Guidance system for self-guided vehicle

A guidance system for a self-guided vehicle features an electrical conductor and permanent magnets embedded together in an elongate groove formed in a vehicle-supporting surface such as a factory or warehouse floor. The electrical conductor provides steering guidance while the permanent magnets embedded with the conductor provide other information without distorting the magnetic field of the electrical conductor. To prevent such distortion, the permanent magnets are selected for their low magnetic permeability and high electrical resistivity. Magnetic field sensors are provided on the vehicle for determining exact position of the vehicle along its path of travel by sensing deviation, or lack thereof, with respect to such permanent magnets.

BACKGROUND OF THE INVENTION 
This invention relates to a guidance system for a self-guided vehicle. More 
particularly, it relates to a system for providing position information to 
the vehicle and for controlling certain aspects of the vehicle's operation 
in response thereto, such as precision stopping at predetermined locations 
as the vehicle travels along a predetermined path. 
Many self-guided vehicles used in industrial applications such as 
warehouses and the like are of the type capable of following a path 
defined by a current-carrying guide wire arranged on, or more typically 
embedded in a groove in, the surface over which the vehicle travels. 
Vehicles of this type, such as those disclosed in Kohls U.S. Pat. No. 
3,411,603, Thompson et al. U.S. Pat. No. 3,768,586, Schnaibel U.S. Pat. 
No. 4,247,896, Nishiki et al. U.S. Pat. No. 4,456,088, Tax et al. U.S. 
Pat. No. 3,669,206 and Taylor U.S. Pat. No. 4,307,329, generally employ 
induction coils to sense the magnetic field caused by the AC current in 
the guide wire, and use steering control signals generated by the coils to 
detect and correct any deviation from the guide wire so as to guide the 
vehicle along the path defined by the guide wire. 
In addition to the steering reference signal provided by the magnetic field 
of the guide wire, other control information, such as vehicle position 
along the path of travel and the location of destinations where the 
vehicle is to stop and pick up or deliver cargo, is needed. Some 
self-guided vehicle systems, such as that disclosed by Tax et al., employ 
sensing loops along the pathway for sensing the approximate position of 
the vehicle as it passes by the sensing loops. Other systems, such as that 
disclosed by Thompson et al., employ a sensor unit on the vehicle to sense 
approximate position data contained in nodes or loops formed by the guide 
wire(s). Still other systems, such as those disclosed by Kohls, and by 
Uemura U.S. Pat. No. 3,653,456, employ magnets, placed alongside the path 
and spaced transversely from the guide wire, to provide approximate 
position information to sensors mounted on the vehicle. 
A major installation problem associated with systems employing a plurality 
of magnets spaced from the conductor, such as disclosed in Kohls, is that 
the magnets must be embedded in the surface over which the vehicle travels 
at locations spaced transversely from the guide wire, requiring a 
plurality of holes to be formed in such surface which interrupt the 
integrity of the surface and raise the installation cost of the system. 
Although it would be much less costly and less disruptive to the surface 
to embed the magnets in the same groove which contains the embedded guide 
wire, this has not been considered possible because the magnets would 
distort the magnetic field of the wire both by shunting the field away 
from the sensing coils on the vehicle and by creating interfering induced 
eddy current fields. Similarly, the guide wire node or loop systems 
described above would also result in a substantial installation expense 
and disruption of the supporting surface. An additional disadvantage to a 
system such as disclosed in Kohls is that an array of magnets arranged 
transversely to the direction of vehicle travel requires a corresponding 
array of sensors on the vehicle. 
Because the foregoing systems provide only approximate position information 
in any case, most of the self-guided vehicles of the type described above 
must employ fifth wheel encoders to provide more precise vehicle position 
information by recording travel distances to enable the vehicle to be 
stopped at precise positions for loading and unloading cargo. For high 
accuracy, however, it is necessary continually to correct or update such 
encoders to compensate for slip, wheel wear, or floor irregularities which 
cause erroneous distance readings. Accordingly, another problem associated 
with such self-guided vehicles is that of providing highly accurate and 
reliable position signals which will enable the vehicle to decelerate and 
stop precisely at prescribed locations. 
