Guido C. H. E. de Croon

Landing with Time-to-Contact and Ventral Optic Flow Estimates

Many recent studies on autonomous spacecraft landing use computer vision methods to improve the accuracy of the state estimates used for landing. Typically, these studies integrate the vision module with other exteroceptive sensors such as laser or radar altimeters. This is a sensible approach for the main landing system of a large spacecraft. However, for a backup emergency system or for much smaller spacecrafts, a solution entirely based on vision and proprioceptive sensors (e.g. gyros) could lead to significant mass savings. Small flying animals are capable of safe and accurate landings while relying only on proprioceptive and visual information. Since this capability holds a promise of landing safely with limited sensors and processing, it has served as inspiration for recent spacecraft landing studies. The focus of these studies has been on the use of ventral optic flow, a measure of the translational velocity divided by the height. Bees are known to use optic flow for controlling their speed and height, also when landing. In particular, when landing, they keep the ventral optic flow constant. Valette et al. study a control law that implements this strategy, simulating landings on the moon. The disadvantages of the sole use of ventral optic flow for landing are two-fold. First, the vertical dynamics of the lander is left free. The ventral flow can have the same constant value for a trajectory in which the lander ascends while accelerating and a trajectory in which the lander descends while decelerating. Thus, one has to directly or indirectly assume some type of descent profile, for example by introducing a pitch law for the spacecraft. Without the use of additional exteroceptive information to compute an optimal pitch profile, this leads to a considerable expense of propellant and to undefined final low-gate conditions. Second, in the case of a straight vertical landing the ventral flow is zero. In such a case, e.g. in the terminal phase of an asteroid landing scenario, the ventral flow does not provide any information on how to land the spacecraft. Advanced Concepts Team, European Space Agency, dario.izzo@esa.int Exteroceptive sensors observe entities external to the spacecraft, while proprioceptive sensors measure quantities within the spacecraft “body”.

Search for a grand tour of the jupiter galilean moons

We make use of self-adaptation in a Differential Evolution algorithm and of the asynchronous island model to design a complex interplanetary trajectory touring the Galilean Jupiter moons (Io, Europa, Ganymede and Callisto) using the multiple gravity assist technique. Such a problem was recently the subject of an international competition organized by the Jet Propulsion Laboratory (NASA) and won by a trajectory designed by aerospace experts and reaching the final score of 311/324. We apply our method to the very same problem finding new surprising designs and orbital strategies and a score of up to 316/324.

Black-box LTI modelling of flapping-wing micro aerial vehicle dynamics

This paper presents the development of black-box linear state-space models for the flight dynamics of a flapping-wing micro aerial vehicle (FWMAV), the DelFly. The models were obtained by means of system identification techniques applied to flight data recorded in a motion tracking chamber and describe the time-averaged dynamics of the vehicle in the proximity of specific stationary points in forward flight. Ordinary least squares and maximum likelihood-based estimation approaches were applied in the time domain, and decoupled models were identified for the longitudinal and the lateral dynamics. The availability of several different datasets additionally allowed for validation and for the estimation and comparison among each other of several separate models. Adequate models were obtained for both the longitudinal and the lateral dynamics. These reproduce the estimation data well and are also capable of predicting the response to validation inputs with a reasonable degree of accuracy, thus allowing for a simulation of the DelFly near the stationary points considered. The identified dynamics are stable and thus in agreement with the observed behaviour of the DelFly in the considered flight regime.

Gust disturbance alleviation with Incremental Nonlinear Dynamic Inversion

Micro Aerial Vehicles (MAVs) are limited in their operation outdoors near obstacles by their ability to withstand wind gusts. Currently widespread position control methods such as Proportional Integral Derivative control do not perform well under the influence of gusts. Incremental Nonlinear Dynamic Inversion (INDI) is a sensor-based control technique that can control nonlinear systems subject to disturbances. This method was developed for the attitude control of MAVs, but in this paper we generalize this method to the outer loop control of MAVs under gust loads. Significant improvements over a traditional Proportional Integral Derivative (PID) controller are demonstrated in an experiment where the drone flies in and out of a fans wake. The control method does not rely on frequent position updates, so it is ready to be applied outside with standard GPS modules.

