arXiv subjects

Stefan M. Goetz

Publications and source records attributed to Stefan M. Goetz.

At least 19 recordsLinked to original sources

arXiv2026

Markerless Head Tracking for Accurate and Accessible Neuronavigation

Neuronavigation is widely used in biomedical research and interventions to guide the precise placement of instruments around the head to support procedures such as transcranial magnetic stimulation. Traditional systems, however, rely on subject-mounted markers that require manual registration, may shift during procedures, and can cause discomfort. We introduce and evaluate markerless approaches that replace expensive hardware and physical markers with low-cost visible and infrared light cameras incorporating stereo and depth sensing, combined with algorithmic modeling of the facial geometry. Validation with 50 human subjects yielded a median tracking discrepancy of only 2.32 mm and 2.01$^\circ$ for the best markerless algorithm compared to a conventional marker-based system, which indicates sufficient accuracy for transcranial magnetic stimulation and a substantial improvement over prior markerless results. The study also suggests that integration of the data from the various camera sensors can improve the overall accuracy further. The proposed markerless neuronavigation methods can reduce setup cost and complexity, improve patient comfort, and expand access to neuronavigation in clinical and research settings.

cs.CV↗

arXiv2025

Entirely Transformerless Universal Direct-Injection Power-Flow Controller

An increasing penetration of renewable energy resources, electric vehicle chargers, and energy storage systems into low-voltage power grids causes several power management and stability problems, such as reverse power flow, (local) overload lines, and over- / under-voltage. Previous power-flow and soft-open-point solutions are bulky and expensive. They need transformers and large magnetics, some on grid frequency, others more compact at high frequency. Even suggested circuits with high-frequency transformers still struggle with cost and size. We present a compact partial power-conversion high-current full-power-flow control circuit without a single transformer. We combine silicon and silicon-carbide, each with their specific advantages for current-dense direct injection. The circuit further needs fewer semiconductors than previous concepts. The circuit links a shunt converter through a non-isolated inverter bidirectionally with low-voltage series modules that practically float with their respective phases can serve between different feeders in low-voltage power grids. We analyze the circuit mathematically and evaluate the operation in simulation and experimental results.

eess.SY↗

arXiv2025

Magnetic Materials for Transcranial Magnetic Stimulation (TMS)

Various coils for transcranial magnetic stimulation (TMS) are widely available for clinical and research use. These coils are almost all designed as air coils, which require large levels of energy to achieve a given magnetic flux density and in turn electric field strength, whereas in other sectors, such as power electronics or electrical machines, magnetic materials have been used for a long time to achieve higher efficiencies. We tested the impact on the electric and magnetic properties of different soft magnetic materials, including various ferrite cores, laminated sheet materials of nonisotropic corn-oriented silicon-steel, non-oriented silicon-steel, as well as cobalt-iron, and soft magnetic compound powder cores with insulated particles. Every material led to a reduction in coil current and voltage for the same target electric field strength. For the same field energy, every material yielded lower losses. Most common materials saturated already at very low currents. More material in thicker layers could shift the saturation point but at the cost of high weight. Due to their low saturation flux density, ferrites appear unsuitable for the high amplitude requirements of TMS. Laminated sheet materials and powder cores reduce the pulse energy, but the laminated sheet material adds more weight for the same effect than powder cores. Thus, appropriate magnetic materials can reduce the required pulse energy. Saturation flux density is the most relevant parameter, whereas the permeability beyond a certain base level is practically irrelevant. Most importantly, the weight of a magnetic-core coil may always be increased compared to an air coil for the same target field.

physics.med-ph↗

arXiv2025

High-Power Dual-Channel Field Chamber for High-Frequency Magnetic Neuromodulation