SUMMARY OF THE INVENTION 
The present invention solves the installation problems of the 
aforementioned previous self-guided vehicle guidance systems by its 
recognition of the fact that embedding permanent magnets of low magnetic 
permeability, and preferably high electrical resistivity, within the 
operative portion of the magnetic field of the guide wire does not 
significantly disrupt or distort such field, either by shunting of the 
field or by creation of interfering eddy current fields, and therefore 
does not interfere with detection of the wire by the vehicle for steering 
purposes. Among the permanent magnet materials having a low magnetic 
permeability (i.e. substantially equal to one or that of free space) 
sufficient to minimize shunting of the field of the guide wire are 
ferrite, rare earth cobalt, and neodymium-iron-boron permanent magnets. 
The ferrite magnets also have the preferred high resistivity to minimize 
induced eddy currents. Other magnets can achieve the preferred high 
resistivity in a resin-bonded powdered state. 
The benefit of placing such permanent magnets in close proximity to the 
guide wire, and particularly embedding them in the same groove as the 
wire, is principally that the integrity of the floor or surface over which 
the vehicle moves need not be disrupted by embedding signal generators 
such as wire nodes or magnets in the surface at locations apart from the 
groove. Also, the benefit of avoiding the expense of creating and 
maintaining numerous holes in, for example, the concrete floor of a 
warehouse to house permanent magnets will be readily appreciated. 
The present invention employs magnetic field sensors, such as Hall effect 
sensors, mounted on the vehicle to sense the magnetic fields of the 
permanent magnets. A simple embodiment of the invention uses one or more 
magnetic field sensors, positioned on the vehicle directly above the guide 
wire groove, merely to detect the polarity or polarities of permanent 
magnets imbedded therein. Normally, an array of such magnets would be 
provided, spaced apart along the guide wire groove at a predetermined 
location, arranged with varied polar orientation of the individual magnets 
yielding a binary signal for each magnet, the combined binary signals 
providing a coded signal. The magnetic field sensor or sensors detect not 
only the polar orientation of each magnet, but also its relative position 
in the array. The information encoded in the array of magnets at a 
particular location might include approximate position information, speed 
information, or information regarding the path directly ahead of the 
vehicle such as turns, stops, or the like. 
Precise positioning information (i.e., the exact position of the vehicle 
along the path of travel) is not obtainable from such a magnetic field 
sensor or sensors, even if arranged to sense the proximity-dependent 
strength of the field of a permanent magnet, because the proximity is 
variable not only with the position of the vehicle along the path of 
travel, but also with the position of the vehicle transverse to the path 
of travel. Also, the strength of the permanent magnet field is normally 
small enough to be distorted by the earth's magnetic field. However, a 
further embodiment of the present invention employs a magnetic field 
sensor assembly mounted on the vehicle in a manner to sense precisely the 
deviation, or lack thereof, along the path of travel of the sensor 
assembly relative to a permanent magnet, irrespective of the vehicle's 
transverse position or angle relative to the path of travel and 
irrespective of the earth's magnetic field. Although different sensor 
arrangements can be employed to accomplish this purpose, the preferable 
arrangement employs two sensors, each between a respective pair of 
elongate, mutually parallel flux concentrators extending in a direction 
other than the direction of polarity of the permanent magnet and 
preferably perpendicular thereto (e.g. along the path of travel for a 
transversely polarized magnet). The sensors are preferably arranged to 
cancel the effect of any homogeneous magnetic field, such as that of the 
earth, by producing signals of opposing but equal magnitude in response to 
such homogeneous field, and to produce simultaneous null signals or equal 
and opposite signals in response to the field of the permanent magnet when 
the sensor assembly is aligned with the magnet. Moreover, their output 
signals are opposite when on opposite sides (along the path of travel) of 
the permanent magnet, thereby indicating the direction in which correction 
of vehicle position is needed to obtain alignment with the magnet. The 
signals from the pair of magnetic field sensors may therefore be used to 
control a servo device associated with the vehicle's primary drive to 
cause the vehicle to stop in direct alignment with the magnet or, if the 
magnet is passed, to reverse and "hunt" for the exact position of the 
magnet until the correct position is obtained. Alternatively, the pair of 
sensors may be used to give precise position information to recalibrate a 
fifth wheel encoder when the sensors are directly aligned with the magnet. 
The foregoing precise positioning signals can, if desired, be combined with 
signals from the same or other magnetic field sensors (such as those which 
sense magnet polarity) indicating general proximity to the permanent 
magnet by sensing field strength. In such case, the vehicle's approach to 
the magnet can be detected and a signal generated to decelerate the 
vehicle as it approaches the magnet, or prepare the vehicle's computer for 
the reception of information. 
