Adjustable compensation ratio feedback system

Abstract

Apparatus for implementing Adjustable Compensation Ratio (ACR) active shielding or control of physical fields (magnetic, electric, electromagnetic, acoustic, etc.), comprising the addition of a secondary internal feedback loop within a conventional primary closed feedback loop topology. Compensation-ratio transfer function order and coefficients adjustment permits accommodating frequency-dependent and frequency-independent effects within a Protected Volume when a system field sensor or sensor array is not at the exact location where external field interference must be optimally canceled. A Laplace polynomial term precisely sets this parameter in a supplementary feedback link by modeling the frequency-dependent characteristic of an Interacting Medium without deleterious effect on other desirable primary closed-loop characteristics. The inventive ACR can be used in advanced active cancellation for magnetic shielding purposes.

Claims

1. An Adjustable Compensation Ratio, closed-loop, negative-feedback system for compensation of physical fields in at least one axis within a Protected Volume of space located within a Volume of space, where such compensation is used for suppression of an undesired physical field, the system comprising: a) an axial subsystem comprising first physical field sensing means performing as a first summation node in a Volume of space, pre-amplification and bandshaping filters representable by a Laplace transform function, a secondary summation node, an adjustable post-amplifier, and an output power driver for providing a compensating physical field and corresponding constant attenuation coefficient per axis; b) parametric extraction means for generating a first ratiometric offset signal; and c) a supplementary direct loop operatively connected to said first summation node, said supplementary direct loop including said secondary summation node for receiving a first ratiometric offset signal to correct for environmental alteration of a compensating signal.

2. The Adjustable Compensation Ratio, closed-loop, negative-feedback system in accordance with claim 1, wherein said first ratiometric offset signal is frequency-dependent.

3. The Adjustable Compensation Ratio, closed-loop, negative-feedback system in accordance with claim 1, wherein said first physical field sensing means is located a predetermined distance from said Protected Volume of space.

4. The Adjustable Compensation Ratio, closed-loop, negative-feedback system in accordance with claim 1, further comprising an intermediary Interacting Medium (IM) surrounding said Protected Volume of space, wherein physical influence due to said Interacting Medium operating on traversing fields is characterized by first and second Laplace transform expressions.

5. The Adjustable Compensation Ratio, closed-loop, negative-feedback system in accordance with claim 4, further comprising a third Laplace transform expression, wherein said third Laplace transform is a function of said first and said second Laplace transform expressions.

6. The Adjustable Compensation Ratio, closed-loop, negative-feedback system in accordance with claim 5, wherein said secondary summation node sums a signal over time represented by said third Laplace transform expression for generating a compensating signal.

7. The Adjustable Compensation Ratio, closed-loop, negative-feedback system in accordance with claim 1, said system compensating at least one set of fields chosen from the group: magnetic fields; electric fields; electromagnetic fields; and acoustic fields.

8. The Adjustable Compensation Ratio, closed-loop, negative-feedback system in accordance with claim 1, further comprising multiple physical field sensing means per axis for adaptive physical field compensation of multiple physical field interference sources.

9. The Adjustable Compensation Ratio, closed-loop, negative-feedback system in accordance with claim 1, further comprising means for introducing cross-coupling compensation terms from orthogonal axial channels.

10. The Adjustable Compensation Ratio, closed-loop, negative-feedback system in accordance with claim 1, further comprising means for directly measuring an Excitation Transfer Coefficient for each axial channel.

11. An Adjustable Compensation Ratio, closed-loop, negative-feedback system for control of physical fields in at least one axis within a Protected Volume of space located within a Volume of space, said control establishing a physical field of predetermined parametric value, the system comprising: a) an axial subsystem comprising first physical field sensing means performing as a first summation node in said Volume of space, pre-amplification and bandshaping filters representable by a Laplace transform function, a secondary summation node, an adjustable post-amplifier, and an output power driver for providing a compensating physical field and corresponding constant attenuation coefficient per axis; b) parametric extraction means for generating a first ratiometric offset signal; c) a supplementary direct loop operatively connected to said first summation node, said supplementary direct loop including said secondary summation node for receiving a first ratiometric offset signal to correct for environmental alteration of a compensating signal; and d) an external input signal channel operatively connected to said secondary node for receiving a parametric control signal for establishing a parametrically-defined physical field within the said Protected Volume of space.

