Printed Spiral Coils Research Articles

Misalignment-Tolerant Planar Spiral Coil Pair Design for 13.56 MHz Inductive Coupling of Wireless Resistive Analog Passive Sensors

Long-term daily-life body signal monitoring offers numerous advantages, such as timely response to health alerts, diseases monitoring, and reducing time and expenses related to clinical trials. Access to physiological data can be achieved with low-cost and comfortable wireless wearable sensors. In our previous publication, we reported a low-cost, easy to implement, and unobtrusive wireless resistive analog passive (WRAP) sensor to provide a feasible bio-signal monitoring technique by using a pair of printed spiral coils (PSC) in a near field connection. Sensitivity, defined as the response to the transducer, is a critical feature in the establishment of a reliable system. In the previous publication, we presented the utilization of a Genetic Algorithm to design a pair of coils and related components to maximize sensitivity. Although the coils’ misalignment can significantly affect the optimized sensitivity, it was not incorporated into the optimization process. This paper focuses on optimizing the coils and components in order to maximize both their sensitivity and their resilience against movements of the PSC pair. In a square-shaped pair comprising a primary coil of 60 mm and a secondary coil of 20 mm dimensions, we found that the sensitivity is maximized at 1.3 mƱ for a 16 mm axial distance. Additionally, it remains above 0.65 mƱ within ±11.25 mm lateral and +14 mm axial displacements.

Geometrical parameter optimization of planner square-shaped printed spiral coil for efficient wireless power transfer system to biomedical implant application

In this paper, the wireless recharging of pacemaker's battery for biomedical implant is considered. Planner square-shaped printed spiral coils (PSCs) are selected as primary (Tx) and secondary (Rx) coil for wireless power transfer (WPT) system. FEM based simulation method is used to optimize the geometrical parameters of square-shaped PSCs. The receiving coil is embedded inside the biomedical implant. The coil track width (w), pitch(s), thickness (t), number of turns(N1,N2) and outer diameter (Dout) of square-shape PCS's are the key parameters for higher coupling coefficient and quality factors. Using magnetic resonance coupling (MRC) based on Series-Parallel (SP) topology; experiments are carried out in an analytical, simulation, and physical environment for four operating frequencies: 300 kHz, 800 kHz, 6.78 MHz, and 13.56 MHz. Efficiency obtained by analytical and simulation method is higher with frequency. The optimum frequency for recharging the pacemaker battery is 6.78 MHz, which provides an efficiency of 85.51% at 100Ω load and is very close to the maximum theoretical efficiency. Voltage gain obtained by physical method is in close approximation with analytical and simulation methods.

Printed Split-Ring Loops with High Q-Factor for Wireless Power Transmission

The use of printed spiral coils (PSCs) as inductors in the construction of Wireless Power Transmission (WPT) circuits can save space and be integrated with other circuit boards. The challenges and issues of PSCs present for WPT mainly relate to maintaining an inductive characteristic at frequencies in Ultra High Frequency (UHF) band and to maximising the power transfer efficiency (PTE) between primary and secondary circuits. A new technique is proposed to increase the Q-factor relative to that offered by the PSC, which is shown to enhance WPT performance. This paper provides four-turn planar split-ring loops with high Q-factor for wireless power transmission at UHF bands. This design enhances the power transfer efficiency more than 12 times and allows for a greater transfer distance from 5 mm to 20 mm, compared with a conventional planar rectangular spiral coil.

Design and Implementation of Printed Spiral Coils used in Wireless Power Transmission Systems for Implantable Biomedical Devices

This paper proposed a small size and efficient square-shaped printed spiral coils (PSCs) at 13.56 MHz to be used for implantable biomedical devices. Detailed modeling of PSCs is presented. A design methodology has been applied to theoretical closed-form equations using MATLAB to optimize the wireless link of a 12×12 mm2 implantable coil example with 10 mm relative distance. All results are validated with the simulation using an electromagnetic field solver HFSS 14.1. Also, the PSCs has been implemented for verification using FR4 printed circuit board. The results show that optimized coil pairs achieved efficiency up to 80% at face to face relative distance of 10 mm in the air.

