B. D. W. Remes

Design, Aerodynamics, and Vision-Based Control of the DelFly

Light-weight, autonomous ornithopters form a promise to observe places that are too small or too dangerous for humans to enter. In this article, we discuss the DelFly project, in which we follow a top-down approach to ever smaller and more autonomous ornithopters. Top-down signifies that the project always focuses on complete flying systems equipped with camera. We give arguments for the approach by explaining which findings on the DelFly I and DelFly II recently led to the development of the DelFly Micro: a 3.07-gram ornithopter carrying a camera and transmitter onboard. These findings concern the design, aerodynamics, and vision-based control of the DelFly. In addition, we identify main obstacles on the road to fly-sized ornithopters.

Design, aerodynamics and autonomy of the DelFly

One of the major challenges in robotics is to develop a fly-like robot that can autonomously fly around in unknown environments. In this paper, we discuss the current state of the DelFly project, in which we follow a top-down approach to ever smaller and more autonomous ornithopters. The presented findings concerning the design, aerodynamics and autonomy of the DelFly illustrate some of the properties of the top-down approach, which allows the identification and resolution of issues that also play a role at smaller scales. A parametric variation of the wing stiffener layout produced a 5% more power-efficient wing. An experimental aerodynamic investigation revealed that this could be associated with an improved stiffness of the wing, while further providing evidence of the vortex development during the flap cycle. The presented experiments resulted in an improvement in the generated lift, allowing the inclusion of a yaw rate gyro, pressure sensor and microcontroller onboard the DelFly. The autonomy of the DelFly is expanded by achieving (1) an improved turning logic to obtain better vision-based obstacle avoidance performance in environments with varying texture and (2) successful onboard height control based on the pressure sensor.

Aerodynamic Experiments on DelFly II: Unsteady Lift Enhancement

Particle image velocimetry measurements and simultaneous force measurements have been performed on the DelFly II flapping-wing MAV, to investigate the flow-field behavior and the aerodynamic forces generated. For flapping wing motion it is expected that both the clap and peel mechanism and the occurrence of a leading edge vortex during the translational phase play an important role in unsteady lift generation. Furthermore, the flexibility of the wing foil is also considered of primary relevance. The PIV analysis shows a strong influx between the wings during the peel but no downward expelling jet during the clap. The force measurements reveal that the peel, oppositely to the clap, contributes significantly to the lift. The PIV visualization suggests the occurrence of a leading edge vortex during the first half of the in- and outstroke, which is supported by a simultaneous augmentation in lift. The early generation of a leading edge vortex during the flex cannot be assessed from the PIV images due to optical obstruction, but is likely to appear since the wing flexing is accompanied with a large increase in lift.

Wing flexibility effects in clap-and-fling

The work explores the use of time-resolved tomographic PIV measurements to study a flapping-wing model, the related vortex generation mechanisms and the effect of wing flexibility on the clap-and-fling movement in particular. An experimental setup is designed and realized in a water tank by use of a single wing model and a mirror plate to simulate the wing interaction that is involved in clap-and-fling motion. The wing model used in the experiments has the same planform with the DelFly II wings and consists of a rigid leading edge and an isotropic polyester film. The thickness of the polyester film was changed in order to investigate the influence of flexibility. A similarity analysis based on the two-dimensional dynamic beam equation was performed to compare aeroelastic characteristics of flapping-wing motion in-air and in-water conditions. Based on the experimental results, the evolution of vortical structures during the clap-and-peel motion is explained. The general effects of flexibility on vortex formations and interactions are discussed. It was observed that the flexibility affects the behavior and orientation of the vortices in relation to the deformation of the wing and interaction with the mirror plate.

Flow visualization and force measurements on a hovering flapping-wing MAV 'DelFly II'

Particle image velocimetry measurements and simultaneous force measurements have been performed on the DelFly II flapping-wing MAV, to investigate the flow-field behavior and the aerodynamic forces generated. For flapping wing motion it is expected that both the clap and peel mechanism and the occurrence of a leading edge vortex during the translational phase play an important role in unsteady lift generation. Furthermore, the flexibility of the wing foil is also considered of primary relevance. The PIV analysis shows a strong influx between the wings during the peel but no downward expelling jet during the clap. The force measurements reveal that the peel, oppositely to the clap, contributes significantly to the lift. The PIV visualization suggests the occurrence of a leading edge vortex during the first half of the in- and outstroke, which is supported by a simultaneous augmentation in lift. The early generation of a leading edge vortex during the flex cannot be assessed from the PIV images due to optical obstruction, but is likely to appear since the wing flexing is accompanied with a large increase in lift.

