Browsing by Department "Department of Electrical Engineering and Automation"
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Item 2-D magnetomechanical transient simulation of a motor with a bar breakage(2017) Climente-Alarcon, Vicente; Arkkio, Antero; Antonino-Daviu, Jose A.; Department of Electrical Engineering and Automation; Polytechnic University of ValenciaThe analysis of the vibration response of electrical machines has importance in noise prediction and more recently, fault diagnosis. This work presents a strongly coupled 2-D magnetomechanical simulation of an induction machine under heavy operational conditions: a direct-on-line startup. Both healthy and broken bar states are simulated in a time span long enough to allow the detailed study of the varying frequency components. The results yield, in addition to the usual electrical and magnetic quantities, electromagnetic induced vibration components in the stator. A comparison with current and vibration experimental data is also performed.Item 2-D Magnetomechanical Transient Study of a Motor Suffering a Bar Breakage(IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2018) Climente-Alarcon, Vicente; Arkkio, Antero; Antonino-Daviu, Jose; Department of Electrical Engineering and Automation; Polytechnic University of ValenciaThe analysis of the vibration response of electrical machines has importance in noise prediction and more recently, diagnosis of electrical faults, especially in the industrial environment, where it is a well-known technique. This work assesses the performance of a strongly coupled 2-D magnetomechanical approach, as directly available in multiphysics software, for the simulation of an induction machine under heavy operational conditions: a direct-on-line startup. Both healthy and broken bar states are simulated in a time span long enough to allow the detailed study of the varying frequency components. The results yield, in addition to the usual electrical and magnetic quantities, electromagnetic-induced vibration components in the stator. A comparison with current and vibration experimental data is also performed showing a good agreement with variable frequency components and certain limitations concerning their amplitude.Item 2D Analytical Model for Computing Eddy-Current Loss in Nonlinear Thick Steel Laminations(IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2022-09) Gurbuz, Ismet; Rasilo, Paavo; Martin, Floran; Osemwinyen, Osaruyi; Belahcen, Anouar; Department of Electrical Engineering and Automation; Computational ElectromechanicsIn this article, we propose an analytical method to compute the eddy-current loss in nonlinear thick steel laminations (3-12 mm) by considering the return path of the eddy currents. Initially, a 2-D finite-element (FE) model is applied to segregate losses measured from toroidal material samples into hysteresis and eddy-current loss components to use them as reference. Afterward, a 2-D analytical time-domain model is proposed for the eddy currents based on the solution of the 2-D field problem. The time-domain model is then used to derive a simple frequency-domain eddy-current loss formulation for the sinusoidal flux density case with the inclusion of a skin-effect correction factor, which accounts for the nonlinearity of the material. Highly accurate results are obtained from the proposed model compared to FE reference results with a mean relative error of 5.1% in the nonlinear region.Item 3-D Eddy Current Modelling of Steel Laminations to Analyze Edge Effects(2016-07-01) Sundaria, Ravi; Rasilo, Paavo; Belahcen, Anouar; Arkkio, Antero; Department of Electrical Engineering and Automation; Tampere University of Technology; Belahcem, Anouar; Demenko, Andrzej; Pietrowski, WojciechThe correct estimation of iron losses is still a challenging task in the numerical analysis of electrical machines. For estimation of eddy current losses, various formulations based on 1-D and 2-D models are mentioned in literature which neglect effect of current density at the edges of steel laminations. This paper compares such simplified 1-D/2-D eddy current loss model with a 3-D model to analyze the effect of edges on eddy current loss calculation. Thickness of the lamination along with frequency of field excitation were determined where considerable deviation in eddy current losses among loss models is observed due to edge assumption.Item 3-D simulation of a rotor suffering a bar breakage(2017) Climente-Alarcon, Vicente; Arkkio, Antero; Antonino-Daviu, Jose A.; Department of Electrical Engineering and Automation; Polytechnic University of ValenciaThe development of Condition-Based Maintenance and Prognostics and Health Management systems can be fostered by models that accurately reproduce the effects and propagation of faults. Conventional methods applied to the analysis and simulation of rotor asymmetries in