GB2236178A

GB2236178A - Monitoring arrangements

Info

Publication number: GB2236178A
Application number: GB8921269A
Authority: GB
Inventors: Roger Martin Langdon
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1989-09-20
Filing date: 1989-09-20
Publication date: 1991-03-27
Anticipated expiration: 2009-09-20
Also published as: GB8921269D0; GB2236178B

Abstract

A monitoring arrangement for determining the position and angular velocity of a mirror 2 includes means for producing a moving interference pattern on the surface of the mirror 2 by combining two laser beams 1 and 3 having slightly differing frequencies to produce the fringe movement. A sensor 5 detects the moving interference pattern after reflection and its phase is compared with that of a moving reference signal at a phase meter 11. The accumulated phase difference gives an indication of the angular position of the mirror 2. The difference in frequency as detected by discriminator 40, between the reference beam and the reflected signal, gives the angular velocity of the mirror 2. A third beam (12 Fig 2) enables independent detection in two planes. To extend the range of measurable mirror angles, a convex pattern reflector (Fig 3) may be used. The mirror is part of a fast scanning optical pointing and tracking system. <IMAGE>

Description

Monitoring Arrangements This invention relates to monitoring arrangements and more particularly, but not exclusively, to arrangements for determining the orientation of a mirror in a fast scanning system.

In fast mirror scanning systems used for optical pointing and tracking, means are required for accurately determining the mirror orientation as it moves rapidly from one orientation to another. This may be done by means of- electromechnical sensors such as potentiometers or shaftangle encoders attached to the mirror mounting. However, under dynamic conditions, the flexure of the mounting itself can introduce considerable inaccuracy due to its limited stiffness.

According to the invention there is provided a monitoring arrangement comprising: means for illuminating an object to be monitored with a moving interference pattern; sensing means arranged to produce an output signal representative of the interference pattern after its reflection from the object; and means for comparing the output signal with a reference signal to determine the position or movement of the object. Such an arrangement is particularly applicable to the monitoring of a mirror in a fast scanning system as the position or movement of the mirrored surface itself may be directly detected. The angular position of the mirror may thus be measured using a non-contact technique which does not involve the mirror mounting or other mechanical components of the apparatus.

In a preferred embodiment, the interference pattern is produced by optical radiation, being in the visible part of the electromagnetic spectrum or infra-red or ultra-violet radiation.

The position of the object may be determined by comparing the phase of the output signal with the phase of the reference signal. Preferably, the reference signal is derived from the same source as the interference pattern, giving a compact arrangement and ensuring good accuracy. It is further preferred that the reference signal is the output of sensing means arranged to receive a fraction of the interference pattern before it is incident on the object.

The moving interference pattern is preferably produced by combining an optical beam of radiation at one frequency with another optical beam of a different frequency. Both beams may be derived from the same source, which is preferably a laser. The output of the laser may be split into two beams and one frequency shifted with respect to the other.

It may be advantageous to monitor the velocity of the object by comparing the frequency of the output signal with that of the reference signal. This is of particular use in systems employing feedback between the angular measurement arrangement and the drive to motors rotating a mirror, for example. The provision of an input to the motor control system which is proportional to the angular velocity of the mirror, in addition to mirror position, enables the response time of the control system to be minimised and angular overshoot to be avoided.

Advantageously, where it is wished to determine the orientation of the object in more than one plane, means are included for illuminating the object with a second moving interference pattern arranged to move in a different direction to the first interference pattern. Both the first and second interference patterns may be produced by combining two beams of radiation having different frequencies, the frequency difference between the two beams producing in the first interference pattern being different to that between the beams arranged to produce the second interference pattern.

In one advantageous embodiment of the invention, the arrangement includes a convex mirror arranged to move with the object and which is illuminated by the, or each, interference pattern.

