The mechanical design, modeling, and partial experimental validation of a prototype Stirling thermocompressor is presented in this paper. The thermocompressor is intended to serve as a compact and quiet, untethered 50 W, pneumatic power supply for an ankle foot orthosis. The goal of high efficiency at the target power density is pursued through the use of novel heat exchangers and high operating temperature and frequency. The motion of the displacer piston is controlled utilizing a brushless DC motor driving a continuous linear reciprocating screw. This paper presents the experimental validation of the heat transfer and pressure dynamics portions of the thermocompressor, leaving the modeling and validation of mass transfer to future work.

This paper presents the design, modeling, and simulated performance of a prototype Stirling thermocompressor. The thermocompressor is intended to be mounted on the back of a user’s lower leg and convert a hydrocarbon fuel source into 50 W of pneumatic power at 650 kPa to power an ankle foot orthosis. Consisting only of a displacer piston, the Stirling thermocompressor’s displacer piston motion is directly controlled by a brushless DC motor so that the frequency of operation can be tuned to output the maximum power. Simulation indicates that this operating frequency is influenced by the intended reservoir pressure and heat transfer properties of the thermocompressor.

This paper presents the design and control of a free-liquid-piston engine compressor (FLPEC).The FLPEC produces low temperature, high-pressure air for use as a high energy density power supply for untethered, compact robots. Internal combustion with a propane-air mixture in the engine combustion chamber drives a high inertance, low mass liquid piston, which compresses air into an onboard reservoir. A real time logical controller is implemented in Matlab to regulate the timings of the air and fuel injection, spark, and exhaust valve and to compensate for misfires. Experimental pressure data is overlaid with the logical state of the injectors, spark plug, and exhaust valve to demonstrate the efficacy of the controller to account for cycle-to-cycle pressure variations in the combustion chamber.Summer 2013 | Volume 9 |

The flow visualization studies around a square cylinder (upstream) and a circular cylinder arranged in tandem configuration is studied experimentally to identify the interference flow patterns. Flow visualization studies are carried out in a re-circulating water channel. The investigations are carried out for tandem arrangement varying the center-to-center distance (L) between the cylinders; L/B ratio is varied from 1 to 5 where B is the characteristic length. Experiments were conduct at a Reynolds number of 2100 (based on B). The results are also obtained for two tandem square cylinder configuration. The flow patterns observed are: Alternate Shear Layer Reattachment with and without gap flow, Simultanous shear layer reattachment and detached shear layer flow pattern. The time of persistence (in percentage) for each flow pattern is estimated over a sufficiently long period of observation time to identify the most influential, predominant flow pattern. Though these patterns are identical for square-square and square-circular tandem configurations, their order of predominance is different for both the configurations.

This article focuses on the use of a free-liquid-piston engine compressor (FPEC) for compact robot power. The FPEC presented in the article combines the engine and the compressor into a single unit. FPEC, a high-power density form of actuation, can help operate human-scale robots. An energy source that provides pneumatic power presents an appealing alternative that alleviates many of the scalability problems of hydraulics while preserving a high actuation power density. The system also presents additional advantages such as power-on-demand with no idle. Taking advantage of the high inertance piston, high-pressure air and high vapor pressure fuel enable the engine to operate in an inject and fire cycle. Dynamically, the FPEC is similar to a bug converter circuit in that the flow is amplified and the high-inheritance piston plays the same energetic role as the inductor. The data suggests that pneumatic systems using the FPEC as a power source would exhibit system energy densities comparable to, if not better than, the best electrochemical systems.