Recursos de colección

UPCommons - E-prints UPC Universitat Politècnica de Catalunya (83.371 recursos)

E-prints UPC cobreix dues finalitats: per una banda, és el dipòsit institucional de la UPC que recull els articles de revista, les comunicacions de congrés i els reports de recerca generats en les activitats de recerca del personal docent i investigador de la universitat; per l'altra, és una eina que permet accelerar la producció científica, allotjant versions de documents prèvies a la publicació en una revista o a les actes d’un congrés.

Articles de revista

Mostrando recursos 1 - 20 de 21

  1. Subharmonic excitation in amplitude modulation atomic force microscopy in the presence of adsorbed water layers

    Santos Hernández, Sergio; Barcons Xixons, Víctor; Verdaguer, Albert; Chiesa, Matteo
    In ambient conditions, nanometric water layers form on hydrophilicsurfaces covering them and significantly changing their properties and characteristics. Here we report the excitation of subharmonics in amplitude modulation atomic force microscopy induced by intermittent water contacts. Our simulations show that there are several regimes of operation depending on whether there is perturbation of water layers. Single period orbitals, where subharmonics are never induced, follow only when the tip is either in permanent contact with the water layers or in pure noncontact where the water layers are never perturbed. When the water layers are perturbed subharmonic excitation increases with decreasing oscillation...

  2. Superregenerative reception of narrowband FSK modulations

    Palà Schönwälder, Pere; Bonet Dalmau, Jordi; López Riera, Alexis; Moncunill Geniz, Francesc Xavier; Águila López, Francisco del; Giralt Mas, Ma. Rosa
    In this paper we investigate the possibilities of narrowband FSK detection using a superregenerative (SR) receiver. Previous SR FM demodulation techniques rely on detecting the amplitude variations caused by the different frequencies involved in FSK modulation. However, this requires relatively high frequency deviations because the frequency response of SR receivers is not very selective. In this paper we take a different approach, exploiting the distinct phase trajectories of FSK modulations resulting from the transmitted data. The well-known fact that the SR oscillator response preserves the phase information of the received signal is successfully exploited to allow the detection of several...

  3. Optimal control of a water distribution network in a supervisory control system

    Cembrano Gennari, Gabriela; Wells, G; Quevedo Casín, Joseba Jokin; Pérez Magrané, Ramon; Argelaguet Isanta, María Rosa
    This paper deals with the use of optimal control techniques in water distribution networks. An optimal control tool, developed in the context of a European research project is described and the application to the city of Sintra (Portugal) is presented.

  4. Decomposition of geometric constraint graphs based on computing fundamental circuits. Correctness and complexity

    Joan Arinyo, Robert; Tarres Puertas, Marta Isabel; Vila Marta, Sebastià
    In geometric constraint solving, Decomposition Recombination solvers (DR-solvers) refer to a general solving approach where the problem is divided into a set of sub-problems, each sub-problem is recursively divided until reaching basic problems which are solved by a dedicated equational solver. Then the solution to the starting problem is computed by merging the solutions to the sub-problems.; Triangle- or tree-decomposition is one of the most widely used approaches in the decomposition step in DR-solvers. It may be seen as decomposing a graph into three subgraphs such that subgraphs pairwise share one graph vertex. Shared vertices are called hinges. Then a...

  5. A low in-band radiation superregenerative oscillator

    Palà Schönwälder, Pere; Bonet Dalmau, Jordi; Moncunill Geniz, Francesc Xavier; Águila López, Francisco del; Giralt Mas, Ma. Rosa
    This brief describes a superregenerative (SR) voltage-controlled oscillator as a building block for SR receivers where most of the oscillator spectrum components are outside the reception frequency band. This allows overcoming one of the main drawbacks of SR receivers, i.e., the potential interference to nearby receivers operating at the same frequency due to oscillator reradiation. We perform a qualitative analysis of the solution of the circuit equations, describe the most relevant parameters for design, and provide some numerical simulation results. Experimental results on a proof-of-concept implementation validating the described principle and a discussion of the observed behavior are provided.

  6. Forward backward asymmetries of lepton pairs in events with a large transverse momentum jet at hadron colliders

    Águila López, Francisco del; Ametller Congost, Lluís; Talavera Sánchez, Pedro
    We discuss forward-backward charge asymmetries for lepton-pair production in association with a large-transverse-momentum jet at hadron colliders. The lepton charge asymmetry relative to the jet direction AjFB gives a new determination of the effective weak mixing angle sin2lept eff M2 Z with a statistical precision after cuts of 10 3 (8 10 3) at LHC (Tevatron). This is to be compared with the current uncertainty at LEP and SLD from the asymmetries alone, 2 10 4. The identification of b jets also allows for the measurement of the bottom-quark–Z asymmetry AbFB at hadron colliders, the resulting statistical precision for sin2lept eff M2 Z...

