U.S. patent application number 13/597535 was filed with the patent office on 2014-03-06 for automatic guided vehicle system and method.
The applicant listed for this patent is Christopher John Murphy. Invention is credited to Christopher John Murphy.
Application Number | 20140067184 13/597535 |
Document ID | / |
Family ID | 50180670 |
Filed Date | 2014-03-06 |
United States Patent
Application |
20140067184 |
Kind Code |
A1 |
Murphy; Christopher John |
March 6, 2014 |
AUTOMATIC GUIDED VEHICLE SYSTEM AND METHOD
Abstract
An apparatus and method for guiding an automatic guided vehicle
along a magnetic pathway and more specifically to an apparatus and
a method capable of accurately and precisely following a weak
magnetic field emitted by a substantially continuous passive
magnetic marker having route junctions.
Inventors: |
Murphy; Christopher John;
(Ann Arbor, MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Murphy; Christopher John |
Ann Arbor |
MI |
US |
|
|
Family ID: |
50180670 |
Appl. No.: |
13/597535 |
Filed: |
August 29, 2012 |
Current U.S.
Class: |
701/23 |
Current CPC
Class: |
G05D 1/0265 20130101;
G05D 2201/0216 20130101 |
Class at
Publication: |
701/23 |
International
Class: |
G05D 1/02 20060101
G05D001/02 |
Claims
1. An automatic guided vehicle (AGV) configured to travel on a
supporting surface and follow a pathway producing a magnetic field,
said AGV comprising: a support assembly having a plurality of
wheels and wherein at least one of said plurality of wheels is
configured to directionally steer the AGV; a first magnetic sensor
assembly coupled to said support assembly, said magnetic sensor
assembly having a first magnetic field detector arranged adjacent
to a second magnetic field detector and wherein said first magnetic
field detector is configured to measure the strength of the
magnetic field produced by the pathway along a first axis and
output a first signal related to the magnetic field strength along
said first axis of said first magnetic field detector and said
second magnetic field detector is configured to measure the
strength of a magnetic field along a second axis and output a
second signal related to the magnetic field strength along said
second axis and wherein said first axis of the first magnetic field
detector is orthogonal to said second axis of the second magnetic
field detector; and a controller capable of receiving said first
and second signals and based on said first and second signals,
determining a steering signal to said at least one of said
plurality of wheels configured to directionally steer the AGV;
wherein said first and second magnetic field detectors are located
adjacent to each other on a single chip and approximately aligned
with the supporting surface.
2. The AGV of claim 1 wherein at least one of said first axis and
second axis is substantially perpendicular to the supporting
surface and the other of said first axis and second axis is
substantially parallel to the supporting surface.
3. The AGV of claim 2 wherein each of said first axis and said
second axis are substantially located within a first plane that is
located substantially perpendicular to the supporting surface.
4. The AGV of claim 3 wherein said first plane is substantially
perpendicular to the pathway.
5. The AGV of claim 1 further including a second magnetic sensor
assembly coupled to said support assembly and wherein said second
magnetic sensor assembly includes a third magnetic field detector
and a fourth magnetic field detector, each having an axis and
wherein said axes of said third and fourth magnetic field detectors
are orthogonal and located substantially within a second plane.
6. The AGV of claim 5 wherein each of said first axis and said
second axis are substantially located within a first plane that is
located substantially perpendicular to the supporting surface and
wherein said second plane is approximately aligned with or within
said first plane.
7. The AGV of claim 1 wherein said field detectors are selected
from the group consisting of hall effect sensors and giant
magnetoresistance sensors.
8. The AGV of claim 1 further including a second magnetic sensor
assembly coupled to said support assembly and wherein said second
magnetic sensor assembly includes a third magnetic field detector
and a fourth magnetic field detector, each having an axis and
wherein said axes of said third and fourth magnetic field detectors
are orthogonal and located substantially within the second plane,
and a third magnetic sensor assembly coupled to said support
assembly and wherein said third magnetic sensor assembly includes a
fifth magnetic field detector and a sixth magnetic field detector,
each having an axis and wherein said axes of said fifth and sixth
magnetic field detectors are orthogonal and located substantially
within a third plane, and wherein the axes of said first, third and
fifth field detectors are substantially aligned and wherein the
axes of said second, fourth and sixth field detectors are spaced
apart and substantially parallel.
9. The AGV of claim 8 wherein said first, second and third planes
are substantially aligned or within the same plane.
10. A method of controlling an AGV, operating on a supporting
surface, along a pathway producing a magnetic field and wherein
said pathway is substantially continuous, said method comprising
the steps of: determining the strength of the magnetic field along
a first axis; determining the strength of the magnetic field along
a second axis, wherein said second axis is orthogonal to said first
axis; calculating a first vector directed at the pathway using the
determined strength of the magnetic field along the first axis and
the determined strength of the magnetic field along the second
axis; calculating a second vector directed at the pathway using the
determined strength of the magnetic field along the first axis and
the determined strength of the magnetic field along the second
axis; determining a point of convergence of the first vector and
the second vector; and determining if the point of convergence of
the first vector and the second vector is within a predetermined
range of the supporting surface.
11. The method of claim 10 further including the step of comparing
deviation of the angle of the calculated vector to a reference
vector angle.
12. The method of claim 10 further including the steps of:
determining the strength of the magnetic field along a third axis;
determining the strength of the magnetic field along a fourth axis,
wherein said fourth axis is orthogonal to said third axis;
calculating a second vector directed at the pathway using the
determined strength of the magnetic field along the third axis and
the determined strength of the magnetic field along the fourth
axis.
13-14. (canceled)
15. The method of claim 10 further including the step of
determining that the determined strength of the magnetic field
along at least one of the first axis and the second axis exceeds a
baseline strength.
16. The method of claim 10 further including the step of
instructing the AGV to stop if the point of convergence is outside
of the predetermined range of the supporting surface.
