Construction of an apparatus for the measurement …[Enter report no.] Construction of an apparatus for the measurement of current induced in a long conductor by local and distant

[Enter report no.]

Construction of an apparatus for the measurement of current induced in a long conductor by local and distant transmitters Final Report

Dunn Engineering Inc. 1280 Old Innes Rd Ottawa, ON K1B 5M7 CSA: Scott Irvine, DRDC Suffield, 403-544-5325 The scientific or technical validity of this Contract Report is entirely the responsibility of the Contractor and the contents do not necessarily have the approval or endorsement of Defence R&D Canada.

Defence R&D Canada Contract Report DRDC Suffield CR 2012-072 April 2012

Defence Research and Recherche et développement Development Canada pour la défense Canada

Construction of an apparatus for the measurement of current induced in a long conductor by local and distant transmitters

Dunn Engineering Inc. Prepared By: Dunn Engineering Inc. 1280 Old Innes Rd Ottawa, ON K1B 5M7 Contractor's Document Number: 12RG01 PWGSC Contract Number: W7702-125297/001/EDM CSA: Scott Irvine, DRDC Suffield, 403-544-5235 The scientific or technical validity of this Contract Report is entirely the responsibility of the Contractor and the contents do not necessarily have the approval or endorsement of Defence R&D Canada.

Defence R&D Canada – Suffield Contract Report DRDC Suffield CR 2012-072 April 2012

© Her Majesty the Queen in Right of Canada, as represented by the Minister of National Defence, 2012

© Sa Majesté la Reine (en droit du Canada), telle que représentée par le ministre de la Défense nationale, 2012

DRDC Suffield CR 2012-072 i

Abstract ……..

The purpose of this study was to design and construct an apparatus to measure the current induced in a long, buried wire by local or distant transmitters. To this end, a brief literature survey was performed to assess state-of-the-art techniques for measurement of local magnetic field, which was used to infer the current present in a buried conductor at a known depth. It was found that the ferrite coil is the most suitable to achieve the task in the frequency range from 300 kHz to 5 MHz. Several ferrite coils were constructed in order to optimize performance within this frequency range. Details are also provided regarding signal conditioning electronics. Finally, a description of the device operation is provided.

The document is comprised of two separate sub-reports. The first consists of sections addressing background survey and initial testing; apparatus design; and apparatus construction, with the second describes the experimental apparatus design and the preliminary test results.

Résumé ….....

La présente étude vise à concevoir et à fabriquer un appareil pour mesurer le courant induit dans un long fil enfoui par des émetteurs locaux ou distants. À cette fin, on a effectué une recherche documentaire rapide pour évaluer des techniques ultramodernes de mesure du champ magnétique local, qui ont été utilisées pour déduire le courant présent dans un conducteur enfoui à une profondeur connue. On a constaté que la bobine à noyau ferromagnétique est la mieux adaptée à la tâche dans la gamme de fréquences de 300 kHz à 5 MHz. Plusieurs bobines à cœur ferromagnétique ont été fabriquées en vue d’optimiser les performances dans cette gamme de fréquences. Le document contient aussi des informations sur les circuits électroniques pour le conditionnement du signal. Enfin, il fournit une description du fonctionnement du dispositif.

Le document comprend deux sous-rapports distincts. Le premier porte sur les tests initiaux et les levés de base, la conception de l’appareil et la fabrication de ce dernier. Le second décrit la conception de l’appareil expérimental et les résultats préliminaires des essais.

ii DRDC Suffield CR 2012-072

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DRDC Suffield CR 2012-072 iii

Executive summary

Construction of an apparatus for the measurement of current induced in a long conductor by local and distant transmitters: Final Report

Dunn Engineering Inc.; DRDC Suffield CR 2012-072; Defence R&D Canada – Suffield; April 2012.

Introduction or background: Improvised explosive devices (IEDs) have been a weapon of choice in recent asymmetric conflicts, with large numbers having been detonated using long command wires. While the command wire provides a large stand-off for triggering such devices, it also provides a weakness that can be exploited for IED detection and location.

The general problem of finding long subsurface conducting objects is also encountered in commercial industry, where underground infrastructure must be mapped for safety in construction and excavation. Detection and location is typically accomplished by using radio frequency (RF) signals to induce electric current in the object of interest and measuring the resulting secondary electromagnetic field with a receiver. Although this is a well-established technology, most research to date has been devoted to either theoretical studies or specific detection technologies. Very few experimental measurements of the current induced in the wire have been performed. Such experiments are critical, not only to provide benchmarking for the aforementioned theoretical studies, but also for providing insight into the underlying physical mechanisms for current induction, with the end goal of maximizing detection capability and fielding improved detectors.

Results: A magnetic field sensor for the measurement of current induced in a wire, by both local and distant transmitters, has been designed and constructed. The contractor determined that a ferrite wound coil was the best choice among available magnetic fields sensors for meeting sensitivity and bandwidth requirements. Details of the optimization of the detector coil, a brief description of the signal conditioning electronics, and a description of the operation and usage of the current sensor have been provided.

Significance: A better understanding of the physics underlying the detection of buried wires will lead to enhanced Canadian Forces IED detection and route clearance capabilities, whether through better assessment of the true capabilities of existing systems under consideration for procurement or through development of new detection systems.

Future plans: A series of experiments utilizing the current sensor are planned. These will include an in-depth analysis of the induction of current in a subsurface wire and assessment of the effects of environmental parameters such as soil moisture content, burial depth, and position of the transmitter inducing the current.

iv DRDC Suffield CR 2012-072

Sommaire .....

Construction of an apparatus for the measurement of current induced in a long conductor by local and distant transmitters: Final Report

Dunn Engineering Inc. ; DRDC Suffield CR 2012-072 ; R & D pour la défense Canada – Suffield; avril 2012.

Introduction : Les dispositifs explosifs de circonstance (IED) ont été des armes de choix au cours des récents conflits asymétriques, et un grand nombre de ceux-là ont été détonnés au moyen de longs fils de commande. Bien que le fil de commande permette de déclencher ces dispositifs à grande distance, il constitue aussi une faiblesse qui peut être exploitée pour détecter et localiser des IED.

L’industrie commerciale connaît aussi le problème général associé au repérage d’objets conducteurs longs souterrains, dans lequel une infrastructure souterraine doit être cartographiée en vue d’assurer la sécurité des travaux de construction et d’excavation. La détection et la localisation sont en général effectuées au moyen de signaux de radiofréquence (RF) qui permettent d’induire un courant électrique dans l’objet d’intérêt et de mesurer le champ électromagnétique secondaire résultant avec un récepteur. Bien que cette technologie soit éprouvée, jusqu’à présent, la plupart des recherches ont porté soit sur des études théoriques, soit sur des technologies particulières de détection. Très peu de mesures expérimentales du courant induit dans le fil ont été effectuées. De telles expériences sont essentielles, non seulement pour fournir une évaluation comparative des études théoriques susmentionnées, mais aussi pour donner un aperçu des mécanismes physiques sous-jacents relatifs à l’induction de courant, visant ultimement à maximiser la capacité de détection et à mettre en service des détecteurs améliorés.

Résultats : Un capteur de champ magnétique pour mesurer le courant induit dans un fil, par des émetteurs tant locaux que distants, a été conçu et construit. L’entrepreneur a déterminé qu’une bobine à cœur ferromagnétique constitue le meilleur choix parmi les capteurs de champs magnétiques disponibles compte tenu de la sensibilité et de la largeur de bande nécessaires. De plus, le rapport contient des renseignements sur l’optimisation de la bobine du détecteur, une courte description des circuits électroniques pour le conditionnement du signal et une description du fonctionnement et de l’utilisation du capteur de courant.

Portée : Une meilleure compréhension des principes physiques sous-jacents à la détection des fils enfouis entraînera l’amélioration, pour les Forces canadiennes, de la détection des IED et des capacités de dépollution le long de l’itinéraire, que ce soit par l’évaluation améliorée des capacités réelles des systèmes existants à l’étude aux fins d’approvisionnement ou par le développement de nouveaux systèmes de détection.

