METHOD AND APPARATUS FOR ELECTROMAGNETIC POSITION AND ORIENTATION TRACKING WITH DISTORTION COMPENSATION

Abstract: Methods and apparatus for position/orientation tracking within a bounded volume employ at least one stationary sensor, called a "witness sensor", having a fixed position and orientation near or within the volume to account for electromagnetic distortion. One or more probe sensors are placed on an object to be tracked within the volume, and the output of each witness sensor is used to compute the parameters of a non-real effective electromagnetic source. The parameters of the effective source are used as inputs to the computation of position and orientation as measured by each probe sensor, as if the object were in the non-distorted electromagnetic field produced by the effective source or sources. In addition to trackers for helmet-mounted displays in aircraft, tank, and armored-vehicle applications, the invention finds utility in any electromagnetic tracking system which might be subject to electromagnetic distortion or interference. Such application areas include electromagnetic motion capture systems, and medical systems and instruments, among others.

Description: METHOD AND APPARATUS FOR ELECTROMAGNETIC POSITION AND ORIENTATION TRACKING WITH DISTORTION COMPENSATION Field of the Invention This invention relates generally to position and orientation tracking and, in particular, to methods and apparatus for accurate position, orientation and motion- tracking within a bounded volume in the presence of electromagnetic distortion. Background of the Invention Existing electromagnetic tracking systems, as well as inertial and combined inertial/optical and optical/magnetic tracers, are sensitive to various kinds of distortion. With respect to electromagnetic trackers, such distortion may arise from eddy currents in metal objects or from ferromagnetic materials, whereas, in the case of inertial trackers, drift or vibration might be the cause. In inertial tracking systems, for example, as described in U.S. Patent No. 5,645,077, requires an additional sensor, or set of sensors, to compensate for drift and movement of a vehicle or aircraft reference frame. Even with these additional sensors, such systems exhibit sensitivity to vibration and temperature instability requiring additional compensation. Inertial tracking systems also experience drift over time periods on the order of minutes to hours. Combination systems, that is, systems which combine optical and magnetic sensing, are designed to compensate for distortion by comparison of the data from two different types of sensors. One such system is described in U.S. Pat. No. 5,831 ,260 to Hansen. The use of combination systems with an optical tracking module (e.g., inertial/optical or magnetic/optical) is restricted to applications where interference associated with night-vision devices is significant, or when parasitic illumination or optical noise is present. In an electromagnetic tracking environment, distortion may arise from eddy currents induced in nearby metal objects or from ferromagnetic materials. Eddy currents, in turn, generate fields that interfere with the field from the source(s) used for tracking purposes. To compensate for distortion of this kind, one solution involves the use of mapping. With mapping, the electromagnetic field in a volume of interest, as distorted by metal objects, is defined in advance and used to solve for position and orientation. Commonly assigned U.S. patent application Serial Mo. 09/215,052, the entire contents of which are incorporated herein by reference, discloses, as part of a preferred embodiment, the use of Green's functions in conjunction with such field mapping. The need remains, therefore, for a simple but effective approach to reducing the effects of distortion in an electromagnetic tracking system. Ideally, such a solution would be useful in a variety of applications, including motion capture and medical instrumentation. Summary of the Invention The subject invention is directed to distortion compensation in electromagnetic position and orientation tracking configurations. Among the advantages over previous approaches, the invention eliminates the need for mapping in advance of actual measurements and, permits real-time adjustment in the event that new field distorters arise. This may be significant in such an environment as aircraft cockpit or tracing of medical/surgery instruments. The invention is specifically applicable to position/orientation tracking systems of the type wherein the components of an AC electromagnetic field are sensed within a bounded volume. As in previous designs, one or more probe sensors are placed on an object being tracked within the volume, each measuring the magnetic induction vector components of the field generated by the source to determine the position, orientation and movement of the object within the volume. However, to compensate for electromagnetic distortion, the invention employs at least one stationary sensor, termed a witness sensor, in addition to the probe sensor(s) disposed on the object. Each witness sensor is supported at a known, fixed position and orientation relative to the reference frame of