Several novel methods, including magnetogenetics and magnetoelectric stimulation, use high frequency alternating magnetic fields to precisely manipulate neural activity. To quantify the behavioral effects of such interventions in a freely moving mouse, we developed a dual-channel magnetic chamber, specifically designed for rate-sensitive magnetothermal-genetic stimulation, and adaptable for other uses of alternating magnetic fields. Through an optimized coil design, the system allows independent control of two spatially orthogonal uniform magnetic fields delivered at different frequencies within a 10 cm x 10 cm x 6 cm chamber. The two channels have nominal frequencies of 50 and 550 kHz with peak magnetic field strengths of 88 and 12.5 mT, achieved with resonant coil drives having peak voltages of 1.6 and 1.8 kV and currents of 1.0 and 0.26 kA, respectively. Additionally, a liquid cooling system enables magnetic field generation for second-level duration, and an observation port and camera allow video capture of the animal's behavior within the chamber. The system generates high-amplitude magnetic fields across two widely separated frequency channels with negligible interference (< 1%). Relatively uniform magnetic field distribution (+/-10% across 94% of the chamber volume) is maintained throughout the chamber, and temperature increase of the inner side of the coil enclosure during the operation is limited to < 0.35 °C/s to ensure in vivo safety. Using cobalt-doped and undoped iron oxide nanoparticles, we demonstrate channel-specific heating rates of 3.5 °C/s and 1.5 °C/s, respectively, validating frequency-selectivity. Both channels can run continuously for four seconds stably.

eess.SY↗

arXiv2025

Ultra-Fast Wireless Power Hacking

The rapid growth of electric vehicles (EVs) has driven the development of roadway wireless charging technology, effectively extending EV driving range. However, wireless charging introduces significant cybersecurity challenges. Any receiver within the magnetic field can potentially extract energy, and previous research demonstrated that a hacker could detect the operating frequency and steal substantial power. However, our approach required time to track new frequencies or precise adjustments of inductance and capacitance, which would be less effective against potential rapid transmitter frequency changes or capacitance drift. As a solution, we enhanced the interceptor and enabled it to intrude as well as steal energy within just three cycles of the high-frequency signal. Moreover, it can work without any circuit parameters or look-up tables. The key innovation is synchronizing the receiver current with the phase of the magnetic sensor voltage. Through MATLAB / Simulink simulations, finite-element analysis, and experimental validation, we demonstrated that our improved method can steal over 76% of the power received by a fully resonant receiver under identical conditions. This attack demonstrates that simple frequency-changing power encryption offers limited protection against such threats.

cs.CR↗

arXiv2025

Fast Energy-Theft Attack on Frequency-Varying Wireless Power without Additional Sensors

With the popularity of wireless charging, energy access protection and cybersecurity are gaining importance, especially in public places. Currently, the most common energy encryption method uses frequency and associated impedance variation. However, we have proven that this method is not reliable, since a hacker can detect the changing frequency and adjust the compensation. However, the previously presented system needed time to follow the updated frequency, while encryption systems may vary the frequency faster to avoid energy theft. Furthermore, the previous system required an additional sensor coil. To solve these problems, we optimized the attack and the associated system, which can intrude and steal energy within 0.2 ms. The key is the elimination of the time-consuming maximum receiver current regulation. Also, we use the main receiving coil rather than any additional sensor antenna to detect the magnetic field. Thus, the new hardware is even simpler. A simulation model and experimental results demonstrate the fast response speed of the attack on encrypted wireless power and steal 65% of the power. Overall, the applicability of the attack is highly improved and leaves less room for hardening the encryption. The results demonstrate that energy access protection needs to be given great attention.

cs.CR↗

arXiv2025

A Highly Compact Direct-Injection Power-Flow Controller and Line-Voltage Regulator with Shared Magnetics and Partial-Power Conversion for Full-Power Control

An increasing integration of photovoltaic units, electric vehicle chargers, heat pumps, and energy storage systems challenges low-voltage power grids and can cause voltage range violation, loss of stability, (local) overload of lines, and power management problems. Research suggested universal power-flow control (UPFC) to solve power management problems. In contrast to bulky, slow, and costly conventional UPFCs with their shunt and series transformers, this paper presents a highly compact and current-dense power-flow controller, which can serve between different feeders in the low-voltage power grids. The enabler is a systematic combination of silicon car-bide (SiC) with silicon (Si) transistors and a strict partial-power topology built around a multi-active bridge. The circuit links an active-front-end converter as a shunt stage through a multi-active-bridge converter bidirectionally with low-voltage series-injection modules floating with their respective phases. The topology can use small power to control high currents through the low-voltage series-injection modules. The multi-active bridge serves as a multi-input-output power router that exchanges energy between all elements. We assess the design as well as the implementation considerations of the proposed power-flow controller mathematically and verify its performance in simulation and real systems.

