20 June 2013
My work focuses on the development of microsystem technologies, in particular, inertial sensors, energy harvesting structures and medical instrumentation sensors. For many years my work studied the implications of using microsystems in implantable devices that help to improve the quality of radiation dose delivery in radiotherapy treatments. My current work involves the design and fabrication of MEMS and VLSI devices and their interaction to become a usable microsystem. Since my current institution also focuses on applied research matters, I have had the opportunity to develop embedded inertial navigation systems and other equipment based on microsystem technologies for aeronautics and pipeline inspection applications. Some of these systems are being employed by major aviation and oil companies in Mexico.
An exact expression for the capacitance between unequal nonparallel plates does not exist. Furthermore, variable capacitances are usually present in electrostatic microelectromechanical structures. Most of the time, these variable capacitances are the way to interpret a given physical phenomenon by means of a developed sensor. Certain fringing capacitances must be categorically considered whereas other fringing effects can be neglected as long as they do not contribute significantly to the primary capacitance value of interest. This work was motivated by the necessity of calculating fringing capacitances in multi-axial comb-drive systems and microactuators driven electrostatically. Our method can contribute to the simplification of the modelling for even more complex electrostatic structures. For instance, a calculation for sections of circular plates was reported in our Electronics Letters paper; nonetheless the method can be applied to several other geometries.
Many well-known calculations of fringing effects are basically numerical solutions of the electric potential or the electric field. Many of these solutions do not provide a straightforward approach to calculate fringing capacitances. However, if a direct computation of the fringing capacitance is possible, it would most probably be a numerical solution. Although our analytical solution is based on a sum of an infinite series, it has been confirmed that the sum can be truncated to n≤5 without significantly affecting the expected results. This way, the calculation of fringing capacitances requires moderate computing resources. It is important to point out that there is no point in applying this truncation to a capacitor with fixed plates. However our method is particularly beneficial when the overlapping plate area varies in time. It does not matter if the plates differ in size, thickness or even if they are nonparallel.
The capacitance between poles and electrodes in a micro-motor can be directly used to estimate the torque that the micro-motor can provide. Torque measurements on micro-motors are not easy to perform. From a more general point of view, direct force measurements on electrostatic actuators are particularly difficult to achieve. Therefore the computation of fringing capacitances can be valuable and straightforward technique to determine forces associated to electrostatic actuators.
Single seismic mass multi-axial capacitive sensors and circular plate bottom-drive micro actuators have been developed using this methodology. A specific micromotor is being fabricated and a number of pole-electrode capacitance measurements will be carried out to validate our analytical method. We have even designed a micro torque meter to further confirm our torque calculations derived from our method.
Our group is currently working on MEMS design as well as their associated VLSI analogue-digital electronics. This is a very time consuming task since most MEMS and VLSI fabrications processes differ significantly from each other. Even so, we have been able to direct these studies to a remarkable variety of applications such as embedded inertial navigation, non-vassal drug delivery systems or harvesting energy systems.
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