interest, at a point near or within the volume of interest, and as close to the sensors on the object being tracked. The outputs of each probe sensor, and each witness sensor, are delivered to a processing unit operative to compute the position and orientation of the object in the presence of electromagnetic field distorters. The processor uses data from each witness sensor to compute parameters, such as the position, orientation, and strength, of an effective electromagnetic source. The effective source(s), which may be treated for the sake of simplicity as a point source or a dipole, would produce the same field of the real source plus field of distortion in the proximity of witness sensors. Note that in the case of a non-distorted quiet environment, the single effective source will coincide with, and will be identical to, the actual real source. This allows the computed parameters of the effective source(s) to be used as inputs to the computation of position and orientation as measured by each probe sensor, as if the object is in the non-distorted electromagnetic field produced by the effective source(s). An electromagnetic position/orientation tracking system according to the invention therefore preferably includes a real source of an AC electromagnetic field; at least one witness sensor measuring electromagnetic induction vector components at a known spatial point; one or more probe sensors measuring electromagnetic induction vector components from the perspective of the object; and a control/processing unit providing the requisite computations. In addition to trackers for helmet-mounted displays in aircraft, tank, and armored-vehicle applications, the invention finds utility in any electromagnetic tracking system which might be subject to electromagnetic distortion or interference. Such application areas include electromagnetic motion capture systems, and medical systems and instruments, among others. Brief Description of the Drawings FIGURE 1 is a drawing which illustrates a preferred embodiment of the invention; FIGURE 2 is a flow diagram that illustrates the steps of a preferred method of carrying out the invention; and FIGURE 3 is a drawing which depicts a specific application of the invention, namely, a helmet-mounted aircraft tracking configuration. Detailed Description of the Invention Figure 1 illustrates, generally at 100, apparatus associated with a preferred embodiment of the invention. An object 102 is to be tracked within a volume of interest 104, wherein electromagnetic distortion may be present. One or more probe sensors 106 are supported on the object to measure magnetic induction-vector components of an AC electromagnetic field 1 1 1 generated by a real, physical source 1 10. A stationary "witness" sensor 1 12 (or a set of sensors) having a known position and orientation is disposed at a known distance R 0 from the source 1 10. The witness and probe sensors 1 12 and 106 are interconnected through paths 1 14 and 116 to a processor unit 120, which outputs the position and orientation of the object 102 within the volume 106. Although multiple, single-axis sensors may be used for the probe or witness sensors, the invention preferably uses multi-axis sensors measuring three components of the induction vector. Magnetic field data from each witness sensor are used by the processor unit 120 to calculate the position, strength and orientation of an "effective" source (or sources) 122 located at a distance R e rr from witness sensor. The effective source 122 is not a real source but model one, which would produce the electromagnetic field in the vicinity of the witness sensor equivalent too the field of electromagnetic distortion which may be present. Depending on the required accuracy and processing time, there are at least two options to treat this effective source: the first one is to consider a single effective source that will create the same field as the actual source plus distortion field; the second options is to consider single or multiple effective sources in addition to the actual source, thus considering the field as a superposition of the real source and the effective source(s) modeling distortion. The second option is more accurate but requires more intensive real-time computations. The result is used to determine the position and orientation of each probe sensor 106 and, hence, the object 102, as measured in the non-distorted field 123 produced by the effective source 122 (or superposition of the field of the real source 1 10 and effective source(s) 122). The distortion is therefore automatically taken into account. The inventive system does not require mapping and, in fact, the witness sensor data are self-calibrating. Although the volume depicted in Figure 1 is a cube, the system works with any geometry. Although mapping approaches benefit from the use of regular geometric volumes, since this invention does not require a priori mapping, arbitrary volumes shapes are readily accommodated. The witness sensor(s) need not be placed within the volume. The only assumption underlying optimum results is that the witness sensor(s) and probe sensor(s) are close enough to one another. Mathematically, the arrangement may be described as follows: Let B(Ro) be the magnetic induction vector measured by a witness sensor located at a distance Ro and a real source of the electromagnetic radiation located at the origin. This magnetic field may be described exactly by the equation: B(R,) = - grad - g ad∑ + RJp(R ϋ + R ώ „ )d'R where G is the Green's function and is equal to the density function. The first term of the above equation represents the field of the source located around the point Ro. The second term describes the field of the spatially distributed distortion in sub-volumes V , st - Note that although the actual source is placed at (0,0,0) in this case, any suitable alternative location may be used through appropriate coordinate transformation. In the case of multiple witness sensors (e.g., N sensors), we will get a set of N similar equations with corresponding substitution of Ro, instead of R () , 1=1..N. The same field B(R o ) may be described approximately as the field of some effective source (the first option, as described above) located around some point R e ιι, as follows: B (R n ) = - grad (R 0 + R e „ + r ) p (R 0 + R c „ + r ) d 3 r The second option, as described above, will yield the equation: B(R,) = + R cl , + r)p(R ϋ + R e „ + r)d'r The solution of these equations with respect to R e ιτ and may be found, for example, by the best-fit method or by the quasi-Newton method. The solution to the above vector equation is the input to the next equation, which describes the electromagnetic field at the location of the probe sensor: B(R cj , + R 0 + δr) - - grad The solution with respect to r, defines the output of the system. The accuracy of the system is enhanced through placement of the witness sensor(s) in close proximity to the probe sensor(s). That is, accuracy is increased when: Multiple witness sensors increase the accuracy of the solution with respect to R e π and, in turn, r. In addition, the volume wherein sufficient accuracy is achievable increases. A flow diagram of the method is presented in Figure 2. At step 202, input data from the witness sensor(s) are gathered. At block 204 the parameters of the effective source(s) are computed. If the data from the witness sensor(s) have changed, this is taken into account at step 220. At step 206, input data from the probe sensor(s) are gathered. At step 208, the position and orientation of the object is computed relative to the coordinates of the volume of interest using the inputs from the witness and probe sensors, and the output is delivered at 210. An advantage of the invention is that the solution for the effective source may be computed at regular intervals, allowing for essentially continuous noise compensation, since witness and probe sensors are preferably at the same approximate location, and therefore nearly equally affected by the external noise (e.g., RF). Figure 3 is a drawing of an aircraft cockpit application of the invention. In this case a pilot 302 moves within a volume 304. A source 306 radiates an AC electromagnetic field within the volume, which is received by at least one witness sensor 308 and at least one probe sensor 310 mounted on the helmet of the operator 302. A processing unit (not shown) receives the inputs from the sensors, and computes the position and orientation of the object, in this case the helmet of the pilot, by applying the effective field generated by source 320 (or superposition of fields from 320 and 306) to the components of the field measured by the probe sensor 310. In addition to trackers for helmet-mounted displays in aircraft, tank, armored- vehicle applications, and tracing of medical/surgery instruments, the invention finds utility in any electromagnetic tracking system which might be subject to electromagnetic distortion or interference. As disclosed in co-pending U.S. patent application Serial No. 09/215,052, such application areas include electromagnetic motion capture systems, and medical systems and instruments, among others. We claim:

Claims: 1. Apparatus for determining the position and orientation of an object within a bounded volume in the presence of electromagnetic distortion, the apparatus comprising: a real source of an electromagnetic field having measurable magnetic induction- vector components; one or more stationary witness sensors supported near or within the volume of interest, each witness sensor being operative to measure the induction-vector components using a fixed known position and orientation; one or more probe sensors supported on the object for measuring the induction- vector components; and a processor in communication with each witness sensor and each probe sensor, the processor being operative to perform the following functions: a) compute the characteristics of the at least one effective electromagnetic source, the effective source being defined as a model source which would produce the field in the vicinity of a each witness sensor similar, or identical, to the field generated by the real source, along with electromagnetic distortion present in the volume, b) receive the induction-vector components measured by each probe sensor, and c) compute the position and orientation of the object by applying the characteristics of the effective source to the induction-vector components measured by each probe sensor. 