eess.SY↗

arXiv2025

High-Power Wide-Bandwidth High-Quality Modular Pulse Synthesizer with Adaptive Voltage Asymmetry in Medical Power Electronics

Noninvasive brain stimulation can write signals into neurons but requires power electronics with exceptionally high power in the mega-volt-ampere range and kilohertz usable bandwidth. Whereas oscillator circuits offered only one or very few pulse shapes, modular cascaded power electronics solved a long-standing problem for the first time and enabled arbitrary software-based synthesis of the temporal shape of stimuli. However, synthesizing arbitrary stimuli with a high output quality requires a large number of modules. We propose an alternative solution that achieves high-resolution pulse shaping with fewer modules by implementing high-power wide-bandwidth voltage asymmetry. Rather than equal voltage steps, our system strategically assigns different voltages to each module to achieve a near-exponential improvement in resolution. The module voltage sequence does also not use just a simple binary pattern other work might suggest but adapts it to the output. Additionally, we introduce a switched-capacitor charging mechanism that allows the modules to charge to different voltages through a single dc power supply. We validated our design in a head-to-head comparison with the state of the art on experimental prototypes. Our three-module prototype reduces total voltage distortion by 13.4% compared to prior art with three modules, and by 4.5% compared to prior art with six -- twice as many -- modules. This paper is the first asymmetric multilevel circuit as a high-precision high-power synthesizer, as well as the first to adaptively optimize asymmetric voltage sequence in modular power electronics.

eess.SY↗

arXiv2025

Frequency Control in Microgrids: An Adaptive Fuzzy-Neural-Network Virtual Synchronous Generator

The reliance on distributed renewable energy has increased recently. As a result, power electronic-based distributed generators replaced synchronous generators which led to a change in the dynamic characteristics of the microgrid. Most critically, they reduced system inertia and damping. Virtual synchronous generators emulated in power electronics, which mimic the dynamic behaviour of synchronous generators, are meant to fix this problem. However, fixed virtual synchronous generator parameters cannot guarantee a frequency regulation within the acceptable tolerance range. Conversely, a dynamic adjustment of these virtual parameters promises robust solution with stable frequency. This paper proposes a method to adapt the inertia, damping, and droop parameters dynamically through a fuzzy neural network controller. This controller trains itself online to choose appropriate values for these virtual parameters. The proposed method can be applied to a typical AC microgrid by considering the penetration and impact of renewable energy sources. We study the system in a MATLAB/Simulink model and validate it experimentally in real time using hardware-in-the-loop based on an embedded ARM system (SAM3X8E, Cortex-M3). Compared to traditional and fuzzy logic controller methods, the results demonstrate that the proposed method significantly reduces the frequency deviation to less than 0.03 Hz and shortens the stabilizing/recovery time.

eess.SY↗

arXiv2025

New Power Decoupling Method for Grid Forming Inverter Based on Adaptive Virtual-Synchronous Machine in Weak Grids

Many countries' policies have shifted rapidly towards using renewable energy for climate reasons. As a result, inverter-based resources are beginning to dominate power systems. Key elements for managing the loss of conventional generators are virtual-synchronous-generator-based grid-forming inverters. Despite the unique advantages of this technology, there are still various challenges, most notably the problem of active-reactive power coupling due to a nonzero power angle and high grid impedance ratio R/X. The effect of power coupling means that any change in the inverter's active power will affect the reactive power and vice versa. This challenge results in grid instability, reduces control performance, and restricts the active power delivery capability of the inverter to the grid. This paper presents a new vision to solve this impact in weak grids by a new power-decoupling method based on adaptive virtual synchronous generator parameters. The power coupling will be studied considering the parameters causing this effect. Fuzzy logic will serve to adjust the parameters of power control loops. Hardware-in-the-loop testing on a real-time simulator (OP4610) and a physical microcontroller verified and validated the proposed method. The results showed the proposed method's effectiveness in eliminating static and dynamic power coupling and improving the grid-forming inverter performance under different operating conditions.

eess.SY↗

arXiv2025

Class E/EF Inductive Power Transfer to Achieve Stable Output under Variable Low Coupling

