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Voliro: An Omnidirectional Hexacopter With Tiltable Rotors
Mina Kamel, Sebastian Verling, Omar Elkhatib, Christian Sprecher, Paula Wulkop, Zachary Taylor, Roland Siegwart, Igor Gilitschenski
TL;DR
The paper addresses the limited maneuverability of conventional multirotors for tasks such as complex inspection and camera motion. It presents Voliro, a tilting-rotor hexacopter with mechanical, control, and allocation designs, and evaluates a prototype experimentally. The experiments demonstrate omnidirectional maneuverability, while slow rotor-tilting dynamics leave position and orientation slightly coupled.
Problem
Conventional multirotors impose maneuverability limits that hinder applications requiring broad pose control, including contact-based inspection and uninterrupted complex camera motions.
Method
Voliro combines a hexacopter with fully tiltable rotors, separate position and attitude controllers, and a computationally simple control-allocation technique.
Results
The prototype demonstrates omnidirectional maneuverability, including transitions from horizontal to upside flight, inclined translations, and physical interaction with a wall.
Takeaways & Limitations
Tiltable-rotor multirotors can broaden maneuverability beyond standard fixed-orientation multirotors while avoiding energy-wasting counteracting forces.
Takeaways & Limitations
Slow rotor-tilting dynamics remain an open issue and leave position and orientation slightly coupled during experiments.
Abstract
from arXiv · showhide
Extending the maneuverability of unmanned areal vehicles promises to yield a considerable increase in the areas in which these systems can be used. Some such applications are the performance of more complicated inspection tasks and the generation of complex uninterrupted movements of an attached camera. In this paper we address this challenge by presenting Voliro, a novel aerial platform that combines the advantages of existing multi-rotor systems with the agility of omnidirectionally controllable platforms. We propose the use of a hexacopter with tiltable rotors allowing the system to decouple the control of position and orientation. The contributions of this work involve the mechanical design as well as a controller with the corresponding allocation scheme. This work also discusses the design challenges involved when turning the concept of a hexacopter with tiltable rotors into an actual prototype. The agility of the system is demonstrated and evaluated in real- world experiments.
I. INTRODUCTION
Voliro addresses limits of conventional multirotors with a hexacopter whose rotors tilt omnidirectionally, enabling independent control of position and orientation. The paper combines mechanical design, control and allocation schemes, and prototype evaluation.
- Motivation: Conventional multirotors limit applications requiring independent control of position and orientation, such as contact-based inspection and uninterrupted camera motion.Inspection may require maintaining sensor contact while adapting orientation to a surface’s geometry.
- Related work: Earlier tilting-rotor approaches improved maneuverability but did not demonstrate omnidirectional flight across the full flight envelope.Related work included simulation, experiments, and restricted flight modes, while one similar design controlled orientation without jointly controlling position and orientation.
- Design challenges: The design targets low weight, broadly useful propeller placement, reduced counteracting forces, and safe operation near surfaces.These mechanical requirements also create controller-design challenges.
- Contributions: Voliro is a hexacopter with fully controllable tilting rotors that decouple position and orientation control.The platform uses a PID controller and a simple allocation scheme translating control outputs into motor configurations.
- Contributions: The work contributes the mechanical design of a fully controllable tilting-rotor hexacopter and its corresponding control and allocation schemes.The prototype concept was evaluated in real-world experiments.
B. Hardware
The prototype integrates customized mechanical and electronic components with flight-control and state-estimation software. Its 3.2 kg platform achieves around 8 min of horizontal flight time, while inclined hovering reduces endurance.
- Hardware: The propulsion system uses 9 inch DJI propellers and KDE2315XF-885 motors, each capable of providing a maximum of 13.7 N of thrust.High thrust-to-weight ratio supports hovering at large inclination angles.
- Hardware: The platform core uses carbon-fiber plates, aluminum spacers and screws, a Pixhawk controller, an UP Board computer, and tilting-motor control boards.The customized core houses the electronics and clamps the rotor-supporting carbon tubes.
