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Open Access Publications from the University of California

Scholarly Works (11 results)

  • Article
  • Peer Reviewed
UC Berkeley Previously Published Works (2021)
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.
    • Thesis
    • Peer Reviewed
    UC Berkeley Electronic Theses and Dissertations (2021)

    In recent years, quadcopters have gained remarkable popularity in a wide range of industries. Given that the utility of quadcopters has already been extensively demonstrated, this dissertation explores changes to the design and control of conventional quadcopters which either enable operation in new environments, allow for novel tasks to be performed, or reduce computational hardware requirements.

    Two novel designs are presented in this dissertation. First, we explore the use of angular momentum to reduce the sensitivity of a vehicle to torque disturbances. We show both theoretically and experimentally how torque disturbance sensitivity monotonically decreases with increasing net angular momentum of the vehicle when using an appropriately designed controller, and discuss how this effect scales with vehicle size. Second, we consider the use of passive (i.e. unactuated) mechanisms to expand the types of tasks a quadcopter can perform. Specifically, we demonstrate how a vehicle with freely rotating arms can change shape mid-flight, allowing for traversal of narrow passageways, simple manipulation, and perching behaviors.

    Next, two algorithms are presented which enable autonomous flight in known and unknown environments. Both algorithms are focused on improving the time required to check whether a given trajectory collides with the environment, reducing the computational power requirements of onboard computers used the fly the vehicle. The first method is used to quickly determine whether a given trajectory collides with a convex obstacle, enabling the avoidance of both static and dynamic obstacles. The second method allows for operation in previously unseen environments by using depth images from an onboard sensor to represent the environment. Rectangular pyramids are used to partition the free-space of the depth image,which enable highly efficient collision checking between trajectories and the environment.

    Each proposed design and algorithm is demonstrated experimentally on custom hardware, and relevant code has been made publicly available where appropriate.

      Cover page: Enhancing Quadcopter Capabilities via Design and Control
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2020)
        Cover page: Flying batteries: In-flight battery switching to increase multirotor flight timeCreative Commons 'BY' version 4.0 license
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2020)
        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 1 m/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.
          Cover page: A flow disturbance estimation and rejection strategy for multirotors with round-trip trajectories
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2023)
          [This corrects the article DOI: 10.3389/frobt.2022.1033715.].
            Cover page: Corrigendum: QUaRTM: A Quadcopter with Unactuated Rotor Tilting Mechanism capable of faster, more agile, and more efficient flightCreative Commons 'BY' version 4.0 license
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2020)
              Cover page: Staging energy sources to extend flight time of a multirotor UAV.Creative Commons 'BY' version 4.0 license
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2022)
              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.
                Cover page: QUaRTM: A Quadcopter with Unactuated Rotor Tilting Mechanism capable of faster, more agile, and more efficient flightCreative Commons 'BY' version 4.0 license
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2021)
                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.
                  Cover page: Staging energy sources to extend flight time of a multirotor UAVCreative Commons 'BY' version 4.0 license
                  • Article
                  • Peer Reviewed
                  UC Berkeley Previously Published Works (2019)
                  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.
                    Cover page: Towards breaching a still water surface with a miniature unmanned aerial underwater vehicle
                    • Article
                    UC Berkeley Previously Published Works (2022)
                    Tethered quadcopters are used for extended flight operations where the power to 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 single tether. We study the design factors that influence the power requirements, such as the electrical resistance of the tether, input voltage, and quadcopters' positions. We present an analysis to predict the required power to be supplied to a series of N tethered quadcopters, with respect to the thrust of each quadcopter which guarantees electrical safety and helps in design optimization. We find that there is a critical boundary of thrusts that cannot be exceeded due to fundamental electrical limitations. We compare the power consumption for one tethered quadcopter and two tethered quadcopters and show that for large quadcopters far enough from the anchor point, a two-quadcopter system consumes lesser power. We show that, for a representative use case of firefighting, a tethered system with two quadcopters consumes 26% less power than a corresponding system with one quadcopter. Finally, we present experiments demonstrating the use of a two-quadcopter tethered system as compared to a one-quadcopter tethered system in a cluttered environment, such as passing through a window and grasping an object over an obstacle.
                      Cover page: Tethered Power for a Series of Quadcopters: Analysis and ApplicationsCreative Commons 'BY' version 4.0 license
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