A low-power portable scanner for body-worn Wireless Resistive Analog Passive (WRAP) sensors for mHealth applications

Collecting multiple physiological signals with an embedded computing device can be more effective with battery-less zero-power sensors. We have previously reported our battery less, body-worn sensors named “Wireless Resistive Analog Passive (WRAP) sensors”. Here, we report design, prototyping and functional validation of a low-power, small and portable signal acquisition (scanner, revision 4) device for these sensors. The scanner couples with these sensors inductively by on-board printed spiral coils (PSC) that rely on impedance mismatch technique for physiological signal collection. The scanner was fully tested, characterized, and validated in lab, as well as through deployment to users’ home. As a test case, we show results of the scanner measuring temperature from WRAP temperature sensors with relative error within 0.01% range. The scanner measurement can distinguish between temperature of 1 °F difference and it shows excellent linear dependence between actual and measured resistance (R2=0.998). The scanner consumes 45.54 mJ of energy per scanning burst, it weighs 110 grams and measures 5.9”X 2.9”X 0.25”. This device has demonstrated the possibility of small, low-power portable scanner for zero-power WRAP sensors that shows promise for real-life data collection from our study participants in Memphis Smart and Connected Community (S&CC).

Design and optimization of printed spiral coils for wireless passive sensors

Monitoring physiological signals during regular life might provide many benefits including early detection of abnormalities and tracking the severities of diseases. A wireless connection between the passive sensor and the scanner eliminates the obtrusive wires, resolves battery-related issues, and makes it easy-to-use. We have previously proposed a wireless resistive analogue passive sensor technique that operates with the help of inductive coupling. The variation of resistive physiological transducer (secondary side) leads to amplitude modulation on the scanner coil (primary side). The design of printed spiral coil (PSC) on printed circuit board, significantly affects the performance of the overall system in terms of sensitivity, the output voltage change as a reflection of the transducer change. To optimize the PSC's profile and maximize the sensitivity, we employ three methods: iterative, analytical, and genetic algorithm (GA). The GA optimized PSCs, as the best result, have been fabricated and the measurement showed a sensitivity of 0.72 mƱ which has 5% (8.8%) deviation from the simulation (theoretical) results. This method can be utilized to design a PSC pair in near-field applications to transfer amplitude modulation with various sizes and fabrication constraints.

A Conformal Split-Ring Loop as a Self-Resonator for Wireless Power Transfer

A conventional printed spiral coil (PSC) has a self-resonant frequency and its equivalent circuit is a parallel inductor-capacitor (LC) circuit. It is desirable to use PSCs in wireless power transfer (WPT) applications; however, these are most commonly constituted by a series-primary and series- or parallel-secondary circuit. This paper proposes a printed conformal split-ring loop with the characteristic of a series LC circuit at its resonance frequency. Regarding the loop as a self-resonator, a magnetic-resonance coupled (MRC) WPT system with series-primary and secondary operating at 433 MHz in the industrial scientific and medical (ISM) band is presented. The maximum measured power transfer efficiency is 87.9% at a transfer distance of 22 mm, the best-reported result for such a configuration.

A Novel Approach to Reach Impedance Matching in Wireless Power Transfer Systems

Wireless power transfer (WPT) using magnetic resonant coupling technology, came into focus promptly by virtue of its long transfer distance, and its non-radiative and high-efficiency power transfer. The impedance matching has been studied in the literature in recent years. However, there is no suitable way to reach the optimum load in reality. In this paper, a new method is proposed to maximize the power transfer ability of a given pair of coupled coils. An analytical calculation of the mutual inductance is presented accurately with respect to the angled concentric multiple-turn printed spiral coils (PSC). In addition, the experimental results were in good agreement with the circuit simulation. Finally, a WPT experiment setup working at 3MHz resonance was established. The experiment results verified that the maximum transfer efficiency at fixed distances can be easily achieved by adjusting the angle to reach impedance matching. Compared to prior to optimization, the maximum improved efficiency was improved by 11%.

