14); L is the pitch of the radiation-sensors (14) ; P is the pitch of the scale marks (11); B iε the angle between the normal (12N) to the grating lines (12) and the alignment (14D) of the radiation-sensors; A is the angle between the scale marks (11) and the grating lines (12G).
8. Scale reading apparatus according to Claims 3, 4 and 5 wherein the phase relationship between the radiation modulations produced by the moire fringes (15) detected by an adjacent pair of radiation-sensors (14) is related to the angular alignments of radiation-sensors (14), grating lines (12G), scale marks (11), and the pitch of the radiation-sensors (14) and the pitch of the scale marks (11), substantially by the expression:
PM = 360*2*L*SIN,B-AΪ*SIN.A. P wherein: PM is the relative phase between the radiation modulations produced by the moire fringes detected by an adjacent pair of radiation-sensors; L is the pitch of the radiation-sensors (14) ; P is the pitch of the scale marks (12G); B is the angle between the normal (12N) to the grating lines (12) and the alignment (14D) of the detectors (14); A is the angle between the scale marks (11) and the grating lines (12G).
9. Scale reading apparatus according to Claims 1, 2, 5 and 6 wherein the prescribed angular alignments of radiation-εenεorε (14), grating lines (12G) and scale marks (11) are related εubεtantially according to the equation:
A = B wherein: B iε the angle between the normal (12N) to the grating lines (12) and the alignment (14D) of the detectors (14) ; A is the angle between the direction of the stripes of the periodic radiation pattern (IIP) and the grating lines (12G).
10. Scale reading apparatus according to Claims 3, 4, 5 and 7 wherein the prescribed angular alignments of radiation-sensors (14), grating lines (12G) and scale marks (11) are related substantially according to the equation: A = B wherein: B is the angle between the normal (12N) to the grating lines (12) and the alignment (14D) of the detectors (14) ; A is the angle between the scale marks (11) and the grating lines (12G).
11. Scale reading apparatus according to Claims 3, 4, 5 and 8 wherein the prescribed angular alignments of radiation-sensors (14), grating lines (12G) and scale marks (11) are related substantially by:
B = 2*A wherein: B is the angle between the normal (12N) to the grating lines (12) and the alignment (14D) of the detectors (14) ; A is the angle between the scale marks (11) and the grating lines (12G).
12. Scale reading apparatuε according to Claims
1, 2, 5, 6, and 11 wherein the phase relationship between the radiation modulations produced by the moire fringes (15) detected by an adjacent pair of radiation-senεorε (14) is related to the angular alignments of radiation-sensors (14), grating lines
(12G), scale marks (11), and the pitch of the radiation-sensors (14) and the pitch of the scale marks (11), substantially by the expresεion:
PM = 360*2*L*SIN.A/2)*SIN.A/2. P wherein: PM is the relative phase between the radiation modulations produced by the moire fringes (15) detected by an adjacent pair of radiation-sensors (14); L is the pitch of the radiation-sensors (14) ; P is the pitch of the εcale marks (11); A is the angle between the direction of the stripes of the periodic radiation pattern (IIP) and the grating lines (12G).
13. Scale reading apparatus according to Claims 3, 4, 5, 6, 7 and 11 wherein the phase relationship between the radiation modulations produced by the moire fringes (15) detected by an adjacent pair of radiation-sensorε (14) is related to the angular alignments of radiation-sensorε (14), grating lines
(12G), scale marks (11), and the pitch of the radiation-senεors (14) and the pitch of the scale marks
(11), substantially by the expression:
PM = 360*2*L*SIN.A/2.*SIN.A/2. P wherein: PM is the relative phase between the radiation modulations produced by the moire fringes
(15) detected by an adjacent pair of radiation-sensors
(14) ; L is the pitch of the radiation-sensors (14) ; P is the pitch of the scale marks (11); A is the angle between the scale marks (11) and the grating lines
(12G).
14. Scale reading apparatus according to Claims 3, 4, 5, 8 and 11 wherein the phase relationship between the radiation modulations produced by the moire fringes (15) detected by an adjacent pair of radiation-sensors (14) is related to the angular alignments of radiation-sensors (14) , grating lines
(12G), scale marks (11), and the pitch of the radiation-εensors (14) and the pitch of the scale marks
(11), substantially by the expression: PM = 360*2*L*SIN.A.*SIN.A.
P wherein: PM is the relative phaεe between the radiation modulations produced by the moire fringes
(15) detected by an adjacent pair of radiation-sensors (14) ; L is the pitch of the radiation-sensors (14) ; P is the pitch of the scale marks (11); A is the angle between the scale marks (11) and the grating lines
(12G).
15. Scale reading apparatus according to Claims 6 to 14 wherein the relative phase, PM, between the radiation modulations produced by the moire fringes (15) detected by an adjacent pair of radiation-sensors (14), is εubstantially 90 degrees.
16. Scale reading apparatus according to Claims 3, 4, 6, 11, 13 and 14 adapted for use with a transmitting scale in which the scale marks (11) are defined by periodically arranged alternate transmitting and opaque stripε.
17. Scale reading apparatuε according to Claims 1 to 16 wherein the alignment (14D) of the detectors (14) is defined by a slit aperture (17) by which radiation is transmitted to the detectors (14) .