16. The method of claim 10 further including the step of correcting
an actual travel path of the AGV to match a desired travel path
aligned with the magnetic marker based on the direction of the
first and second vectors.
Description
TECHNICAL FIELD
[0001] The present invention is directed to an apparatus and method
for guiding an automatic guided vehicle (AGV) along a magnetic
pathway and more specifically to an apparatus and a method capable
of accurately and precisely following a weak magnetic field emitted
by a substantially continuous passive magnetic marker having route
junctions.
BACKGROUND OF THE INVENTION
[0002] AGVs having automated guidance systems and thereby capable
of operating without a human operator are increasingly common in
industrial facilities. AGVs are used for a variety of tasks and
functions, of which the most common function is to transport
material along predetermined routes. To ensure precise and accurate
guidance, AGVs use many guidance methods, including dead reckoning,
electrified guide wires, optical systems, inertial guidance
systems, magnetic markers, as well as a variety of other systems.
Each of these systems has a variety of drawbacks, typically related
to system cost and complexity. The largest components of the system
costs are the initial installation costs and the cost of each AGV.
Issues related to the complexity of the system typically include
limited guide path revision flexibility, high costs associated with
any guide path revisions, complex and heavy processor use during
operation, and for some systems, limited operational accuracy.
[0003] In an electrified guide wire system, a conductive wire is
buried in the floor of a facility and produces a strong magnetic
field. More specifically, the AGV's guidance system senses and
tracks an active magnetic field generated by current passing
through the buried wire. These active magnetic fields are very easy
to accurately and precisely track during operation and therefore
have minimal AGV costs in comparison to other types of guidance
systems that require more complex sensors and control units on the
AGV. However, the initial installation costs of these electrified
guide wire systems within the facility in which the AGVs operate
are extremely high in comparison to other guidance systems and an
electrified guide wire system has extremely limited guide path
revision flexibility and high costs associated with any such
revisions to the guide paths. More specifically, any change in a
route the AGV follows requires tearing up the floor of the
facility, removing existing electrified guide path wires and
replacing them with rerouted guide path wires, all resulting in
significant inconvenience or down time for the facility. As
manufacturing and distribution facilities are increasingly
implementing flexible techniques to allow switching between
products being produced or distributed, the AGVs in these
facilities are commonly using optical or inertial guidance systems,
which do not require extensive facility renovations every time a
minor change is made to the guide path of the AGVs.
[0004] Optical guidance systems have made significant strides in
improved accuracy, but typically require expensive sensors and
controllers with substantial processing power to process the
graphical images used in guidance of the AGV, which increases the
cost and complexity of each AGV. Inertial guidance systems, similar
to optical guidance systems, have significantly improved their
capabilities, however, inertial guidance systems using encoders and
gyroscopes also typically require significant processing power. As
such, the guidance systems for these AGVs are generally expensive
and complex in comparison to the guidance systems for AGVs that
follow an electrified guide wire. While optical and inertial
guidance systems do not require tearing up the floor of a facility
to make route changes, similar to most guidance systems, any route
changes may require substantial time and costs for the system to
learn the new route. In optical and inertial guidance systems, at
least one AGV typically must be taught the new route and in some
systems, a new map of the facility must be created, which can be
time consuming and require specialized expertise. As such, the
components on the AGV relating to the guidance system may have
substantially greater costs and complexity than similar components
on any AGV in an electrified guide wire system. The costs
associated with the AGVs learning new routes are much higher for
inertial and optical guidance systems than electrified guide wire
systems, when the costs relating to facility renovations related to
new routes are excluded. In comparison to AGVs that follow
electrified guide wires, the advantage of AGVs with inertial and
optical guidance systems is that they allow high guide path
flexibility, and route changes require little facility renovations.
For example, at most, some optical guidance systems only require
new markers, targets or reflectors to be added to the facility in
relation to any guide path changes.
[0005] As the use of AGVs in the material handling industry has
increased, there has been a corresponding growth and desire for
lower cost AGVs that include flexibility regarding route revisions
within a facility, similar to inertial and optical guidance
systems, with the lower sensor costs and ease of learning new paths
for AGVs used in following electrified guide wire systems. In an
attempt to meet these desires, some manufacturers are using AGVs
that follow a passive magnetic pathway, which instead of using an
expensive buried electrified guide wire, uses passive magnetic
materials typically applied to the floor of the facility. Examples
of such passive magnetic materials include magnetic tapes, paints,
bars, markers and other magnetic materials. Due to the weaker
magnetic field generated by these passive magnetic materials, some
AGVs have experienced issues in accurately and precisely tracking
the guide paths formed by the passive magnetic materials. While the
installation costs are much lower and route revisions are much
cheaper than electrified guide wire systems, the weaker magnetic
fields of these passive magnetic materials may limit the ability of
traditional magnetic sensors on the AGV, previously used with
electrified guide wires, to determine the precise location of the
weaker magnetic signal, especially in facilities having strong
background or ambient magnetic fields.
[0006] Two types of passive magnetic markers are commonly used,
continuous magnetic pathways or discrete magnetic markers. One
problem with discrete magnetic markers is that the AGV must be
presumed to be in alignment with the path defined by the discrete
magnet markers. Therefore, the AGV must first be manually aligned
very carefully with the pathway of magnet markers. During
operation, if errors compound, such as a heading error increasing
to a point where correction is not possible, the sensors on the AGV
may be too far displaced from the next marker to obtain a valid
reading of the next marker. Missing or displaced magnetic markers
are also problematic, as once the AGV is misaligned with the actual
path, most current methods of using discrete markers are unable to
correct the AGV's path sufficiently to detect the next correctly
located discrete marker in the pathway. More specifically, even
when the AGV is traveling along a straight pathway, if the next
expected magnetic marker is displaced to one side from its proper
position, the AGV may initiate a turn based on the location of the
displaced marker, which may cause the heading of the AGV to be too
far deviated from the correct pathway to locate the subsequent
properly placed marker. These issues are particularly acute in
areas where an AGV must make a turn.