Recherches futures : Une série d’expériences utilisant des capteurs de courant sont prévues. Elles comprennent l’analyse approfondie de l’induction du courant dans un fil souterrain et l’évaluation des effets des paramètres environnementaux, comme le pourcentage d’humidité du sol, la profondeur d’enfouissement et la position de l’émetteur qui induit le courant.

DRDC Suffield CR 2012-072 v

Table of contents

Abstract …….. ................................................................................................................................. i Résumé …..... ................................................................................................................................... i Executive summary ........................................................................................................................ iii Sommaire ..... .................................................................................................................................. iv Table of contents ........................................................................................................................... v List of figures ................................................................................................................................ vii List of tables ................................................................................................................................. viii 1 Current sensor report 1 ............................................................................................................. 1

1.1 Introduction ................................................................................................................... 1 1.1.1 Purpose ................................................................................................................ 1 1.1.2 Scope ................................................................................................................... 1 1.1.3 Applicable documents, codes and standards ........................................................ 1

1.2 Background survey ........................................................................................................ 2 1.2.1 System modelling and estimation of magnetic field strength .............................. 2 1.2.2 Methods of measuring magnetic field strength .................................................... 2

1.2.2.1 Anisotropic magnetoresistance (AMR) ..................................................... 3 1.2.2.2 Giant magnotoresistance (GMR) ............................................................... 3 1.2.2.3 Proton precession magnetometer (PPM) .................................................... 3 1.2.2.4 Flux gate magnetometer ............................................................................. 3 1.2.2.5 Superconducting Quantum Interference Device (SQID) ........................... 4 1.2.2.6 Hall effect................................................................................................... 4 1.2.2.7 Induction coil ............................................................................................. 4

1.2.3 Selected method ................................................................................................... 5 1.3 Ferrite coil sensor theory of design ............................................................................... 6

1.3.1 The transfer function ............................................................................................ 6 1.3.2 Constants and sensor parameters ....................................................................... 10 1.3.3 Design considerations ........................................................................................ 11

1.3.3.1 Resonant frequency .................................................................................. 11 1.3.3.2 Inductance ................................................................................................ 11 1.3.3.3 Capacitance .............................................................................................. 12 1.3.3.4 Terminating resistance and sensitivity ..................................................... 13 1.3.3.5 Optimization considerations .................................................................... 14

1.4 Test sensor design and evaluation ............................................................................... 15 1.4.1 Introduction........................................................................................................ 15 1.4.2 Test sensor #1 .................................................................................................... 16 1.4.3 Test sensor #2 .................................................................................................... 19 1.4.4 Test sensor #3 .................................................................................................... 21

vi DRDC Suffield CR 2012-072

1.4.5 Conclusion ......................................................................................................... 23 1.5 Proposed apparatus design .......................................................................................... 24

1.5.1 Overview and operation ..................................................................................... 24 1.5.2 Sensor coil design .............................................................................................. 24 1.5.3 Signal conditioning electronics .......................................................................... 26

1.5.3.1 Sensor wand mounted signal conditioning .............................................. 26 1.5.3.2 Electronic enclosure ................................................................................. 27

1.5.4 Data display ....................................................................................................... 27 2 Current sensor report 2 ........................................................................................................... 28

2.1 Introduction ................................................................................................................. 28 2.1.1 Purpose .............................................................................................................. 28 2.1.2 Scope ................................................................................................................. 28 2.1.3 Applicable documents, codes and standards ...................................................... 28

2.2 Experimental apparatus ............................................................................................... 29 2.2.1 Overview............................................................................................................ 29 2.2.2 Sensor coil design .............................................................................................. 30 2.2.3 Signal conditioning electronics .......................................................................... 31 2.2.4 Data display ....................................................................................................... 32 2.2.5 Operation ........................................................................................................... 33

2.3 Preliminary results ....................................................................................................... 35 2.3.1 Sensitivity .......................................................................................................... 35 2.3.2 Current sensor calculator utility ......................................................................... 36 2.3.3 Verification ........................................................................................................ 36

Annex 1 A- The B-field in sensor cross section ............................................................................ 38 Annex 1 B– Sensor orientation ...................................................................................................... 40 Annex 2A - Preliminary test data .................................................................................................. 41

DRDC Suffield CR 2012-072 vii

List of figures Figure 1 : Summary of magnetometer types. .................................................................................. 5

Figure 2: Field geometry. ............................................................................................................... 9

Figure 3: Table of relevant constants and main sensor parameters. ............................................. 10

Figure 4: Coil layout. .................................................................................................................... 11

Figure 5: Equivalent circuit of sensor and terminating resistance. ............................................... 13

Figure 6: Sensitivity as a function of frequency and terminating resistance. ............................... 14

Figure 7: General Arrangement of Test Bed. ............................................................................... 16

Figure 8: Preliminary test sensor, Test Sensor # 1,using R-050750 ferrite core. ......................... 17

Figure 9: Test sensor # 1 output using R-050750 core, single coil section design. ...................... 18

Figure 10: Test sensor # 1 output with distance from conductor. Separation through air.Ro =1 k . ............................................................................................................................ 18

Figure 11: Test sensor # 1 output with 1 MHz current in conductor. Sensor to conductor distance is 5.5 cm. ....................................................................................................... 19

Figure 12: Test sensor # 2 schematic using R-050750 ferrite core. ............................................. 20

Figure 13: Test sensor # 2 output with frequency. ....................................................................... 20

Figure 14: Test sensor # 2 output voltage with current at 1 MHz. ............................................... 21

Figure 15: Test sensor # 3 schematic............................................................................................ 22

Figure 16: Test sensor # 3 output with frequency. ....................................................................... 22

Figure 17: Test sensor # 3 output with current. ............................................................................ 23

Figure 18: Recommended Sensor design schematic. ................................................................... 25

Figure 19: Graph of sensor output at 2 MHz. .............................................................................. 26

Figure 20: Data display. ............................................................................................................... 27

Figure 21: Experimental apparatus. .............................................................................................. 29

Figure 22: Experimental apparatus in carrying case. ................................................................... 30

Figure 23: Sensor design schematic. ............................................................................................ 30

Figure 24: Wand mounted signal conditioning schematic. .......................................................... 31

Figure 25: Sensor electronic enclosure schematic. ....................................................................... 32

Figure 26: Typical display. ........................................................................................................... 33

Figure 27: Graph of sensitivity at 2 MHz. .................................................................................... 35

Figure 28: Current sensor calculator screenshot. .......................................................................... 36

Figure 29: Field through cross section of sensor. ........................................................................ 38

Figure 30: Ratio of average and exact cross sectional sensor flux. ............................................. 39

viii DRDC Suffield CR 2012-072

List of tables

Table 1: PC current sensor calculator utility. ............................................................................... 37

DRDC Suffield CR 2012-072 1

1 Current sensor report 1

1.1 Introduction

1.1.1 Purpose The purpose of the project is the construction of an apparatus for the measurement of current induced in a long conductor by local and distant transmitters.

This is the final part of the first of two reports.

Part I - Background Survey; Part II – Sensor Design Theory and Test Sensor Design and Evaluation; and Part III - Apparatus Design.

The second report is to follow which will include:

experimental apparatus; and preliminary results.

1.1.2 Scope It is required to develop an apparatus to measure the local current induced in long buried conductors by local and distant transmitters.

The buried conductors are wire (of various geometries) and of ~75m in length. They are buried in various soils up to 0.1m in depth. The RF Transmitter has a frequency range of 0.3 to 5MHz. The typical induced current range is:

@ 400kHz: 0.01 to 0.1mA

@ 4 MHz : 0.1 mA to 1.0mA.

It is further required to calibrate the apparatus using a surface laid wire and current sensor.

The apparatus must be portable and suitable for field use.

Although a rugged, reliable, and repeatable apparatus is required, it is for experimental purposes and the designed choices (cost vs. engineering effort, etc.) should reflect this.

