CA1045703A

CA1045703A - Object tracking and orientation determination means, system and process

Info

Publication number: CA1045703A
Application number: CA203,986A
Authority: CA
Inventors: Jack Kuipers
Original assignee: POLHEMUS NAVIGATION SCIENCES
Current assignee: POLHEMUS NAVIGATION SCIENCES
Priority date: 1973-07-30
Filing date: 1974-07-03
Publication date: 1979-01-02
Also published as: JPS5045667A; FR2239690B1; SE7409509L; SE396145B; IL45177A; GB1465134A; DE2436641A1; JPS6321157B2; DE2436641B2; IT1017347B; FR2239690A1; US3868565A; IL45177A0; DE2436641C3

Abstract

OBJECT TRACKING AND ORIENTATION DETERMINATION
MEANS, SYSTEM AND PROCESS
ABSTRACT
A field (e.g., a magnetic field) which nutates about a pointing vector is used to both track or locate an object in addition to determining the relative orientation of this object. Apparatus for generating such a field includes mutually orthogonal coils and circuitry for supplying an unmodulated carrier, hereafter called DC signal, to one coil and an AC modulated carrier signal, hereafter called AC
signal, to at least one (usually two) other coil, such that the maximum intensity vector of a magnetic field produced by the currents in the coils nutates about a means axis called the pointing vector direction of the field. The generated field is sensed in at least two orthogonal directions at the object to be tracked and whose orientation is to be de-termined. The sensed signals provide an indication of the direction and orientation of the object relative to the coordinates of the generating means.

Description

~45~3 1 B~CKGROUND OF T~IE INVENTION
I. Field of the Invention This invention relates to an object locating or tracking system or process in which a vector field which is caused to nutate about an axis called the pointing vector, is used to locate or track a remote object. It also relates to an apparatus for generating such a nutating field, more particularly a nutating magnetic field, which nutates about an axis called the pointing vector. More particularly, the invention relates to such a system or process which is capable of determining bo~h the relative translation and the relative angular orientation of the co-; ordinate frame of a remote object, relative to the reference coordinate frame of the apparatus which generates and points the nutating field.
II. Description of the Prior Art The use of orthogonal coils for generating and sensing magnetic fields is well known. Such apparatus has received wide attention in the area of mapping magnetic zo fields to provide a better understanding of their characteristics, for example. If a magnetic field around generating coils ;~
can be very accurately mapped through use of sensing coils, it has also been perceived that it might be possible to determine the location of the sensing coils relative to the generating coils based on what is sensed. However, a problem associated with doing this is that there is more than one -location and/or orientation within a usual magnetic dipole field that will provide the same characteristic sensing signals in a sensing coil. In order to use a magnetic field Eor this purpose, additional information must therefore be provided.

, ~ 57(:~3 l ~ne approach to provide the additional information required for this purpose is to have the generating and sensing coils move with respect to each other, such as is taught in U.S. Patent 3,644,825, entitled MAGNETIC DETECTION
SYSTEM FOR DETECTING MOVEMENT OF AN OBJECT UTILIZING SIGNALS
` DERIVED FROM TWO ORTHOGONAL PICKUP COILS, issued February 22, 1972 to Paul D. Davis, Jr. and Thomas E. McCullough. The motion of the coils generates changes in the magnetic field, and the resulting signals then may be used to determine direction of the movement or the relative position of the generating and sensing coils. While such an approach removes some ambiguity about the position on the basis of the field sensed, its accuracy is dependent on the relative motion9 and it cannot be used at all without the relative motion.
, Another approach that has been suggested to provide the additional required information is to make the magnetic field rotate as taught in Kalmus, "A New Guiding and Tracking System", IRE Transactions on Aerospace and Navigational Electronics, March 1962, pages 7-10. To determine the distance between a generating and a sensing coil accurately, that approach requires that the relative orientation of the coils be maintained constant. It therefore cannot be used to determine both the relative translation and relative orientation of the generating and sensing coils.
While the art of locating and tracking remote objects is a well developed one, there still remains a need for a way to determine the relative angular orientation of a remote object in addition to locating or tracking the object. Further, there is a need for a means, system OT
process which operates on the signals detected by one f.

lO~S~3 sensor, those signals resulting from the nutating field generated by one generating means, which is capable of deter-mining continuously the location of or tracking the remote object and sensor, in addition to simultaneously determining continuously the relative angular orientation of the remote object and sensor.
Accordingly, it is an object of this invention to ;provide a system and process capable of determining both relative translation and relative orientation of remote objects through the use of a vector field.
As here described one can determine relative trans-lation and orientation of remote objects through use of a field in a continuous manner, so that translation and orientation may be tracked and therefore determined continuously.
There is further described a system and process for locating an object precisely relative to a reference coordinate frame of the vector field generating means.
Further, a system is described in which a pointing vector defined by a modulated field is used to track an object very precisely.
A generator capable of producing electronically a field which nutates about a specified pointing vector is des-cribed, which field can be used in the above system and process.
Also described therefore is an efficient signal processing technique which results in the measure of the relative translation of a remote object ~two angles) in addition to the -simultaneous measure of the relative angular orien-tation of the - remote object (three angles). That is, the teaching herein can provide a means for measuring five independent angular measure-ments utilizing only one field generating means and only one sensing means at the remote moving object.
The above and related objects may be attained through , - 5 -1~457(1;~
use o~ the sy~tem process ~nd ;Eield generating apparatus described herein. This invention is based on the realizati~n that the only pbsitions in anutating dipole field where the field strength is magnitude invariant lie along the axis of nutation, herein called the pointing vector. This phenomenon ~ allows very precise location or tracking of a remote object `~ that is free to undergo not only changes in position but also changes in angular orientation.
In accordance with one aspect of the inv~ntion there is provided,an object locating system which comprises:
(a) means for radiating a nutating field about an electrically directable pointing vector;
(b) means located at the object to be tracked, for sensing the nutating field; and (c) means electrically interconnected with said sensing means for determining the direction to the object relative tothe coordinate frame of the radiating means.
In accordance with a second aspect of the invention there is provided, a process for tracking an object, which - 20 comprises:
; (a) radiating a nutating field about a pointing vector;
(b) sensing the generated field in at least two axes of the object to be tracked; and (c) determining from the sensed field after appro-priate coordinate transformation processing, the directional position of the object from the radiator.
~- In accordance with a third aspect of the invention there is provided, a process for tracking an object relative to ; 30 the coordinate frame of a radiator, which comprises:
(a) radiating a nutating field about a pointing vector;

