The second part of this work discusses the fabrication and testing of a fuel-powered shape memory alloy actuator system (FPSMAAS) and its main components. Fuel (propane) is burned in a combustor and its heat is transferred to a working fluid medium, which in turn transfers the heat to the SMA element to drive its martensite-to-austenite phase transformation. For the austenite-to-martensite transformation, the heat is removed from the SMA element by a cooling fluid, from which the heat is then removed via a heat exchanger. The process of implementing the FPSMAAS consisted of three phases of increasing complexity, corresponding to three generations of the actuator system. For the final generation of the FPSMAAS, the SMA element was housed in a rectangular channel, featuring an innovative way of separating the cold from the hot fluid medium that alternately come in contact with the SMA element, in order to minimize the mixing between them. To meet our goal of miniaturization, a multi-channel combustor/heat exchanger and a micro-tube heat exchanger were developed and tested. The final actuator system was composed of pumps, solenoid valves, check valves, bellows, a micro-tube heat exchanger, a multi-channel combustor/heat exchanger and a control unit. The experimental tests of the final system resulted in 250 N force with 2.1 mm displacement and 1.0 Hz actuation frequency in closed-loop operation. The test results for the individual components as well as the final assembled actuator are compared with the results of the numerical analyses conducted in Part I and a good agreement has been demonstrated.

This work (Part I) discusses the numerical analysis of a fuel-powered shape memory alloy actuator system (FPSMAAS) that utilizes fuels with high energy densities, such as propane, as its energy source and thus reduces or eliminates the dependence on electrical power supplies, such as batteries. The main component of the actuator, a shape memory alloy (SMA) element, operates as a heat engine and converts the thermal energy of fuel combustion to mechanical energy. The incorporation of the high energy density fuel and actuator control hardware and software inside the unit makes for a compact actuator system, requiring only low-power, digital actuator control signals as input. Due to the relatively high recovery stress and strain of SMAs, the compact actuator can provide significant force and stroke. Convection heating and cooling of the SMA also results in relatively high actuation frequencies. Finally, if the compact actuator is utilized in the context of a larger system producing excess parasitic heat, the energy density of the actuator subsystem increases. The numerical analysis of the SMA element/strip was conducted using commercial software, including FLUENT and ABAQUS. The goal of the analysis was to estimate actuation frequency and actuation strain. In addition, a multi-channel combustor and a micro-tube heat exchanger were designed and analyzed. These results were compared to experimental tests and measurements (Part II).

This work discusses the numerical analysis, the design and experimental test of the fuel-powered compact SMA actuator along with its capabilities and limitations. Convection heating and cooling using water actuate the SMA element of the actuator. The energy of fuels, having a high energy density, is used as the energy source for the SMA actuator in order to increase power and energy density of the system, and thus in order to obviate the need for electrical power supplies such as batteries. The system is composed of pump, valves, bellows, heater (burner), control unit and a displacement amplification device. The experimental test of the first designed SMA actuator system results in 150 M Pa stress (force: 1560N) with 3 % strain and 0.5 Hz actuation frequency. The actuation frequency is compared with the prediction obtained from numerical analysis. For the first designed fuel-powered SMA actuator system, the results of numerical analysis were utilized in determining design parameters and operating conditions.

This paper discusses the design and experimental results of the improved fuel-powered compact SMA actuator system and its comparison with the first-generation design. The K-alloy SMA strip (12 mm x 0.9 mm), actuated by a forced convection heat transfer mechanism, is embedded in a rectangular channel. In this channel, a rectangular piston, with a slot to accommodate the SMA strip, runs along the strip and prevents mixing between the hot and cold fluid in order to increase the efficiency of the system. The main energy source is fuel, such as propane, in order to achieve high energy and power densities of the system. Numerical analysis was performed to determine optimal channel geometry and to estimate maximum available force, strain and actuation frequency of the SMA actuator. The combustor/heat exchanger was designed to achieve higher heat transfer rates to the hot fluid from the energy source. The SMA actuator system is composed of pumps, valves, bellows, radiator, combustor/heat exchanger and control unit. The experimental testing of the SMA actuator system resulted in 735 N force with 2.5% strain and 0.25 Hz actuation frequency in closedloop operation.