Inductance Calculation of Multilayer Circular Printed Spiral Coils

In the past few years, implanted biomedical devices have been widely used in healthy and medical applications and wireless power transfer (WPT) techniques used in these devices have been become a research hot topic. The printed spiral coils (PSCs), which have characteristics of small volume, low cost, high precision and high-integration, are better suited for WPT system. It is noteworthy that mutual inductance value of PSCs is a significant parameter which can influence the transfer efficiency of WPT system. Considering the geometry of multilayer circular PSCs and the distance between two adjacent layers, the previous methods are not suitable for calculating the mutual inductance of multilayer circular PSCs. In this study, an approximate calculation formula of mutual inductance between two single-layer circular PSCs is presented based on the existing research. On this basis, the self-inductance values of double-layer circular PSCs are calculated by MATLAB, the calculation results are verified by the ANSYS Q3D simulation and measurement results.

Design of coils on printed circuit board for inductive power transfer system

Due to the different application prospects, the coils of inductive power transfer (IPT) system varies. The coils on printed circuit boards (PCB) have drawn considerable attention gradually. However, the applications mainly involve portable equipment and implantable medical devices operating at a high frequency, not fit for IPT system. In this paper, an IPT system based on PCB coils were designed. All the characteristics of single turn coils on a single-layer, multi turns coils on a single-layer and multi turns coils on multi-layers of printed spiral coils (PSCs) were analysed and calculated. The self-inductance and mutual inductance of the series windings on PSCs were modeled and calculated firstly. An equivalent circuit of coils were proposed to simplify the calculation and facilitate the analysis. Furthermore, in order to increase mutual inductance and transfer more power, the multi-layer coils connecting in series were designed for the first time and the inductance parameters can be acquired by the means of proposed method. The analyses on coils model were verified by simulations using finite element method and measuring. Finally, the coil characteristics on IPT system were verified by the circuit simulation and experiment results and the designed coils can meet the desired requirements.

Sensitivity optimization of Printed Spiral Coil for Wireless Resistive Analog Passive (WRAP) Sensors using Genetic Algorithm.

Body-worn battery-less Wireless Resistive Analog Passive (WRAP) sensor can be unobtrusive while collecting physiological data continuously. Inductive connection between a pair of Printed Spiral Coils (PSC) eliminates the intrusive wires. Inductive connection of primary and secondary PSC enabled us to probe the body signals using the inductive link. The primary side voltage is modulated by the sensed body signal at the secondary PSC. The coil physical characteristics influence the sensitivity which is defined as observed voltage changes over the sensor variation. We have previously reported an iterative method to optimize the coil specifications for maximum sensitivity with constrained coil prof ile size by maximizing the power transfer efficiency from primary to secondary. In this study sensitivity is maximized by first, driving an analytical multivariable equation of circuit components and physical characteristics, and then using Genetic Algorithm (GA) to maximize it with considering the size and fabrication constraints. The results are compared to the other methods that shows a higher result in the range of 102 comparing to the best alternate methods (sqp). It helps us to detect smaller physiological signals in the noisy environment.

무선주파수 간섭 측정용 차폐된 Printed Spiral Coil(PSC) 프로브의 대역폭 개선 설계

Herein, electromagnetic shielding structures to reduce the external noise coupling to printed spiral coils (PSCs) and a design method for improving the bandwidth of shielded PSCs have been proposed. It has been demonstrated that the bandwidth of shielded PSCs is limited due to the parasitic capacitance between the coils and the shielding structures and is confirmed by the transfer function simulation of the shielded PSCs with a transmission line as the radio-frequency interference noise source. A design method for the bandwidth improvement of the shielded PSCs has been proposed based on the equivalent circuit model analysis and the case studies depending on PSC designs with a three-dimensional field simulation. With the design method, an optimized shielded PSC design has been presented and successfully confirmed by experimental verification in that the optimized design results in a significant bandwidth improvement.

