Multi-legged and Multi-limbed Robots

Hexapod-to-quadruped (Hexaquad) Robot for Land-to-Underwater Application

Hexaquad robot is inspired from a hybrid of biological anatomy of selected living creatures, which include insects, crustacean and peristaltic organism, as shown in Figure 1. As one of the multi-purpose active suspension vehicle (ASV), Hexaquad robot is expected not only to be able to walk on irregular and mountainous terrain, but also able to perform other tasks such as picking and placing obstacles as well as climbing. With reference to the Figure 2(a), the original conceptual model of Hexaquad is inspired from the combination of three group of creatures; insect, crustesean and worm. Moreover proposed prismatic spine design was inspired by the peristaltic movement of a worm, while the 3 degree-of-freedom (DoF) foot-to-gripper (FTG) leg configuration is inspired by the movement of a crab as shown in Figure 1. The two flexible and movable legs on the prismatic spine allows the Hexapod robot to identify its center of gravity (CoG) and center of pressure (CoP) to realize the Hexa-Quad transformation. On the other hand, the arthropod configuration in a crustacean is considered as a hybrid of hexapod and decapod creatures as the configuration has the ability to optimize a leg as an arm in a particular situation. In order to optimize the use of actuators and energy, Hexaquad robot is modeled with the same capability as an arthropod creature. The foot is fully utilized in hexapod mode as shown in Figure-3(a) compared to that in quadruped mode as shown in Figure 3(c), in which FTG is utilized.

Figure 1: Proposed Hexaquad robot with hybrid bio-inspired configuration [patent applied]

In fabrication phase, some modification has been done due to the practical balance for Hexaquad structure. On the other hand, the modification also consider a type of material to be used for the frame, payload for leg minimum torque selection for the motor/actuator, as well as water resistance (anti-corrosion) since the robot will be deployed for underwater operation. Overall frame structure material of the robot are aluminum alloy 6061 and this material is suited to the Hexaquad robot requirement as shown in Table-1. The concept used in remodeling the Hexaquad robot is called Actuating Frames, in which the actuators/motors used are a part of the body frame, hence reducing major frame fabrication. As shown in solid model in Figure-4, tubular actuator is used as Link 2, linear motor with spur gear as parallel actuator for Link 1, and bipolar stepper motor as shoulder motor.

In fabrication phase, some modification has been done due to the practical stability for Hexaquad structure. The modification had considered a type of material used for the frame, payload for leg minimum torque selection for the motor/actuator, as well as water resistance (anti-corrosion) since the robot will be deployed for underwater operation. Overall frame structure materials of the robot are aluminum alloy 6061 and this material is suited to the Hexaquad robot requirement as shown in Table-1. The concept used in remodeling the Hexaquad robot is called Actuating Frames, in which the actuators/motors used are a part of the body frame, hence reducing major frame fabrication. As shown in solid model in Figure-4, tubular actuator is used as Link 2, linear motor with spur gear as parallel actuator for Link 1, and bipolar stepper motor as shoulder motor.

Figure 4: Hexaquad Leg Mechanism inspired from Crab leg, (a) Fabrication model, (b) Conceptual model

The scissoring concept is applied for the Foot-to-Gripper (FTG) actuation, as shown in Figure 5, which produces high torque using optimum-sized linear type actuator. The linear movement of the shaft will cause the tips to come together (foot) or split (gripper) as shown in Figure 5(a) and 5(b), respectively. Tubular actuator, an indirect drive DC motor that is available in high torque capacity, is used in the actual Hexaquad robot as part of the leg frame (Link 2) as well as a puller shaft to split the tips as shown in Figure 5(b).

Figure 5: FTG with scissoring mechanism (a) Foot mode (gripper closed), (b) Gripper mode

For the case of control system, various effort has been done in joint precision as listed in publication list below. The precision is essential in leg/arm motion as premier works for dynamic motion control. As shown in Figure 6, the coordination of Hexaquad is notified and the motion for each leg was designed with semi-oval as shown in Figure 7. Figure 8 shows the overall control architecture for Hexaquad with tripod walking pattern as a main trajectory for its locomotion.