U.S. patent number RE30,492 [Application Number 06/031,618] was granted by the patent office on 1981-01-27 for reverse direction guidance system for lift truck.
This patent grant is currently assigned to Logisticon, Inc.. Invention is credited to Thomas R. Blakeslee.
United States Patent |
RE30,492 |
Blakeslee |
January 27, 1981 |
Reverse direction guidance system for lift truck
Abstract
An automatic guidance device for a self-powered cargo moving
vehicle operated by a vehicle-borne sensor which follows a buried,
energized wire path and which includes sensor means mounted either
between the fixed and steerable axles or in front of the fixed axle
of the vehicle for guiding the vehicle when it travels in a
direction such that the fixed axle precedes the sensor by
effectively generating a position error signal relative to the
direction of travel. In one preferred embodiment the sensor
includes a pair of sensing coils whose outputs are combined to
generate an error signal V=(1+K)R-KF where R and F are the
difference of outputs of a pair of rear and forward sensors,
respectively, and K is equal to the ratio of the distance of the
rear pair of coils to the virtual sense point and the distance
between the rear and forward pairs of coils.
Inventors: |
Blakeslee; Thomas R. (Woodside,
CA) |
Assignee: |
Logisticon, Inc. (Sunnyvale,
CA)
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Family
ID: |
27363919 |
Appl.
No.: |
06/031,618 |
Filed: |
April 19, 1979 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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629491 |
Nov 6, 1975 |
4040500 |
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Reissue of: |
644549 |
Dec 29, 1975 |
04043418 |
Aug 23, 1977 |
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Current U.S.
Class: |
180/168;
318/587 |
Current CPC
Class: |
B62D
1/28 (20130101); G05D 2201/0216 (20130101) |
Current International
Class: |
B62D
1/28 (20060101); B62D 1/00 (20060101); B62D
001/28 () |
Field of
Search: |
;180/79.1,98,167,168
;318/580,587 ;250/202 ;364/424 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1801967 |
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Jun 1970 |
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DE |
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1808442 |
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Jun 1970 |
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DE |
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1955758 |
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May 1971 |
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DE |
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2192520 |
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Feb 1974 |
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FR |
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2216161 |
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Aug 1974 |
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FR |
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2235035 |
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Jan 1975 |
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FR |
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Primary Examiner: Sheridan; Robert G.
Assistant Examiner: Siemens; Terrance L.
Attorney, Agent or Firm: Limbach, Limbach & Sutton
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part of my copending
application Ser. No. 629,491 filed Nov. 6, 1975, .Iadd.now U.S.
Pat. No. 4,040,500 .Iaddend.and entitled GUIDANCE SYSTEM FOR LIFT
TRUCK.
Claims
What is claimed is:
1. An improved, self-guided vehicle of the type which automatically
follows an externally 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 actuator means attached to the ground engaging
steering wheel for steering the vehicle in response to a steering
control signal to the steering actuator means, and steering circuit
means supplied with the position error signal for generating a
first steering control signal for the steering actuator means to
cause the steering actuator means to automatically steer the
vehicle along the external path, wherein the improvement
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.
2. An improved self-guided vehicle as recited in claim 1 wherein
the path is defined by a buried, energized wire and wherein the
sensor means comprise a pair of forward sensor coils and a pair of
reverse sensor coils with the terms forward and rear taken in the
sense of the direction of forward vehicle travel, each of the pairs
of coils being mounted on the vehicle so as to normally straddle
the path and each pair of the coils producing an output signal
representative of the difference of the outputs of the coils of
each pair, and means for generating the position error signal (V)
with respect to the virtual sense point according to the
formula:
where
R=difference of outputs of the rear pair of sensor coils;
F=difference of outputs of the forward pair of sensor coils;
and
K=constant=ratio of the distance between the rear pair of coils and
the virtual sense point to the distance between the rear and
forward pairs of coils.
3. An improved self-guided vehicle as recited in claim 1 wherein
the path is defined by a buried, energized wire and wherein the
sensor means comprise right (R) and left (L) error coils mounted on
the vehicle on opposite sides of the path and a direction sensing
coil for sensing the angle the vehicle makes with respect to the
wire, the direction sensing coil being aligned with the wire so
that its output (.phi.) is proportional to the tangent of the angle
made with the wire for small angles, and means for generating the
position error signal (V) with respect to the virtual sense point
according to the formula
where
K is a constant determined in part by the ratio of the distance
between the direction sensing coil and the virtual sense point to
the maximum output of the direction sensing coil
L=output of left error coil
R=output of right error coil
.phi.=output of the direction sensing coil.
4. An improved self-guided vehicle as recited in claim 1 further
comprising
enabling logic means supplied with the sensor signal for producing
an enabling output signal when the vehicle is being manually
steered toward the external path and has crossed it, or passed
within a predetermined distance of it, and thereafter is moving
away from the external path, and
switching means supplied with the enabling output signal to supply
the first steering control signal to the steering actuator means
only after the enabling output signal is generated, whereby the
vehicle will become automatically guided along the external path
only after it has been manually steered across or just up to the
external path and is starting to move away from it.
5. An improved self-guided vehicle as recited in claim 4 wherein
the external path is defined by an energized guide wire and the
improvement to the vehicle further comprises:
coil means within the sensor means and straddling the wire to
produce output signals proportional to the vehicle's lateral
distance from both sides of the wire, the coil means including
means for producing an error signal from the difference of the coil
means output signals, and
means within the enabling logic means for triggering in part the
enabling output signal when the arithmetic signs of the slope and
polarity of the error signal are equal, thereby indicating that the
vehicle is moving away from the wire.
6. An improved self-guided vehicle as recited in claim 5 wherein
the coil means further include means for producing a reference
signal from the sum of the coil means output signals and the
enabling logic means include means for sensing when the reference
signal reaches a predetermined magnitude and for thereafter
generating a threshold signal which triggers in part the enabling
logic means to produce the enabling output signal.
7. An improved self-guided vehicle as recited in claim 6 wherein
the enabling logic means will continue to produce the enabling
output signal when once triggered provided that the threshold
signal continues to be generated.
8. An improved self-guided vehicle as recited in claim 5 wherein
the coil means comprise at least a pair of sensing coils on
opposite sides of the wire and a multilayered assembly for
interconnecting the coils and for mounting the coils horizontally,
the multilayered assembly including a printed circuit board, a
sheet of insulating material, a mu metal strip and a strip of
ferrous material, the printed circuit board having a metallic
circuit printed thereon and the coils having leads attached to the
printed circuit board and interconnected by the printed circuit,
the mu metal strip providing a low reluctance return path for lines
of magnetic flux generated by the energized wire and passing
through the coils.
9. An improved self-guided vehicle as recited in claim 4 wherein
the steering actuator means include an electric motor for
positioning the ground engaging steering wheel and the steering
circuit means include
an electric tachometer for sensing the voltage drop across the
motor and the electrical current flowing through it for generating
a tachometer feedback signal representative of the response of the
motor to the steering control signal, and
means for subtracting the tachometer feedback signal from the
steering control signal before it is supplied to the motor.
