Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1989-08-15
1990-12-04
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
250237G, 356374, G01D 534
Patent
active
049755714
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to apparatus for measuring displacement between a scale having a periodic structure and a read head.
A known type of such apparatus comprises a plurality of gratings, one of which is defined by the periodic structure of the scale. An illumination means interacts with one or more of the gratings to produce an interaction product including a spatially periodic fringe field. Another of the gratings forms an analyser grating situated in a position co-planar with the fringe field and having a periodicity which is substantially the same as the periodicity of the fringe field, whereby the fringe field and the analyser grating co-operate to produce light modulations when, in operation, the scale and the read head are displaced one relative to the other. Means are provided for detecting the light modulations in order to measure the displacement. Examples of such apparatus are shown in U.S. Pat. No. 3,812,352 (MacGovern) and in Diffraction Gratings, M. C. Hutley, Academic Press, 1982, pages 293-304.
For example, the known apparatus may include a source of light arranged for illumination of the periodic structure of the scale and interacting therewith to produce the interaction product including the fringe field, the analyser grating being provided in the read head. Alternatively, the apparatus may comprise a source of light arranged for illumination of the periodic structure of the scale and interacting therewith to produce a first interaction product, an index grating positioned in the light path of the first interaction product and interacting therewith to produce a second interaction product including said fringe field, both the index grating and said analyser grating being provided in the read head.
It is expensive to provide gratings in the read head, and so it would be advantageous to avoid the use of a grating in the read head or to reduce the number of such gratings.
SUMMARY OF THE INVENTION
The present invention is characterised in that at least two of said gratings are constituted by said periodic structure of the scale; and in that the read head includes reflector means positioned in the light path of at least one of the interaction products to reflect said interaction product back onto the scale.
BRIEF DESCRIPTION OF THE DRAWINGS
Examples of apparatus according to this invention will now be described with reference to the accompanying drawings wherein:
FIG. 1 is an elevation of a first example of the apparatus,
FIG. 2 is a plan view of FIG. 1,
FIG. 3 is an unfolded view of the optical path through the apparatus,
FIG. 4 is a view similar to FIG. 1 but shows a second example,
FIG. 5 is a view similar to FIG. 1 but shows a third example,
FIG. 6 is an elevation of the apparatus according to a fourth example,
FIG. 7 is a plan view of FIG. 6,
FIG. 8 is a plan view of a fifth example,
FIG. 9 is an elevation of a sixth example, corresponding to FIG. 6 but showing an improvement,
FIG. 10 is a plan view of FIG. 9,
FIG. 11 is a more detailed view of FIG. 10, showing ray paths,
FIG. 12 is an elevation corresponding to FIG. 9 but showing a further modification,
FIG. 13 is an elevation of a seventh example, and
FIG. 14 is an elevation of an eighth example.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIGS. 1 to 3, a reflective scale 10 is defined by periodic light and dark marks 11 provided on an elongate scale member 12 orientated with reference to directions X,Y,Z. The marks 11 lie in the X-Y plane and effectively form a diffraction grating. A readhead 13 comprises a light source 14 for projecting a beam L of light on to the scale, a lens 15 for collimating the light, a prism 16 having flat reflective surfaces 17,18 for reflecting the light between them and the scale, and a lens 19 for focussing the light finally reflected from the scale on to an opto-electronic transducer 20. The angle between the surfaces 17,18 is 90.degree.. The readhead is so positioned that the surfaces 17,18 each lie at 45.degree. to the X-Y plane and each lie at 90.degree.
REFERENCES:
patent: 3573468 (1971-04-01), Lang
patent: 3812352 (1974-05-01), MacGovern
patent: 4115008 (1978-09-01), Shepherd
patent: 4143268 (1979-03-01), Marold
Patent Abstracts of Japan, vol. 9, No. 40, pp. 336, 1763, (Feb. 1985) JP 58-182312.
Patent Abstracts of Japan, vol. 10, No. 160, pp. 465, 2216, (Jun. 1986), JP 61-17016.
Hutley, M. C., "Diffraction Gratings", Academic Press, 1982, pp. 293-304.
"Intererence-Generated Moire Fringes in a System of Three Reflection Gratings" Optical Technology vol. 36, No. 10, Oct. 1971, pp. 588-590.
McMurtry David R.
Pleydell Mark E.
Stephens William F. N.
Hsia Sherrie
Nelms David C.
Renishaw plc
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