Design Optimization of Multiple-Layer PSCs With Minimal Losses for Efficient and Robust Inductive Wireless Power Transfer

Inductive wireless Power Transfer (IPT) is a promising technology for remote powering of a wide variety of applications of electronic devices. To design IPT systems with the highest power transfer efficiency and the maximal robustness to coupling factor variations between transmitter and receiver of printed spiral coils (PSCs), high quality factors ( Q-Factor ) of the utilized PSCs are required. Designing PSCs with high Q-Factor is limited by the eddy current, the proximity effect, and parasitic losses. In this paper, PSC parasitic losses are carefully analyzed and specific design solutions are proposed. Genetic algorithm optimizations are developed to accommodate the proposed design solutions in minimizing losses. Single and multiple layer variable width PSCs are optimally designed with eddy current and proximity effect losses minimized. The designed PSCs are fabricated and experimental measurements are performed. The validity of the proposed approach to largely improve both IPT efficiency and robustness are confirmed. Using multiple coil layers, the robustness to axial and lateral coupling variations between coils is highly improved. For a triple-layer PSC design case, up to 3.5-fold improved robustness are obtained in reference to conventional IPT systems. Compared with the previous state of the art IPT topologies, the highest Figure-of-Merit value is obtained using the proposed design solutions.

무선주파수 간섭 측정을 위한 Printed Spiral Coil (PSC) 프로브의 고주파 모델링

In this paper, a new high-frequency equivalent circuit model of printed spiral coils (PSCs) for radio-frequency interference (RFI) measurement has been proposed. To achieve high-frequency modeling, the proposed model consists of distributed components designed based on the design parameters of the PSCs. In addition, an analytic model for PSCs based on T-pi conversion has been proposed. To investigate the feasibility of the proposed model for RFI measurement, the transfer function between a microstrip line and a PSC has been extracted by combining the proposed model and mutual inductance. The self-impedances of the proposed model and the transfer function have been successfully validated using three-dimensional field simulation and measurements, revealing noticeable correlations up to a frequency of 6 GHz. The proposed model can be employed for high-frequency probe design and RFI noise estimation in the gigahertz range wireless communication bands.

Efficient Four-Coil Wireless Power Transfer for Deep Brain Stimulation

This paper demonstrates a closed-loop wireless power transfer (WPT) system for deep brain stimulation-implanted biomedical applications. A novel two-layer PCB FR4 coil design is proposed as a second coil for enlarging the WPT efficiency. High-efficiency implantable WPT technology was achieved by applying an inner dual-layer printed spiral coil (PSC) and outer helical coil and this coordinated with a T-type impedance-matching network. To reduce the loading effect, which lowers Q factor in implanted devices, four-coil WPT systems are recommended compared with two- or three-coil WPT systems. A practical four-coil system was implemented for verification, for which a dual-layer printed circuit board composed of a PSC, with a load loop on the other side of FR4, was designed and implemented around the system on a chip. PSCs for biomedical implants can be optimized by designing the geometric parameters. The implant coil occupies an area measuring $5 \times 5$ mm, and is implemented on both an FR4 board and an implantable flexible substrate with the optimized geometric parameters. Finally, the transmission efficiencies of using FR4 and flexible printed circuit boards, at a distance of 10 mm, were 19.1% and 14.8% through the air and 11.7% and 7.7% through the tissue, respectively, thus enabling the supply of milliwatts of power to the stimulation circuits, and successfully demonstrating the stimulation tissue model with a V/I output current of up to 180 $\mu \text{A}$ .

Inductive power transmission to millimeter-sized biomedical implants using printed spiral coils.

The operation frequency (f) has been a key parameter in optimizing wireless power transmission links for biomedical implants with millimeter (mm) dimensions. This paper studies the feasibility of using printed spiral coils (PSCs) for powering mm-sized implants with high power transmission efficiency (PTE) at different fps. Compared to wire-wound coils (WWCs), using a PSC in the implant side allows batch fabrication on rigid or flexible substrates, which can also be used as a platform for integrating implant components. For powering an implant with 1 mm diameter, located 10 mm inside the tissue, the geometries of transmitter (Tx) and receiver (Rx) PSCs were optimized at different fPs of 50 MHz, 200 MHz, and 500 MHz using a commercial field solver (HFSS). In simulations, PSC- and WWC-based links achieved maximum PTE of 0.13% and 3.3%, and delivered power of 65.7 μW and 720 μW under specific absorption rate (SAR) constraints at the optimal fp of 50 MHz and 100 MHz, respectively, suggesting that the performance of the PSC-based link is significantly inferior to that of the WWC-based link.