10. An improved, self-guided vehicle of the type which
automatically follows a path defined by a buried, energized wire
and which has at least one ground engaging steerable wheel, and a
pair of wheels on a fixed axle, sensor means mounted on the vehicle
for generating a sensor signal representative of both the lateral
position of the vehicle with respect to the wire and the
inclination of direction of vehicle travel with respect to the
wire, steering actuator means attached to the ground engaging
steering wheel for steering the vehicle in response to a steering
control signal to the steering actuator means, and steering circuit
means supplied with the sensor signal for generating a first
steering control signal for the steering actuator means to cause
the steering actuator means to automatically steer the vehicle
along the wire path, wherein the improvement comprises:
the sensor means is mounted on the vehicle between the steerable
wheel and the fixed axle and guides the vehicle along the wire path
when the vehicle is traveling in a direction such that the fixed
axle wheels precede the steerable wheel, the sensor means including
a pair of first sensor coils and a pair of second sensor coils, the
first sensor coils being mounted closest to the fixed axle wheels
and each of the pairs of coils being mounted on the vehicle so as
to normally straddle the wire path and each pair of the coils
producing an output signal representative of the difference of the
outputs of the coils of each pair, and means for generating the
position error signal (V) with respect to a virtual sense point,
located beyond the fixed axle wheels and in the direction of
vehicle travel, according to the formula:
where
A=difference of outputs of first pair of sensor coils;
B=difference of outputs of the second pair of sensor coils; and
K=constant=ratio of distance between the first pair of sensor coils
and the virtual sense point to the distance between the first and
second pairs of coils. .Iadd. 11. An improved, self-guided vehicle
of the type which automatically follows an externally defined path,
and which has at least one ground engaging steerable wheel, a pair
of wheels on a fixed axle, a path sensor mounted on the vehicle for
generating a position error signal representative of both the
lateral position of the vehicle with respect to the path and the
inclination of direction of vehicle travel with respect to the
path, a steering actuator attached to the ground engaging steerable
wheel for steering the vehicle in response to a steering control
signal to the steering actuator, and a steering circuit controller
supplied with the position error signal for generating said
steering control signal for the steering actuator to cause the
steering actuator to automatically steer the vehicle along the
path, wherein the improvement comprises a first and a second pair
of sensors, both pairs of sensors being mounted on the vehicle
between the steerable wheel and the fixed axle to guide the vehicle
along the path when the vehicle is traveling in a direction such
that the fixed axle wheels precede the steerable wheel, the first
pair of sensors being mounted closest to the fixed axle wheels and
each of the first and second pairs of sensors being mounted on the
vehicle so as to normally straddle the path, each of the first and
second pairs of sensors producing an output signal representative
of the difference of the outputs of the sensors of each pair, and
means supplied with output signals of the sensor pairs for
generating a position error signal (V) with respect to a virtual
sense point, located beyond the fixed axle wheels and in the
direction of vehicle travel, according to the formula:
where A=difference of outputs of first pair of sensors;
B=difference of outputs of the second pair of sensors; and
K=constant=ratio of the distance between the first pair of sensors
and the virtual sense point to the distance between the first and
second pairs of
sensors, this ratio being greater than 1. .Iaddend..Iadd. 12. An
improved, self-guided vehicle as recited in claim 11 wherein the
improvement further comprises separate automatic gain controlled
amplifying means for the first and second pairs of sensors whereby
the first and second pairs of sensors are desensitized to
variations in the vertical distance between the path and each of
the sensor pairs. .Iaddend.
Description
BACKGROUND OF THE INVENTION
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 platform incorporates the vehicle
control so the operator can ride on the platform. The aisle widths
are extremely narrow and may be as narrow as 4 feet. In the
applicant's copending application the guidance system allows the
operator to select between manual, power steering guidance of the
vehicle or automatic guidance of the vehicle. In the automatic
guidance mode, the vehicle follows an energized wire which is
buried in the floor over which the vehicle travels.