12. The Adjustable Compensation Ratio, closed-loop, negative-feedback system in accordance with claim 11, wherein said external input signal channel comprises at least one bandshaping filter representable by Laplace transform functions.

13. The Adjustable Compensation Ratio, closed-loop, negative-feedback system in accordance with claim 11, wherein said first ratiometric offset signal is frequency-dependent.

14. The Adjustable Compensation Ratio, closed-loop, negative-feedback system in accordance with claim 13, wherein said first physical field sensing means is located a predetermined distance from said Protected Volume of space.

15. The physical field Adjustable Compensation Ratio, closed-loop, negative-feedback system in accordance with claim 14, further comprising an intermediary Interacting Medium surrounding said Protected Volume of space, wherein physical influence due to said Interacting Medium operating on traversing fields is characterized by first and second Laplace transform expressions.

16. The Adjustable Compensation Ratio, closed-loop, negative-feedback system in accordance with claim 15, further comprising a third Laplace transform expression, said third Laplace transform expression being a function of said first and said second Laplace transform expressions.

17. The Adjustable Compensation Ratio, closed-loop, negative-feedback system in accordance with claim 16, wherein said secondary summation node sums a signal over time represented by said third Laplace transform expression for generating a compensating signal.

18. The Adjustable Compensation Ratio, closed-loop, negative-feedback system in accordance with claim 11, said system creating at least one set of fields chosen from the group: magnetic fields; electric fields; electromagnetic fields; and acoustic fields.

19. The Adjustable Compensation Ratio, closed-loop, negative-feedback system in accordance with claim 11, further comprising means for introducing cross-coupling compensation terms from orthogonal axial channels.

20. The Adjustable Compensation Ratio, closed-loop, negative-feedback system in accordance with claim 11, further comprising means for directly measuring an Excitation Transfer Coefficient for each axial channel.

21. An Adjustable Compensation Ratio, closed-loop, negative-feedback system for combined compensation and control of physical fields in at least one axis within a Protected Volume of space located within a Volume of space, wherein such compensation and control simultaneously suppress an undesired physical field and establish a physical field of predetermined parametric value, the system comprising: a) an axial subsystem comprising first physical field sensing means performing as a first summation node in said Volume of space, pre-amplification and bandshaping filters representable by a Laplace transform function, a secondary summation node, an adjustable post-amplifier, and an output power driver for providing a compensating physical field and corresponding constant attenuation coefficient per axis; b) parametric extraction means for generating a first ratiometric offset signal; c) a supplementary direct loop operatively connected to said first summation node, said supplementary direct loop including said secondary summation node for receiving a first ratiometric offset signal to correct for environmental alteration of a compensating signal; and d) an external input signal channel operatively connected to said secondary node for receiving a parametric control signal for establishing a defined physical field within said Protected Volume of space.

22. The Adjustable Compensation Ratio, closed-loop, negative-feedback system in accordance with claim 21, wherein said external input signal channel includes at least one bandshaping filter representable by Laplace transform functions.

23. The Adjustable Compensation Ratio, closed-loop, negative-feedback system in accordance with claim 22, wherein said first ratiometric offset signal is frequency-dependent.

24. The Adjustable Compensation Ratio, closed-loop, negative-feedback system in accordance with claim 22, wherein said first physical field sensing means is located a predetermined distance from said Protected Volume of space.

25. The Adjustable Compensation Ratio, closed-loop, negative-feedback system in accordance with claim 22, further comprising an intermediary Interacting Medium surrounding said Protected Volume of space, wherein the physical influence due to said Interacting Medium operating on traversing fields is characterized by first and second Laplace transform expressions.

26. The Adjustable Compensation Ratio, closed-loop, negative-feedback system in accordance with claim 25, further comprising a third Laplace transform expression being a function of said first and second Laplace transform expressions.

27. The Adjustable Compensation Ratio, closed-loop, negative-feedback system in accordance with claim 26, wherein secondary summation node sums a signal over time represented by said third Laplace transform expression for generating a compensating signal.

28. The Adjustable Compensation Ratio, closed-loop, negative-feedback system in accordance with claim 22, said system simultaneously compensating and controlling at least one set of fields chosen from the group: magnetic fields; electric fields; electromagnetic fields; and acoustic fields.