[0007] Methods of using discrete magnet markers typically require a
variety of external apparatuses and complex methods to follow
changes in the path such as curves and junctions or divergences in
the path. To adjust and correct for potential issues, many methods
have been proposed that encode information about path changes in
the magnetic markers. These encoding systems are typically limited
to a small amount of information and may even require many
individual magnets to form a single marker to encode enough
information. The requirements for these individual, specialized
magnets increases the cost of installation and overall complexity
of the system, while the practicality of the system is limited in
view of the limited amount of information capable of being
encoded.
[0008] While some systems have attempted to replace the discrete
magnetic markers in turns with a continuous marker, such as a
magnetic bar, these bars are typically expensive to install, may
require facility renovations and may require expensive custom
radius magnetic bars in the turns. To reduce the initial
installation cost, eliminate the problems described above with
discrete markers and provide increased flexibility relating to
changes in the routes of the AGVs within a facility, some systems
are using continuous magnetic markers such as magnetic tape,
magnetic paint, and adhesive materials having magnetic properties
or any combination thereof. However, due to the typically weaker
magnetic fields emitted by these continuous markers, as compared to
electrified guide wires and even discrete magnetic markers, the
magnetic sensors used on these AGVs have increased in cost and the
processing power required for tracking has also increased to ensure
accurate and precise tracking similar to the tracking obtained in
electrified guide wire systems with a stronger magnetic field. For
example, many current systems require the use of twelve different
sensors in an array, such that the magnetic field strength from
each sensor may be used to determine which sensors are over the
path marker. Based on the determination of which of the twelve
sensors are over the marker, the AGV's processing unit then
determines the offset of the vehicle from the centerline of the
magnetic marker path and adjusts the travel path of the AGV. The
cost of these systems is increased due to the number of sensors
used, and the amount of processing power required to process the
twelve different signals. The AGV's cost is also increased in that
typically at least two sensors are also used to detect stray,
ambient, or background magnetic fields. The effectiveness of these
background magnetic sensors is limited since these sensors only
detect one component of the three components in the magnetic
field.
[0009] Another problem with current magnetic sensor assemblies
following a passive and continuous magnetic marker is that the
algebraic relationship used to calculate the location of the AGV is
not only sensitive to the width of the marker, but also the height
of the sensor from the magnetic marker. As such, minor deviations
in the width of the marker or height of the sensor on the AGV may
cause variations in locating a magnetic marker. As the height of
the sensor from the magnetic marker is required in calculating the
position of the magnetic marker, it is very important that the
height of the sensor assembly during the assembly of the AGV
precisely matches the specified height, raising costs of assembly
or requiring specific calibration of each AGV in relation to
height. The requirement for the height to be precisely known is
also problematic as it may vary depending on the amount of load the
AGV is carrying or due to the variations in the floor of the
facility. Additionally, the requirement for the width of the
magnetic marker to be consistent over its entire length is
problematic. For example, if magnetic paint is used, it is possible
for variations in width to occur over the miles of routes typically
found in a facility, especially over time in view of the wear
caused by the AGVs and other vehicles and people continually
passing over the magnetic marker.
[0010] Similar to the above described system and method, other
magnetic systems and methods have been developed wherein at least
two sensors are placed a known distance apart on the AGV and angled
such that their sense axes meet at approximately the location of
the centerline of the magnetic tape, if the AGV is centered along
the magnetic marker. This method is also subjective to ambient
magnetic fields, the width of the magnetic tape as well as the
height of the vehicle, similar to a sensor directly placed over the
center of the marker. Very careful alignment and calibration is
also required to ensure precise and accurate guidance by the AGV as
well as high precision during manufacturing regarding the height of
the sensor assembly, the distance apart of the two magnetic
sensors, and relative angles of the sensors.
SUMMARY OF THE INVENTION
[0011] The present invention is directed to an apparatus and method
for guiding an AGV along a magnetic pathway and more specifically
to an apparatus and a method capable of accurately and precisely
following a weak magnetic field emitted by a substantially
continuous passive magnetic marker having route junctions.
[0012] The present invention is further directed to an AGV
configured to travel on a supporting surface and follow a pathway
producing a magnetic field. The AGV includes a support assembly
having a plurality of wheels and wherein at least one of the
plurality of wheels is configured to directionally steer the AGV,
and a first magnetic sensor assembly coupled to the support
assembly. The magnetic sensor assembly has a first magnetic field
detector arranged adjacent to a second magnetic field detector and
the first magnetic field detector is configured to measure the
strength of the magnetic field produced by the pathway along a
first axis and output a first signal related to the magnetic field
strength along the first axis of the first magnetic field detector.
The second magnetic field detector is configured to measure the
strength of a magnetic field along a second axis and output a
second signal related to the magnetic field strength along the
second axis. The first axis of the first magnetic field detector is
orthogonal to the second axis of the second magnetic field
detector.
[0013] The AGV further includes a controller capable of receiving
the first and second signals and based on the first and second
signals, capable of determining a steering signal to provide to the
at least one of the plurality of wheels configured to directionally
steer the AGV.
[0014] At least one of the first axis and second axis is
approximately perpendicular to the supporting surface and the other
of the first axis and second axis is approximately aligned with the
supporting surface. In addition, each of the first axis and the
second axis are preferably located within a first plane that is
located approximately perpendicular to the supporting surface. The
first plane is also approximately perpendicular to the pathway.
[0015] The AGV may include a second magnetic sensor assembly
coupled to the support assembly. Similar to the first magnetic
sensor assembly, the second magnetic sensor assembly includes a
third magnetic field detector and a fourth magnetic field detector,
each having an axis and wherein the axes of the third and fourth
magnetic field detectors are orthogonal and are located preferably
within a second plane. In addition, each of the first axis and the
second axis are substantially located within a first plane that is
located perpendicular to the supporting surface and wherein the
second plane is approximately aligned with or within the first
plane.