- Hardware: 3.2 kg total weight and around 8 min flight time are reported for horizontal orientation.Flight time decreases significantly when hovering at a large inclination angle.
- Flight controller: PX4 fuses IMU data with external Vicon or GNSS pose information to estimate position for the controller.Desired pose trajectories are sent from an external computer through the UP Board and Pixhawk.
- Modelling: The system model combines rigid-body dynamics with established aerodynamic modeling techniques to support model-based control synthesis and testing.The paper defines inertial, body, and rotor-unit coordinate frames for the vehicle model.
2) Notation convention:
The paper uses coordinate-system subscripts to identify vector representations and rotation matrices, while modeling Voliro under simplifying rigid-body, rotor, and low-velocity assumptions.
- Notation convention: Left-hand subscripts identify the coordinate system in which a vector or rotation matrix is represented.For example, R_AB rotates a vector from body frame F_B to inertial frame F_A.
- Notation convention: A hat over a state symbol denotes its estimate.
- Model assumptions: The model assumes a rigid, symmetric body with rotors aligned in height and axis through the center of gravity.
- Model assumptions: Rotor thrust and drag torque scale with squared rotor speed, while body and tilting dynamics are simplified by low-velocity and independence assumptions.
- Model assumptions: Rotor interference is likely significant, but flight tests found that a well-tuned controller can treat it as an unmodeled disturbance.
C. Rigid Body Model
The rigid-body model combines Newton–Euler body dynamics, first-order actuator models, and an allocation matrix that maps rotor speeds and tilts to six-axis forces and moments.
- Rigid Body Model: Newton–Euler formalism models the body dynamics using the system’s moment of inertia and the 3-dimensional identity matrix.
- Rigid Body Model: The rotor DC motors and closed-loop tilting velocities are modeled as first-order systems with respective time constants, including a saturation on tilting speed.The maximum tilting velocity is denoted ω_α,max.
- Rigid Body Model: Aerodynamics maps rotor angular velocities and tilting angles into forces and moments acting at the body’s center of gravity.
- Rigid Body Model: Rotor forces and reaction torques act along each rotor frame’s z-axis, then rotate into the body frame and combine with moment arms.The lift and drag coefficients are denoted μ and κ.
- Rigid Body Model: The 6 × 6 allocation matrix depends on tilting angles and, when full rank, permits independent production of all forces and moments.This enables independent control of platform translation and rotation, except in singular configurations.
A. Control structure
Voliro’s control structure separates position and attitude tracking, then allocates desired six-degree-of-freedom forces and torques across rotor speeds and orientations.
- Control structure: The omnidirectional actuation allows separate position and attitude controllers because forces and torques are independent.
- Control structure: Changing each propeller’s angular velocity and orientation produces desired forces and torques, creating a 6-DoF output with 12 control inputs.
- Position control: A PID position controller with feed-forward terms generates a desired force vector in the vehicle body frame from position error.
- Position control: The position control law uses proportional, derivative, and integral gains together with gravity, vehicle mass, desired acceleration, and frame rotation.
- Control structure: A cascade structure is used for attitude control onboard the PixHawk flight controller.
C. Attitude control
Voliro’s attitude controller uses cascaded quaternion-based attitude and angular-rate loops to generate desired moments from orientation and rate errors.
- Attitude control: The attitude controller has an outer loop that generates desired body rates and a rate loop that generates desired moments.
- Attitude control: Desired body rates are computed from the vector part of the quaternion attitude error.
- Attitude control: The sign of the quaternion error’s real part is selected to avoid unwinding phenomena.
- Attitude control: Desired moments combine rate error feedback with a center-of-mass offset term and gyroscopic compensation using the estimated angular velocity and inertia matrix.
D. Control allocation
Voliro’s control allocation transforms an orientation-dependent nonlinear problem into a linear force-allocation problem, then recovers rotor speeds and tilting angles. The approach enables fast computation but retains constraints from actuator dynamics and physically inadmissible speeds.