Some ways in which the invention may be performed are now described by way of example with reference to the accompanying drawings, in which: Figure 1 schematically illustrates a monitoring arrangement in accordance with the invention; Figure 2 schematically illustrates another monitoring arrangement providing positional information in two planes; and Figure 3 schematically illustrates a further measuring arrangement utilising reflection from a convex surface.

With reference to Figure 1, a uniphase, monochromatic laser beam 1 is incident on a mirror 2, whose angular orientation is to be measured. A second uniphase, monochromatic laser beam 3 is also incident on mirror 2 at an angle of incidence slightly different to that of beam 1.

The frequency of laser beam 3 is shifted by a small amount from that of laser beam 1 by, for example, passing it through an acousto-optic modulator (not shown). The combination of beams 1 and 3 on the surface of mirror 2 produces interference fringes which move transversely across the mirror surface at a velocity dependent on the difference in frequency between beams 1 and 3.

The beams reflected from mirror 2 are both incident on a small pinhole 4, behind which is placed a photodetector 5. The interference fringes formed in the plane of the pinhole 4 move across the pinhole, producing a sinusoidally modulated intensity on photodetector 5 at a frequency which is equal to the difference frequency between laser beams 1 and 3. Before reaching the mirror 2, beams 1 and 3 pass through a beam splitter 6, which reflects a small amount of the incident light onto a second pinhole 7, which is placed in front of a second photodetector 8. The signals from photodetectors 5 and 8 are amplified by pre-amplifiers 9 and 10 and are then compared in phase at a phasemeter 11. As the angular orientation of mirror 2 changes, the output from the phasemeter 11 alters by an amount approximately proportional to the change in angle.As the mirror 2 is moved from a known fixed reference angle to another unknown angle, the total phase shift between signals from photodetectors 5 and 8 is recorded by phasemeter 11, which adds 24 to the result every time the phase changes from 2Ar to zero, and subtracts K from the result every time the phase changes from zero to ZG7. The total phase shift, including the integer number of 2's is characteristic of the total angular movement of mirror 2.

Angular velocity measurement may be provided by incorporating a frequency discriminator 40 as shown in Figure 1. The outputs from the the pre-amplifiers 9 and 10 are passed to the inputs of the frequency discriminator 40, which produces an output proportional to the difference in frequency between the inputs. This difference frequency is proportional to the angular velocity of mirror 2.

The arrangement as shown in Figure 1 provides a means for measuring the angular orientation of the mirror 2 in the plane of the paper. Components of angular movement of the mirror 2 in the plane perpendicular to the paper may be measured simultaneously using the modified system shown in Figure 2.

In the arrangement of Figure 2, a third laser beam 12 is provided in addition to laser beams 1 and 3, making a small angle with respect to beam 1 in a plane perpendicular to the plane of the paper. Beam 12 is a uniphase monochromatic laser beam, shifted in frequency with respect to beam 1, for example by a second acousto-optic modulator (not shown). The frequency difference between beams 1 and 12 is different to the frequency difference between beams 1 and 3. The combination of beams 1 and 12 incident on the surface of the mirror 2 causes interference fringes to be produced which move in a direction perpendicular to the paper. The fringes are incident on the pinhole 4 and on pinhole 7, causing modulated photocurrents to appear in the outputs from photodetectors 5 and 8.These modulated photocurrents contain a frequency component f y characteristic of the interference fringes moving perpendicular to the plane of the paper, as well as a different frequency component x characteristic of the movement of fringes in the plane of the paper.Angular movement of the mirror 2 in a plane perpendicular to the plane of the paper causes a change in the phase difference between the output signals of the photodetectors 5 and 8 at frequency fy, whereas angular movement of mirror 2 in the plane of the paper causes a phase difference between outputs from photodetectors 5 and 8 at frequency fx. In order to separate the two components of angular movement, the output from photodetector 5 is passed through narrow-band electrical filters 13 and 14 tuned to frequencies fx and f respectively, whereas the output from the photodetector 8 is passed through narrow-band electrical filters 15 and 16 tuned to frequencies fx and y respectively.