2. The apparatus of claim 1 , wherein: the object moves within the volume; and at least functions b) and c) performed by the processor are repeated at regular intervals to track the motion of the object. 3. The apparatus of claim 1 , wherein the computed characteristics of the effective source include the strength, position and orientation of the effective source. 4. The apparatus of claim 1 , wherein the bounded volume includes at least a portion of a vehicle interior, and wherein each probe sensor is supported relative to an operator of the vehicle. 5. The apparatus of claim 4, wherein vehicle interior forms part of an aircraft cockpit, and wherein each probe sensor is mounted on a helmet worn by the operator. 6. The apparatus of claim 1 , wherein at least one probe sensor is positioned on a medical instrument. 7. The apparatus of claim 1 , wherein at least one probe sensor is positioned on a person's body. 8. The apparatus of claim 1, including a plurality of witness sensors, and wherein the characteristics of the effective source are determined by a best-fit approximation. 9. The apparatus of claim 1, including a plurality of witness sensors, and wherein the characteristics of the effective source are determined by an error minimization or optimization procedure. 10. The apparatus of claim 1 , wherein the effective source is treated as a point source. 1 1. The apparatus of claim 1 , wherein the effective source is treated as a dipole source. 12. The apparatus of claim 1, wherein each witness sensor and each probe sensor measures components of the magnetic induction vector. 13. A method of determining the position and orientation of an object within a bounded volume in the presence of electromagnetic distortion, comprising the steps of: a) generating an electromagnetic field having magnetic induction-vector components from a stationary source; b) measuring the components of the electromagnetic field at one or more stationary points near or within the volume; c) measuring the components of the electromagnetic field at one or more points relative to the object; d) computing the characteristics of an effective electromagnetic field, which would be produced by the stationary source in the vicinity of a given witness sensor along with electromagnetic distortion present in the bounded volume; e) receiving the induction-vector components measured relative to the object; and f) computing the position and orientation of the object by applying the characteristics of the effective field to the induction-vector components. 14. The method of claim 13, wherein: the object moves within the volume; and at least steps e) and f) are repeated at regular intervals to track the motion of the object. 15. The method of claim 13, wherein the step of computing the characteristics of the effective electromagnetic field include computing the strength, position and orientation of the effective field. 16. The method of claim 13, wherein the object is positioned within a vehicle interior. 17. The method of claim 13, wherein the object is supported on the body of an individual. 18. The method of claim 13, wherein the step of computing the characteristics of the effective electromagnetic field includes the step of performing a best-fit approximation. 19. The method of claim 13, wherein the step of computing the characteristics of the effective electromagnetic field includes the step of performing an error minimization or optimization procedure. 20. The method of claim 13, further including the step of treating the effective source as a point source. 21. The method of claim 13, further including the step of treating the effective source as a dipole source. 22. The method of claim 13, wherein three components of the induction vector are measured at each stationary point. 23. The method of claim 13, wherein components of the induction vector are measured at each point relative to the object. 24. In a position/orientation tracking system of the type wherein AC electromagnetic radiation is received by a sensor disposed on an object being tracked in the presence of electromagnetic distortion, the improvement comprising: at least one witness sensor having a fixed position and orientation within the field relative to the source of the electromagnetic distortion; and processor means in communication with the witness sensor for: a) computing a non-distorted virtual field proximate to the witness sensor, and b) computing the position and orientation of the object as if the object were located within the non-distorted virtual field. 25. The improvement of claim 24, wherein the witness sensor is in close proximity to the object. 26. The improvement of claim 24, further including a plurality of spaced-apart witness sensors. 27. The improvement of claim 24, further including a plurality of sensors on the object being tracked. 28. The improvement of claim 24, wherein the object is supported on a person's body.

Inventors: JONES HERBERT R JR, KHALFIN IGOR, JONES, HERBERT, R., JR., KHALFIN, IGOR
Applicants: POLHEMUS INC, POLHEMUS INC.
Application year: 2000
Main classification: G01R
IPC codes: A61B19, G01B7, G01S1, G01S5, G01V3, G02B27, G06F3