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ABSTRACT: The fourth-order converter known as superbuck is often used to interface the solar arrays into the rest of the power system due to the continuous input current it provides. Peak-current-mode (PCM) control is applied. The small-signal models of such a converter do not exist in public domain literature and its dynamical features are not known. The paper introduces the dynamical modeling based on consistent methods. The characterization shows that the PCM-controlled superbuck converter may have similar features characteristic to the conventional buck converter such as high input-voltage-noise attenuation, effectively first-order output dynamics, and high insensitivity to source interactions, when the inductor-current feedback is properly compensated. The converter is, however, more susceptible to the input-filter instability due the shape of the ideal input impedance than the conventional buck converter. Experimental evidence is provided based on a 440-kHz converter.
IEEE Transactions on Power Electronics 06/2008; · 4.65 Impact Factor
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ABSTRACT: Limitations of traditional magnetoencephalography (MEG) exclude some important patient groups from MEG examinations, such as epilepsy patients with a vagus nerve stimulator, patients with magnetic particles on the head or having magnetic dental materials that cause severe movement-related artefact signals. Conventional interference rejection methods are not able to remove the artefacts originating this close to the MEG sensor array. For example, the reference array method is unable to suppress interference generated by sources closer to the sensors than the reference array, about 20-40 cm. The spatiotemporal signal space separation method proposed in this paper recognizes and removes both external interference and the artefacts produced by these nearby sources, even on the scalp. First, the basic separation into brain-related and external interference signals is accomplished with signal space separation based on sensor geometry and Maxwell's equations only. After this, the artefacts from nearby sources are extracted by a simple statistical analysis in the time domain, and projected out. Practical examples with artificial current dipoles and interference sources as well as data from real patients demonstrate that the method removes the artefacts without altering the field patterns of the brain signals.
Physics in Medicine and Biology 05/2006; 51(7):1759-68. · 2.83 Impact Factor
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ABSTRACT: The reliability of biomagnetic measurements is traditionally challenged by external interference signals, movement artifacts, and comparison problems caused by different positions of the subjects or different sensor configurations. The Signal Space Separation method (SSS) idealizes magnetic multichannel signals by transforming them into device-independent idealized channels representing the measured data in uncorrelated form. The transformation has separate components for the biomagnetic and external interference signals, and thus, the biomagnetic signals can be reconstructed simply by leaving out the contribution of the external interference. The foundation of SSS is a basis spanning all multichannel signals of magnetic origin. It is based on Maxwell's equations and the geometry of the sensor array only, with the assumption that the sensors are located in a current free volume. SSS is demonstrated to provide suppression of external interference signals, standardization of different positions of the subject, standardization of different sensor configurations, compensation for distortions caused by movement of the subject (even a subject containing magnetic impurities), suppression of sporadic sensor artifacts, a tool for fine calibration of the device, extraction of biomagnetic DC fields, and an aid for realizing an active compensation system. Thus, SSS removes many limitations of traditional biomagnetic measurements.
IEEE Transactions on Signal Processing 10/2005; · 2.63 Impact Factor
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J. P. Mäkelä,
A. Ahonen,
M. Hämäläinen,
R. Hari,
R. Llmoniemi,
M. Kajola,
J. Knuutila,
O. V. Lounasmaa,
L. McEvoy,
R. Salmelin,
O. Salonen,
M. Sams, J. Simola,
C. Tesche,
J-P. Vasama
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ABSTRACT: We recorded auditory evoked magnetic fields from nine healthy subjects with a 122-channel whole-head SQUID magnetometer. This type of measurement ensures that responses from both hemispheres are collected in the same stage of vigilance. The stimuli were 50-ms 1-kHz tones, delivered alternately to the two ears at interstimulus intervals (ISIs) of 1, 2, 4, 8, and 16 s per ear. In both hemispheres, the prominent 100-ms response (N100m) was, on average, 22% larger and 9 ms earlier for contralateral than ipsilateral tones. The N100m amplitude increased as a function of ISI and saturated at ISIs of 8–16 s. The average ISI dependence of N100m was similar over both hemispheres and for both contralateral and ipsilateral stimuli, implying that tones leave neural traces of similar duration in both hemispheres. Differences were observed as well: the response waveforms were clearly asymmetric in one subject, and four subjects displayed an additional right-hemisphere-dominant 250-ms deflection (N250m). © 1993 Wiley-Liss, Inc.
