Get Back in Shape ! A Hardware Prototype Self-Reconfigurable Modular Microrobot that Uses Shape Memory Alloy

Self-reconfigurable robotic systems composed of multiple modules have been investigated intensively with respect to their versatility, flexibility, and fault-tolerance. In particular, recent studies examined their feasibility through hardware and software experiments [1]-[6]. Homogeneity enables the system to adapt itself to the external environment by changing its configuration. It can also repair itself if some part becomes faulty or damaged, as any module can function as any part in the system. This article focuses on a microsized model of a self-reconfigurable homogeneous modular robot that opens up many applications, such as inspection robots in hazardous environments or microscale simple manipulators. One example of an application is a microrobot that moves around inside pipes in nuclear or chemical plants by changing its shape and reorganizing itself as a manipulator to execute repairing tasks when it detects a fault (Fig. 1). Other applications include a robot that searches for survivors through narrow spaces in buildings destroyed by natural disasters and space applications like microsize planetary exploring robots, solar panels, or satellite antennas. Recently developed lightweight self-reconfigurable modular robots [2], [4], [6] use conventional electromagnetic motors that have limitations in microsizing; they become ineffective because the power-weight ratio decreases significantly on microscales. Moreover, self-reconfigurable microrobots require actuators that yield enough torque and motion range to transport other modules. These severe requirements have been major barriers to developing self-reconfigurable microrobots. Although some microscale self-assembly systems have been reported [7], they are passively assembled to predetermined shape by surface tension in an irreversible manner and cannot form arbitrary shapes. To develop a modular microrobot that can actively reconfigure itself, we adopt an actuating mechanism driven by a shape memory alloy (SMA). One of the advantages of an SMA actuator is that it keeps a higher power-weight ratio on microscales than electromagnetic motors [8]. It is especially difficult to find micromotors that have comparable size and torque to our second micromodule model, which is described later in this article. The simplicity of the overall actuator mechanism is another advantage, whereas microsize electromagnetic mechanisms require microsize gear reductions that are difficult to fabricate. In addition, the slow response (especially in cooling) becomes less significant as the ratio of surface area to volume becomes large on microscales. Although several types of SMA microactuators have been developed [8], [9], it is still difficult to provide the sufficient torque and wide motion range required for self-reconfigurable microrobots. Therefore, we devised a rotational actuator mechanism using SMA torsion coil springs that satisfies both requirements of torque and motion area. Using this SMA actuator, we designed two-dimensional (2-D) modules that measure 4 × 4 × 8 cm and weigh 80 g, respectively. Each module is equipped with a microprocessor and an intermodule communications device. The self-reconfiguration capacity of the modules will be verified by a multimodule experiment. To confirm the ease of microsizing SMA actuators, we developed a half-size model of the first model. The half-size module measures 2 cm and weighs 15 g without the control unit. Its self-reconfiguration function