Study On Diamagnetic Bearings For MEMS

Micro-Electro-Mechanical Systems (MEMS) is the integration of mechanical elements, sensors, actuators and electronics on a common silicon substrate through the utilization of microfabrication technology. Due to its diminutive dimension, many physical phenomena that are negligible at the macro-scale could find novel application as size shrinks down. One of the winners of miniaturization is diamagnetic levitation.Diamagnetism is a magnetic property inherent to many materials such as water, protein, silver, bismuth, graphite, etc. Unlike stated in Earnshaw's theorem, which eliminates the possibility of stable levitation of a ferromagnetic or paramagnetic body in a static magnetic field, diamagnetic materials are repelled by the magnetic field and attracted to the local field minimum due to their characteristic negative magnetic susceptibility. Since local minima can exist in the free space, diamagnets could thus be employed in combination with other magnetic sources, such as permanent magnets (PM), to construct passive and stable levitation at room temperature, with PM providing the lift capacity to counterbalance gravity and diamagnetic materials providing stability to keep levitation within reasonable limit.Although such mechanism has already proved its worth for several high-precision scientific sensors and the clean room transportation, it is of little use to bearing systems at the macro-scale mainly because the weak diamagnetic effect of materials available currently is not capable of providing enough stability and bearing pressure for most mechanical applications. According to the electromagnetic theory, the magnitude of the magnetic field mapped around a PM remains unchanged after size reduction, while the field gradient scales inversely with characteristic dimensions. This means that for interaction elements involving magnets and soft magnetic materials, for which the volumic force acting on each particle is governed by the relation of B ? B, the force-to-volume ratio improves strongly as scale shrinks down and therefore diamagnetic material could be utilized with PM to construct simple bearing solution for MEMS.Engineering of diamagnetic bearings requires a complex mix of disciplines, including electromagnetic theory, kinetics, material science, design theory and etc. The objective of this thesis is to study its basic characteristics so as to gain a systematic understanding of such a novel bearing mechanism. The major content includes:(1) Based on the electromagnetic theory, the micro-scale effectiveness of diamagnetic levitation is demonstrated and the magnetomechanical interaction in terms of load-carrying capacity and the restoring in the axial, lateral and inclination directions are evaluated.(2) The eddy current effect on the performance of diamagnetic bearing is investigated. Due to the good electrical conductivity of some of diamagnetic materials or the intentionally introduced eddy current damper for vibration control, eddy current could be induced in the diamagnetic bearing with viscous force opposing the relative motion between the rotor and the stator. Such damping mechanism is analyzed with a thin sheet model and the image method and compared with that due to aerodynamic effects so as to give an idea of its significance at the micro-scale.(3) Based on the derived dynamic coefficients, linear vibration characteristics and nonlinear parametric resonance of the rotor-bearing system are studied. Results obtained indicate that there are two major instability problems for diamagnetic bearings.(4) Due to the malmanufacturing of the bearing, magnetic asymmetry could occur either on the PM part or the diamagnetic part. The impact of such non-ideal working condition on the performance of the bearing is investigated. The rotational loss induced thereby is also studied to show the acceleration characteristics of the bearing.(5) Basic vibration characteristics of the bearing are assessed experimentally. Compared to other bearing mechanisms for microsystems, diamagnetic approach has the advantage of being completely passive, hereby eliminating the design and operational cost associated with active electromagnetic or electrostatic approaches; and it is more compact in configuration as compared to air bearings, which require connection to the external source of pressurized gas. This thesis represents a first step towards a systematic understanding of a potentialy interesting application of magnetic bearings, and the findings herein would pave the way for further extensive study.