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Description  |
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BACKGROUND OF INVENTION
1. Field of the Invention
The invention relates to a non-contact sensing system for monitoring the
position and orientation of a rigid body in space. More specifically, the
invention relates to such a system which can monitor the position and
orientation of a pilot's helmet within the cockpit of a high performance
tactical aircraft simulator.
2. Description of Prior Art
In order for aircraft simulators to provide a realistic simulation of
aircraft flight, it is necessary to display a simulated surrounding
environment to the simulator pilot. In currently available flight
simulators, the displays are provided on the windscreens in the cockpit
areas. These systems require one CRT for each such windscreen. With such a
system, images of equal quality must be generated in each CRT regardless
of where the pilot is looking. This is costly both in terms of hardware
and computer time.
However, by projecting the image directly onto the visor of a pilot's
helmet, a significant cost saving can be realized since the field of view
is greatly reduced so that both the quality of the image (i.e. resolution)
and the frame rate can be reduced. The field of view would be head slaved
in order to provide an image which is instantaneously limited to the
pilot's viewing area but which can be redirected to permit the pilot to
"look" in any direction. It is also contemplated to track eye position or
point of regard and provide a higher resolution image slaved to the eye's
foveal area. In such a system, the pilot would perceive high resolution
imagery wherever he looks. However, at any instant, the system is only
required to produce high resolution images over a 15.degree. cone.
One of the difficulties associated with the use of helmet mounted visor
displays is the maintenance of image stability. In order for the computer
generated image to appear stationary as the pilot moves his head, the
position of the pilot's helmet must be known relative to the simulator's
fixed axis. This position is defined by three translation parameters (X,
Y, Z) and three rotation parameters (Euler Angles). This information is
also necessary in order to align the computer generated image with the
frame of the windscreen. Thus, a (helmet) position sensing system is
needed.
Currently available non-contact position sensing systems comprise a
magnetic system as described in Raab, F. H., Blood, E. B., Steiner, T. O.,
and Jones, H. R., "Magnetic Position and Orientation Tracking System",
IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-15, No. 5,
September 1979, pp. 709-718, and a V-slit system as described in Lewis, C.
J. G., "Helmet Mounted Display and Sight Development", Proceedings of the
American Helicopter Society, May 1979, pp. 79.17.1-79.17.13. However, both
of these systems fail to meet required specifications.
SUMMARY OF INVENTION
It is therefore an object of the invention to provide a non-contact
position sensing system which meets the required specifications.
In accordance with the invention, the system monitors the position
orientation of a body in space which has at least three distinct point
light sources mounted thereon. The system includes at least two position
sensor heads each of which includes a position sensitive detector.
BRIEF DESCRIPTION OF DRAWINGS
The invention will be better understood by an examination of the following
description, together with the accompanying drawings, in which:
FIG. 1 is a schematic diagram of the inventive system;
FIG. 2A is a schematic diagram of a position sensitive detector;
FIG. 2B is an electron flow diagram of the position sensitive detector; and
FIG. 3 illustrates an analog position processing circuit for processing
data of the position sensitive detector.
DESCRIPTION OF PREFERRED EMBODIMENTS
Referring to FIG. 1, 1 is a rigid object in space whose position and
orientation must be monitored. In the particular embodiment illustrated, 1
is a helmet which is worn by a pilot in the cockpit of a high performance
tactical aircraft simulator.
A mounting plate 3 carries at least three point light sources, preferably
LED's, and the mounting plate is mounted on the helmet. Obviously, the
LED's could be mounted directly on the helmet instead of on a mounting
plate.
Fixedly mounted in the cockpit are position sensor heads 5 and 7. The
sensor heads are mounted within detection range of the light from all of
the LEDs, and are directed at the LEDs. Camera lenses 6 and 8 are disposed
in front of the position sensor heads for reasons explained below.
As the position sensor head is sensitive to a wide spectrum of light,
bandpass interference filters, illustrated schematically at 9 and 11, are
placed in front of each lens to minimize the effect of ambient light. The
filters are spectrally matched to the light of the LEDs.
Each position sensor head includes a position sensitive detector which will
be discussed in association with FIGS. 2 and 3 below.
The output of each position sensor head is fed to an analog-to-digital
converter 13, and the digital data is then fed to a computer 15. One
output of 15 is fed to a controller 17 which provides a host computer with
the position and orientation information of the helmet as computed in the
computer.
As is known, and will be discussed below, at least three LEDs coupled with
two position sensor heads are needed to determine the position and
orientation of the helmet in all six degrees of freedom. In accordance
with the invention, the LEDs are turned on one at a time, and in
sequential arrangement, and the three dimensional position of each LED is
determined by a triangulation scheme using the data from the two position
sensor heads.
It is also necessary to maintain a constant light intensity of each LED,
when it is turned on, regardless of the position or orientation of the
helmet.
LED controller 19 maintains a constant light intensity of the turned on
LEDs and also turns on the LEDs one at a time and in sequential
arrangement. As can be seen, the controller 19 is controlled by the
computer 15 which determines the required LED current to maintain a
constant light intensity as a function of the position of the helmet. The
computer also selects the particular LED to be turned on, and takes into
account the fact that it was this turned on LED when computing the
position and orientation of the helmet.
