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Description  |
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BACKGROUND OF THE INVENTION
I. Field of the Invention
This invention relates generally to apparatus for accurately measuring the
orientation of an unconstrained body with respect to a fixed reference
system, and more particularly to a system used principally in military
aircraft wherein the aiming point is designated by sighting through a
viewing system incorporated into the pilot's helmet.
II. Description of the Prior Art
A principal use for the present invention is in target designation. When
the pilot identifies a target to be attacked, he causes a reticle in his
helmet visor system to coincide with the target and indicates that
coincidence by means of a push-button switch control. At that instant, the
orientation measurement system must be able to precisely identify the
orientation of the helmet with respect to a fixed reference, typically the
air frame.
It is very desirable that the orientation measurement system operates in a
manner which in no way constrains the movement of the pilot's head. Two
non-contact methods for performing this measurement are well known in the
prior art, namely, optical and magnetic. Optical systems, such as that
disclosed in the LaRussa U.S. Pat. No. 4,439,755 employ a collimated
optical source fixed to the pilot's helmet and precisely oriented with
respect to the line-of-sight axis. An optical receiver, which is sensitive
to the angle of arrival of the optical beam transmitted from the helmet,
is used to determine the orientation of the helmet. A commonly used method
for determining the angle of arrival of the beam is to employ a dual-axis
photo-detector in which X and Y axis analog voltages provide an electrical
output which is proportional to the angle of arrival of the beam of light
impinging on t he photo-detector.
Such optical systems, however, suffer a number of significant limitations.
First of all, an unobstructed line of sight must be maintained between the
optical transmitter and receiver for all possible orientations of the
helmet. This requirement often may be difficult to achieve in modern
cockpit configurations. While oftentimes all optical systems can maintain
a high degree of accuracy over limited field-of-view, the accuracy is
diminished over a practical field-of-view of .+-.70.degree. of azimuth or
pitch and .+-.120.degree. of azimuth or yaw which is typically required.
U.S. Pat. No. 4,396,885 to Constant describes a magnetic system which is
capable of an increased dynamic range of operation, which thereby
translates into higher accuracy over a large field-of-view. In a typical
magnetic system, such as that exemplified by the Constant patent, the
helmet transmitter is comprised of three mutually orthogonal magnetic
coils, one each for the X, Y and Z axis with respect to the line-of-sight
reference of the helmet reticle. Likewise, the receiver is comprised of
three mutually orthogonal coils, one each for the X, Y and Z axis with
respect to the fixed reference of the air frame. An operational system, as
will be subsequently described in more detail, relies on an accurate
measurement, axis-by-axis, of the mutual magnetic inductive coupling
between a transmitter coil and its respective corresponding receiver coil.
As in the system of the above-referenced Constant patent, the measurement
is generally accomplished using a single AC frequency for exciting the
three transmitter coils in sequence and tuning the receiver to reject
everything but that single frequency and, thus, generally rejecting all
stray magnetic signals which might otherwise introduce error. The AC
excitation frequency determines the tracking rate of the measurement
system. An excitation frequency of the order of 10 KHz is required for a
system to accurately track the rapid head motions that would normally be
encountered. Unfortunately, at this range of frequency, the position
accuracy of the system may be severely degraded due to eddy current
errors. That is to say, at higher frequencies, the AC magnetic field
generated by any of the transmitter coils may induce eddy currents in all
of the surrounding conductive material, such as the aircraft fuselage.
These eddy currents, in turn, generate an associated magnetic field. The
net result is that the surrounding conductive material distorts the
transmitted magnetic field, so as to introduce an error in the position
measurement. There is, thus, no practical prior art system which combines
the very desirable features of a wide field-of-view, accurate position
determination, and rapid tracking of the helmet motion.
It is the principal object of this invention to perform the direction
measurement in such a way that all of the above-mentioned desirable
features are simultaneously and effectively realized.
SUMMARY OF THE INVENTION
As was previously described, eddy current errors can significantly degrade
the accuracy of a magnetic measurement system at practical excitation
frequencies. To maintain a high tracking rate and to adequately reject
stray magnetic signals, which themselves could be alternate sources of
error, it is imperative to maintain an excitation frequency on the order
of 10 KHz. Thus, a means must be provided to accurately perform a
measurement at this higher frequency and then apply a correction factor to
the measurement, which totally accounts for magnetic fields occasioned by
eddy currents, to provide a result equivalent to the theoretical limit
afforded by DC excitation. This would be a relatively simple matter if the
eddy current error were only a function of the angular orientation of the
helmet, i.e., roll, pitch and yaw, since, in this case, the eddy current
correction could be computed directly from a measured set of x, y and z
components. However, the induced error is a function of both rotational
orientation and X, Y and Z coordinate position relative to the receiver.