~ - 6 -~
' "'' 457C~3 (b~ sensing the xadiated field in at least two orthogonal axes at the object, to produce an output signal for each axis; and (c) determining from the sensed field after appro-priate coordinate trans~ormation processing, the directional position of the object and also its angular orientation, both relative to the coordinate ~rame o~ the radiating means.
In accordance with a ~ourth aspect of the invention there is proYlded, an object tracking s~stem comprising means for radiating a directa~le nutating field about a pointing vector which 1s the axis of nutation; means located at the object for sensing the nutating field; and, means for deriving a signal, hased on the sensing of the nutating field, which is related to the misdirection, i an~, of said pointing vector, means for redirecting said pointing vector toward said object in accordance with said error signal.

If the system is used to locate the object only, say for small perturbations in pointing angle, means is ~: .
provided for generating a signal based on the sensed ield for indicating the location of the object. If the system is used to track the object, a signal generating means is connected between the sensing means and the fiel~ generating means whic]l provides a signal to the Eield generating mealls, based on the sensed ield, for moving the pointing vector o the nutating field toward the sensing means. Preferably, orthogonal coils are used both in the genera~ion of the nutating fielrl -- in which case it is an electromagnetic field -- and in the s~nsing o~ the resulting Çield.

.... .

- 6a ' ' , r~
-..................................... . . .

~45~3 ; Fig. 1 describes the geometry of a simple coordi-nate transformation called a rotation;
Fig. 2 is the block diagram representation of a single rotation operator, as in Fig. 1, called a Resolver;
Fig. 3 shows the circuit giving 360 degree pointing freedom to ~he two-dimensional nutating magnetic field in the plane;
Fig 4a shows the pointing angles defined for three-dimensional pointing;
Fig. 4b illustrates the circuit corresponding to the pointing angles o-f Fig. 4a;
Fig. 5 is a schematic representation of a prior . art magnetic field generating and sensing system;
Fig. 6 is a representation o-f signals sensed in the system of Fig. 5;
Fig. 7 is a schematic representation of a system which will allow practice of the invention for determining location and orientation of an object which moves in two-dimensions;
Fig. 8 is a representation of signals sensed in the system o-f Fig. 7;
Fig. 9 is a representation of a simplified two-dimensional system using a two-coil generator and a two-coil sensor;
Fig. 10 is a schematic representation of a system in accordance with the invention which will track the location and the angular orientation of an object free to ,,~
move in two-dimensions; and Fig. 11 is a schematic representation o-f a system in accordance with the invention which will track the .,.~, '. ~

L57~3 1 location or direction and tlle relative angular orientation of an object free to move in three-climensions, subject to certain restraints.
DETAILED D~SCRIPTION OF SPECIFIC EMBODIMENTS OF THE IMVENTICN
Ap~aratus for generating a direetable, nutating, magnetie Eield along a pointing vector includes at least two orthogonally posi-tioned coils through which excitation currents can be passed. T]lis excitation current will usually be operating at some specified carrier frequellcy which is modulated by a direct current ~DC) signal and/or an alternating current (AC) signal. ~lereinafter, these modulation envelopes will be referred to only as DC signal or ~C signal. The AC
signal is at the nutation ~requency. Circuitry for supplying a DC current through one of the coils and an AC current through at least one additional orthogonally positioned coil -~ produces a nutating magnetic fleld whose pointing vector is in the direction of the axis o-f the DC coil, or more properly stated, in the direction of the axis of the DC field. The amplitude of the nutation depends Oll the relative amplitude of AC and DC signals, in most cases taken to be equal in amplitude. If the object can move in only two dimensions, the nutation need only be a simple nodding in the plane of the motion. This can be produced by a DC signal in one of the coils and an AC signal in the second coil, with both coils in the plane of the motion. If the object is free to move in three dimellsiolls, -the nutation desirably describes a conical motion about the pointing vector Or the field, the concial apex at the intersection of the coi~s. Such a ., .
nutating field can be generated by the combination of a DC
signal in one of the coils, an ~C signal in a second coil, .... .

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57~;~
1 and another ~C signal haYing a phase in quadrature with the ; phase of the first AC signal, passed through the third coil, all three coils being mutually, spacially orthogonal.
In both the 2-D and 3-D nutating fields descTibed above, the pointing vector is fixed to the direction o-f the axis of the DC field. To make this nutating field direct-able, a signal processing means known as a coordinate transformation circuit must operate on the reference AC and ; DC excitation signals in order to point the nutating field 10 in the desired direction. A brief discussion of the coordi-nate transformation known as a rotation is presented as background in order to properly teach the principles under-lying the techniques employed in this invention.
A vector transformed by pure rotation from one 15 coordinate frame into another coordinate frame is also said to be resolved from the one into the new coordinate frame.
Resolve and resolution in this context are synonyms for ;~ transform and transformation. The operator which transforms the components of a given vector in one coordinate frame 20 into its components in another coordinate frame where the two coordinate frames are related by a simple angular rotation we will define as a resolver. The equations ; governing this transformation are:

~` X 2 = X IcosA + y lsillA
Y2 = ylcosA - xlsinA

. Z2 Z 1 where in this case the zl axis is the axis of rotation. The equations are readily verified from the geometry illustrated in Fig. 1. Note that when the two components operated on by ~` 30 the resolver are ordered positively (zxyzxy.. ) then the first component of the positively ordered pair always has g ,~
. . .