The outputs from the filters 13 and 15 are compared in phase by a phasemeter 17, and the outputs from the filters 14 and 16 are compared in phase by a phasemeter 18.

The outputs from the phasemeters 17 and 18 are characteristic of angular movements of mirror 2 in planes parallel and perpendicular to the plane of the paper respectively. Absolute angular measurement of the movement of the mirror 2 in the two orthogonal planes may be made by a procedure similar to the described with reference to Figure 1.

If mirror 2 is planar, and beams 1, 3 and 12 are approximately parallel or only weakly divergent, the angular range of movement of the mirror 2 for which interference fringes reflected from the mirror 2 are intercepted by the pinhole 4 and the photodetector 5 may be very limited. In order to enable measurements to be made over a large angular range, the mirror 2 may be replaced by a small convex mirror 19, which is attached to the moving assembly at some convenient point 20, (as shown in Figure 3). Laser beams 1, 3 and 12, reflected from the surface of the convex mirror 19, are spread over a wide angular range.

Thus when the mirror 19 is tilted over a wide angular range, part of the fringe pattern generated by beams 1, 3 and 12 is always incident on the pinhole 4 and the photodetector 5, enabling angular measurements to be made.

The mirrored surface which reflects the interference fringes may be the rear or front face of a scanning mirror. In the latter case, the wavelengths of the illuminating radiation must be chosen such that there is no interference between it and the detected radiation from a scanned scene.

Claims

1. A monitoring arrangement comprising: means for illuminating an object to be monitored with a moving interference pattern; sensing means arranged to produce an output signal representative of the interference pattern after its reflection from the object; and means for comparing the output signal with a reference signal to determine the position or movement of the object.

2. An arrangement as claimed in claim 1 wherein the phase of the output signal is compared with the phase of the reference signal to determine the position of the object.

3. An arrangement as claimed in claim 1 or 2 wherein the reference signal is derived from the same source as the interference pattern.

4. An arrangement as claimed in claim 3 wherein the reference signal is the output of sensing means arranged to receive a fraction of the interference pattern before it is incident on the object.

5. An arrangement as claimed in any preceding claim wherein the moving interference pattern is produced by combining an optical beam of radiation of one frequency with another optical beam of radiation of a different frequency.

6. An arrangement as claimed in claim 5 wherein both beams are derived from the same source, the output of which is split into two beams and the frequency of one beam being shifted relative to the other.

7. An arrangement as claimed in claim 6 wherein the source is a laser.

8. An arrangement as claimed in any preceding claim wherein the frequency of the output signal is compared with that of the reference signal to determine the velocity of the object.

9. An arrangement as claimed in any preceding claim and including means for illuminating the object with a second moving interference pattern arranged to move in a different direction to the first interference pattern to determine the orientation of the object.

10. An arrangement as claimed in claim 9 wherein the first interference pattern is produced by combining first and second beams of radiation of respective different frequencies and the second interference pattern by combining third and fourth beams of radiation of respective different frequencies, the frequency difference between the first and second beams being different to that between the third and fourth beams.

11. An arrangement as claimed in claim 10 wherein the first beam is also the third beam.

12. An arrangement as claimed in claim 10 or 11 and including first and second filters having passbands, the centres of which lie at substantially the frequency difference between the first and second beams and the third and fourth beams respectively.

13. An arrangement as claimed in any preceding claim wherein the object is a mirror.

14. An arrangement as claimed in claim 13 wherein the mirrored surface is arranged to reflect the, or each, interference pattern and a viewed scene.

15. An arrangement as claimed in any preceding claim and including a convex mirror arranged to move with the object and which is arranged to be illuminated by the, or each, interference pattern.

16. An arrangement as claimed in any preceding claim wherein the, or each, interference pattern is produced by optical radiation.

17. A monitoring arrangement substantially as illustrated in and described with reference to the accompanying drawings.