Lenaerts / Puers Omnidirectional Inductive Powering for Biomedical Implants

1. Auflage 2008
ISBN: 978-1-4020-9075-2
Verlag: Springer-Verlag
Format: PDF
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)

E-Book, Englisch, 222 Seiten

investigates the feasibility of inductive powering for capsule endoscopy and freely moving systems in general. The main challenge is the random position and orientation of the power receiving system with respect to the emitting magnetic field. Where classic inductive powering assumes a predictable or fixed alignment of the respective coils, the remote system is now free to adopt just any orientation while still maintaining full power capabilities. Before elaborating on different approaches towards omnidirectional powering, the design and optimisation of a general inductive power link is discussed in all its aspects. Special attention is paid to the interaction of the inductive power link with the patient’s body. Putting theory into practice, the implementation of an inductive power link for a capsule endoscope is included in a separate chapter.

Autoren/Hrsg.

Lenaerts, Bert

Puers, Robert

Inhaltsverzeichnis

1;Abstract;6
2;List of Abbreviations;8
3;List of Symbols;9
4;Contents;12
5;Introduction;16
5.1;Wireless Power Transmission;16
5.2;Types of Wireless Power Transmission;17
5.2.1;Radiative;17
5.2.2;Conductive;18
5.2.3;Capacitive;19
5.2.4;Inductive;20
5.3;A Biomedical Perspective;21
5.3.1;Instrumentated Implants;21
5.3.2;Transcutaneous Powering;22
5.3.2.1;Radiative;22
5.3.2.2;Conductive/Capacitive;23
5.3.2.3;Inductive;24
5.4;Inductive Links;24
5.4.1;Powering System;24
5.4.2;Data Communication;25
5.5;Conclusions;26
5.6;What to Expect;27
6;Magnetic Induction;28
6.1;Maxwell's Equations;28
6.1.1;Time-Domain, Integral Form;28
6.1.2;Time-Harmonic, Differential Form;29
6.1.3;Constitutive Relations and Ohm's Law;29
6.1.4;Magnetic and Electric Potential;30
6.1.5;Current and Flux;31
6.2;Conductive Wire;32
6.3;Inductance;33
6.4;Inductor Models;36
6.5;Finite Element Modelling;39
6.5.1;Axisymmetric Geometries;40
6.5.1.1;DC Models;42
6.5.1.2;AC Models;44
6.5.2;2-D Wire Models;47
6.5.3;3-D Models;48
6.5.4;Mutual Inductance;51
6.6;Conclusions;51
7;Inductive Link Design;53
7.1;Link Equations;53
7.1.1;Important Quantities;53
7.1.2;Equivalent Two-Port Representations;56
7.1.3;Secondary Resonance;57
7.1.4;Transmitted Power;59
7.1.5;Link Efficiency;60
7.1.6;Link Gain and Critical Coupling;62
7.1.7;Parallel vs. Series Resonance;64
7.1.8;Summary;67
7.2;Loose-Coupling Approximation;67
7.3;Tertiary Circuits;69
7.3.1;Conductive Objects;70
7.3.2;Coupled Resonators;75
7.4;Link Optimisation;79
7.4.1;Operating Frequency: omega;79
7.4.1.1;Technical;79
7.4.1.2;Exposure and EMC Regulations;80
7.4.2;Magnetic Design: M0;80
7.4.2.1;Calculation;80
7.4.2.2;Optimal Coil Dimensions;80
7.4.2.3;Ferromagnetic and Conductive Materials;81
7.4.3;Winding Losses: R10 and R20;82
7.4.3.1;Copper Volume and Wire Diameter;82
7.4.3.2;Inter-winding Capacitance;84
7.4.3.3;Litz Wire;85
7.4.4;Secondary Coil Optimisation: N2;85
7.4.4.1;Optimisation for Link Efficiency;86
7.4.4.1.1;Exhaustive Method;86
7.4.4.1.2;Iterative Method;87
7.4.4.1.3;Series vs. Parallel;88
7.4.4.2;Optimisation for Transmitted Power;88
7.4.4.3;Critical Coupling;89
7.4.5;Secondary Coil and Capacitance Tapping;89
7.4.6;Advanced Gain Stabilisation;91
7.5;Misconceptions About k and Q;92
7.6;Conclusions;94
8;Power Converters and Voltage Regulators;96
8.1;Rectifiers;96
8.1.1;Diodes;97
8.1.2;Peak Rectifiers;98
8.1.2.1;Half-Wave Rectifier;98
8.1.2.2;Bridge Rectifier;100
8.1.2.3;Full-Wave Rectifier with Voltage Doubling;100
8.1.3;Class D Rectifiers;101
8.1.3.1;Current-Driven Half-Wave Rectifier;102
8.1.3.2;Current-Driven Bridge Rectifier;103
8.1.4;Class E Rectifiers;104
8.1.5;To Conclude;104
8.2;Inverters;105
8.2.1;Semiconductor Switches;106
8.2.2;Saturating Class C Inverter;107
8.2.3;Class D Inverters;109
8.2.4;Class E Inverters;110
8.2.4.1;Principle of Operation;110
8.2.4.2;Circuit Analysis and Component Values;112
8.2.5;Alternative Load Networks;115
8.2.6;Design of an Inductive Link Driver;117
8.2.6.1;Class E Design Flow;117
8.2.6.2;Capacitor Types;119
8.2.6.3;MOSFET Drivers and Harmonic Suppression;120
8.2.6.4;Data Modulation;122
8.2.6.5;A 1050 kHz, 130 Ampere-Turns Class E Coil Driver;122
8.3;Voltage Regulators;125
8.3.1;Linear Regulators;126
8.3.2;Switching Regulators;128
8.4;Conclusions;129
9;Omnidirectional Coupling;131
9.1;Problem Definition;131
9.2;Multiple Primary Coils;132
9.2.1;System Concept;132
9.2.2;Worst-Coupling Map;133
9.2.3;Capsule Endoscope with One Secondary Coil;135
9.3;Multiple Secondary Coils;139
9.3.1;Power-Combining Rectifiers;139
9.3.2;Worst-Case Conditions;141
9.3.2.1;Transmitted Power;141
9.3.2.2;Efficiency;144
9.3.3;Capsule Endoscope with Three Secondary Coils;146
9.4;Conclusions;149
10;Biological Tissue Interaction;151
10.1;Electromagnetic Fields in Biological Tissue;151
10.1.1;Near Field;151
10.1.1.1;Conservative Electric Field Ec;152
10.1.1.2;Magnetic Field H and Induced Electric Field Em;153
10.1.2;Far Field;154
10.2;Health Effects of Electromagnetic Fields;155
10.2.1;Directly Observable Effects;155
10.2.1.1;Direct Physiological Effects;155
10.2.1.2;Thermal Effects;156
10.2.1.3;Contact Currents (Shocks and Burns);156
10.2.2;Other Biological Effects;157
10.2.2.1;Long-Term, Low-Level Effects;157
10.2.2.2;Direct Interaction with Magnetic Fields;157
10.3;Exposure Limits and Regulations;157
10.4;Examples from Biomedical Engineering Practice;160
10.5;Conclusions;162
11;An Inductive Power Link for a Capsule Endoscope;163
11.1;Wireless Endoscopy;163
11.2;Design: Choices and Motivation;164
11.2.1;Specifications;164
11.2.2;Operating Frequency;164
11.2.3;Coil Configuration;165
11.2.4;Voltage Regulator and Rectifier;165
11.2.5;Power Optimisation;166
11.2.6;Verification;167
11.3;Fabrication;168
11.4;Measurement;172
11.5;Biological Tissue Interaction;175
11.5.1;ICNIRP Compliance;177
11.5.2;Link Efficiency;181
11.5.3;Class E Tuning;182
11.5.4;Secondary Resonance Tuning;184
11.6;Conclusions;184
12;A Class E Driver for Deformable Coils;186
12.1;Class E ZVS Inverter with Transductor;186
12.2;Control Loop;190
12.3;Measurement Results;194
12.3.1;Deforming the Primary Coil;195
12.3.2;Varying the Frequency;197
12.4;Conclusions;198
13;Conclusions;201
13.1;Comprehensive Summary;201
13.2;Main Contributions and Achievements;203
13.3;Further Research;204
14;Coil Measurements;206
14.1;Single Coil Characterisation;206
14.1.1;General Considerations;206
14.1.1.1;Self-inductance;207
14.1.1.2;Self-resonance;208
14.1.1.3;Equivalent Series Resistance;208
14.1.2;One-Port S11 Measurement;208
14.1.3;Two-Port Q' Measurement;209
14.1.4;Impedance Analysers and LCR Meters;212
14.2;Coupling Characterisation;213
15;References;219
16;Index;227