1.1.3 Applicable documents, codes and standards

The applicable documents are:

Contract No. W7702-125297/001/EDM

2 DRDC Suffield CR 2012-072

1.2 Background survey

1.2.1 System modelling and estimation of magnetic field strength As required, the system being modeled is a long ( 75 m), cable (18 AWG, copper) buried in up to

10 cm of various types of soil. The magnetic field B, a distance R around a current carrying wire is given by:

= 4 (1)

The relative magnetic permeability for most soils, defined as = / ranges from =1.00001 1.0011, therefore it is common to assume most soils have a permeability equal to that of free space, = 4 10 / . Only in extreme cases (such as in bedrock or iron mining areas) will deviate significantly from unity.

The minimum and maximum magnetic fields given by the current range 0.01 1 mA at a distance R =0.1 m from the wire will therefore be:

= 20 (2)

= 2 (3)

For comparison, the background from earth’s magnetic field at 50 latitude is 58 , so the secondary field will be very weak requiring a very sensitive detector.

It is assumed that the primary field will be applied in such a way to minimize interference with the secondary field and sensor. This may be achieved through distance or driving the secondary current from the end of the wire. For this reason we will ignore the presumed negligible effects of the primary field.

1.2.2 Methods of measuring magnetic field strength There are six types of magnetic field sensor:

Anisotropic Magnetoresistance (AMR); Giant Magnetoresistance (GMR); Proton Precession Magnetometer (PPM); Flux Gate Magnetometer; Superconducting Quantum Interference Device (SQUID); Hall Effect; and Coil.

1 James H. Scott. Electrical and magnetic properties of rock and soil. United States Department of the Interior GeologicalSurvey,1996. 2Albrecht Jander, Carl Smith, Robert Schneider, Owen Hall. Magnetoresistive sensors for non-destructive evaluation.


1.2.2.1 Anisotropic magnetoresistance (AMR) Some ferromagnetic materials display a change in resistance (2-3%) in a direction perpendicular to an applied magnetic field. These sensors are usually made of nickel-iron thin films deposited on a silicon wafer. The response to field frequency is very fast, not being limited by coil inductance or oscillating frequencies (DC-5 MHz range). The detectable field range is 1-500 nT.

AMR devices are a common and well developed technology with several commercial examples available2. While the range and bandwidth of AMR sensors appear appropriate for the application, it is not clear from the literature at this point if they can be built with adequate sensitivity at high frequency. All reports found on the performance of existing AMR devices omitted performance details above 10 kHz. There is little scope for improving their performance beyond what is already achieved345, therefore if all current AMR sensors are inadequate, there is unlikely one could be created for this application.

Beyond these performance issues, fabrication of these devices often involves thin film deposition or lithography, which is impractical for this apparatus.

1.2.2.2 Giant magnotoresistance (GMR) GRM uses a similar principle to AMR except it exploits quantum effects between thin, ferromagnetic multi-layers to increase the change in resistivity in the presence of a magnetic field to 10-20%. These sensors can be used in hard drives and other applications using very small scale magnetic domains as well as macroscopic applications. The advantage of GMR is that it can be used to detect very strong fields, on the order of Teslas, with very high resolution, but cannot detect very weak fields67.

1.2.2.3 Proton precession magnetometer (PPM)

A direct current flowing in a solenoid around a hydrogen rich fluid (kerosene, water, etc.) causes the magnetic moments of some of the protons to align with the applied field. Once the solenoid’s field is removed, the protons with their aligned magnetic fields precess at a frequency which is directly proportional to the remaining primary field to be detected8. The bandwidth is very narrow and restricted to the low frequency range, DC-2 Hz.

1.2.2.4 Flux gate magnetometer Best for DC or low frequency field measurements. The basis of operation is sending an alternating current around a magnetic core, where an outside field will strengthen the flux in one direction through the core and coil by biasing the domains in the permeable core. This bias will create an 3 Steven A. Macintyre. Magnetic field measurement.Technical report, Macintyre Electronic Design, 2000. 4S. Andreev and P.Dimitrov. Anisotropic-magnetoresistance integrated sensors. Journal of Optoelectronics and Advanced Materials, 2005. 5C.Y.Tan, X.Chen, Y.G.Mal, Y.Liu, Z.W. Liu, and C.K. Ong. Ferromagnetic resonance frequency tuning of FeTaN thin films by strips patterning with angular displacements. Journal of Physics D, 2007. 6L.Perez, C.Dolabdjian, W. Wach, and L.Butin. Advances in magnetoresistance magnetometer performances applied in eddy current sensor arrays. 7Carl H. Smith and Robert W. Schneider.Low-field magnetic sensing with GMR sensors. 8Short review of nuclear precession scalar magnetometers.Technicalreport,GEMSystems,Inc.


asymmetry in a secondary AC voltage measured in a passive coil surrounding the primary coil and core. Measuring this voltage gives a measure of the external field910. A commercially available example of such a device is here, note the low maximum frequency. A general limit of 10 kHz in the drive frequency creates an upper limit on the detectable field frequency of 1 kHz, which is unsuitably low.

1.2.2.5 Superconducting Quantum Interference Device (SQID) A sensitive, ultra-low field detector capable of detecting a single quanta of magnetic flux. The device is based on superconducting loops containing Josephson junctions, which allow quantum tunnelling of electron pairs. Magnetic flux from an external field will bias this tunnelling current, creating a measurable voltage that is related to the external field strength.

While extremely sensitive, the frequency range SQUIDs can detect is only DC-5 Hz. Furthermore, the superconducting ring requires cryogenic cooling below the superconducting transition temperature, making the SQUID a cumbersome and expensive magnetometer11.

1.2.2.6 Hall effect Hall Effect devices use the Lorentz force on charge carriers in a semiconductor to detect magnetic fields. The force on a moving charge in a magnetic field B is given by = ( × ). In a Hall Effect device, a voltage is applied longitudinally to a thin slab of semiconductor to create a current. If a magnetic field is present parallel to height of the thin slab, the charge carriers will move to the top or bottom of the slab as they travel the length. This creates a new voltage, called the Hall voltage, across the top and bottom of the slab proportional to the applied field.

Although very simple in principle, Hall Effect devices are not an ideal option where a high level of sensitivity is required12. The Hall Effect is a small effect13, and the difficulties with high precision/frequency are caused by second order and parasitic effects, such as non-linearity of the sensitivity, material imperfections in the conducting strip, noise, and piezoresistive effects.

The field range for a Hall effect device is mT-T with a broad frequency range from DC-MHz14. The lack of sensitivity found in these devices make them inappropriate for the current sensing requirements of this apparatus.

1.2.2.7 Induction coil Induction coil sensors come in two main varieties: air coil sensors and ferromagnetic core sensors. Both types operate on Faraday’s law of induction to produce a voltage in the coil when a magnetic flux is present inside the coil. A ferromagnetic core inside the coil will greatly magnify the flux

9 P. Ripka, F. Primdahl, O.V. Nielsen, J.R. Petersen, and A. Ranta. A.C. magnetic-field measurement using the fluxgate. Sensors and Actuators A: Physical,46(1-3):307{311,1995. 10M.v. KREUTZBRUCK, K. ALLWEINS,and C. HEIDEN. Fluxgate-magnetometer for the detection of deep lying defects. In 15th World Conference on Non-Destructive Testing 15-21 October 2000 in Rome PROCEEDINGS. 11Dr. Andreas P. Friedrich and HelmuthLemme.The universal current sensor. 12 R.S. Popovic. Hall-Effect Devices.Sensors and Actuators, 17, 39-53, 1998. 13W.J. Grubbs. Hall Effect Devices. The Bell System Technical Journal, May, 1959. 14J.E. Lenz. A Review of Magnetic Sensors. Proceedings of the IEEE, 78,6, 1990,


inside the coil by a factor up to tens of thousands, depending on the design, allowing such a sensor to obtain the same flux as a much larger air coil. There are some trade-offs to using a ferromagnetic core, such as reduced linearity in the response function of the sensor, added weight and frequency limitations.