induction machines are based in the classical 2-D finite element approach, which is unable to reflect its complex effects, such as interbar currents and their interaction with the stator flux density wave. Thus, this work applies a 2D-3D multiphysics finite element procedure to assess the impact of a bar breakage on a rotor cage during stationary operation, with reduced computational cost compared to a full motor 3-D simulation. Initial results improve previous ones obtained for transient operation conditions, showing the potential of the approach and confirming previous analytical studies.Item A 3D dynamic lumped parameter thermal network of air-cooled yasa axial flux permanent magnet synchronous machine(2018-04-01) Mohamed, Abdalla Hussein; Hemeida, Ahmed; Rashekh, Alireza; Vansompel, Hendrik; Arkkio, Antero; Sergeant, Peter; Ghent University; Department of Electrical Engineering and AutomationTo find the temperature rise for high power density yokeless and segmented armature (YASA) axial flux permanent magnet synchronous (AFPMSM) machines quickly and accurately, a 3D lumped parameter thermal model is developed and validated experimentally and by finite element (FE) simulations on a 4 kW YASA machine. Additionally, to get insight in the thermal transient response of the machine, the model accounts for the thermal capacitance of different machine components. The model considers the stator, bearing, and windage losses, as well as eddy current losses in the magnets on the rotors. The new contribution of this work is that the thermal model takes cooling via air channels between the magnets on the rotor discs into account. The model is parametrized with respect to the permanent magnet (PM) angle ratio, the PM thickness ratio, the air gap length, and the rotor speed. The effect of the channels is incorporated via convection equations based on many computational fluid dynamics (CFD) computations. The model accuracy is validated at different values of parameters by FE simulations in both transient and steady state. The model takes less than 1 s to solve for the temperature distribution.Item 3D Printing of Superhydrophobic Objects with Bulk Nanostructure(WILEY-V C H VERLAG GMBH, 2021-11-11) Dong, Zheqin; Vuckovac, Maja; Cui, Wenjuan; Zhou, Quan; Ras, Robin H.A.; Levkin, Pavel A.; Department of Applied Physics; Department of Electrical Engineering and Automation; Department of Bioproducts and Biosystems; Soft Matter and Wetting; Robotic Instruments; Karlsruhe Institute of TechnologyThe rapid development of 3D printing (or additive manufacturing) technologies demands new materials with novel properties and functionalities. Superhydrophobic materials, owing to their ultralow water adhesion, self-cleaning, anti-biofouling, or superoleophilic properties are useful for myriad applications involving liquids. However, the majority of the methods for making superhydrophobic surfaces have been based on surface functionalization and coatings, which are challenging to apply to 3D objects. Additionally, these coatings are vulnerable to abrasion due to low mechanical stability and limited thickness. Here, a new materials concept and methodology for 3D printing of superhydrophobic macroscopic objects with bulk nanostructure and almost unlimited geometrical freedom is presented. The method is based on a specific ink composed of hydrophobic (meth)acrylate monomers and porogen solvents, which undergoes phase separation upon photopolymerization to generate inherently nanoporous and superhydrophobic structures. Using a desktop Digital Light Processing printer, superhydrophobic 3D objects with complex shapes are demonstrated, with ultralow and uniform water adhesion measured with scanning droplet adhesion microscopy. It is shown that the 3D-printed objects, owing to their nanoporous structure throughout the entire volume, preserve their superhydrophobicity upon wear damage. Finally, a superhydrophobic 3D-printed gas-permeable and water-repellent microfluidic device and a hierarchically structured 3D-printed super-oil-absorbent are demonstrated.Item A 3D Terrain Generator: Enhancing Robotics Simulations with GANs(2024-02-16) Arellano, Silvia; Otero, Beatriz; Kucner, Tomasz Piotr; Canal, Ramon; Department of Electrical Engineering and Automation; Nicosia, Giuseppe; Ojha, Varun; La Malfa, Emanuele; La Malfa, Gabriele; Pardalos, Panos M.; Umeton, Renato; Mobile Robotics; BarcelonaTechSimulation is essential in robotics to evaluate models and techniques in a controlled setting before conducting experiments on tangible agents. However, developing simulation environments can be a challenging and time-consuming task. To address this issue, a proposed solution involves building a functional pipeline that generates 3D realistic terrains using Generative Adversarial Networks (GANs). By using GANs to create terrain, the pipeline can quickly and efficiently