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OmniRotor: An Agile, Coaxial, All-Terrain Vehicle

A coaxial, versatile, modular, tilt-rotor, all-terrain vehicle that can apply its full thrust in any direction.

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A coaxial, versatile, modular, tilt-rotor, all-terrain vehicle that can apply its full thrust in any direction. The OmniRotor platform can be used for search and rescue, environmental monitoring, infrastructure inspection, and aerial manipulation applications. More information can be found at the following videos:

Demonstration Video 1: https://www.youtube.com/watch?v=1dOp-dNc0_U
Demonstration Video 2: https://www.youtube.com/watch?v=JN7Ji_fr_0w

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. 

OmniRotorCAD.zip

CAD files of the coaxial omnirotor, all-terrain vehicle platform.

x-zip-compressed - 18.43 MB - 10/11/2021 at 13:08

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Coaxial Benchmarking Platform - CAD.zip

CAD files of the coaxial benchmarking platform.

x-zip-compressed - 2.12 MB - 10/05/2021 at 20:22

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Coaxial Benchmarking Platform - BOM.xlsx

Bill of materials of the coaxial benchmarking platform that we are using for modelling purposes.

sheet - 9.64 kB - 10/05/2021 at 20:22

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Coaxial Benchmarking Platform - LabView Code.rar

Code of the coaxial benchmarking platform. We are working on a new version that will not be relying on Labview.

RAR Archive - 11.86 MB - 10/05/2021 at 20:22

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  • Some Experimental Results

    Joao Buzzatto3 days ago 0 comments

    To further validate the platform's practical feasibility experimentally, and assess its performance, the hybrid vehicle prototype was subjected to two tests. The first experiment demonstrates the concept's capabilities as an aerial vehicle. The prototype was required to start from rest, take off, and then track some desired path, maintaining a fixed heading orientation. Specifically, the path used forms an inclined eight shape in 3D space. The second test was designed to demonstrate the concept's capabilities as a UAV and a ground vehicle. On it, the prototype performs an emulated search-and-rescue scenario.
    The PX4 open-source flight stack of a Pixracer, a flight controller board commonly used for small racing quadrotors, was modified to incorporate the control strategy and inverse kinematics described in the previous section. The original attitude/rate controllers and mixer were adequately modified, while the middleware and other essential parts of the flight stack were kept, e.g., position controller, land/takeoff detector,  real-time task scheduling, and drivers for embedded sensors. The controller's output commands were mapped to the external actuators, i.e., Dynamixel servo motors, by creating an interface to the flight controller using the MAVLink messaging protocol and an OpenCM/Arduino board. The two 2050Kv brushless motors with 7-inch propellers were controlled directly by the flight controller and 4-in-1 ESC. Furthermore, MATLAB/Simulink running on an offboard PC was employed for handling telemetry, parsing and creating MAVLink messages, and transmitting position and orientation data as UDP packets to the flight controller. Pose measurements were obtained using a Vicon motion capture system.

    The desired orientation is set to a fixed value of 90 degrees. That orientation was precisely maintained with 1.3 degrees RMS error. Overall, the results are promising and validate the vehicle's ability to track complex 3D paths with satisfactory performance. However, such faults observed in the performance can not be overlooked. Some possibilities could explain the undesired behaviours the system displayed. The vanes' model between counter-rotating coaxial rotors is unknown, and the authors believe that once a model is obtained, control could be improved considerably. Furthermore, a design iteration that favors better rigidity, employing higher-end materials such as carbon fiber instead of PLA for some parts of the design, would reduce low-frequency vibrations and improve sensor readings. The considerable weight present in the Core link can also influence performance by transferring reaction torques due to its movement, disturbing the system. Such an effect could be reduced by increasing the distance between the CG and the thrust's application point with design changes. This would increase the attitude control authority, minimizing the impact of disturbances. Nevertheless, the prototype proved to be consistent in its flight performance and capable of tracking many other trajectories shapes, which are not presented here.

