Device for determining the spatial position of an object

A device for determining the spatial position of an object (11) including a plurality of sources (A1, A2, B1, B2, C) emitting beams of the polarized light in different directions and disposed on the object and a plurality of stationary detection devices (21, 22, 23), each having means for measuring the angular orientation of the direction of polarization of a beam of polarized light which they receive relative to a corresponding reference direction. At least two of the sources (A1, B1) emit beams having distinguishable characteristics, but the same direction of polarization, and are arranged so close to each other that a specific detection device (22) always receives at least one or the other of the beams. Means associated with the detection device distinguishes the beams from each other as a function of the characteristics.

BACKGROUND OF THE INVENTION 
The invention relates to a device for remotely determining the spatial 
position of a rotatable object. The movement of the object relative to a 
fixed trihedral reference frame is resolved into rotational components. 
Such positional determinations are especially useful in applications where 
variations in the orientation of the head of a pilot of an aircraft 
relative to the cockpit should be followed, for example in order to obtain 
an indication of said movements for a subsequent study or in order to 
provide automatic control in accordance with changes in the pilot's line 
of sight. For this type of applications the fixed reference frame is 
represented by the cockpit and the movable object in the space of said 
cockpit is generally constituted by the helmet on the pilot's head, which 
helmet moves in accordance with the direction of visual observation of the 
pilot. 
Devices are known which have been designed in order to enable the angular 
position of the pilot's line of sight to be determined by remote measuring 
means, without any mechanical linkage between the pilot's helmet and the 
cockpit and employing polarized light. Such a device is described in U.S. 
Pat. No. 3,867,629. The pilot's helmet is provided with a reflector and a 
polarizer, which together constitute a source of polarized light, the 
direction of the polarizing vector varying with the angular position of 
the helmet relative to the axis of propagation of the beam. Said beam is 
directed at a device for detecting the angular position of the polarizing 
vector, which device comprises a rotating analyser and an optical sensor. 
Said sensor is sensitive to the luminous intensity of the beam which it 
receives via the analyser, which analyzer is mounted on a rotary disc 
which is perpendicular to the axis of propagation of the beam. This 
detection device is stationary with respect to the object being observed. 
For the specific use considered the disc carrying the analyser is 
rotatably mounted on the cockpit and the optical sensor is rigidly 
connected to said cockpit behind the analyser. This means that the 
direction of the beam should remain fixed. Measuring is effected by 
determining the angle through which the disc has rotated from an angular 
reference position when a maximum light transmission through the analyser 
indicates the coincidence of its direction of polarization with that of 
the light beam from the helmet, said last-mentioned direction, being 
related, as already set forth, to the angular position of the helmet 
relative to the axis of propagation of the beam. 
British Patent Specification No. 1,045,994 discloses how, in a device of 
the same type, variations in luminous intensity as a function of the 
angular distance existing at any instant between the analyser and the 
polarizer may be used in a differential measurement. A polariser mounted 
on the pilot's helmet cooperates with a system comprising a plurality of 
analysers mounted on one and the same rotary disc with differently 
oriented polarizing axes. The system is made to rotate around the light 
beam, its angular position being referred to the cockpit until the 
luminous intensities of the beams transmitted by the differently oriented 
analysers are equal. As in the system described in the previously 
mentioned U.S. patent, observation is limited to rotational movements of 
the helmet about a single axis, which is fixed relative to the cockpit, in 
accordance with which axis the light beam used for the measurements 
propagates. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to obtain a substantial extension 
of the use of known devices. In order to reach such a goal, it does not 
suffice to combine devices as described in the prior-art documents. 
Although an arbitrary rotational movement can be resolved into three 
elementary rotational movements about the three axes of, for example, a 
trirectangular reference frame, problems remain to be solved. First, the 
three rotational components cannot be treated separately. Second, the 
measurable characteristics of each of them depend on the values observed 
for the previous ones and on the selected reference direction. Third, an 
arbitrary rotational movement in space does not have any fixed direction 
in which the beam used in the known devices can be directed. It is another 
object of the invention that the measurements of a rotation in space be 
effected relative to a three-dimensional reference frame by very simple 
means having a high reliability of operation, which is particularly useful 
in devices intended for mounting in aircraft. 