29. The Adjustable Compensation Ratio, closed-loop, negative-feedback system in accordance with claim 22, further comprising multiple sensing means per axis for adaptive physical field compensation of multiple physical field interference sources.

30. The Adjustable Compensation Ratio, closed-loop, negative-feedback system in accordance with claim 22, further comprising means for introducing cross-coupling compensation terms from orthogonal axial channels.

31. The Adjustable Compensation Ratio, closed-loop, negative-feedback system in accordance with claim 22, further comprising means for directly measuring an Excitation Transfer Coefficient for each axial channel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A complete understanding of the present invention may be obtained by reference to the accompanying drawings, when considered in conjunction with the subsequent detailed description, in which:

(2) FIG. 1 is a schematic diagram of a generalized unity-ratio active compensation system of the prior art;

(3) FIG. 2 is a schematic diagram of the Adjustable Compensation Ratio (ACR) generalized system in accordance with the present invention; and

(4) FIG. 3 is a schematic diagram of the ACR system configured for parametric control.

DESCRIPTION OF THE PREFERRED EMBODIMENT

(5) An Adjustable Compensation Ratio, closed-loop, negative-feedback system is provided for suppressing magnetic fields, for example, in a large, Operational Volume of space. An axial subsystem has a first magnetic field sensing probe in a Protected Volume of space. A parametric sensor, located a predetermined distance from the first magnetic field sensing probe, generates a first ratiometric offset signal. A supplementary direct loop is operatively connected to the first summation node. The supplementary direct loop includes a secondary summation node for receiving the first ratiometric offset signal to correct for alteration of the compensating signal. An intermediary Interacting Medium surrounds the Protected Volume of space. The physical influence due to the Interacting Medium operating on traversing fields is characterized by first and second Laplace transform expressions.

(6) A generalized form of the ACR invention implementing these features for active-feedback shielding is described in FIG. 2. This generalized diagram differs from the prior art system depicted in FIG. 1 in that it adds the extraction and frequency-domain processing of a supplementary signal for injection into a secondary summation node, 2. Functionally, the injection of a supplementary signal permits the outer closed-loop to correct for alteration of the compensating signal due to either a field gradient or, separately, physical interaction that may occur due to an IM between the location of the sensing means and the PV. Such correction by means of a supplementary loop frequency-dependent additive signal, provides arbitrarily error-free spatial and temporal mapping of the compensating field onto the interfering field under the aforesaid conditions, dependent only on the order of a correction polynomial (in S), the accuracy of its term coefficients, and proper adjustment of the offset scaling parameter, .

(7) Accurately minimizing or, alternatively, controlling a residual field (e.g., magnetic, electric, electromagnetic, acoustic) within a PV that is both physically separate and partly isolated from the sensing means requires that the compensating field signal be pre-processed in a manner specific to the invention that results in an accurate replication of the interfering field within the PV. Referring to FIG. 2, it may be noted that subtraction of the compensating field, .sub.0, from the interfering field, , within the PV is accomplished by inversion of the compensation field, either by the generating means or inversion of the signal prior to the compensating field generating means. The superposition of the two signals is thus a subtraction process yielding a residual field with the magnitude error term, . In practical active-shielding installations, the average value of through the PV is the parameter defining the effective shielding coefficient, which is set by requirements of a specific application. The key feature of the inventive ACR feedback system is that may be constrained to any arbitrarily low value to meet the application performance requirements.

(8) Observing that the Protected Volume (PV) may be separated from the Operating Volume (OV) by an Interacting Medium (IM) as illustrated in FIG. 2, and that the physical influence due to a given IM operating on the traversing fields can be fully characterized by Laplace transform expressions S.sub.1 and S.sub.2, the optimum Laplace signal transform function S.sub.3, required within the supplementary loop, can be analytically determined. Moreover, the analytic form demonstrates that in all cases a specific expression for S.sub.3 that corresponds to a spatial and temporally-independent null within the PV is realizable. Under such conditions, the resulting average value of will then be determined solely by the gradient and time-dependent differential within the PV between the interfering and compensating fields.