[0016] The field detectors are selected from either hall effect
sensors or giant magnetoresistance sensors. It is expected that the
first and second sensors are located adjacent to each other on a
single chip and approximately aligned with the supporting
surface.
[0017] A third magnetic sensor assembly may be coupled to the
support assembly. The third magnetic sensor assembly includes a
fifth magnetic field detector and a sixth magnetic field detector,
each having an axis and wherein the axes of the fifth and sixth
magnetic field detectors are orthogonal and located substantially
within the a third plane. The axes of the first, third and fifth
field detectors are also preferably aligned and the axes of the
second, fourth and sixth field detectors are spaced apart and
preferably parallel. In addition, the first, second and third
planes are preferably aligned or within the same plane.
[0018] The present invention also relates to a method of
controlling an AGV operating on a supporting surface and along a
pathway producing a magnetic field and wherein the pathway is
substantially continuous. The method includes the steps of
determining the strength of the magnetic field along a first axis;
determining the strength of the magnetic field along a second axis,
wherein the second axis is orthogonal to the first axis;
calculating a first vector directed at the pathway using the
determined strength of the magnetic field along the first axis and
the determined strength of the magnetic field along the second
axis; and determining that the determined strength of the magnetic
field along at least one of the first axis and the second axis
exceeds a baseline strength.
[0019] The method may further include the step of comparing
deviation of the angle of the calculated vector to a reference
vector angle. If the AGV includes additional field detectors or
sensor assemblies, the method may include the steps of determining
the strength of the magnetic field along a third axis; determining
the strength of the magnetic field along a fourth axis, wherein the
fourth axis is orthogonal to the third axis; and calculating a
second vector directed at the pathway using the determined strength
of the magnetic field along the third axis and the determined
strength of the magnetic field along the fourth axis. The point of
convergence of the first and second, as well as any additional
vectors may be calculated. The method then determines if the point
of convergence of the first vector and the second vector is within
a predetermined range of the supporting surface.
[0020] Further scope and applicability of the present invention
will become apparent from the following detailed description,
claims, and drawings. However, it should be understood that the
detailed description and specific examples, while indicating
preferred embodiments of the invention, are given by way of
illustration only, since various changes and modifications within
the spirit and scope of the invention will become apparent to those
skilled in the art.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] The present invention will become more fully understood from
the detailed description given here below, the appended claims, and
the accompanying drawings in which:
[0022] FIG. 1 is a perspective view of an AGV positioned in
operative alignment with a magnetic marker on the floor and the
magnetic sensor assembly in an exemplary position;
[0023] FIG. 2 is a perspective view of an AGV positioned in
operative alignment with a magnetic marker on the floor and
approaching divergent route junction with a magnetic sensor
assembly exemplarily positioned on the front of the AGV;
[0024] FIG. 3 is an exemplary schematic view of an AGV with three
magnetic sensor assemblies centered over the magnetic marker and
illustrating the determined vectors;
[0025] FIG. 4 is a schematic exemplary view of an AGV with three
magnetic sensor assemblies offset to a first side of the magnetic
marker and illustrating the position of the determined vectors for
each sensor assembly;
[0026] FIG. 5 is a schematic exemplary view of an AGV with three
magnetic sensor assemblies offset to a second side of the magnetic
marker and illustrating the corresponding determined vectors for
each sensor assembly;
[0027] FIG. 6 illustrates the vector positions of a three-part
sensor assembly when positioned over a uniformly magnetized
floor;
[0028] FIG. 7 illustrates the data received from the magnetic
detectors in a sensor assembly as the AGV travels along a path and
the relationship between the angle of detection as well as the
magnitude of the magnetic strength;
[0029] FIG. 8 illustrates the tape path of a divergent route
junction with a left path being followed and the relative degrees
of the individual sensors from a three-part sensor detector;
[0030] FIG. 9 is a side view schematic representation of the plane
in which the detectors sit versus the plane in which the magnetic
marker is located; and
[0031] FIG. 10 is a schematic representation of the sense axes for
the two orthogonal detectors in a sensor assembly on an AGV.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0032] The present invention is directed to an AGV 10 capable of
following a magnetic marker 20 along predetermined routes within a
facility, and a related method for determining position of the
magnetic marker 20 relative to the AGV 10 and correcting the actual
travel path of the AGV to match a desired travel path aligned with
the magnetic marker 20. The AGV 10 is generally illustrated in
FIGS. 1 and 2 and may be formed in any size, shape, style, type, or
configuration of a vehicle that is capable of driving along
predetermined routes without a human operator. The AGV 10 may vary
from small automatic carts up to large vehicles capable of carrying
many of tons of material.
[0033] The magnetic sensor assembly 20 includes at least two
magnetic field sensors or detectors 22 having orthogonal sense
axes. The magnetic sensor assembly 20 includes or is in contact
with a controller 30 on the AGV 10. The controller 30, which
receives input from the magnetic field sensors 22, processes such
input to determine the location of the AGV 10 relative to the
magnetic marker 40, and provides steering and course adjustments to
follow the magnetic marker 40. The sensor assembly 20 and
controller 30 are also configured to accurately follow divergent
paths at junctions 50 in the magnetic marker 40. The controller 30
provides signals or control outputs to the wheels or steering
assembly relating to course corrections that were determined from
processing signals regarding the magnetic field of the magnetic
marker 40. The control 30 outputs are used in steering the AGV 10
to ensure that the actual travel path is substantially the same as
the desired travel path, typically centered along the magnetic
marker 40.
