Paper: https://ieeexplore.ieee.org/abstract/document/9635995
Micro Aerial Vehicles (MAV) with Vertical Takeoff and Landing (VTOL) capabilities, such as quadrotors, have offered significant value to many research fields and markets. However, only recently, MAV began to be explored as systems capable of interacting with the environment, performing manipulation tasks, and participating in more versatility-demanding operations. Pursuing the goal of turning flying machines into more versatile instruments, many researchers have resorted to using tilting rotor mechanisms to create new aerial vehicle concepts. Nevertheless, most such new concepts are bulky and lack the required versatility, and are restricted to particular applications. In this work, we address these issues by proposing a novel coaxial, versatile, modular tilt-rotor, all-terrain vehicle concept.
The OmniRotor platform can apply its full thrust in any direction, regardless of the frame's orientation where it is mounted. The platform does not have any limitations regarding rotation's range. It can change its thrust direction continuously without needing to unwind back to a specific configuration. With the addition of control surfaces between the coaxial rotors, the OmniRotor is turned into a functional VTOL MAV with hovering capabilities that can be used as a ground vehicle, a UAV, and an all-terrain vehicle.
The OmniRotor can be used for search and rescue, environmental monitoring, infrastructure inspection, and aerial manipulation applications.
Fig. 1: The Omnirotor platform. A hybrid, all-terrain robotic UAV developed employing the proposed agile, coaxial, omnidirectional rotor module.
Given the aim of developing a rotor module capable of applying its full thrust in any direction, the more obvious solution is to place the rotors on a structure that rotates according to two perpendicular joints. Such a mechanism is called a gimbal, and it is largely employed in camera systems. The immediate limitation that arises from such an approach, however, is the wiring. Every active rotor tilting mechanism is limited to turn by how much its wire connections can wind around the rotating axis. The standard solution for these wiring constraints is solved by employing slip-rings, which are essentially rotary electric connections. Once again, this is common on gimbal camera stabilization systems. However, brushless electric motors used for propulsion systems tend to draw large amounts of electric current, leading to large and heavy slip-rings. This extra weight is undesirable and would limit the flight time and payload capacity of the vehicle considerably.
To avoid using two heavy slip-rings on the two DoF gimbal mechanism, we proposed the solution of changing the location of the electric components on the system circuit. Instead of placing the battery away from the rotors and at the geometric center of the UAV, as it is commonly done for most multirotor UAVs, the proposed solution places the battery together with rotors. This approach shortens the wire length for the power connections (mainly from the battery to the ESCs and rotors), leaving only low current wire connections routing toward the vehicle's base and the control boards. By employing the slip-rings in this configuration, the proposed design can use both DoF continuously. This means that the base's orientation does not affect the direction to which the thrust is applied. Additionally, by placing the battery with its wider side along the rotors' axis, the propellers' occlusion effect is minimized. In our design, the dimension of this occlusion is only slightly wider than the motors' diameter.
Fig. 2: Exploded view of the rotor-on-gimbal mechanism. With two perpendicular DoF, it can direct the full thrust of the coaxial rotors in any direction. The use of slip-rings allows for continuous operation regardless of number of turns. The control vanes are positioned in between the coaxial rotors, and they move perpendicular to the core tilting axis. The introduction of these control elements provides improved control when hovering.
With the proposed rotor-on-gimbal design, the system's increased capability leads to a variety of applications. Given the ability to apply thrust in any direction without being limited by the base mount's orientation, one possible application is to place this modular rotor concept on a structure with wheels. The result is a vehicle able to roll on the ground and fly, acting both as a ground robot and an Unmanned Aerial Vehicle (UAV). With the flexibility of the omnidirectional thrust vectoring, the system can easily overcome obstacles by taking-off the ground. Furthermore, having the base mount attached above the rotor module, i. e., in an upside-down manner, brings the center of gravity of the system closer to the ground. This reduces the torque the rotor's thrust generates when the wheels are in contact with the ground, making it stable for operation as an Unmanned Ground Vehicle (AGV). Carbon fibre rods form a hexagon structure connected with 3D-printed PLA connectors. The use of carbon fiber rods ensures that the frame will have adequate strength without a significant increase in the overall weight of the UAV. This structure is connected to a laser-cut acrylics sheet mount, where the rotor module base is attached. The carbon fibre structure is fitted with a lightweight (7 g) injected-molded, Pololu ball caster with 3/4 inches Plastic Ball. With electronics and propellers inside the cage-like structure, the impact of collision incurred damages to the UAV, is minimized.