GB2251750A

GB2251750A - An optical system for the determination of the position of an object

Info

Publication number: GB2251750A
Application number: GB9100383A
Authority: GB
Inventors: John Malcolm Morrison
Original assignee: GEC Ferranti Defence Systems Ltd
Current assignee: Leonardo MW Ltd
Priority date: 1991-01-09
Filing date: 1991-01-09
Publication date: 1992-07-15
Anticipated expiration: 2011-01-09
Also published as: GB2251750B; GB9100383D0

Abstract

To determine the position of an object, photodiodes A - C on the object respond when illuminated by scanning horizontal and vertical fan-shaped beams emanating from source S. The beams are produced by the action of a laser beam through a Bragg cell or other electro-optic or acousto-optic device driven at a variable r.f. drive frequency. As an alternative, A - C may be replaced by light emitting diodes and the source S by a single photodiode. <IMAGE>

Description

An Optical System for the Determination of the Position of an Object This invention relates to an optical system for the determination of the position of an object.

The invention is especially applicable to optical systems for detecting the position, including orientation, of an pilot's helmet in a cockpit of an aircraft or a helicopter.

The invention provides an optical system for determining the position of an object, comprising scanning means for scanning markings on the object either by illuminating the markings by means of a scanning light beam or by viewing the illuminated markings by means of a scanning window, processing means for calculating the position of the object from the output of photosensitive means responsive to the illuminated markings and from the known spacing of the markings on the object, the scanning means including an optical medium for deflecting the light beam or the viewing window by means of the electro-optic or acousto-optic effect.

The arrangement permits accurate location while avoiding the need for a moving mechanical scanning mechanism.

The optical medium preferably forms part of a so-called Bragg cell i.e. the optical medium is attached to a piezo-electric transducer for converting r.f. electrical signals to ultrasonic vibrations, which cause the optical medium to behave like a diffraction grating, the spacing of the grating varying as the frequency of the r.f. signal is varied. However, it would be possible to use an electro-optic material, that is, a material which changes its refractive index in accordance with changes in the potential of two electrodes attached to either side of the electro-optic medium.

In the case where a light beam scans, the markings may be photodiodes or other photosensors which are connected to the processing means and respond when illuminated by the light beam. In the case of a scanning window, the markings may be photo-emitters such as light emitting diodes (LEDs), and the scanning means may include a single photodiode or other photosensor to respond when the illuminated LEDs come into view.

Advantageously, the scanning entails planar repeated sweeps across the object in different directions, preferably orthogonal directions i.e. the beam or the scanning window are planar, preferably, fan-shaped. The sweeps in different directions may be sequential, simultaneous, or simultaneous but alternating.

An optical system constructed in accordance with the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of the scanning of the object; Figures 2a, 2b are schematic representations of sequential horizontal and vertical sweeps over the object; Figure 3a is a representation of an alternative form of scanning in which the horizontal and vertical sweeps take place simultaneously; Figure 3b illustrates a typical output of photosensitive means for the scanning arrangement of Figure 3a; Figure 4 is a schematic representation of a practical arrangement for performing the sweeps illustrated in Figures 2a, 2b, and 3a; and Figure 5 is a schematic representation of the Bragg cell used in the embodiment of Figure 4.

Referring to Figure 1, three photosensors in the form of photodiodes A, B, C are mounted on an object e.g. a pilot's helmet in a cockpit, and are capable of movement within a cubic region 1. The region 1 is scanned by two fan-shaped beams 2, 3, originating from a source 4, the beam 2 intersecting the nearest face of the volume in a horizontal line and repeatedly scanning from top to bottom, and the beam 3 intersecting the nearest face of the volume in a vertical line and repeatedly scanning from left to right. A reference photodiode R is mounted in a known location in the volume.

Referring to Figures 2a, 2b, the outputs of photodiodes A - C are connected to processing means, and the times at which the photodiodes respond give, in view of the uniform rate of scanning of the beams, the spatial position of the photosensors, separately in the horizontal direction (Figure 2a) and in the vertical direction (Figure 2b). The two scans thus give the projection of the three diodes A - C onto the x, y co-ordinate system of the nearest face of the volume 1 and, knowing the actual spacing of the photodiodes on the helmet (usually not in one plane), the processing means is able to calculate the relative position including orientation of the helmet in space and, because of the reference photodiode R, its absolute position.

