Adaptation and Optimization of Planar Coils for a More Accurate and Far-Reaching Magnetic Field-Based Localization in the Near Field

In this publication, further elements of the newly developed inductive localization in the near field are presented. The advantage of inductive localization is the usage of the magnetic fields, which have a very low influence of non-metallic materials in the environment and thus follows good applications in the area of medicine and biochemistry. This allows a precise localization of sensor platforms in inhomogeneous mixtures of materials, where classical methods have major problems with inhomogeneous dielectric conductivity or density. The calculation of the localization of the searched object differs from other methods such as ultrasound or electromagnetic waves due to the source-free propagation of the magnetic field. Therefore, new mathematical evaluation methods and systematic adaptations are necessary, which are presented in this paper in circuit analysis. For this purpose, the exact circuit influences of one coil and the influence of another coil are investigated and which resonance circuit should be selected for both coils for a inductive localization with optimized signal strength.

I. Introduction

Inductive localization is based on the induction of an alternating voltage from a transmitting coil to a receiving coil and extends the application of inductive wireless energy transmission in the near field. The amplitude of the induced voltage depends on the operating frequency w , the current of the transmitting coil i ₁ and the mutual inductance M ₁₂ ( u _ind,12 = j ωM ₁₂ i ₁ ). The mutual inductance describes the coupling between two coils and is dependent on the distance, orientation and permeability, which in a metal-free environment is μ _r ≈ 1. [1] So the mutual inductance indirectly obtains information about the locating reference of the other coil. For a normal 3D localization at least six locating coils are necessary, which determine the mutual inductance of the coil by induced voltages. It should be noted that all coils influence each other, which also leads to unwanted mutual inductances/induced voltages at each coil. [2] To locate different coils at the same time, a backscatter method can be used where each coil has its own backscatter frequency. Depending on the efficiency, this method causes a reduction in signal strength. In this work, the influence of coils is investigated for the optimization of signal strengths. Since only a signal strength-based solution is useful for inductive tracking due to its application in the near field, high signal strengths are preferred for high ranges and good accuracies. The reason for the desired high signal strength is the characteristic of the near field, where the amplitude of a magnetic antenna decreases with r ⁻³ [2] , while for EM waves it has an damping of only r ⁻¹ . [3] Because the strength of the magnetic field or the current that generates the magnetic field is limited by circuit requirements or costs, thermal issues and EMC restrictions, the locating coils and the respective transmitting and receiving circuits must be optimized to achieve high receiving power. In addition, environmental factors affecting the behavior of the locating coils, such as resonance shift or frequency dependence, must be considered in the locating calculation, which may be caused by the influence of the other coils.