Browsing by Author "Goetz, SM"
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Item Open Access A model of variability in brain stimulation evoked responses.(Conference proceedings : ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference, 2012) Goetz, SM; Peterchev, AVThe input-output (IO) curve of cortical neuron populations is a key measure of neural excitability and is related to other response measures including the motor threshold which is widely used for individualization of neurostimulation techniques, such as transcranial magnetic stimulation (TMS). The IO curve parameters provide biomarkers for changes in the state of the target neural population that could result from neurostimulation, pharmacological interventions, or neurological and psychiatric conditions. Conventional analyses of IO data assume a sigmoidal shape with additive Gaussian scattering that allows simple regression modeling. However, careful study of the IO curve characteristics reveals that simple additive noise does not account for the observed IO variability. We propose a consistent model that adds a second source of intrinsic variability on the input side of the IO response. We develop an appropriate mathematical method for calibrating this new nonlinear model. Finally, the modeling framework is applied to a representative IO data set. With this modeling approach, previously inexplicable stochastic behavior becomes obvious. This work could lead to improved algorithms for estimation of various excitability parameters including established measures such as the motor threshold and the IO slope, as well as novel measures relating to the variability characteristics of the IO response that could provide additional insight into the state of the targeted neural population.Item Open Access A Modular Multilevel Series/Parallel Converter for a Wide Frequency Range Operation(IEEE Transactions on Power Electronics, 2019-10-01) Li, Z; Ricardo Lizana, F; Yu, Z; Sha, S; Peterchev, AV; Goetz, SMWhen providing ac output, modular multilevel converters (MMCs) experience power fluctuation in the phase arms. The power fluctuation causes voltage ripple on the module capacitors, which grows with the output power and inversely to the output frequency. Thus, low-frequency operations of MMCs, e.g., for motor drives, require injecting common-mode voltages and circulating currents, and strict dc voltage output relative to ground is impossible. To address this problem, this paper introduces a novel module topology that allows parallel module connectivity in addition to the series and bypass states. The parallel state directly transfers power across the modules and arms to cancel the power fluctuations and hence suppresses the capacitor voltage ripple. The proposed series/parallel converter can operate at a wide frequency range down to dc without common-mode voltages or circulating currents; it also allows sensorless operation and full utilization of the components at higher output frequencies. We present detailed simulation and experiment results to characterize the advantages and limitations of the proposed solution.Item Open Access A Reduced Series/Parallel Module for Cascade Multilevel Static Compensators Supporting Sensorless Balancing(IEEE Transactions on Industrial Electronics, 2020) Li, Z; Motwani, JK; Zeng, Z; Lukic, SM; Peterchev, AV; Goetz, SMItem Open Access An Estimation-Based Solution to Weak-Grid-Induced Small-Signal Stability Problems of Power Converters(IEEE Journal of Emerging and Selected Topics in Power Electronics, 2020) Fang, J; Yu, J; Zhang, Y; Goetz, SMItem Open Access Circuit topology and control principle for a first magnetic stimulator with fully controllable waveform.(Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Annual International Conference, 2012-01) Goetz, SM; Pfaeffl, M; Huber, J; Singer, M; Marquardt, R; Weyh, TMagnetic stimulation pulse sources are very inflexible high-power devices. The incorporated circuit topology is usually limited to a single pulse type. However, experimental and theoretical work shows that more freedom in choosing or even designing waveforms could notably enhance existing methods. Beyond that, it even allows entering new fields of application. We propose a technology that can solve the problem. Even in very high frequency ranges, the circuitry is very flexible and is able generate almost every waveform with unrivaled accuracy. This technology can dynamically change between different pulse shapes without any reconfiguration, recharging or other changes; thus the waveform can be modified also during a high-frequency repetitive pulse train. In addition to the option of online design and generation of still unknown waveforms, it amalgamates all existing device types with their specific pulse shapes, which have been leading an independent existence in the past years. These advantages were achieved by giving up the common basis of