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[This corrects the article DOI: 10.3389/frobt.2022.1033715.].

Tethered quadcopters are used for extended flight operations where the powerto the system is provided via a tether connected to an external power source.In this work, we consider a system of multiple quadcopters powered by a singletether. We study the design factors that influence the power requirements, suchas the electrical resistance of the tether, input voltage, and quadcopters'positions. We present an analysis to predict the required power to be suppliedto a series of N tethered quadcopters, with respect to the thrust of eachquadcopter which guarantees electrical safety and helps in design optimization.We find that there is a critical boundary of thrusts that cannot be exceededdue to fundamental electrical limitations. We compare the power consumption forone tethered quadcopter and two tethered quadcopters and show that for largequadcopters far enough from the anchor point, a two-quadcopter system consumeslesser power. We show that, for a representative use case of firefighting, atethered system with two quadcopters consumes 26% less power than acorresponding system with one quadcopter. Finally, we present experimentsdemonstrating the use of a two-quadcopter tethered system as compared to aone-quadcopter tethered system in a cluttered environment, such as passingthrough a window and grasping an object over an obstacle.

We present QUaRTM - a novel quadcopter design capable of tilting the propellers into the forward flight direction, which reduces the drag area and therefore allows for faster, more agile, and more efficient flight. The vehicle can morph between two configurations in mid-air, including the untilted configuration and the tilted configuration. The vehicle in the untilted configuration has a higher pitch torque capacity and a smaller vertical dimension. The vehicle in the tilted configuration has a lower drag area, leading to a higher top speed, higher agility at high speed, and better flight efficiency. The morphing is accomplished without any additional actuators beyond the four motors of a quadcopter. The rigid connections between the quadcopter frame and the quadcopter arms are replaced with sprung hinges. This allows the propellers to be tilted when high thrusts are produced, and recover to the untilted configuration when the thrusts are brought low. The effectiveness of such a vehicle is demonstrated by running experiments on a prototype vehicle with a shape similar to a regular quadcopter. Through the use of tilting, the vehicle is shown to have a 12.5% higher maximum speed, better high-speed agility as the maximum crash-free cruise speed increased by 7.5%, and a better flight efficiency as the power consumption has dropped by more than 20% in the speed range of 15-20 m s-1.

This paper presents a round-trip strategy of multirotors subject to unknown flow disturbances. During the outbound flight, the vehicle immediately utilizes the wind disturbance estimations in feedback control, as an attempt to reduce the tracking error. During this phase, the disturbance estimations with respect to the position are also recorded for future use. For the return flight, the disturbances previously collected are then routed through a feedforward controller. The major assumption here is that the disturbances may vary over space, but not over time during the same mission. We demonstrate the effectiveness of this feedforward strategy via experiments with two different types of wind flows; a simple jet flow and a more complex flow. To use as a baseline case, a cascaded PD controller with an additional feedback loop for disturbance estimation was employed for outbound flights. To display our contributions regarding the additional feedforward approach, an additional feedforward correction term obtained via prerecorded data was integrated for the return flight. Compared to the baseline controller, the feedforward controller was observed to produce 43% less RMSE position error at a vehicle ground velocity of 1m/s with 6 m/s of environmental wind velocity. This feedforward approach also produced 14% less RMSE position error for the complex flows as well.

Energy sources such as batteries do not decrease in mass after consumption, unlike combustion-based fuels. We present the concept of staging energy sources, i.e. consuming energy in stages and ejecting used stages, to progressively reduce the mass of aerial vehicles in-flight which reduces power consumption, and consequently increases flight time. A flight time vs. energy storage mass analysis is presented to show the endurance benefit of staging to multirotors. We consider two specific problems in discrete staging - optimal order of staging given a certain number of energy sources, and optimal partitioning of a given energy storage mass budget into a given number of stages. We then derive results for a continuously staged case of an internal combustion engine driving propellers. Notably, we show that a multirotor powered by internal combustion has an upper limit on achievable flight time independent of the available fuel mass. Lastly, we validate the analysis with flight experiments on a custom two-stage battery-powered quadcopter. This quadcopter can eject a battery stage after consumption in-flight using a custom-designed mechanism, and continue hovering using the next stage. The experimental flight times match well with those predicted from the analysis for our vehicle. We achieve a 19% increase in flight time using the batteries in two stages as compared to a single stage.

This paper presents a round-trip strategy of multirotors subject to unknownflow disturbances. During the outbound flight, the vehicle immediately utilizesthe wind disturbance estimations in feedback control, as an attempt to reducethe tracking error. During this phase, the disturbance estimations with respectto the position are also recorded for future use. For the return flight, thedisturbances previously collected are then routed through a feedforwardcontroller. The major assumption here is that the disturbances may vary overspace, but not over time during the same mission. We demonstrate theeffectiveness of this feedforward strategy via experiments with two differenttypes of wind flows; a simple jet flow and a more complex flow. To use as abaseline case, a cascaded PD controller with an additional feedback loop fordisturbance estimation was employed for outbound flights. To display ourcontributions regarding the additional feedforward approach, an additionalfeedforward correction term obtained via prerecorded data was integrated forthe return flight. Compared to the baseline controller, the feedforwardcontroller was observed to produce 43% less RMSE position error at a vehicleground velocity of 1 m/s with 6 m/s of environmental wind velocity. Thisfeedforward approach also produced 14% less RMSE position error for the complexflows as well.

