Mechatronics Methods for Mitigating Undesirable Effects of Pre-motion Friction in Nanopositioning Stages with Mechanical Bearings

Nanopositioning (NP) stages are used to for precise positioning in a wide range of nanotech processes, ranging from substrate patterning to micro additive manufacturing. They are often used for point-to-point (P2P) motions, where the stage is commanded to travel to and settle within a pre-specified window of the target position, and for tracking motions, where the stage is commanded to follow a reference trajectory. The settling time, in-position stability and tracking accuracy of NP stages directly affects productivity and quality of the associated processes or manufactured products. NP stages can be constructed using flexure, fluidic, magnetic or mechanical bearings (i.e., sliding and, especially, rolling-element bearings). Of these choices, mechanical bearings are the most cost-effective, and are currently the only commercially viable option for a growing number of NP applications that must be performed in high vacuum environments. However, mechanical-bearing-guided NP stages experience nonlinear pre-motion (i.e., pre-sliding/pre-rolling) friction which adversely affects their precision and speed. Control-based compensation methods, commonly used to address this problem, often suffer from poor robustness and limited practicality due to the complexity and extreme variability of friction dynamics at the micro scale. Therefore, this dissertation proposes three novel mechatronics methods, featuring a combination of mechanical design and control strategy, as more effective and robust solutions to mitigate the undesirable effects of pre-motion friction. The first approach is vibration assisted nanopositioning (VAN), which utilizes high frequency vibration (i.e., dither) to mitigate the low speed (slow settling) of mechanical-bearing-guided NP stages during P2P motions. VAN allows the use of dither to mitigate pre-motion friction while maintaining nanometer-level positioning precision. P2P positioning experiments on an in-house built VAN stage demonstrates up to 66% reductions in the settling time, compared to a conventional mechanical bearing NP stage. A major shortcoming of VAN is that it increases the cost of NP stages. To address this limitation, a friction isolator (FI) is proposed as a simple and more cost-effective method for mitigating pre-motion friction. The idea of FI is to connect the mechanical bearing to the NP stage using a joint that is very compliant in the motion direction, thus effectively isolating the stage from bearing friction. P2P positioning tests on a NP stage equipped with FI prototypes demonstrate up to 84% reductions in the settling time. The introduction of FI also enables accurate and robust reductions of motion errors during circular tracking tests, using feedforward compensation with a simple friction model. One pitfall of FI is that it causes increased error of the stage during in position. Therefore, a semi-active isolator (SAI) is proposed to mitigate the slow settling problem using the FI, while maintaining the benefits of friction on in-position stability. The proposed SAI, which connects the bearing and NP stage, is equipped with solenoids to switch its stiffness from low, during settling, to high once the stage gets into position. P2P experiments demonstrate up to 81% improvements in the settling time without sacrificing in-position stability. The proposed mechatronics methods are compared and FI stands out as a result of its simplicity, cost-effectiveness and robust performance. Therefore, the influence of design parameters on the effectiveness of FI are investigated to provide design guidelines. It is recommended that the FI should be designed with the smallest stiffness in the motion direction, while satisfying other requirements such as in-position stability and off-axis rigidity.PHDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/155296/1/terrydx_1.pd