Cristina Marghescu

Simulation of a wireless sensor network using OPNET

We aim to evaluate the performance of a ZigBee wireless sensor network using OPNET. The network is intended for a general application and should be able to collect data from many nodes, every node connected to several sensors. One possible application for such a network is in the medical area. At first the network will be used to display the evolution of blood pressure; the body temperature, the ambient temperature and pressure and the level of the transfusion liquid can be collected as well. The network coordinator will transmit the data to a local server for further processing. The simulation will be used to evaluate the general parameters of the wireless network and to optimise it.

Modelling and simulation of energy harvesting with solar cell

The paper focuses on modelling and simulation of energy consumption of an autonomous embedded system with a solar cell as energy source. Mathematical models for energy consumption/generation for solar cell, microcontroller and its peripherals are elaborated, tested and coupled together for complete system simulations that allow system configuration and optimization for specific cases.

Analysis of current carrying capacity of silver-based conductive pastes for PCB repair

This paper presents some investigations on the current carrying capabilities of PCB traces repaired with silver-based conductive paste. Such PCB traces with different parameters (width, gap and substrate) are tested at different currents and electrical and thermal measurements are performed. An automated measurement system is used for observing any change in trace resistance for the time span of each test.

Study on optimization of capacitive pressure sensor using coupled mechanical-electric analysis

The object of the paper is a pressure sensor realized in thick-film technology developed at “Jožef Stefan” Institute. This pressure sensor is also the object of a common project which has the goal to optimize the sensor design acting mainly on layout parameters. Some of the sensor parameters are determined by the LTCC process, for instance the thickness of diaphragm whose thickness is a multiple of foil thickness. The novelty in the proposed paper is the use of specific simulation tools to give an estimate of sensor output characteristics. Using this technique it is possible to optimize the layout and construction of the sensor. The finite-element simulation is used to outline the output characteristic: capacitance versus pressure. This simulation implies the coupled field analysis from mechanical deflection of sensor diaphragm to electrical field quantities, in this case the capacitance. The analysis will take into account beside geometrical parameters the temperature dependence of materials properties. This fact will give a better appreciation of real life behavior of manufactured sensors, and can suggest some specific packaging methods.

FE modeling of capacitive pressure sensors realized in LTCC technology

Although the pressure sensor market is dominated by silicon based sensors, in recent years ceramic based sensors have proved interesting and have been increasingly researched due to their material properties which qualify them for use in harsh environments. This paper describes the use of finite element analysis (FEA) to determine the functioning of a diaphragm-type ceramic capacitive pressure sensor starting from mechanical pressure and obtaining the output signal in form of an electrical capacitance. The capability of the software (Ansys/Multiphysics) to realize multi-field analysis will be used here in a very intensive way. Of major concern is the possibility to anticipate the influence of different liquid media on this category of pressure sensors developed using LTCC (Low Temperature Co-fired Ceramic).

EDLC characterization platform

Technological improvements have led to the development of low resistivity materials with higher surface which are capable of storing more energy in the form of electrical charge. These developments as well as a better theoretical understanding of the process of charge transfer, a process that appears in double layer type materials, led to the development of capacitors with high capacitance, known as EDLC (Electrochemical Double-Layer Capacitor). It is important for the user to be able to predict the behaviour of the EDLC in different conditions. This paper introduces a platform which can be used to obtain the capacitance as a function of voltage, time or temperature.

Implementing a Capacitive Pressure Sensor Realized on LTCC

LTCC (Low Temperature Co-Fired Ceramic) has great potential in the field of sensors and transducers due to its thermal, electrical and mechanical properties. The paper describes work concerning a capacitive pressure sensor realized on LTCC with thick-film deposition technologies. A capacitive pressure sensor converts a change in the position of the conductive plates to an electrical signal; for this a deformable diaphragm is used. In the presented case one electrode is bonded to the deformable diaphragm (an edge-clamped, circular diaphragm) and the other electrode is fixed. The signal from the sensor is processed by the AD7745 circuit. This circuit is a high resolution digital capacity-signal converter with high performances, high linearity ±0.01% and very good accuracy ±4 fF. The circuit also encompasses a voltage reference and a temperature sensor with a resolution of 0.1°C. The external connection is made through the I2C interface, using a signal control unit which processes the signal and sends the information to an LCD (liquid crystal display) and/or to a computer which, in turn, records the information for later use through the USB interface.

Study of environmental influences on the behavior of a capacitive pressure sensor

In the frame of a bilateral project Romania-Slovenia (partners: University “Politehnica” of Bucharest and “Jožef Stefan” Institute from Ljubljana) a capacitive pressure sensor with air using LTCC (low temperature co-fired ceramic) and thick-film materials and technologies was developed. After establishing the optimal dimensions from the point of view of the sensitivity as well as costs, question arise concerning the functioning of the sensor in real employment conditions. It has been observed that temperature as well as humidity influence the way the sensor behaves, requiring additional calibration actions. The influence of water vapors was also of interest. In order to fully characterize the sensor, all this influences need to be studied. This paper focuses on characterizing the sensor by highlighting these effects through measurements and, where possible, through simulations.

Comparison between Zubieta model of supercapacitors and their real behavior

The supercapacitor is a relatively young electronic component which bases its operation on the principle of the Helmholtz double layer of charge formed on the surface of an activated carbon (with an extremely high equivalent surface) and an electrolyte which allows both a short distance between the electric layers as well as a very large useful area. For optimal use of an EDLC (Electrochemical Double Layer Capacitor) it is necessary to know the time behavior, specific electrical characteristics (discharge current, charge current, voltage variation at the terminals, equivalent series resistance, and especially leakage current (self-discharge current), as well as behavior over a long time - days or even months - under electric charge at different voltages, etc.) and the influence of environmental operating conditions (temperature, humidity, vibration, etc.). For modeling in more detail the behavior of an EDLC in time and voltage a comparison between the Zubieta model and actual behavior was performed. The research is focused on the accuracy of the Zubieta model related to the real behavior of supercapacitors.

Investigation on current capabilities of Ni-based conductive pastes for PCB repair

The paper presents some electrical and thermal tests and experiments related to the current capabilities of PCB traces repaired with Nickel-based conductive paste. The lack of available scientific information in this topic motivated us to perform this investigation regarding the heating of a PCB trace segment made from conductive paste during current flowing through it. Based on the experimental results, the authors want to offer a reliable technical resource in the case of traces repaired with Ni-based conductive paste or other applications where traces made of conductive paste are used.