[0034] The AGV 10 generally includes a supporting frame body or
assembly 14 to which a plurality of wheels 16 are attached as well
as a guidance system having a magnetic sensor assembly 20 and a
controller 30. The guidance system will have the ability through
any known technique to provide steering and directional control to
or through the wheels 16. The magnetic sensor assembly 20 generally
includes at least two magnetic field detectors 22 capable in
combination of allowing the controller 20 to determine at least one
vector in the direction of a magnetic marker 40. The operating
environment of the AGV 10 may be any environment in which the
supporting surface 12, such as a floor of a manufacturing facility,
is capable of including a magnetic marker 40 that defines a
pathway.
[0035] The magnetic marker 40 is generally desired to be continuous
although unintended or intentional small gaps may be included, such
as those occurring from wear over time in a facility, e.g.,
displacement of magnetic tape or magnetic paint from high traffic
areas. Use of a continuous magnetic marker allows substantially
constant sensing of the magnetic field by the magnetic sensor
assembly 20. The magnetic marker 40 may be any type of elongated,
substantially continuous material or item with minimal breaks or
gaps applied to or set within the supporting surface 12 of the
facility. Of course, even though not preferred, the present
invention allows AGVs 10 to also work with electrified guide wire
systems which is useful for facilities already having electrified
guide wires. It is expected that for ease of installation, ease of
repair, and ease of route changes that materials similar in
application to magnetic tape and magnetic paint will primarily be
used. In the case of magnetic tape, paint or the like, the magnetic
marker 40 generally includes a first edge 42, a center 44, and a
second edge 46. The magnetic marker 40 emits a magnetic field 48
similar to that illustrated in FIGS. 3-6. While the magnetic sensor
assembly 20 specifically does not track the edges 42, 46 of the
magnetic marker 40, it is configured to use the magnetic field 48
to determine approximately the center 44 of the magnetic marker 40.
Variations in the width of the magnetic marker 40 are not an issue
for the present invention, as the width of the magnetic marker 40
is not used in any of the steps of determining the location of the
magnetic marker 40. The magnetic marker 40 may be routed through
the facility around equipment, storage racks, machinery, and the
like to provide the desired functions throughout the facility. As
such, the magnetic marker 40 at times will need to include
junctions 50 such as illustrated in FIGS. 2 and 8, specifically
including a first path 52 and a second path 54 and at times, a
third path 56, or additional paths as desired. Therefore, the AGV
10 may follow the original path 58 of the magnetic marker 40 to the
junction 50 wherein the preprogrammed route of which path 52, 54,
56 it is to follow is used in evaluating the magnetic fields 48
related to the junction.
[0036] As discussed above, the structure of the AGV 10 may be of
any size, shape, type, or configuration. For example, the AGV 10
may be a loader, hi-lo, or other material handling vehicle.
Depending upon the desired AGV 10 configuration, the number and
orientation of the wheels 16 may vary. Typically, the AGV 10 will
have at least three or more wheels, preferably in most embodiments,
four or more wheels. At least one or two of the wheels 16 will be
directional wheels for steering input received from the controller
to provide the directional control of the AGV 10. However, in some
embodiments, the directional control of the AGV may be provided
through skid steer control wherein wheels 16 on one side of AGV 10
rotate either faster or slower than the wheels 16 on the other side
of the AGV causing the AGV 10 to turn. The configuration of the
wheels 16 may vary depending upon the type of facility and
supporting surface 12 on which the AGV 10 operates.
[0037] The support structure, frame, or body 14 of the AGV 10 may
also be formed in any size, shape, type or configuration and
typically will vary depending upon the desired functional task to
be performed by the AGV 10. The AGV 10 includes, mounted to support
structure or frame either directly or indirectly, the guidance
system which typically includes magnetic sensor assembly 20,
controller or processor 30 and though not illustrated, the steering
control for the wheels 16.
[0038] The guidance system receives an input of magnetic field
strength in certain directions, which is then provided to a
controller or processor 30 to determine the exact location of the
magnetic marker 40 and whether the AGV needs to make course
corrections. The magnetic sensor assembly 20 generally includes the
magnetic field sensors or detectors 22. At least two magnetic field
detectors 22 are included in the magnetic sensor assembly 20 and
while the invention only needs one assembly 20 having two detectors
22, most of the figures illustrate three magnetic sensory
assemblies 20 having each at least two detectors 22. Exemplary
magnetic field detectors 22 may be hall effect sensors or giant
magnetoresistance sensors. Of course, any other sensor capable of
providing the necessary information as described below,
specifically relating to the ability to determine the vector from
the strength of the magnetic field along orthogonal axes, may be
used in the present invention. The hall effect sensors or giant
magnetoresistance sensors should be configured or designed for low
field magnetic sensing unless the AGV is used with an electrified
guide wire system. The detectors 22 are arranged with at least two
sense axes being orthogonal. To maintain the orthogonal
relationship between the two axes of the sensors and limit
installation costs, it is preferable that the at least two
detectors 22 are on a single chip. The use of hall effect sensors
on the same chip allows for advantages over coil based magnetic
sensors in that they are extremely sensitive to low field
magnitudes, such as from 120 microgauss to 6 gauss and are
solid-state magnetic sensors. Of course, more than two detectors 22
could be included on a single chip on the AGV 10 and multiple
magnetic sensor assemblies 20 may be used, each having at least two
sensors 22, and more specifically, each with at least two of the
detectors 22 having orthogonal axes. The chips, including the
detectors 22, can be very small and the present invention, by using
two magnetic sensors on the same chip with orthogonal sensing axes,
eliminates the need of the prior art for precise displacement a
certain distance to each side of the magnetic marker of the
sensors. The present invention also eliminates the prior art
requirement that the sensors be a certain height away from the
magnetic marker, except that it is desirable for the mounting
height to be greater than the width of the magnetic marker to
ensure accuracy of the determined vectors. As discussed in the
background, in the prior art, magnetic sensors usually need to be
carefully centered or placed a careful distance from each other to
each side of the centerline of the AGV as well as a very specific
distance from the supporting surface and changes in load weight at
times cause variance in this distance, thereby reducing the
effectiveness and accuracy of the prior sensor arrangements.
[0039] Giant magnetoresistance sensors typically use thin film
structures composed of alternating ferrous magnetic layers and
non-magnetic layers. The magnetic field 48 is typically measured as
a change in the electrical resistance depending on whether the
magnetization of an adjacent ferrous magnetic layer is in parallel
or anti-parallel alignment. More specifically, the overall
resistance may be relatively low for parallel alignment and
relatively high for anti-parallel alignment. Spacer material, such
as copper, is interspaced as the non-magnetic layer between the
ferrous magnetic layers which may be an iron or an iron alloy such
as a nickel iron.
[0040] The magnetic sensor assembly 20 may be located anywhere
desired on the AGV 10. However, it is typically believed that for
improved reaction times and ease of controlling the direction of
the AGV as well as ease of access to the magnetic sensor assembly
20 that the sensor assembly 20 preferably is orientated in the
direction of travel. However, for some AGVs 10 that commonly switch
between directions of travel, it may be desired to locate it closer
to the center of the AGV, such as illustrated in FIG. 1 or use two
different placements for sensor assemblies on the AGV. As such, the
magnetic sensor assembly 20 can be located anywhere on the AGV so
long as it is in a serviceable position, preferably along the
centerline of the AGV and its ability to sense the magnetic field
produced by the magnetic marker 40 is not adversely affected by
such placement. Of course, the sensor assembly 20 may be offset
from the center line of the AGV 10, so long as the controller knows
the approximate envelope of the AGV 10 on each side of the sensor,
to avoid collisions and ensure proper guidance of the AGV 10.
[0041] The orientation of the sense axes 24 of the individual
detectors 22 that form the sensor assembly 20 are preferably in the
same plane 26, orthogonal to each other with the individual
detectors 22 as illustrated in FIG. 10 and arranged approximately
along a line. The detectors 22 are typically arranged along a line
approximately parallel to the supporting surface 16, but preferably
perpendicular to the direction of travel as well as the centerline
44 of the magnetic marker 40. Such an arrangement of the magnetic
sensors 22 puts the sensors 22 at an approximate uniform height
above the supporting the surface 16. FIGS. 3-6 illustrate the
vector positions 32 determined by the controller 30 reading the
signals and do not represent the sensor axes of the individual
detectors 22. These figures also illustrate the multiple sensor
assemblies 20 being substantially aligned. A specific orientation
of each detector 22 is such that the axes of at least two of the
detectors are orthogonal to each other. Also, so long as the sense
axes are orthogonal to each other, the vector analysis as described
below may be performed, such that the exact orientation of the
sense axes relative to the supporting surface for the magnetic
marker may be determined. More specifically, because the present
invention uses a novel vector analysis to determine the location of
the magnetic marker 40 with orthogonally orientated sense axes of
at least two detectors 22, the vectors will always point to the
centerline of the magnetic marker 40 and as such, the position and
height relative to the supporting surface 12 have little effect or
may be accounted for by the controller 30. In addition, even though
the present invention describes the orientation of at least one
sense axis perpendicular to the floor and one sense axis parallel
to the floor, the present invention through its use of vectors can
adjust easily for other alignments as the vector always points to
the center of the magnetic marker no matter the orientation,
however the at least two sense axes are perpendicular. If the sense
axes 24 are not aligned as described above relative to the
supporting surface and marker 40, the AGV 10 will need to be
calibrated to automatically adjust for a vector that while it
points to the centerline of the marker 40 would not center the AGV
10 over the marker 40 due to the misalignment of the sensor axes
24, as described below in greater detail. Of course, it is also
preferable that the plane containing the sense axes be
approximately perpendicular to the magnetic marker 40. In addition,
to ensure consistent and uniform measurement of the magnetic field
to determine a vector, it is preferable that the sensors having
orthogonal axes be directly adjacent to each other as any
significant spacing could cause different readings and thereby
reduce accuracy of the vector. More specifically as the strength of
the magnetic field is measured along two orthogonal axes, any
displacement other than adjacent may reduce the accuracy of the
system. For ease of use, it is expected that the magnetic sensor
assembly 20 will be installed on the AGV such that one of the sense
axes 24 from a first detector 22 is perpendicular to the supporting
surface 12 while the other sense axes 24 is parallel to the
supporting surface 12 and with a two sense axes being orthogonal to
each other and located in a single plane. Of course, minor
variations in the orientation of the sense axes as well as
orientation of the sensor assembly 20 may be allowed without need
for calibration, however, these are expected to be minimized. FIGS.
9 and 10 illustrate direction of the sense axes relative to the AGV
as well as the supporting surface 12.
[0042] The present invention needs little to no calibration in
comparison to AGVs of the prior art. Previously, AGVs required very
specific and critical placement of the magnetic sensor assembly in
height over the floor as well as the location of any sensors from
each other, and the width of the marker 40. More specifically,
prior methods to determine a magnetic pathway required the
determination of a linear offset of the sensor from the magnetic
marker 40 and such calculation of the offset requires that the
controller have information about the expected, precise distance of
the magnetic sensor to the marker and width of the magnetic marker
at that spot. Any misadjustment of the sensor or variations in the
magnetic marker or supporting surface, as well as variations in
height of the sensor due to the amount of load being transported,
would cause error in the height of the AGV body and thereby cause
errors in locating the magnetic marker. Because the present
invention uses vectors and measures the angle of the vectors as
well the magnitude, path offset or height information is not
required, and no matter where the sensor assembly 20 is located on
the AGV, the magnetic marker 40 is accurately and precisely
found.
[0043] The present invention also allows relatively easy insertion
of an AGV onto the magnetic marker path. The present invention does
not require careful insertion and alignment, but instead an AGV is
simply placed on the path and pointed roughly in the required
direction. The algorithm and steering mechanism work to rapidly
acquire tracking of the magnetic marker in an accurate and precise
fashion, and guide the AGV along the magnetic marker 40.
[0044] The only requirement regarding placement of the magnetic
sensor assembly 20 is that the height of the detector 22 must be
adequate to detect the magnetic field. Also, while the present
invention is not sensitive to variations in the width of the marker
or the distance between the detector 22 and the marker, the width
of the marker needs to be narrow enough to define an unambiguous
pathway, particularly in areas with junctions. As the width of the
magnetic marker increases, the present invention generally requires
an increase in height of the sensor assembly from the supporting
surface to accurately and precisely determine the centerline of the
marker, but the controller does not use this height in any
calculations. Therefore, the height is at least greater than the
widest point of the magnetic marker. Conversely, the height should
be limited as the strength of the magnetic field generally
decreases as the distance increases. In addition, as the height
increases the same displacement of the AGV 10 in position from the
center of the maker 40 causes decreasing amounts of measured angle
of deviation from the expected vector direction.
[0045] The sensor assembly 20 includes the magnetic field detectors
22 with their axes 24 in a single plane 26 and aligned orthogonal
or substantially perpendicular to each other. An orthogonal
alignment of the detectors 22 is used in the determination of a
vector pointing to the center of the magnetic marker 40. More
specifically, the magnetic sensor assembly 20 includes magnetic
field sensors or detectors 22 which send data relating to at least
two orthogonal components of the magnetic field to the controller
30 to determine the vector direction and strength of the magnetic
field in the plane of the sensors. As stated above, the axes 24 of
the detectors 22 fall substantially within a single plane 26 and
include typically one perpendicular to the supporting surface 10 as
well as the magnetic marker 40 and one parallel to the supporting
surface, but perpendicular to the centerline of the magnetic marker
40. Therefore, each detector 22 detects the strength of the
magnetic field in the direction of its axis 24. Using the vector
analysis described below, the processor 30 is able to determine the
direction of the magnetic field in the plane 26 containing each
sensors axis 24.
[0046] To ensure that the AGV 10 is reading a magnetic field
emanating from a magnetic marker 40 and not background magnetic
fields, the AGV 10 requires the magnitude of the magnetic field to
exceed a specified predetermined level on at least one of the
magnetic field detectors or sensors 22. For example, the system is
configured so that the Earth's magnetic field does not affect or
has only a minimal affect on locating the marker 40. More
specifically, the system requires at least one detector 22 to have
a minimum magnetic strength level that eliminates the potential
issue of an AGV displaced from the magnetic marker 40 traveling
freely using the Earth's magnetic field. In addition, some
supporting surfaces 12 may include materials that have magnetic
properties which may emit a stronger magnetic field than the
Earth's magnetic field, such that they may be measurable along the
AGV's route. For example, a concrete floor may include various
rocks having magnetic properties that may be unevenly dispersed
throughout the floor. These typically only have low strength
magnetic fields emanating therefrom and as such requiring at least
one of the detectors 22 to have a minimum threshold of magnetic
strength along its axis 24 eliminates most issues. Therefore, the
potential for misguidance due to variations in the magnetic
properties of the supporting surface 12 is reduced.
[0047] In addition, the vector analysis of the present invention
include methods for dealing with ambient magnetic fields as well as
variations in the magnetic field, such as due to variations in the
magnetic properties of the floor or even magnetic fields produced
by industrial machinery in the facility in which the AGV operates.
Previously, in facilities that processed materials or had
industrial processes that caused magnetic fields, such as machines
using significant amounts of power, the magnetic fields emanating
from these machines and supporting surfaces could confuse the
magnetic sensors on the AGVs. Therefore, AGVs using magnetic
guidance systems had limitations of what types of facilities or
around what types equipment or machinery the AGV could operate. In
the prior art, there is no sufficient method for easily determining
if something other than the magnetic marker is affecting the AGV's
determination of its present location, relative to the magnetic
marker. Because the present invention uses a vector analysis,
determining errors due to magnetic fields emanating from other than
the magnetic marker are easily determined. First, because the
present invention uses determined vectors in guiding the AGVs, the
AGV can be programmed to stop if the vector is pointing in a
direction outside a specified range. For example, a temporary large
magnetic field generated by an industrial process may cause the
vector to point almost sideways, which the controller could easily
determine to be an invalid reading and stop operation of the AGV.
Second, through the use of two spaced apart sensor assemblies 20
having each two detectors 22 with orthogonal sense axes 24, the
controller 30 can determine the point of vector convergence or
intersection as an error check. The vector associated with each
sensor assembly 20 is determined using each of the detectors 22 and
the controller 30 analyzes at least two vectors to determine a
point of convergence of the vectors. Therefore, not only may error
checks be performed using individual vectors 32, but if the height
is approximately known to the magnetic marker 40, the point of
convergence should approximate the distance to the magnetic marker
40. More specifically, two magnetic sensor assemblies 20 each
having at least two magnetic field detectors 22 with their axes
orthogonal to each other are used. The processor or controller 30
determines the vector related to each magnetic sensor assembly 20
and the direction indicated by each vector should converge on the
center of the marker. Therefore, the controller can use the signals
to analyze the vectors for convergence and in the presence of
distributed background magnetism that is substantially uniform, the
marker 40 will still be the point of convergence of the vectors. In
comparison, upon receiving an unexpected result, such as the point
of convergence for multiple sensors is not logically the magnetic
marker, the AGV can stop operation. Examples of points of
convergence determined as incorrect include when the point of
convergence is displaced higher than the plane of the supporting
surface 12 or substantially below the supporting surface. The point
of convergence must be below or above the surface by at least a
specified amount, before the AGV is instructed to stop. In some
circumstances, if the vectors are determined to not converge, then
the controller also would stop the AGV. This point of convergence
allows for a quick and easy determination that the magnetic sensor
assembly 20 as well as the controller 30 is operating accurately
and precisely to locate the magnetic marker 40 so that the guidance
system may match the actual travel path of the AGV 10 to the
desired travel path of the AGV along the magnetic marker. An
example of magnetic detectors 22 having vectors 32 that do not
converge would be in areas of a supporting surface where the
magnetic field of the supporting surface is greater than or
sufficient in strength to confuse the magnetic sensor assemblies.