An induction coil cannot measure a static field, but can detect fields over a range from Hz-GHz using special design. Using very large loops or a highly permeable core also makes possible the detection of extremely weak fields (~0.1 ).

The output voltage will be reasonably linear in response to field frequency, but above the resonant frequency of the circuit, this linearity will decrease. Above the resonant frequency, the coil’s self or parasitic capacitance will cause the output voltage to drop15,16. These sensor characteristics depend heavily on the coil geometry, and by careful design, a sensor can be built to minimize these negative effects while maximizing the performance in the areas needed.

1.2.3 Selected method The frequency and dynamic range of each method is summarized in the table below:

Magnetometer Bandwidth

(Hz)

Resolution

(nT)

Range

(T)

Anisotropic magnetoresistor DC - 10 10 10 10

Giant magnetoresistance DC - 10 1 10 10

Proton precession DC - 10 0.05 10 10

Fluxgate DC - 10 0.1 10 10

SQUID DC - 10 10 10 10

Hall Effect DC - 10 100 10 10

Induction Coil 10 10 Varies 10 10

Figure 1 : Summary of magnetometer types.

15SlawomirTumanski. Induction coil sensors – a review. Measurement Science and Technology, pages R31-R46, 2007. 16 A. Roux, O. LeContel, C. Coillot, A. Bouabdellah, B. de la Porte, D. Alison,S. Ruocco,and M.C. Vassal. The search coil magnetometer for THEMIS.


Given our requirements for frequency, portability, and sensitivity, the most viable sensor appears to be a ferrite core induction coil. The device properties are highly customizable through specific geometries and electronics, and the varied application of coil inductors is well documented. An appropriate cored coil sensor can probably be built to suite this specific application at a reasonable cost compared to all other sensor types.

An AMR device appears close to fulfilling the requirements as well, but does not convincingly possess the high frequency accuracy needed in this application. Furthermore, the technical difficulties in manufacturing a custom AMR make it significantly less feasible in comparison to a cored coil sensor.

It follows from the above that a ferrite coil sensor will be selected for this apparatus.

1.3 Ferrite coil sensor theory of design

1.3.1 The transfer function The voltage output of an induction coil sensor is given by its transfer function, which is depends upon many parameters of the coil. Designing a coil sensor to suit specific requirements needs careful consideration of this transfer function.

The most general transfer function for a conducting loop is given by

= / (1)

where = is the magnetic flux through the loop.

Assuming a sinusoidally oscillating magnetic field of the form

= sin( ) (2)

through a coil of diameter with turns, the maximum voltage generated each cycle can be written as =

= 0.5 (3)

To increase the sensitivity of the coil, a highly permeable core will be used. A magnetically permeable core will multiply the field strength inside the coil by a factor equal to the relative permeability of the core:

= (4)

Dropping the `max' subscripts, the output voltage becomes

= 0.5 (5)


To maximize flux through a coil while minimizing sensor size, many layers of coil windings may be used. In this case, = = ( + )/2, where and are the outer and inner diameter of the coil windings.

It is important to note that the apparent permeability of the core, , is not necessarily equal to the permeability of the core material, , at high frequency. This is due to a property of magnetic materials called the demagnetizing factor, , which is related to the tendency of magnetic domains to resist rapid changes in their orientation. The demagnification factor depends on the internal field in the material which in turn depends on the geometry of the core. This property will generally reduce the permeability of a material in a rapidly oscillating magnetic field.

The relative permeability of the core in terms of the demagnetizing factor is15,17

= ( ) (6)

For large values of , 1/ . However, cores suitable for high frequency applications tend to have lower values (: 100), therefore the full expression for will be used. For a core to operate well at high frequency, resistive losses due to eddy currents need to be minimized. This can be accomplished by embedding magnetic particles in a ceramic to form a ferrite, which is essentially an insulating material. This material will reduce losses at high frequency but will have reduced permeability compared to some other materials18.

The demagnetizing factor for cylindrical geometry, with core length is19

( ) (7)

Next, taking into account the number of turns, , for a single layer coil

= / (8)

and for a multi-layer coil15,

= ( )2

= ( ) (9)

where is the length of the core wrapped in wire (it will be shown that generally < ), is number of coil sections, is the packing factor of the winding ( 0.85), and is the diameter of wire. If there are multiple sections of winding on the core separated by a distance , we use as

17H.C.SeranandP.Fergeau.An Optimized Low-Frequency Three-Axis SearchCoil Magnetometerfor Space Research. Reviewof Scientic Instruments,2005. 18L.Dixon.Magnetic Core Properties,1994. 19J.A.Osborn.Demagnetizing Factorsofthe General Ellipsoid.PhysicalReview,1945.


the length of each coil section (= if one section), and for the number of winding sections such that = .

Thus the transfer function for a cylindrical, cored, multi layered sensor, in terms mostly related to the sensor geometry, is

= 0.5 ( ) (10)

The above equation assumes a homogeneous field throughout the length of the coil. The field from a wire along a line perpendicular to the axis of the wire will not be homogeneous, as the ends of the coil will effectively be farther away from the wire. To incorporate this effect into the design calculations, the average field along the length of coil will be used in place of .

Using the geometry laid out in figure 1 to calculate the average field along the length of the sensor, let

=

= d (11)

Using = cos

= cos (12)

and

cos = / (13)

= + (14)

gives a functional form for the average field along the length of a the sensor:

= 2 // 1+ d

= arctan (15)


Figure 2: Field geometry.

It should also be noted that in addition to the field varying along the length of the sensor, the field over any cross sectional area of the sensor is inhomogeneous as well, as the top of the sensor is further from the source than the bottom. For very large separations between source and sensor, the field becomes approximately homogeneous. For the geometries of interest in this application, it is shown in Appendix A that assuming a homogeneous field over the cross sectional area of the coil is an adequate approximation.

Finally we arrive at the transfer function for a multi-layer cylindrical sensor a distance above a current source, oriented perpendicular to the wire to maximize flux:

= 0.5 ( ) ( ) arctan (16)

which for a single layer, single section coil sensor reduces to

= 0.5 ( ) arctan (17)

These equations describe the open circuit voltage across the ends of the coil and will partly guide the sensor design by indicating whether or not an adequate voltage will be produced for for a given sensor geometry and field strength . Discussed below are several other factors which will play a role in the sensor design.


1.3.2 Constants and sensor parameters Below is a table of all relevant parameters and a reference schematic for general coil construction. The top section of the table contains the parameters which will be directly specified, the rest are constants or dependent variables.

Core length

Coil length

Inner diameter of coil

Outer diameter of coil

Number of coil sections

Length of coil sections

Spacing between coil sections ( )

Diameter of wire (including insulation)

Relative permeability of core material

Relative permittivity of section spacing material

Relative permittivity of wire insulation

Permeability of vacuum

Apparent permeability of core

Permittivity of vacuum

Number of turns

Number of wire layers in coil

Thickness between layers (2 × wire insulation thickness)

Wire packing factor

Effective area of multilayer coil

Figure 3: Table of relevant constants and main sensor parameters.


Figure 4: Coil layout.

1.3.3 Design considerations

1.3.3.1 Resonant frequency A crucial parameter of an induction coil is its resonant frequency, . This is the frequency of signal which will travel with the least impedance through the circuit. Above this frequency, the self capacitance of the coil will reduce the output voltage and create non-linear effects in the transfer function. Therefore is generally desired to be above the operating frequency range of the sensor. The resonant frequency is given by = (18)

where is the inductance of the coil and is the total capacitance of the coil. Obtaining a high enough resonant frequency will be a major consideration for this project. As can be inferred from the above formula, increasing the resonant frequency requires reducing the inductance and/or capacitance of the sensor. This usually entails reducing the geometry of the sensor, which can come at the cost of sensitivity. Therefore the resonant frequency, sensitivity, and geometry of the sensor must be carefully considered as interrelated parameters. Existing designs are able to achieve balance between these factors without difficulty if they operate in a low frequency regime20,21.

