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Biomimetic Millisystems Lab

RoACH: A Robotic Autonomous Crawling Hexapod
The smallest untethered legged robot until 2011*
RoACH robot pictured next to US quarter The robot pictured to the right is the Robotic Autonomous Crawling Hexapod (RoACH). Fabricated from long fiber-reinforced composites, this robot makes use of Smart Composite Microstructures and integrated shape memory alloy (SMA) wire actuators. In addition to having novel kinematics that are enabled by the SCM process, all power, control, and communication electronics are carried onboard and the entire robot is powered with a 20maHr Lithium-polymer battery from the Full River corporation. It's capable of walking at about 1 body length per second (3 cm/sec) for 9.5 minutes.

Hoover IROS 2008    Movie of RoACH
* HAMR3 in A. Baisch et al. IROS 2011 reports a 1.7 gram autonomous robot at 3 cm/sec.
The importance of kinematic design
As robots get smaller, their design becomes more difficult. In general, the surface area to volume ratio becomes larger, and this has important ramifications. Surface forces become large compared to interial forces, so phenomena such as friction (in bearings or pin joints, for example) play a more significant role than they did at a larger scale. It also means that traditional actuator technologies like DC motors are necessarily less efficient and less power-dense at this scale.

The SCM process goes a long way toward addressing these difficulties. It enables the fabrication of complicated, functional folded structures that move using elastic deformation rather than through the use of traditional mechanical elements like pin joints or bearings. It also integrates well with novel actuator technologies like piezo-electric material or shape memory alloy.

However, effective, autonomous millirobots using SCM still require skill on the part of the designer to create viable designs that use a minimal number of actuators. Minimizing the number of required actuators minimizes the power required to drive the robot, making it more likely to be capable of sustained autonomy. This design goal therefore forces some of the control of the robot to be integrated into the kinematic design.Kinematic model of RoACH robot

RoACH is an example of an autonomous millirobot in which the kinematics of locomotion (an alternating tripod gait, in this case) drove the design of the robot. Pictured to right, RoACH has only two degrees of freedom and is therefore actuated using only actuators. The central blue plate forms a linkage known as the Sarrus linkage. When it contracts, the yellow legs are lifted and the purple legs are lowered. When it expands, the purple legs are lifted and the yellow legs lowered. When the blue plate is moved forward along the long axis of the body (with respect to the outer aqua-colored plates), the yellow legs swing forward while the purple legs swing backward. When it is moved backward, the purple legs swing forward while the yellow legs swing backward.

Proper timing of the actuation of these two degrees of freedom allows the robot to walk forward, backward, and even turn.

RoACH is fabricated using a version of the SCM process that has been adapted to work with prepregnated S2 glass fiber reinforced composites. In this process, a layer of polymer film is sandwiched between laser-micromachined outer layers of composite and the whole structure is cured. In places where the composite has been cut away, the polymer layer acts a compliant hinge, allowing two rigid composite links to bend with respect to each other. This process enables the creation of complicated folded robot designs and can even be scaled up to work with inexpensive materials for scaled rapid prototyping.
Power and Control
Control electronicsThe RoACH robot is powered using a 20maHr Lithium-polymer battery from Full River. Control is accomplished with the electronics shown to the right. The primary components of the board are a 10Mips PIC LF2520 microprocessor and an IrDA communication module for wireless communication with a PC.

Power electronics

The power electronics are pictured on the left and consist of a standard boost converter topology for increasing the battery voltage to the level necessary to provide sufficient current to heat the SMA wire actuators.

RoACH weighs: 2.5 grams

Dimensions: 35mm long x 14 mm wide x 13 mm tall

Maximum walking speed: 1 body length (3cm) per second

Maximum battery life: 9min 30secs

Specific power: 15mW/mms-1

For more details see the conference paper on RoACH.

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