~ ~ 4 ~ ~ ~ 3 1 the positivc sine term when tl-lc angle o~ rotation is posi-tive. If -the angle of rotation is negative then the sign of the sine terms reverses. A conveniellt notation for a resolver is the block Sl10WIl ill l'ig. 2 wherc t}le rotation in S this case is shown as negative about the y-axis. The y component is therefore not affected by the transformation and this ~act is indicated in -this notation by passing that component directly through the box as shown whereas, the resolver block representing l;ig. 1 woul~ show the zl axis ln passing directly through the box. This notation should be regarded as a signal flow or block diagram for vector componellts, particularly useful in describing the computa-tional strategy employed.
One process includes the generat.ion o~ directable, nutating field, nutating about an axis calle~ the pointing vector. In the

2-D case, a single resolver operates on the AC and DC
orthogonal components of the reference nutation excitation vector in order to produce the proper mixture of AC and DC
., on each of the two generator coils such that the pointing vector, along with the entire nutating magnetic field structure, is directed so as to make an angle A with the reference X-axis, as shown in l~ig. 3. The excita-tion for the two generator coils necessary to direct the pointing vector in the required direction defined by the angle A is given by the equations:
rxcitation for X-coil = (DC)cosA - (AC) sinA
rxcitation for Y-coil = (AC)cosA ~ (DC) sinA
The computational circuitry necessary for precisely directing or pointing the nutating magnetic field for the 3-D case operates, in principle mucll the same as in the 2-D

r~

. .
.. ..

case. rl`he re~erence nutation excitation vector now consists oE three conll~oncllts: a I)C an~ two ~C signals (~ua~rature related. Tlle pointing vector and its entire nutating magnetic field structure are pointed in any desired direc-tion defined in terms of angles A and B, in this case.
- Figure ~ illustrates the pointing geometry and the computa-tional coordinate transEormatioll circuitry necessary for achieving the desired pointing direction by operating on the ; given three reEerence excitatioll signals. ~ more detailedexplanation of coordinate transformations, calculations and applications is contained in Kuipers, J., Solutions and Simulation of Certain Kinematics and Dynamics Problems Using .

Resolvers, I'roceedings oE the FiEth Congress of -tlle Inter-national Association for Analog Computation, Lausanne, Switzerland, August 28 - September 2, 1967, page 125 to page 134.
A process for traeking an objeet in-cludes the generation of a field which nutates about a pointing vector. The generated field is sensed in at least 2Q two axes at the object to be located or tracked. From the processed relationship between the Eield components sensed in each of the orthogonal axes, the position of the object relative to the pointing vector oE the field is determined to locate the object. To track the object, the pointing vector of the nutating field is moved until the field sensed on the two axes, aEter appropriate coordinate transformation processing, indicates tllat the object lies along the pointing vector. This has taken place when the processed signal resulting ~rom the sensed nutating field is magnitude invariant over the nutation cycle. If a pointing error exists, then the amplitude of the modulation sellsed in the . .
' ~.
:

~ 5~3 l pointing direction i5 proportional to the angular displace-ment of the object -from the pointing vector. More speci-fically, the relative phase of the detected and processed signals co~pared to the reference field generating signals is proportional to the direction o-f the object relative to the pointing vector. The modulation amplitude o-f the sensed and processed signal, in the pointing vector direction is proportional to the angular displacement from the pointing vector.
The above discussion explains that the pointing vector can continuously track the object. This results in two angular measures defining the location of the object.
Determination of the anugular orientation of the object is however an independent matter. The orientation of the object is specified in general by three Euler (see Kuipers' referenced paper) angles measured relative to the re:Eerence coordinate frame at the generator. Two of the error measures of angular orientation are proportional to whatever non-zero ;; projections of the sensed and processed DC field component exist in the coordinate directions of the plane perpendi-cular to the pointing direction. The third angular error i measure is proportional to the relative phase of the sensed ; and processed nutation signals in this orthogonal plane, compared to the nutation reference excitation at the generator means.
This system, apparatus for generating a nutating field about a pointing vector, and process allows a remote object to be located and tracked very precisely, both as to position and angular orientation. While the invention should find application in a wide variety of situations where remote object location or tracking coordinates, in ~ ~ .

~04~3 1 a~lition ~o tllc orintatio~ lcs oE thc ol~jcct, is re-~uired, it is par-ticularly adap~ed in its prcsent preferre~
Eorm ror use in tracking the position an(l angular orienta-tion of an observer's head, morc speci~ically his line-o~-sight, for visually-coupled control system applicatiolls. In this limited application, the pilot's line-of-sight is continuously and precisely defined relative to the coordi-nates of the aircraft. Many other applications such as automatic landing or docking, remotely piloted vehicles, automatically negotiated air-~o-air refuclling, formation control, etc. are all applications operating over much larger domains. In general, any situation involving two or more independent bodies or coordinate frames, wherein it is desired not only that the relative distance or location of the frames be measured, tracked and controlled precisely but also that it is desired simultaneously and with the same device to precisely measure, track and control -the relative angular orientation of the two frames, is a potential application of the invented sub~ect matter.
Referring now particularly to ~ig. 5, the elements of a prior art magnetic field generating and sensing system which cannot be used to locate, track or determine the orientation of an object, are shown. Included is a magnetic field generator 10 having a coil lZ wound oE copper or other conducting wire on a magnetic, preferably isotropic, core 14. A source 16 o-f current i at some convenient carrier Ere(luency~ is connected to the coil 12 hy lea~s 18 and 20.
Sellsor 22 has a coil 24 wound preferably also on a maglletically isotropic core 25, as in the case of the generating coil 12.
Sense circuits 26 are connected to the coil 24 by leads 28 and 30.