Human Brain Mapping 10/2004; 1(1):48 - 56. · 5.88 Impact Factor
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ABSTRACT: Stationary SQUID sensors record time-varying magnetic fields only. Any DC sources, such as magnetic impurities on the scalp or physiological DC currents, are invisible in conventional MEG with stationary sources and sensors. However, movement of the subject relative to the measurement device transforms the DC fields into time-varying MEG signals, which are either signals of interest from biomagnetic sources, or movement artifacts when caused by magnetic residue on the head. These signals can be demodulated to DC by tracking the head movement and by using this recorded information to decompose the signals into a device-independent source model. To do this we have used the signal space separation method (SSS) along with a continuous head position monitoring system. From time variations of the recorded signal, a linear equation is obtained relating the averaged MEG signal variation, the DC-source in the head, and the varying external interference. In this way an unbiased estimate is obtained for the DC source as it is automatically separated from external interference. The method was tested by feeding DC current in an artificial current dipole on a phantom head and by continuously moving and rotating this phantom randomly with a motion amplitude of several centimeters. After the SSS based movement demodulation and reconstruction of the signal from inside of the helmet, the location of the DC current dipole in the phantom could be determined with an accuracy of 2 mm. It is concluded that the method enables localization of DC sources with MEG using voluntary head movements.
Neurology & Clinical Neurophysiology 02/2004; 2004:35.
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ABSTRACT: Brain mechanisms extracting invariant information from varying auditory inputs were studied using the mismatch-negativity (MMN) brain response. We wished to determine whether the preattentive sound-analysis mechanisms, reflected by MMN, are capable of extracting invariant relationships based on abstract conjunctions between two sound features. The standard stimuli varied over a large range in frequency and intensity dimensions following the rule that the higher the frequency, the louder the intensity. The occasional deviant stimuli violated this frequency-intensity relationship and elicited an MMN. The results demonstrate that preattentive processing of auditory stimuli extends to unexpectedly complex relationships between the stimulus features.
Psychophysiology 04/2001; 38(2):359-65. · 3.29 Impact Factor
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ABSTRACT: We recorded somatosensory evoked magnetic fields from ten healthy, right-handed subjects with a 122-channel whole-scalp SQUID magnetometer. The stimuli, exceeding the motor threshold, were delivered alternately to the left and right median nerves at the wrists, with interstimulus intervals of 1, 3, and 5 s. The first responses, peaking around 20 and 35 ms, were explained by activation of the contralateral primary somatosensory cortex (SI) hand area. All subjects showed additional deflections which peaked after 85 ms; the source locations agreed with the sites of the secondary somatosensory cortices (SII) in both hemispheres. The SII responses were typically stronger in the left than the right hemisphere. All subjects had an additional source, not previously reported in human evoked response data, in the contralateral parietal cortex. This source was posterior and medial to the SI hand area, and evidently in the wall of the postcentral sulcus. It was most active at 70-110 ms.
Experimental Brain Research 02/1994; 99(2):309-15. · 2.39 Impact Factor
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ABSTRACT: Two tone stimuli, one frequent (standard) and the other infrequent (a slightly higher, deviant tone), were presented in random order and at short intervals to subjects reading texts they had selected. In different blocks, standards were either 250, 1,000, or 4,000 Hz, with the deviants always being 10% higher in frequency than the standards of the same blocks. Magnetic responses elicited by the standard and deviant tones included N1m, the magnetoencephalographic equivalent of the electrical N1 (its supratemporal component). In addition, deviant stimuli elicited MMNm, the magnetic equivalent of the electrical mismatch negativity, MMN. The equivalent dipole sources of the two responses were located in supratemporal auditory cortex, with the MMNm source being anterior to that of N1m. The dipole orientations of both sources in teh sagittal plane depended on stimulus frequency, suggesting that the responses are generated by tonotopically organized neuronal populations. The tonotopy reflected by the frequency dependence of the MMNm source might be that of the neural trace system underlying frequency representation of auditory stimuli in sensory memory.
Psychophysiology 10/1993; 30(5):537-40. · 3.29 Impact Factor
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ABSTRACT: Neuromagnetic responses were recorded over the left hemisphere to find out in which cortical area the heard and seen speech are integrated. Auditory stimuli were Finnish/pa/syllables presented together with a videotaped face articulating either the concordant syllable/pa/(84% of stimuli, V = A) or the discordant syllable/ka/(16%, V not equal to A). In some subjects the probabilities were reversed. The subjects heard V not equal to A stimuli as/ta/ or ka. The magnetic responses to infrequent perceptions elicited a specific waveform which could be explained by activity in the supratemporal auditory cortex. The results show that visual information from articulatory movements has an entry into the auditory cortex.
Neuroscience Letters 07/1991; 127(1):141-5. · 2.11 Impact Factor
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ABSTRACT: It is shown that DC-SQUID read-out electronics can be realized
utilizing positive feedback without deteriorating the SQUID noise
performance. The required gain rise is achieved by interconnecting the
SQUID output and a flux modulation coil via a cooled FET acting as a
voltage-controlled resistor; different SQUIDs with different types of
FETs have been studied experimentally. Possible ways of adaptively
controlling the feedback gain produced by the positive feedback are
briefly discussed
IEEE Transactions on Magnetics 04/1991; · 1.36 Impact Factor
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ABSTRACT: In this paper we describe the instrumentation for biomagnetic measurements available in our laboratory. The focus is on our 24-channel planar gradiometer system. In addition, a 122-channel system under construction will be discussed.
Clinical Physics and Physiological Measurement 02/1991; 12 Suppl B:39-44.
