Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1~73544 :
BACKGROVND OF THE INVEN~ION
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This invention relates to guidance devices for self-
powered vehicles and more particularly to a wire-following
guidance device for an order picking vehicle.
In the material handling industry, high-rise order
picker vehicles (OPVs) permit narrow aisle storage and retrieval
: operations of nonpalletized case or item storage. Such OPVs
carry an operator on a lifting platform who picks orders from
either a pallet or a storage module. The lifting plat~orm
incorporates the vehicle control so-the operator can ride on
- the platform. The aisle widths are extremely narrow and may
be as narrow as four feet. In the applicant's copending
Canadian application Serial No. 264,680 filed November 2, 1976
and entitled "Bolt-on Guidance System for Lift Truck" the
guidance system allows the operator to select between manual,
power steering guidance of the vehicle or automatic guidance
of the vehicle. In the sutomatic guidance mode, the vehicle
follows an energized wire which i6 buried in the floor over
which the vehicle travels.
In many self-guided vehicle systems, includine the
system described in the applicant's copending application
referred to above, the vehicle has a pair of wheels on a fixed,
that is, non-steerable axle and a steerable wheel which is
usually located in the front of the vehicle with respect to
the normal direction of travel. The device for sensing the
buried, energized wire then includes at least a ~air of coils ~ ~
which straddle the wire and which are mounted on the vehicle ~ -
ahead of the fixed axle. The purpose in mounting the coils
ahead of the fixed axle is to obtain servo stability. If it
is desired to move the vehicle in the reverse direction, in
order to retain stability, it is then necessary to mount an
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auxiliary pair of sensor coils on the vehicle in a position
such that they precede the fixed, non-steerable axle when the
vehicle travels in the reverse direction.
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See, for example, U.S. Patent No. 3,667,564 (Schnell) which
describes an elaborate mechanism for having two pairs of
steerable wheels, one of which is mechanically moved out of
position when the vehicle is traveling in the forward direction
and which is lowered to the ground and steerable when the
vehicle travels in the reverse direction. The normal steerable
wheel must be made to be nonrevolvable around its vertical
axis. The reverse direction sensor coils are also lowered
into and out of position depending on the direction of travel
of the vehicle. This mechanism is rather complicated and
clumsy. A further problem is that in the event a load is to
be carried behind the fixed axle, that is, on the forklift
itself, the load when it is lowered to the ground will inter-
fere with the sensor coils positioned underneath the forklift
and behind the axle.
.
SUMMARY OF THE INVENTION
The above disadvantages of the prior art are overcome
by the present invention of an improved, self-guided vehicle
of the type which automatically follows an e~ternally defined
path in a forward direction and which has at least one ground ~ ~-
; engaging steerable wheel, sensor means mounted on the vehicle
for generating a position error signal representative of the
position of the vehicle with respect to the path, steering
motor means attached to the ground engaging steering wheel ~
for steering the vehicle in response to a steering control ,
signal to the steering motor means, and steering circuit means
supplied with the position error signal for generating a first
steering control signal for the steering motor means to cause
the;steering~motor means to automatically steer the vehicle
along the external path. ~he improvement of the invention
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comprises sensor means for generating the position error signal
relative to a virtual sense point to the rear of the vehicle
to guide the vehicle when it travels in a backward direction
along the path.
In the preferred embodiment of the invention the
path over which the vehicle travels is defined by a buried,
energized wire. The sensor means comprise a pair of forward
sensor coils and a pair of reverse sensor coils with each of
the pairs of coils being mounted on the vehicle so as to
normally straddle the buried wire. Each pair of coils produces
an output signal representative of the difference of the outputs
of the coils of each pair. The sensor means further include
means for generating the position error signal (V) with respect
to the virtual sense point according to the formula:
V = (l+K)R-KF
where R = the difference of the outputs of
the rear pair of sensor coilsi
F = the difference of the outputs of
the forward pair of sensor coils; and
K = a constant which equals the ratio of
the distance of the rear pair of coils
to the virtual sense point divided by
the distance between the rear and
forward pairs of coils.
In a less advantageous embodiment the virtual sense
point position error signal is generated by right and left
error coils which are mounted on the vehicle on opposite sides
of the buried wire together with a direction sensing coil for
sensing the angle the vehicle makes with respect to the wire~
The direction sensing coil is aligned with the wire so that
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its output is proportional to the tangent of the angle made
with the wire for small angles. The sensor means further
includes means for generating the position error signal (V)
with respect to the virtual sense point according to the
formula
V=L-R+K~
where K is a constant determined in part by the maximum -
output of the direction sensing coil and the distance from
the position sensing coils to the virtual sense point, L
and R, of course, being the outputs of the left and right
error coils, respectively. This embodiment is less
advantageous because it is overly sensitive to variations
in the straightness of the buried wire. A slight wiggle in
the wire is greatly magnified and causes a spurious error
signal.
It is therefore an object of the present
invention to provide a guidance device for a self-powëred
cargo moving vehicle which allows the vehicle to travel in
the reverse direction without the necessity for having sensor
coils preceding the fixed axle of the vehicle;
It is another object of the present invention to
provide a guidance device for a self-powered vehicle which
senses the position and angle of the vehicle with respect
to the buried wire and generates a position error signal
relative to a virtual sense point which is ahead of the
j vehicle.
The foregoing and other objectives, features
' and advantages of the invention will be more readily
understood upon consideration of the following detailed
description of certain preferred embodiments of the
invention, taken in conjunction with the accompanying
drawings.