1.3.3.2 Inductance The inductance of a coil can be calculated from several different empirical formulae [7, 8], and it can be shown that the expressions quickly converge for realistic coil lengths.

The expression used to calculate inductance in this work will be15

= / (19)

20Y. Cheng, S. Wang, and M. Zhang. Research of Miniature Magnetic Coil Sensor used for Detecting Power Cables Under-ground. In Electrical and Control Engineering (ICECE), 2011 Interna-tional Conference on, pages 6065-6068, Sept.2011. 21P.Wang,P.Lewin,K.Goddard,andS.Swingler.Designand Testingofan Induction Coilfor Measuringthe MagneticFieldsof UndergroundPowercables.2010.


1.3.3.3 Capacitance Along with , may be estimated to give a measure of coil's resonant frequency. There are several sources of capacitance in a coil sensor, but the major contributors are coil self capacitance, coil-core capacitance, and section-section capacitance between groups of coil windings (if more than one)17. While section-section capacitance can be reduced with fewer sections at the expense of larger coil self capacitance, the overall coil capacitance will be greater if it is wound in one continuous section. For high frequency ( 1 MHz) and low field strength applications, a multi-section winding may be appropriate. However, spacing the coil sections may require layering the turns to maintain the number of turns and sensitivity. Having multiple layers of winding per section also increases the self capacitance, lowering the resonant frequency. This means that to obtain good performance at high frequency, a middle ground must be found between the capacitance effects of coil length, number of sections, number of layers, and wire thickness.

The estimation of coil capacitance is quite difficult, and relies on various equations based on empirical observation and theory, combined17,22,23. There appears to be no thorough theoretical model for the self capacitance of a solenoid coil.

Reasonably accurate capacitance values can be calculated for multi layer, multi section coils using the following [8], where is the self capacitance of one coil section, and is the capacitance for sections: = ( 1)( + ) (20)

= (21) The total inter-section capacitance, , can be calculated, but is much less than the self capacitance of each section, and can therefore be neglected17.

The coil to core capacitance is the last significant quantity in the capacitance, and is given by

= (22)

where is the coil to core capacitance of each section. No formula exists to calculate , but an approximate (order of magnitude) value can be found by calculating the capacitance of two concentric cylinders equal in length to the coil section,

= (23)

where the cylinder outer diameter is taken to be the average coil diameter.

Indicative of the lack of applicable theory and the inexact nature of calculating capacitance for a coil, these expressions do not apply to the case of a single layer coil ( = 1). Therefore, for a 22D.W.Knight.The Self-Resonanceand Self-Capacitanceof Solenoid Coils.Technical Report,2010. 23R.G.Medhurst.H.F. Resistanceand Self Capacitanceof Single-Layer Solenoids.WirelessEngineer,1947.


single layer coil, an expression originally derived by fitting empirical capacitance data and later refined will be used here22,23.

= 4 1 + 1 + (24)

where and are the relative permittivity external and internal to the coil, respectively, in metres, in Farads, and

= 0.717 + 0.933 / + 0.106 (25)

1.3.3.4 Terminating resistance and sensitivity So far, considerations for the output voltage of the sensor have applied only to the open circuit case for the coil. In practice, a measurement device must be connected to the coil, although its equivalent resistance may be very high to approximate the open circuit case. Figure 5 shows the equivalent circuit of the sensor and the terminating resistance, , where the output voltage will be measured24.

Figure 5: Equivalent circuit of sensor and terminating resistance. The output voltage of the sensor in this equivalent circuit, let us call it the operational sensitivity of the sensor, can be expressed in the form24

= ( ) ( ) (26)

24H.Ueda.Several ProblemsAbout Sensitivityand Frequency Responseof an Induction Magnetometer. Sci.Rep.TohokuUniv,Ser.5,Geophysics,1975.


where the absolute sensitivity is given by equation 17, = / , = / and = / . In full form this appears as

= . ( )( ) ( ) (27)

Figure 6 shows the effect of changing the terminating resistance for a representative test sensor. It can be seen that the greatest sensitivity near resonance is achieved when the terminating resistance is greatest. As the terminating resistance decreases, the frequency response flattens out at the expense of sensitivity.

Figure 6: Sensitivity as a function of frequency and terminating resistance.

1.3.3.5 Optimization considerations Regardless of the size of induction coil to be used, certain geometric relationships exist between parts of the sensor to optimize the output for any given size, and this section will address these considerations.

Coil length vs. core length:

The first optimization parameter to consider is the ratio of coil length to core length. Using tools such as magnetostatic solving software, it can be shown17 that the apparent permeability of a long rod is maximized in the middle of its length, while the permeability reaches almost unity (no gain over vacuum) at the ends. This justifies leaving a length of core at the ends unwrapped for several reasons. The ends of the core are the furthest away from the field source in this application, and combined with low permeability, wrapping here gives little relative gain. Additional turns increase


the length and therefore the capacitance of the coil, which will limit the maximum operational frequency of the coil. Minimizing turns also reduces weight.

Depending on design requirements such as size and weight, the recommended ratio of coil length to core length, / , is 0.4-0.915,17.

Outter diameter vs inner diameter:

In the case of a multilayer coil, the thickness of the coil may be considered to reduce weight, size and electronic noise. An optimal value for the inner diameter of the coil has been determined to be = 0.3 15, where the outter diameter, may be taken as large as is practical. The optimization presented by Lukoschus25 to minimize weight and signal noise, calculated for the general geometry considered here, is = 0.46 .

1.4 Test sensor design and evaluation

1.4.1 Introduction To verify the design theory given above and to refine the design of the Apparatus sensor several test sensors were built and evaluated,

Three different models of off-the-shelf, high permeability ferrite cores were purchased for the test sensors:

125 μr R-050750 ferrite core (7.5” Rod) from Bytemark (www.cwsbytemark.com)

r VFR8J ferrite core (8” Rod) from Stormwise (www.stormwise.com)

r VFR10J ferrite core(10” Rod) from Stormwise (www.stormwise.com)

For evaluation purposes a test bed was constructed. This consisted of:

Current Carrying Conductor – Belden 8522. Tinned copper 18 AWG (stranded 16x30). PVC insulation, 0.08” OD.

Current Source – A programmable signal generator (configured for sine wave output) was connected to the conductor and a series 1k shunt resistor. An oscilloscope was connected across the 1k shunt, effectively measuring current. The signal generator output voltage was adjusted to give the required conductor current.

Current Sensor – Ferrite core sensor of several designs (see below) built into a wand with terminating resistor and sensor amplifier.

Sensor Amplifier – Two stage, non-inverting op-amp (LT1222) with a gain of 100. The op-amps were compensated to roll-off at 17MHz. They were power by +/-12V, 0V bench

25DieterG.Lukoschus.Optimization Theoryfor Induction-Coil Magnetometersat Higher Frequencies. IEEETransactionsonGeoscienceElec-tronics,1979.


supply using a shielded (0V) twist pair (+/-12V) cable. The output of the amplifier was by coax cable to the Signal Detector.

Sensor Signal Detection – A Tektronix TDS 2014B 200MHz Oscilloscope in FFT spectrum mode was used as a detector. The FFT window was adjusted to be centered on the frequency of the conductor current.

The general test bed arrangement is shown below.

Figure 7: General Arrangement of Test Bed.

1.4.2 Test sensor #1

The first test sensor was constructed using a simple, one layer, one section coil design on the R-050750 ferrite core. Detailed specifications are shown in figure 8. A relatively low number of turns was used to ensure a resonant frequency reasonably positioned relative to the operational range, in this case calculated to be 2.89 MHz.


Figure 8: Preliminary test sensor, Test Sensor # 1,using R-050750 ferrite core.

The theoretical sensitivity of this sensor, given by equation 27 and using = 100 , is

( , , ) = . × . ×. . × . × (28)

Performance data for this sensor is shown in figures 9, 10, and 11. With such low terminating resistance the output is expected to be flat with frequency over the operating range. However, the data shows a distinct peak in the region of the calculated resonant frequency. The signal to noise ratio of this design was very low. With various terminating resistances, noise bursts would appear or the signal would cut off beyond certain frequencies, indicating this design would not be suitable for accurate measurements under these conditions (i.e. weak fields). However, this sensor does provide a proof of design principle in that it is sensitive to frequency, distance from the source, current, and the calculated and measured resonant frequency agree, as seen in the following figures.