This paper develops an inductive power transfer(IPT)system with stable output power based on a Class E/EF inverter. Load-independent design of Class E/EF inverter has recently attracted widespread interest. However, applying this design to IPT systems has proven challenging when the coupling coefficient is weak. To solve this issue, this paper uses an expanded impedance model and substitutes the secondary side's perfect resonance with a detuned design. Therefore, the system can maintain stable output even under a low coupling coefficient. A 400 kHz experimental prototype validates these findings. The experimental results indicate that the output power fluctuation remains within 15% as the coupling coefficient varies from 0.04 to 0.07. The peak power efficiency achieving 91%

eess.SY↗

arXiv2025

Conveniently Identify Coils in Inductive Power Transfer System Using Machine Learning

High-frequency inductive power transfer (IPT) has garnered significant attention in recent years due to its long transmission distance and high efficiency. The inductance values L and quality factors Q of the transmitting and receiving coils greatly influence the system's operation. Traditional methods involved impedance analyzers or network analyzers for measurement, which required bulky and costly equipment. Moreover, disassembling it for re-measurement is impractical once the product is packaged. Alternatively, simulation software such as HYSS can serve for the identification. Nevertheless, in the case of very high frequencies, the simulation process consumes a significant amount of time due to the skin and proximity effects. More importantly, obtaining parameters through simulation software becomes impractical when the coil design is more complex. This paper firstly employs a machine learning approach for the identification task. We simply input images of the coils and operating frequency into a well-trained model. This method enables rapid identification of the coil's L and Q values anytime and anywhere, without the need for expensive machinery or coil disassembly.

eess.SY↗

arXiv2024

Hacking Encrypted Wireless Power: Cyber-Security of Dynamic Charging

Recently, energy encryption for wireless power transfer has been developed for energy safety, which is important in public places to suppress unauthorized energy extraction. Most techniques vary the frequency so that unauthorized receivers cannot extract energy because of non-resonance. However, this strategy is unreliable. To stimulate the progress of energy encryption technology and point out security holes, this paper proposes a decryption method for the fundamental principle of encrypted frequency-varying wireless power transfer. The paper uses an auxiliary coil to detect the frequency and a switched-capacitor array to adaptively compensate the receiver for a wide frequency range. The switched-capacitor array contains two capacitors and one semi-conductor switch. One capacitor compensates the receiver all the time while the other's active time during one wireless power transfer cycle is regulated by the switch. Thus, the proposed hacking receiver controls the equivalent capacitance of the compensation and steals energy. Finally, a detailed simulation model and experimental results prove the effectiveness of the attack on frequency-hopping energy encryption. Although any nonnegligible energy extracted would be problematic, we achieved to steal 78% to 84% of the energy an authorized receiver could get. When the frequency changes, the interceptor is coarsely tuned very quickly, which can hack fast frequency-varying encrypted system.

eess.SY↗

arXiv2023

Hairpin Motors for Electromobility: Twists and Bends of a Technological Breakthrough that Initially Arrived A Century Too Soon

There is currently a major trend to hairpin-winding motors for small and medium drives with increased power, specifically more torque density in the automotive industry. Practically all large players in the field either already use this winding technology or have announced doing so soon. However, hairpins, bar windings, and other segmented winding techniques are not purely a material and production issue. Instead their application to small drives influences all aspects of the design of machines, which are currently explored and studied by the industry. These range from not obvious gaps in the theory, parameter studies for maxima of efficiency, possible as well as advantageous winding schemes, thermal design, and ways to control ac losses to specific materials and process difficulties. Despite the apparent novelty of the trend, however, designers could revisit a widely forgotten knowledge base of more than 100 years for many of those questions. This old knowledge base and the understanding that many recently presented concepts have been developed earlier may speed up the technological development and appear to be a key to further innovation. Instead, many problems need to be solved again and technologies re-invented. Furthermore, as this technology has recently become merely industry-driven, a substantial portion of the information and technological developments are not available to the public -- a state that to our eyes may harm the innovation capacity of the drives community.

eess.SY↗

arXiv2023

A Highly-Compact Direct-Injection Universal Power Flow and Quality Control Circuit

