U.S. patent number 3,589,454 [Application Number 04/792,893] was granted by the patent office on 1971-06-29 for mole guidance system.
This patent grant is currently assigned to Bell Telephone Laboratories, Incorporated. Invention is credited to James C. Coyne.
United States Patent |
3,589,454 |
Coyne |
June 29, 1971 |
MOLE GUIDANCE SYSTEM
Abstract
A subterranean missile is equipped with three mutually
orthogonal coils in its body and a fourth suitably fixed in an
articulatable steering member. The missile is guided along any
desired underground trajectory defined with reference to a
coordinate system including the plane of a dipole-quadrupole
antenna system laid on the ground above. Voltages induced in the
noncenterline body coils are used in a closed feedback loop to
maintain the resultant magnetic field circularly polarized at the
mole location. Heading errors are then revealed as a voltage phasor
on the centerline coil. Rotation of the steering member
sufficiently to bring the voltage induced in its coil into phase
with the centerline coil voltage also aligns the steering member
such that its articulation will reduce the heading error to zero.
Any new trajectory may be defined as a suitable new voltage added
to the centerline coil voltage.
Inventors: |
Coyne; James C. (New
Providence, NJ) |
Assignee: |
Bell Telephone Laboratories,
Incorporated (Murray Hill, NJ)
|
Family
ID: |
25158384 |
Appl.
No.: |
04/792,893 |
Filed: |
December 27, 1968 |
Current U.S.
Class: |
340/853.5;
175/45; 175/61; 175/73 |
Current CPC
Class: |
E21B
7/04 (20130101); E21B 7/26 (20130101) |
Current International
Class: |
E21B
7/04 (20060101); E21B 7/00 (20060101); E21B
7/26 (20060101); E21b 007/04 () |
Field of
Search: |
;175/26,45,61,73
;340/183,195 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Novosad; Stephen J.
Assistant Examiner: Favreau; Richard E.
Claims
What I claim is:
1. A method for controlling the heading of a missilelike burrowing
device having a longitudinal axis, within a soil bulk comprising
the steps of:
generating in said bulk a rotating magnetic vector that bears a
known geometric relationship to a desired trajectory with respect
to which said heading is to be controlled,
detecting the phase angle of said vector when it aligns with the
device longitudinal axis, and
manipulating a steering member that is movably connected to the
mole to an attitude of known relation to said phase angle such that
the new device trajectory will include a heading direction that is
perpendicular to said magnetic vector.
2. A method for controlling the heading of a missilelike burrowing
device having a longitudinal axis, with respect to a desired
trajectory, comprising the steps of:
generating a rotating magnetic field vector at the mole location,
said vector bearing a known geometric relationship to said
trajectory,
detecting the phase difference between signals induced by said
vector in two magnetometers whose sensitive axes lie respectively
along the device longitudinal axis, and selectively fixed within a
steering member that is movably connected to the mole, and
manipulating said steering member until said phase difference is a
specified value corresponding to a steering attitude that will
bring said mole into said desired trajectory.
3. A heading control method pursuant to claims 1 or 2, wherein said
vector generating step comprises generating a circularly polarized
field vector.
4. A heading control method pursuant to claims 1 or 2, wherein said
generating step further comprises generating a circularly polarized
magnetic field; and said method comprises the further steps of
generating a substantially sinusoidal signal having a magnitude and
phase proportional to polar coordinates of a specified target
point, adding said phasor to the induced voltage phasor of said
longitudinal magnetometer, and adjusting the resulting phasor by a
factor proportional to the present device position, thereby
specifying a new device trajectory.
5. A heading control system for a steerable, missilelike burrowing
device operating within a soil bulk comprising:
means for generating a rotating magnetic field vector at the device
location,
a first magnetometer mounted with its sensitive axis substantially
coincident with the device longitudinal axis,
a second magnetometer having its sensitive axis mounted in a
predetermined orientation within a positionable steering
surface,
means for detecting the phase difference between the voltages
induced by said rotating field in said magnetometers,
means for positioning said steering surface to reduce said phase
difference to a specified value, and
means for affecting a steering attitude in said steering surface
such as maintains said phase difference at said specified
value.
