An artificial muscle for a human arm-like manipulator with high strain and high power density are under development,
and an SMA(Shape memory alloy) spring is a good actuator for this application. In this study, an artificial muscle
composed of a silicon tube and a bundle of SMA(Shape memory alloy) springs is evaluated. A bundle of SMA springs
consists of five SMA springs which are fabricated by using SMA wires with a diameter of 0.5 mm, and hot and cool
water actuates it by heating and cooling SMA springs. A faucet-like valve was also developed to mix hot water and cool
water and control the water temperature. The mass of silicon tube and a bundle of SMA springs is only 3.3 g and 2.25 g,
respectively, and the total mass of artificial muscle is 5.55 g. It showed good actuating performance for a load with a
mass of 2.3 kg and the power density was more than 800 W/kg for continuous valve switching with a cycle of 0.6 s. The
faucet-like valve can switch a water output from hot water to cold water within 0.3s, and the artificial muscle is actuated
well in response to the valve position and speed. It is also presented that the temperature of the mixed water can be
controlled depending on the valve position, and the displacement of the artificial muscle can be controlled well by the
mixed water. Based on these results, SMA spring-based artificial muscle actuated by hot and cool water could be
applicable to the human arm-like robot manipulators.
The important required characteristics of an artificial muscle for a human arm-like manipulator are high strain and high power density. From this viewpoint, an SMA (shape memory alloy) spring is a good candidate for the actuator of a robotic manipulator that utilizes an artificial muscle. In this study, the maximum power density of an SMA spring was evaluated with respect to the input power. The spring samples were fabricated from SMA wires of different diameters ranging between 0.1 and 0.3 mm. For each diameter, two types of wires with different transition temperatures were used. The relationship between the transition temperature and maximum power density was also evaluated. Each SMA spring was stretched downward by an attached weight and the temperature was increased through the application of an electric current. The displacement, velocity, and temperature of the SMA spring were measured by laser displacement sensors and a thermocouple. Based on the experimental data, it was determined that the maximum power densities of the different SMA springs ranged between 1,300 and 5,500 W/kg. This confirmed the applicability of an SMA spring to human arm-like robotic manipulators. The results of this study can be used as reference for design.
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