For example, an area of the supporting surface having a strong
magnetic field may cause the vectors to point straight down as
illustrated in FIG. 6 such that they will not converge. Of course,
when using the point of convergence as an error check, the height
of the detector 22 from the magnetic marker 40 is approximately
used by the controller, but is not used for location, and is
specifically not used in calculating the vectors 32.
[0048] Although the present invention is configured such that maps
of the facility and routes are not needed, a technician during the
calibration process of the AGV may in some instances, where the
supporting surface 12 is problematic with stray or ambient magnetic
fields, map the AGV 10 along the magnetic path to determine problem
areas. These problem areas are then identified and can be adjusted
for when operating the AGV. For example, if one particular piece of
industrial equipment emanates a strong magnetic field such that the
AGV as it approaches the industrial equipment and has the vectors
pointing at the piece of equipment versus the magnetic marker 40,
use of stored map allows the AGV to adjust for this background
magnetic field and properly follow the magnetic marker 40. Because
the present invention uses vector directions to determine location
of the marker 40, it eliminates the issues of the prior art in
which when the strength of the background magnetism is larger than
the threshold for detecting off marker condition, and the AGV
believes that it is on the correct path, even though it is not
following the marker.
[0049] The present invention determines a vector output by each
magnetic sensor assembly 20 having at least two magnetic sensors or
detectors 22. This is determined by taking the strength of the
magnetic field along one axis 24 and combining it, using an
algorithm, with a strength of the magnetic field along the second
axis 24, which is orthogonal to the first axis and then calculating
a vector. More specifically, the vector's direction is given
by:
Angle=offset+ARCCOS(Signal A/Signal B) (1)
The offset is a constant based upon the rotational orientation of
the sensors. Therefore, if the sensors are not orthogonally aligned
such that in the illustrated examples, one is perpendicular to the
floor with its axis and the other axis is parallel to the floor,
the offset allows for an adjustment. The offset also allows for
differences in the manufacturing process of the AGV 10 so that the
AGV 10 can be calibrated to ensure accurate and precise tracking of
the magnetic marker 40. It is important to note that the offset is
a constant based upon the rotational orientation of the detectors
22 specifically their axes 24 relative to each other and not the
deviations in side-to-side in location on the AGV. It should be
noted that in some embodiments an AGV could also be configured not
to follow the line along the center of the AGV, but for example
follow a line offset from center about 25% to one side of the AGV,
in which an additional offset variable would need to be added.
[0050] To ensure that the overall strength of the magnetic field is
great enough, the field strength of the magnetic field is
calculated. More specifically, the field strength is calculated
as:
FIELD STRENGTH= {square root over ((signal a).sup.2+(signal
b).sup.2)}{square root over ((signal a).sup.2+(signal b).sup.2)}
(2)
Therefore, the vector indicates the direction to the center of the
guidepath magnetic marker. If the AGV is directly over the marker
40, and if the magnetic sensor assembly 20 is centered on the AGV,
the vector is pointed directly perpendicular to the floor 12. As
stated above, if the magnetic sensor assembly 20 is intentionally
offset from the center of the AGV, yet the AGV is to follow the
magnetic marker along the center of the AGV, an additional offset
can be added to the formula to adjust for the variance of the
vector from pointing directly perpendicular to the floor 12 to
pointing at a particular angle. In fact, such an additional offset
is just a simple adjustment to the expected angle. When the vehicle
is offset from the magnetic marker 40 and it is desirable to resume
precise and accurate tracking, for a centered sensor assembly 20,
the vector will have an angle that is not perpendicular to the
floor, the AGV is then steered to align the vector to be
perpendicular to the supporting surface 12. Of course, if the
magnetic sensor assembly 20 is intentionally configured to not be
directly over the magnetic marker, any deviation from the expected
vector angle will be noted as an error in position so that after
correction of the location of the AGV, with the AGV centered over
the magnetic marker, the vector 32 will have the assigned angle.
The magnetic sensor assembly 20 provides output to the controller
30 which calculates the vector 32 as well as field strength. The
controller 30 in processing the angle of each vector 32 computes an
angle error proportional to the difference between the measured
angle and the desired angle, which is typically perpendicular. The
controller 30 uses that angle error signal to determine a steering
control signal which is sent to the vehicle's steering system. The
controller 30 adjusts the steering to keep the vector 32 pointed at
the desired angle, which is typically perpendicular to the
floor.
[0051] The present invention allows for easy following of a desired
passive magnetic marker pathway even when the pathway includes
divergence or junctions in the pathway. At a divergence, if the AGV
needs to follow a track to either the left, right or center of the
marker. If the AGV follows a marker to the left of center, the AGV
will follow the left divergence of the path compared to the right
of center or center of the path. To follow an exemplary left
divergence at a junction, the control 30 subtracts the angle of the
magnetic field sensor from a reference angle to the right of center
to calculate the error angle. The controller then controls the
steering to minimize the error signal. The effect is that the AGV
may look off to the right to see the path yet keeping the vehicle
to the left. A similar process is used for the right
divergence.
[0052] The foregoing discussion discloses and describes an
exemplary embodiment of the present invention. One skilled in the
art will readily recognize from such discussion, and from the
accompanying drawings and claims that various changes,
modifications and variations can be made therein without departing
from the true spirit and fair scope of the invention as defined by
the following claims.
* * * * *