• How can a legged robot fly?

    Carl Bugeja05/17/2017 at 19:33 0 comments

    FlyPod is a compact hybrid robot designed to operate in air and land. It has twelve angular actuators to control the angles of each joint thus being able to perform a walking gait. But how can it maintain stability during flight using the same actuators?

    The shape of the quadruped robot is symmetrical and thus the Center Of Gravity (COG) is roughly located in the middle. By assuming that the platform’s mass is equally distributed, the following two hypothetical aerial steering methods can be utilized:

    • By actuating the trajectory points of the legs in a common direction, the mass of the linkages could cause the robot’s COG to shift. As shown in the image, a rotational force is generated when one leg is shifted towards the pivot point (middle of the robot) and the opposite leg is extended from the pivot point.

    • Another similar control technique is shown in the image bellow. In contrast to the first concept, the four legs are now all spread out. By closing one of the legs, a turning moment is produced from the mass of the opposite extended leg.

    For these concepts to generate the necessary moment, a large portion of the robot’s mass is required to be located in the Tibia links. Heavy components such as the robot’s onboard battery can unbalance the system. This problem was resolved by dividing the battery into four identical cells, to obtain a more symmetrical weight distribution. These batteries were placed in the Tibia link of each leg to provide a larger moment and improve the robot’s flight maneuverability.

    FlyPod Robot

    This innovative COG steering concept reduces additional weight, costs and power consumption related with the flaps or swashplate mechanism, making the hybrid system more efficient. A similar steering technique was also investigated by the US Army in the 1950s when developing the Hiller VZ-1 Pawnee flying platform. This small hovering aircraft was steered using the pilot’s reflexes to manipulate the vehicle’s COG by shifting his body mass.

    Stay tuned for FlyPod’s first flight stability test video!

  • Why use a Coaxial-Rotor?

    Carl Bugeja04/30/2017 at 10:53 0 comments

    When one starts to design the architecture of an Unmanned Hybrid Vehicles (UHV), it is a safe design choice to use the quad-rotor configuration. These drones utilise a simple thrust vectoring control technique to maintain stability during flightby adjusting the speed of the four propellers. Having multiple propellers will obviously generate more thrust, for a given propeller size and thus allow the drone to carrying a heavier payload. However, the use of multiple rotors reduces the energy efficiency of the system, which can lead to a limited time of operation.

    By utilising the same sensors and actuators for both locomotion, the robot’s power consumption and overall mass can be decreased. This reduction in mass would permit the use of a compact coaxial-rotor design.

    Although multi-legged robots are expensive and complex to control, their adaptive locomotion allow the system to walk on uneven terrains. The leg’s symmetrical design also allow the robot to move in any direction, without the necessity of reorienting itself. It was decided that each leg will consists from three revolute joints, to enhance the workspace of the foot-tip. The leg’s kinematic chain was formed from three linkages, known as the Coxa, Femur and Tibia links. By actuating these three linkages during flight, the robot’s centre of mass can be manipulated to control the raw and pitch axes. The yaw angle of the robot could be regulated by simply adjusting the speed of the twin propellers.

    To increase the compactness of the robot, the chassis was composed from a duct that housed a pair of contra-rotating propellers. Although the duct increases the overall mass of the platform, it also reduces tip losses of the propellers. Thus, a larger net thrust could be generated by minimising the clearness between the duct’s inner diameter and the rotor. The duct also protects the surrounding environment from the high speed rotating propellers, offering a safer design.