. . p . ., ",~

~ S7l;~3 1 In use in ~ccordance with prior art techniques, the passage of current i through coil 12 creates a magnetic field 32. Coil Z4 of sensor 22 is moved to different points around the generating coil 12, and currents induced in the coil 24 provide a measure of the strength o-f the magnetic field 32 at the different points. With reference to the reference coordinate axes 34, 36, and 38, in addition to simple translation of coil 24 in the directions X, Y and/or Z, the coil 24, whose coordinate axes are 33, 35 and 37, may assume different relative angular orientations by rotations about these axes x, y and/or z.
Fig. 6 shows the output signal and coil 24 measured by sense circuit 26 for a given field 32 generated by current i flowing through the coil 12, as coil 24, is rotated for 360 degrees about either the y axis 35 or the z axis 37. In fact, coil 24 could be translated to uncountably many points around coil 12 where the above rotations of coil 24 would again give the same output signal shown in Fig. 6.
This demonstrates simply why the prior art apparatus cannot be used to uniquely define the relative position of nor the relative angular orientation of the sensing coils 24 with ; respect to coil 12.
i; F:igs. 7 and 8 show coil 12 which nutates the field 32 in a simple nodding motion, induced by nutating means 44 connected to coil 12 by line 46, through a predetermined angle 48, e.g., 45 degrees, and the resulting output curves as sensed by circuits 26. The translation and rotation motions to be considered are restricted to the ~-Y plane.
The curves of Fig. 8 illustrate the basis underlying stra-tegy i~ t~ ~ubi~t-invcnti~n-. In Fig. 7 note that two ,~.~.,;t orthogonal angular orientations are shown for the sensor ;:

, ~45~3 1 coil 24. In each of these two orientations there is, in general, an AC and a DC component induced in coil 24. When coil 24 is aligned with the y-axis which is assumed to be orthogonal to the pointing axis 50, the induced signal consists of a zero AC component at the fundamental nutation frequency and a zero DC component. When coil 24 is aligned with the x-axis, which is coincident with the pointing axis - 50, the induced signal consists of the entire DC componentand again zero AC at the fundamental nutation frequency.
The two pertinent signals for determination of relative orientation and translation, are the DC signal induced in coil 24 when in the y-position and the AC signal when in the x-position. Both are zero as illustrated in the first two curves of Fig. 8 when there is no orientation or translation error.
~ If a translation error exists then the sensor coil ; 24, in the x-position, will sense some AC signal 47 at the Eundamental nutation frequency. The magnitude of this signal will be proportional to the magnitude of the translation error; its phase, either 0 degrees or 180 degrees, will indicate the direction of the error.
If an orientation error exists, then the sensor coil 24 in the y-position will sense some DC signal 45. The magnitude and polarity oE this DC signal will indicate the magnitude and direction of the orientation error, respectively.
The apparatus of Fig. 7 will allow practice of the process of the i~4~ion to determine the location and 'Y.~t~
orientation of coil 24 by alternately positioning the coil 24 along the x and y axes, assuming freedom to move or orientate the coil 24 alternately to coincide with the x and y axes. If movement occurs in each of the X, Y and Z

. :~

l directions, -that is, in all three dimensions, then more than a simple planar nutation in the X-Y plane is required to - characterize that movement, as will be considered in more detail below. In the X-Y plane, however, rather than a successive positioning of the coil 24, it is far simpler to utilize two orthogonal coils, as in the apparatus of Fig. 9.
Therefore, coil 24 of Fig. 7 has been replaced by ortho-gonally positioned coils 52 and 54, each connected to sense circuits 26 by leads 56 and 58, and 60 and 62, respectively.
While nutation of the field 32 in Fig. 7 through the angle 48 can be accomplished by any convenient method, such as by means 44 giving a mechanical nutating motion of the coil 12 in Fig. 7, it is best accomplished electrically, utilizing a pair of coils 64 and 66, also orthogonal. Current sources 68 and 70 are connected to each of these coils by leads 72 and 74 and 76 and 78, respectively. As shown, current source 68 supplies a DC signal i to coil 64, and current source 70 supplies an AC signal, say Msin wt, to coil 66.
These signals can be either simple DC and AC or may be both superimposed on a suitable carrier frequency such as lO
kilohertz, in which case the terms AC and DC pertain to the modulation envelope defining each curve. In either case, the resulting magnetic field in the apparatus of Fig. 9 will nutate about a pointing axis 80 which is always coincident with the axis of the coil 64 as the AC signal in coil 66 produces an alternating magnetic field which adds vectorially to the magnetic field generated by the DC signal in coil 64.
In practice, an object having orthogonal sensing coils 52 and 54 mounted on it is free to move anywhere in - 30 the plane defined by the axes of the coils. If the system is to track the object, generating coils 64 and 66 should ~J l, ''~