6. A heading control system pursuant to claim 5, wherein said
generating means generates a circularly polarized magnetic field at
the device location.
7. For a missilelike burrowing device having a centerline axis and
a surface placeable into a steering attitude with respect to said
axis, a guidance system comprising in combination:
means for generating and maintaining at the mole location a
circularly polarized magnetic field whose rotating vector is
perpendicular to a reference trajectory;
means for detecting the phase of first signals developed by said
field in the direction of said centerline axis;
means for detecting the phase of second signals developed by said
field in the direction of the present permissible steering attitude
of said surface;
means for rotating said surface about said centerline to bring said
signals into phase; and
means for thereafter placing said surface into a steering attitude
such as maintains said signals in phase, whereby mole heading
errors with respect to said reference trajectory are corrected.
8. For a mole having a centerline axis and a steering tail
rotatable about said axis and articulatable with respect to a pivot
pin perpendicular to said axis, a guidance system comprising in
combination:
means for generating and maintaining at the mole location a
circularly polarized magnetic field whose rotating vector is
perpendicular to a reference trajectory;
means for detecting the phase of first signals developed by said
field in the direction of said centerline axis;
means for detecting the phase of second signals developed by said
field in the direction which said tail would move if then
articulated;
means for rotating said tail to bring said signals into phase;
and
means for articulating said tail to bring said mole into a
trajectory that reduces the magnitude of said first signals to
zero.
9. A mole guidance system pursuant to claims 8, 9, or 10, wherein
the means for generating said circularly polarized field comprises
a dipole and a quadrupole antenna loop, driven by AC currents of
the same frequency but differing by a phase factor;
means for monitoring the phase and magnitude of the dipole and
quadrupole fields at the mole location; and
means for adjusting the antenna driving currents and magnitudes so
as to create a resultant vector at the mole location which rotates
with constant magnitude and angular velocity.
10. For a mole having a cylindrical elongated body, a trio of
magnetometers fixed in said body, with their sensitive axes
mutually orthogonal, a first said magnetometer being axially
coincident with the centerline of said body, a steering tail
rotatable about said centerline and articulatable about a pivot pin
normal to said centerline, and a fourth magnetometer fixed in said
tail at right angles to said pivot pin, a guidance system
comprising in combination:
means for generating and maintaining at the mole location a
circularly polarized magnetic field whose rotating vector bears a
known geometric relation to a reference trajectory;
means for detecting the phase difference between voltages developed
by said field in said first and fourth magnetometers;
means for rotating said tail about said centerline to reduce said
phase difference to zero; and
means for thereafter articulating said tail in a direction that
places said mole into a trajectory which will reduce the magnitude
of said first magnetometer voltages to zero.
Description
This invention relates to subterranean missiles, and specifically
concerns a guidance system for such devices.
BACKGROUND OF THE INVENTION
In the patent application of Coyne-Elia-Southworth, Ser. No.
764,750, filed Oct. 3, 1968, there is described a system for
continuously detecting the position and attitude of a subterranean
missile or "Mole" with respect to a fixed coordinate system
relative to a pair of antennae laid on the ground over the desired
trajectory. In that system, the position coordinates of the mole in
the magnetic flux plane of the antennae as well as its pitch, yaw,
and roll angles all are continuously calculated from voltages
detected in a trio of mutually orthogonal magnetometers fixed in
the mole body. From these voltages appropriate steering
instructions are given the mole to cause it to assume the desired
heading.
This earlier system recognized the advantages of a
dipole-quadrupole antenna. For example, calculation of the mole
depth is a simple function of the antenna half spacing. Mole
distance from the center wire, measured in multiples of the antenna
half spacing, is equal to the ratio of dipole to quadrupole field
strength, as measured by magnetometers in the mole at the mole
location. Also, the polar angle at the origin is exactly equal to
the included angle between the field vectors, as measured by the
mole magnetometers.