(9) Analytic determination of the Laplace transform term S.sub.3 is derived from the fundamental equality relating the residual field E within the PV to the IM-modified interfering and compensating signals. By inspection, the residual field is determined as:
.sub.o=.sub.iS.sub.1+(.sub.c)S.sub.2(1)
Equivalently,
.sub.o=.sub.iS.sub.1.sub.cS.sub.2(2)
where:

(10) For .sub.o0,

(11) .sub.o=error term corresponding to residual interfering signal within the Protected Volume (PV),

(12) .sub.i=frequency-domain representation of interfering signal (magnetostatic, electrostatic, electromagnetic or acoustic),

(13) S.sub.1=transfer Laplace transform of Interacting Medium for interfering signal,

(14) .sub.c=frequency-domain representation of compensating signal (field type as per .sub.i, above), and

(15) S.sub.2=transfer Laplace transform of Interacting Medium for compensating signal.
.sub.iS.sub.1.sub.cS.sub.2=0(3)
Or,

(16) $\begin{array}{cc}{}_i={}_c\left(\frac{S_2}{S_1}\right),& \left(4\right)\end{array}$
=condition of maximum interference cancellation.

(17) This is the system equation that describes the necessary compensating field condition for minimizing interfering field effects within the PV, or, alternatively, optimizing the accuracy of control effects therewithin. For the ACR system to attain a condition of maximum interference cancellation it is therefore necessary to introduce a compensating term that includes information inherent in the frequency-dependent transfer function

(18) $\left(\frac{S_2}{S_1}\right).$
In the ACR topology shown in FIG. 2, this term is introduced by means of a supplementary feedback loop consisting of the parameter extraction module (PEM), that is, a parametric extraction means such as a transductor for developing a signal proportional to the parameter being monitored, and the S.sub.3 signal processing section. Determination of the requisite supplementary feedback transfer function is treated analytically by first deriving the secondary, or ratiometric, error term .sub.2 corresponding to the condition of ideal interfering field cancellation at a point within the PV:
.sub.2=(.sub.1R)(5)
where:

(19) .sub.2=ratiometrically-adjusted error term incorporating compensation for Interacting Medium (IM),

(20) .sub.1=field compensation error term observed at the sensor output for sensor external to Protected Volume (PV) but within Operational Volume (OV) defined as effective operating volume of compensation means,

(21) R=ratiometric compensation Laplace transform required for condition of .sub.o0,

(22) =ratiometric adjustment parameter employed for site-tuning purposes; its normalized value range is >1, with the effective range of this adjustment determined by the range of ratiometric difference that is inherent in the expression for S.sub.3 due to incorporation of the Interacting Medium (IM), and system transfer constants. This adjustment parameter has positive magnitude for |S.sub.2|>|S.sub.1|, negative magnitude for |S.sub.1|>|S.sub.2|.

(23) Compensation excitation required to achieve the condition of ideal interfering field cancellation or control accuracy can be determined in relation to the input error and the ratiometric terms by inspection and substitution for .sub.2 to establish the following equality:

(24) $\begin{array}{cc}\left({\mathrm{.Math.}}_1-R\right)S_4{T}_A=Y_c=\frac{{}_c}{k_2}& \left(6\right)\end{array}$
where:

(25) S.sub.4=signal transfer Laplace transform defined by controller processor (usually tailored to and non-variant for a specific ACR embodiment),

(26) T.sub.A=amplifier frequency-independent transfer characteristic (i.e., gain coefficient),

(27) Y.sub.c=compensation means excitation value,

(28) .sub.c=compensation means induction value,

(29) k.sub.2=compensation means transfer/conversion coefficient.

(30) Rearranging to express system main feedback loop parameters in terms of R, the supplemental loop signal processing function:

(31) $\begin{array}{cc}R={\mathrm{.Math.}}_1-\frac{{}_c}{S_4{T}_A{k}_2}& \left(7\right)\end{array}$

(32) For derivation of the optimum ratiometric compensation factor R, the adjustment parameter, , which normally provides an overall scaling factor primarily for convenience in matching an expected range of site conditions but can optionally include scaling of frequency-determining parameters, is set to unity and an equivalence expression incorporating the ratiometric adjustment Laplace transform S.sub.3 is introduced:

(33) $\begin{array}{cc}R={\mathrm{.Math.}}_1-\frac{{}_c}{S_4{T}_A{k}_2}=S_3{}_c\frac{k_1}{k_2}& \left(8\right)\end{array}$
where:

(34) S.sub.3=idealized transfer Laplace transform defined by the necessary frequency-dependent ratiometric function,

(35) k.sub.1=parameter extraction module (PEM) transfer coefficient.

