[article-cris] Sähkötekniikan korkeakoulu / ELEC

Permanent URI for this collectionhttps://aaltodoc.aalto.fi/handle/123456789/21538

Browse

Browsing [article-cris] Sähkötekniikan korkeakoulu / ELEC by Issue Date

Filter results by year or month
Now showing 1 - 20 of 4665
  • Photoreflectance study of photovoltage effects in GaAs diode structures
    (1992) Airaksinen, V-M.; Lipsanen, H.K.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Photoreflectance measurements of a GaAs p‐i‐n diode as a function of temperature (50–450 K) are reported. The photovoltage in the structure is obtained from the electric field strength which is determined from the Franz–Keldysh oscillations in the photoreflectance spectrum. The results are compared to those from an n‐GaAs structure where the Fermi level is pinned at the surface. In this case the photovoltage can be determined only by fixing the energy of the Fermi pinning level. The theoretical photovoltages are also calculated from the ideal p‐n and Schottky diode equations. This study clearly shows the importance of the photovoltage effects in the photoreflectance measurements.
  • Luminescence from excited states in strain-induced In_(x)Ga_(1-x)As quantum dots
    (1995-05-15) Lipsanen, H.; Sopanen, M.; Ahopelto, J.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We have fabricated quantum dots by locally straining InxGa1−xAs quantum wells with self-organized growth of nanometer-scale InP stressors on the sample surface. The structure is completed in a single growth run using metalorganic vapor-phase epitaxy. Photoluminescence from the dots is redshifted by up to 105 meV from the quantum-well peak due to the lateral confinement of excitons. Clearly resolved luminescence peaks from three excited states separated by 16–20 meV are observed when the quantum well is placed at the depth of 1–10 nm from the surface of the sample. The observed redshift and peak separation are in agreement with simple calculations using a finite-element method and two-dimensional parabolic potential model. This structure is easily fabricated and offers a great potential for the optical study of relaxation and recombination phenomena.
  • Fabrication of InGaAs quantum disks using self-organized InP islands as a mask in wet chemical etching
    (1996) Sopanen, M.; Lipsanen, H.; Ahopelto, J.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    GaInAs quantum disks are fabricated by wet chemical etching from a GaInAs/GaAs near‐surface quantum well using self‐organized InP islands as an etch mask. InP islands are formed in coherent Stranski–Krastanow growth mode by metalorganic vapor phase epitaxy. The free‐standing GaInAs/GaAs columns, produced by a three‐step etching process, are overgrown at 550 °C. The luminescence efficiency per emitting area from the regrown quantum disks is one order of magnitude larger than that from a regrown reference quantum well.
  • Zeeman effect in parabolic quantum dots
    (1996-07-08) Rinaldi, R.; Guigno, P.; Cingolani, R.; Lipsanen, Harri; Sopanen, M.; Tulkki, J.; Ahopelto, J.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    An unprecedentedly well resolved Zeeman effect has been observed when confined carriers moving along a closed mesoscopic path experience an external magnetic field orthogonal to the orbit plane. Large Zeeman splitting of excited higher angular momentum states is observed in the magnetoluminescence spectrum of quantum dots induced by self-organized InP islands on InGaAs/GaAs. The measured effect is quantitatively reproduced by calculations including the vertical quantum well confinement and strain induced, nearly parabolic, lateral confinement, together with the magnetic interaction.
  • Copper flotation profit and control-system accuracy
    (1996-11) Tenno, R.; Jämsä-Jounela, Sirkka-Liisa
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Copper flotation profit can be estimated online utilising the assay of the on-stream analysers. High and low limits of the profit can be calculated, and the range between limits can be used for an evaluation of the return on investment, which has to be done during the implementation of a new control system or analyser. The range depends on the target values for concentrate and tailings, and on the accuracy of the controls and analyser. The separate pay-back values from any improvement in the controls or the analyser accuracy are estimated. The economic effect is analysed and demonstrated on data collected from two flotation plants.