While thus constituting a fully workable location detection scheme,
the earlier system nevertheless has several drawbacks. For one, the
computations of position and heading data requires extensive logic
circuitry. More importantly, the system does not provide for the
mole to seek out a prescribed trajectory. Moreover, the trajectory
itself is not superimposable onto the system.
Accordingly, the principal object of this invention is to simplify
the control of a subterranean missile in its movement from one
point to another along a prescribed path.
Another object of the invention is to automatically guide a mole to
a specified terminal point within a reference frame defined by a
pair of magnetic field-generating antennae.
A further object of the invention is to achieve an automatic mole
guidance system in which the terminal point which the mole seeks
can be respecified at will.
SUMMARY OF THE INVENTION
The present invention, broadly, is grounded in the recognition of
the advantages of a rotating magnetic field vector at the mole
location. Mole heading errors can be directly detected by phase
comparison of the induced voltages in an axial magnetometer in the
mole body with that in a magnetometer mounted in an articulatable
steering surface.
The heading error is the angle between the mole longitudinal axis
and a line parallel to the magnetic field propagating antenna. The
heading error is zero when the mole axis is parallel to the antenna
wire. In this position, no voltage is included in the axial
magnetometer; but with heading error from any mole orientation
whatsoever, a small but detectable voltage is induced in the axial
magnetometer. The trajectory which would reduce the heading error
to zero lies in the plane defined by the mole axis and a line
through the mole parallel to the antenna wire. Thus, a steering
surface articulated in that plane will cause the heading error to
reduce to zero. The steering surface is placed in that plane by
manipulating the surface until, pursuant to the invention, the
aforementioned induced voltages are in phase with each other.
The rotating magnetic field can be produced by any convenient
antenna configuration energized to produce a rotating vector. An
antenna configuration which is preferred because of the simple
geometric relationships which it provides, is the dipole-quadrupole
scheme of the aforementioned patent application Ser. No. 764,750.
For that antenna configuration, as well as for certain others, a
preferred antenna energization is with signal currents of the same
frequency but differing by a phase factor.
For heading error correction pursuant to the invention, the
magnetic field vector must merely rotate. It need not describe a
circular path and, in fact, can be widely elliptical with no
performance penalty.
However, pursuant to a further and major facet of the invention, a
field maintained in circular polarization at the mole location
permits control of the mole's position in three-dimensional space
through the expedient of heading control. A mole trajectory
originating from its present position and extending to a desired
terminal point is specified according to this aspect of the
invention by summing an appropriate voltage phasor with the output
signal of the mole axial magnetometer. The specified trajectory can
be one parallel to the antenna, or diverging or converging with
respect thereto. The superimposed voltage phasor plus the induced
voltage in the axial magnetometer produces a voltage phasor output
identical to the phasor that would have been induced had the
antenna been actually moved to a position directly above and
parallel to the chosen trajectory. Steering then proceeds in
exactly the same manner as above. The steering surface is
manipulated until voltages in its magnetometer and in the mole
axial magnetometer are in phase; and the surface is articulated so
as to keep them in phase. The heading error to be reduced to zero
now, however, is with respect to the line between the mole's
present position and the desired terminal point.
One feature of the invention, therefore, is the use of a rotating
magnetic vector which provides a phase comparison way to find the
correct position of a mole steering surface to counteract any
heading error.
A further feature of the invention involves defining new mole
trajectories which the mole seeks out by heading control using the
same phase comparison methods, thereby providing the capability of
steering toward any desired terminal point.
The invention, its further objects, features and advantages will be
fully apprehended from a reading of the description to follow of an
illustrative embodiment.
DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic perspective diagram of the mole and antenna
system;
FIG. 1A is a perspective diagram depicting certain geometric
relations;
FIG. 2 is a schematic perspective diagram of the mole articulating
tail;
FIGS. 3, 3A, 3B, and 3C are schematic diagrams, taken with the
plane of flux lying in the paper, and showing the mole and tail in
various working positions with resulting voltages induced in
certain coils;
FIG. 4 is a diagram, taken in the flux plane of the antenna system
and showing certain critical geometric relationships between the
mole and a magnetic field vector;
FIGS. 5A, 5B, 5C, and 5D are diagrams illustrating the critical
geometric relationships of mole heading in terms of a new desired
trajectory; and
FIG. 6 is a block diagram of an overall mole guidance system
pursuant to the invention.
DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT
The mole working environment is shown schematically in FIG. 1.
Current-carrying conductors in the form of a dipole antenna 10 and
a quadrupole antenna 11 are laid on the ground over the desired
route. Their construction and mounting details are described in the
aforementioned patent application Ser. No. 764,750. AC voltages are
applied to antennae 10, 11 by power amplifiers 12, 13,
respectively. The voltages are maintained at the same frequency,
for example, 5 kHz., but, importantly, differ in phase in a manner
to be described.
FIG. 2 shows a mole 14 equipped with suitable components for use in
the present guidance system. Mole 14 consists of a body 15 and a
tail assembly 16. Assembly 16 is mounted for rotational movement a
full 360.degree. about the centerline axis 17 of body 15. The tail
itself, designated 18, is pivotable about a fixed clevis pin 19
which is perpendicular to axis 17. Control of rotation of assembly
16 and articulation of tail 18 are affected by conventional
servomechanisms consisting of rotary actuator 20 and articulation
actuator 50 which are shown in FIG. 6 schematically.
FIGS. 1 and 2 show three magnetometers such as induction coils 21,
22, 23, fixedly mounted in mole 14. Their sensitive axes A, B, C
are mutually orthogonal. As installed, axis A of coil 21 is
coincident with the mole centerline axis 17. A fourth magnetometer
such as coil 24 is mounted in tail 18 with its sensitive axis D
normal to the extended centerline axis 17 and also normal to clevis
pin 19. Electrical connections run from each magnetometer to the
processing circuitry which will be fully explained with reference
to FIG. 6.
CIRCULAR POLARIZATION
As two distinct magnetic fields exist in the soil bulk, the net
field illustrated in FIG. 4 at a specified point and time is given
by the vectorial addition of the separate fields, the latter
denoted D, Q. Consider their vectorial addition for an arbitrarily
chosen position of mole 14 as seen in the flux plane. The field
vectors D, Q have peak values of D, Q, respectively. As the fields
are out of phase, the peaks occur at different times. At the given
time, their instantaneous values are d, q. In FIG. 4, vector d was
arbitrarily taken at its maximum value D. The vector resultant of
the instantaneous vectors d, q, is b.
It is seen that b rotates in space with passing time, and in
general describes an ellipse. Resultant vector b passes through the
sensitive axes, A, B, C, D of coils 21--24; and voltages are
induced in each coil. When the component of b along a coil is
maximum, the induced voltage in that coil is a maximum. These
sinusoidal voltages are denoted V.sub.1, V.sub.2, V.sub.3, V.sub.4
respectively for the coils 21--24; voltages V.sub.1 and V.sub.4 are
schematically depicted in FIGS. 3A and 3B.
It may be desired, as for certain purposes of the present
invention, to provide a circularly polarized magnetic field, i.e.,
to cause b to rotate with constant angular velocity and constant
magnitude. The following considerations then apply. FIG. 4
illustrates that D and Q differ in direction by the geometric angle
.PSI.. Further, d and q differ in phase by the angle .beta. which
is the phase difference between the applied sinusoidal currents
i.sub.d and i.sub.q to antennae 10, 11. Omitting proof, the
conditions for circular polarization are:
D=Q;
.beta.=.pi.-.PSI.
such conditions are maintained, for example, by monitoring the
voltages induced in coils 22, 23 whose axes B, C lie in or near the
place of flux. When the field is circularly polarized, the voltages
V.sub.2, V.sub.3 induced by b coils 22, 23 have equal magnitude and
90.degree. phase difference. This is achieved, for example, by
fixing the dipole antenna current i.sub.d and by providing means
for controlling the phase and magnitude of the quadrupole current
i.sub.q so that the field at the mole location is circularly
polarized.
If now the mole undergoes a position change, the field becomes
slightly elliptical, and the output voltages V.sub.2, V.sub.3 of
magnetometers 22, 23 will change slightly in both amplitude and
phase. A small correction of both magnitude .DELTA.I.sub.q and
phase .DELTA..beta. of i.sub.q is required to reestablish the
circular field and is achieved in the antenna current regulator 60
shown in FIG. 6.