  7. PEM fuel cell fault diagnosis via a hybrid methodology based on fuzzy and pattern recognition techniques

    Escobet Canal, Antoni; Nebot Castells, M. Àngela; Múgica Álvarez, Francisco
    In this work, a fault diagnosis methodology termed VisualBlock-Fuzzy Inductive Reasoning, i.e. VisualBlock-FIR, based on fuzzy and pattern recognition approaches is presented and applied to PEM fuel cell power systems. The innovation of this methodology is based on the hybridization of an artificial intelligence methodology that combines fuzzy approaches with well known pattern recognition techniques. To illustrate the potentiality of VisualBlock-FIR, a non-linear fuel cell simulator that has been proposed in the literature is employed. This simulator includes a set of five fault scenarios with some of the most frequent faults in fuel cell systems. The fault detection and identification...

  8. Phase contrast and operation regimes in multifrequency atomic force microscopy

    Santos Hernández, Sergio
    In amplitude modulation atomic force microscopy the attractive and the repulsive force regimes induce phase shifts above and below 90°, respectively. In the more recent multifrequency approach, however, multiple operation regimes have been reported and the theory should be revisited. Here, a theory of phase contrast in multifrequency atomic force microscopy is developed and discussed in terms of energy transfer between modes, energy dissipation and the kinetic energy and energy transfer associated with externally driven harmonics. The single frequency virial that controls the phase shift might undergo transitions in sign while the average force (modal virial) remains positive (negative). ©...

  9. Unlocking higher harmonics in atomic force microscopy with gentle interactions

    Santos Hernández, Sergio; Barcons Xixons, Víctor; Font Teixidó, Josep; Verdaguer Prats, Albert
    In dynamic atomic force microscopy, nanoscale properties are encoded in the higher harmonics. Nevertheless, when gentle interactions and minimal invasiveness are required, these harmonics are typically undetectable. Here, we propose to externally drive an arbitrary number of exact higher harmonics above the noise level. In this way, multiple contrast channels that are sensitive to compositional variations are made accessible. Numerical integration of the equation of motion shows that the external introduction of exact harmonic frequencies does not compromise the fundamental frequency. Thermal fluctuations are also considered within the detection bandwidth of interest and discussed in terms of higher-harmonic phase contrast...

  10. A superregenerative QPSK receiver

    Palà Schönwälder, Pere; Bonet Dalmau, Jordi; Moncunill Geniz, Francesc Xavier; Águila López, Francisco del; Giralt Mas, Ma. Rosa
    In this paper we present a description and experimental verification of a superregenerative receiver (SR) for QPSK signals. Exploiting the fact that a conventional SR generates pulses which preserve the input phase information, we take $N$ 1-bit samples of each generated pulse. A suitable choice of the sampling frequency gives as a result a bit vector containing a sub-sampled version of each PSK pulse. Extremely simple digital processing of the vectors from two consecutive pulses allows symbol decision, together with information on signal quality and frequency displacements. Although presented for the QPSK case, the principle may be applied to the...

  11. Enhanced sensitivity and contrast with bimodal atomic force microscopy with small and ultra-small amplitudes in ambient conditions

    Santos Hernández, Sergio
    Here, we introduce bimodal atomic force microscopy operated with sub-nm and ultra-small, i.e., sub-angstrom, first and second mode amplitudes in ambient conditions. We show how the tip can be made to oscillate in the proximity of the surface and in perpetual contact with the adsorbed water layers while the second mode amplitude and phase provide enhanced contrast and sensitivity. Nonlinear and nonmonotonic behavior of the experimental observables is discussed theoretically with a view to high resolution, enhanced contrast, and minimally invasive mapping. Fractions of meV of energy dissipation are shown to provide contrast above the noise level.

  12. Improved switching strategy for selective harmonic elimination in DC-AC signal generation via pulse-width modulation

    Palacios Quiñonero, Francisco; Vicente Rodrigo, Jesús; Molina Hernández, M. Antonia; Karimi, Hamid Reza
    We present an advanced design methodology for pulse-width-modulated (PWM) DC-AC signal generation. Using design methods based on the Walsh transform, AC sinusoidal signals can be approximated by suitable PWM signals. For different AC amplitudes, the switching instants of the PWM signals can be efficiently computed by using appropriate systems of explicit linear equations. However, the equation systems provided by conventional implementations of this approach are typically only valid for a restricted interval of AC amplitudes and, in general, a supervised implementation of several equation systems is necessary to cover the full AC amplitude range. Additionally, obtaining suitable equation systems for designs with a large number...