  • Carrier relaxation dynamics in quantum dots: scattering mechanisms and state-filling effects
    (1997-02-15) Grosse, S.; Sandman, J.; von Plessen, G.; Feldman, J.; Lipsanen, H.; Sopanen, M.; Tulkki, J.; Ahopelto, J.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Stressor-induced InxGa1−xAs quantum dot structures of high structural quality allow a detailed experimental investigation of carrier relaxation between distinct zero-dimensional quantized states. Time-resolved photoluminescence studies combined with appropriate model calculations show that state filling effects, Coulomb scattering, and acoustic phonon scattering determine the relaxation scenario in a way characteristic for a zero-dimensional electronic system. These investigations allow a quantitative estimation of the inter-dot-level relaxation rates mediated by (i) Coulomb scattering and (ii) acoustic phonon scattering.
  • Magneto-optical properties of strain-induced InxGa1−xAs parabolic quantum dots
    (1998-04-15) Rinaldi, R.; Mangino, R.; Cingolani, R.; Lipsanen, H.; Sopanen, Markku; Tulkki, J.; Brasken, M.; Ahopelto, J.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We have investigated the Zeeman splitting in strained InxGa1−xAs quantum dots with different quantization energies by means of magnetoluminescence. A multifold splitting of Π and Δ states is observed due to lifted degeneracy of m>0 states. The experimental data were systematically compared to the diamagnetic and Zeeman shift of the single-particle states, taking into account the details of the valence-band structure and the spin splitting. Our data indicate that excitonic effects are negligible and that the magneto-optical properties of these strongly confined dots can be described rather accurately within the single-particle model.
  • Doping and carrier transport in Ga1−3xIn3xNxAs1−x alloys
    (2001-08-29) Li, Wei; Pessa, M.; Toivonen, J.; Lipsanen, H.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Si- and Be-doped Ga1−3xIn3xNxAs1−x(0<~x<~3%) layers are grown on GaAs substrates by gas-source molecular beam epitaxy with a nitrogen radical beam source. The carrier concentration and mobility are observed to decrease substantially with increasing nitrogen content in both p- and n-type GaInNAs films. After rapid thermal annealing at 750 °C, the Be dopants are almost fully activated in p-type material; yet only a small fraction of the Si dopants are activated in n-type GaInNAs films. At low temperature a broad photoluminescence band centered at 1.041 eV (about 120 meV below the band gap) is observed in n-type GaInNAs, which suggests the possible compensating centers present in Si-doped GaInNAs.
  • Efficiency of a voltage sourced inverter with controllable intermediate DC-voltage
    (2002) Pinho, Paulo; Kyyrä, Jorma
     A4 Artikkeli konferenssijulkaisussa
    The efficiency of a low cost three-phase PWM (Pulse-Width-Modulated) VSI (Voltage Source Inverter) is evaluated in this paper based on the fact that a controllable DC-link is available. The study was conducted considering that a VSI should supply a three-phase low-voltage induction motor, with rated power of 500 kW supplied with a industrial voltage of 690 V. Results presented in this paper are based on computer simulations and analytical expressions for calculation of losses. The goal is to improve the overall efficiency on the inverter part of the ASD (Adjustable-Speed-Drive), especially at low speed region without harming the current and torque quality, which should be kept within acceptable values. The circuit topology is a conventional two level IGBT-based three-phase voltage source inverter. A two-level natural sampling PWM with sine-wave modulation was used in order to generate the control pulses for the IGBT’s and hence allowing the output voltage level to be varied. Scalar control of the inverter output voltage was implemented in order to allow the maximum air gap flux in the motor. Using an ideal (without ripple) and controllable intermediate DC-link voltage stage, several computer simulations are accomplished and waveforms are used to evaluate the inverter performance. The objective of the so-called open-loop simulations is to verify how the circuit behaves and to have a simple indication on trend of the results. The final results clearly show that by lowering the DC-link voltage, the switching losses of the inverter can be reduced, especially at low speeds. Also torque quality and total harmonic distortion of the output current are slightly improved.
