This work proposes a method to estimate the electrical constriction resistance of two mating metallic rough surfaces based on the finite element method (FEM). The FEM-based method generates a random array of three-dimensional orthogonal parallelepipeds to simulate the stochastic distribution of the asperities across the contact interface. The effect of the contact pressure is studied in detail, since once the contact materials and the topology of the contact area are settled, the contact pressure plays a critical role in determining the electrical constriction resistance. The proposed model is based on two critical variables, the contact pressure and the surface roughness of the mating surfaces, which must be measured in the laboratory to calibrate the model. Results provided by the FEM-based model are compared with experiments for three geometries, thus validating the accuracy of the proposed approach. Although the apparent contact areas of the analyzed specimens have a rectangular shape, the proposed method is also applicable to determine the electrical constriction resistance of other geometries. It is also proved that depending on the pressure applied to the contact interface, the electrical constriction resistance can be almost independent of the apparent area of contact. Although the aim of this work was to generate an electrical constriction resistance model for power connectors, it is also applicable to many other power devices.
In this paper we present low-power seismic acquisition equipment for long-range telemetry, developed as a compact system for easy transportation. This acquisition system has been developed to detect the seismic activity of volcanoes, and represents the achievement of joint work from different scientific and technological disciplines such as geophysics, electronics, communications, mechanics, and information technology. The paper includes the laboratory validation results by means of automated measuring systems and calibration procedures, and the final validation results of different measurement campaigns on the El Hierro and Tenerife islands (Canary Islands). The seismic acquisition equipment includes an on board signal processing system developed to reduce the size of seismic data transmitted and to increase the autonomy of the equipment. Moreover, the seismic acquisition equipment has been designed based on a low cost electromagnetic sensor, which has been conditioned to achieve our goals. In a volcanic seismic survey, a series of seismic acquisition systems are placed near the area under study, where they record the natural seismic activity of volcanoes.
Sarria, D.; Sbragaglia, V.; Gomariz, S.; Garcia, J.; Artero, C.; Aguzzi, J.; Sardà, F.; Manuel, A. Measurement Vol. 69, p. 180-188 DOI: 10.1016/j.measurement.2015.03.002 Data de publicació: 2015-06-01 Article en revista
Patterns of how the Norway lobster (Nephrops norvegicus L.) emerges from its burrow in relation to sea currents at different times of the day-night cycles is currently unknown. The degree of influence of these variables when simulated in controlled laboratory conditions entails great technical and technological difficulties. This paper presents the development of an integrated control system that manages a set of aquariums generating water currents and patterns of light. The solution proposed combines the use of commercial equipment and a custom electronic design in order to achieve the required specifications with cost-minimization. The control system has circulation pumps and visible and infrared LED lighting to recreate different currents and light conditions. Preliminary results show the control system works properly and all components responded as expected during the experiments. (C) 2015 Published by Elsevier Ltd.
At present, pumped storage is the only system to store huge amounts of energy. The surplus of energy in the electrical grid is used for pumping water from a lower to a higher reservoir (pump operation). At peak hours or in case of emergency this water goes down generating electrical energy (turbine operation). In the last years, due to the massive entrance of wind power, the number of start and stop cycles have increased dramatically as well as the operation at very low loads. In this new scenario more cases of damage have been reported and an effective vibration monitoring is necessary.
Reversible pump-turbines (RPT) are high performance machines that have to change operation from pump to turbine mode (reversing runner rotation and flow direction) in a short time. Due to their special design characteristics (large power concentration) and operating conditions, they generate large pressure pulsations and forces when in operation.
In this paper the vibration monitoring of large RPT units have been analysed. The results obtained after 15 years of monitoring several power plants with this type of machines have been used for the analysis. In the last years, most of the supervised power plants have been monitored on-line remotely.
First the monitoring system used is described. The main types of excitation forces expected during the operation of these machines are studied especially the most characteristic and distinctive that are the pressure pulsations due to the rotor/stator interaction.
The vibration signatures measured in machines with different designs are analysed in detail and compared with the signatures of conventional turbines. The influence of the operation type (as a pump and or as a turbine) and on the power delivered has been studied and the changes of the signatures presented. After the analysis of the vibration signatures the procedure used for the selection of spectral bands is explained.