(36) To complete the closed-loop analytic characterization, it is necessary to specify a sensor transfer coefficient:
.sub.1=(.sub.i.sub.c)k.sub.p(9)
where:

(37) k.sub.p=sensor transfer coefficient (e.g., V/G for magnetic probe).

(38) Substituting and rearranging:

(39) $\begin{array}{cc}{\mathrm{.Math.}}_1=\left({}_c\frac{S_2}{S_1}-{}_c\right)k_p={}_c{k}_p\left(\frac{S_2}{S_1}-1\right)& \left(10\right)\end{array}$

(40) The above expression may then be substituted into the previous expression for the idealized (i.e., .sub.o.sub.min0) ratiometric compensation factor R:

(41) $\begin{array}{cc}R={}_c{k}_p\left(\frac{S_2}{S_1}-1\right)-\frac{{}_c}{S_4{T}_A{k}_2}& \left(11\right)\end{array}$
Or, equivalently:

(42) 0$\begin{array}{cc}R={}_c{k}_p\left(\frac{S_2}{S_1}-1-\frac{1}{S_4{T}_A{k}_2{k}_p}\right)& \left(12\right)\end{array}$

(43) Substituting the equivalent expression (8), above, for R:

(44) $\begin{array}{cc}{}_c{k}_p\left(\frac{S_2}{S_1}-1-\frac{1}{S_4{T}_A{k}_2{k}_p}\right)=S_3{}_c\frac{k_1}{k_2}& \left(13\right)\end{array}$
which leads directly to a definition of the desired compensation factor, S.sub.3,

(45) $\begin{array}{cc}{S}_3=\left(\frac{S_2}{S_1}-1-\frac{1}{S_4{T}_A{k}_2{k}_p}\right)\frac{k_p{k}_2}{k_1}& \left(14\right)\end{array}$

(46) Noting that the term

(47) $\left(\frac{1}{S_4{T}_A{k}_2{k}_p}\right)0$
when T.sub.A>>1, and that in practice T.sub.A is typically in the range of 30-40, the equivalent expression for the desired compensation factor, S.sub.3 simplifies to:

(48) $\begin{array}{cc}{S}_3\left(\frac{S_2}{S_1}-1\right)\frac{k_p{k}_2}{k_1}& \left(15\right)\end{array}$
where the primary term,

(49) $\left(\frac{S_2}{S_1}-1\right),$
is a Laplace transform expression describing the required compensation with regard to the difference between the transfer functions into the PV; and the secondary term,

(50) $\frac{k_p{k}_2}{k_1},$
is a scalar expression describing the magnitude of the required compensation with regard to the scaling coefficients of the sensor probe, the compensation means transfer/conversion coefficient and the parameter extraction module (PEM) transfer coefficient. Equation 15 describes the essence of the invention as employed for arbitrarily accurate compensation or control of physical processes influenced by fields extending through and affected by an Interacting Medium.

(51) Referring still to FIG. 2, the main, or outer loop of the ACR system is a canonic implementation of classical negative feedback loop that consists of the sensor, a signal processing unit applying optional corrective frequency-dependent transfer function S.sub.4, and power driver T.sub.A. A simple feedback loop of this type is typically employed in the prior art for Active Compensation Shielding, with the operative range of its frequency response appropriate for the field type (e.g., magnetic, electric, electromagnetic, acoustic) being compensated.

(52) In the ACR topology, a supplementary signal voltage representing the affecting feedback quantity, in the present example a compensating magnetic field, is developed by the parameter extraction module (PEM), modified by frequency-dependent transfer function S.sub.3 derived in Equation 15 and added as a scalar quantity to the error signal .sub.1 that is produced by the first summation node.

(53) The summation node producing error signal .sub.1 is in fact the field sensor, or a single channel of the field sensor in the case of a multi-axial system, that generates a signal equivalent to the superposition of the corresponding interfering field and compensating field components at the sensor location.

(54) By the addition of summation node 2, it is possible to relocate the point of minimum difference between the interfering field and the compensating field to a position arbitrarily removed from the sensing probe, assuming that an interfering field gradient condition exists, or that an IM exists that exerts a differential effect on the interfering and compensating fields.

(55) As previously discussed, the addition of summation node 2 also permits a frequency-dependent compensating term to be introduced, allowing the compensating field to exactly match and subtract from the interfering field in the case where an IM introduces a time-dependent differential between the two quantities.