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BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a perspective view of an order-picking
vehicle equipped with a guidance system according to the present
invention;
Figure 2 is a side-elevational view of an order- :
picking vehicle equipped with the guidance system of the
invention;
Figure 3 is a plan view of the buried wire guide path
for the order-picking vehicle of the invention;
Figure 4 is a plan, diagrammatic view of tbe sensor
coil arrangement of the embodiment depicted in Figure l;
Figure 5 is an enlarged, vertical view, partly in
section and with portions broken away, of one of the sensor
coil assemblies of the invention;
Figure 6 is an enlarged, diagrammatic view of one of
the sensor coil arrangements of the in~ention; . :
Figures 7A and 7B together are a schematic block
diagram of the electronic guidance~system of the invention;
Figures 8 and 9 are, together, a detailed schematic
diagram of portions of the circuit depicted in block diagram
form in Figures 7A and 7B;
Figure 10 is a waveform diagram of the forward sensor
coil outputs of the guidance system depicted in Figures 7A and
7B;
Figure 11 is a plan view of an alternative coil
arrangement of a second embodiment according to the invention;
, Figure 12 is a circuit diagram of a modification of
! the circuit depicted in block diagram form in Figure 7 to : -
accommodate the second embodiment of the invention; and
. Figure 13 is a detailed schematic diagram of the
direction sensing logic portion of the circuit depicted in
Figures 7A and 7B.
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DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS
For convenience in relating this application to the
applicant's copending application referred to above, the -
reference numerals used throughout this application correspond
wherever possible to the reference numerals used for the same
elements in the applicant's copending application.
Referring now more particularly to Figures 1, 2 and 3
the basic order-picking vehicle 10 utilized in the guidance
system of the invention is of a conventional type. It has a
rear portion 12 which houses the motor and storage batteries
which drive the order-picking vehicle. As viewed in Figure 2,
the leftmost wheel is a ground-engaging guide wheel 14 which
is pivotable in a horizontal plane about a vertical axis and
which is driven by the motor within the housing 12. A pair
of horizontally spaced apart members 16 extend from the right-
hand end of the OPV, as viewed in Figure 2, and each supports
a pair of ground-engaging roller wheels 18 on nonpivotable,
fixed axles. A forklift assembly 20 is supported on a vertical
rack 22 extending above the horizontal members 16. The forklift
assembly 20 includes an operator cubicle 24 and a control
console 26 mounted within the cubicle 24. A steering mode
selector-handwheel 74 allows the operator to manually steer,
power steer, or automatically guide the vehicle. The operation
of this mechanism is explained in detail in applicant's
copending application. The forklift mechanism 20 is raised and
lowered on the rack 22 under the opexator's control by
conventional means which will, therefore, not be described in
further detail. The angular alignment of the ground-engaging
; steering wheel 14 is depicted visually above the OPV 10 by a
rotatable indicator 28 on top of the rear housing 12.
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The description of the OPV 10 to this point has been
of a conventional OPV. To modify the OPV 10 for the guidance
system of the present invention, a forward direction or front
sensing coil assembly 30 is mounted on the bottom surface of
the OPV imme~iately behind the wheels 14 and along the axis
of symmetry of the OPV 10. The normal, forward direction of
travel for the vehicle 10 is to the left in Figure 2, as
indicated by the arrow 5. A feedback sensor 32 is mounted on
' top of the indicator 28 to sense the angular position of the
indicator 28. A reverse direction or rear sensing coil assembly
31 is mounted on the bottom surface of the OPV along the OPV's
axis of symmetry and between the coil assembly 30 and the
fixed axle, nonsteerable wheels 18. The OPV 10, when operating -
in the automatic guidance mode, straddles a buried wire 34 in
the floor 36. The wire 34 is connected to a 6.3 KHz line dri~er
unit 38 which sends high frequency signals along the wire 34.
As will be explained in greater detail hereinafter, the OPV 10
when operating in the automatic guidance mode is centered over
the wire 34 and the sensing coil''.assemblies 30 and 31, straddl~
ing th~ wire, pick up the wire signals and feed them to an
'electronic guidance system. The guidance system, through a
motorized unit to be described in greater detail hereinafter,
rotates the ground-engaging steering wheel 14 in a manner so
: as to steer the OPV 10 along the wire 34.
The layout of the wire 34 in a typical installation
is depicted in Figure 3, which shows the wire 3'4 serpentined
through a plurality of storage aisles 40. The OPV 10 is
manually power steered, in a manner to be described in greater
detail hereinafter, into the storage facility until it
approaches the wire 34 at which point the operator switches
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the guidance mechanism to its automatic mode as it is approach-
ing the wire 34. When the OPV has either passed over the wire
34 and is heading away from it, or has come relatively close to ;-
the wire 34 and is heading away from it, the guidance system
will electronically lock onto the wire and guide the OPV over
the wire 34 and down between the storage aisles 40 until the
operator stops the OPV 10.
Referring now more particularly to Figures 4, 5 and ~`
6, the electronic sensor portion of the guidance device of the
present invention will be described in greater detail. The -
electro~.magnetic field transmitted by the alternating current
traveling through the buried wire 34 is distributed radially
along the wire as is illustrated by the magnetic flux lines
156 in ~igure 6. The front magnetic coil sensor assembly 30
carried by the OPV 10 comprises a pair of right and left
. . .
reference coils 158 and 160 and a pair of right and left error
coils 162 and 164, respectively. The terms right and left are
taken in Figure 4 looking in the direction of forward travel
.
as indicated by the arrow 5 in Figure 6 as though the observer
were standing behind the coils and lookin~ toward the direction
of travel (left). The reference coils 158 and 160 are spaced
apart by approximately 7.5 inches, that is, they are each
approximately 3.75 inches horizontally from the buried wire 34.