The foregoing and other objectives, features and advantages of the present 
invention will be more readily understood upon consideration of the 
following detailed description of the invention taken in conjunction with 
the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
With reference to FIGS. 1 and 2, a self-guided vehicle 10 has a 
conventional microprocessor-based controller 12 controlling all of its 
operating systems, including its drive system (forward, reverse, 
decelerate, stop, etc.), its steering system and its cargo-handling 
implements, as well as any other special-purpose functional components 
with which it may be equipped, in a conventional manner. The vehicle 10 
travels upon a supporting surface 14 which may be a warehouse or factory 
floor of concrete or similar magnetically-impermeable material. Formed in 
the floor 14 is an elongate groove 16 corresponding to the intended path 
of travel of the self-guided vehicle. An electrically-conductive, 
insulated cable 18 is embedded in the groove and carries alternating 
current so as to maintain a surrounding alternating magnetic field. The 
portion of the magnetic field referred to herein as the "operative 
portion" is the portion 20 which is sensed by a conventional steering 
sensor 22 on the vehicle 10. Such steering sensor 22 normally comprises a 
pair of coils (not shown) spaced transversely with respect to the path of 
travel of the vehicle. If the vehicle is centered on the conductor 18, 
signals of equal magnitude are induced by the operative portion of the 
field in the pair of coils indicating to the controller 12 that no 
steering correction is needed. However if the vehicle deviates to one side 
or the other of the conduit 18, signals of unequal magnitude are induced 
in the respective coils indicating the need for steering correction. 
The unique guidance system of the present invention is employed in 
conjunction with the abovedescribed conventional steering control system. 
The guidance system includes a plurality of permanent magnets, such as 24 
and 25, positioned in close proximity to the conductor 18 so as to be 
within the operative portion 20 of the conductor's magnetic field. The 
direction of polarity of the magnets is preferably transverse to the path 
of travel of the vehicle and the direction of the groove 16. As used 
herein, the "direction of polarity" of a permanent magnet refers to the 
direction between the poles of the magnet itself or, if pole pieces are 
utilized which alter such direction, the direction between the poles of 
the pole pieces. 
As shown in FIGS. 1 and 2, the permanent magnets 24, 25 are preferably 
embedded in the same groove 16 in which the electrical conductor 18 is 
embedded, positioned vertically above or below the conductor 18. In order 
that the permanent magnets do not shunt the operative portion 20 of the 
conductor's magnetic field away from the steering sensor 22, the magnets 
are composed of permanent magnet material having a magnetic permeability 
substantially equal to that of free space, i.e. substantially equal to 
one. Magnets having such low permeability include, for example, the 
various ferrite permanent magnets, rare earth cobalt magnets, and 
neodymium-iron-boron magnets. Alnico magnets are excluded because of their 
high permeability. 
Moreover, it is preferable that the magnets 24, 25 be of a relatively high 
electrical resistivity (i.e. significantly higher than that of rare earth 
cobalt magnets in their normal sintered form) so that eddy currents and 
their resultant magnetic fields cannot be induced in the magnets in any 
significant magnitude by the alternating current in the conductor 18. Such 
eddy current fields could otherwise distort the operative portion 20 of 
the conductor's magnetic field. The ferrite magnets in their normal 
sintered form have the preferred high electrical resistivity. On the other 
hand, magnets such as rare earth cobalt, having normally low electrical 
resistivity, should preferably be in resin-bonded, powdered form to 
satisfy the high resistivity requirement. 
Each permanent magnet has a magnetic field indicated generally as 26 (for 
magnet 24) in FIGS. 1 and 2. The magnetic fields of the individual magnets 
can be used to supply different types of information to magnetic field 
sensors on the vehicle 10. For example, if an array of permanent magnets 
are distributed along the groove 16 and have nonuniform polar 
orientations, such as indicated by the magnets 24 and 25 shown in FIG. 1, 
the vehicle 10 can receive a digitally-coded message from such magnets as 
it travels over them along the path of travel. The vehicle senses such a 
message by means of one or more polarity-sensing magnetic field sensors 
such as Hall effect sensors 28 and 30 (FIG. 2) having elongate flux 
collector pairs 28a, 28b and 30a, 30b, respectively, extending parallel to 
the direction of polarity of the magnets 24 and 25, and parallel to each 
other. It will be noted that the flux collectors 28a, 28b are connected to 
their Hall effect sensor 28 in a reverse relationship to the connection of 
flux collectors 30a and 30b to their Hall effect sensor 30. This 
represents one way to eliminate the effect of the earth's magnetic field 
on the sensing function of the sensors 28 and 30. For example, if the 
earth's magnetic field, represented as 32 in FIG. 1, is applied at any 
angle to the flux collectors, the component thereof sensed by sensor 30 
will be opposite, but equal in magnitude, to that sensed by sensor 28. 
