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This paper addresses the stability problem of pump controlled asymmetric hydraulic actuators and proposes a physical solution for it. The system under consideration utilizes a shuttle valve to compensate for unequal flow rates due to the asymmetry in the actuator. Possible hydraulic circuit configurations resulting from various valve positions are defined on the load pressure versus velocity plane and a generalized linear model of the system is derived. The investigation shows that there exists a critical load pressure region in which any equilibrium point requiring a partially open spool position is unstable during the retraction of the actuator. A particular valve underlap is proposed in order to avoid the instability and a shuttle valve selection guide is presented. Theoretical findings are validated by both numerical simulations and experimental tests. Results show that the undesired pressure oscillations are removed up to certain actuator velocities with the use of an underlapped shuttle valve.

This paper presents the design, optimization and implementation of an electromagnetic type vibration-to-electrical micro energy harvester. The proposed harvester implements a new design employing array of parylene cantilevers on which planar gold coils are fabricated. The micro harvester generates voltage by virtue of the relative motion between the coils and a stationary magnet. The coils are connected electrically in series to sum up the voltage output from individual cantilevers. The number of cantilevers can be adjusted to improve the generated power without significantly increasing the overall device volume. Another forthcoming feature of this study is the investigation of the phase-shift phenomenon, which is the effect of natural frequency mismatches between the cantilevers due to fabrication related nonuniformities. A detailed mathematical modeling and optimization of the design for various cases, together with the phase and frequency shifts between the cantilevers, are carried out. The proposed harvester is implemented on a microscale and mathematical modeling is verified through extensive tests. The fabricated device occupies a volume of 9.5 × 8 × 6 mm 3 . A single cantilever of this device can generate a maximum voltage and power of 0.67 mV and 56 pW, respectively, at a vibration frequency of 3.4 kHz. These values can be improved considerably by increasing the coil turns and natural frequency of the cantilevers. However, our test results show that any mismatch between the series cantilevers results in significant degradation of the overall output.

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