18. Scale reading apparatus comprising a scale member (10) having regularly spaced marks (11), a radiation source (13), at least two radiation-sensors (14), a grating (12) having lines (12G); the marks. when radiation is incident upon them,producing a periodic radiation pattern, which cooperates with the grating to form moire fringeε, the radiation-senεors (14) being positioned to receive radiation from the radiation source (13) , the radiation received by the radiation-senεorε (14) being modulated by the moire fringes wherein radiation modulations falling on the respective radiation-sensors (14C,14D) occur in phase shifted relationship, characterised in that the radiation-senεors and radiation source have a geometrical relationship such that the εaid phase shifted relationship varies with variation in relative angular position from parallelism, between the grating (12) and a plane containing the scale marks (11).
19. Scale reading apparatus according to claim 18 in which the variation in relative angular position is effected by moving the grating and keeping the scale marks stationary or vice versa, the movement being a rotation of the moving part about rotation axiε (llX) which is parallel to the plane of the stationary part.
20. Scale reading apparatus according to Claim 19, wherein the rotation axis (llX) is substantially parallel with the conventional roll axis of scale reading devices.
21. Scale reading apparatuε according to Claim 19- or 20, wherein the rotation axiε (11X) iε substantially parallel with the conventional pitch axis of scale reading devices.
22. Scale reading apparatus according to any one of Claims 18 to 21, having the radiation source (13) and radiation-sensors (14) all situated on one side of a plane containing the grating lines (12G), and with the scale member (10) lying on the side of the grating (12) remote from both the radiation source (13) and radiation-sensors (14), in order that radiation from the radiation source (13) passes first through the grating (12) to impinge upon the scale marks (11), whereupon the radiation is diffracted by the scale marks (11) and at the same time the radiation being reflected back towards the grating (12) and forming a periodic radiation pattern, in the form of diffraction fringes, in the plane of the grating lines (12G), the radiation thereafter passing back through the grating (12) to fall upon the radiation-εe εors (14) .
23. Scale reading apparatus according to any one of Claims 18 to 21, wherein the said periodic radiation pattern is composed of diffraction fringes.
24. Scale reading apparatus according to any one of Claims 18 to 22, wherein the geometrical relationship between radiation source and radiation-sensors is such that, when shown in plan view, the radiation-sensors have differing off-sets from the rotation axis.
25. Scale reading apparatus according to any one of Clai ε 18 to 23, wherein the geometrical relationεhip between radiation εource and radiation-sensors is defined in plan by a triangle (24) with the radiation-εenεors lying εpaced apart on one side (18) of the triangle, and with the radiation source positioned at a corner (19) opposite the side containing the radiation-sensors.
26. Scale reading apparatus according to Claim 25 wherein the geometrical relationship between radiation source and radiation-sensors is defined in plan by an isosceles triangle with the radiation source positioned at the apex of the triangle and with two radiation-sensors positioned at either end of the base line of the triangle; the grating lines being substantially perpendicular to the base line of the triangle.
27. Scale reading apparatus according to Claim 25 wherein the geometrical relationship between radiation source and radiation-senεorε is defined in plan by an isosceles triangle with the radiation source positioned at the apex of the triangle and with two radiation-sensors positioned at either end of the base' line of the triangle, with a third radiation-sensor positioned substantially at the halfway point of said base line; the grating lines being substantially perpendicular to the base line of the triangle.
28. Scale reading apparatus according to Claim 25 wherein the geometrical relationship between radiation source and radiation-sensors is defined in plan by an isosceles triangle with the radiation source positioned at the apex of the triangle and with two radiation-sensorε poεitioned at either end of the baεe line of the triangle; with further radiation-sensors positioned along the base line of the triangle spaced apart such that the phases of radiation modulations detected by the radiation-senεors are separated equally between all adjacent pairs of radiation-senεors; the grating lines being substantially perpendicular to the base line of the triangle.
29. Scale reading apparatuε according to any one of Claims 18 to 24 wherein the geometrical relationship between radiation source and radiation-sensors is defined in plan by a straight line in which said radiation source and radiation-sensors lie; the grating lines being substantially perpendicular to said εtraight line.
30. Scale reading apparatus according to any one of Claims 18 to 29 wherein the scale marks and the grating lines are substantially parallel in plan view.
31. Scale reading apparatus according to any one of the preceding claims wherein means is provided for effecting said variation in relative angular position between the grating and a plane containing the scale marks, from nominal parallelism.
32. Scale reading apparatus according to any one of Claims 18 to 20, or 22 to 31 wherein there is provided a transmiεsive scale member having refractive properties.
PCT/GB1990/001375 1989-09-06 1990-09-06 Measurement device Ceased WO1991003712A1 (en)