In many self-guided vehicle systems, including the system described
in the applicant's copending application referred to above, the
vehicle has a pair of wheels on a fixed, that is, nonsteerable 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
pair 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 auxiliary pair
of sensor coils on the vehicle in a position such that they precede
the fixed, nonsteerable axle when the vehicle travels in the
reverse direction. See, for example, U.S. Pat. No. 3,667,564
(Schnell) which described 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 non-revolvable 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
interfere 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 externally 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. The improvement of the invention
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:
where
R=the difference of the outputs of the rear pair of sensor
coils;
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 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
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-powered 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 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, take in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of an order-picking vehicle equipped
with a guidance system according to the present invention;
FIG. 2 is a side-elevational view of an order-picking vehicle
equipped with the guidance system of the invention;
FIG. 3 is a plan view of the buried wire guide path for the
order-picking vehicle of the invention;
FIG. 4 is a plan, diagrammatic view of the sensor coil arrangement
of the embodiment depicted in FIG. 1;
FIG. 5 is an enlarged, vertical view, partly in section and with
portions broken away, of one of the sensor coil assemblies of the
invention;
FIG. 6 is an enlarged, diagrammatic view of one of the sensor coil
arrangements of the invention;
FIGS. 7A and 7B together are a schematic block diagram of the
electronic guidance system of the invention;
FIGS. 8 and 9 are, together, a detailed schematic diagram of
portions of the circuit depicted in block diagram form in FIGS. 7A
and 7B;
FIG. 10 is a waveform diagram of the forward sensor coil outputs of
the guidance system depicted in FIGS. 7A and 7B;
FIG. 11 is a plan view of an alternative coil arrangement of a
second embodiment according to the invention;
FIG. 12 is a circuit diagram of a modification of the circuit
depicted in block diagram form in FIG. 7 to accommodate the second
embodiment of the invention; and
FIG. 13 is a detailed schematic diagram of the direction sensing
logic portion of the circuit depicted in FIGS. 7A and 7B.
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 FIGS. 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 FIG. 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 righthand end of the OPV, as viewed in FIG. 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 operator'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 indictor 28 on top of the
rear housing 12.
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 immediately
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 FIG. 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 driver unit 38
which sends high frequency signals along the wire 34. As will be
explained in greater detail hereinafter, the OPV 10 when the
operating in the automatic guidance mode is centered over the wire
34 and the sensing coil assemblies 30 and 31, straddling the 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
FIG. 3, which shows the wire 34 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 unit it approaches the wire 34 at which point the
operator switches the guidance mechanism to its automatic mode as
it is approaching 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 FIGS. 4, 5 and 6, the electronic
sensor portion of the guidance device of the present invention will
be described in greater detail. The electromagnetic 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 FIG. 6. The front magnetic coil
sensor assembly 30 carried by the OPV 10 comprises a pair of right
and left reference coil 158 and 160 and a pair of right and left
error coils 162 and 164, respectively. The terms right and left are
taken in FIG. 4 looking in the direction of forward travel as
indicated by the arrow 5 in FIG. 6 as though the observer were
standing behind the coils and looking 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 from the buried wire 34. The rear sensor coil
assembly 31 is spaced approximately two .[.feed.]. .Iadd.feet
.Iaddend.behind the front sensor coil assembly 30 and comprises a
pair of right and left sensor coils 33 and 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 inches
horizontally from the wire. The rear coil assembly is also eight
feed forward of a hypothetical virtual sense point 29.
Referring now to FIG. 4, assume that the vehicle deviates from the
guide path so that the buried wire is at an angle 100 with respect
to the vehicle's axis of symmetry, as indicated by the wire 34'
shown in dashed line fashion in FIG. 4. The method by which the
guidance circuit of the invention provides an error signal when the
vehicle is traveling 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 traveling
backward, however, then by geometric construction if can be seen
that the error (V) in position between the virtual sense points 29
and 29' is determined by the formula ##EQU1## 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.]. .Iadd.For .Iaddend.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 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.times.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 drive motor. A very linear
and repeatable sensor with good noise rejection is obtained by
constructing the sensor in a rigid sandwich as shown in FIG. 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 inch thick, a 2 inch.times.7 inch strip 163 of mu metal
which is 0.006 inch thick, and a 1/8 inch 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 guides 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 steel truck
components are not matched front to back. The resulting unmatched
nonlinearilities in sensor characteristics could cause greatly
magnified nonlinearities in the resulting virtual sensor
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 FIGS. 7A and 7B the operation of
the guidance system of the invention will be described in greater
detail. The oppositely phased output RF and --LF from the right and
left reference coils 158 and 160, respectively, are fed through
separate variable gain transconductance amplifiers 102 and 101,
respectively, are fed to the minus and plus inputs of a summing
junction 104, respectively. 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 control rectifier/amplifier 106.