Thus, a single set of measured x, y and z components is insufficient to
make the error correction.
The present invention overcomes this problem by performing coordinate
component measurements, concurrently, at least two discrete, preselected
frequencies. Since the magnitude of the eddy current error is a function
of frequency, these measurements provide sufficient information to compute
the error allowing it to be removed from the computed orientation of the
fixed and movable objects. While the preferred embodiment of this
invention discloses the use of two sets of excitation frequencies, it is
understood that even greater accuracy might be realized by employing
additional sets of such frequencies.
The foregoing principal objects as well as other features and advantages of
the present invention will become apparent to those skilled in the art
from the following detailed description of a preferred embodiment,
especially when considered in conjunction with the accompanying drawings
in which like numerals in the several views refer to corresponding parts.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram representation of the preferred embodiment;
FIG. 2 is a three-dimensional vector diagram useful in an understanding of
the present invention;
FIG. 3 represents graphically a multi-frequency transmitter drive signal;
and
FIG. 4 is a detail block diagram of the circuitry for implementing the
amplifier/multiplexer/A-to-D converter module of the block diagram of FIG.
1.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 is a block diagram of the coordinate measurement system comprising
the present invention. The transmit coil assembly 10 is comprised of three
mutually orthogonal coils 12, 14 and 16, and, as indicated by the legends,
each is aligned with the X.sub.T, Y.sub.T and Z.sub.T axes of an Eulerian
coordinate system, respectively. The transmit coil assembly is assumed to
be permanently fixed to the measurement reference platform. For example,
in an aircraft helmet sight application, the X.sub.T, Y.sub.T and Z.sub.T
axes would correspond to the roll, yaw and pitch axes of the aircraft,
respectively. Excitation is sequentially provided to the three transmitter
coils. This excitation consists of two or more frequency components. The
lowest of these frequency components, termed the fundamental component,
would be as lo as possible consistent with the desired update rate for the
overall system. A typical fundamental frequency might be 200 Hz. Any
additional frequency components would be harmonics of the fundamental and
would be selected to span a significant frequency range. In a typical
application, the second frequency would be 2,000 Hz or the tenth harmonic
of the fundamental. Since the error reduction will be accomplished by
extrapolating the trend from higher to lower frequencies, the use of
additional intermediate harmonics will enhance the error reduction
process. As is well understood in the signal processing art, the
frequencies employed should be judiciously chosen using the Nyquist
criteria to avoid interference or aliasing in subsequent signal processing
operations.
The receive coil assembly 20 is comprised of mutually orthogonal coils 22,
24, 26, also aligned with the X.sub.R, Y.sub.R and Z.sub.R axes of the
reference coordinate system. The receive coil coordinate system
corresponds to the movable assembly of the measurement system. For
example, in a helmet sight system, the X.sub.R axis would correspond to
the pilot's line-of-sight through the helmet sight reticle.
The amplifier/multiplexer/A-to-D converter 28 converts the analog signals
from the three receive coils into digital samples which are presented to
the signal processor 30. Specifically, at each sample interval, the
instantaneous value of the voltage signals coupled into the receive coils,
x or y or z is converted to a digital value and provided as input values
to the signal processor 30. The sample rate is preferably chosen such that
in a typical case in a 5 millisecond time period, 64samples of all three
channels are provided. With this 5millisecond sample interval, the motion
of the movable body (the helmet) during this time is negligible and the
data can be considered as concurrent, representing a measurement at a
single instant in time.
For each sample interval, the signal processor 30 performs a fast Fourier
transform (FFT) to compute a set of x, y and z components for each set of
excitation frequencies. For example, if a fundamental and one harmonic
frequency are used with multiple frequency source 18 (one relatively high
frequency and one relatively low frequency per axis), signal processor 30
will produce two independent sets of x, y and z components, one for the
relatively high harmonic frequency and another for the relatively
fundamental frequency. The three components, x, y and z, which are outputs
of the signal processor 30, are sufficient to identify the physical
orientation of the receive coils 20 with respect to the transmit coils 10.
Of specific interest are the angles .alpha. and .theta., which represent
the rotation of the receive coils in yaw and pitch, respectively, from the
reference orientation of the transmit coils 10. Coordinate computation 32
computes a pair of angles .alpha. and .theta. for each set of x, y, z
components generated by signal processor 30. As described before, each set
of angles thus produced will exhibit a degree of eddy current error in
each of them, where this error is a function of the excitation frequency.