Leseproben

Thus far, fixed coil parameters have been assumed for in design and optimisation of an inductive power link. If the exact value of some of these parameters was hard to define, a worst-case value is used instead. The underlying idea is that when a link meets the specifications for this worst-case scenario, it also does under more favourable circumstances. The exact value of the mutual inductance between the primary and secondary coil for instance, heavily depends on their relative position. As such, uncertainties in coil alignment give rise to a certain tolerance on the mutual inductance value.

Designing an inductive link for a freely moving secondary system goes one step beyond coping with tolerances. Indeed, the magnetic coupling can actually become zero when the secondary coil can adopt just any orientation. Multiple coils at either the primary or secondary side are necessary to eliminate the eventuality of zero coupling. This chapter investigates the link performance that can be achieved with each of both approaches. The worst-case conditions are identified, since they determine the lower performance limits. The application of capsule endoscopy serves as example by which the developed quantitative techniques are illustrated. Novel research work is presented in this chapter, published in [97] and [94, 95]. The actual realisation of an omnidirectional power link for a capsule endoscope is treated in Chap. 7.

5.1 Problem Definition

A remote system is assumed that is free to move within a confined space and that is to be inductively powered. Freedom of movement applies to the position as well as to the orientation of the secondary system, hence to the full six degrees of freedom. It can be proven that at any position of a secondary coil in a primary magnetic field, there exists an orientation yielding zero mutual inductance M. The proof is given for a secondary winding with a point-symmetric geometry, with the point of symmetry defining the centre of the coil. All turns are assumed to lie in parallel planes. The winding axis is defined as the straight line perpendicular to these planes and passing through the centre of the coil.

Rotating such a secondary coil over 180 along an axis through its centre, perpendicular to its winding axis, reverses the sign of the captured magnetic flux. In the absence of conductive media, and certainly at DC, M is a real quantity. Since the mutual inductance is a continuous function of the secondary coil orientation, it hence follows that M crosses zero when rotating the secondary coil over 180 along any axis through its centre perpendicular to its winding axis.

The fact that the mutual inductance can become zero implies that inductive power transmission cannot be guaranteed for a freely moving secondary coil. In order to overcome this limitation, imposed by the directionality of magnetic coupling, different approaches are conceivable. One is to vary the primary magnetic field with the position and orientation of the secondary coil. This can be achieved with a moving primary coil or by use of multiple, individually driven primary coils. The dual approach is to make use of a secondary coil that is movable with respect to the remote system, or by making use of multiple secondary coils.