This paper analyzes the ground effect in multirotors, that is, the change in the thrust generated by the rotors when flying close to the ground due to the interaction of the rotor airflow with the ground surface. This effect is well known in single-rotor helicopters but has been assumed erroneously to be similar for multirotors in many cases in the literature. In this paper, the ground effect for multirotors is characterized with experimental tests in several cases and the partial ground effect, a situation in which one or some of the rotors of the multirotor (but not all) are under the ground effect, is also characterized. The influence of the different cases of ground effect in multirotor control is then studied with several control approaches in simulation and validated with experiments in a test bench and with outdoor flights.

The proximity between the multirotor blades and the environmental obstacles restricts the application of aerial manipulators in inspection tasks due to the risk of impacts, the limitation in the reach of the arm, and the physical interactions. This paper presents a long reach aerial manipulator consisting of a hexarotor platform equipped with a 2-DOF compliant joint arm attached at the tip of a one-meter-length link in passive pendulum configuration. The arm integrates magnetic encoders for force/torque estimation-control based on joint deflection, a range sensor in the forearm link for measuring the distance to the contact point, and a camera for visual inspection. A 2-DOF wearable exoskeleton interface has been developed, allowing the teleoperation of the arm with visual feedback in a more intuitive way. The paper also covers the kinematics and dynamics of the aerial manipulator, including the dynamics of the flexible long reach link. The developed system has been evaluated in testbench and in outdoor flight tests.

This paper presents a robotic system using Unmanned Aerial Vehicles (UAVs) for bridge-inspection tasks that require physical contact between the aerial platform and the bridge surfaces, such as beam-deflection analysis or measuring crack depth with an ultrasonic sensor. The proposed system takes advantage of the aerodynamic ceiling effect that arises when the multirotor gets close to the bridge surface. Moreover, this paper describes how a UAV can be used as a sensor that is able to fly and touch the bridge to take measurements during an inspection by contact. A practical application of the system involving the measurement of a bridge’s beam deflection using a laser tracking station is also presented. In order to validate our system, experiments on two different bridges involving the measurement of the deflection of their beams are shown.

The inspection of bridges and other infrastructure with UAVs when the sensors need to be in contact with the surface (i.e. ultrasound for crack inspection or reflector prism for beam deflection) is a great challenge due to the coupling which occurs between the aerial and the inspection problems. This paper presents a new design of a multirotor UAV that can be used in some of these applications to eliminate the coupling and to be able to carry out the inspection by contact in a more effective way. The proposed solution uses the so-called ceiling effect to maintain in contact a specially designed aerial platform to the ceiling. So, the coupling disappears because the multirotor is still in contact with the ceiling in a fixed position while performing inspection. Moreover, the presented results show that making use of the ceiling effect also improved the maximum flight time of the platform. The solution is presented with experimental results in a test stand and flight tests representing a bridge inspection application (Fig. 1). A video of the experiments is also included.

This paper presents the design, modelling and control of a multirotor for inspection of bridges with full contact. The paper analyzes the aerodynamic ceiling effect when the aerial robot approaches the bridge surface from below, including its aerodynamic characterization using Computational Fluid Dynamics (CFD). The proposed multirotor design takes the modelled aerodynamic effects into account, improving the performance of the aerial platform in terms of the stability and position accuracy during the inspection. Nonlinear attitude and position controllers to manage the aerodynamic effects are derived and tested. Last, outdoor experiments in a real bridge inspection task have been used to validate the system, as well as, the controller and the aerodynamic characterization. The experiments carried out also include a complete autonomous mission of the aerial platform during a structural assessment application.