The error coils 162 and 164 are spaced approximately 14.5 inches
apart, that is, 7.25 inches horizontally ~rom the buried wire
34. The rear sensor coil assembly 31 is spaced approximately
two feet behind the front sensor coil assembly 30 and comprises
a pair of right and left sensor coils 33 ana 35, respectively,
spaced apart approximately 7.5 inches and on opposite sides of
the buried wire 34. That is, each coil is approximately 3.75
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inches horizontally from the wire. The rear coil assembly is
also eight feet forward of a hypothetical virtual sense point
2g.
Referring now to Figure 4, assume that the vehicle
deviates from the guide path so that the huried wire is at an
angle ~ with respect to the vehicle's axis of symmetry, as
indicated by the wire 34' shown in dashed line fashion in
Figure 4. The method by which the guidance circuit of the
invention provides an error signal when the vehicle is travel-
ing forward in the direction of the arrow 5 is described in
detail both in the applicant's copending application referred
to above and hereinafter. If the vehicle is trav~ling back-
ward, however, then by geometric constiuction it can be seen
that the error (V? in position between the virtual sense points
29 and 29' is determined by the formula:
V = R - (F - R) Dv
V = (1 + Dv/Ds)R _ Dv F
where F = error at front sensor 30, i.e., -
difference of outputs of coils 160 and ~ -
; 158;
R - error at rear sensor 31, i.e.,
difference of outputs of coils 35 and 33;
Dv = distance from rear coil assembly 31 to
the virtual sense point 29; and ,
Ds = distance between the coil assemblies
30 and 31.
For example, if the sensors 30 and 31 are spaced 2 feet apart
and the virtual sense point must be 10 feet behind the rear
sensor 31 the projection ratio is 10/2 = 5 so the virtual
sensor signal would be six times the rear sensor signal minus
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five times the front sensor signal.
Since the error signal is the difference between two
large numbers, both sensor characteristics must be very exact.
A 5% nonlinearity in the back sensor characteristic, for example,
results in a 5 x 5 = 25~ nonlinearity in the virtual sensor
output. Since variations in sensor slope cause variations in
loop gain, a linear sensor characteristic is important if servo
stability and accuracy are required. Another problem caused by
the high sensor gain is increased sensitivity to noise from the
~rive motor. A very linear and repeatable sensor with good
noise rejection is obtained by constructing the sensor in a
rigid sandwich as shown in Figure 5. The coils, such as coil
158, are all mounted horizontally on a printed circuit board
157 by their leads 159. The coil leads are interconnected by
the printed circuit in a manner to be described below. The
: side of the board 157 opposite to the coils is pressed hard
against an assembly of sheets comprised of a sheet of rubber
161, 1/8" thick, a 2" x 7" strip 163 of mu metal which is
.006" thick, and a 1/8" thick strip 165 of cold rolled steel.
The mu metal strip 163 ends near the inside end of the sensor
coil and provides a low reluctance, horizontal return path for
the lines of flux 156 from the guide wire 34. This increases
sensor output and reduces response to noise from the vehicle
drive motor. The mu metal also linearizes the sensor
characteristic and reduces variations in sensor characteristic
caused by steel truck components. This is very important as
distortions of the magnetic field from the wire due to stee~
truck components are not matched front to back. The resulting
unmatched nonlinearities in sensor characteristics could cause
greatly magnified nonlinearities in the resulting virtual sensor
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characteristic. It also makes it possible to have a very thin
sensor package. In one embodiment the sensor coils are 50 mHy.
R.F. choke coils. It should be understood that this same
construction is used for both of the sensor assemblies 30 and
31.
Referring now more particularly to Figures 7A and 7B
the operation of the guidance system of the invention will be - -
described in greater detail. The oppositely phased outputs RF
and -LF from the right and left reference coils 158 and 160,
respectively, are fed through separate variable gain trans-
conductance amplifiers 102 and 101, respectively, are fed to
the minus and plus inputs of a summing junction 104, respec-
tively. The resultant output LF~RF from the summing junction
104 is fed through a 6.25 kHz band pass filter 105 to the
input of an automatic gain controi rectifier/amplifier 106.
The output from the AGC rectifier/amplifier 106 is supplied
to the DC gain control inputs to the variable gain trans-
conductance amplifiers 101 and 102. The output of the
amplifiers 101 and 102 is in proportion to the DC gain control
current input. The result of this loop circuit is to adjust
the gain of the amplifiers 101 and 102 to keep the sum of the
left and right reference coils outputs (as seen at the output
o~ the band pass filter 105) constant. The difference be~een
the left and right reference sensor coil outputs is thus made
less sensitive to distance to the buried wire 34 and is also
made more linear. In a similar fashion the oppositely phased
outputs -RR and LR from the rear right and left sensor coils
33 and 35, respectively, are supplied through variable ~ain
transconductance amplifiers 107 and 108, respectively, to the
plus and minus inputs of a summing junction 109, respectively.
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The resulting ~output LR+RR from the summing junction 109 is
fed through a 6.25 kHz band pass filter 110 to the input of
an automatic gain control rectifier/amplifier 111. The DC
output from the AGC rectifier/amplifier lll is fed to the
moving "contact arm" of a single pole, double throw electronic
switch 112. The switch 112 is operated by a vehicle direction
sensor logic circuit 114. When the vehicle is moving in the
reverse direction the equivalent of the contact arm 112 is
connected to supply the output of the automatic gain cont~ol
rectifier/amplifier 111 to the DC gain control input to the
transconductance ampliiers 107 and 108. When the vehicle is
moving in the forward direction the output from the automatic
gain control rectifier/amplifier is supplied to the variable
gain control inputs to a pair of transconductance amplifiers :
100 and 103. The ampliier 100 is supplied with the output ~:
from the l:eft error coil 164 and the amplifier 103 is supplied
with the output from the right error coil 162. The respective
outputs LFO and -RFO from the amplifiers 100 and 103 are also
supplLed to t~e minus and plus inputs of the summing junction
109. Since when the switch 112 is in the forward mode the
amplifiers 107 and 108 receive no DC gain control current, .they
are effectively turned off and only the LFO and -RFO signals
are supplied to the summing junction 109. Conversely, when the
switch 112 is in the reverse mode the amplifiers 100 and 103
' receive no DC gain control signals and are effectively shut off : -
so that only the LR and -RR signals are supplied to the summing
junction 109. As in the case of the amplifiers 101 and 102,
the pbrpose of the loop involving the band pass filter 110 and
. the automatia gain control rectifier/amplifier 111 is to adjust
the gains at the respective amplifiers to keep the sum of the ~-
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left and right sensor coils signals supplied to the summing
junction 109 (as seen at the output of the band pass filter
110) constant.