Figure 9: Test sensor # 1 output using R-050750 core, single coil section design.

Figure 10: Test sensor # 1 output with distance from conductor. Separation through air. =1 .


Figure 11: Test sensor # 1 output with 1 MHz current in conductor. Sensor to conductor distance is 5.5 cm.

Proceeding from here we build and test a design using multiple coil sections and various cores of similar geometries but from different manufacturers. We expect to develop a test sensor with a higher resonant frequency, higher sensitivity, and improved signal to noise ratio.

1.4.3 Test sensor #2 Changes made in Test Sensor # 2 were to use thinner wire to allow more turns per unit length (AWG 22), and spacing the coil into three sections to reduce total self-capacitance to maintain or increase the resonant frequency. The same model of core was used as in Test Sensor # 1, the R-050750. See figure 12 for specifications.

The output across frequency rises sharply from 300 kHz to ~ 1.2 MHz, and beyond that oscillates and shows no strong dependence with frequency. It appears that the resonant frequency of this coil is actually near 1.2 MHz, far from the theoretical 20 MHz, and that the coil capacitance is not entirely behaving as three coils in series. This could be due to the spacing of the coil sections being inadequate for this effect to manifest. A future design may use spacing at least as large as the lengths of the adjacent coil sections.


Figure 12: Test sensor # 2 schematic using R-050750 ferrite core.

Figure 13: Test sensor # 2 output with frequency.


Figure 14: Test sensor # 2 output voltage with current at 1 MHz. Figure 14 shows that output is directly related to current, as expected. As well, the signal is quite strong and can be increased even further with a large terminating resistance.

1.4.4 Test sensor #3 This build shares the general characteristics of Test Sensor # 2, but is scaled onto a geometrically similar but higher permeability core (800 r vs 125 r), the VFR10J. See figure 15 for construction details.

Similar to the previous build, this sensor shows a resonant frequency at ~ 1.3 MHz, but with apparently greater senstivity across the frequency range of interest.


Figure 15: Test sensor # 3 schematic.

Figure 16: Test sensor # 3 output with frequency.


Figure 17: Test sensor # 3 output with current.

1.4.5 Conclusion The test sensors built so far were able to deliver the required sensitivities to a nearby current carrying conductor, with future work to focus on increasing the resonant frequency of the sensor past the operational ceiling of 5.0 MHz. The sensors were able to detect the change in distance from the conducting wire through the decreasing magnetic field strength. Similarly, they were also sensitive to changes in the amount of current flowing in the conductor at a fixed distance.

Test sensor # 1, using larger gauge wire than the other builds, demonstrated significant noise levels in certain frequency ranges. The larger gauge wire decreases the number of turns per unit length, and therefore the sensitivity of the coil. This comes with an expected gain in resonant frequency due to less stray capacitance in the coil. The coil did demonstrate the expected resonant frequency, with adequate voltage output, but the noise in the signal was at times too high to generate accurate measurements.

Test sensors # 2 and # 3 achieved the goal of a much improved output signal through use of smaller gauge wire and more turns, but failed to produce a higher resonant frequency than test sensor # 1 despite separation of the coil sections, as would be expected from capacitors in series. The resonant frequency dropped slightly due to the increased total coil length (and increased capacitance thereof), but not as much compared to a coil of equal length with no spacing. This indicates that the spacing likely had an effect, but not enough for the coil sections to completely behave as separate capacitors in series.


The final design will incorporate these features into a coil that will have a high signal to noise ratio and appropriate sensitivity in the operating range of currents, frequencies and distances required in this application.

Additionally, the results show that the sensor probe itself will not exceed the approximate size of the cores themeselves (~20 x 200 mm), which will make the final design highly portable.

1.5 Proposed apparatus design

1.5.1 Overview and operation The proposed apparatus will be comprised of:

Sensor Wand (handheld cylinder approx. 350mm long x 35mm dia.) with integral 2m cables;

Electronics Enclosure (approx. 150mm x 100mm x 30mm) with connectors;

PicoScope 4227 Precision USB Oscilloscope (100MHz, 250MS/s, 16-bits);

Laptop (supplied by others) with PicoScope software (Windows 7, with two USB 2.0 ports); and

Cables (BNC to BNC, 2 x USB)

The Sensor Wand is attached by two integral cables to the Electronics Enclosure. The Electronic Enclosure is connected by USB (for power only) to the laptop and also by BNC cable to one channel of the Pico Scope. The Pico Scope is attached by USB cable to the Laptop. The Laptop, running the Pico Scope software is used to measure signal amplitude.

To use the apparatus to measure the depth or current in a buried current carrying conductor the sensor wand is moved over the surface (while parallel to it) until a local peak signal amplitude is found. The Sensor Wand is than rotated (still parallel to the surface) until the orientation is found with the peak amplitude. The buried conductor should now be directly under the Sensor Wand and perpendicular to it. The buried conductor depth (assuming the RF frequency and current are known) or current (assuming the RF frequency and depth are known) can be calculated using the peak signal amplitude and a sensitivity equation.

1.5.2 Sensor coil design The recommended sensor design is shown in Figure 18. This design uses a single coil section to minimize noise and provide a clear signal at extremely low conductor currents while having a high resonant frequency to allow measurements over the entire frequency range.


Figure 18: Recommended Sensor design schematic.

It has been observed that efforts to raise the resonant frequency are qualitatively effective, but not quantitatively in agreement with theory. This once again highlights the shortcomings of current theoretical models for coil self-capacitance and how their behaviour is deviant from that of ideal capacitors. Therefore to raise the resonant frequency to 5.0 MHz or beyond, a more empirical approach to refining the sensor output has been taken.

The sensitivity of the sensor with a 500 terminating resistance is:

= 0.935 tan 0.0597 / × Where:

V is the sensor coil output in Vrms F is the frequency of the conductor current in Hz I is the conductor current in Arms R is the distance from the conductor to the center of the sensor in m.

This relationship has been empirically determined and differs from the earlier derived theoretical sensitivity equation. The sensor output at 2 MHz is shown in Figure 19.


Figure 19: Graph of sensor output at 2 MHz.

The maximum output of the sensor, before amplification is 9.9 mVrms from a 1.0 mArms current, 5.0 cm away at 5 MHz. The minimum sensor output is 3.2 Vrms from a 0.01 mArms current, 20.0 cm away at 300 kHz.

1.5.3 Signal conditioning electronics The proposed signal conditioning electronics are in two parts: Sensor Wand mounted and Electronic Enclosure mounted.

1.5.3.1 Sensor wand mounted signal conditioning The proposed circuit consists of a three op-amp, fully differential amplifier (instrumentation style) with a gain of 10 operating from +/- 12V. The input is connected to the sensor coil terminating resistor and the output is a balanced 50R line driver.

The power and signal cables are both shielded, twisted pairs. The power cable shield is 0V (USB power GND) and signal shield is not connected.


1.5.3.2 Electronic enclosure The proposed circuit consists of a balance differential receiver and a differential to signal ended converter (gain of 10). A 5V to +/-12V converter is included to power the circuit and the Sensor Wand.

1.5.4 Data display The signal conditioning electronics will feed one channel of the PicoScope.

The PicoScope software will be configured to FFT mode. This will be used to detect and measure peak signal output.

The expected display will be similar to that shown below. The output in dBu can be converted to mVrms.

Figure 20: Data display.

The expected sensitivity of the input to the scope in terms for frequency, depth and current is 100 times that given above for the sensor.


