GB2120880A

GB2120880A - Optical transducers

Info

Publication number: GB2120880A
Application number: GB08311240A
Authority: GB
Inventors: David Niall Macfadyen
Original assignee: Thales Optronics Ltd
Current assignee: Thales Optronics Ltd
Priority date: 1982-05-11
Filing date: 1983-04-25
Publication date: 1983-12-07
Also published as: GB2120880B; GB8311240D0

Abstract

An optical transducer (10) comprises an achromatic light beam delivery system (11,12,13) with means (14) at the output thereof for spatially separating the delivered light beam into a plurality of wavelength separated light beams which are incident on a pair (15) of moire plates (15A,15B) one being fixed and the other movable. The grating line spacing in each channel of the plates (15A,15B) is substantially less than the spot size of the light beam which interrogates that channel. The encoded light beams from the plates (15A,15B) are gathered by a collection system (16,17) and delivered to an analyser (18) which determines the extent of displacement of the movable plate according to the encoding of the light beams. <IMAGE>

Description

SPECIFICATION Optical transducers This invention relates to optical transducers and in particular to the type of transducer in which the displacement of an element is sensed by optical means.

The recent development of passive optical transducers for use in industrial control, especially in hazardous environments promises to provide faster, more efficient, cheaper and safer control than the hitherto conventional electrical, hydraulic or pneumatic systems as a consequence of the inherent advantages of optical fibres, and various passive optical transducers have already been proposed.

In one proposed transducer a displaceable element in the form of a grey-coded plate is interrogated by a set of incident light beams emergent from one or more optical fibres giving rise to a set of encoded light beams which are collected by one or more optical fibres and delivered to an analyser which determines the extent of displacement of the plate according to the encoding of the encoded light beam. This transducer suffers from a number of disadvantages which limit its practical application for high resolution use.For example, to maximise the efficiency of each channel in the grey code the spot diameter of the interrogating light beam requires to be just less than the spacing between the bars in that channel and since the plate is grey-coded this means employing a different spot size for each channel which in turn means employing a different set of precision optics for each channel. As a consequence of the attendant cost and complexity this solution has not been implemented so far as we are aware. Instead, the known transducer utilises the same spot size for each channel, the selected spot size being chosen to maximise efficiency in the channel where the bar spacing is least. However this size is in turn limited to about 50 cm as a consequence of the optical fibres used to deliver the incident light beam so that resolution of the transducer is limited accordingly.Furthermore by virtue of the variation in light intensity occurring over the area of the light spot of the interrograting beam encoding occurs in a non-iinear manner which is overcome by the analyser operating on a binary basis which requires a great many channels if a given sensitivity in measuring plate displacement is to be achieved. For example, if sensitivity is to be measured to 10-3 x maximum displacement 10 channels are required.

It is an object of the present invention to provide an improved form of optical transducer in which the foregoing disadvantages are obviated or mitigated.

According to the present invention there is provided an optical transducer having a movable element the displacement of which is sensed by optical means, said transducer comprising an anchromatic optical fibre light beam delivery system, means at the output of the delivery system for spatially separating the delivered light beam into a plurality of incident light beams which are wavelength separated, a pair of multi-channel moiré grating plates the channels of which are individually interrogated by a respective incident light beam, one of said plates being the movable element of the transducer and wherein the grating line spacing in each channel is substantially less than the spot size of the pertaining interrogating light beam, an optical fibre collection system for gathering the encoded light beams emanating from the grating plates, and an analyser at the output of the collection system for determining the extent of displacement of the movable element according to the encoding of the light beams.

By virtue of the present invention there is no requirement for precision optics to provide a careful lytailored spot size in the interrogating light beam and there is no requirement for the light beams to have different spot sizes from channel to channel.

Resolution of the transducer is determined by the grating line spacing in the channel where line spacing is least. Furthermore, variations in light intensity over the area of the light spot do not affect encoding of the light beam which remains linear so that the analyser no longer requires to operate on a binary basis and accordingly the necessary number of channels to achieve a given sensitivity can be greatly reduced. For example, only three channels are required to achieve 10-3 X maximum displacement sensitivity when the analyser operates on a decimal basis.

The achromatic optical fibre light delivery system may conveniently comprise a white light source or a 50 nm bandwidth light emitting diode (LED) delivering into an optic fibre or fibre bundle the output of which is passed through a diffraction grating to spatially separate the wavelengths. The individual light beams may be formed by a stop or this function may be performed by physical separation of the individual channels of the moiré grating plates. Prior to the diffraction grating it is preferable to reduce the divergence of the beam emanating from the fibre or fibre bundle and this may be effected by a collimator but with the diffraction grating the light beams incident on the moir6 grating plates are divergent.

An alternative arrangement which enables the incident light beams to be collimated would be to use a graded spectrum filter instead of a diffraction grating. The same effect could be achieved using individual wavelength-selective filters for the respective channels but in this case the majority of the source light would be lost by reflection and/or absorption.

The analyser may conveniently comprise a dispersive element such as a diffraction grating to spatially separate the wavelengths of the encoded light beams from the respective channels, the spatially separated beams then being incident on a photodiode array the electrical output signals from which are then fed to one or more decoding threshold-level comparators for each channel.Alternatively the anaylser may be single channel and arranged to decode temporarily separated encoded light beams in which case the incident light beams at the moiré grating plates require to be temporarily separated for example by having a variable wavelength light source and chirping the wavelength thereof or alternatively by providing the optical fibre collection system with delay lines for the respective channels so that the simultaneously encoded light beams will arrive sequentially at the analyser.