It should be understood that the invention broadly involves the use
of a rotating magnetic field vector. Accordingly, while the antenna
signal frequencies have in the illustrative embodiment been
specified to be the same while differing by a phase factor, other
schemes exist for realizing a rotating vector. Also, while the
dipole-quadrupole antenna described in applicant's preferred mode,
other antenna structures can be envisioned for realizing the
rotating vector. The advantages of the dipole-quadrupole
configuration are that with the driving current described earlier
and circular polarization, a simple determination of mole position
is afforded. That is, the ratio of quadrupole antenna peak current
to dipole antenna peak current equals the distance from the antenna
center wire 11a (measured in multiples of the antenna half
spacing). Also, the supplement of the phase difference between
dipole and quadrupole antenna currents equals the polar angle. This
point and others will now be expanded.
AUTOMATIC HEADING CONTROL
The rotating magnetic vector b, pursuant to a major facet of the
invention, enables finding the correct position of a steering
control surface such as tail 18 to counteract any heading
error.
The coordinate system notations used below and alluded to elsewhere
also bear summarization at this point. The XYZ system is with
reference to the stationary antenna as seen in FIGS. 1 and 1A, for
example; the X-axis is coincident with the center wire 11a, and the
XY plane is defined by the antenna wires 10, 11, 11a. Positions in
the soil bulk are measured conveniently with reference to the XYZ
system. The X'Y'Z' system is a coordinate system located with
reference to the XYZ system, and comes into play when a new
trajectory is to be specified in a manner to be described. The
X"Y"Z" system moves with the mole, its axes being always parallel
to the respective axes of the XYZ system.
Heading error is any orientation of mole 14 which brings the
centerline axis 17, and the coincident magnetometer axis A, out of
parallelism with the X" axis, as illustrated in FIG. 1A. In the
earlier patent application Ser. No. 764,750, heading error included
a pitch component and a yaw component .theta.. Ascertaining these
angles required substantial added equipment complexity. As will
shortly be seen however, the present invention renders it
completely unnecessary, in correcting for heading error, to
calculate the values of either; nor is it necessary to compute a
roll angle.
In FIGS. 1 and 3 the mole 14 is depicted as being on trajectory,
moving toward a target point X.sub.2 Y.sub.2 Z.sub.2 with zero
heading angle. Thus, mole 14 is directly under the antenna center
wire 11a. Rarely in practice, however, is the mole ideally
positioned.
FIG. 1A illustrates the more usual situation. The mole is shown as
deviating from the desired heading, namely the X" axis. This
heading deviation is measured by the various angles shown, and now
defined.
called the pitch angle, is the angle between the mole axis 17 and
its projection on the X"Y" plane.
.theta., called the yaw angle, is the angle between the projection
of the mole axis 17 on the X"Y" plane and the X" axis.
H, called the heading angle, is the angle between the mole's axis
17 and the X" axis.
M is the angle between the projection of the mole's axis 17 on the
Y"Z" plane and the negative Z" axis.
.theta. and are related to H and M by the following equations:
are
H.sup.2 =.theta..sup.2 + .sup.2
tan M=.theta./-
Accordingly, FIG. 3A depicts the mole as deviating in position and
also from the desired heading by an arbitrary heading error angle H
which, it will be understood, consists of a pitch and a yaw
component. The object is to find a rotational position for tail 18
such that, with articulation, the mole follows a trajectory that
not only reduces H, but reduces H to zero.
Consider that when the mole tail 18 is articulated, the mole
trajectory lies in a plane defined by the mole centerline axis and
the axis of tail 18. The heading error angle H can go to zero only
if said plane also includes the X" axis. Accordingly, the problem
reduces to articulating tail 18 in the plane defined by the X" axis
and the mole centerline axis 17.