The analysis of the results obtained with the monitoring is shown. First the evolution of overall levels and the comparison with vibration Standards is discussed. Second the main types of damage found are introduced. Damage in the generator, in bearings, in supporting structure and abnormal operation with cavitation are shown. The changes in vibration signatures and spectral bands for these types of damage are also given. Finally the specific case of damage in runners is introduced.
There is a growing interest in accurately measuring the timing of the J peak of the ballistocardiogram (BCG) in order to obtain cardiovascular function markers non-invasively, especially in modern home healthcare applications. In this paper we have studied the effect that some common uncertainty sources have in the time measurement of the J peak. This is a necessary step towards the standardization of modern ballistocardiography systems equivalent to that available for ECG systems. We conclude that, to reduce J peak time uncertainty below the measured intrinsic uncertainty of about +/- 2 ms, the minimal bandwidth should be from 1.5 Hz to 22.5 Hz; the sampling frequency can be decreased up to 50 Hz when using cubic spline interpolation; 5 bits are required to quantify the signal, and signal-to-noise ratio (SNR) and signal-to-interference ratio (SIR) should be over 40 dB and 3 dB respectively. (C) 2014 Elsevier Ltd. All rights reserved.
There is a growing interest in accurately measuring the timing of the J peak of the ballistocardiogram (BCG) in order to obtain cardiovascular function markers non-invasively, especially in modern home healthcare applications. In this paper we have studied the effect that some common uncertainty sources have in the time measurement of the J peak. This is a necessary step towards the standardization of modern ballistocardiography systems equivalent to that available for ECG systems. We conclude that, to reduce J peak time uncertainty below the measured intrinsic uncertainty of about ±2 ms, the minimal bandwidth should be from 1.5 Hz to 22.5 Hz; the sampling frequency can be decreased up to 50 Hz when using cubic spline interpolation; 5 bits are required to quantify the signal, and signal-to-noise ratio (SNR) and signal-to-interference ratio (SIR) should be over 40 dB and 3 dB respectively.
This article describes a wireless sensor network (WSN) designed and implemented to monitor and assist composting processes. After framing the problem, the objectives and characteristics of the WSN are discussed and a detailed description of the hardware and software of network components are presented. The article ends with the presentation and discussion of results obtained with the network.
Layers of AlN and CNx were investigated as suitable films to coat thin film anemometers because they are simultaneously electrical isolators and thermal conductors. Thermal time constants were measured and thermal equivalent models are proposed in this work. AlN performed better in the experiments.
Turo, A.; Chavez-Dominguez, J. A.; Garcia, M.; Bulkai, A.; Tomek, P.; Tóth, G.; Girones, A.; Salazar, J. Measurement Vol. 46, num. 3, p. 1101-1108 DOI: 10.1016/j.measurement.2012.10.016 Data de publicació: 2013-04 Article en revista
The demand for powder metallurgy (P/M) parts in its traditional automotive market is predicted to grow, but future sector expansion depends directly upon its capability to manufacture zero-defect parts for industries such as aerospace and medicine. The lack of adequate inspection systems has important implications from the point of view of quality assurance, since it increases the costs, time and wasted material. In recent years the applicability of several techniques for the inspection of P/M parts has been investigated, such as Eddy current testing, computer tomography or X-ray imaging, but studies have revealed that all of them have deficiencies that make them unavailable for a complete and reliable flaw detection and density defect recognition. A new inspection tool has been developed based on pulse echo ultrasonic technology combined with robotics, which makes it possible to provide a global density map of sintered.
A recent frequency estimation method was proposed in a Measurement paper for use in portable DSP based instrumentation. This method is especially important due to its low computational effort making it suitable for fast and low cost instrumentation. It has been found that many parameters can affect the frequency estimation using the proposed method. In this paper, the performance of the method in the presence of additive noise is evaluated, and the influence of signal frequency, initial phase, additive noise and number of analog-to-digital converter bits on the estimator is studied. Some caveats of the practical use of the method are addressed. Detailed numerical simulations based on a Monte Carlo procedure are presented in order to highlight the range of applicability of the method and determine the estimation bias.
Electronic engineering is showing a definite trend towards differential circuits. However, most measurement instruments are single-ended. Consequently, engineering graduates often lack the skills to perform correct differential measurements. This paper describes the theoretical background, development and limitations of three experiments that help students to gain insight into differential measurements and the origin and consequences of a low common-mode rejection ratio. The experiments require only common equipment and suit both electrical and non-electrical engineering students.