Note: Descriptions are shown in the official language in which they were submitted.
2150582
METHOD AND APPARATUS FOR GUIDING A DRIVERLESS
VEHICLE USING A SENSOR TRACKING A CABLE
EMITTING AN ELECTROMAGNETIC FIELD
TECHNICAL ~IELD
This invention relates to apparatus and
method for guiding a driverless vehicle along a
guide cable buried in a road surface and more
particularly to the use of two detecting coils
placed in an X configuration on the vehicle for
sensing an electromagnetic field direction vector
independent of spacial and electromagnetic field
magnitudes which information is used to measure
lateral displacement of the vehicle relative to the
guide cable to steer the vehicle so as to track the
guide cable.
BACKGROUND ART
Perpendicularly disposed coils have been
mounted on a driverless vehicle and used to detect
the electromagnetic field surrounding a guide cable
for automatically guiding the driverless vehicle
along the cable. In known apparatus, one coil is
disposed vertically and the other coil is disposed
horizontally. Voltages induced in the coils are
compared and used to determine the lateral location
of the coils relative to the guide cable. This
location information is processed and used to steer
the vehicle.
2150582
The output voltage associated with these
coils varies proportionately with current frequency
in the guide cable, guide cable current magnitude,
radial distance from the guide cable, coil core
S size, number of coil wire turns and the angle found
between the major axis of the coil relative to a
line from the cable to the center of the coil,
referred to as angle beta.
As each coil is rotated in a plane
perpendicular to the cable generating the
electromagnetic field, it's output will be~come
maximum when the coil core is parallel to the
circular lines of flux. Its output will become
minimum (zero) when the coil core is perpendicular
to the flux, i.e. pointing to, or away from, the
wire. Thus the relative effectiveness of the coil
varies as the 'sin' of the angle beta.
The sin (beta) term, reflects the ratio
of the radial to circular field sensed at each coil
location and affects the sensor output. Therefore,
the difference in magnitude sensed between the two
coils is based on the radius term, the respective
distances between each coil and the cable current.
Changes in any of these factors have a profound
effects on the output signal. Additionally,
depending on angle beta, some of either the radial
and/or circular field information associated with
conventional apparatus must be discarded, resulting
in a less than ideal signal to noise ratios.
Another disadvantage with conventional coil
arrangements is that the information provided
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by the coils only indicates the approximate lateral
displacement from the guide cable and not the
measure or distance of the lateral displacement
Therefore, vehicle steering correction can only be
made in the direction opposite the displacement and
not with precision.
DISCLOS~RE O~ INVENTION
An object of the present invention is to
provide an apparatus for guiding a driverless
vehicle along a guide cable disposed in a road
surface that measures the lateral deviation of the
vehicle from the guide cable.
Another object of the present invention
is to provide an apparatus for guiding a driverless
vehicle along a guide cable disposed in a road
surface having an increased 'field of view~ of the
cable over conventional apparatus.
Another object of the present invention
is to provide an apparatus including
perpendicularly disposed coils for providing off-
center displacement measurement information for
guiding a driverless vehicle on the basis of sensed
lateral displacement from a guide cable disposed in
a road surface wherein the sensed lateral
displacement is determined solely from an
electromagnetic field direction vector and is
independent of all electromagnetic field magnitude
components
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Another object of the present invention
is to provide a method for sensing an
electromagnetic field direction vector at a single
spacial point for providing an error signal that
defines a measurement for guiding a driverless
vehicle over a guide cable disposed in a road
surface.
In carrying out the above objects and
other objects the apparatus for guiding a
driverless vehicle along a path defined by a guide
cable disposed in a road surface, wherein the guide
cable carries a current generating an
electromagnetic field in the space surrounding the
guide cable, includes a sensor for sensing the
direction and magnitude of the electromagnetic
field surrounding the cable. The sensor is defined
by first and second spaced detecting coils having
major axes being mounted in an X-coil configuration
on said vehicle such that the major axes are
intersecting in the direction of current and are
oriented generally at +/-45 degrees relative to the
road surface. Each detecting coil senses both the
radial and circular field vectors of the magnetic
field at a given point and a processor in
communication with the first and second detecting
coils compares the magnitude of the radial vector
with the magnitude of the circular vector whereby
the measurement of the lateral position of the
sensor relative to said guide cable is determined.
A method for guiding the driverless
vehicle along a path defined by a guide cable
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disposed in a horizontal road surface, wherein the
guide cable carries a current generating an
electromagnetic field in the space surrounding the
guide cable, includes the steps of;
mounting a first coil, having a major
coil axis, at + 45 degrees relative to the
horizontal on said vehicle;
mounting a second coil, having a major
coil axis, at - 45 degrees relative to the
horizontal on said vehicle such that the axes of
the first and second coils intersect in the
direction of current;
sensing both the radial and circular
field vectors of the electromagnetic field with
each coil;
comparing the magnitude of the radial
vector with the magnitude of the circular vector to
establish the lateral position of the intersection
point of the axes of the coils relative to the path
off-center displacement information; and
communicating this information to a
steering assembly to steer the vehicle.
The above objects and other objects,
features, and advantages of the present invention
are readily apparent from the following detailed
description of the best mode for carrying out the
invention when taken in connection with the
accompanying drawings.
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BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a schematic diagram of the
front or rear of a driverless vehicle being guided
over a guide cable;
FIGURE 2 is a diagram illustrating first
and second coils mounted generally at +/- 45
degrees relative to the horizontal accordance with
the present invention, field lines and voltage
vectors in the electromagnetic field due to the
guide cable which carries alternating current;
FIGURE 3 is a block diagram illustrating
the various voltages occurring in the detecting
coils and their conversion to an output off center
distance measurement signal; and
FIGURE 4 is a schematic block diagram of
one embodiment of the invention.
BEST MODE FOR CARRYING OUT THE INVENTION
With reference to FIGURE 1, a driverless
vehicle 10 is seen from the front or rear for
following a guide cable 12 disposed in a horizontal
road surface 14. Mounted on the vehicle 10 is a
sensor 16 defined by two detecting coils 18,20.
One coil 18 is mounted at +45 degrees relative to
the road surface 14, and the other coil 20 is
mounted at -45 degrees. The terms +45 and -45
degrees refer to the angle of the axis of the coil
or its core relative to the road surface 14
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2150~82
or horizontal The axes of the coils 16,18
intersect in the direction of current flow The
guide cable 12 carries an alternating current
which, when undisturbed, generates an
electromagnetic field having circular magnetic
field lines which generate in the coils 18 and 20
voltages that may be used for measuring the lateral
off center position of the vehicle 10 relative to
the guide cable 12 and subsequently for steering
the vehicle to follow the cable
The manner in which the driverless
vehicle 10 is guided over the cable 12 will now be
discussed with reference to FIGURES 2-4. The
dashed lines in FIG. 2 are intended to represent
the circular vectors of the magnetic field
surrounding the guide cable 12. These field lines
are intended to represent the case where the
vicinity of the cable 12 is free of ferromagnetic
objects and other current carriers which would
distort the circular cross section of the field
lines. The elevational location or height h of the
detector coils 18,20 remains constant.
Due to the mounting geometry of the coils
18,20 wherein each major axis is positioned at +/-
45 degrees to the horizontal, each coil produces anoutput voltage which is the vector sum of both the
horizontal and vertical portions of the guide
cable's 12 cylindrical electromagnetic field When
the sensor 16 is centered over the cable each coil
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18,20 sees equal horizontal and vertical signal
magnitudes. However, the signs of each signal are
opposite, as each coil 18,20 sees the source on
different sides of their major axis When the
sensor 16 is moved laterally to the left or right
of the guide cable 12, the output of each coil 18,
changes as a function of the sum of itls
orientation to the horizontal plane (+/- 45
degrees) and the error angle Theta, in the vertical
plane.
Either coil's signal or voltage is
maximum when it's major axis is perpendicular to
the guide cable 12 and alternatively is minimum
(zero) when it's major axis is pointing toward the
guide cable. Since these coils 18, 20 move in a
plane parallel to the cable 12, while cutting it's
cylindrical electromagnetic field, the form of
their output signals are proportional to the SIN of
(Beta Coil 1 or 2).
Accordingly, the true position of the
sensor 16 or error position information for a
feedback loop as hereinafter described, is found by
dividing the radial vector difference by the
circular vector sum sensed by the coils 18,20. The
actual signs imply that it is the sum of the radial
vectors that is divided by the difference of the
circular vectors, but to preserve a hardware phase
comparator operation for monitoring 'guide-safe',
the polarity of the coils 18,20 is chosen such that
when the sensor 16 is centered over the cable 12,
the output of the coils are 180 degrees out
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of phase. Thus, when the sensor 16 is centered
over the cable 12, vectors being equal but of
opposite sign, the radial vectors cancel in the
numerator, while the circular vectors add to
double, in the denominator. As the height h
remains positive, i.e. the sensor 16 stays above
the cable 12, the denominator will never reach zero
and cause an invalid divide operation.
With reference to FIGURE 2, the following
equations apply:
Signal Coil 1 = K1 ~ SIN ~Beta Coil 1)
Signal Coil 2 = K1 ~ SIN (Beta Coil 2)
Therefore:
Signal of Coil 2 ~ Signal of Coil 1
Di6tance (+/- Err) = Height ~ ---------------------------------
Signal of Coil 2 - Signal of Coil 1
The following is a mathematical proof for the above
linear measurement equation.
Where by definition:
Clock Wise (CW) Angles, from ver~ical, are
positive.
Radius r is defined as the line/distance
between the center of the coils 18, 20 and the
center of the cable 12
Current is defined as the electrical current
in the wire.
Frequency is defined as the frequency of the
Current
Height h is the vertical distance from the
cable to the horizontal plane in which the
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coils 18,20 can move
Distance d (+/-) is the horizontal distance
from the cable to the center of the coils
Alpha Coil 1 is the off-vertical angle of coil
l (+45)
Alpha Coil 2 is the off-vertical angle of coil
2 (-45)
Theta is the error angle between the radius
and the height
and
K1 is proportional to current, frequency and
inductance and inversely proportional to
radius
Signal Coil 1 = Rl ~ SIN (Beta Coil 1) (eq. 1)
Signal Coil 2 = Kl ~ SIN (Beta Coil 2~ ( eq. 2)
As seen in FIGURE 2:
Beta Coil 1 = Theta - Alpha Coil 1
and
Beta Coil 2 = Theta - Alpha Coil 2
since when Theta = Alpha Coil 1, Beta Coil 1 goes
to zero and when Theta = Alpha Coil 2, Beta Coil 2
goes to zero.
Therefore:
Signal Coil 1 = Kl ^ SIN (Theta - Alpha Coil 1) (eq 3)
Signal Coil 2 = Kl ~ SIN (Theta - Alpha Coil 2)( eq 4)
From the 'SIN(A-B) = SIN (A)*COS(B) - COS(A)*SIN
(B)' trigonometric identity, equations 3 and 4
above can be expanded as:
Signal Coil 1 = Kl ~ (SIN(Theta)~COS(Alpha Coil 1)
COS(Theta)~SIN(Alpha Coil 1)) ( eq 5)
Signal Coil 2 = Kl ~ (SIN(Theta)~COS(Alpha Coil 2) -
COS(Theta)^SIN(Alpha Coil 2)) ( eq 6)
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Since, by definition:
Alpha Coil 1 = + 45 degree6 and Alpha Coil 2 = - 45 degree6
we see: SIN(Alpha Coil 1) = COS(Alpha Coil 1) =
-SIN (Alpha coil 2) = COS(Alpha Coil 2)
s with all magnitudes being equal to (Square Root of
2)/2 which is approximately equal to 0.707...
Further referring to the geometry illustrated in
FIGURE 2, it can also be seen:
SIN(Theta~ = Di~tance / Radius ~ d / r
and
COS(Theta) - ~eight / Radiu~ ~ h / r
Equations 5 and 6 can now be simplified by
substitution:
Signal Coil 1 ~ Kl. ~ (d/r ^ 0.707 - h/r ~ 0 707)
Signal Coil 2 ~ (d/r ~ 0.707 - h/r ~ -.707)
or when:
Signal Coil 1 = K' ~ (d - h) K~ = (Kl ~ 0.707) /r
Signal Coil 2 = K' ~ (d + h)
To solve for the desired error Distance (+/-d) in
terms of the sensor's height (h), the two coil
outputs are combined as follows:
Signal Coil 2 Signal Coil 1 ___ ((d + h) (d - h))
Signal Coil 2 - Signal Coil 1 K' ~ ((d + h) - (d - h))
K' ~ (d + h + d - h) 2d d Di6tance
____________________ = ___ = __ = _ _ ____
K' ~ (d h - d + h) 2h h Height
Therefore:
Signal Coil 2 Signal Coil 1
Di6tance (+/- Err) = Height ~ ------------------------------
Signal Coil 2 - Signal Coil 1
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12
Use of this solution. In combination
with known devices for a driverless vehicle
steering, can be implemented as illustrated in the
block diagram of FIGURE 3.
With reference to FIGURE 3, it can be
seen that the single add, subtract, divide and
multiply are all that is required for
implementation of this solution, and can be handled
by either analog or digital electronics
Additional signal conditioning, i.e. filtering, AC
demodulation, A/C conversions can be easily
accomplished
In order to use sensor 16, the output of
the coils 18,20 has to be synchronously
demodulated, in order to preserve the sign
information.
FIGURE 4 illustrates in block diagram
form one method of implementing the 'X-Coil' sensor
16 into the steering system of a driverless vehicle
10.
With reference to FIGURE 4, an example of
the herinabove described 'X-Coil' configuration was
designed for vehicle 10 steering for a working
height of 3 inches above a 100 milliampere
guidepath cable, yielding a hardware 'guide-safe'
width of +/- 3 inches However, for those cases
where the sensor height h has to be either higher
or lower, and the software 'guide-safe~ feature
hereinafter below described may be employed so that
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the sensor 16 will easily cover the 1 to 6 inch
height range With use of AGC, a digital steering
package can handle a cable current range of 20 to
400 milliamperes, a height range of 1 to 6 inches
and a horizontal displacement range of t /- 12
inches.
Using high Q filters and synchronous
demodulators, all external signals, not synchronous
in frequency and phase, are rejected One signal
that will not be rejected, however, is the same
wire nearby, such as with a 'return cut' in the
path. A return cut will distort the electromagnetic
field, thereby shifting the null as seen by the
sensor. This distortion will cause a null shift
that is a direct proportion of the distances of the
sensor to each of the cables, i e if the height is
3 inches and the return cut is 24 inches, the null
will shift 3/24 times the unity output position (3
inches at a 3 inch height), or 3/24 * 3 = 3/8 of an
inch, with the direction of the shift depending on
the phase of the 'return cut' At one half the
distance between the two cables, everything fails
as the resulting circular vector, the
division, goes to zero Linearity also falls off
at the 10 to 12 inch range due to return cuts' as
close as 20 feet For these reasons, the current
package scales the sensor's output signal to +/-
full range at +/-8 inches of horizontal
displacement.
30Accordingly the field of view seen by
sensor 16 is greatly increased, without losing port
21~05~2
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and starboard directions. This improves the 'off-
wire maneuver' recapture procedure Also with the
vastly improved linearity, and current provisions
for entry of the vehicles sensor height, all
outputs become a uniform ratio of +/- 8 inches,
thereby providing off-wire steering adjustments of
+/- 4 inches, to account for 'return cuts' and/or
to center on loads.
Both the wider 'field of view' and the
linearity, provide variable programmed 'guide-safe'
limits. This feature has proven very useful in
monitoring 'off-wire maneuvers', as the ANSI
specifications allows a +/- 6 inch guide-safe
window for such operations. The internal hardware
phase comparators provide a primary +/- 'guide
safe' output This window, however, is not set to
+/- 3 inches for all sensor heights
With the 'x-coil' configuration the +/-
'guide safe' window becomes equal to the height h
of the coils 18, 20 above the wire, i e for a
sensor 16 centered 3 inches above the wire, the
'guide-safe' window will be +/- 3 inches and will
vary +/- 1/2 inch if the wire depth changes +/-1/2
inch. Under control of a vehicle microprocessor,
the digital steering software controls which one,
or both, of the 'guide-safe' signals are active
While the best mode for carrying out the
invention as been described in detail, those
familiar with the art to which this invention
relates will recognize various alternative designs
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and embodiments for practicing the invention as
defined by the following claims.