Conversely, the portion of the magnetic field 26 of the permanent magnet 
24 sensed by sensor 30 will be opposite to, but of less flux density than, 
that sensed by sensor 28 because of the closer proximity of sensor 28 to 
the magnet 24. Alternative ways to eliminate the effect of the earth's 
magnetic field include using excitation currents of opposite polarities 
for two otherwise identically oriented sensor and flux collector 
assemblies, or orienting the two sensors oppositely with otherwise 
identical flux collector arrangements and excitation current polarities. 
With reference to FIG. 4, the Hall effect sensors 28 and 30 are supplied 
with excitation current from constant current sources 33 and 34, 
respectively, while their output signals, which are proportional to sensed 
flux density but of opposite sign, are summed at junction 38. Thus, the 
equal and opposite components of the output signals of the sensors 28 and 
30, resulting from the earth's magnetic field, cancel each other. 
Conversely, the unequal and opposite components of the output signals, 
resulting from the magnetic field 26 of the permanent magnet 24, are 
summed at junction 38 producing a resultant signal whose sign is dependent 
on the polarity of the magnet 24. The resultant signal is presented to the 
inverting input of an operational amplifier 40. The output of the 
amplifier 40 is thus positive or negative depending upon the polarity of 
the magnet 24, and is transmitted to the vehicle controller 12 which 
processes the output signal in a conventional manner to receive 
information indicative of approximate vehicle position, speed, the path 
ahead of the vehicle, or the like. 
Moreover, if desired, the summed output signals from the polarity-sensing 
sensors 28 and 30 can be used as an approximate indicator of proximity to 
the magnet 24. Since the output signals of the Hall effect sensors 28 and 
30 are each proportional to the sensed flux density of the magnetic field 
26 of the permanent magnet 24, which, in turn, is dependent on the 
proximity of each sensor to the magnet, the output signal from amplifier 
40 has a magnitude proportional to such proximity, regardless of its sign. 
Accordingly, a comparator 44 is provided to transmit a positive output 
signal to an OR gate 46 when amplifier 40 produces a positive output 
signal of a magnitude exceeding a threshold level set by resistor 48, 
while a comparator 50 similarly transmits a positive output signal to the 
OR gate 46 in response to a negative output signal from amplifier 40 of a 
similar magnitude exceeding a threshold level set by resistor 52. A 
positive output from either comparator 44 or comparator 50 causes OR gate 
46 to deliver a positive output to the vehicle controller 12, 
approximately indicating a predetermined proximity to the permanent magnet 
along the vehicle's path of travel. This signal can be used for numerous 
purposes, such as decelerating the vehicle in preparation for stopping at 
the magnet, or preparing the vehicle's computer to accept the coded input 
of data from an array of permanent magnets. 
The sensors 28 and 30, while providing a signal proportional to sensed flux 
density, are not well suited for accurate position sensing even though the 
sensed flux density is proportional to their proximity to the permanent 
magnet. The reasons why such sensors are inaccurate position indicators is 
that the flux density which they sense is a function not only of the 
proximity of the sensors to the magnet along the path of travel, but also 
of the proximity of the sensors transverse to the path of travel, which 
varies in response to steering control. Moreover, the sensors 28 and 30 
will give identical outputs on either side of a permanent magnet along the 
path of travel, so that the direction of deviation or desired correction 
relative to the magnet will not be known. 
Accordingly, a different pair of magnetic flux sensors, i.e. Hall effect 
sensors 54 and 56, are provided to indicate exact position of the vehicle 
relative to a permanent magnet such as 24 by producing a signal indicative 
of the deviation, or lack thereof, of the sensors with respect to the 
permanent magnet along the path of travel. It will be noted that the flux 
collectors 54a, 54b and 56a, 56b, respectively, extend longitudinally in a 
nonparallel relationship to the direction of polarity of the permanent 
magnet 24. Preferably they extend perpendicular to the direction of 
polarity of the magnet, i.e. parallel to the groove 16 for a 
transversely-magnetized magnet such as 24, and are parallel to each other. 
