Robert K. Messenger

Piezoresistive feedback for improving transient response of MEMS thermal actuators

We examine exploiting the inherent piezoresistivity of a polysilicon compliant mechanism to provide feedback sensing of the mechanism displacement. As the piezoresistive compliant mechanism deflects to produce motion its resistance changes producing a usable signal. The goal of this work is to improve the transient response of a thermal actuator through piezoresistive feedback control. Implementing feedback control significantly improves the actuators transient response. The actuator response time to step inputs is reduced from 800μs to 230μs with proportional control alone. The system bandwidth was increased from 500~Hz to 4~kHz with proportional control. The large overshoot in the step response or the resonant peak in the frequency response can be reduce by an appropriately tuned 2~kHz notch prefilter.

FEEDBACK CONTROL OF A THERMOMECHANICAL INPLANE MICROACTUATOR USING PIEZORESISTIVE DISPLACEMENT SENSING

Feedback control has proven useful in improving reliability and performance for a variety of systems. However there has been limited success implementing feedback control on surface micromachined MEMS devices. The inherent difficulties in sensing microscale phenomena complicate the development of an economical transducer that can accurately monitor the states of a surface micromachined system. We have demonstrated a simple and effective sensing strategy that uses the piezoresistive property of the polysilicon thin film of which surface micromachined MEMS devices are fabricated. The states of the device are monitored by measuring the change in resistance of flexible members which deflect as the device moves. Measurement of the output displacement of an in-plane thermal actuator is presented as a candidate application. While there still is a noise issue to be dealt with, this approach provides adequate signal strength to implement feedback control using off-chip analog circuitry. Implementation of proportional/integral control on the system is successfully demonstrated.Copyright

Improved Nanopositioning Resolution Through Piezoresistive Feedback Control of a MEMS Thermal Actuator

Utilizing the piezoresistive properties of polysilicon as an on-chip sensing mechanism facilitates the implementation of feedback control on surface-micromachined MEMS devices. We have performed nanopositioning resolution tests on a MEMS thermal actuator, both open and closed loop, to demonstrate the performance improvements possible with feedback control. A thermomechanical in-plane microactuator (TIM), fabricated using the MUMPS fabrication process, was used in this study. The actuator was coupled to a piezoresistive displacement sensor (PRDS) that was fabricated as part of the same process. Measurements of the actuator output, taken using a scanning electron microscope, show that nanopositioning repeatability improved from ±59 nm to ±31 nm when feedback control is employed.Copyright