  13. Stability, resolution, and ultra-low wear amplitude modulation atomic force microscopy of DNA: small amplitude small set-point imaging

    Santos Hernández, Sergio; Barcons Xixons, Víctor; Christenson, Hugo K.; Billingsley, Daniel J.; Bonass, William A.; Font Teixidó, Josep; Thomson, Neil H.
    A way to operate fundamental mode amplitude modulation atomic force microscopy is introduced which optimizes stability and resolution for a given tip size and shows negligible tip wear over extended time periods ( 24 h). In small amplitude smal l set-point (SASS) imaging, the cantilever oscillates with sub-nanometer am plitudes in the proximity of the sample, without the requirement of using large drive forces, a s the dynamics smoothly lead the tip to the surface through the water layer. SASS is demonstrat ed on single molecules of double-stranded DNA in ambient conditions where sharp silicon tips (R 2–5 nm) can resolve the right-handed double helix

  14. Optimization of oxygen transfer through venturi-based systems applied to the biological sweetening of biogas

    Rodriguez, Ginesta; Dorado Castaño, Antonio David; Bonsfills Pedrós, Anna; Sanahuja Moliner, Ricard; Gabriel, David; Gamisans Noguera, Javier

  15. Investigation of nanoscale interactions by means of subharmonic excitation

    Santos Hernández, Sergio; Phillips, M.A.; Verdaguer, Albert; Font Teixidó, Josep; Chiesa, Matteo; Gadelrab,, K.; Stefancich, M.; Armstrong, P.; Li, G.; Souier, T.; Thomson, Neil H.; Barcons Xixons, Víctor
    Multifrequency atomic force microscopy holds promise as a method to provide qualitative and quantitative information about samples with high spatial resolution. Here, we provide experimental evidence of the excitation of subharmonics in ambient conditions in the regions where capillary interactions are predicted to be the mechanism of excitation. We also experimentally decouple a second mechanism for subharmonic excitation that is highly independent of environmental conditions such as relative humidity. This implies that material properties could be mapped. Subharmonic excitation could lead to experimental determination of surface water affinity in the nanoscale whenever water interactions are the mechanism of excitation.

  16. Quantification of dissipation and deformation in ambient atomic force microscopy

    Santos Hernández, Sergio; Gadelrab,, K.; Barcons Xixons, Víctor; Stefancich, M.; Chiesa, Matteo
    A formalism to extract and quantify unknown quantities such as sample deformation, the viscosity of the sample and surface energy hysteresis in amplitude modulation atomic force microscopy is presented. Recovering the unknowns only requires the cantilever to be accurately calibrated and the dissipative processes occurring during sample deformation to be well modeled. The theory is validated by comparison with numerical simulations and shown to be able to provide, in principle, values of sample deformation with picometer resolution.

  17. Spatial horizons in amplitude and frequency modulation atomic force microscopy

    Font Teixidó, Josep; Santos Hernández, Sergio; Barcons Xixons, Víctor; Thomson, Neil H.; Verdaguer, Albert; Chiesa, Matteo
    In dynamic atomic force microscopy (AFM) the cantilever is vibrated and its dynamics are monitored to probe the sample with nanoscale and atomic resolution. Amplitude and frequency modulation (AM and FM) atomic force microscopy have established themselves as the most powerful, robust and reliable techniques in the field. Nevertheless, it is still debatable whether one or the other technique is preferred in a given medium or experiment. Here, we quantitatively establish the limitations in resolution of one and the other technique by introducing the concept of space horizon SH and quantifying it. The SH is the limiting space boundary beyond which collective atomic interactions do not affect the...

  18. A high-resolution UWB IR superregenerative receiver front end with an SRD quench shaper

    Moncunill Geniz, Francesc Xavier; Bonet Dalmau, Jordi; Palà Schönwälder, Pere; Águila López, Francisco del; Giralt Mas, Ma. Rosa
    We present a simple receiver front end that makes use of the baseband superregeneration principle to detect ultrawideband (UWB) impulse radio signals. The UWB antenna is directly connected to the core circuit consisting of a resistor-capacitor (RC) network coupled to a negative resistance that varies under the control of an external quench generator. Due to a step-recovery-diode quench shaper, 50-ps time-domain sensitivity windows are generated that filter the received pulses and reject noise and interference. The circuit achieves high gain, exhibits automatic gain control, and directly demodulates binary phase modulations.

  19. The intrinsic resolution limit in the atomic force microscope: implications for heights of nano-scale features

    Santos Hernández, Sergio; Barcons Xixons, Víctor; Christenson, Hugo K.; Font Teixidó, Josep; Thomson, Neil H.
    Peer Reviewed

  20. How localized are energy dissipation processes in nanoscale interactions?

    Santos Hernández, Sergio; Barcons Xixons, Víctor; Verdaguer, Albert; Font Teixidó, Josep; Thomson, Neil H.; Chiesa, Mateo
    We describe fundamental energy dissipation in dynamic nanoscale processes in terms of the localization of the interactions. In this respect, the areal density of the energy dissipated and the effective area of interaction in which each process occurs are calculated for four elementary dissipative processes. It is the ratio between these two, which we term M that provides information about how localized the interactions are. We show that neither the phase lag, nor the magnitude of the energy dissipated alone provide information about energy localization but M has to be considered instead.

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