- Control-allocation challenge: The allocation problem must determine six rotor speeds and angular positions despite over-actuation and nonlinear dependence on rotor orientations.These properties make solutions non-unique and complicate direct allocation.
- Linearized allocation: Variable transformation decomposes each rotor’s force into vertical and lateral components, converting the nonlinear allocation problem into a linear one.The transformed variables are collected into a 12 × 1 vector of vertical and lateral forces.
- Linearized allocation: A 6×12 static allocation matrix independent of rotor orientation maps the decomposed forces to the system wrench, and its Moore-Penrose pseudo-inverse computes the force solution.Because the system is under-determined, the pseudo-inverse provides the minimum-norm solution.
- Optimization objective: Minimizing the decomposed-force norm produces more consistent, evenly distributed angular velocities and reduces power consumption.The norm is proportional to the sum of the fourth powers of rotor speeds.
- Command recovery: Actual rotor speeds and orientations are recovered by solving the transformed force equations, allowing motor and tilt commands to be computed through matrix-vector multiplication.The calculation can run at several hundred Hz on the Pixhawk micro-controller.
- Limitation: Tilting angles and angular velocities are nonlinearly coupled, so the allocation cannot constrain their slower and faster dynamics separately.At a 90° roll hover, rapid 180° tilting commands can exceed tilting-motor dynamics and generate uncommanded forces and moments.
A. Simulation
The system was modeled and tested in a Gazebo-based simulation environment using RotorS, MAVLink, PX4 SITL, and ROS. Experiments then demonstrated upside-down rotation, inclined horizontal translation, and interaction with a wall using external motion capture for state estimation.
- Simulation setup: The simulation uses Gazebo as the physics environment and models the tilting motors, while RotorS simulates sensors and thrust motors.Sensor data is sent through MAVLink to PX4 software-in-the-loop, with commands delivered through ROS.
- Simulation setup: Figure 8 presents the model of the system in the simulation environment.The figure documents the simulated system model used in the controller test.
- Experimental evaluation: Experimental state estimation combines external motion-capture pose information with onboard IMU data in the flight controller.The motion-capture pose is sent at 10 Hz and fused with the IMU.
- Experimental evaluation: The experiments also show Voliro interacting with and moving along a wall, while both demonstrated maneuvers are described as infeasible for a standard multicopter.This evaluation targets the platform’s ability to operate through configurations unavailable to standard multicopters.
- Experimental evaluation: The first maneuver rotates the vehicle about its body y-axis from horizontal to upside down and back while holding other desired rotations and positions at zero.The maneuver is performed slowly to demonstrate stabilization across rotations around the y-axis.
- Experimental evaluation: A second maneuver translates horizontally in x and y while maintaining a 50° rotation about the x-axis and zeroing the other rotations.The results are reported in Figure 11.
2) Wall Interaction Experiments:
Voliro demonstrates omnidirectional maneuverability through wall interaction, including vertical transition, compliant contact, and motion along a wall. Experiments also reveal coupling and actuator-saturation limitations.
- Wall interaction setup: A passively compliant three-sphere module enables stable wall contact and reduces oscillation during interaction.The module is mounted on Voliro and can roll passively.
- Interaction sequence: Voliro transitions to vertical flight at pitch 90° and approaches the wall while maintaining that orientation until contact.The sequence comprises transition, approach, and contact establishment.
- Wall motion: A proportional controller generates force vectors that let the platform drive in any direction on the wall, including tracking a circle.The wall-driving behavior is illustrated in Figure 13c.
- Observed maneuverability: The experiments demonstrate omnidirectionality: the vehicle reaches all orientations along one rotation axis and translates while inclined.Position and orientation remain slightly coupled because the controller does not counteract all translational and rotational dynamics.
- Limitations and demonstrations: At pitch 90°, allocation can demand inadmissible rotor speeds and saturate two thrust motors, although the system still tracks position and orientation.The conclusion identifies slow rotor-tilting dynamics as an open issue and reports horizontal-to-upside flight and wall interaction demonstrations.