' ~4~703 l have the capability to generate a magnetic field which ; nutates about a pointing vector 80, with a peak-to-peak~ angular nutation amplitude 49, in which the pointing vector80 does not coincide with the axis of coil 64. Such a magnetic field can be created by supplying the appropriate . mixtures of the AC and DC signals to coil 64 and to coil 66.
.............. As was described earlier, the amplitude 44 o-f nutat.ion angle depends upon the relative amplitude of the reference DC and AC sources, 68 and 70, respectively. The angle that the pointing vector 80 makes with the the reference x-axis of the coil 64 is governed by the mi.xing process performed by : the resolver circuit or process suggested in the discussion related to Fig. 3, inserted in the leads 72, 74, 76 and 78 between sources 68 and 70, and coils 64 and 66, respec-tively. The resolver operates on the fixed re-ference DC and : AC signals from sources 68 and 70, such that the processed signals received from the resolver for exciting the genera-tor coils 64 and 66 now have the capability of directing the ~: pointing vector 80 of the nutating field, at any desired angle A, through a full 360 degrees, in accordance with the . equations Excitation of coil 64 = ~DC)cosA - ~AC)sinA
Excitation of coil 66 = ~AC)cosA + ~DC)sinA
In order to provide sufficient information for tracking in a plane the position and the angular orientation of an object having sensing coils 52 and 54 mounted on it, ; the sense circuits 26 should have the capability to detect, after coordinate rotation processing of the signals induced in the sensing coils 52 and 54, the AC error component in - 30 the pointing vector direction and the DC error component in : the direction orthogonal to the pointing vector. The ~`.'3 ~al91L5~3 . .
1 relative phase and amplitude of the above mentioned AC error is proportional to the direction and magnitude of the pointing error. The polarity and magnitude o-f the above mentioned DC error component is proportional to the direction and magnitude of the error in the computed orientation angle of the remote object. These two error signals, which are proportional to the angular error in the pointing angle and to the angular error in the relative orientation angle of the object, respectively~ are used to make corrections in the previous measure of these two angles. The change in the pointing angle will shift the pointing vector until the sensor coils 52 and 54 lie along it, at which time the AC
error signal, measured in the direction of the pointing vector 80, will be zero. The indicated change required in the orientation angle will improve or correct the computed orientation angle which represents the relative coordinate relationship between the coordinate frame of the generator coils 64 and 66, and the coordinate frame of the sensor coils 52 and 54. If this relationship is properly repre-sented in the signal processor, by the orientation angle resolver ~, then the DC error signal detected in the direc-tion orthogonal to the pointing vector 80, will be zero.
In summary, and with added reference to ~ig. 10, ; ~ apparatus ~ ~66~dance with tho invent-i~n, for continuously tracking the relative lccation or direction and the relative angular orientation between two independent bodies in a plane, is described. The reference coordinates of the plane are defined by the X-axis 84 and the Y-axis 86 which are coincident with the field generating coils 64 and 66, respectively. Both the translation and the orientation angles will be measured with respect to this reference jr ,~ ,Sj ' :
~lO~ 3 l coordinate frame. The sensor coils 52 and 54 are fixed to the remotely moving object, and their mutually orthogonal axes 90 and 92 define the coordinate frame of the object to be tracked both as to location and orientation. In order to generate a nutating magnetic field pointed in a prescribed direction relatîve to the fixed coordinate frame of the generator coils 64 and 66, a particular mixture of DC and AC
excitation signals is required in each of the generating coils. The resolver 102 processes the reference DC and AC
excitation signals received on leads 104 and 106 from sources 68 and 70, respectively, in accordance with the presumed input pointing angle A 82, to give the appropri-ately mixed resolver output excitation signals which are ~: connected by leads 108 and 110 to the generator coils 64 and 66, respectively, such that the pointing vector 80 and its attendant nutating field structure makes the angle A with respect to the reference X-axis. The generated nutating - field points nominally at the sensor coils 52 and 54. The peak-to-peak amplitude of the nutation 88 is fixed, usually 20 45 to 90 degrees, and depends upon the relative magnitude of the two fixed reference DC and AC excitation signals from sources 68 and 70. It is clear that the signals induced in the sensor coils 52 and 54 depend not only on the pointing angle A but also on the relative orientation angle ~ 94. It ; 25 is ~or this reason that the induced signals in coils 52 and 54 are connected by leads 112 and 114 to resolver 96 and to be processed by resolver 96 which removes or unmixes that .: part of the AC and DC mixing of the two signals that is attributable to the non-zero orientation angle ~ 94. The two output signal components from resolver 96 are connected by leads 116 and 118 to resolver 98 which further unmixes ` ~;94~j7~1~

l the DC and AC signals mixing that was necessary to achieve the desired pointing angle A 82. If the presumed pointing angle A and the presumed orientation angle ~ are correct, then the output components from resolver 98 will be totally unmixed. That is, there will be no AC modulation error on the nominal DC output signal 120 which indicates that there is no pointing error, and also there will be no DC component on the nominally AC signal 122 which indicates that the ` computed orientation angle is corect. In the event that the angles ~ and/or A are incorrect, as will be the case, since very small errors are expected, when operating under dy-namically changing circumstances, then sense circuits 26 will detect the AC and DC errors on lines 120 and 122, respectively, relate them to errors in the angles ~ and A, respectively, and on leads 124 and 126 introduce the cor-responding incremental changes accumulated by Angle Mea-suring Circuit 100, in the respective angles. These im-proved angle measures o-f ~ and A are connected to the ; appropriate resolvers employed in this embodiment on leads 132, 134 and 136 in a stable feedback arrangement. That is, the corrections made in the outputs 128 and 130 tend to reduce the errors measured on components 124 and 126. These principles can be extended to applications in three dimen-sions by employing the system shown in Figure 11.
As in the system of Figure 10, the system of Figure 11 includes magnetic field generating coils 64 and 66 and magnetic field sensing coils 52 and 54. A third magnetic field generating coil 158, which is mutually orthogonal to coils 6~ and 66, and a third magnetic field sensing coil ; 30 248, which is mutually orthogonal to coils 52 and 54, is provided in order to measure information in the third ~ ''~ .

,; . . . . .

