This invention relates to apparatus for optically sensing the position of a member.
Many known optical position sensors direct light onto a member whose position is to be sensed and measure the intensity of the light after reflection from the member, some suitable arrangement being made to make the intensity of the reflected light dependent on the position of the member. An example of one such arrangement is described in UK Patent Specification GB 2160311A.
A problem associated with sensors of the aforementioned type is that errors can occur as a result of variations in the transmittivity of the optical fibres.
Also, variations in the output of the light source may not be distinguishable from variations due to movement of the member whose position is being sensed. Specification GB 2160311A suggests a complex technique for correcting such errors.
This invention provides apparatus for sensing the position of a member constrained to move in a particular manner comprising: beam producing-means designed to generate a beam of radiation and to cause it to sweep across the member; and timing means for detecting a time period beginning and/or ending at an instant when the beam illuminates a point on the said member, the manner of movement of the member and the manner in which the beam is, during operation of the apparatus, caused to sweep being such that the said instant and therefore the said time period is a function of the position of the member.
Because the invention relies on the detection of a time value rather than an intensity value the problem previously explained in relation to the prior art can, at least in principle, be eliminated.
In one possible arrangement the beam can be made to sweep across two points, one or both of which are on the said member, the time between illumination of these two points being measured. However another possibility is to measure the time between successive illuminations of the same point on the said member. Providing the focus of movement of the point is not radial, with respect to a centre point of the sweeping action, this time will likewise be related to the position of the point and therefore of the member.
Preferably the means for causing the beam to sweep is designed to do so cyclically at a constant period.
However, provided the rate of sweep at any given time is known, it is possible to use a beam which sweeps cyclically with a varying period or even a beam which performs just a single sweep for each measurement.
PreferablyjS the means for causing the beam to sweep comprises a reflector which is mounted on or which is part of a resonant mechanical oscillator. The beam could however be swept in other ways such as by refraction in an oscillating refractive element or in a Bragg cell whose refractive index varies periodically. The mechanical oscillator may be driven in any desired way for example using photoacoustic, capacative or piezo-electric techniques.
In an arrangement which detects the time between illumination of two separate points, these two points are preferably both on the member because this gives the maximum change in the said time period for a given change in the position of the member. the second is mounted on the member.
It may be advantageous to measure the time taken for the light beam to perform a sweep or a full cycle and to make suitable adjustments either to keep this constant or to correct the time measurements to compensate for changes in the period of the sweeping action which may occur due to ageing or changing external influences.
Preferably the or each said point is defined by a reflector. A reflector is suitable because it need not significantly interfere with the movement of the member.
As an alternative to a reflector, it may be possible to use a photo-detector or the end of a fibre optic link to a photo-detector.
Where a reflector is employed some arrangement needs to be provided to receive the radiation after reflection from it. One suitable arrangement includes a light source which directs light via a beam splitter or fibre Ycoupler to the means for causing the beam to sweep. When the beam illuminates the reflector (which is preferably retroreflective) the light is reflected back along the same path from which it arrived to the beam splitter or fibre Y- coupler which diverts it to an optical sensor.
Two ways in which the invention may be performed will now be described by way of example with reference to the accompanying drawings in which :
Figure 1 is a schematic illustration of a pressure sensor employing the invention; and
Figure 2 is a similar illustration of another pressure sensor also employing the invention.
The pressure sensor shown in Fig. 1 comprises a sealed evacuated box 1 and a diaphragm 2, which forms a part of one side of the box 1. The difference in pressure between the vacuum inside the box and atmospheric pressure outside the box causes diaphragm 2 to be deformed so that a central portion of it moves in the direction of the arrow. A displacement sensor, which will now be described, is used to measure the movement; which is a function of the atmosphere pressure.
A light source 3 at a location L remote from the box 1 produces a light beam. Light from the light source 3 travels along an optical fibre 4, which extends into the box 1. Light from the fibre passes through a beam splitter 5 to a reflective surface of a resonating mechanical oscillator 6. The resonation produces a changing angle of incidence of the light on the reflective surface and hence a reflected beam 7 which sweeps through an arc 6f angle A measured at the point P1 where the beam 7 is reflected. It should be noted that the point P1 moves during the oscillation of the oscillator 6 but this movement is very small and point P1 will always have the same position for any given direction of reflected beam 7.
Thus the position of P1 can be derived as required for any position of the beam in the sweep.