The RF and -LF outputs of the amplifiers 102 and 101,
respectively, are also supplied to the inputs of a subtraction
junction 113 whose output, LF-RF, is supplied to the input of
a transconductance amplifier 115. The output of the trans~
conductance amplifier 115 is suppliea to a summing junction 116.
The summing junction 116 has the property that its ou~put is
five times the valve of its inputs.
The minus and plus inputs to the summing junction 109
are also supplied to a second subtraction junction 117 whose
output is supplied to a transconductance amplifier 118. The
output of the transconductance amplifier 118, which in the
reverse mode is RR-LR and which in the forward mode is RFO-L~O,
is also supplied to one of the inputs of the summing junction
;116. The output of the amplifier 118 is also supplied to
another summing junction 119. Another input to the summing
junction 119 is an electronic, CMOS switch 120 connected to
the output of the summing junction 116. The switch is closed
in the reverse mode and is open in the forward mode.
Thus in the reverse direction mode the output of the
summing junction 119 is six times the rear sensor signal minus -
five times the front sensor signal for a-projec~ion distance to
the virtual sense point of five times the spacing between the
sensor assemblies 3n and 31. The differencing ~s obtained by
adding LF-RF to the signal RR-LR which is the equivalent of
substracting RF-LF from the signal RR-L~. Trimpot adjustments
are provided to allow for the elimination of variations in the
sensor coil output and circuit gains. In the forward dlrection
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mode the signal RFO-LFO corresponding to the difference of
the right and left error coils 103 and 100 is supplied at
the ou~put of the summing junction 119. The filtered sum of
the right and left reference coil signals RF and LF is used
as a reference signal for a synchronous detector 180 to be
described in greater detail hereinafter.
The direction of the vehicle is determined from the
armature voltage of the driving motor and the forward and
reverse switches, symbolized by switch 121, in the motor control.
See also Figure 13. When the vehicle direction command îs
reversed, inertia causes the vehicle to continue moving in the -~
same direction for some time. During this time the armature
voltage will be reversed. Actual vehicle direction is thus
sensed by setting a logic latching circuit 124 to the
direction commanded by the control switch 121 only when the
armature voltage is not réversed. This is sensed by the
direction sense logic circuit 114 whose output is true when
the vehicle is going in the direction which has been commanded.
To prevent unnecessary power consumption by the
vehicle steering motor when the vehicle is stopped, a time-out
circuit 122 operating under the control of the direction sense
logic circuit 114 enables the steering control signal via an
electronic, CMOS switch 123 only when the armature voltage
indicates that the vehicle is moving. This circuit turns on
rather quickly, but only turns the guidance circuit off after
a 15 ~econd delay.
The output of the ban pass filter 105 is suppliea to
the input of an inverting amplifier 172 and to one arm of a
potenti~meter 174. The output of the amplifier 172 is labeled
3~ "-(LF+RF)" and is fed to the other arm of the potentiometer 174.
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The movable contact of the potentiometer 174 is mechanically
connected to the ground engaging steering wheel 14 as repre- :~
sented by the dashed line to the motor 44. Thus the angular
orientation of the ground engaging steering wheel 14 is
reflected in the position of the movable contact arm of the
potentiometer 174. The potentiometer 174 together with the
mechanical linkage indicated in dashed line form as being :
connected to the motor 44 actually represents the sensor 32
mounted on the indicator 28 on the back of the OPV housing
12 as shown in Figures 1 and 2.
If the ground engaging steering wheel 14 is turned
; to the right as far as it will go the movable contact arm of
the potentiometer 174 will be moved to the position where it
receives the signal -(LF+RF). If the ground engaging steering :
wheel is turned as far as it will turn to the left the mova~le. :~
contact arm of the potentiometer 174 will be at the opposite
end to receive the signal (LF+R~). The signal output from
the movable contact arm is labeled FB, because it is a :
negative feedback signal, and thls signal FB is added at the
summing junction 119. The output of the summing junction is : .
thus "R-L+FB" where R-L is 6 (RR-LR)-5(RF-LF) in the reverse
direction mode and is RFO-LFO in the forward direction mode. . .
This signal is supplied to the input of an operational
amplifier 176.
The amplifier signal ~-L+FB from the output of the
amplifier 176 is fed through a loop gain variab~e resistance
178 to one input of a synchronous detector 180. The ampli~ied
.
reference signal -(LF+RF) from the output of the amplifier 172
is fed to another input of the synchronous detector 180. The
synchronous detector detects signals which are coherent to the
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reference signal, that is when the reference signal is less
. than 180 out of phase with the error signal the synchronous
detector integrates the error signal R-L+FB. ~hen the
reference signal is more than 180 out of phase with the
error signal the synchronous detector inverts and integrates .:
the error signal R-L+FB. In this way spurious noise signals
are averaged out to nothing. The output from the synchronous
detector 180 is a DC si.gnal whose magnitude is representative
of the position error of the OPV 10 and whose polarity
indicates on which side of the wire the OPV 10 is positioned. :
This output is fed through a 5 Hz low pass ~ilter 184 to filter
out any high frequency pulses and the output of the filter 184
is fed to a .1-1.2 H~ lead filter which introduces an
approximately 60 lead in phase to prevent oscillation in the
feedback loop. The output from the lead filter 186 is fed to
one terminal of a single pole double throw electronic switch
188.
The other terminal of the switch 188 is connected
to the power steering tachometer 46 which is rotated by the
handwheel 74. The output o~ the lead filter 186 is also
supplied to an enable logic circuit 190. Another input to
the enable logic circuit 190 is from the output of a signal
amplitude detector 192 whose input is supplied from the
inverting amplifier 172.
The purpose of the enable logic circuit 190 is to
determine when the guidance system has "acquired" the buried
wire 34. The output of the signal amplitude detector 192
represents.a threshold signal.which is.simply an ampli~ied
version of the reference signal -(LF+RF). This threshold
signal together with the signal from the lead filter 186 .-
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allows the enable logic circuit 190 to detect whether the
signal is strong enough to guide the circuit and, from the
signs of the slope and the polarity of the error signal,
whether or not the OPV 10 has either crossed over the wire
and is heading away from it or has closely approached the
wire and is heading away from it.
Referring more particularly to Figure 10 a waveform
graph of the reference signal -(LF+RF) and the error signal
R-L is depicted with respect to the buried wire 34. As is
readily apparent from the figure the reference signal has a
slight dip in amplitude when the OPV 10 is centered over the
buried wire 34. The error signal udergoes a zero crossing
when the OPV 10 is centered over the wire 34. When the error
. .
signal and the reference signal lie on the same side of the
abcissa they are in phase and when the error signal is on the
opposite side of the abcissa the error and reference signals
are out of phase. At the point where the OPV 10 is about to
' cross the wire the polarity of the output of the synchronous
detector is changing from one polarity to another and the
slope of the error signal is approaching zero. It is this
condition which triggers the enable logic circuit 190 to
activate the electronic switch 188 to connect the output of
the lead filter 186 to the plus input of a summing junction 194.
Until this condition is reached, the enable logic circuit 190
connects the power steering tachometer to the plus input of
.
the summing junction 194. The manual-auto mode switch 146 is
'~l also connected to the enable logic circuit 1~0, thereby allow-
' ing the operator to manually cause the switch 188 to connect
' the power steering tachometer 46 to the summing junction 194
when the handwheel 74 is in its intermediate position. The
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enable logic circuit 190 also lights the "Auto" light 136 when
the switch 198 is in the position connecting the lead filter
186 to the summing junction 194.
The output of the summing junction 194 goes to a
pulse width modulated power driver circuit 196. The output of
the power driver 196 is a series of pulses whose width is
proportional to the magnitude of the error signal and whose
polarity corresponds to the polarity of the output signal from
the synchronous detector 180, that is the polarity is dependent
upon which side of the buried wire 34 the OPV 10 is standing.
One output lead from the power driver 196 is fed directly to
the motor 44. The other output lead is fed to the motor 44
through a low resistance 198. An electronic tachometer 200
has three inputs which are connected to the output of the
power driver 196 and the motor 44 so as to be able to sense
both the voltage drop across the motor 44 and the voltage drop
across the resistance 198. The motor 44 in effect acts like a
generator. By knowing how much of the voltage drop across the
motor is due to resistance losses in the armature it is possible
by sensing the current through the motor, as represented by
the voltage drop across the resistor 198, to calculate the
true back EMF generated by the motor 44. This information is
calculated in the electronic tachometer in analog fashion to
produce a feedback signal which is subtracted at the junction
. 194. This negative feedback signal provides a damping to
prevent the motor from oscillating due to overshoot which
might otherwise occur because of the major negative feedback
loop through the potentiometer 174. :
Referring now more particularly to ~igures 8 and 9,
a more detailed description will be given of the circuit
.
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depicted in Figures 7A and 7B. The same components depicted
in Figures 7A and 7B have been encircled with dashed lines
and are referred to by the same reference numbers. The ;
-(LF+RF) reference signal is fed to one input of a differential
amplifier 180 arranged in the circuit to act as the synchronous
detector. The output of the synchronous detector 180 is fed
to a low pass 5 Hz filter 184 comprised of a capacitor
connected between the output of the synchronous detector 180
and the circuit ground and a resistor connected between the
output of the synchronous detector 180 and the signal ground.
It should be noted that some of the components in the circuit
to be described are connected to the circuit ground while-
others are connected to the signal ground. The reason for this
is, as will be observed in the lower portion of Figure 8, that
the power supply designated generally as 202 has a plus 12 volt
output with respect to the circuit ground and a plus 7 volt
output connected to the signal ground.
The output of the lowpass filter 184 is supplied to
one input of a differential amplifier 204 whose other input
is supplied with the output of the lead filter 186 comprised
of a parallel RC circuit connected in feedback configuration
to the amplifier 204.
The output of the differential amplifier circuit 170
is also supplied to the signal amplitude detector 192 which is
comprised of an input resistance 206 connected to the cathode
of a diode 208 whose anode is connected to the input of a
differential amplifier 210. The other input of the amplifier
210 is connected through a resistance 212 to the circuit
ground and through a capacitor 214 to the anode of the diode
30 208. Plus 12 volts bias is supplied through a resistor 216 to -
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the anode of the diode 208. The output signal from the
amplifier 210 may be designated as the threshold signal and
it is supplied via a line 220 to the enable logic circuit 190.
The "MAN." terminal of the single pole double throw switch 146
is connected to the line 220. The contact arm of the switch
146 is connected to the circuit ground. Thus when the switch
146 is in the "MAN." position the line 220 is grounded and no
threshold signal is supplied to the enable logic circuit 192
just as if no threshold signal had been produced. Both of
these conditions will be designated for the purposes of this
discussion as a logic low.
The line 220 is connected to the input of an inverter
222 whose output is fed to one input of a NOR gate 224. The
output of the NOR gate is fed to one input of a second NOR gate
226 and to the controlling input of a CMOS switch 228 and the
input of an inverter 230. The other input of the NOR gate
224 is the output of the NOR gate 226. The.output of the
inverter 230 is connected through a resistance 232 to the base .
of an NPN transistor 234. The emitter electrode of the
20. transistor 234 is connected to the circuit ground. The LED
138 is connected in.series between the plus 24 volt supply and
the collector of the transistor 234. .
The output of the inverter 230 is also connected to ~.
the controlling input of a second CMOS switch 236 whose input
is supplied with the output of the power steering tachometer
.
46. The outputs of the CMOS switches 228 and 236 are combined
and fed to one input o~ a differential amplifier 238.
The other input of the NOR gate 226 is supplied from
the output of an exclusive OR gate 240. As will be explained
30 . in greater detail hereinafter, the output.of the OR gate 240
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is a signal representative of whether or not the signs of the
slope and polarity of the error signal after æynchronous
detection are the same to "enable" the logic, i~e. to make
the guidance system acquire the buried wire 34.
As was stated before, when the switch 146 is in the
manual mode or when no threshold signal is present on the line
220, a log1c high is placed on the corresponding input to the
NOR gate 224. When this happens the NOR gates 224 and 226 act
as a flip-flop in which the h~igh input from the inverter 222
to the NOR gate 224 is an overriding reset. The result is that
the output of the NOR gate 224 will be a logic low and the
output of the NOR gate 226 will be a logic high reyardless of
the output of the exclusive OR gate 240. The logic low
appearing at the output of the NOR gate 224 will cause the
transistor 234 to become conductive to energize the LED 138.
This same logic low will also cause the CMOS switch 228 to be
open and, because of the inverter 230, the CMOS switch 236
will be closed.
With the CMOS switch 228 open and the CMOS switch
236 closed the output from the power steering tachometer 46 ~ -
will be fed to the input of the differential amplifier 238.
The output of the amplifier 238 may be taken as the volocity
command or, in effect the steering control signal to the motor.
The polarity of the signal will determine which way the motor
control rotates.
If the switch 146 is switched to the auto position,
as shown in Figure 8, and a threshold signal appears on the
line 220, the output of the inverter 222 will be a logic low.
Assuming that the output from the exclusive OR gate 240 is
also a logic low, indicating that the sign of the slope is
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not equal to the sign of the polarity of the synchronously
detected error signal, and that the output of the NOR gate
224 continues to be a logic low, then the output of the NOR
gate 226 will be a logic high. At this point, even though
the switch 146 is at "AUTO", the OPV 10 will continue under
the power steering mode until the signs of the slope and
polarity of the modified error signal are equal. When this
happens the output of the exclusive OR gate 240 will be a
logic high, causing the output of the NOR gate 226 to be a
logic low. With two logic lows to the input of the NOR gate
224 its output will change to a logic high and latch the flipr
, flop. ;'
A logic high at the output of the NOR gate 224 will
cause the CMOS switch 228 to become conductive-and ths CMOS
switch 236 to become non-conductive. The LED 138 supplied from
the outpu,t of the inverter 230 will also be extinguished. Thus
the input signal to the amplifier 238 will be the guidance
control input derived from the sensing coils and the OPV lQ
will be steered automatically.
In order to determine the polarity and slope of the
error signal the output of the amplifier 204 is fed to one
input of an amplifier 242 whose other input is connected to
the chassis ground and whose output is fed to one input of
the exclusive OR gate 240. The output from the amplifier 204
is also fed to one input of a differential amplifier 244 and,
through a resistor 246 to the other input of the differentia~
amplifier 244. This other input is also'connected to the
circuit ~round through a capacitor 248. The output of the ' ,
amplifier 244 is supplied to the other input of the exclusive
OR gate 240. The output of the amplifier 242 is representative
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of the polarity of the output of the amplifier 204 and the
output of the amplifier 244 is representative of the slope
of the same signal. When the oPV 10 has come sufficiently
close to the buried wire 34 for the threshold signal to be
established at the output of the amplifier 210 then the two
amplifiers 242 and 244 together with the exclusive OR gate
240 will determine whether the sign of the slope of the error
signal is equal to the sign of its polarity, indicating th~t
the OPV 10 is going away from the wire. When this happens
the output of the exclusive OR gate 240 will be a logic high.
It should be noted that the guide flip-flop made up
of the NOR gates 224 and 226 is effectively a latching flip-
flop. Once the flip-flop 224 has gone into the auto mode,
it will only reset on a change in state of the signal applied
from the output of the inverter 222, which indicates either
that the switch 146 has been thrown to the manual mode or that
the threshold signal has been lost. Provided the threshold -
signal is present and the switch 146 is in the auto positi~n,
; no changes at the output of the exclusi~e OR gate 240 will
affect the state of the flip-flop.
In order to warn the operator that the guide flip-
flop has changed state, such as if the threshold signal shoula
somehow be lost, the output from the NOR gate 224 is fed
through a series RC circuit 246 to one input of a low true
NAND gate 248. This same input of the NAND gate 248 is also
supplied with appropriate plus 12 volt bias. The other input
of the NAND gate is connected directly to the auto terminal
of the switch 146 a~d through a resistance 250 to the LE~ 136.
The output of the NAND gate 248 i~s supplied to the base
electrode of an NPN transistor 252 whose emitter electrode i5
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connected to the circuit ground and whose collector electrode
is connected in series with an alarm 254 to a plus 24 volt
source.
In operation, the input to the NAND gate 248 supplied
by the switch 146 is a logic low. When the output of the NOR
gate 224 also goes to a logic low, indicating that the guide
flip-flop has somehow reset itself, then the output of the
NAND gate 248 will become a logic high, triggering the alarm
254 through the transistor 252. An amplifier 256 having one
lead connected through a diode 258 to the plus 12 volt source
and its output connected through a resistance 260 to the base
electrode of the transistor 252 will activate the alarm 254 if
there is a power failure.
Referring more particularly now to Figure 9, the
velocity command output signal from the amplifier 238 is fed
to one inpuk of a comparator 262 and to the corresponding inpu~
of a second comparator 264. The output of the amplifier 262 is
fed to one input of an exclusive OR gate 266, the input o~ an
inverter 268 and through a parallel diode resistance circuit
270 to one input of an amplifier 272. The same input of the
amplifier 272 is connected through a capacitor 274 to the
circuit ground. The output of the inverter 268 is connected
through a similar parallel diode resistance circuit 276 to
one input of an amplifier 278. This same input of the amplifier
is connected through a capacitor 280 to the circuit ground.
The other inputs of the amplifier 272 and 278 are connected
to a plus 12 volt source.
The output of the amplifier 278 is connected to
the base electrode of an NPN transistor 282 whose collector
electrode is connected through a resistance 284 to the
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collector electrode of a PNP transistor 286. The emitter
electrode of the transistor 286 is connected directly to the
plus 24 volt source for the motor. The base electrode of
the transistor 286 is forwardly ~iased through appropriate
resistances from the plus 24 volt source.
The output from the amplifier 272 is connected to
the base electrode of an NPN transistor 288 whose collector
electrode is connected through a resistance 290 to the
collector electode of a PNP transistor 292. The emitter
electrode of th~ transistor 292 is connected directly to the
plus 24 volt motor source and its base electrode is forwardly
biased by appropriate resistance from the plus 24 volt source.
The base electrodes of the transistors 286 and 292 are also
connected to the collector electrode-of an NPN transistor 294
whose emitter electrode is connected to the circuit ground.
To control the direction of the current supply to
the motor 44 the collector electrode of the transistor 286
is connected to the base electrode of a PNP transistor 296
whose emitter electrode is connected to the plus 24 volt
motor source. The collector electrode of the transistor 296
is connected to a junction point 298 and to the collector
electrode of an NPN transistor 300. The base and emitter
electrodes of the transistor 300 are connected to the
~ollector of the transistor 288 and to a junction point 302,
respectively. The emitter electrode of the transistor 282
is connected to the base electrode of an NPN transistor 304
, whose emitter electrode is connected to the point 302 and
whose collector electrode is connected to a junction point
, 306.
The collector electrode of the transistor 292 is
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connected to the base electrode of a PNP transistor 308 whose
emitter electrode is connected to the plus 24 volt battery
source for the motor. The collector electrode of the trans-
istor 308 is connected to the junction point 306. The point
302 is connected in series with a very low resistance wire
310 to the minus terminal of the motor battery. The motor 44
is connected at one side to the junction point 298 and
through the resistor 198 to the junction point 306.
The output of the exclusive OR gate 266 is fed to
one input of a NOR gate 314. The output of the NOR gate 314
is supplied to a combination of inverters and operational
amplifiers designated generally as 316 which convert the NOR
gate 314 into a 200 microsecond, one shot multi-vibratar.
The output of the NOR gate 314, which is effectively the
output of the multi-vibrator, is fed through an inverter
318 to the base electrode of the NPN transistor 294.
If any of the inputs to the NOR gate 314 is a logic
high its output is a logic low and the transistor 294 will be
conductive to forwardly bias the transistors 286 and 292.
When the transistors 286 and 292 are forwardly biased, i.e.
conductive, they short together the base and emitter electrodes
of the transistors 296 and 308, respectively, making them
non-conductive so that the motor will not run. As long as all
the inputs to the NOR gate 314 are logic lows, its output will
be a logic high and the transistors 286 and 292 will be
non-conductive.
Assuming that the output of the amplifier 262 is a
logic high, the output of the amplifier 272 will cause the
transistor 288 to become conductive thereby making the PNP
transistor 308 and the NPN transistor 300 conductive by
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connecting their base electrodes together through the
resistor 290, which can have a value of 600 ohms, for example.
It can be seen that this causes a current path to flow from
the 24 volt battery source through the transistor 308, the
resistor 198, the motor 44, the transistor 300 and the resistor
310 to the minus terminal of the battery. Thus the motor will
run in a preordained direction determined by the path of current
flow. Similarly, when the output of the amplifier 262 is the
equivalent of a logic low, these same transistors will be
turned off and, through the inverter 268 and the amplifier
278, the transistors 282, 304 and 296 will becsme conductive ~`
to supply current to the motor 44, though in the opposite
direction to cause the motor to rotate in the opposite direction.
Thus, the polarity of the output of the amplifier 262 is
determinative of the direction in which the motor will run.
As will be described in greater detail hereinafter, the
polarity of the output signal from the amplifier 262 depends
; on the polarity of the velocity command signal from the
amplifier 238 as well as the output of the electronic tacho-
meter 200.
As explained above in reference to Figures 7A and
7B, the electronic tachometer 200 is connected in parallel
with the motor and across the resistance l98. As shown in
Figure 12, these connections are made by way of lines 312,
Z 320 and 322 connected to points 298, the junction of the
motor and the resistor 198, and the point 306, respectively.
The lines 312, 320 and 322 are the three inputs to the
electronic tachometer 200, which is comprised of a differential
amplifier 324 whose inputs are supplied by the lines connected
to the motor and whose output is connected to the inputs of the
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amplifiers 262 and 264 other than the inputs connected to the
output of the amplifier 238. As mentioned above, the outputs
of the amplifiers 262 and 264 are supplied to the inputs of
an exclusive OR gate 266. The exclusive OR gate acts as a
controlled inverter whose output will be low whenever the
absolute magnitude of the velocity command signal exceeds the
absolute magnitude of the tachometer output signal, provided
that the two signals are of the same polarity. If the two
signals are of opposite polarity, then the output of the OR --
gate 266 will be low. For any other condition the output of
the OR gate 266 will he a logic high with the result that
the motor 44 will be turned off. The minimum time during
which the motor 44 will be turned off is approximately 200
micrGseconas, which is determined by the circuit values within
the multi-vibrator circuit 31~. The duration ~uring which the
motor 44 will be turned on is determined by the length of
time required for the output signal from the electronic
tachometer 200 to match the velocity command signal from the
amplifier 238. In order to guard against the possibility
that a pair of series connected power transistors such as
transistors 296 and 300 or 308 and 304 might be simultaneously
made conducting the parallel resistor diode circuits 276 and
270 together with their associated capacitors 280 and 274
insure that when there is a change in polarity of the velocity
! command signal that all the power transistors will be turned
off before any other set is turned on.
A differential amplifier 326 has its two inputs
connected in parallel with the resistor 310 to act as a torque i
limiting sensor to shut off the motor in the event that,
becausq of some physical binding in the guide wheel mechanism,
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the motor is forced to draw an excess of current which would
damage the motor. When the voltage across the resistor 310
increases beyond a predetermined value the output of the
amplifier 326 reaches what amounts to a logic high which is
fed to one input at the NOR gate 314. This logic high will
cause the motor to be deenergized. Similarly, the power
failure signal from the output of the amplifier 25Ç is also
supplied to one input of the NOR gate 314 to shut off the
motor in the event there is a failure in power to the guidance
circuit.
Referring now more particularly to Figures 11 and 12,
a modified embodiment of the reverse guidance system accordi~g
to the invention is illustrated. In this embodiment there are
no rear sensing coils, but instead there is a single angle
sensing coil 125 which is positioned more or less between the
right and left reference coils 158' and 160', respectively.
The direction sensing coil 125 may take the form of a long
; ferrite coil not unsimilar to the type which is sometimes used
as an antenna coil in portable radios or portable radio
direction finders. The coil 125 is positioned on the bottom
of the vehicle to be normally directly over the buried wire 34.
Assuming that the vehicle is positoned correctly over the wire
34 but is heading in a direction at an angle ~ to the wire, the
output of the coil 125 will be A sine ~, where A is the maximum
amplitude of the coil output. It will be appreciated that for
very small angles of ~ the sine of ~ will approximately equal
the tangent of ~.
If it is desired to steer the vehicle with respect to a
virtual sense point 29' spaced to the rear of the vehicle and
beyond the fixed axle wheels when the vehicle makes an angle
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with the wire 34 this virtual sense point will be displaced
off of the wire by a distance d. If the virtual sense point
is at a distance Dp from the center of the coil 125, by ~ .
geometric construction the error d is equal to approximately
Dp sine ~, or
_~ . A sin~ = d
Since A sin~ is simply the output of the coil 125 and since Dp :
is a constant, the error signal can be rewritten as R~ where ~
is the output of the coil 125. Added to this must be the normal,
positional error signals from the coils 162 and 164 which
. indicate how far away the vehicle is from the buried wire 34.
Thus the final error signal is R-L+K~ where R and L are the
outputs from the error coils 162 and 164. . ..
Referring now more-particularly to Figure 12, the .
circuit depicted in Figure 7A is modified so that the circuit
elements 107, 108, 113, 115, 116 and 120 are omitted. Further- ....
more the gain control signal for the amplifiers 100 and 103 is ; :.
20. taken directly from the output of the AGC rectifier/amplifier
111 rather.than from the terminal of the switch 112. The
output of the direction sensing coil 125 is fed to the inputs
I of a transconductance amplifier 126~-whose output is connected to
the plus input terminal of the summing junction 109 and to the
l plus input terminal of the summing junction 117. Thus it can
.:, be appreciated that when the switch 112 is in the forward
direction mode, the output from the amplifier 118 will be : ~
RF0-LF0 and when the switch 112 is in the reverse direction ~ .-
mode, thus supplying a gain signal to the amplifier 126, the :
output of the amplifier 118 will be RF0-LFO+K~. As in the
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embodiment depicted in Figure 7A the output of the amplifier
118 is supplied to the summing junction 119 where it is
combined with the feedback signal FB. The output of the
junction 119 goes to the switch 123 as in the embodiment
depicted in Figure 7A.
As mentioned at the beginning of this application,
this embodiment is somewhat less advantageous than the multicoil
arrangement depicted in Figure 7A because this embodiment is
overly sensitive to variations in the straightness of the
~0 buried wire. A slight wiggle in the wire is greatly magnified
and causes a spurious error signal. These effects can be
reduced somewhat by using a long direction sensing coil 125
and by only using the system on a vehicle with a relatively
small wheel base. This has the effect of reducing the projection
distance Dp with the consequence that variations in the
straightness of the buried wire are not magnified as ~reatly.
This is apparent from the fact that the constant K is actually
equal to Dp. Thus any reduction in Dp will also reduce the
effect of any variations in the straightness of the buriea wire.
Although the foregoing invention has been described
in some detail by way of illustration and example for purposes -
of clarity of understanding, it is understood that certain
changes and modifications may be practiced within the spirit
of the invention as limited only by the scope of the appended
claims.
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