~045~
1 dimension. For ease of understanding, the three coils in each case have been shown as spacially separa~ed. In actuality, the magnetic axes of both the generator coils and the sensor coils intersect in a mutually orthogonal rela-tionship as shown by the cartesian coordinate frames 84, 86, 160, and 90, 92, 170, respectively. It should also be noted that an additional AC reference excitation signal has been provided such that ACl and AC2 are quadrature related or 90 degrees phase related. They may be considered as sinusoids of equal amplitude but 90 degrees out of phase, although the two reference ACl and AC2 signals need not necessarily be sinusoidal in the practical embodiment of the system.
Reference is again made to Fig. 4 which was related to the earlier discussion of coordinate transformation circuitry and which shows the three dimensional pointing geometry. As in the case of the two dimensional embodiment shown in Fig.
10, the ability to point the pointing vector 180 in any direction in which the assembly of sensing coils 52, 54 and 248 are free to move enables the sensing coils to be tracked.
The re~erence excitation DC, ACl and AC2 signals from sources 68, 70 and 140, respectively, define a conically nutating 164 magnetic field about a pointing axis 180 which is coincident with the axis of the DC component of the field. It should be emphasized again that the pointing of the vector 180 is accomplished electrically by the circuit to be described while the generating coils 64, 66 and 158 maintain a fixed orientation physically. DC source 68 and AC2 source 140 are connected by leads 142 and 144, respec-tively, to resolver 220, whose output lead 148 and output lead 146 from ACl source 70 are connected to resolver 222.
The output leads 154 and 156 provide the excitation signals . .

~0~ 3 ., 1 from resolver 222 to generator coils 64 and 66, respec-tively. Generator coil 158 is excited through connection 152 from the output of resolver 220. The two angles A and B
of resolver 222 and 220, respectively, are thus operating on the reference nutating field vector input whose components are the reference excitations from sources 68, 70 and 140, so as to point the pointing vector 180 and its attendant nutating field structure in accordance with the geometry shown in Fig. 4. The pointing vector 180 is presumed to be pointing nominally at the sensor which is Eixed to the remote object to be tracked by the system. This sensor con-sists of the three mutually orthogonal sensor coils 52, 54 and 248, which are fixed to the remote object and in the preferred embodiment are aligned to the principal axes of the remote object, so that in the process of determining the orientation of the sensor triad the orientation of the remote object is therefore determined. As in the discussion of the two dimensional case, illustrated in Fig. 10, the ~ signals induced in the sensor coils 52, 54 and 248 depend on ; 20 the orientation of their sensor coordinate frame, defined by the mutually orthogonal coordinate axes 90, 92 and 170, relative to the pointing axis 180 and its two orthogonal nutation components of the nutating field. In other words, the particular mixing of the three reference excitation signals DC, ACl, and AC2 from sources 68, 70 and 140, induced in each of the three sensor coils 52, 54 and 248, depends not only upon the two pointing angles A and B which govern the composite pointing coordinate transformation ~' circuit 252 but also upon the three Euler angles defining ,; 30 the relative angular orientation of the remote object and which govern the composite orientation coordinate trans-.'.~ ~-':, 7~)3 l formation circuit 250. The principal function of the two coordinate transformation circuits 250 and 252 in the overall computational strategy of the system is that the transformation circuit 250 unmixes that part of the re-ference signal mix induced in the sensor coils attributable to the relative orientation of the remote object, and coordinate transformation circuit 252 unmixes the remaining part of the reference signal mix that was due to the pointing angles. If the three orientation angles defining coordinate transformation circuit 250 and the two pointing angles defining the coordinate transformation circuit 252 properly represent the physical relationship between the sensor and generator coordinate frames, then the signals sensed by the sense circuits 26 will correspond to the unmixed reference signals DC, ACl and AC2, respectively, from sources 68, 70 and 140.
The sensor coils 54 and 248 are connected to ~, resolver 224 by leads 168 and 172, respectively. The output of sensor coil 52 and one output from resolver 224 connect to resolver 226 by leads 166 and 174, respectively. One output from resolver 224 and one output from resolver 226 connect to resolver 228 by leads 176 and 178, respectively.
The two outputs from resolver 228 are connected to resolver 230 by leads 186 and 188, respectively. One output from resolver 226 and one output from resolver 230 connect to resolver 232 on leads 184 and 190, respectively. One output from resolver 230 and the two outputs from resolver 232 provide the processed signal inputs to sense circuits 26 by connections 192, 194 and 196, respectively. Sense circuits 26 operates on the three input signals provided by leads ; 194, 192 and 196, to sense deviations from their nominally - r~_ 1 correct values which should correspond to the reference excitation signal components 68, 70 and 140, respectively.
The signal sensed on lead 194 should be nominally DC. If lead 194 contains an AC error signal at the nutation fre-quency then a pointing error exists, that is, the pointing vector 180 is not pointing precisely at the sensor coils 52, 54 and 248. That portion of the AC error signal, detected ; on lead 194 that is of the same absolute phase as the excitation signal 146, is proportional to an error in the pointing angle A. This pointing angle error in A is con-nected to the angle measuring circuits 100 by lead 200.
That portion of the AC error signal detected on lead 194 that is of the same absolute phase as the excitation signal 144, is proportional to an error in the pointing angle B.
This detected error in pointing angle B is connected to the angle measuring circuits 100 by lead 202. The signal that appears on lead 192 should be nominally AC at the nutation frequency and no DC signal. Whatever DC signal appears on lead 192 is proportional to an orientation angle error in the angle ~, called the relative bearing angle. This detected error in the relative bearing angle ~, is connected to the angle measuring circuits by lead 208. The signal that appears on lead 196 should also be nominally AC at the nutation frequency and should contain no DC. Whatever DC
signal is present on signal lead 196 is proportional to an error in the relative orientation angle ~, called the relative elevation angle. This error in the relative - elevation angle ~, is connected to the angle measuring circuits 100 by lead 206. As mentioned above, the nominal signals appearing on leads 192 and 196 are not only char-acterized as being AC at the nutation frequency but also -2~-, ~45~03 1 quadrature related as are their nominal reference signal counterparts ACl and AC2. Moreover, whatever phase dif-ference exists between the signal on lead 192 and signal source 70, or alternatively, whatever phase dif-ference exists between the signal on lead 196 and signal source 140, is proportional to an error in the relative orientation angle ~, called the relative roll angle. This error in the relative roll angle ~, is connected to the angle measuring circuit 100 by lead 204. The function of the angle measuring circuits 100 is to provide correct or corrected measures of ~ the two pointing angles A and B on leads 210 and 212, : respectively, based upon the angular errors sensed by sense ; circuits 26. Another function of the angle measuring circuits 100 is to provide correct or corrected measures of ; 15 the three relative orientation angles ~, ~ and ~, on leads 214, 216, and 218, respectively. These continuously im-proved angle measures, appearing on leads 210~ 212, 214, . 216, 218, are connected by leads 234 and 240, 236 and 238, 246, 244, 242, to resolvers 222 and 230, 220 and 232, 224, 226, 228, all respectively, in a stable feedback arrange-.~ ment. That is, the corrections made in the respective angles by the angle measuring circuits 100 tend to reduce to ~ zero the error signals detected by sense circuits 26 appearing on leads 194, 192 and 196.
It should be pointed out that the sequence of angles and their corresponding axes of rotation, for both ! the pointing coordinate transformation circuit 252 and the relative orientation coordinate transformation circuit 250, i~ are not unique. That is, other angle definitions and rotation sequences can be used for either of the two trans-formations subject to their having the required pointing and . - . . . . ~