Optic-flow based slope estimation for autonomous landing

Micro Air Vehicles need to have a robust landing capability, especially when they operate outside line-of-sight. Autonomous landing requires the identification of a relatively flat landing surface that does not have too large an inclination. In this article, a vision algorithm is introduced that fits a second-order approximation to the optic flow field underlying the optic flow vectors in images from a bottom camera. The flow field provides information on the ventral flow (Vx/h), the time-to-contact (h/ – Vz), the flatness of the landing surface, and the surface slope. The algorithm is computationally efficient and since it regards the flow field as a whole, it is suitable for use during relatively fast maneuvers. The algorithm is subsequently tested on artificial image sequences, hand-held videos, and on the images made by a Parrot AR drone. In a preliminary robotic experiment, the AR drone uses the vision algorithm to determine when to land in a scenario where it flies off a stairs onto the flat floor.

Controlled Flight Maneuvers of a Flapping Wing Micro Air Vehicle: a Step Towards the Delfly II Identification

The Delfly II Flapping Wing Micro Air Vehicle was flown in an external tracking chamber. It was possible to perform controlled flight-test maneuvers with an ornithopter that is capable of hover and forward flight, for system identification purposes. This was achieved by programming its autopilot to deflect the a control surface, while keeping the other surfaces at trimmed condition. Step, doublet and triplet inputs of 1/3, 2/3 and 4/3 of a second on the elevator, rudder and flapping frequency actuators were performed to excite the Delfly’s eigenmodes. These tests were carried out at different flight speeds, ranging from -0.5 to 8 m/s and with the ornithopter’s center of gravity at 83%, 74%, 44% and 42% of the wing root chord. As a result, it was possible to cover the Delfly’s flight envelope and collect data that will be used to build a dynamic and aerodynamic model of the Delfly. The selected inputs have shown to excite the Delfly in dampened oscillatory modes that can be compared to phugoid and short period for the longitudinal dynamics. The Delfly is highly affected by the rudder deflections. The results also reveal an unstable lateral mode similar to a spiral.

Sky Segmentation Approach to obstacle avoidance

The capability to visually discern possible obstacles from the sky would be a valuable asset to a UAV for avoiding both other flying vehicles and static obstacles in its environment. The main contribution of this article is the presentation of a feasible approach to obstacle avoidance based on the segmentation of camera images into sky and non-sky regions. The approach is named the Sky Segmentation Approach (SSA). The central concept is that potentially threatening static obstacles protrude from the horizon line. The main challenge for SSA is automatically interpreting the images robustly enough for use in various environments and fast enough for real-time performance. In order to achieve robust image segmentation, machine learning is applied to a large database of images with many different types of skies. From these images, different types of visual features are extracted, among which most of the features investigated in the literature. In the interest of execution speed and comprehensibility, decision trees are learned to map the feature values at an image location to a classification as sky or non-sky. The learned decision trees are fast enough to allow real-time execution on a Digital Signal Processor: it is run onboard a small UAV at ∼ 30 Hz. Experiments in simulation and preliminary experiments on a small UAV show the potential of SSA for achieving robust obstacle avoidance in urban areas.

Experimental Investigation on the Aerodynamics of a Bio-inspired Flexible Flapping Wing Micro Air Vehicle

An experimental investigation on a 10 cm bio-inspired flexible Flapping-Wing Micro Air Vehicle (FWMAV) was conducted in both hovering and forward-flight conditions with the objective to characterize its aerodynamic performance. The measurements in hovering conditions were performed with the particular objective to explore the effect of different wing configurations (i.e. different aspect ratios and wing flexibilities), whereas forward flight tests in a wind tunnel were carried out to assess the aerodynamic performance of the FWMAV as a function of flow speed, flapping frequency and body angle. The cyclic variation of forces (lift and thrust) generated as a result of the wing flapping was captured by means of a high-resolution force sensor, in combination with high-speed imaging to track the wing motion. Results of measurements in hover show that the flapping frequency, aspect ratio and wing flexibility have a crucial impact on the efficiency and the force generation during the flapping cycle. An estimated flight envelop for the MAVs operation is defined from the data obtained in the wind tunnel measurements. Furthermore, additional tests on several brushless DC motors provide a feasible option in future engine selection and design.

Flow visualization in the wake of flapping-wing MAV ‘DelFly II’ in forward flight

Time-resolved velocity field measurements in the wake of the flapping wings of the DelFly II Micro Aerial Vehicle (MAV) in forward flight configuration were obtained by Stereoscopic Particle Image Velocimetry (Stereo-PIV). The PIV measurements were performed at several spanwise planes in the wake of the flapping wings and at a high framing rate to allow a reconstruction of the temporal development of the three dimensional wake structures throughout the flapping cycle. The wake reconstruction was performed by interpolating between the measurement planes through a Kriging interpolation procedure. First, the general wake topology of the DelFly II model is described in conjunction with the behavior of the distinctive flow structures, in particular, tip vortex, trailing edge vortex, and root vortex. Second, the effect of reduced frequency is investigated by changing the flapping frequency. Comparison of the three dimensional wake structures for different cases of reduced frequency reveals major differences in both formation and interaction of vortical structures.