Curve extrapolation 34 uses the multiple samples provided by coordinate
computation 32 to effectively estimate the eddy current error component
and then remove it to generate the relatively error-free computed sight
angle output on line 36. Well-known curve fitting techniques may be
employed to provide the best estimate of this resulting angle. The
functions represented by blocks 30, 32 and 34 in FIG. 1 all involve
well-understood digital processing algorithms and may be accomplished with
various configurations of general purpose or special purpose digital
processors.
FIG. 2 shows a graphical presentation of the system operation. Vectors
X.sub.T, Y.sub.T and Z.sub.T represent the orthogonal axes of the
reference platform for transmit coils 10. For clarity, only one octant of
a sphere is shown. However, the principles can be extended to all other
space. One set of x, y and z components based on high frequency excitation
results in a predicted axis, X2, which is established by angles
.alpha..sub.2 and .theta..sub.2. A second set of x, y and z components
based on lower frequency excitation results in a different axis, X1 which
is established by angles .alpha..sub.1 and .theta..sub.1. The intersection
of lines X2 and X1 with a spherical surface establishes a trajectory which
can be extrapolated to point P. The line from the origin, O, to the point,
P, is thus the computed sight angle.
Referring next to FIG. 3, it shows a particularly efficient way for
generating a multiple frequency excitation for the transmit coils 10. The
induced voltage pattern is generated by summing 200, 600, 1,000, 1,400,
1,800 and 2,200 Hz frequency components. This voltage pattern when applied
to the transmit coils 10 results is a pseudo square wave flux pattern
which is shown in dashed lines.
FIG. 4 shows a detailed block diagram implementation of the
amplifier/multiplexer/A-to-D converter circuitry shown as block 28 in FIG.
1. Multiplexer 38 may comprise a 4.times.2 differential multiplexer. It
receives differential input signals from receive coils 22, 24, 26 on
inputs 40-45, respectively. Differential inputs 46 and 47 provide an
attenuated transmitted signal which is useful for calibration purposes.
The operational amplifier 48 provides a linear amplification of the signal
selected by multiplexer 38. Bandpass filter 52 rejects out-of-band noise
while one or more notch filters 54 may be added to provide additional
rejection at predetermined frequencies which may be emanating from a stray
signal source within the area occupied by the system of the present
invention. For example, the frequency of the horizontal sweep of the
magnetic deflection circuitry typically used in cockpit displays can be an
overwhelming source of noise for the system if not specifically rejected
by a notch filter or the like.
Processor 58 controls the selection of the input signal via multiplexer 38
and its conversion to a digital input. Analog signals may be prescaled
before being converted to make maximum use of the dynamic range of the A/D
converter. In this regard, an 8-bit scaling value is transmitted to a
buffer register 60 which controls the state of an 8-bit ladder network 62.
Ladder 62 is placed in the feedback loop of an AGC amplifier 63 so that
the resulting gain of this stage is proportional to the value stored in
buffer 60.
In generating one digital sample, the following steps are executed: (1)
Processor 58 selects the input channel 40-45 to be sampled; (2) It
transmits the scaling value to buffer register 60; (3) It captures the
analog output of amplifier 63 in sample-and-hold circuit 64 and presents
this value as the analog input to A/D converter 68; (4) The A/D converter,
in turn, transforms this value to a 12-bit digital number which is
provided as an operand input to processor 58.
The signal processor 30 in FIG. 1 preferably includes a high-speed fast
fourier transform chip, such as the Type 32010 device sold by the Texas
Instruments Company, which is found to allow a sufficiently high system
update rate and an adequate number of samples to match with the A-to-D
converter employed which accommodates a 25 millisecond cycle time. As
earlier mentioned, using an FFT computation algorithm, the processing
computer 30 extracts from each axis the value of the multiple frequency
components used to drive the transmitter coils 10. Once the information
from the three transmitter axes have been received and processed, a matrix
of nine components are available for each frequency component employed.
This allows computation of the location and angular orientation of the
helmet coils with respect to the transmitter coils to be accomplished.
This invention has been described herein in considerable detail in order to
comply with the Patent Statutes and to provide those skilled in the art
with the information needed to apply the novel principles and to construct
and use such specialized components as are required. However, it is to be
understood that the invention can be carried out by specifically different
equipment and devices, and that various modifications, both as to
equipment details and operating procedures, can he accomplished without
departing from the scope of the invention itself.
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