generate detailed surfaces, saving researchers time and effort in developing simulation environments for their experiments. The proposed model utilizes a Deep Convolutional Generative Adversarial Network (DCGAN) to generate heightmaps, which are trained on a custom database consisting of real heightmaps. Furthermore, an Enhanced Super-Resolution Generative Adversarial Network (ESRGAN) is used to improve the resolution of the resulting heightmaps, enhancing their visual quality and realism. To generate a texture according to the topography of the heightmap, chroma keying is used with previously selected textures. The heightmap and texture are then rendered and integrated, resulting in a realistic 3D terrain. Together, these techniques enable the model to generate high-quality, realistic 3D terrains for use in robotic simulators, allowing for more accurate and effective evaluations of robotics models and techniques.Item A 50 Hz magnetic field affects hemodynamics, ECG and vascular endothelial function in healthy adults(Public Library of Science, 2021-08-05) Okano, Hideyuki; Fujimura, Akikatsu; Kondo, Tsukasa; Laakso, Ilkka; Ishiwatari, Hiromi; Watanuki, Keiichi; Saitama University; Department of Electrical Engineering and Automation; Soken Medical Co. Ltd.Application of exposure to 50/60 Hz magnetic fields (MFs) has been conducted in the treatment of muscle pain and fatigue mainly in Japan. However, whether MFs could increase blood flow leading to muscle fatigue recovery has not been sufficiently tested. We investigated the acute effects of a 50 Hz sinusoidal MF at Bmax 180 mT on hemodynamics, electrocardiogram, and vascular endothelial function in healthy young men. Three types of regional exposures to a 50 Hz MF, i.e., forearm, upper arm, or neck exposure to MF were performed. Participants who received three types of real MF exposures had significantly increased ulnar arterial blood flow velocity compared to the sham exposures. Furthermore, after muscle loading exercise, MF exposure recovered hemoglobin oxygenation index values faster and higher than sham exposure from the loading condition. Moreover, participants who received real MF exposure in the neck region had significantly increased parasympathetic high-frequency activity relative to the sham exposure. The MF exposure in the upper arm region significantly increased the brachial artery flow-mediated dilation compared to the sham exposure. Computer simulations of induced in situ electric fields indicated that the order-of-magnitude estimates of the peak values were 100-500 mV/m, depending on the exposure conditions. This study provides the first evidence that a 50 Hz MF can activate parasympathetic activity and thereby lead to increase vasodilation and blood flow via a nitric oxide-dependent mechanism.Item AC Loss Analysis Approaches for Hairpin Winding Configuration: Analytical, Hybrid Model, and FEA(2023) Ghahfarokhi, Payam Shams; Kallaste, Ants; Podgornovs, Andrejs; Cardoso, Antonio J.Marques; Belahcen, Anouar; Vaimann, Toomas; Tallinn University of Technology; Riga Technical University; University of Beira Interior; Department of Electrical Engineering and Automation; Department of Electrical Engineering and AutomationOne of the effective options to achieve higher power density and lower-weight electric motors for electric vehicle (EV) applications is to replace the conventional winding with a hairpin configuration. This novel concept has several advantages, but the biggest drawback is high AC loss. Therefore, as this type of winding utilizes in EV motors for high-speed application, the correct estimations of this loss are essential during the design procedure. This paper presents three primary approaches to model and calculate the AC loss of hairpin windings: analytical, hybrid model, and FEA methods. In addition, the FEA method is used to validate and evaluate the accuracy of two other methods. Accordingly, both analytical and hybrid model results agree with FEA results. However, the hybrid model has higher accuracy rather than the analytical method.Item AC Losses in Windings: Review and Comparison of Models with Application in Electric Machines(IEEE, 2023-12-19) El Hajji, Taha; Hlioui, Sami; Louf, François; Gabsi, Mohamed; Belahcen, Anouar; Mermaz-Rollet, Guillaume; Belhadi, M'Hamed; Department of Electrical Engineering and Automation; Computational Electromechanics; CY Cergy Paris Université; University of Paris-Saclay; StellantisIndustry is increasingly adopting high-speed electric machines due to their high-power density and low cost. However, increasing the speed leads to many challenges that are worth to study since they have relevant impact on the design of the machine. One of these challenges is the losses in stator winding assembly. At low frequency, losses in windings are usually assumed equal to DC losses. However, at high frequency