Optimisation and characterisation of a thin MEMS micro generator
Creators: name: Devadoss, Surendran
Publisher: name: RMIT University
Year: 2024

description: Electrochemical batteries are limited by low gravimetric &amp; volumetric energy density (ED) and not suitable for scaling. Studies of MEMS micro-engine coupled generator power supplies for portable applications are undertaken by many research institutes to develop an alternative to electrochemical batteries. Micro systems that have been reported in the current literature produce output power in the range of 100µW to 1mW at rotor speeds of 30,000 to 100,000 rpm, but challenges and issues still remain in the area of engine and generator development to overcome low system efficiency and low energy density. The main contributions of the thesis are in the areas of thin micro generator development suitable for planar MEMS technologies which can be integrated with micro combustion engines. The development of a thin MEMS axial generator topology to achieve high power density is presented in the thesis. A customized lumped parameter analysis methodology using parametric equations is developed to select suitable parameters for the MEMS Micro generator. The methodology takes inputs from magnetic circuit and planar coil analysis used in the thesis. The magnetic circuit analysis is done using a Permeance coefficient method to determine air gap flux density (Bg), force and torque. The planar coil analysis is performed using an original analytical method to determine winding parameters like resistance, coil length, and no of coils to operate the system within practical current density and current loading level. ANSYS FEA analysis is used to validate the theoretical method prediction of air gap flux density for the 5mm rotor radius micro generator model which showed accuracies of 93% to FEA prediction. Equivalent circuit model of the micro generator with the selected parameters is developed for characterisation. The equivalent resistance and reactance of the equivalent circuit model is found to be 573KΩ and 489Ω respectively. Characterisation for no load, full load and variable load is performed and an estimated useful power output of 1.6mW is achievable at 15000 rpm. The micro generator when operated at an increasing temperature of 100oC would see reduction in O/P power from 1.6.mW to 1.3mW due to temperature effect on both flux density at air gap and winding resistance. Generalised equations for the engine are derived and linked to the micro generator lumped parameter equations to form the analysis model for integrated engine-generator system. The model was analysed for attainable energy density for a system volume of 40e-3*70e-3*5e-3mm. The estimated energy density from the analysis was 42.3 WH/Kg. This estimated energy density is three times the energy density that could be achievable from existing micro generators for the same volume. A small improvement in engine efficiency of 3-5% is believed to increase the energy density of the module closer or higher than existing batteries. The capacity of the micro engine –generator system to be able to operate for 3.9 hours at 1.6mW output power would still make this system a significant alternative source of power for portable applications with a huge scope for further improvement