In the figures they are shown oppositely arranged relative to their 
respective sensors 54 and 56 such that the earth's magnetic field 32 
causes equal but opposite output signals from the sensors 54 and 56, 
regardless of its angle of incidence relative to the sensors (but as 
before, opposite excitation current polarities or opposite sensor 
orientations could accomplish the same purpose). With respect to their 
sensing of the magnetic field 26 of the permanent magnet 24, the outputs 
of the two sensors are both zero when the sensors are aligned transversely 
along the path of travel with respect to the magnet 24. Alternatively, if 
the vehicle is angled with respect to the path of travel, their outputs 
are substantially equal and opposite when the sensor assembly is aligned 
transversely with the magnet 24 (i.e. when the two sensors are equidistant 
from the magnet in a direction along the path of travel) so that their sum 
provides a null signal. Thus, the sensors together provide an accurate, 
position-sensing null signal, indicating alignment with the magnet 24, 
which is relatively insensitive to transverse offsets or angles of the 
vehicle relative to the path of travel. When the sensor assembly is not so 
aligned with the magnet 24, the combined output of the sensors is of one 
sign or the other, such sign depending on the direction of deviation of 
the sensor assembly from the magnet along the path of travel. 
With respect to FIG. 3, the two sensors 54 and 56 are supplied with 
excitation current from constant current sources 58 and 60, while their 
outputs are connected in series by circuit 62 between the inverting and 
noninverting inputs of a comparator 64. As in the case of sensors 28 and 
30, the equal but opposite components of the sensors' outputs resulting 
from the earth's magnetic field cancel each other. Moreover, when the 
sensors are aligned with the permanent magnet along the path of travel, 
they produce either zero output signals, or equal and opposite output 
signals (if the vehicle is angled relative to the path of travel), from 
their sensing of the magnetic field 26 of the permanent magnet. 
Accordingly, at alignment the output of comparator 64 is likewise zero. On 
the other hand, when the sensors 54 and 56 deviate to one side on the 
other of the magnet along the path of travel, their series-connected 
outputs, whether equal or unequal, are of one sign or the other depending 
on the direction of deviation. Accordingly, the output from the comparator 
64 is likewise of one sign or the other depending on the direction of 
deviation. 
Output signals of either sign from the comparator 64 are sensed by the 
vehicle controller 12 through line 66 and used to instruct the vehicle's 
drive system as to the direction of correction of the vehicle's position 
needed to bring the sensors into alignment with the magnet. Such signals 
could also be used to indicate proximity to the magnet, as a less 
preferable alternative to the proximity signals from sensors 28 and 30, 
for purposes of decelerating the vehicle preparatory to stopping in 
alignment with the magnet. 
The zero output of comparator 64, indicating precise alignment of the 
sensors with the permanent magnet along the path of travel, is sensed 
through lines 68a and 68b respectively. Each of these lines is connected 
to an OR gate 70 through a respective positive edge triggered, one-shot 
multivibrator 72 or 74. When the output signal from comparator 64 crosses 
through zero toward positive polarity, indicating alignment with the 
permanent magnet, the one-shot multivibrator 72 transmits a positive 
output signal to the OR gate 70 causing the gate to deliver a positive 
output signal to the vehicle controller 12. Conversely, when the output 
signal from comparator 64 passes through zero toward negative polarity, 
likewise indicating alignment with the magnet, inverter 76 causes the 
other one-shot multivibrator 74 to transmit a positive output signal to 
the OR gate 70, which likewise causes the gate to deliver a positive 
output signal to the vehicle controller 12. The positive output signal 
from the gate 70 can be used by the vehicle controller 12 for any of 
several purposes, such as instructing the vehicle drive system to stop the 
vehicle in precise alignment with the magnet along the path of travel, or 
updating and correcting a fifth wheel encoder if the vehicle is so 
equipped. 
The terms and expressions which have been employed in the foregoing 
specification are used therein as terms of description and not of 
limitation, and there is no intention, in the use of such terms and 
expressions, of excluding equivalents of the features shown and described 
or portions thereof, it being recognized that the scope of the invention 
is defined and limited only by the claims which follow.