some phenomena become noticeable and worth evaluating since they lead to noticeable losses. AC losses in windings are due to skin effect, proximity effect, and circulating currents. The main models of AC losses in windings in the literature are presented for both circular and rectangular wires. These models are compared using both interior and surface-mounted permanent magnet synchronous machines. Results show that for proximity effect, analytical model is convenient when considering armature reaction only, however, when considering on load case, hybrid model with simple FEA is suitable thanks to its interesting trade-off between accuracy and computation time, which makes it suitable for large-scale optimization. For circulating currents effect, finite element model is unavoidable. Moreover, two important properties concerning circulating currents are stated as well as their mathematical proofs.Item AC Magnetic Loss Reduction of SLM Processed Fe-Si for Additive Manufacturing of Electrical Machines(MDPI AG, 2021-02-24) Tiismus, Hans; Kallaste, Ants; Belahcen, Anouar; Tarraste, Marek; Vaimann, Toomas; Rassõlkin, Anton; Asad, Bilal; Ghahfarokhi, Payam Shams; Department of Electrical Engineering and Automation; Computational Electromechanics; Tallinn University of TechnologyAdditively manufactured soft magnetic Fe-3.7%w.t.Si toroidal samples with solid and novel partitioned cross-sectional geometries are characterized through magnetic measurements. This study focuses on the effect of air gaps and annealing temperature on AC core losses at the 50 Hz frequency. In addition, DC electromagnetic material properties are presented, showing comparable results to conventional and other 3D-printed, high-grade, soft magnetic materials. The magnetization of 1.5 T was achieved at 1800 A/m, exhibiting a maximum relative permeability of 28,900 and hysteresis losses of 0.61 (1 T) and 1.7 (1.5 T) W/kg. A clear trend of total core loss reduction at 50 Hz was observed in relation to the segregation of the specimen cross-sectional topology. The lowest 50 Hz total core losses were measured for the toroidal specimen with four internal air gaps annealed at 1200 ◦C, exhibiting a total core loss of 1.2 (1 T) and 5.5 (1.5 T) W/kg. This is equal to an 860% total core loss reduction at 1 T and a 510% loss reduction at 1.5 T magnetization compared to solid bulk-printed material. Based on the findings, the advantages and disadvantages of printed air-gapped material internal structures are discussed in detail.Item Accurate Computational Prediction of Core-Electron Binding Energies in Carbon-Based Materials: A Machine-Learning Model Combining Density-Functional Theory and GW(AMERICAN CHEMICAL SOCIETY, 2022-07-13) Golze, Dorothea; Hirvensalo, Markus; Hernández-León, Patricia; Aarva, Anja; Etula, Jarkko; Susi, Toma; Rinke, Patrick; Laurila, Tomi; Caro, Miguel A.; Department of Applied Physics; Department of Electrical Engineering and Automation; Department of Chemistry and Materials Science; Computational Electronic Structure Theory; Microsystems Technology; Physical Characteristics of Surfaces and Interfaces; Centre of Excellence in Quantum Technology, QTF; Department of Applied Physics; University of ViennaWe present a quantitatively accurate machine-learning (ML) model for the computational prediction of core-electron binding energies, from which X-ray photoelectron spectroscopy (XPS) spectra can be readily obtained. Our model combines density functional theory (DFT) with GW and uses kernel ridge regression for the ML predictions. We apply the new approach to disordered materials and small molecules containing carbon, hydrogen, and oxygen and obtain qualitative and quantitative agreement with experiment, resolving spectral features within 0.1 eV of reference experimental spectra. The method only requires the user to provide a structural model for the material under study to obtain an XPS prediction within seconds. Our new tool is freely available online through the XPS Prediction Server.Item Accurate Insulating Oil Breakdown Voltage Model Associated with Different Barrier Effects(MDPI AG, 2021-04-09) Ghoneim, Sherif S.M.; Dessouky, Sobhy S.; Boubakeur, Ahmed; Elfaraskoury, Adel A.; Sharaf, Ahmed B. Abou; Mahmoud, Karar; Lehtonen, Matti; Darwish, Mohamed M. F.; Department of Electrical Engineering and Automation; Power Systems and High Voltage Engineering; Taif University; Port Said University; National Polytechnic School, Algeria; Egyptian Electricity Holding Company; Mataria Technical CollegeIn modern power systems, power transformers are considered vital components that can ensure the grid’s continuous operation. In this regard, studying the breakdown in the transformer becomes necessary, especially its insulating system. Hence, in this study, Box–Behnken design (BBD) was used to introduce a prediction model of the breakdown voltage (VBD) for