2 Current sensor report 2

2.1 Introduction

2.1.1 Purpose The purpose of the project is the construction of an apparatus for the measurement of current induced in a long conductor by local and distant transmitters. This is second report which includes:

Experimental Apparatus Preliminary Results

The first report was in three parts:

Part I - Background Survey Part II – Sensor Design Theory and Test Sensor Design and Evaluation Part III - Apparatus Design

2.1.2 Scope It is required to develop an apparatus to measure the local current induced in long buried conductors by local and distant transmitters.

The buried conductors are wire (of various geometries) and of ~75m in length. They are buried in various soils up to 0.1m in depth, The RF Transmitter has a frequency range of 0.3 to 5MHz. The typical induced current range is:

@ 400kHz: 0.01 to 0.1mA

@ 4 MHz : 0.1 mA to 1.0mA.

It is further required to calibrate the apparatus using a surface laid wire and current sensor.

The apparatus must be portable and suitable for field use.

Although a rugged, reliable, and repeatable apparatus is required, it is for experimental purposes and the designed choices (cost vs. engineering effort, etc.) should reflect this.

2.1.3 Applicable documents, codes and standards

The applicable documents are:

Contract No. W7702-125297/001/EDM

Current Sensor Report – Parts I, II and III, Rev 3.1.


2.2 Experimental apparatus

2.2.1 Overview The experimental apparatus is comprised of:

Sensor Wand (handheld cylinder approx. 400mm long x 32mm dia.) with integral 2m cables.

Electronics Enclosure (approx. 130mm x 105mm x 35mm) with connectors.

PicoScope 4227 Precision USB Oscilloscope (100MHz, 250MS/s, 16-bits)

Laptop (supplied by others) with PicoScope software (Windows 7, with two USB 2.0 ports)

Cables (BNC to BNC, 2 x USB)

Figure 1 shows the equipment connected and Figure 2 shows the equipment in its carrying case.

Figure 21: Experimental apparatus.


Figure 22: Experimental apparatus in carrying case.

2.2.2 Sensor coil design The sensor design is shown in Figure 3. The design uses a single coil section to minimize noise and provide a clear signal at extremely low conductor currents while having a high resonant frequency to allow measurements over the entire frequency range.

Figure 23: Sensor design schematic.


2.2.3 Signal conditioning electronics The signal conditioning electronics are in two parts: Sensor Wand mounted and Electronic Enclosure mounted.

2.2.3.1 Sensor wand mounted signal conditioning

The circuit consists of a three op-amp, fully differential amplifier (instrumentation style) with a gain of 10 operating from +/- 12V. The input is connected to the sensor coil terminating resistor and the output is a balanced 50R line driver.

The power and signal cables are both shielded, twisted pairs. The power cable shield is 0V (USB power GND) and signal shield is not connected.

The schematic is shown in Figure 4.

Figure 24: Wand mounted signal conditioning schematic.

2.2.3.2 Electronic enclosure mounted signal conditioning

The circuit consists of a balanced differential receiver and a differential to signal ended converter (gain of 20). A 5V to +/-12V converter is included to power the circuit and the Sensor Wand.


The schematic is shown in Figure 5.

Figure 25: Sensor electronic enclosure schematic.

2.2.4 Data display

The signal conditioning electronics feed channel A of the PicoScope.

If configured as described in the Operation section below, the PicoScope software is in spectrum analyser mode with signal amplitude given in dBV (where 0 dBV is 1 Vrms). The display includes all signals picked up by the current sensor, including AM radio stations. Only the peak of the frequency being used by the transmitter is relevant.


The display will be similar to that shown in figure 6 below.

Figure 26: Typical display.

2.2.5 Operation

The Sensor Wand is attached by two integral cables to the Electronics Enclosure. The Electronic Enclosure is connected by USB (for power only) to the laptop and also by BNC cable to one channel of the Pico Scope. The Pico Scope is attached by USB cable to the Laptop. The Laptop, running the Pico Scope software is used to measure signal amplitude.

The set-up instructions are:

Start the PicoScope application on the laptop.

Plug in the PicoScope USB cable when prompted.

Connect the Current Sensor probe cables, power and output, to the Electronic Enclosure.

Connect the Electronic Enclosure USB cable to the laptop. A green LED will illuminate when powered.

Connect the BNC cable from the amplifier box to the Picoscope channel A.


The PicoScope operating instructions are, from the application:

1. Select AC coupling and auto scaling for channel A.

2. Set the mode to Spectrum Analyzer.

3. Choose Spectrum Options:

Spectrum bins – 16384;

Window function - Flattop;

Display mode – Magnitude;

Scale – Log; and

Log unit – dBV.

Select the frequency range appropriate to the transmitter frequency.

Add measurements:

a. Frequency at peak; and

b. Amplitude at peak.

When making a measurement ensure that the frequency at the peak is the same as the transmitter frequency.

Refer to the application Help function for assistance in using PicoScope.

To use the apparatus to measure the current in a buried current carrying conductor of known depth, move the sensor wand over the surface (while parallel to it) until a local peak signal amplitude is found at the frequency of the transmitter. Then rotate the Sensor Wand (still parallel to the surface) until the orientation is found with the peak amplitude. The buried conductor should now be directly under the Sensor Wand and perpendicular to it. The buried conductor current can be calculated using the peak signal amplitude and a sensitivity equation (see preliminary results for the sensitivity equation).


2.3 Preliminary results

2.3.1 Sensitivity

Appendix A gives preliminary test data acquired using the experimental apparatus. Using curve fitting from the data the Sensitivity of the experimental apparatus was determined to be: = 489 arctan . exp ( . ) + (17.5 9) exp .

Where: V is the sensor coil output in Vrms f is the frequency of the conductor current in Hz I is the conductor current in Arms R is the distance from the conductor to the center of the sensor in m.

The measurement of the PicoScope is in dBV. To convert dBV to Vrms use: = 20 log 1.0 Figure 7 shows a three axis plot (Output verses current and depth) of the above sensitivity equation at 2MHz.

Figure 27: Graph of sensitivity at 2 MHz.


2.3.2 Current sensor calculator utility A PC utility has been provided to simplify the calculation of conductor current in mArms from the measured PicoScope output in dBV, the frequency in MHz and the depth in cm. Figure 8 below shows a screen shoot of the DRDC Current Sensor Calculator.

Figure 28: Current sensor calculator screenshot.

2.3.3 Verification To verify the sensitivity equation, the output for 13 different frequency, current and depth points where recorded. The results are given in the table below as well as the calculated current (for the PC Current Sensor Calculator Utility) and the error between the calculated and actual current. This verification was done using the test setup described in the first report.


Table 1: PC current sensor calculator utility.

Distance (cm)

Frequency (MHz)

Current (mArms)

Output (dBV)

Calculated Current (mArms)

Current Error

(mArms)

5 0.4 0.020 -46.9 0.026 0.006

10 0.6 0.050 -42.6 0.061 0.011

15 0.8 1.000 -19.2 1.136 0.136

6 1.0 0.050 -38.5 0.052 0.002

9 1.2 0.100 -34.1 0.105 0.005

12 1.5 0.200 -29.1 0.207 0.007

7 2.0 0.050 -36.1 0.051 0.001

11 2.2 0.500 -19.1 0.498 -0.002

14 2.6 0.100 -34.1 0.105 0.005

8 3.0 0.400 -16.8 0.487 0.087

13 3.5 0.150 -28.3 0.209 0.059

10 4.0 0.900 -13.2 0.956 0.056

14 4.0 0.300 -25.5 0.311 0.011


Annex 1 A- The B-field in sensor cross section

Here it will be shown that using the field at the center of the sensor cross section to calculate the total flux over the whole cross section is a valid approximation for simplifying calculations. This will be shown using square sensor cross section of width L, which greatly simplifies the integrals compared to circular cross section and gives very similar results. We will compare the flux, , through a cross section of the sensor assuming the field throughout is equal to that of the field at the centre, to the actual flux, , integrated over the cross section.

is simply = = (1) Referring to figure 18, the exact, integrated flux is = 2 1 d = ln // (2) where = + has been used to change the initial integral, and again to remove from the final form. The ratio of these expressions, shown in figure 19, shows convergence for large enough values of / , or when the distance to the source is large compared to the characteristic width of the sensor. The present design will be working in the region / = 6.3.

Figure 29: Field through cross section of sensor.


Figure 30: Ratio of average and exact cross sectional sensor flux.

Therefore, to simplify calculations and analysis, an expression for flux of the form 1 will be used.


Annex 1 B– Sensor orientation

It was observed that each test sensor displayed some degree of output regardless of orientation with respect to the conductor. The highest output is expected and observed to be when the long axis of the sensor is perpendicular to the conductor, where the field lines will pass at right angles through the area of the coils, maximizing flux.

The output diminishes as the sensor is rotated parallel to the conductor, but the flux does not drop to zero. When the sensor is parallel to the conductor, and the magnetic field lines from the conductor are parallel to the plane of each coil, there is no direct flux passing through the area of the coils.

This output even at parallel orientation can be explained by viewing the behaviour of the core as an ensemble of individual, microscopic magnetic moments who's directions are oscillating 180o because of fluctuations in the external magnetic field. On average, there will be no preferred path or arc for the moment vectors to follow between initial and final orientations. Therefore, regardless of how a coil, or any circle with the moment vector anchored in the center, is oriented, the flux through the area of the coil will actually be the same. The reason why the sensor is then sensitive to direction at all is due to the permeability of the core being a function of orientation.

Via analysis of the demagnetizing factors of various shapes, it is well known that the permeability of a cylinder is maximized along its long axis, which was the initial motivating choice for a cylindrical core. Using the demagnetizing factors of a cylinder to derive the permeability, , in a direction parallel to one of its short axes, it was found that = 61.5

where is the permeability along the long axis.

Taking this into account, along with that when the sensor is parallel to the conductor, the ends of the sensor will be closer to the conductor than in the perpendicular case, the theoretical output ratio reduces to ( )( ) = 52.5

The actual sensors displayed an even lower output ratio of 3.5 – 5.7. This is likely caused by the ferrite not being a continuous magnetic material, chosen to reduce losses at high frequencies, but which may not possess internal demagnetization to the same degree as a pure, continuous substance.

The low ratio between parallel and perpendicular outputs means that regardless of orientation, the sensor will detect the wire. This can certainly be seen as an advantage in this sensor application where initially locating a buried wire of unknown orientation may be necessary.


Annex 2A - Preliminary test data

Test Data Set #1 - Distance 10cm, Current 1.0 mArms FREQUENCY AMPLITUDE FREQUENCY AMPLITUDE

kHz dBV kHz dBV 300 -17.6 2700 -10.6 400 -17.4 2800 -10.5 500 -17.2 2900 -10.5 600 -16.9 3000 -10.6 700 -16.7 3100 -10.7 800 -16.3 3200 -10.9 900 -16.0 3300 -11.1

1000 -15.6 3400 -10.9 1100 -15.3 3500 -10.7 1200 -15.0 3600 -10.8 1300 -14.7 3700 -11.1 1400 -14.3 3800 -11.6 1500 -13.9 3900 -12.1 1600 -13.5 4000 -12.5 1700 -13.2 4100 -12.6 1800 -12.9 4200 -11.6 1900 -12.7 4300 -9.6 2000 -12.6 4400 -7.2 2100 -12.3 4500 -4.6 2200 -11.6 4600 -2 2300 -11.2 4700 0.5 2400 -10.9 4800 2.9 2500 -10.7 4900 5.2 2600 -10.6 5000 7.6


Annex 2A continued….. Test Data Set #2 - Distance 10cm, Current 0.1mArms

FREQUENCY AMPLITUDE FREQUENCY AMPLITUDE kHz dBV kHz dBV 300 -37.7 2700 -30.3 400 -37.6 2800 -30.3 500 -37.4 2900 -30.3 600 -37.0 3000 -30.4 700 -36.7 3100 -30.6 800 -36.4 3200 -30.8 900 -36.1 3300 -31.0

1000 -35.7 3400 -30.7 1100 -35.3 3500 -30.5 1200 -35.0 3600 -30.7 1300 -34.6 3700 -31.0 1400 -34.2 3800 -31.5 1500 -33.8 3900 -32.1 1600 -33.5 4000 -33.3 1700 -33.2 4100 -32.3 1800 -32.8 4200 -31.4 1900 -32.6 4300 -29.5 2000 -32.4 4400 -27.0 2100 -32.1 4500 -24.4 2200 -31.5 4600 -21.8 2300 -31.1 4700 -19.5 2400 -30.9 4800 -17.2 2500 -30.7 4900 -15.0 2600 -30.5 5000 -12.8


Annex 2A continued….. Test Data Set #3 - Distance 10cm, Frequency 2.0MHz

CURRENT AMPLITUDE mArms dBV

0.01 -51 0.02 -46.3 0.04 -40.3 0.06 -36.9 0.08 -34.4 0.1 -32.4 0.2 -26.5 0.3 -23 0.4 -20.5 0.5 -18.8 0.6 -17.1 0.7 -15.7 0.8 -14.5 0.9 -13.6 1 -12.6

Test Data Set #4 - Frequency 2.0MHz

1.0mArms 0.5mArms DISTANCE AMPLITUDE AMPLITUDE

cm dBV dBV 5 -8.5 -14.7

10 -12.6 -18.8 15 -15.5 -21.5 20 -18.5 -24.4 25 -21.1 -27.2


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1. ORIGINATOR (the name and address of the organizationpreparing the document. Organizations for who the documentwas prepared, e.g. Establishment sponsoring a contractor'sreport, or tasking agency, are entered in Section 8.)Dunn Engineering Inc.1280 Old Innes Rd, Ottawa, ON, K1B 5M7

2. SECURITY CLASSIFICATION(overall security classification of the document, including specialwarning terms if applicable)UNCLASSIFIED(NON-CONTROLLED GOODS)DMC OF REVIEW: GCEC JUNE 2010

3. TITLE (the complete document title as indicated on the title page. Its classification should be indicated by the appropriate abbreviation(S, C or U) in parentheses after the title).

Construction of an Apparatus for the Measurement of Current Induced in a Long Conductor by Local andDistant Transmitters

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Dunn Engineering Inc.5. DATE OF PUBLICATION (month and year of publication of

document)April 2012

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Contractor Report

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Defence R&D Canada-Suffield

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12RG01

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W7702-125297/001/EDM

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CR-2012-072

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highly desirable that the abstract of classified documents be unclassified. Each paragraph of the abstract shall begin with an indication of the security classification of the information in the paragraph (unless the document itself is unclassified) represented as (S), (C) or (U). It is not necessary to include here abstracts in both official languages unless the text is bilingual).

The purpose of this study was to design and construct an apparatus to measure the current induced in a long, buried wire by local or distant transmitters. To this end, a brief literature survey was performed to assess state-of-the-art techniques for measurement of local magnetic field, which was used to infer the current present in a buried conductor at a known depth. It was found that the ferrite coil is the most suitable to achieve the task in the frequency range from 300 kHz to 5 MHz. Several ferrite coils were constructed in order to optimize performance within this frequency range. Details are also provided regarding signal conditioning electronics. Finally, a description of the device operation is provided.

The document is comprised of two separate sub-reports. The first consists of sections addressing background survey and initial testing; apparatus design; and apparatus construction, with the second describes the experimental apparatus design and the preliminary test results.

14. KEYWORDS, DESCRIPTORS or IDENTIFIERS (technically meaningful terms or short phrases that characterize a document and could be helpful in cataloguing the document. They should be selected so that no security classification is required. Identifies, such as equipment model designation, trade name, military project code name, geographic location may also be included. If possible keywords should be selected from a published thesaurus, e.g. Thesaurus of Engineering and Scientific Terms (TEST) and that thesaurus-identified. If it is not possible to select indexing terms which are Unclassified, the classification of each should be indicated as with the title.) Current Sensor Sensing Utility Detection Wire Detection

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Construction of an apparatus for the measurement …[Enter report no.] Construction of an apparatus for the measurement of current induced in a long conductor by local and distant