Turning now to FIGS. 2A and 2B, as can be seen, the position sensitive
detector is a planar photodiode 21 with very uniform resistive layers
formed over both the top and bottom surfaces. The camera lenses 6 and 8
are used to focus the infrared light from the LEDs onto the active area of
this detector.
This results in the generation of electron-hole pairs in the depletion
layer directly under the light spot. The electrons migrate to the N-layer
where they are channelled between two electrodes. Since the N-layer has a
uniform resistivity, the current flowing to the pair of Y electrodes will
depend on its distance from the point of incidence of the light.
The same basic principles apply to the P-layer, i.e., holes will migrate to
the P-layer. The current signals are then processed using, for example,
the circuitry illustrated in FIG. 3.
Turning now to FIG. 3, the processing circuit comprises preamps 23
connected to each one of the X and Y electrode pairs. Adders 25A add the
contents of the X electrodes and adders 25B add the contents of the Y
electrodes. Subtractors 27A subtract the contents of the X electrodes and
subtractors 27B subtract the contents of the Y electrodes. The outputs of
25A and 27A are fed to background cancellation circuits 29A and the
outputs of 25B and 27B are fed to background cancellation circuits 29B,
and the outputs of cancellation circuits 29A and 29B are fed to dividers
31A and 31B, respectively.
With filtering, as per filters 9 and 11, the resolution of the position
sensor head can be as high as 0.02% of full scale. The filtering is
necessary because of the high frequency jitter which is normally present.
With the dynamic performance of the inventive device, it can be used for
vibration measurements at frequencies of up to 50 kHz.
It is necessary to use two sensors viewing the same LED patterns (at least
three LEDs) to uniquely determine the helmet position in all six degrees
of freedom. The image on the sensitive area of the detector of each LED is
a two-dimensional perspective projection of an LED moving in
three-dimensional space. Consequently, the three-dimensional position of
the LED cannot be recovered from its image coordinates. At best, only the
direction of the vector between the LED and the sensor can be determined.
If each LED is viewed from two locations, a triangulation scheme can be
used to compute the position of each LED in three dimensions. Therefore,
with a minimum of three LEDs and two sensors, the helmet position can be
uniquely determined in all six degrees of freedom.
The addition of more LEDs results in an overdetermined system of equations
which can increase the accuracy of the measurement through the application
of least squares analysis as taught in Roach, J. W. and Aggarwal, J. K.,
"Determining the Movement of Objects from a Sequence of Images", IEEE
Transactions on Pattern Analysis and Machine Intelligence, Vol. PAMI-2,
No. 6, November 1980, pp. 554-562.
Data processing tasks for the optical helmet position sensing systems are
accomplished, in one embodiment, by an INTEL iSBC 86/12 single board
computer. This Board's capabilities are summarized as follows:
______________________________________
CPU 8086
CPU SPEED 5 MHz
RAM (BASE BOARD) 32K
EPROM (BASE BOARD) 16K
SERIAL I/O 1
PARALLEL I/O LINES 24
TIMERS 2
INTERRUPTS 8
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In order to determine the helmet position and orientation, a large number
of floating point operations must be performed. Several trigometric
functions are also required. The floating point operations can be
performed by the 8087 Numeric Processor. The 8087 has the ability to
perform the high speed floating point mathematics required in complex
control algorithms.
Helmet position and orientation data are, as above-mentioned, transferred
from the computer to a host computer. In the preferred embodiment, the
transfer from the INTEL 86/12 is to a SEL 32/55 host computer. In this
embodiment, the control 17 comprises a MULTIBUS-SELBUS Controller (MBSEL).
The MBSEL board is an intelligent DMA controller which communicates with
the SELBUS via a high speed data interface board (HSD). Because of the
86/12 board's dual port RAM, the MBSEL can access the position and
orientation data directly via the MULTIBUS. Data transfers are transparent
to both the SEL and 8086 processors.
Again, in the particular embodiment, a custom-built analog board 13 is used
to read the output signals from the position sensor heads and to convert
them into digital signals. In order to minimize the conversion time, two
independent 14-bit analog-to-digital converters were included on this
board.
Methods for the mathematical analysis of the data are taught in, for
example, Lenox, J. B., Six Degree of Freedom Human Eyeball Movement
Analysis Involving Steriometric Techniques, Ph.D. Thesis, Stanford
University, Stanford, Calif., 1976, Schut, G. H., "On Exact Linear
Equations for the Computation of the Rotational Elements of Absolute
Orientation", Photogrammetria, Vol. 17, No. 1, 1960, pp. 34-37, Schut, G.
H., "Formation of Strips from Independent Models", AP-PR 36, NRC-9695,
Division of Physics, National Research Council of Canada, Ottawa, July
1967, and Thompson, E. H., "An Exact Linear Solution of the Problem of
Absolute Orientation", Photogrammetria, Vol. 15, No. 4, 1959, pp. 163-179.
In comparison with other non-contact position sensing systems, the
inventive system offers high resolution and speed at a relatively low
cost. These features suggest other possible applications of the inventive
system in the areas of robot vision, industrial control, kinesiology and
laboratory instrumentation. Accordingly, the inventive system has uses
outside of the environment as described above in the preferred embodiment.
Although a particular embodiment has been described, this was for the
purpose of illustrating, but not limiting, the invention. Various
modifications, which will come readily to the mind of one skilled in the
art, are within the scope of the invention as defined in the appended
claims.
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