The output from the AGC rectifier/amplifier 106 is supplied to the
DC gain control inputs to the variable gain transconductance
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 of the band pass filter 105) constant. The
difference between 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 right and left sensor coils 33
and 35, respectively, are supplied through variable gain
transconductance amplifiers 107 and 108, respectively, to the plus
and minus inputs of a summing junction 109, respectively. 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 111 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 control rectifier/amplifier 111 to the DC gain control input
to the transconductance amplifiers 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 amplifier 100 is supplied with the output from the left
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 supplied to the 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 purpose of the loop involving the band
pass filter 110 and the automatic gain control rectifier/amplifer
111 is to adjust the gains at the respective amplifiers to keep the
sum of the 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 transconductance
amplifier 115 is supplied to a summing junction 116. The summing
junction 116 has the property that its output is five times the
value 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--LFO, is also supplied to one
of the inputs of the summing junction 116. The output of the
amplifer 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 projection distance to the virtual
sense point of five times the spacing between the sensor assemblies
30 and 31. The differencing is obtained by adding LF--RF to the
signal RR--LR which is the equivalent of subtracting RF--LF from
the signal RR--LR. Trimpot adjustments are provided to allow for
the elimination of variations in the sensor coil output and circuit
gains. In the forward direction mode the signal RFO--LFO
corresponding to the difference of the right and left error coils
.[.103.]. .Iadd.162 .Iaddend.and .[.100.]. .Iadd.164 .Iaddend.is
supplied at the output 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 FIG. 13.
When the vehicle direction command is 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 reversed. 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 second delay.
The output of the band pass filter 105 is supplied to the input of
an inverting amplifier 172 and to one arm of a potentiometer 174.
The output of the amplifier 172 is labeled "--(LF+RF)" and is fed
to the other arm of the potentiometer 174. The movable contact of
the potentiometer 174 is mechanically connected to the ground
engaging steering wheel 14 as represented 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.]. .Iadd.front
.Iaddend.of the OPV housing 12 as shown in FIGS. 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 movable contact arm of the
potentiometer 174 will be at the opposite end to receive the signal
(LF+RF). The signal output from the movable contact arm is labeled
FB, because it is a negative feedback signal, and this 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 R--L+FB from the output of the amplifier 176
is fed through a loop gain variable resistance 178 to one input of
a synchronous detector 180. The amplified 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 reference signal, that is
when the reference signal is less than 180.degree. out of phase
with the error signal the synchronous detector integrates the error
signal R--L+FB. When the reference signal is more than 180.degree.
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 signal 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 filter 184
to filter out any high frequency pulses and the output of the
filter 184 is fed to a 0.1-1.2 Hz lead filter whih introduces an
approximately 60.degree. 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 of 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 amplified version of the reference signal
-(LF+RF). This threshold signal together with the signal from the
lead filter 186 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 FIG. 10 a waveform graph of the
reference signal -(LF+RF).Iadd., denoted in the figure L'+R',
.Iaddend.and the error signal .[.R-L.]. .Iadd.L-R .Iaddend.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 undergoes 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
.[.in.]. 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 asumming junction 194. Until
this condition is reached, the enable logic circuit 190 connects
the power steering tachometer to the plug input of the summing
junction 194. The manual-auto mode switch 146 is also connected to
the enable logic circuit 190, thereby allowing 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 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 FIGS. 8 and 9, a more detailed
description will be given of the circuit depicted in FIGS. 7A and
7B. The same components depicted in FIGS. 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 FIG. 8, that the power supply
designated generally as 202 has a plus 12 volt output with respect
to the circuit ground and a plus.Badd..[. 7.]..Baddend. .Iadd.5
.Iaddend.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.].
