MicroGimbals provide a unique solution to image stabilization system for micro air vehicles (MAVs). The MicroGimbal is a Microelectromechanical System (MEMS) device designed for military officials and doctors who need the ability to control the position, stability and attitude of micro-scale imaging devices in otherwise harsh environments. The MicroGimbal uses electrostatic actuation to achieve controlled angular deflection along two axes. Unlike standard MEMS optical devices (DMD, resonant frequency oscillators, etc.), the MicroGimbal designed herein uses the multi-layer, copper/polymer, PolyStrata®process. This study presents one novel MicroGimbal design that is optimized within its process material limits for two degrees of freedom and other device performance requirements such as maximum angular swing and payload capacity. By varying key device feature dimensions—such as torsional flex pivot (TFP) cross-section, beam length, gap spacing and widths–allowed the MicroGimbal design to converge on an optimal solution.
However, after design and optimization, the performance of the device was determined to be limited by its material properties; namely insufficientyield stress. Other strategies to improve the PolyStrata®process to increase mechanical behavior and strength while allowing torsional compliance were investigated but not included in the optimization analyses. These investigations included enhancing the electroplated copper via nanoparticle deposits, strain hardening, and/or utilizing composite materialsWhile these methods are potential solutions, they may not be a feasible process change for the PolyStrata® process. Therefore, further analysis to design a working gimbal device by meeting the material constraints was pursued and greater efforts were made to incorporate the design constraints. These constraints were clearly in competition with each other and required various performance tradeoffs.
Based on the selection of a working value for yield strength of Copper of 66.7MPa (using a 1.5 FOS), the design rules for PolyStrata®, and other design requirements, the optimal torsional flex pivot geometry was determined to be: height* = 100um, width* = 10um, and the length* = 1.15um (where * implies the optimal solution set). This optimization can be scaled up to define geometry of the outer sets ofTFPs because the inner roll mode structural mass is insignificant compared to the CCD load that will ultimately be placed on the platform. The maximum predicted displacement with this geometry is approximately 5 degrees.
Additionally, the electrostatic actuation made possible by using the doubly stacked comb drive actuators in torsion (enabled by the PolyStrata® Process)are expected to create a force that could displace each axis about 2 degrees. This analysis assumes that a charge pump circuit could be designed and integrated into the structure such that our approximate 5V can be amplified to about 24V [8]. The bandwidths of such circuits are believed to meet or exceed the bandwidths needed for imaging (above 1 kHz).
In order to quantify the dynamic response of such a gimbal structure, a finite element analysis was performed to determine the mode resonant frequencies for both roll and pitch. These were determined to be 418 Hz for roll and 706 Hz for pitch.