A light-driven motor driven by a single source of light is demonstrated for the first time. The system utilizes a NiTi shape memory alloy (SMA) wire initially trained straight shape. A rotational speed of 135 to 140 rpm was observed at the smaller wheel for a loop of 0.3 mm-diameter SMA wire.

Introduction

An light driven actuator (LDA) using shape memory alloy (SMA) has been proven its usefulness with its high energy conversion efficiency of 1 % *. LDA provides a reciprocal motion while rotational motion is preferred for some applications (Propella, fan, winch, can easily be converted into translational motion by wheels,  mobile robot, light-driven car)

We apply the same combination (SMA + Photothermal effect) to realize a rotational motion.

The system utilizes a wire of NiTi shape memory alloy (SMA) initially trained straight shape. The SMA wire is formed in a loop and put around two wheels. (Idler & drive wheel). As the wire rotates, it undergoes a change in the radius of curvature. Light irradiates the point where the wire is leaving the drive wheel. SMA wire undergoes phase transition and straighten: causing a rotational force to the wheel. The system requires only one light source. And it can utilize a broad spectrum range, so it is suited for solar powered devices.

Theory

The energy produced per unit length of wire is expressed as follows.

Ea and Em are Young’s modulus for the austenite and martensite states.

Experiment

An experimental setup is shown below. A cw diode-pumped laser at a wavelength of 1064 nm was used. The laser beam was slightly expanded to a 1/e2 diameter of 2.5 mm at the location of the irradiation on the SMA wire.

Result

Up to 60 rpm for continuous operation, and 135 to 140 rpm for non-continuous operation was observed. The experimental results were consistent with the theory. Rotational speed was proportional to the light power, confirming the photothermal effect. 

For details please read: H. Okamura, "Light driven motor" , Optical engineering Letters, 50 (2), 020503-1-3 (2011) DOI:10.1117/1.3542042    Download from here

The light-driven motor in action.

The driving portion of the motor observed by a CCD camera.

* H. Okamura, K. Yamaguchi, and R. Ono, "Light-Driven Actuator with Shape Memory Alloy for Manipulation of Macroscopic Objects,“ International Journal of Optomechatronics 3, 277-288 (2009).

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