To achieve these objects, the device according to the invention, comprises 
a plurality of polarized-light sources whose beams are oriented in 
different directions. The sources are disposed on the object whose 
rotational position in space is to be determined, for example a pilot's 
helmet. The device further comprises a plurality of associated stationary 
detection devices, which are for example connected to the pilot's aircraft 
cockpit. Each of said detection devices may be similar to as those used in 
either of the above-identified prior-art arrangements or to any other 
device which enables determination of the angular orientation of the 
direction of polarization of a polarized-light beam which it receives 
relative to a reference direction. During movements of the object being 
examined, one detection device can thus receive beams from different 
sources attached to the object at different instants. The different beams 
are distinguished by the characteristics of the light of said beams, which 
are not affected by the movements of the object. In this respect it is 
advantageous if the polarized light is emitted in the form of periodic 
pulses, which are shifted in time for the different sources. 
In accordance with the invention, the device comprises a plurality of 
sources which emit beams of polarized light in different directions and 
are disposed on the object and a plurality of stationary detection 
devices, each comprising means for measuring the angular orientation of 
the direction of polarization of a beam of polarized light which it 
receives relative to an associated reference direction. At least two of 
said sources emitting beams have the same direction of polarization and 
are arranged in such a way relative to each other that a specific 
detection device permanently receives at least the one or the other of the 
beams of said two sources. Means are associated with said detection device 
for distinguishing the beams from each other as a function of the 
characteristics of the light, which are independent of the orientation of 
the light, and for selecting the beam to which each measurement relates. 
It should be appreciated that in the foregoing the detection devices which 
are said to be stationary are stationary only relative to the object. 
Furthermore, said devices, which are known per se, generally comprise a 
rotating analyser, which is mounted for rotation about a fixed axis and 
the angular orientation which is determined is that of the direction of 
polarization which rotates about said axis depending on the rotational 
movements of the object. It should also be appreciated that, generally 
speaking, complete measurements for an object whose orientation may vary 
in all directions requires three angular measurements, by means of three 
stationary detection devices defining a fixed trihedral reference frame. 
In practice, this is suitably a trirectangular trihedral, relative to 
which the position of a similar trirectangular trihedral, which is rigidly 
connected to the object is to be defined. The sources disposed on an 
object then suitably comprise at least a first source on an axis which is 
perpendicular to a plane containing a series of sources radiating in said 
plane and being regularly spaced around said axis at angular distances 
which are sufficiently small to ensure that each of the two stationary 
detection devices permanently receives one of the beams emitted by said 
series of sources. All the beams emitted by the sources of said series 
have a direction of polarization which is perpendicular to the plane in 
which said sources are disposed. For example, if for the radiation sources 
use is made of light-emitting diodes whose beams, after having traversed 
an associated polarizer which is rigidly connected to the object, cover a 
solid angle of 90.degree., it suffices to use four diodes spaced at 
90.degree. from each other in one plane in order to guarantee that the 
detection devices can follow the rotational movements of the object about 
the perpendicular axis. 
For most practical uses and in particular those which concern the 
determination of rotational movements of the head of a pilot in an 
aircraft, it is necessary that there is also provided a series of sources 
emitting in the direction of the third detection device, in accordance 
with the axis which is perpendicular to the plane containing the 
aforementioned series of sources. Indeed, in practice the rotational 
movements to be observed remain within limited ranges around certain axes 
of the trirectangular trihedral. It may then be advantageous to select the 
plane of the series of diodes so that it is perpendicular to the axis 
about which the rotations are most complete. Then it suffices to use a 
single source on said axis when the components of the rotation about the 
other axes normally remain within such limits that the beam emitted by the 
said single source is permanently incident on the associated stationary 
detection device. 