  • Zero-speed operation of sensorless induction motors using full-order flux observer
    (2002-08-12) Hinkkanen, Marko; Luomi, Jorma
     A4 Artikkeli konferenssijulkaisussa
    This paper deals with the flux estimation for sensorless induction motor drives. Stable zero-speed operation is achieved by using a speed-adaptive full-order flux observer and a simple software compensation of inverter nonlinearities. No voltage feedback is needed for the compensation and hardware costs can be lowered. Experimental results including stable zero-speed operation are shown.
  • Digital implementation of full-order flux observers for induction motors
    (2002-09-09) Hinkkanen, Marko; Luomi, Jorma
     A4 Artikkeli konferenssijulkaisussa
    This paper deals with flux estimation for induction motor drives by using a full-order flux observer. A problem of full-order flux observers is their need for computationally demanding discretization methods in order to work stably and accurately at high speeds. An implementation of the full-order flux observer using the stator and rotor fluxes as state variables in the stator reference frame and in the rotor reference frame, respectively, was recently proposed. This paper describes how an observer gain can be included in this structure. It is shown that discretization errors of the proposed implementation are small and that there is more freedom to choose an observer gain, even if the simple forward Euler discretization is used.
  • Control of pulp levels flotation cells
    (2003) Jämsä-Jounela, Sirkka-Liisa; Dietrich, Maija; Halmevaara, Kalle; Tiili, Otso
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The control of pulp levels in flotation cells is a complex control task. Many different aspects such as cell structure and valve sizing, and their effects on control, have to be taken into account. Control becomes more and more difficult as the complexity of the cell configurations and the number of cells used in configuration increase. In this paper the mathematical model of a flotation cell is first developed and several configurations of cells are constructed. Simulations are then performed in order to determine suitable control parameters for PI and feed-forward controllers of the cell levels. Finally, the effects of valve sizing and cell structure are presented and discussed.
  • Parameter sensitivity of full-order flux observes for induction motors
    (2003-07-28) Hinkkanen, Marko; Luomi, Jorma
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    This paper deals with flux estimation for induction motor drives. The equations of the parameter sensitivity of both the rotor flux estimation and the torque production are derived for a full-order flux observer. Based on the parameter sensitivity analysis, practical methods of designing robust observer gains combining the current model and the voltage model are proposed. The proposed gains are easy to tune and lead to a simple observer structure. Experimental results show that for inaccurate parameter estimates, both the steady-state and dynamic errors in the produced torque are small as compared to the current model. Furthermore, high-speed operation is possible without modeling the magnetic saturation even if motor parameters are highly erroneous.
  • Modified integrator for voltage model flux estimation of induction motors
    (2003-07-28) Hinkkanen, Marko; Luomi, Jorma
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    This letter deals with voltage model flux estimators for sensorless induction motor drives. In order to eliminate the drift problems, the pure integrator of the voltage model is replaced with a first-order low-pass filter, and the error due to this replacement is compensated in a very simple way.
  • Polarizability of conducting sphere-doublets using series of images.
    (2004) Wallen, Henrik; Sihvola, Ari
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The classical electrostatic problem of two nonintersecting conducting spheres in a uniform incident electric field is considered. Starting from the basic Kelvin’s image principle, the two spheres are replaced with equivalent series of image sources, from which the polarizability is calculated. Explicit expressions for the axial and transversal components of the polarizability dyadic are found by solving the recurrence equations. Efficient numerical evaluation of the different series is also discussed.
  • Combination of voltage model and high-frequency signal injection for sensorless permanent magnet synchronous motor drives
    (2004-09-05) Piippo, Antti; Hinkkanen, Marko; Luomi, Jorma
     A4 Artikkeli konferenssijulkaisussa
    The paper proposes a method for the rotor speed and position estimation of permanent magnet synchronous motors, applicable in a wide speed range including standstill. A modified voltage model is used at high speeds, and it is combined with a high-frequency signal injection technique at low speeds. The fast dynamic response of the voltage model is thus augmented with the steady-state accuracy of the high-frequency signal injection technique. The stability and robustness of the combined observer are demonstrated by simulations and experiments.