This paper presents a novel direct-injection modular universal power flow and quality control topology exclusively using lower power components. In addition to conventional high-voltage applications, it is particularly attractive for the distribution and secondary grids, e.g., in soft open points, down to low voltage as it can exploit the latest developments in low-voltage high-current semiconductors. In contrast to other concepts that do not interface the grid through transformers, it does not need to convert the entire line power but only the injected or extracted power difference. The proposed power flow and quality (f/q) controller comprises a shunt active front end, together with high-frequency links serving as a power supply for a series floating module per phase. Each of the floating modules is in series with one phase of the line, floating with the electric potential of that particular phase, avoiding any ground connection. Omitting bulky and dynamically limited line transformers of conventional universal power flow controllers, the presented direct-injection f/q controller enables exceptionally small size and volume, high power density, high frequency content, and fast response. In contrast to direct-injection concepts with full back-to-back converters, it only needs to handle a fraction of the power. The circuit combines grid-voltage low-current electronics in the shunt unit and low-voltage high-current modules in the floating series injection units. Simulations and experiments demonstrate and validate the concept.

eess.SY↗

arXiv2023

A Smart Adaptively Reconfigurable DC Battery for Higher Efficiency of Electric Vehicle Drive Trains

This paper proposes a drive train topology with a dynamically reconfigurable dc battery, which breaks hard-wired batteries into smaller subunits. It can rapidly control the output voltage and even contribute to voltage shaping of the inverter. Based upon the rapid development of low-voltage transistors and modular circuit topologies in the recent years, the proposed technology uses recent 48 V power electronics to achieve higher-voltage output and reduce losses in electric vehicle (EV) drive trains. The fast switching capability and low loss of low-voltage field effect transistors (FET) allow sharing the modulation with the main drive inverter. As such, the slower insulated-gate bipolar transistors (IGBT) of the inverter can operate at ideal duty cycle and aggressively reduced switching, while the adaptive dc battery provides an adjustable voltage and all common-mode contributions at the dc link with lower loss. Up to 2/3 of the switching of the main inverter is avoided. At high drive speeds and thus large modulation indices, the proposed converter halves the total loss compared to using the inverter alone; at lower speeds and thus smaller modulation indices, the advantage is even more prominent because of the dynamically lowered dc-link voltage. Furthermore, it can substantially reduce the distortion, particularly at lower modulation indices, e.g., down to 1/2 compared to conventional space-vector modulation and even 1/3 for discontinuous pulse-width modulation with hard-wired battery. Other benefits include alleviated insulation stress for motor windings, active battery balancing, and eliminating the vulnerability of large hard-wired battery packs to weak cells. We demonstrate the proposed motor drive on a 3-kW setup with eight battery modules.

eess.SY↗

arXiv2022

Ageing Mitigation and Loss Control Through Ripple Management in Dynamically Reconfigurable Batteries

Dynamically reconfigurable batteries merge battery management with output formation in ac and dc batteries, increasing the available charge, power, and life time. However, the combined ripple generated by the load and the internal reconfiguration can degrade the battery. This paper introduces that the frequency range of the ripple matters for degradation and loss. It presents a novel control method that reduces the low-frequency ripple of dynamically reconfigurable battery technology to reduce cell ageing and loss. It furthermore shifts the residual ripple to higher frequencies where the lower impedance reduces heating and the dielectric capacitance of electrodes and electrolyte shunt the current around the electrochemical reactions.

eess.SY↗

arXiv2022

Synchronous Multistep Predictive Spectral Control of the Switching Distortion in DC--DC Converters

In automotive power electronics, distortion and electromagnetic interference (EMI) generated by the switching action of power semiconductors can be a significant challenge for the design of a compact, lightweight vehicle. As semiconductor switching frequencies increase, e.g., through the introduction of new materials, such as gallium nitride and silicon carbide, this problem becomes more severe. We present a control scheme for an automotive dc-to-dc converter that reduces the EMI generated by shaping switching distortion predictively at the run time. The multistep model-predictive control scheme chooses the subsequent switching state that optimizes the output spectrum according to predefined criteria. To achieve real-time operation, it evaluates the possible switching state candidates for the next modulation step without explicitly solving a single Fourier transform. In addition, the switching rate and voltage ripple are controlled in a single unified control law. We present and experimentally validate that the control scheme can indeed run at real time already with currently available mid-range hardware. The results demonstrate that the largest spectral peak of the switching distortion can be decreased by 48 dB compared to conventional pulse-width modulation. Furthermore, spectral gaps can be implemented in the output distortion and altered in real-time, allowing certain frequency bands -- e.g., bands used by other sensitive electronics such as sensors, communication busses, or tuners -- to be kept free of EMI from the converter's switching actions.

eess.SY↗