It can be seen by reference to FIGS. 3A--3C that the correct
rotational position of tail 18 occurs when the voltage induced by b
in coils 21 and 24 are in phase. With any heading error, axis A of
coil 21 has a projectable component in the plane of flux, as seen
in FIG. 3A. In such case, as seen in the bottom diagram of FIG. 3A
vector b induces a voltage V.sub.1 in coil 21 which peaks when the
component of vector b on axis A is maximum. If the field were
exactly circular this would occur when vector b and axis A were
coincident. Tail coil 24 when viewed in the flux plane has a
voltage V.sub.4 induced by vector b.
Owing to the initial random rotation of tail 18 with respect to the
mole body, however, the induced voltages in coils 21 and 24 differ
by a phase factor. This phase difference is monitored by suitable
circuitry such as depicting in FIG. 6. Rotary actuator 20 then is
servoed until the phase difference is zero as shown in FIG. 3B.
Coil axes A and D now lie in a plane that includes the X"-axis.
Articulation of tail 18 around pin 19 at this time as depicted in
FIG. 3C, will place the mole on a trajectory that at some time is
assured of bringing the mole parallel to the X-axis, thus reducing
the heading error angle H to zero.
When the tail 18 articulates, the magnitude of voltages induced in
coil 24 will decrease, but the phase will remain unchanged.
Further, as the mole 14 proceeds along its curved trajectory in
seeking a zero heading angle error, the phase of voltage V.sub.4
will continue to remain unchanged provided no disturbances roll the
missile or divert it out of the desired trajectory plane.
If the mole is diverted, however, then the voltage phasers V.sub.1
and V.sub.4 will be once again out of phase, and the inventive
heading correction procedure will continuously correct. If the
articulation angle is too large that the rotary unit 20 cannot turn
the tail against soil forces, the tail is straightened and then
rotated until V.sub.1 and V.sub.4 are in phase; and then the tail
is rearticulated. If the articulation angle is not great, a new
rotary position of tail assembly 16 can be sought without
straightening the tail.
The articulation angle itself is chosen to the desired turning
curvature and does not affect the phase matching of voltages
V.sub.1 and V.sub.4. The change in magnitude of voltage V.sub.4 as
tail 18 articulates does provide a means of measuring the
articulation angle, however.
The voltages V.sub.1 and V.sub.4 are depicted in FIGS. 3A--3C as
being equal in magnitude merely for ease of illustration. They need
not be; and in practice rarely are.
It should be understood that the critical inventive point thus far
is the correction of heading angle errors by phase comparison
techniques, in which the phase of the voltage induced in the axial
magnetometer 21 by a rotating magnetic field vector is compared to
the phase of the voltage induced in the magnetometer 24 which is
fixed in the steering member in a predetermined orientation. This
orientation is with respect to the sensitive axis D and the
corresponding steering attitude which the steering member
exhibits--or as in the instant embodiment, would exhibit, if
articulated. Accordingly it is clear that the phase comparison does
not require that there be zero phase difference between the induced
voltages. It requires only that the phase difference be known which
corresponds to the orientation of the steering member magnetometer
axis D (and thus also the steering member itself) that will reduce
the heading angle error to zero. To this end, the choice of a zero
phase difference is convenient.
HEADING CONTROL PLUS POSITIONAL CONTROL
In the system so far described, control of the mole's heading angle
enables the mole's centerline axis 17 to be kept parallel to the
antenna center wire 11a. In many field situations, heading control
in the above terms with respect to the stationary center wire would
suffice as a complete guidance system if, among other
considerations, the mole is originally planted parallel to the
antenna center wire and if further the trajectory is not long.
There are, however, an infinite number of lines parallel to the
center wire, any one of which could be assumed by the mole
centerline axis to satisfy the requirement of zero heading angle
error. Accordingly, the likely positional error at the terminal
point increases with increasing trajectory length until it becomes
intolerably large.
Pursuant therefore to a further major aspect of the invention,
positional control is incorporated into the guidance scheme by
causing the system to guide--again, by heading control--along any
new chosen straightline path from the mole, and not just a path
parallel to the antenna center wire. The new path advantageously
can be one which links the mole and any desired target point.