1 relative orientation ~ree~om.
It should l~e pointed out that the system Ilere described can use state~of-the-art tech-niclues using digital, analog or hybrid circuitry.
It should also bc pointed out that whereas tlle des-cribed system might be also regarded as a unique five degree-of-frcc~om transducing system between two remotely separated independcllt coordinate frallles, employing only one generating source in one of the coordinate frames and only one sensor in the otller coordinate frame, the system can easily be extended to provide a measure of the full six degrees-of-freedom by using two generating means. Ihe second generating means would or could be located at another point in tlle coordinate frame of the first ~enerating means, operating cooperatively Witll the first generating means on a time shared basis, thereby allowing the third translation coor~i-nate, that of relative range, to be determined by triangu-larization, using the same computational techniques employed ~ in the invention.
i 20 It should also be emphasized that the teachinq herein applies to a wide range of applications operable ; in ~omains from a few cubic :Eeet or less to applications operable in domains of several cubic miles.
In the ~iscussion above it is to be understood that the sense circuits 26 are internally supplied with the components o-f the reference excitation signals from sources 68, 7Q and 140 in ordcr to logically perform the discrimi-nating sensing function required of their sensing circuits 26.
The resolvers which form components of the circuitry described herein may be fabricated, by way of example, in .r3.

~.

,~
:
L57~3 1 accordallce wi-th tlle teachings Or llnited states Patent No.

3,1~7,169, entitlecl LLI~CTRONIC RESOI.V[,R, issue~ June 1, 1965 to Robert D. Irammell, Jr. and I~obert S. Johnson, and United States Patent No. 2,927,734, entitled CO~IPUTING SYSTE~1 FOR
EIECTRONIC RESOLVI R, issued ~larcll 8, 1960 to Arthur W.
Vance. The sensing circuits, again by way of example, may be fabricated in accordance with the teachings of a circuit diagram appearing at page 67 of the book entitled "Ilec-tronics Circuit Designers Casebook", published by Ilec-tronics, ~Ic(,raw Ilill, No. 14-6. The angle measuring cir-cuitry may take the form of any of a vast number of well-known Type I Servomechanisms. There are, of course, numerous alternate constructions available or each of these com-ponents as will be readily appreciated by those skilled in the art.
It should now be apparent that a remote object tracking and orientation determination system capable of attaining the stated objects has been ; provided. The system and process deseribed utilizes a field for the purpose of determining tracking and orienta-tion angles of a remote object very precisely relative to -the coordinate frame of the apparatus which generates the field. With a two-dimensional nutation of the generated field, the tracking and orientation angles of the remote ; 25 object in the plane of nutation may be determined. Witll a three-dimensional nutation, the direction to and the orienta-tiOIl of a remote object may be determined.
It will be appreciated by those skilled in the art, additionally, that (a) the raw output ~,rom the angle measuring circuitry will be useful in certain situations in an open looped systenn although ordinarily, for ~, ~ and ~ to . . ~ . . . ~, .~ ,~, ;--. -. .
, . . . ~ , ~ :

7 ~ 3 1 be aceLIrcl~e, -tlle ~enercltor mnst be po.illting cl.irectly at the sensing means; ancl (b) absolute locatio]l and orientation (including ~istance) of an object relat:ive to the reference source can be determined by utilizing two physically dis-.- 5 placed generators such as tllat shown in Fig. 11 with appro-priate receiving ancl output circuitry at the object.
While a system has been clescribed in detail for tracking the movement ancl angular orienta-tion of a generalized remote object, it sllould be readily apparent to one art-skillecl that the diselosu~ may be used in a variety of object locating, tracking and orientation angle determination applications. One application currently in development is tracking the movement and orientation of :- an observer's head, or more specifically, his line-of-sight for use in a Visually-Coupled-Control System. Other poten-~ tial applications: a two-climens:ional system might be employed with surface modes of transportation, such as in the docking of ships or maintaining proper distances between passenger cars in an automated public transportation system.
Other ai.rcra-ft navigation problems suitable for handling with the invention include airborne alignment of missle systems, automated coupling of boom-nozzle and receptacle for in-flight refuelling of aircra:Et, formation :flying, instrument landing of vertical take-off and landing craft, and the like.
:: While the above descripti.on treats preferred eml)od:illlellts Or t.he :inventioll, :it should be rea~ily ,lpl)(lrellt that a vari.ety of modifications may be made in the system and process witllin the scope of the appenclecl c].aims.

, .

