Optical system for measuring orientation with cubic wedge and mask
The field of the invention is that of optical devices making it
possible to measure the orientation of an object in space without contact.
Diverse possible fields of application exist, but the main application is the
detection of aircraft pilot helmet posture, thus making it possible to project in
5 his visor an image superimposed exactly on the exterior landscape or to
slave various systems of the craft to his gaze. The precision sought in such
systems is of the order of a milliradian.
Various optical techniques making it possible to undertake on-
10 helmet orientation measurement exist. Generally, spottable elements are
installed on the helmet and are pinpointed by a system of cameras. The
position of the images of these spottable elements makes it possible to
determine the orientation of the helmet by calculation.
These elements may be passive or active. Passive elements are
15 illuminated by an external source. To this end, retroreflecting cubic wedges
may be used, which make it possible to reduce the problems of stray light,
due to solar illumination. It suffices to dispose the optical emission and
reception members on the same axis.
The active elements are generally light-emitting diodes. The
20 cameras have a fixed fine-tuning distance and consequently a necessarily
limited depth of field.
This technique presents a certain number of drawbacks. The
quality of the image of each point imaged on the detector depends on the
position of the helmet and its orientation, thus limiting the precision of the
25 system if it is desired to cover a significant measurement volume or an
appreciable range of rotation.
The system according to the invention remedies these two
drawbacks. It essentially comprises, mounted on a fixed framework of known
30 orientation, a single optical device of telecentric type for emitting and
receiving parallel light beams. The beams emitted emanate from a point
source; the beams received originate from the retroreflection of the light
coming from the source by a retroreflector mounted on the mobile object
whose orientation it is sought to determine . The retroreflector comprises a
mask of particular shape . The analysis of the image of the mask of the
5 reflector by a matrix detector disposed in the optical device makes it possible
to retrieve the orientation of the retroreflector and consequently of the mobile
object.
It may be demonstrated that, with this detection system, the quality
of the measurement is, by construction, independent of the position of the
10 helmet and of its orientation. Furthermore, its other advantages are as
follows:
- A very simple algorithm for determining orientation;
- A possible adaptation of the direction of illumination to the position
of the helmet;
15 - A great insensitivity to solar illumination;
- The use of entirely passive helmet-mounted devices requiring
neither linking cable, nor electrical power supply cable.
More precisely, the subject of the invention is a system for
20 detecting the posture of a mobile object in space comprising an electrooptical
fixed device of known position and orientation comprising at least one
first point emission source and a photosensitive matrix sensor; and an
assembly comprising an optical cubic wedge disposed on the mobile object,
characterized in that:
25 the electro-optical fixed device comprises a telecentric optic
essentially comprising a projection objective, a reception objective and a
semi reflecting optical element which are arranged in such a way that the first
point emission source is disposed at the focus of the projection objective by
reflection or by transmission through the semi-reflecting optical element and
30 that the image of the first point emission source is disposed at the focus of
the reception objective by transmission or by reflection through the semireflecting
optical element;
the input face of the cubic wedge comprises a mask in the shape
of a parallelogram, each side of the parallelogram comprising a geometric
35 marking making it possible to identify it, the image of the mask on the
photosensitive matrix sensor, by reflection on the faces of the cubic wedge,
being the intersection of the projection of the mask and of the projection of its
image by the cubic wedge with respect to the centre of the cubic wedge.
Advantageously , the markings are simple geometric shapes of
5 small dimension with respect to the dimensions of the sides and situated in
the vicinity of the ends of each side.
Advantageously, the geometric shapes form lugs and /or notches.
Advantageously, the first point emission source or its image is
disposed on the optical axis common to the projection objective and to the
10 reception objective.
Advantageously, the fixed device comprises a matrix of point
emission sources , the said sources being turned on as a function of the
position of the mobile object.
The invention also relates to a first pilot helmet , characterized in
15 that it comprises an optical cubic wedge whose input face comprises a mask
in the shape of a parallelogram, each side of the parallelogram comprising a
geometric marking making it possible to identify it , the said cubic wedge
being intended to operate in a system for detecting the posture of a mobile
object as described hereinabove.
20 The invention also relates to a second pilot helmet, characterized
in that it comprises an electro -optical fixed device of known orientation with
respect to the helmet , the said device comprising at least one first point
emission source , a telecentric optic and a photosensitive matrix sensor, the
said device being intended to operate in a system for detecting the posture of
25 a mobile object as described hereinabove.
Finally, the invention relates to a weapon system comprising
sighting means , characterized in that the said means comprise an optical
cubic wedge whose input face comprises a mask in the shape of a
parallelogram , each side of the parallelogram comprising a geometric
30 marking making it possible to identify it, the said cubic wedge being intended
to operate in a system for detecting the posture of a mobile object as
previously described . The weapon system may be a firearm carried by an
infantryman.
