The HexRoller project aims to show that locomotion can be achieved by programming the stiffness of the actuators in a robotic system. The HexRoller is a robotic hexagon capable of rolling motion by changing the stiffness of its six joints. By programming stiffness instead of position, we eliminate the need for the precise coordination of joint motions in a closed kinematic chain. Each joint may be actuated without regard for the position of the other joints in the chain.
The HexRoller is composed of six printed circuit board faces linked together by six flexible joints printed on an Objet 3D printer. The joints are actuated by laser-cut sheets of shape memory alloy (SMA). Each joint is controlled by the neighboring PCB. Each PCB is fitted with an Atmel microcontroller, a two-axis accelerometer, and the power circuitry required to drive a fixed current through the attached SMA actuator. One of the six PCBs is populated with a Nordic nRF24L01+ wireless transceiver which provides a communication link to a desktop PC. This 'master' PCB communicates with the other other five 'slave' PCB using an I2C bus. The I2C bus and the power rails are routed through a one-layer flex circuit which connects all six PCBs.
In order to drive enough current through each SMA actuator, power is shared between all six PCBs. While each PCB has an attached 130mAh LiPo battery, the flex circuit allows these batteries to be grouped in pairs. Each pair of batteries is connected in series. The three resulting groups are then connected in parallel generating 7.4V with a total capacity of 390mAh.
The actuators are built using thin sheet SMA (0.05mm). Each actuator is consists of a set of unit-cells that can expand when energy is applied. Some of the configurations that can be built are shown. The hexroller uses the surface actuator version.