all magnetic stimulation devices so far, i.e., the high-voltage oscillator. Distributed electronics handle the high power dividing the high voltage and the required switching rate into small portions.Item Open Access Grid Impedance Estimation through Grid-Forming Power Converters(IEEE Transactions on Power Electronics, 2020) Fang, J; Deng, H; Goetz, SMItem Open Access Low-Frequency Scheduler for Optimal Conduction Loss in Series/Parallel Modular Multilevel Converters(IEEE Transactions on Power Electronics, 2021) Tashakor, N; Iraji, F; Goetz, SMItem Open Access Magnetic stimulation with arbitrary waveform shapes(IFMBE Proceedings, 2013-04-16) Goetz, SM; Singer, M; Huber, J; Pfaeffl, M; Marquardt, R; Weyh, TDevice technology for magnetic stimulation is still extremely limited regarding waveform dynamics and flexibility. Existing systems are well-known to be very inefficient from an energy perspective. In addition, neither a noninvasive analysis of different neuron dynamics nor an adjustment of the pulse waveform for a more specific stimulation is possible with existing equipment as a matter of principle. The uncontrollable high power in the Megawatt range obstructs such aims with classical means. This contribution introduces a novel stimulator technology which gives up the traditions from classical pulse-source topologies. The design forgoes any high-voltage devices in the actual pulse circuitry, but is based on mass-produced high-power lowvoltage components instead. It enables the generation of almost arbitrary waveforms, including all classical waveforms in magnetic stimulation, with a single device. For any of these pulses the field energy, except the unavoidable basic ohmic losses, can be retrieved from the stimulation coil and be fed back into the internal energy storages. This also applies to classical monophasic pulses, which converted all their energy into heat in classical systems. The power requirements of this technology are comparably low accordingly. The combination of switching control and big highly flexible energy storages moreover enables even high pulse trains as in theta-burst protocols with one pulse source. © 2013 Springer-Verlag.Item Open Access Modular Multilevel Series/Parallel Converter for Bipolar DC Distribution and Transmission(IEEE Journal of Emerging and Selected Topics in Power Electronics, 2020) Lizana F., R; Rivera, S; Li, Z; Dekka, A; Rosenthal, L; Bahamonde I., H; Peterchev, AV; Goetz, SMItem Open Access Modulation and Control of Series/Parallel Module for Ripple-Current Reduction in Star-Configured Split-Battery Applications(IEEE Transactions on Power Electronics, 2020) Li, Z; Lizana, R; Yu, Z; Sha, S; Peterchev, AV; Goetz, SMItem Open Access Multilevel Converters with Symmetrical Half-Bridge Submodules and Sensorless Voltage Balance(IEEE Transactions on Power Electronics, 2020) Fang, J; Li, Z; Goetz, SMItem Open Access Noninvasive Detection of Motor-Evoked Potentials in Response to Brain Stimulation Below the Noise Floor-How Weak Can a Stimulus Be and Still Stimulate.(Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Annual International Conference, 2018-07) Goetz, SM; Li, Z; Peterchev, AVMotor-evoked potentials (MEP) are one of the most important responses to brain stimulation, such as supra-threshold transcranial magnetic stimulation (TMS) and electrical stimulation. The understanding of the neurophysiology and the determination of the lowest stimulation strength that evokes responses requires the detection of even smallest responses, e.g., from single motor units, but available detection and quantization methods are rather simple and suffer from a large noise floor. The paper introduces a more sophisticated matched-filter detection method that increases the detection sensitivity and shows that activation occurs well below the conventional detection level. In consequence, also conventional threshold definitions, e.g., as 50 μV median response amplitude, turn out to be substantially higher than the point at which first detectable responses occur. The presented method uses a matched-filter approach for improved sensitivity and generates the filter through iterative learning from the presented data. In contrast to conventional peak-to-peak measures, the presented method has a higher signal-to-noise ratio (≥14 dB). For responses that are reliably detected by conventional detection, the new approach is fully compatible and provides the same results but extends the dynamic range below the conventional noise floor. The underlying method is applicable to a wide range of well-timed biosignals and evoked potentials, such as in electroencephalography.Item Open Access State-Space Modeling and Control of Grid-Tied Power Converters with Capacitive/Battery Energy Storage and Grid-Supportive Services(IEEE Journal of Emerging and Selected Topics in Power Electronics, 2021) Fang, J; Deng, H; Tashakor, N; Blaabjerg, F; Goetz, SM