Why use a Coaxial-Rotor?

A project log for FlyPod

A robot capable of walking and hovering

Carl Bugeja 04/30/2017 at 10:530 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.