(56) Before introduction as the supplementary additive term to summation node 2, it is necessary to modify the proportional PEM output voltage derived from the compensating means drive signal by the product of frequency domain transfer function S.sub.3 and scalar ratiometric adjustment parameter , which is in essence a fine tuning adjustment that assumes practical importance in typical site installations.

(57) As the principal modifying term, Laplace transform S.sub.3 may assume the form of a frequency-independent spatial offset, a frequency-dependent compensating term, or a combination of the two. For example, in the special case where a) the sensor must be located away from the desired PV location of maximum interfering field attenuation, b) an appreciable interfering field gradient exists, and c) no frequency-dependent differential exists between the interfering and compensating fields through the PV, then the term becomes a frequency-independent constant proportional to the product of field gradient and distance between the sensor probe and PV location of maximum interfering field attenuation.

(58) In the special case where the interfering field gradient is negligible but environmental alteration of the interfering and compensating fields exists due to environmental factors such as an IM or physically influential objects in or near the OV that interact differently with the interfering and compensating fields, proper selection of the S.sub.3 order and coefficients prevent residual field within the PV from exhibiting significantly varying magnitude response to the range of frequency spectra that constitute the time-varying interfering field.

(59) An active-compensation system of the prior art lacking this frequency-dependent compensation scheme could only adequately compensate two distinct spectra simultaneously present in the interfering field if the spectral transfer function of the compensating field were exactly that of the interfering field from sensor to PV. That is, in FIG. 2, compensation would occur if S.sub.1=kS.sub.2, where k is an arbitrary constant corresponding to the field gradient and distance from probe to PV. However, if the physical environment near or between the sensor and PV includes an IM that physically interacts with the fields to produce a different frequency response with regard to the interfering and compensating constituents (e.g., a MRI magnet as previously discussed), then the terms S.sub.1 and S.sub.2 will vary differentially with frequency, conditions under which a conventional prior art ACS system is not capable of providing satisfactory cancellation.

(60) In the ACR system, the frequency-dependent information is coded in Laplace transform term S.sub.3 by the relationship defined in Equation 15, above, consequently modifying the compensation field response so that the interfering and compensating fields match in spectral content and thus temporally in the PV. Without this feature, good compensation can be obtained for one of the interfering field spectra but not simultaneously for both. In the general case where both an interfering field gradient and anisotropic IM may be present, the modifying term S.sub.3 provides a linear combination of gradient offset and frequency-dependent information, addressing the two orthogonal dimensions of the problem.

(61) Typical ACR magnetic ACS operating values for MRI service are:

(62) Operational frequency range: 0.001 to 1,000 Hz

(63) Open loop gain (k.sub.pS.sub.4T.sub.A):35

(64) Closed-loop PV (isocenter) field attenuation at optimum calibration: >46 dB (factor of 200)

(65) The fundamental ACR invention principle may also be implemented in the form of an anisotropically adaptive system that discriminates between or among extraneous field sources on the basis of directionality and gradient by means of a multiple sensor array for one or more orthogonal axes. The array would consist of at least two sensors for each implemented axis, thereby providing gradient information to the ACR system that, with appropriate processing, identifies the axial constituents of a vector describing the field source direction and gradient magnitude or, more generally, the composite vector describing a superposition of all axially-sampled field source directions and magnitudes. For an OV free of an Interacting Medium (IM), the constituent matrix elements of this vector are then employed as the basis of a computation that describes the drive signal required for each element of a single axial channel's compensating field sources. The net field produced by the compensating sources in each axis thus provides adaptive gradient compensation in the one or more active axes throughout the OV.

(66) In the case where an IM of significant influence exists, the aforementioned information describing the vector field source direction and gradient magnitude is utilized as the basis of a computation that includes a model of the IM with a description of the IM anisotropic interactive properties. The intermediate result of the computation is a matrix of constituent Laplace transform terms describing superimposed IM properties from the perspective of all active source directions. This result may then be utilized in a further computation to determine the effective values of axial Laplace transform terms S.sub.3 thereby providing unique frequency- and gradient-dependent compensation for fields from different source locations traversing an anisotropic IM within the OV.