the transformer insulating oil in the presence of different barrier effects for point/plane gap arrangement with alternating current (AC) voltage. Interestingly, the BBD reduces the required number of experiments and their costs to examine the barrier parameter effect on the existing insulating oil VBD. The investigated variables were the barrier location in the gap space (a/d)%, the relative permittivity of the barrier materials (εr), the hole radius in the barrier (hr), the barrier thickness (th), and the barrier inclined angle (θ). Then, only 46 experiment runs are required to build the BBD model for the five barrier variables. The BBD prediction model was verified based on the statistical study and some other experiment runs. Results explained the influence of the inclined angle of the barrier and its thickness on the VBD. The obtained results indicated that the designed BBD model provides less than a 5% residual percentage between the measured and predicted VBD. The findings illustrated the high accuracy and robustness of the proposed insulating oil breakdown voltage predictive model linked with diverse barrier effects.Item Accurate schemes for calculation of thermodynamic properties of liquid mixtures from molecular dynamics simulations(2016-12-28) Caro, Miguel A.; Laurila, Tomi; Lopez-Acevedo, Olga; Department of Applied Physics; Department of Electrical Engineering and Automation; Computational Electronic Structure Theory; Computational Soft and Molecular Matter; Microsystems TechnologyWe explore different schemes for improved accuracy of entropy calculations in aqueous liquid mixtures from molecular dynamics (MD) simulations. We build upon the two-phase thermodynamic (2PT) model of Lin et al. [J. Chem. Phys. 119, 11792 (2003)] and explore new ways to obtain the partition between the gas-like and solid-like parts of the density of states, as well as the effect of the chosen ideal “combinatorial” entropy of mixing, both of which have a large impact on the results. We also propose a first-order correction to the issue of kinetic energy transfer between degrees of freedom (DoF). This problem arises when the effective temperatures of translational, rotational, and vibrational DoF are not equal, either due to poor equilibration or reduced system size/time sampling, which are typical problems for ab initio MD. The new scheme enables improved convergence of the results with respect to configurational sampling, by up to one order of magnitude, for short MD runs. To ensure a meaningful assessment, we perform MD simulations of liquid mixtures of water with several other molecules of varying sizes: methanol, acetonitrile, N, N-dimethylformamide, and n-butanol. Our analysis shows that results in excellent agreement with experiment can be obtained with little computational effort for some systems. However, the ability of the 2PT method to succeed in these calculations is strongly influenced by the choice of force field, the fluidicity (hard-sphere) formalism employed to obtain the solid/gas partition, and the assumed combinatorial entropy of mixing. We tested two popular force fields, GAFF and OPLS with SPC/E water. For the mixtures studied, the GAFF force field seems to perform as a slightly better “all-around” force field when compared to OPLS+SPC/E.Item Achieving low-temperature wafer level bonding with Cu-Sn-In ternary at 150 °C(Elsevier Limited, 2023-01-01) Golim, Obert; Vuorinen, Vesa; Ross, Glenn; Wernicke, Tobias; Pawlak, Marta; Tiwary, Nikhilendu; Paulasto-Kröckel, Mervi; Department of Electrical Engineering and Automation; Electronics Integration and Reliability; Department of Electrical Engineering and Automation; EV Group E. Thallner GmbHIn this work, a low-temperature wafer-level bonding process at 150 °C was carried out on Si wafers containing 10 µm-sized microbumps based on the Cu-Sn-In ternary system. Thermodynamic study shows that addition of In enables low-melting temperature metals to reach liquid phase below In melting point (157 °C) and promotes rapid solidification of the intermetallic layer, which are beneficial for achieving low-temperature bonding. Microstructural observation shows high bonding quality with low amount of defect. SEM and TEM characterization concludes that a single-phase intermetallic formed in the bond and identified as Cu6(Sn,In)5 with a hexagonal lattice. Mechanical tensile test indicates that the bond has a mechanical tensile strength of 30 MPa, which are adequate for 3D heterogeneous integration.Item Acoustic noise computation of electrical motors using the boundary element method(MDPI AG, 2020-01-03) Sathyan, Sabin; Aydin, Ugur; Belahcen, Anouar; Department of Electrical Engineering and Automation; Computational ElectromechanicsThis paper presents a numerical method and computational results for acoustic noise of electromagnetic origin generated by an induction motor. The