4
The invention will be better understood and other advantages will
become apparent on reading the description which follows given without
limitation and by virtue of the appended figures among which:
Figure 1 represents a general view of the posture detection
5 system according to the invention;
Figure 2 represents a view of a cubic wedge according to the
invention with its mask;
Figure 3 represents the formation of the image MO of an object
point M through the cubic wedge and of their images M' and M'O projected
10 onto the detector;
Figure 4 represents the geometric construction of the image of a
mask by reflection on the faces of the cubic wedge;
Figure 5 represents an example of a marking system for the mask;
Figure 6 represents the geometric construction of the image of a
15 mask marked by reflection on the faces of the cubic wedge;
Figure 7 represents a system according to the invention with
matrix of luminous sources.
By way of first exemplary embodiment , Figure 1 represents a first
20 embodiment of the detection system according to the invention in the
simplest case , that is to say comprising a single point source S and within the
context of detection of the orientation of a pilot helmet H . This first
configuration can be adapted very easily to other applications.
The system essentially comprises two sub-assemblies , an electro-
25 optical fixed device Ot and a helmet whose orientation it is sought to
determine . It is referenced in a coordinate frame (0, x, y, z).
The electro-optical fixed device Ot is situated in the cockpit of an
aircraft and occupies a known position and orientation with respect to the
frame of reference of the aircraft.
30 The electro-optical device Ot comprises a point-like light source S.
This source may be a light-emitting diode or a laser diode.
It also comprises a telecentric optical system comprising a
projection objective L, a reception objective L ' and a semi-reflecting optical
element LSR. The projection objective L like the reception objective L' may
35 consist either of simple lenses or of groups of lenses. The semi-reflecting
optical element LSR may be either a simple plane plate treated as
represented in the various figures or a splitter cube. The assembly of the
projection and reception objectives constitutes an afocal system, that is to
say their focus is common.
5 The image of the source S is disposed at the focus of the
projection objective L by reflection on the semi-reflecting optical element LSR.
Consequently, the image of S is collimated at infinity by the objective L which
thus emits a parallel light beam in a direction x0 as indicated in Figure 1. It is
equally possible to use the semi-reflecting optical element LSR in reflection on
10 the emission pathway and in transmission on the reception pathway or vice
versa.
The helmet H is equipped with a single retro-reflector of cubic
wedge type CO. A diaphragm or mask, of known shape and position with
respect to the cubic wedge, is fixed on the input face of the cubic wedge. The
15 collimated return flux emanating from the cubic wedge retroreflector CO is,
whatever its position or its orientation, projected at PO parallel to x0 onto the
vertical detector De in a bird's eye projection through the afocal objective
consisting of the optics L and U. On the basis of the shape of the image
gathered on the detector, a simple calculation provides the instantaneous
20 orientation of the helmet.
The principle of orientation measurement on the basis of the
device of Figure 1 is described hereinbelow. The cubic wedge CO is
represented in Figure 2 in an orthogonal coordinate frame (0, P, Q, R). The
three orthogonal reflecting faces are denoted POQ, POR and ROQ. In front
25 of the vertex 0 is positioned a transparent mask MK in the shape of a
parallelogram, the positions of whose vertices A, B, C and D with respect to
the vertex 0 are known.
Figure 3 represents the image, given by the cubic wedge CO, on
the detector D of a point M of the mask. When a parallel light beam is shone
30 onto the cubic wedge CO, each point M of the contour of the mask and its
image MO through the reflector are, in principle, symmetric with respect to the
vertex 0 of the reflector.
Since the bird's eye projection preserves symmetry, the projected
images M' and M'O of the points M and MO on the detector D are symmetric
35 with respect to the projection 0' of O. In the same manner, the mask ABCD
and its image AOBOCODO through the reflector CO are symmetric with respect
to 0. Since the symmetry preserves parallelism, AOB0OOD0 is a
parallelogram.
Since the bird's eye projection preserves parallelism, the image
5 projected on the detector of the mask ABCD is a parallelogram A'B'C'D'; the
projected image of the image AOBOCODO is also a parallelogram
A'OB'OC'OD'0. Since the bird's eye projection preserves symmetry, the
projected parallelogram A'B'C'D' is symmetric with respect to 0' of the
projected parallelogram A'OB'OC'OD'O as seen in Figure 4.
10 The real luminous image actually obtained on the detector D is the
area common to the two areas A'B'C'D'and A'OB'OC'OD'O. For a direction of
the axis x0 inside the angle of vertex 0 and of base ABCD, this area has the
points A', S, NO and T as contour as seen, in Figure 4 where this area is
represented by bold lines. It corresponds to the luminous flux incident in the
15 direction x0, filtered by the diaphragm ABCD, then reflected by the reflector
of vertex 0, filtered by the contour AOBOCODO, symmetric with the contour
ABCD and finally projected onto the detector D parallel to x0.
This luminous area gathered on the detector has as contour a
parallelogram A'-S-A'0-T which is centred on 0', the meeting point of the
20 diagonals, and which consists in the general case of one of the vertices of
the projection A'B'C'D' of the mask ABCD and of a fraction of each of the
sides A'B' and A'D' of the projection of the mask ABCD.
If the mask of the cubic wedge is a simple parallelogram as
indicated in Figures 2 and 4, it is impossible to discriminate in the image A'-
25 S-A'O-T the vertices and sides of the mask as seen in Figure 4. Hence, as
indicated in Figure 5, a specific marking is added at the end of each side of
the mask to differentiate the sides and therefore the vertices of the mask.
This marking is a shape coding.
By way of nonlimiting example, the coding of Figure 5 is:
30 - coding of the side AB: notches Al and B2 outwards from the
contour of triangular shape;
- coding of the side BC: lugs Bl and C2 inwards from the contour
of rectangular shape;
- coding of the side CD: lugs Cl and D2 of triangular shape;
35 - coding of the side DA: notches Dl and A2 of rectangular shape.
7
It is understood that, while remaining within the context of this
invention, an infinity of possible codings exists.
5
This marking allows, on the projected image,
- mutual differentiation of the four sides of the diaphragm, so as
to identify the projected vertex;
- differentiation between the vertices of the parallelogram
A'B'C'D' characterized by a marking on the sides of the vertex,
in proximity to the vertices and one of the "new" vertices S or T
characterized by an absence of marking in proximity to the
10 vertices.
The four elements utilized or, the luminous parallelogram imaged
on the detector are, in the example of Figure 6:
the positions of the centre O', and of the vertex A' that are
pinpointed by virtue of the lug A'l and A'2,
15 - the directions of one of the straight lines bearing the sides A'S
and A'T.
In this Figure 6, the common area is represented by bold lines as
in Figure 4.
It seems that there is still a last ambiguity to be resolved. It relates
20 to a side and its image through the reflector. As seen in Figure 6, there is no
differentiation on the image projected between the vertex A' of the
parallelogram A'B'C'D' and its homologue NO of the symmetric quadrilateral
A'OB'OC'OD'O. In fact, there is none. Indeed, in the great majority of
applications, the angular swings are limited, on the one hand by the limits of
25 the angular acceptance of the reflector: maximum rotation in each direction,
about the y and z axes of the fixed coordinate frame, always less than 90
degrees and on the other hand by the operational limits of the orientation of
the reflector which are the maximum rotations in each direction, about the x
axis of the fixed coordinate frame, always less than 90 degrees. Thus, the
30 point A is always "on the left" of B, the point D is always "on the left" of C, the
points A and B are always "above" C and D.
It is possible for the two projected parallelograms to have fractions
of sides in common. In this case, the vertex used for the analysis may be
either A', or B'.
35
Analysis of the image produced on D then makes it possible to
determine the orientation of the retro-reflector CO. This image analysis
provides, in the coordinate frame of the detector, the following three
elements:
5 - the positions of the projections of two known points of the reflector:
o the vertex 0 of the cubic wedge;
o one of the four vertices of the contour of the mask;
- the orientation of the projection of a known direction of the reflector,
namely one of the sides of the contour passing through the previous
10 vertex.
The orientation of two directions of the cubic wedge and therefore
of the helmet are obtained with the aid of these three elements.
The parallelogram general shape of the mask is preferable. Other
simple geometric shapes would be possible such as quadrilaterals or
15 triangles, but they may lead under certain orientation conditions either to
complex shapes of images such as hexagons, or to shapes of images in
which no vertex is the projection of a vertex of the mask.
As seen in Figure 1, if a point source disposed on the optical axis
20 is used, the measurement is possible only in the collimation beam given by
the lens L. To illuminate more significant measurement zones without using
optics of overly large dimension, it is necessary to displace the source of the
optical axis so as to obtain off-axis collimated beams. The orientation of
illumination is then modified to permit significant lateral displacements of the
25 mobile object. Various opto-mechanical means exist for ensuring this
displacement.
A simpler way is represented in Figure 7. The source S is replaced
by a matrix MSL of luminous sources S' positioned on the image by the LSR
semi-reflecting plate in the focal plane of the lens L.
30 A single source S' is turned on on the matrix M. When the image
PO arrives at the edge of the detector, configuration that can be pinpointed by
a simple image processing, the source S' is turned off and another source S'
of the matrix is turned on so as to recentre the image of the cubic wedge on
the detector D.
35
The advantages of the system according to the invention are as
follows. The cubic wedge fixed on the mobile object and serving for detection
is lightweight, compact, passive since it does not comprise any electrical link,
is insensitive to solar illumination, and is insensitive to metallic masses. The
5 optical emission-reception device is simple both in the opto-mechanical
design and in the digital processing of the images of the cubic wedge.
The assembly makes it possible to precisely determine the
orientation of a mobile object independently of its position. In the previous
examples, the mobile object is a pilot helmet and the fixed device is
10 positioned in an aircraft cabin. It is of course possible to fix the cubic wedge
on an operator, for example on his head' or on his hand.
It is also possible to use the system in the guise of remote sighting
system. It operates in the following manner. A weapons system has its
sighting line equipped with a cubic wedge according to the invention. It is
15 thus possible to ascertain the orientation of the sighting line by means of an
emission-reception device. A fixed camera films a scene at infinity. This
scene is reproduced on a monitor. The orientation of a direction of the
weapons system is symbolized by a mobile reticle inlaid into the image on
the monitor. An operator can thus precisely control the orientation of his
20 weapons system with respect to a target in the scene without having to sight.
Consequently, the alignment of his eye, of the target and of the mobile object
is no longer necessary; it suffices to control in the image the superposition of
the target and of the reticle. The position of the operator is no longer a
constraint.
25 In another variant, the illuminating and picture-capturing device is
carried by the helmet of an infantryman. The helmet is equipped with an
imager presenting at infinity a mobile reticle superimposed on the real scene
at infinity; the infantryman's weapon is equipped with the reflector; its precise
orientation with respect to the helmet is symbolized by the position of the
30 mobile reticle displayed in the helmet's imager. The infantryman thus carries
out sighting remotely in his helmet. The previous advantages are obtained
again. The alignment of the eye of the infantryman, of the target and of the
weapon is no longer necessary; the alignment of the eye of the infantryman,
of the target and of the reticle is sufficient; the position of the weapon with
35 respect to the head is no longer a constraint.
10
It is also possible to use the system according to the invention as
a lightweight, amagnetic and precise measurement toolkit to carry out fast
mapping of electromagnetic helmet position detection; fast mapping toolkit.
Moreover, the fixed part of the toolkit comprising the illumination source and
5 the camera part can be sited remotely, away from the piloting cabin.
11
CLAIMS
1. System for detecting the posture of a mobile object in space
5 comprising an electro-optical fixed device (Ot) of known orientation
comprising at least one first point emission source (S) and a photosensitive
matrix sensor (D); and an assembly comprising an optical cubic wedge (CO)
disposed on the mobile object,
characterized in that:
10 the electro-optical fixed device comprises a telecentric optic
essentially comprising a projection objective (L), a reception objective (L')
and a semi-reflecting optical element (LSR) which are arranged in such a way
that the first point emission source is disposed at the focus of the projection
objective by reflection or by transmission through the semi-reflecting optical
15 element and that the image of the first point emission source is disposed at
the focus of the reception objective by transmission or by reflection through
the semi-reflecting optical element;
the input face of the cubic wedge comprises a mask (MK) in the
shape of a parallelogram, each side of the parallelogram comprising a
20 geometric marking making it possible to identify it, the image of the mask on
the photosensitive matrix sensor (D), by reflection on the faces of the cubic
wedge, being the intersection of the projection of the mask and of the
projection of its image by the cubic wedge with respect to the centre (0) of
the cubic wedge.
25
2. System for detecting the posture of a mobile object according to
Claim 1, characterized in that the markings are simple geometric shapes of
small dimension with respect to the dimensions of the sides and situated in
the vicinity of the ends of each side.
30
3. System for detecting the posture of a mobile object according to
Claim 2 , characterized in that the geometric shapes form lugs (A2, B1, C2,
D1) and/or notches (Al, B2, C1, D2,1.
12
4. System for detecting the posture of a mobile object according to
Claim 1, characterized in that the first -point emission source or its image is
disposed on the optical axis common to the projection objective and to the
reception objective.
5
5. System for detecting the posture of a mobile object according to
Claim 1, characterized in that the fixed device comprises a matrix (MSL) of
point emission sources, the said sources being turned on as a function of the
position of the mobile object.
10
6. System for detecting the posture of a mobile object according to
one of the preceding claims, characterized in that the mobile object is a pilot
helmet (H).
15 7. System for detecting the posture of a mobile object according to
one of Claims 1 to 5, characterized in that the electro-optical fixed device (Ot)
of position and of orientation is mounted on a pilot helmet (H).
8. System for detecting the posture of a mobile object according to
20 one of Claims 1 to 5, characterized in that the mobile object is a weapon
system comprising sighting means.
9. System for detecting the posture of a mobile object according to
Claim 8, characterized in that the weapon system is a firearm.