For example, for determining the position of the head of a pilot, it may 
generally be assumed that the azimuth varies over the widest angular 
range, whilst the angle of sight hardly varies more than 60.degree. to 
both sides of the rest position, and the angle of roll even to a smaller 
extent. Furthermore, in the final measurement results the angle of sight 
and the azimuth are of primary interest, but the determination of the 
angle of roll is useful in various data-processing operations by means of 
which the rotation expressed in the azimuth angle and angle of sight can 
be derived effectively from the measuring results, which initially yield 
the angles of rotation about the axes of the movable trihedral associated 
with the object, through the orientations of the directions of 
polarization about the axes of propagation of the detected light beams. 
Suitably, the light sources which are used are controlled so that the 
effective intensity, through each polarizer, is emitted in the form of 
periodic pulses, the pulses from each source being shifted in time 
relative to those from the other sources. In combination with this 
arrangement, the device in accordance with the invention is adapted to 
process the results of measurements of the angular orientation of the 
directions of polarization differently depending on whether the 
measurement, which at a specific instant is effected by a detection 
device, relates to the one or the other of the polarized beams which may 
be incident on it. A distinction can easily be made in accordance with the 
shift of the pulses. Naturally, said shift, as well as the pulse width, 
should be small relative to the time intervals between successive pulses. 
It is to be noted that in the preferred embodiment of the invention, the 
polarized beams emitted by sources in one plane with the same direction of 
polarization, are interchangeable up to the final determination of the 
three angles which define the orientation of the object in space, and that 
consequently each detection device may be used for measuring a specific 
angle and always plays the same part in the calculations, regardless of 
the beam which it detects. It suffices that at each instant the signals 
are processed in such a way that for each detection device only a single 
beam, which differs for each device, is taken into account. The signals 
relating to different sources may be separated in accordance with the 
shift of the pulses and for further processing the signal having the 
highest amplitude may be selected. It should also be noted that the 
various angular measurements are not influenced by any translatory 
movements of the object.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the particular embodiment described, the device is mounted in an 
aircraft and is adapted to measure at any instant the position of the 
pilot's line of sight, expressed as an angle of sight .phi. and an azimuth 
angle .theta. relative to a fixed reference frame connected to the 
aircraft. The movements of the head are in fact defined by three 
rotational movements expressed as angles of azimuth, sight, and roll 
respectively, that is .theta., .phi. and .rho., which are defined by the 
position of an orthonormal reference frame OXYZ, associated with the 
pilot, relative to a fixed reference frame Oxyz, associated with the 
aircraft cockpit. It is assumed that in practice the movements of the head 
are such that the aximuth .theta. may vary between -120.degree. and 
120.degree., or even through a complete circle between -180.degree. and 
+180.degree. on both sides of the longitudinal axis of the aircraft, 
whilst the sight and roll components of the rotation each remain within an 
angle of 90 degrees from the rest position in which the component is zero. 
In FIG. 1 the position of the pilot's head is represented by the position 
of his helmet 11 when he is in an initial or rest position, in which all 
the components of the rotation are zero, the head being perpendicular to 
the vertical axis Oz and facing the front of the aircraft along the 
longitudinal axis, opposite to the axis Oy. The angle .theta. is also 
shown, which represents the azimuth of the line of sight when the pilot 
turns his head, for example, to the left, keeping it straight. 
The same angle .theta. is found in the plan view of FIG. 2, between the 
axis Ox of the fixed reference frame and the axis OX of the helmet. 
On the helmet 11 various light-emitting diodes are arranged on the axes of 
the movable reference frame. In front of each of them there is arranged a 
polarizer, which is also rigidly connected to the helmet 11, such as the 
polarizer 12, which is shown in FIG. 1 in front of the diode C disposed on 
the top of the helmet. Together with an associated polarizer each diode 
constitutes a source of polarized light. Such a source may also be formed 
in any other known manner. In the particular case described, the emission 
cone of each source covers a solid angle of approximately 90 degrees whose 
axis is situated on the radius of the helmet which passes through the 
diode. 
There are provided at least five sources thus arranged on the helmet. The 
first one is that already mentioned with reference to FIG. 1, whose diode 
C is disposed at the top of the helmet. Four others are arranged in the 
plane perpendicular to the axis through the diode C. These are the diodes 
A1, B1, A2, B2 of FIG. 2, which are regularly spaced at 90 degrees from 
each other. The corresponding beams radiated from the helmet each cover a 
solid angle which may be estimated at approximately 90 degrees. 
In the special case described, there are also provided two further diodes 
D1 and D2 in the same plane, which are disposed on the line of sight OY' 
at 45 degrees on both sides of the diode B2. In order to simplify the 
measuring operations and the processing of the results yielding the 
desired angles, the polarization vectors of the beams emitted in the plane 
OXY are all parallel to each other and are oriented perpendicularly to 
said plan, in accordance with the axis OZ, whilst the polarization vector 
of the beam emitted by the diode C is parallel to the plane OXY, and is 
specifically oriented in accordance with OX. 
Associated with the array of diodes on the helmet is an electronic control 
device 15 (FIG. 3), which supplies a drive current with periodic pulses to 
these diodes. The frequency is the same for all the diodes, but the pulses 
from the different diodes are shifted by a small fraction of a period 
relative to each other, except for the diametrically opposed diodes which 
are driven simultaneously, A1 and A2 on the one hand and B1 and B2 on the 
other hand. By way of example, the clock 16 supplies control pulses having 
a frequency of 30 kHz, which are sequentially transferred to the diode C 
without delay, to the diodes A1 and A2 with a delay introduced at 17, to 
the diodes B1 and B2 with a double delay introduced at 18, and to the 
diodes D1 and D2 with a triple delay introduced at 19. 
Furthermore, on the cockpit, the device comprises three electro-optical 
detection devices for the polarized beams, which devices are associated 
with the fixed reference frame and are respectively disposed on the axes 
Ox, Oy and Oz. The detection device 21, which is disposed above the pilot 
in order to receive the beam radiated from the diode C, is shown in FIG. 
1. The two other detection devices are referenced 22 and 23 in FIG. 2, but 
are not shown in more detail. They are identical to the device 21, each 
being disposed on the appropriate axis. 
As is shown in FIG. 1, the detection device 21 comprises a disc 30, which 
is mounted for rotation about its own axis, which is fixed in accordance 
with the axis Oz of the reference frame. The central portion of the disc 
is constituted by an analyser 31, whilst the annular peripheral portion is 
formed with two apertures 32 and 33, which are diametrically opposed in 
accordance with the direction of polarization of the analyser. On the axis 
of the disc 30 a photodiode 36 detects the intensity of the luminous flux 
which it receives via the analyser 31. A light-emitting diode 34 and a 
photo-detector diode 35 are disposed on opposite sides of the disc 30, at 
the location of the annular peripheral portion. The diode 35 detects the 
luminous flux from the light-emitting diode 34, whilst as the disc rotates 
each of the two apertures 32 and 33 passes between them. Thus, upon each 
half revolution of the disc, a fixed reference orientation in time is 
obtained for the direction of polarization of the analyser 31. This 
reference orientation is that of the axis Oy in the case of FIG. 1. It is 
parallel to the axis Oz for the detection devices 22 and 23. The speed of 
rotation of the discs is constant and for example fixed at 30 revolutions 
per second. 
The angle .alpha. which the polarization vector of the incident beam makes 
with the fixed reference orientation is defined by the orientation of the 
disc at the instant at which a maximum intensity of the luminous flux 
transmitted by the analyser indicates that the polarization vectors of the 
beam and the analyzer are parallel. In fact, for reasons of detection 
accuracy, it is preferred to detect the minimum. This means a comparison 
between the detected fluxes for successive pulses in the emitted beam. 