Enhanced planar wireless power transfer using strongly coupled magnetic resonance

A novel planar wireless power transfer (WPT) system using strongly coupled magnetic resonance is proposed. Additional layers of printed spiral coils (PSCs) are applied to the transmitter/receiver resonator of the 4-coil planar WPT system to improve the quality factor and transmission efficiency. In addition, by connecting multilayer transmitter/receiver resonators together using conductive shorting walls, the transmission efficiency is further improved. Five planar WPT systems using coplanar PSCs and printed loops have been investigated. The measured results show that the proposed design with two additional layers of resonators and shorting walls could improve the transmission efficiency from 77.27 to 84.38%.

A Planar Magnetically Coupled Resonant Wireless Power Transfer System Using Printed Spiral Coils

A fully planar wireless power transfer (WPT) system via strongly coupled magnetic resonances is presented. In it, both the transmitter and the receiver are planarized with the use of coplanar printed spiral coils (PSCs) and a printed loop. An equivalent circuit model of the proposed planar WPT system is derived to facilitate the design, and a flowchart is provided for the optimization of the system with given size constraints. To realize high peak power transfer efficiency, the quality factor of individual loop or resonator, the mutual coupling between resonators, and the frequency splitting phenomenon of the system are analyzed in addition to the effect of the input impedance of the system on the transmission efficiency. Furthermore, parallel current paths are created by applying auxiliary strips to the backside of the substrates and connecting to the prime resonators using vias to decrease the resistance and to increase the quality factor of the PSC resonators, and this in turn further improves the transfer efficiency of the proposed planar WPT system. The measured results show that the proposed WPT system is able to provide a stable wireless power transfer with up to 81.68% efficiency at a distance of 10 cm. The planar structure and the high transfer efficiency make the proposed design a suitable candidate for wireless power transfer of small portable electronic devices.

Optimization of Data Coils in a Multiband Wireless Link for Neuroprosthetic Implantable Devices

We have presented the design methodology along with detailed simulation and measurement results for optimizing a multiband transcutaneous wireless link for high-performance implantable neuroprosthetic devices. We have utilized three individual carrier signals and coil/antenna pairs for power transmission, forward data transmission from outside into the body, and back telemetry in the opposite direction. Power is transmitted at 13.56 MHz through a pair of printed spiral coils (PSCs) facing each other. Two different designs have been evaluated for forward data coils, both of which help to minimize power carrier interference in the received data carrier. One is a pair of perpendicular coils that are wound across the diameter of the power PSCs. The other design is a pair of planar figure-8 coils that are in the same plane as the power PSCs. We have compared the robustness of each design against horizontal misalignments and rotations in different directions. Simulation and measurements are also conducted on a miniature spiral antenna, designed to operate with impulse-radio ultra-wideband (IR-UWB) circuitry for back telemetry.

Modeling and Optimization of Printed Spiral Coils in Air, Saline, and Muscle Tissue Environments

Printed spiral coils (PSCs) are viable candidates for near-field wireless power transmission to the next generation of high-performance neuroprosthetic devices with extreme size constraints, which will target intraocular and intracranial spaces. Optimizing the PSC geometries to maximize the power transfer efficiency of the wireless link is imperative to reduce the size of the external energy source, heating of the tissue, and interference with other devices. Implantable devices need to be hermetically sealed in biocompatible materials and placed in a conductive environment with high permittivity (tissue), which can affect the PSC characteristics. We have constructed a detailed model that includes the effects of the surrounding environment on the PSC parasitic components and eventually on the power transfer efficiency. We have combined this model with an iterative design method that starts with a set of realistic design constraints and ends with the optimal PSC geometries. We applied our design methodology to optimize the wireless link of a 1-cm (2) implantable device example, operating at 13.56 MHz. Measurement results showed that optimized PSC pairs, coated with 0.3 mm of silicone, achieved 72.2%, 51.8%, and 30.8% efficiencies at a face-to-face relative distance of 10 mm in air, saline, and muscle, respectively. The PSC, which was optimized for air, could only bear 40.8% and 21.8% efficiencies in saline and muscle, respectively, showing that by including the PSC tissue environment in the design process the result can be more than a 9% improvement in the power transfer efficiency.