Such target point normally is directly beneath the center wire and
represents the terminal point of mole travel. The target point can,
however, be both horizontally and vertically removed from the
nominal terminal point directly below the center wire end. Further,
the target point can be respecified any number of times.
Implementation of position control is depicted in FIG. 6; and
involves generating a sinusoidal voltage, denoted R.sub.0 cos
(.omega.t-.psi..sub.o). The latter voltage has a magnitude and
phase equal to the polar coordinates R.sub.o, .psi..sub.o (FIG. 5B)
of the target point X.sub.o Y.sub.o Z.sub.o (FIG. 5A). The
quantities R.sub.o and .psi..sub.o are set, either manually or
otherwise, into a phase and magnitude control 65.
Control 65 derives another input directly from the quadrupole
antenna 11. Antenna current regulator 60, as already described,
monitors the voltages V.sub.2 and V.sub.3 induced in coils 22, 23
and adjusts the antenna current i.sub.q to values that maintain
circular polarization.
The sinusoid R.sub.o cos (.omega.t-.psi..sub.o) is then added in
summing amplifier 70 to the sinusoidal quadrupole antenna voltage R
cos (.omega.t+.beta.). The gain of amplifier 70 is proportional to
1/X-X.sub.o, the forward distance of the target point.
The summing amplifier 70 output is phase shifted in circuit 71 by
the factor 3(.pi./2-.beta.) to place the zero phase reference on
the -Z.sub.1 axis; and then added in summing amplifier 73 to the
voltage induced in magnetometer 21. The latter has been normalized
in circuit 72 by the field strength as measured by the RMS voltage
V.sub.2. Normalization can as well be achieved with the RMS voltage
V.sub.3 in the same fashion. The output of the normalizing
operation is a sinusoidal voltage whose amplitude is the heading
angle H measured in radians, and whose phase is the angle M as seen
in FIGS. 1A and 5B.
The output of summing amplifier 73 yields a new voltage phasor,
denoted H' cos (.omega.t+M') whose amplitude H' is the heading
angle of the mole with respect to the new desired trajectory X'
seen in FIGS. 5A--5C, and whose phase is the angle M' shown in FIG.
5D.
The output of summing amplifier 73 is fed to two comparators 74 and
75. In comparator 74 the phase of the voltage in coil 24 is
compared with the phase of output of summing amplifier 73. The
difference error signal .DELTA.M' is used to drive the rotary
actuator 20 until the error signal goes to zero. This operation
places the tail 18 in a position such that when articulated the
heading angle with respect to the X' axis will go to zero.
In the second comparator 75 the amplitude of the output of summing
amplifier 73 is compared with two reference voltages, H' max. and
H' min. These are, for example, set in manually and represent the
dead zone of the on-off control of the tail articulation actuator
50. When H' exceeds H' max. the valve motor 80 is turned on,
opening valve 81 to hydraulic supply pressure which drives actuator
50 through suitable hydraulic connections 82. With tail 18
articulated, the mold assumed a trajectory that begins to reduce
the heading angle H' toward zero. When H' drops below H' min.,
comparator 75 signals valve motor 80 to remove supply pressure; and
the fluid pressure is relieved as through return line 83. The mole
tail straightens out in about a tail length of travel.
The capability of specifying and respecifying the mole target point
has numerous highly useful ramifications. It is, for example,
possible to correct for the natural tendency of the mole to surface
due to the greater strength of soil below the mole bottom, simply
by moving the target point closer in toward the mole. Obstructions
such as rocks, drain pipes and the like, whose locations are known,
are readily avoided by specifying a series of safe trajectories. It
further is possible to steer straight paths under bushes, for
example, where laying the antenna in a straight path is precluded.
Also, the antenna does not require any relocation in the process of
specifying the trajectory. Additionally, the mole can be caused to
fare into a desired trajectory in a sweeping arc, simply by leaving
the gain of amplifier 70 constant. Aiming at a fixed point in
contrast requires changing of the amplifier gain.
Numerous additional advantages, variations and embodiments of the
invention will be readily envisioned by persons skilled in the art.
The spirit of the invention is limited only by the scope of the
claims to follow.
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