~, . ~

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

An object locating system which comprises:
(a) means for radiating a nutating field about an electrically directable pointing vector;
(b) means located at the object to be tracked, for sensing the nutating field; and (c) means electrically interconnected with said sensing means for determining the direction to the object relative to the coordinate frame of the radiating means.

The object locating system of claim 1 in which the object is free to translate and to orient in two dimensions, and in which the said field radiating means comprises orthogonal radiators and said field sensing means comprises orthogonal sensors, said radiators and sensors being in the plane of the two dimensions and sources of a DC reference signal and an AC reference signal to be passed through the two radiators and means for measuring and processing the signals induced thereby in the sensors to determine the translation and orientation of said object.

The object locating system of claim 1 in which the object is free to translate and orient in three dimensions and in which said field radiating means comprises three orthogonal radiators and said sensing means comprise three orthogonal sensors and sources of a DC reference signal, a first AC reference signal and a second AC reference signal phase quadrature related to the first AC signal to be passed through each of the three radiators and means for measuring and processing the signals induced in the sensor coils to determine the translation and orientation of said object.

A process for tracking an object, which comprises:
(a) radiating a nutating field about a pointing vector;
(b) sensing the generated field in at least two axes of the object to be tracked; and (c) determining from the sensed field after appropriate coordinate transformation processing, the directional position of the object from the radiator.

The process of claim 4 in which the object is free to translate and orient in two dimensions and in which the field is sensed in two dimensions.

The process of claim 4 in which the object is free to translate and orient in three dimensions and in which the field is sensed in three dimensions.

A process for tracking an object relative to the coordinate frame of a radiator, which comprises:
(a) radiating a nutating field about a pointing vector;
(b) sensing the radiated field in at least two orthogonal axes at the object, to produce an output signal for each axis; and (c) determining from the sensed field after appropriate coordinate transformation processing, the directional position of the object and also its angular orientation, both relative to the coordinate frame of the radiating means.

The process of claim 7 in which the object is free to translate and to orient in two dimensions, the field nutates by nodding in the plane of the two dimensions, and in which the field is also sensed in the two dimensions.

The process of claim 7 in which the object is free to translate and to orient in three dimensions, the field nutates by describing a conical motion about the point-ing vector, and in which the field is sensed in the three dimensions.

The process as set forth in claim 7 wherein said determining step includes the steps of deriving a signal, based on the sensing of the nutating field, which is related to the misdirection, if any, of the pointing vector and applying the signal to the radiator.

An object tracking system comprising means for radiating a directable nutating field about a pointing vector which is the axis of nutation; means located at the object for sensing the nutating field; and, means for deriving a signal, based on the sensing of the nutating field, which is related to the misdirection, if any, of said pointing vector, means for redirecting said pointing vector toward said object in accordance with said error signal.

The system as set forth in claim 11 wherein: said radiating means includes means for generating a pointing vector having at least two independent components which are a function of at least two primary reference signals and means for transforming said primary reference signals in accordance with presumed pointing angle inputs to direct said pointing vector in a direction corresponding to said presumed pointing angle inputs; wherein said sensing means comprises means for sensing components of the field generated by the nutating field about said pointing vector and means for transforming the sensed components in accordance with said presumed pointing angle inputs to yield a set of reconstructed reference signals; and, wherein said deriving means comprises means for comparing at least some of said reconstructed reference signals with at least some of said primary reference signals, deriving pointing angle error signals and altering the pointing angle input signals of said transforming means in accordance therewith to tend to null the pointing error.

The system as set forth in claim 12 wherein said directable field is an electromagnetic field, said radiating means including orthogonal radiators, said primary reference signals being a DC signal and a first AC signal.

The system as set forth in claim 13 wherein said sensing means includes orthogonal sensors.

The system as set forth in claim 14 wherein the object is free to translate in two dimensions and to orient in the same two dimensions, and in which there are at least two orthogonal radiators and sensors in each of said radiating and sensing means, respectively, the radiators and sensors being in the plane of the two dimensions.

The system as set forth in claim 14 wherein the object is free to translate in three dimensions and to orient in three dimensions, in which there are three orthogonal sensors and radiators in each of the sensing and radiating means, respectively, and in which a second AC signal in phase quadrature with said first AC signal is one of said primary reference signals.

The system as set forth in claim 12 which further comprises means for determining the orientation of said object.

The system as set forth in claim 17 wherein said directable field is an electromagnetic field, said radiating means including three orthogonal radiators; said sensing means including three orthogonal sensors, said primary reference signals being a DC signal, a first AC signal and a second AC signal in phase quadrature with said first AC
signal.

The system as set forth in claim 18 wherein said means for determining the orientation of said object comprises means for analyzing two of said reconstructed reference signals to determine two degrees-of-freedom of orientation and means for determining the phase relationship between at least one primary reference AC signal and the corresponding reconstructed AC reference signal to determine the third degree-of-freedom of orientation.

The system as set forth in claim 19 wherein said pointing error signals are determined by the presence or absence of a reference signal variant in one of said reconstructed reference signals and wherein said two degrees-of-freedom of orientation are determined by detecting the presence of a DC signal in the other two of said reconstructed reference signals.

The object tracking system as set forth in claim 11 which further includes means tracking the angular orientation of the object, said system comprising: means for generating a series of three primary reference signals;
first means for transforming said primary reference signals in accordance with a first transformation representing a presumed pointing angle; means including three orthogonal radiators for radiating said transformed primary reference signals; means including three orthogonal sensors for sensing the transformed primary reference signals, said sensing means being rigidly affixed to the object; second means for transforming said sensed signals in accordance with a second transformation which is the inverse of a presumed set of orientation angles; third means for transforming said sensed signals in accordance with a third transformation which is the inverse of said first transformation; means for comparing the sensed signals so transformed with said primary reference signals and altering, if necessary, the presumed pointing and orientation angles.