Referring to Figures 4 and 5, the optical medium used for executing the scanning of the light beam is a Bragg cell shown in Figure 5 and incoming laser beam from the source S is deflected in direction 4a by interaction with ultrasonic vibrations (schematically shown as 5) set up in the optical medium 6 by means of a piezo-electric transducer 7 driven by a r.f. driver 8 and bonded to the optical medium by means of a bonding layer 9. The opposite end of the optical medium has an absorbing surface 10a. The ultrasonic vibration set up in the optical medium produce a variation of refractive index in the medium, and the result is an action like a diffraction grating, and it is for this reason that the device is known as a Bragg cell by analogy with Bragg scattering of X-rays from the lattice planes of a crystal.The angle of deflection of the beam e is a linear function of the frequency of the r.f. signal driving the piezo-electric transducer 7.

Referring to Figure 4, a beam from a laser diode 4 (providing a collimated beam) is incident on the Bragg cell 10, the r.f. driver which is connected to a voltage controlled oscillator 11 (a linear voltage controlled oscillator driven with a sawtooth waveform) in order to produce a scanning deflection of the beam in the plane of the paper.

To produce two scanning beams, the scanned beam is incident on a beam splitter 12.

The transmitted beam, still scanning in the plane of the paper passes through an electro-optic switch Eol to a cylindrical lens 13 which is divergent in a direction perpendicular to the plane of the paper to produce, from the laser beam moving from side to side in the plane of the paper, a fan having a plane perpendicular to the plane of the paper and scanning from left to right i.e. as shown in Figure 2b.

The beam reflected from the beam splitter 12 passes through a Dove prism 14 which converts the side to side scanning in the plane of the paper into an up and down scanning into and out of the plane of the paper, this being reflected by a mirror 15 via a second electro-optical switch E02 through a cylindrical lens 16. The latter is arranged at 900 to the cylindrical lens 13, but its also divergent in order to convert the laser beam scanning into and out of the plane of the paper into a horizontal fan-shaped beam scanning from above to below the plane of the paper i.e.

the beam 2a. This is reflected in mirror 17 and recombined at combiner 18, which acts as the source S for scanning the volume.

The electro-optic switches EO1, E02 are alternately on and off so that a horizontal scan is performed then a vertical scan then another horizontal scan etc.

The photodiodes A - C produce an output when illuminated by the scanning light beam.

The arrangement enables the object to be accurately located and there are no moving parts in the scanning mechanism.

The electro-optic switches EO1, E02, may be omitted, in which case the light beam scan simultaneously and the photodiodes A - C produce two outputs as they are crossed first by one of the beams and then by the other beam. That is, unless the photodiode lies on a diagonal, in which case a single output such as that for C is illustrated. Indeed, reference diodes may be placed at the ends of the diagonal for alignment purposes. As another alternative, there may be rapid switching of the electro-optic switches over the course of a single scan to distinguish between the two scans.

Both alternatives i.e. alternate or simultaneous scans i.e with or without the electro-optic switches EO1, E02, may be modified by replacing the photodiodes A - C by light emitting diodes and then replacing the laser diode 4 by a single photodiode. In this case, two fan-shaped acceptance slits scan the region 1 in horizontal and vertical directions and the photodiode responds when one of the light emitting diodes is crossed by the acceptance slit.

It is to be understood that the terms optical, photosensor, photodiode, light and illuminate, are to be understood as being not limited to visible electro-magnetic radiation but to include ultra-violet and infra-red electro-magnetic radiation.

Claims

1. An optical system comprising scanning means for scanning markings on the object either by illuminating the markings by means of a scanning light beam or by viewing the illuminated markings by means of a scanning window, processing means for calculating the position of the object from the output of photosensitive means responsive to the illuminated markings and from the known spacing of the markings on the object, the scanning means including an optical medium for deflecting the light beam or the viewing window by means of the electro-optic or acousto-optic effect.

2. An optical system as claimed in claim 1, in which a piezo-electric transducer arranged to be driven at a variable r.f. frequency is connected to the optical medium to induce ultrasonic vibrations in the optical medium.

3. An optical system as claimed in claim 2, in which the optical medium forms part of a Bragg cell.

4. An optical system as claimed in any one of claims 1 to 3, in which the scanning means is arranged to scan the region of the object in two scanning directions inclined to each other

5. An optical system as claimed in claim 4, in which the two scanning directions are orthogonal.

6. An optical system as claimed in any one of claims 1 to 5, in which photosensors on the object are illuminated by a light beam.

7. An optical system as claimed in any one of claims 1 to 5, in which the markings are photo-emitters which illuminate a photosensor via the scanning window.

8. An optical system substantially as hereinbefore described with reference to the accompanying drawings.