Two retroreflectors 17 and 18 are mounted at points
P2 and P3 on the diaphragm 2, within the angle A of sweep of the beam 7. An angle (a) subtended at the point P1 by points P2 and P3 depends on the position of the diaphragm 2. The resonant oscillator 6, and thus swept beam 7, moves cyclically with a constant period, thus the time between illumination of the retroreflectors 17 and 18 is dependent on the position of the diaphragm 2. A method for measuring this time interval will now be described.
When a retroreflector 17 or 18 is illuminated it directs the received light back along the path from which it came, via the point P1 on the resonant oscillator 6 to the beam splitter 5. The beam splitter 5 passes the reflected light, which is in the form of a pulse, into a fibre 8 which passes it to a photo-detector 9 at the remote location L. The light pulses from P2 and P3 cause photo-detector 9 to generate electrical pulses which respectively open and close a gate 10 which controls the passage of high frequency pulses from an oscillator 11 to a counter 12. The latter thus accumulates a count which is directly proportioned to the time- taken for the beam to sweep between the points P2 and P3.
A light source 13 at the remote location L produces light pulses at the resonant frequency of the element 6.
These pulses travel along a fibre 14 and are used to drive the resonant oscillator 6 photoacoustically: i.e. the pulses of light cause local heat expansion and hence resonation of element 6.
In the pressure sensor shown in Fig. 2 two additional retroreflective members, at points P4 and P5, are attached to the sides of the box 1 where they remain static with respect to the measuring system. Since the distance between P4 and P5 and thus the angle B subtended by them at the point P1 are constant, they can be used as a reference. A reference is desirable because the resonant oscillator 6 may, over a period of time, alter its resonant frequency due to changes in external parameters.
The retroreflective members at points P4 and P5 cause pulses of light to travel to the photodetector 9 in the same way as do the retroreflective members at points P2 and P3. The pulses of electricity produced by photo detector 9 are used to control a gate 15, which controls the passage of high frequency pulses from an oscillator 15 to two counters 12 and 16.
tIn operation, when the gate 15 receives the electrical pulse corresponding to the passage of the swept beam 7 across point P4 it is set to pass the high frequency pulses from oscillator 11 to counter 16. When the gate 15 receives the electrical pulse corresponding to the passage of the swept beam 7 across point P2 it is set to pass the high frequency pulses from the oscillator 11 to counter 12. When the gate 15 receives the electrical pulse corresponding to the passage of the swept beam 7 across point P3 it stops the passage of the pulses from oscillator 11 to the counter 12 and when it receives the electrical pulse corresponding to the passage of swept beam 7 across point P5 it stops the passage of:the pulses from oscillator 11 to counter 16.Thus the number of pulses counted by the counter 12 will be proportional to the time taken for the beam to sweep from P2 to P3 and the number of pulses counted by counter 16 will be proportional to the time taken for the beam to sweep from
P4 to P5. Because the angle (b) subtended at P1 by P4 and P5 is constant the figure in counter 16 can be used as a scaling factor to remove changes in the time taken to scan from P2 to P3 due to changes in the frequency of the resonant oscillator 6.
It will be appreciated that the illustrated pressure sensors have been described only by way of example and that many other methods of making use of the invention are possible. In one such alternative method, which is currently under consideration, optical power is transmitted along a fibre between locations in a manner similar to that shown in Fig. 2. The modulated light emerging from the fibre 14 is however applied to an optical-to-electrical transducer within the casing of the pressure sensor. The electrical output of this transducer is then used (a) to cause oscillation of a reflector by the piezoelectric effect and (b) to power a light source inside the casing which directs a beam of light onto the reflector as previously described.
Another proposal which is currently being considered is to use a single retroreflective point like point P3 of
Fig. 2 on the member 2 and to measure the time period between a first instant when the beam illuminates this point a first time and a second instant when it illuminates the beam a second time. Providing the point
P3 is not constrained to move in the direction of the beam when the beam illuminates it (i.e. radially with respect to the beam) this time period is dependent on the position which it is required to sense. It is possible to use a detector or retroreflector at a fixed point (or at a point at the centre of the movable diaphragm to provide a reference signal to monitor and correct for variations in the scan period.
Another Proposal under consideration is to use: a reflector of finite size and to measure the time taken for the beam to scan across it. In such an arrangement the point or points referred to in the accompanying claims is/are defined by the edge(s) of the reflector. Yet another possibility would be to substitute the reflective parts on the member with non-reflective parts and viceversa.