(67) A further refinement of the basic ACR invention is inclusion of compensation for the effects of cross-axial coupling that may occur with ACR systems consisting of more than one operative axis. Such compensation may be provided in the ACR system by introducing a matrix describing the corrective axial cross-coupling terms into the compensation signal computational process, as is well known in the art shown, for example, in the aforementioned reference of Buschbeck, et al. Because the unwanted cross-coupling effects are caused by fixed environmental factors, for example nearby ferromagnetic objects in the case of magnetic fields or nearby high dielectric constant objects in the case of electric fields, or geometric misalignment of compensating sources and sensors, these effects do not change significantly over time. Importantly, they also do not change significantly with the magnitude or direction of interfering or compensating fields. For these reasons, the cross-coupling correction terms for a specific site or non-orthogonal compensation source geometry may be programmed into the ACR on a long-term basis. Determination of proper axial cross-coupling correction coefficients for the ACR system is made prior to system installation by OV survey data collection and analysis, or after system installation by inclusion of a calibration algorithm that operates on the basis of providing known excitation signals to compensation source elements while sampling axial sensor responses, as described below.

(68) A second refinement to the ACR invention is the inclusion of a calibration algorithm providing known excitation signals to compensation source elements while sampling axial sensor responses, the algorithm then calculating the value of the Excitation Transfer Coefficient (ETC) for each axial channel. An axial channel is one discrete signal processing signal path through the ACR feedback invention as illustrated in FIGS. 2 and 3, for each of one or more operative axes as defined by the required invention configuration.

(69) The term Excitation Transfer Coefficient (ETC) is specific to the present system. ETC is defined in that context as the ratio of the magnitude of the physical effect induced at or in the vicinity of the responsive sensor location to the magnitude of the compensating means input excitation. For magnetic fields, the ETC values have dimensions of Teslas per Ampere. Determination of ETC is important for system calibration, system-to-system comparative studies and improved estimation of materials requirements for installations. Axial magnetic field ETC measurements are performed by applying, sequentially, a calibrated coil excitation current to one set of axial coils at a time for determination of the corresponding ETC component. Post-installation ETC measurements provide a convenient and reliable method for identifying abnormal field situations due to installation errors or ferromagnetic materials in the site environment that can significantly affect system performance.

(70) The resultant values may be examined to determine that the specified ACR compensating signal sources are adequate for the size of the OV to be either protected or subject to an ACR-controlled field, and the maximum value of the field to be compensated or generated. A variant of this capability permits utilizing the excitation of a given single axis while observing the ETC induced in one or more orthogonal axes to determine site-specific axial cross-coupling coefficients.

(71) Control Configuration

(72) Parametric control of superimposed fields within a PV, removed from and/or shielded from the sensing means location by an intervening IM, can be accomplished with an adaption of the ACR invention as illustrated in FIG. 3. Here, field control rather than field compensation is the desired capability.

(73) To implement an external field control function requires two modifications of the FIG. 2 topology: a) the addition of a control port to summation node 2 with, optionally, frequency-dependent signal processing in the control signal path, and b) configuration of the compensation transfer function S.sub.3 order and coefficients for control functionality. Operation of the ACR loops remains as previously described for the general active compensation functionality. However, the supplementary ratiometric compensation loop transfer function now provides correction for IM interaction with, and modification of the controlled (or compensating) field rather than correcting for the differential between an interfering field and a compensating field.

(74) When term S.sub.3 is correctly defined for the system environment, control signal (t) applied at the control port will effect a proportionate change in the controlled field within the PV, with optimal frequency and linear group phase response tailored by S.sub.3 across the frequency range of the control passband, thus minimizing step function over- or undershoot, poor rise time, and other frequency-dependent control artifacts. The signal conditioning transfer function S.sub.5, optionally applied to the control input signal, permits an arbitrary frequency response to be achieved within the PV, independent of the correction transfer function S.sub.3.

(75) A hypothetical example application of the ACR control configuration would be control of magnetic, electric or electromagnetic fields affecting a core fusion reaction within a strongly ionized quasi-shielding plasma sheath.

(76) Since other modifications and changes varied to fit particular operating requirements and environments will be apparent to those skilled in the art, the invention is not considered limited to the example chosen for purposes of disclosure, and covers all changes and modifications which do not constitute departures from the true spirit and scope of this invention.

(77) Having thus described the invention, what is desired to be protected by Letters Patent is presented in the subsequently appended claims.