An adaptive energy-harvesting circuit with low power dissipation is presented and demonstrated, which is useful for efficient ac/dc voltage conversion of a piezoelectric micropower generator. The circuit operates stand-alone, and it extracts the piezoelectric strain energy independent of the load and piezoelectric parameters without using any external sensor. The circuit consists of a voltage-doubler rectifier, a step-down switching converter, and an analog controller operating with a single supply voltage in the range of 2.5-15 V. The controller uses the piezoelectric voltage as a feedback and regulates the rectified voltage to adaptively improve the extracted power. The nonscalable power dissipation of the controller unit is less than 0.05 mW, and the efficiency of the circuit is about 60% for output power levels above 0.5 mW. Experimental verifications of the circuit show the following: 1) the circuit notably increases the extracted power from a piezoelectric element compared to a simple full-bridge diode rectifier without control circuitry, and 2) the efficiency of the circuit is dominantly determined by its switching converter. The simplicity of the circuit facilitates the development of efficient piezoelectric energy harvesters for low-power applications such as wireless sensors and portable devices.

Rotating stall and surge have been stabilized in a transonic single-stage axial compressor using active feedback control. The control strategy is to sense upstream wall static pressure patterns and feed back the signal to an annular array of twelve separately modulated air injectors. At tip relative Mach numbers of 1.0 and 1.5 the control achieved 11 and 3.5 percent reductions in stalling mass flow, respectively, with injection adding 3.6 percent of the design compressor mass flow. The aerodynamic effects of the injection have also been examined. At a tip Mach number, Mtip, of 1.0, the stall inception dynamics and effective active control strategies are similar to results for low-speed axial compressors. The range extension was achieved by individually damping the first and second spatial harmonics of the prestall perturbations using constant gain feedback. At a Mtip of 1.5 (design rotor speed), the prestall dynamics are different than at the lower speed. Both one-dimensional (surge) and two-dimensional (rotating stall) perturbations needed to be stabilized to increase the compressor operating range. At design speed, the instability was initiated by approximately ten rotor revolutions of rotating stall followed by classic surge cycles. In accord with the results from a compressible stall inception analysis, the zeroth, first, and second spatial harmonics each include more than one lightly damped mode, which can grow into the large amplitude instability. Forced response testing identified several modes traveling up to 150 percent of rotor speed for the first three spatial harmonics; simple constant gain control cannot damp all of these modes and thus cannot stabilize the compressor at this speed. A dynamic, model-based robust controller was therefore used to stabilize the multiple modes that comprise the first three harmonic perturbations in this transonic region of operation.

A single-crystal silicon micromachined air turbine supported on gas-lubricated bearings has been operated in a controlled and sustained manner at rotational speeds greater than 1 million revolutions per minute, with mechanical power levels approaching 5 W. The device is formed from a fusion bonded stack of five silicon wafers individually patterned on both sides using deep reactive ion etching (DRIE). It consists of a single stage radial inflow turbine on a 4.2-mm diameter rotor that is supported on externally pressurized hydrostatic journal and thrust bearings. This work presents the design, fabrication, and testing of the first microfabricated rotors to operate at circumferential tip speeds up to 300 m/s, on the order of conventional high performance turbomachinery. Successful operation of this device motivates the use of silicon micromachined high-speed rotating machinery for power microelectromechanical systems (MEMS) applications such as portable energy conversion, micropropulsion, and microfluidic pumping and cooling.