This paper presents a model to predict aero-dynamic disturbances during close formation flight of two multirotors. The model is based on a propeller velocity field model and characterization of change in propeller thrust under oncoming flow. Using this model, we predict forces and torques on one mutirotor due to downwash of the other with respect to relative separation of the multirotors. We conduct proximity flight experiments using two types of quadcopters to measure forces and torques using accelerometer and rategyro. Predictions from the model and results from experiments match well for vertical separations greater than seven times the vehicle size. This verifies the range of fidelity of the model.

Unmanned aerial-underwater vehicles (UAUVs) provide the potential for working on missions in complex multi-domain environments. To achieve amphibian mobility, current UAUV designs rely on additional mechanical components such as multiple layers of propeller blades, water ballast, buoys or wings. This paper presents a miniature UAUV which has a simple mechanical design that resembles a traditional quadcopter. The paper discusses the dynamic modelling, state estimation and control strategy for this UAUV, as well as a detailed characterization of the quadcopter blades operating in the air and water regimes. A strategy for the UAUV to breach calm water surface is then proposed and experimentally tested. The results demonstrate that the UAUV can successfully breach the still water surface, but also show tracking error and breaching delay that are not fully characterized by the model. This suggests the need to carry out further analysis on the dynamics of the UAUV both underwater and in the transition regime.

Redundant attitude representations are often used in Kalman filters for estimating dynamic states that include an attitude. A minimal three-element attitude deviation is combined with a reference attitude, where the deviation is included in the filter state and has an associated covariance estimate. This paper derives a reset step that adjusts the covariance matrix when information is moved from the attitude deviation to the reference attitude. When combined with the extended or unscented Kalman filter prediction and measurement steps, the reset allows one to easily construct a Kalman filter for a system for which the state includes an attitude. This algorithm is closely related to (and a correction to) the multiplicative extended Kalman filter or the unscented quaternion estimator, depending on whether the reset is combined with an extended or unscented Kalman filter. In comparison to the multiplicative extended Kalman filter, it is more general and includes a reset after the measurement update, as well as a reset after both the prediction and measurement update steps of the unscented quaternion estimator. This reset step is derived by tracking the mean and covariance through a linearization, similar to an extended Kalman filter prediction step. The reset step is validated using Monte Carlo sampling. [ABSTRACT FROM AUTHOR]

This paper presents a relaxed definition of hover for multicopters with propellers pointing in a common direction. These solutions are found by requiring that the multicopter remain substantially in one position, and that the solutions be constant when expressed in a coordinate system attached to the vehicle. The vehicle’s angular velocity is then shown to be either zero or parallel to gravity. The controllability of a vehicle’s attitude about these solutions is then investigated. These relaxed hover solutions may be applied as an algorithmic failsafe, allowing, for example, a quadrocopter to fly despite the complete loss of one, two, or three of its propellers. Experimental results validate the quadrocopter failsafe for two types of failure (a single propeller and two opposing propellers failing), and a nonlinear simulation validates the remaining two types of failure (two adjacent and three propellers failing). The relaxed hover solutions are also shown to allow a multicopter to maintain flight in spite of extreme center of mass offsets. Finally, the design and experimental validation of three novel vehicles is presented. [ABSTRACT FROM AUTHOR]

A method is presented for the rapid generation and feasibility verification of motion primitives for quadrocopters and similar multirotor vehicles. The motion primitives are defined by the quadrocopter's initial state, the desired motion duration, and any combination of components of the quadrocopter's position, velocity, and acceleration at the motion's end. Closed-form solutions for the primitives are given, which minimize a cost function related to input aggressiveness. Computationally efficient tests are presented to allow for rapid feasibility verification. Conditions are given under which the existence of feasible primitives can be guaranteed a priori . The algorithm may be incorporated in a high-level trajectory generator, which can then rapidly search over a large number of motion primitives which would achieve some given high-level goal. It is shown that a million motion primitives may be evaluated and compared per second on a standard laptop computer. The motion primitive generation algorithm is experimentally demonstrated by tasking a quadrocopter with an attached net to catch a thrown ball, evaluating thousands of different possible motions to catch the ball. [ABSTRACT FROM PUBLISHER]

We present QUaRTM - a novel quadcopter design capable of tilting the propellers into the forward flight direction, which reduces the drag area and therefore allows for faster, more agile, and more efficient flight. The vehicle can morph between two configurations in mid-air, including the untilted configuration and the tilted configuration. The vehicle in the untilted configuration has a higher pitch torque capacity and a smaller vertical dimension. The vehicle in the tilted configuration has a lower drag area, leading to a higher top speed, higher agility at high speed, and better flight efficiency. The morphing is accomplished without any additional actuators beyond the four motors of a quadcopter. The rigid connections between the quadcopter frame and the quadcopter arms are replaced with sprung hinges. This allows the propellers to be tilted when high thrusts are produced, and recover to the untilted configuration when the thrusts are brought low. The effectiveness of such a vehicle is demonstrated by running experiments on a prototype vehicle with a shape similar to a regular quadcopter. Through the use of tilting, the vehicle is shown to have a 12.5% higher maximum speed, better high-speed agility as the maximum crash-free cruise speed increased by 7.5%, and a better flight efficiency as the power consumption has dropped by more than 20% in the speed range of 15-20 m s -1 ., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Tang, Jain and Mueller.)