computation of noise incorporates three levels of numerical calculation steps, combining both the finite element method and boundary element method. The role of magnetic forces in the production of acoustic noise is established in the paper by showing the magneto-mechanical and vibro-acoustic pathway of energy. The conversion of electrical energy into acoustic energy in an electrical motor through electromagnetic, mechanical, or acoustic platforms is illustrated through numerical computations of magnetic forces, mechanical deformation, and acoustic noise. The magnetic forces were computed through 2D electromagnetic finite element simulation, and the deformation of the stator due to these forces was calculated using 3D structural finite element simulation. Finally, boundary element-based computation was employed to calculate the sound pressure and sound power level in decibels. The use of the boundary element method instead of the finite element method in acoustic computation reduces the computational cost because, unlike finite element analysis, the boundary element approach does not require heavy meshing to model the air surrounding the motor.Item Acoustophoresis of monodisperse oil droplets in water: effect of symmetry breaking and non-resonance operation on oil trapping behavior(American Institute of Physics, 2023-12-01) Bazyar, H.; Kandemir, Mehmet; Peper, J.; Andrade, M. A.B.; Bernassau, A. L.; Schroën, K.; Lammertink, R. G.H.; Department of Electrical Engineering and Automation; Robotic Instruments; Delft University of Technology; University of Twente; Universidade de São Paulo; Heriot-Watt UniversityAcoustic manipulation of particles in microchannels has recently gained much attention. Ultrasonic standing wave (USW) separation of oil droplets or particles is an established technology for microscale applications. Acoustofluidic devices are normally operated at optimized conditions, namely, resonant frequency, to minimize power consumption. It has been recently shown that symmetry breaking is needed to obtain efficient conditions for acoustic particle trapping. In this work, we study the acoustophoretic behavior of monodisperse oil droplets (silicone oil and hexadecane) in water in the microfluidic chip operating at a non-resonant frequency and an off-center placement of the transducer. Finite element-based computer simulations are further performed to investigate the influence of these conditions on the acoustic pressure distribution and oil trapping behavior. Via investigating the Gor’kov potential, we obtained an overlap between the trapping patterns obtained in experiments and simulations. We demonstrate that an off-center placement of the transducer and driving the transducer at a non-resonant frequency can still lead to predictable behavior of particles in acoustofluidics. This is relevant to applications in which the theoretical resonant frequency cannot be achieved, e.g., manipulation of biological matter within living tissues.Item Active Incremental Learning of a Contextual Skill Model(IEEE, 2019) Hazara, Murtaza; Li, Xiaopu; Kyrki, Ville; Intelligent Robotics; Department of Electrical Engineering and AutomationContextual skill models are learned to provide skills over a range of task parameters, often using regression across optimal task-specific policies. However, the sequential nature of the learning process is usually neglected. In this paper, we propose to use active incremental learning by selecting a task which maximizes performance improvement over entire task set. The proposed framework exploits knowledge of individual tasks accumulated in a database and shares it among the tasks using a contextual skill model. The framework is agnostic to the type of policy representation, skill model, and policy search. We evaluated the skill improvement rate in two tasks, ball-in-a-cup and basketball. In both, active selection of tasks lead to a consistent improvement in skill performance over a baseline.Item Active Robot Learning for Temporal Task Models(ACM, 2018) Racca, Mattia; Kyrki, Ville; Department of Electrical Engineering and Automation; Intelligent RoboticsWith the goal of having robots learn new skills after deployment, we propose an active learning framework for modelling user preferences about task execution. The proposed approach interactively gathers information by asking questions expressed in natural language. We study the validity and the learning performance of the proposed approach and two of its variants compared to a passive learning strategy. We further investigate the human-robot-interaction nature of the framework conducting a usability study with 18 subjects. The results show that active strategies are applicable for learning preferences in temporal tasks from non-expert users. Furthermore, the results provide insights in the interaction design of active learning robots.