.Iadd.172 .Iaddend.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 208.
Plus 12 volts bias is supplied through a resistor 216 to 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.]. .Iadd. 190 .Iaddend.just as if no threshold
signal has 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 NPN transistor 234. The emitter
electrode of the 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 of 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 in greater detail
hereinafter, the output of the OR gate 240 is a signal
representative of whether or not the signs of the slope and
polarity of the error signal after synchronous 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 logic 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
high 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 regardless of the output of the exclusive OR gate 240.
The logic low apearing 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 velocity 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
FIG. 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 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 a "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 flip flop.
A logic high at the output of the NOR gate 224 will cause the CMOS
switch 228 to become conductive and the CMOS switch 236 to become
non-conductive. The LED 138 supplied from the output 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 10 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.].
.Iadd.signal .Iaddend.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 differential amplifier
244. This other input is also connected to the circuit ground
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 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 that the
OPV 10 is goind 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 position, no changes at the output of the exclusive
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 should 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 and through a resistance 250 to the LED
136. The output of the NAND gate 248 is supplied to the base
electrode of an NPN transistor 252 whose emitter electrode is
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 FIG. 9, the velocity command
output signal from the amplifier 238 is fed to one input of a
comparator 262 and to the corresponding input of a second
comparator 264. The output of the amplifier 262 is fed to one input
of an exclusive OR gate 266, the input of 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 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 biased 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 electrode of a
PNP transistor 292. The emitter electrode of the 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.].
.Iadd.PNP .Iaddend.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
collector 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 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 transistor 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-vibrator. 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.].
.Iadd.PNP .Iaddend.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 now 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 62 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 NPN transistor
300 conductive by 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
become 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 tachometer 200.
As explained above in reference to FIGS. 7A and 7B, the electronic
tachometer 200 is connected in parallel with the motor and across
the resistance 198. As shown in FIG. 9, these connections are made
by way of lines 312, 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 cutput is connected to the inputs of the
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 be 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 microseconds,
which is determined by the circuit values within the multi-vibrator
circuit 316. The duration during 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 limiting sensor
to shut off the motor in the event that, because of some physical
binding in the guide wheel mechanism, 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 256 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 FIGS. 11 and 12, a modified
embodiment of the reverse guidance system according 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 positioned
correctly over the wire 34 but is heading in a direction at an
angle .phi. to the wire, the output of the coil 125 will be A sine
.phi., where A is the maximum amplitude of the coil output. It will
be appreciated that for the very small angles of .phi. the sine of
.phi. will approximately equal the tangent of .phi..
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 .phi. 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
D.sub.p from the center of the coil 125, by geometric construction
the error d is equal to approximately D.sub.p sine .phi., or
##EQU2##
Since A sin .phi. is simply the output of the coil 125 and since
(D.sub.p /A) is a constant, the error signal can be rewritten as
K.phi. where .phi. 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.phi. where R and L are the
outputs from the error coils 162 and 164.
Referring now more particularly to FIG. 12, the circuit depicted in
FIG. 7A is modified so that the circuit elements 107, 108, 113,
115, 116 and 120 are omitted. Furthermore the gain control signal
for the amplifiers 100 and 103 is 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 of a transconductance amplifier 126 whose output is
connected to the plus input terminal of the summing junction 109
and to the 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 RFO-LFO
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 RFO-LFO+K.phi.. As in the embodiment depicted
in FIG. 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 FIG. 7A.
As mentioned at the beginning of this application, this embodiment
is somewhat less advantageous than the multicoil arrangement
depicted in FIG. 7A because this embodiment 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. 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 D.sub.p with the consequency that
variation in the straightness of the buried wire are not magnified
as greatly. This is apparent from the fact that the constant K is
actually equal to (D.sub.p /A). Thus any reduction in D.sub.p will
also reduce the effect of any variations in the straightness of the
buried 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.
* * * * *