This comparison is effected by an electronic processor for the signals 
supplied by the various detection devices, which processor also effects 
sampling and computation of the angles. 
This arrangement is described with reference to the block diagram shown in 
FIG. 3. Three circuits respectively serve for calculating the angles 
.alpha., .beta. and .gamma. between the fixed reference frame Oxyz and the 
reference frame OXYZ of the helmet on which the light-emitting diodes are 
arranged. Sampling circuits 41 to 45 supply the signals obtained from the 
detection devices to the three processing circuits in accordance with the 
time delay of the pulses. Thus, they receive the clock signals H, H1 and 
H2, H being synchronized with the control signal of the diode C, H1 with 
that of the diodes A1 and A2, and H2 with that of the diodes B1 and B2. 
Each sampled signal is transferred to an analog-to-digital converter, such 
as 46, for the circuit corresponding to the diode C, subsequently to a 
comparator, such as 47, which receives the information corresponding to 
the minimum flux that is detected. The signal is then transferred from the 
comparator 47 to a pulse counter 48, which operates with a frequency which 
is substantially higher than that of the signals which control the drive 
of the diodes the sampling, and which is reset to zero by the signal 49, 
which is supplied by the electro-optical detection device for the 
information which provides the reference orientation of the polarization 
vector. 
For the angles .beta. and .gamma., the signal coming from the detection 
devices 22 and 23 normally contains information on two beams from one of 
the diodes A1 or A2 and one from the diodes B1 or B2, between which a 
distinction is to be made. To this end, the conversion into digital 
signals is effected by 51 and 52, both for the signal sampled on the clock 
signal H1 and for the signal sampled on the clock signal H2. An amplitude 
comparator 53 selects that signal which has the highest amplitude and 
corresponds to the diode nearest the axis of the detection device in order 
to transmit it to the following circuits. The minimum is found by means of 
the comparator 54, and the pulse counter 55 supplies the value of the 
angular orientation of the polarization vector relative to the reference 
orientation. Regardless of the source of origin A1, A2, B1 or B2, the 
beams sampled on H1 and H2 are interchangeable in the measurements and the 
subsequent calculations. The value found at 55 through detection by the 
device is assigned to the angle .beta.. The circuits associated with the 
detection device 23 are formed in a similar way and the value supplied by 
the pulse counter 58 is assigned to the angle .alpha.. 
The information representing the values of the angles .alpha., .beta. and 
.gamma. is subsequently processed in the computer 60 for deriving the 
values of the azimuth angle .theta., the angle of sight .phi. and the 
angle of roll .rho.. In the present example, it is desired to determine 
the value of the angle of sight .phi. with the greatest accuracy. Its 
value is therefore calculated first and is directly constituted by .beta., 
whilst .rho. is derived from .phi. and .gamma. at 61 and the azimuth 
.theta. is derived, at 62, not only from .alpha. but also from .phi. and 
.rho.. The equations used are standard equations and are as follows: 
##EQU1## 
The azimuth .theta. is thus defined, except for .pi., whilst it may vary to 
a greater extent without consequent loss of information at the detection 
devices 22 and 23. These devices receive the beams from the diodes D1 and 
D2, in order to enable a different sign to be attributed to the angle 
.theta. depending on its direction relative to the reference direction, 
and to enable it to be thus defined at 65, except for 2.pi.. To this end, 
the detection signal of the device 22 is sampled at 63 on a clock signal 
H3 in synchronism with the control pulses for the diodes D1 and D2, and 
the circuit 64 converts the absence or presence of a corresponding 
illumination into a binary signal D, which is applied to the circuit 65 of 
the computer 60, which defines the sign of the angle .theta.. In the case 
described, the arrangement of the diodes D1 and D2 in conjunction with 
their emission angle ensures that the device 22 is illuminated by the one 
or the other diode when .theta. is positive from 0 to 180 degrees.