    Finally, to illustrate the Omnirotor platform's use as a hybrid vehicle, the concept was tested on a potential search-and-rescue environment. In the first part of this scenario, the vehicle transverses a track on the ground, starting from rest and crossing below a narrow, near-the-floor passage. This part of the experiment is performed with teleoperation, where a pilot controls the vehicle driving on the ground. The inputs from the pilot to the system are sent with a regular radio controller. To generate enough effort to move on the ground, the pilot commands low throttle values but large values of pitch and row. In this mode, the same control mapping and mixer used for flying are employed.

    In the second part of the scenario, while traversing the track through the soil, the vehicle is faced with an obstacle it supposedly can not cross as a ground vehicle. The Omnirotor then changes mode switching to...

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  • Control Surfaces in Between Coaxial Rotors

    Joao Buzzatto10/05/2021 at 05:42 0 comments

    A drawback of the gimbal mechanism is its kinematic singularities points. These are points in its workspace where, when performing inverse kinematics to track a path, minimal changes on its pointing direction results in large motion of the joints. This results in the rotor-on-gimbal mechanism's inability to perform small force corrections quickly without introducing disturbances to the system when operating near such singularity points. Unfortunately, in the proposed all-terrain vehicle concept, which utilizes only one of the rotor-on-gimbal module, the Center of Gravity (CG) of the system is always aligned with the kinematic singularity for both of its stable configurations (see Fig. 1).

    Fig. 1: Illustration of the two possible hovering stable points for the all-terrain vehicle concept with one rotor-on-gimbal module. For the vehicle to hover stably, the vehicle's weight must be aligned with the thrust produced by the rotor. This happens when the CG is either above the rotors (inverted equilibrium) or below it (hanging equilibrium). Such direction coincides with the gimbal kinematic singularity, which is aligned with the axis of the first joint.

    A possible solution to such a problem is to add control surfaces downstream to the propellers, arranged perpendicular to the second DoF. Such addition provides enough control authority in all directions even when operating at the gimbal's singularity points. However, given the limited space to fit such addition and to keep a small footprint for the rotor-on-gimbal design, it was decided to place the control surfaces between the coaxial rotors, as shown below (Fig. 2). Despite the considerable added complexity to the system, especially in the aerodynamics sense, this solution approach is lightweight, easy to incorporate, and inexpensive.

    Fig. 2: Exploded view of the Omnirotor central module. This `Core' part includes several critical components of the design. More precisely, it consists of the battery, the rotors, and the control vanes. The backbone of the module has two attachment points for the rotors. The side panels hold both the control vanes and the servos that actuate them. The side panels and the backbone structure were designed to lock the battery inside and provide rigidity to the assembly.

    For the prototype, two 9 g HD-1900A micro servos actuate the control surfaces. They are mounted to 3D-printed PLA holders, which are glued to the battery case lids. The control surfaces, or vanes, are 3D-printed PLA NACA 0006 airfoil profiles, with 100 mm chord length and 90 mm spam. These vanes hinge about 4 mm carbon fiber rods, which are press-fitted into 3D-printed PLA holders glued to the battery case lids. The servo horns and the vanes are connected by 0.8 mm steel rods, and together they form a parallel four-bar linkage. Such parallel linkage arrangement guarantees that the motion of the control surfaces is equal to the servos' motion but translated by their separation distance.

    As mentioned before, when the vehicle is at hovering its CG is aligned with the gimbal's kinematic singularity. To control the UAV in this operating point, we maintain the first DoF locked, and the vanes movement coordination performs compensations on the pitch axis. An important note here is that the control of the vanes is not based on a model. In contrast to the well-known and studied rotor typical on multirotor UAVs, at the time of writing, the authors could not find in the literature any study which describes the system used here, i. e., a counter-rotating coaxial rotor system with control surfaces placed in between them.

    Implementation-wise,  control of the vanes is done by the use of a properly set mixer on the flight controller. Given that each vane is controlled by an independent servo motor, here we consider that they can contribute to the regulation of the system in two ways: i) Help to regulate yaw, and ii) control the vehicle's pitch. To control yaw, the mixer sends commands to move...

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  • Designing an all-terrain, hybrid vehicle

    Joao Buzzatto10/05/2021 at 05:13 0 comments



    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.

  • The Rotor-On-Gimbal Mechanism

    Joao Buzzatto10/05/2021 at 04:17 0 comments

    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.

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