  • Analysis and design of full-order flux observers for sensorless induction motors
    (2004-10-04) Hinkkanen, Marko
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    This paper deals with the flux estimation for sensorless induction motor drives. The linearized model of the speed-adaptive full-order flux observer is applied to help choosing the observer gain and the speed-adaptation gains. It is shown that the linearized model reveals potential instability problems that are difficult to find by other means. An observer gain and a method to vary the speed-adaptation gains in the field-weakening region are proposed. Experimental results show stable operation in a very wide speed range.
  • Stabilization of regenerating-mode operation in sensorless induction motor drives by full-order flux observer design
    (2004-11-22) Hinkkanen, Marko; Luomi, Jorma
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    This paper deals with the full-order flux observer design for speed-sensorless induction motor drives. An unstable region encountered in the regenerating mode at low speeds is well known. To remedy the problem, a modified speed-adaptation law is proposed. Instead of using only the current estimation error perpendicular to the estimated flux, the parallel component is also exploited in the regenerating mode. Using current estimation error loci in steady state, a linearized model, simulations, and experiments, it is shown that the observer using the proposed speed-adaptation law does not have the unstable region. It is also shown that the effect of erroneous parameter estimates on the accuracy of the observer is comparatively small.
  • Development issues on the life cycle management of mineral grinding processes and equipment
    (2005) Remes, Antti; Suontaka, Ville; Saloheimo, Kari; Jämsä-Jounela, Sirkka-Liisa
     A4 Artikkeli konferenssijulkaisussa
    The requirements set on the performance of industrial equipment during their life-time are increasing due to the growing demand to minimize expenses and maximize production as a result of tightening global competition. The demand to optimize the equipment performance is making the ability to utilize different services during the equipment more important. Equipment performance optimization includes integration of the process and maintenance information. In this paper the life-cycle management concepts are reviewed and a general framework and a case study in the mineral processing field is presented and discussed.
  • The WEBT campaign to observe AO 0235+16 in the 2003-2004 observing season. Results from radio-to-optical monitoring and XMM-Newton observations
    (2005) Raiteri, C.M.; Villata, M.; Ibrahimov, M.A.; Larionov, V.M.; Kadler, M.; Aller, H.D.; Aller, M.F.; Kovalev, Y.Y.; Lanteri, L.; Nilsson, K.; Papadakis, I.E.; Pursimo, T.; Romero, G.E.; Teräsranta, H.; Tornikoski, M.; Arkharov, A.A.; Barnaby, D.; Berdyugin, A.; Böttcher, M.; Byckling, K.; Carini, M.T.; Carosati, D.; Cellone, S.A.; Ciprini, S.; Combi, J.A.; Crapanzano, S.; Crowe, R.; di Paola, A.; Dolci, M.; Fuhrmann, L.; Gu, M.; Hagen-Thorn, V.A.; Hakala, P.; Impellizzeri, V.; Jorstad, S.; Kerp, J.; Kimeridze, G.N.; Kovalev, Yu.A.; Kraus, A.; Krichbaum, T.P.; Kurtanidze, O.M.; Lähteenmäki, A.; Lindfors, E.; Mingaliev, M.G.; Nesci, R.; Nikolashvili, M.G.; Ohlert, J.; Orio, M.; Ostorero, L.; Pasanen, M.; Pati, A.; Poteet, C.; Ros, E.; Ros, J.A.; Shastri, P.; Sigua, L.A.; Sillanpää, A.; Smith, N.; Takalo, L.O.; Tosti, G.; Vasileva, A.; Wagner, S.J.; Walters, R.; Webb, J.R.; Wills, W.; Witzel, A.; Xilouris, E.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä