Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1988-02-18
1989-11-07
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
250226, H01J 516, H01J 4014
Patent
active
048794627
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to opto-electronic scale-reading apparatus. International Publication No. WO86/03833 published July 3rd, 1986 discloses an opto-electronic scale-reading apparatus comprising a scale defined by spaced lines, a read head, the read head and the scale being movable one relative to the other in the direction of spacing of the lines, the read head comprising projecting means for projecting light on to the scale along a first path, the scale co-operating to pass on the light from the projecting means along a second path, the light so passed on having a pattern corresponding to the spacing of the lines, receiving means for receiving the light as passed on by the scale along said second path, and spatial filter means arranged in said second path for determining that, regarding the pattern passed on from the scale, substantially only lines of a given periodicity are transmitted by the spatial filter means.
SUMMARY OF THE INVENTION
It is an object of this invention to improve the efficiency of spatial filtering in an opto-electronic scale-reading apparatus.
The invention provides an opto-electronic scale reading apparatus wherein a read head comprises a lens adapted to project a beam of collimated light through a spatial filter on to a scale having lines. Light reflected from the scale passes back through the filter and through the lens to a phase discrimination circuit. The filter comprises two gratings co-operating to produce light modulations corresponding to the scale lines. Since the light passes twice through the filter the latter has improved ability to exclude scale lines not having the periodicity demanded by the filter.
BRIEF DESCRIPTION OF THE DRAWINGS
An embodiment of apparatus according to this invention will now be described with reference to the accompanying drawings wherein:
FIG. 1 is a sectional side view of a read head and scale combination together with a diagrammatic layout of an opto-electronic phase detection system.
FIG. 2 is a section of the read head and scale of FIG. 1 together with a phase diagram of a moire fringe.
FIG. 3 is a view similar to FIG. 2 showing a modified grating.
FIG. 4 is an enlarged detail of FIG. 1 and shows the optical geometry of a spatial filter being part of the apparatus.
FIG. 5 is a circuit diagram.
DESCRIPTION OF A PREFERRED EMBODIMENT
Referring to FIGS. 1 and 2, a scale 10 has scale lines 10A spaced in the direction, X, of the length of the scale. A light source 12 is connected by an input optical fibre 13 to a read head 11 where an end 15 of the fibre is positioned to emit incident light on to a collimating lens 16 having an axis 16A. The incident collimated beam, denoted 17, passes through a colour filter 18 having three sections 19,20,21 of different colours, and further passes through a first grating 24 and a second grating 25. Light reflected from the scale 10 passes back through the gratings, the colour filter 18 and the lens 16, and is focussed by the lens onto one end 31 of an output optical fibre 30 situated in close proximity with the end 15 of the fibre 13, the ends 15,31 being situated at opposite sides of the axis 16A. The fibre 30 connects the read head 11 to a colour-discriminator 35 having a first dichroic filter 36 for reflecting the colour of the section 19 on to a sensor 37 while transmitting the other two colours, a second dichroic filter 38 for reflecting the colour of the section 20 on to a senosr 39, and sensor 40 for sensing the remaining colour, i.e. that of the section 21. The sensors 37,39,40, whose outputs are denoted A,B,C, are connected to a phase detection circuit 41 whose outputs 42,43 represent the sine and cosine terms of light modulations 44 (FIG. 1) produced by the gratings 24,25. The colour filter 18 constitutes a phase determining means 18. The filter 18 and the discriminator 35 constitute a phase detection system.
Referring to FIG. 5, the circuit 41 comprises differential amplifiers 51,52 for producing signal values B-A and B-C, and further differential amplifiers co
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Nelms David C.
Oen William L.
Renishaw plc
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