The moiré grating plates may conveniently be provided with control and checking stations. For example one such station could have grating lines perpendicular to the grating lines of the channels previously discussed so that non-lineal movement of the movable plate would give rise to an identifiable signal from this checking station. Another example of such stations would be to provide one or more reference threshold levels for decoding purposes, such as 0,5,9 in the decimal scale, when these stations are subject to the same illumination level as the individual channels.

An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 illustrates an optical transducer according to the present invention in schematic form; and Figure 2 illustrates components of Figure 1 in greatly enlarged format.

In the drawings the transducer 10 comprises a light source 11 which delivers a bandwidth of light or white light to an optical fibre 12 so as to form an achromatic optical fibre light beam delivery system.

At the output of the fibre 12 there is located a collimator or divergence limiting element 13, which as previously explained is optional, and a wavelength separating element 14which is illustrated as a simple grating but may take other forms as previously discussed. The light beam radiation emanating from element 14 is in the form of angularly dispersed wavelengths individually collimated so that they are in the form of parailel beams incident on a pair 15 of multichannel moir6 grating plates 15A, 15B, which will be explained in greater detail with reference to Figure 2. One plate of the pair 15 is fixedly mounted whilst the other is movable in the direction of arrow A.The light transmitted through the pair 15 is collected by an optical fibre collection system comprising an element 16 to focus or concentrate the encoded light onto an output fibre 17 which delivers to an analyser 18.

Figure 2 illustrates the plates 1 so, 1 SB, separately in the interests of clarity. Each plate consists of six channels A,B,C,D,E,F individually bounded (or mutually separated) by a non-transmissive zone 20 which solely in the interests of clarity is depicted by stippling. Each of channels A,C and E consists of a set of non-transmissive bars or lines 21 separated by transmissive zones 22 the line spacing of channel A being much less than that of channel C which in turn is much less than that of channel E. The plates in Figure 2 are enlarged to enable the individual lines 21 in channel A to be visible to the naked eye. The relative line spacing in channels A,C and E is that required for decimal decoding (three decade).

The light spot previously referred to which is incident on each of channels A,C,E is laterally defined by the zones 20 and its depth is set by the delivery system approximately to equal this width and to provide for acceptable linearity of encoding the line spacing actually illustrated in channel A of Figure 2 would be the maximum.

With regard to channels B,D and F these are arranged to provide the control and checking stations discussed previously. Firstly it will be noted that the bars or lines 24 extend perpendicularly to lines 21 and are adjoined in each channel by one or two transmissive zones 25 according to which of channels B,C and F is being considered. Secondly it will be noted that in channels B and Fthe lines 24 are differently disposed on the plates 1 so, 1 SB, whereas in channel D the lines 24 are identicaliy disposed on the two plates 15A, 15B. Accordingly when the plates 1 5A, 1 SB are correctly orientated even during movement of one in the transducing direction indicated by arrow A of Figure 1 channels B,D and F are used as threshold level monitors. Channel F provides zero transmission; channel D provides 50% transmission; and channel B provides 25% transmission. Furthermore the correct orientation of the plates 15A, 15B can be monitored by comparing the relative transmission of channels B,D and F. Furthermore if the two plates 1 5A, 1 SB become misaligned either by lateral displacement or rotation the relative individual channel transmissions will alter so that a warning can be established in analyser 18thatthe three transducing channels A,C and E are erroneous.

It will be evident that the particular arrangement of lines 24 and zones 25 can be varied to provide different transmission factors whilst retaining the orientation of the lines 24 therein perpendicular to the lines 21 of the transducing channels A,C and E.

Claims

1. An optical transducer having a movable element the displacement of which is sensed by optical means, said transducer comprising an achromatic optical fibre light beam delivery system, means at the output of the delivery system for spatially separating the delivered light beam into a plurality of incident light beams which are wavelength separated, a pair of multi-channel moiré grating plates the channels of which are individually interrogated by a respective incident light beam, one of said plates being the movable element of the transducer and wherein the grating line spacing in each channel is substantially less than the spot size of the pertaining interrogating light beam, an optical fibre collection system for gathering the encoded light beams emanating from the grating plates, and an analyser at the output of the collection system for determining the extent of displacement of the movable element according to the encoding of the light beams.

2. An optical transducer as claimed in claim 1, wherein said delivery system comprises a white light source.

3. An optical transducer as claimed in claim 1, wherein said delivery system comprises a lightemitting diode of 50 nm bandwidth.

4. An optical transducer as claimed in any one of claims 1-3, wherein said spatially-separating means comprises a diffraction grating.

5. An optical transducer as claimed in any one of claims 1-3, wherein said spatially-separating means comprises a graded-spectrum filter.

6. An optical transducer as claimed in any one of claims 1-5, wherein said analyser comprises a dispersive element to separate spatially the wavelengths of the encoded light beams, and detecting means sensitive to the respective spatiallyseparated wavelengths.

7. An optical transducer as claimed in any one of claims 1-5, wherein said collection system comprises means for temporarily separating simultaneously encoded light beams, and said analyser comprises detecting means sensitive to the band of wavelengths.

8. An optical transducer as claimed in any one of claims 1-7, wherein said moir6 grating plate comprise control stations arranged to provide reference signals when the control stations are subjected to the same illumination level as the individual channels.

9. An optical transducer as claimed in any one of claims 1-8, wherein said moir6 grating plates comprise checking stations arranged to provide error signals indicative of non-lineai movement of the movable plate.

10. An optical transducer substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.