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
GB8920082.8 1989-09-06
GB898920082A GB8920082D0 (en) 1989-09-06 1989-09-06 Grating device
GB909002771A GB9002771D0 (en) 1990-02-08 1990-02-08 Grating device
GB9002771.5 1990-02-08

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WO1991003712A1 true WO1991003712A1 (en) 1991-03-21

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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1997005457A1 (en) * 1995-07-26 1997-02-13 Carl Zeiss Jena Gmbh Photo-electric distance- and angle-measurement system for measuring the displacement of two objects with respect to each other
JP2014153175A (en) * 2013-02-07 2014-08-25 Mitsutoyo Corp Separate encoder
US10667896B2 (en) 2015-11-13 2020-06-02 Cardiac Pacemakers, Inc. Bioabsorbable left atrial appendage closure with endothelialization promoting surface

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3604813A (en) * 1968-11-28 1971-09-14 Gerrit Hendrik Te Kronnie Position-indicating instrument
DE2126774A1 (en) * 1970-06-02 1971-12-16 Bendix Corp Apparatus and method for determining the magnitude of displacement movements using interference fringes
FR2124824A5 (en) * 1971-01-25 1972-09-22 Bendix Corp
FR2189706A1 (en) * 1972-06-21 1974-01-25 Asahi Kogaru Kogyo Kk
WO1989005440A1 (en) * 1987-12-12 1989-06-15 Renishaw Plc Opto-electronic scale-reading apparatus

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3604813A (en) * 1968-11-28 1971-09-14 Gerrit Hendrik Te Kronnie Position-indicating instrument
DE2126774A1 (en) * 1970-06-02 1971-12-16 Bendix Corp Apparatus and method for determining the magnitude of displacement movements using interference fringes
FR2124824A5 (en) * 1971-01-25 1972-09-22 Bendix Corp
FR2189706A1 (en) * 1972-06-21 1974-01-25 Asahi Kogaru Kogyo Kk
WO1989005440A1 (en) * 1987-12-12 1989-06-15 Renishaw Plc Opto-electronic scale-reading apparatus

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
MESSTECHNIK. vol. 81, no. 7, July 1973, MUNCHEN DE pages 226 - 229; Meissner: "Längen- und Winkelmessgeräte;Geber für die elektrische Messung nicht-elektrischer Grössen" see page 226, column 2, paragraph 2 - page 227, column 2, paragraph 2; figure 18 *

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1997005457A1 (en) * 1995-07-26 1997-02-13 Carl Zeiss Jena Gmbh Photo-electric distance- and angle-measurement system for measuring the displacement of two objects with respect to each other
JP2014153175A (en) * 2013-02-07 2014-08-25 Mitsutoyo Corp Separate encoder
US10667896B2 (en) 2015-11-13 2020-06-02 Cardiac Pacemakers, Inc. Bioabsorbable left atrial appendage closure with endothelialization promoting surface

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