Closed loop control of a MEMS floating element to detect shear stress

This thesis describes the modeling, design, and microfabrication of a floating element shear stress sensor as well as open-loop testing and closed-loop control design and implementations.;Two sensors have been designed to detect wall shear stress in flow by measuring a change of capacitance. A microfabrication procedure has been developed and successfully applied for the manufacturing of the two designs. The microfabrication process uses an electroplated nickel structural layer and copper sacrificial layer on a glass substrate.;An open-loop testing setup for the sensor chip has been developed, which controls the air flowing across the sensor surface while reading out changing capacitance. The theoretical maximum achievable surface shear in the test setup is 0.4 Pa with a resolution of approximately 0.001 Pa.;Static out-of-plane stiffness testing of the two designs shows reasonable agreement with theoretical expectations, indicating that the devices have been successfully released and are not stuck down. In-plane testing in the flow cell shows clear response of the sensor to changes in flow rate. However, considerable drift of the output signal is experienced, and capacitance output is not linear in the volume flow rate.;A theoretical design for a closed-loop force feedback controller has been carried out. The controller achieves a 12.5 kHz bandwidth of electrostatic force divided by shear force, a phase margin of 21.7 degrees and a damping ratio of 0.7 for the closed loop system. Both continuous-time and discrete implementation have been considered. An analog circuit implementation of the controller has been implemented and demonstrates excellent agreement with the theoretical transfer function. The closed loop controller will be applied to the physical plant in future work.