Morphologic and Aerodynamic Considerations Regarding the Plumed Seeds of Tragopogon pratensis and Their Implications for Seed Dispersal

Tragopogon pratensis is a small herbaceous plant that uses wind as the dispersal vector for its seeds. The seeds are attached to parachutes that increase the aerodynamic drag force and increase the total distance travelled. Our hypothesis is that evolution has carefully tuned the air permeability of the seeds to operate in the most convenient fluid dynamic regime. To achieve final permeability, the primary and secondary fibres of the pappus have evolved with complex weaving; this maximises the drag force (i.e., the drag coefficient), and the pappus operates in an “optimal” state. We used computational fluid dynamics (CFD) simulations to compute the seed drag coefficient and compare it with data obtained from drop experiments. The permeability of the parachute was estimated from microscope images. Our simulations reveal three flow regimes in which the parachute can operate according to its permeability. These flow regimes impact the stability of the parachute and its drag coefficient. From the permeability measurements and drop experiments, we show how the seeds operate very close to the optimal case. The porosity of the textile appears to be an appropriate solution to achieve a lightweight structure that allows a low terminal velocity, a stable flight and a very efficient parachute for the velocity at which it operates.

An evolutionary robotics approach for the distributed control of satellite formations

We propose and study a decentralized formation flying control architecture based on the evolutionary robotic technique. We develop our control architecture for the MIT SPHERES robotic platform on board the International Space Station and we show how it is able to achieve micrometre and microradians precision at the path planning level. Our controllers are homogeneous across satellites and do not make use of labels (i.e. all satellites can be exchanged at any time). The evolutionary process is able to produce homogeneous controllers able to plan, with high precision, for the acquisition and maintenance of any triangular formation.

Stereo Vision Based Obstacle Avoidance on Flapping Wing MAVs

One of the major challenges in robotics is to develop a fly-like robot that can autonomously fly around in unknown environments. State-of-the-art research on autonomous flight of light-weight flapping wing MAVs uses information such as optic flow and appearance variation extracted from a single camera, and has met with limited success. This paper presents the first study of stereo vision for onboard obstacle detection. Stereo vision provides instantaneous distance estimates making the method less dependent than single camera methods on the camera motions resulting from the flapping. After hardware modifications specifically tuned to use on a flapping wing MAV, the computationally efficient Semi-Global Matching (SGM) algorithm in combination with off-board processing allows for accurate real-time distance estimation. Closed-loop indoor experiments with the flapping wing MAV DelFly II demonstrate the advantage of this technique over the use of optic flow measurements.

Autonomous Wind Tunnel Free-Flight of a Flapping Wing MAV

A low-cost high performance control system is developed to enable autonomous untethered flight inside a wind tunnel. Such autonomous flight is desirable for aerodynamic experiments on flapping wing MAVs, since fixing the fuselage has been shown to significantly alter wing deformations, air flow and performance on vehicles with a periodically moving fuselage. To obtain autonomous untethered flight, 3D position information is obtained from off-board WiiMote infrared tracking sensors with a total system accuracy of 0.8mm and an update rate of 80Hz in a quarter cubical meter control box. This information is sent to a 1.5 gram onboard autopilot containing communication, inertial measurements as well as onboard infrared tracking of an in-tunnel LED to achieve the high performance control needed to position itself precisely in the wind tunnel flow. Flight tests were performed with the 16 gram flapping wing MAV DelFly II. The achieved control performance is shown to be sufficient for many new research purposes, like researching the influence of a fixed fuselage in flapping wing aerodynamic measurements and obtaining more precise performance characteristics.

Free flight force estimation of a 23.5 g flapping wing MAV using an on-board IMU

Despite an intensive research on flapping flight and flapping wing MAVs in recent years, there are still no accurate models of flapping flight dynamics. This is partly due to lack of free flight data, in particular during manoeuvres. In this work, we present, for the first time, a comparison of free flight forces estimated using solely an on-board IMU with wind tunnel measurements. The IMU based estimation brings higher sampling rates and even lower variation among individual wingbeats, compared to what has been achieved with an external motion tracking system in the past. A good match was found in comparison to wind tunnel measurements; the slight differences observed are attributed to clamping effects. Further insight was gained from the on-board rpm sensor, which showed motor speed variation of ± 15% due to load variation over a wingbeat cycle. The IMU based force estimation represents an attractive solution for future studies of flapping wing MAVs as, unlike wind tunnel measurements, it allows force estimation at high temporal resolutions also during manoeuvres.

Local histogram matching for efficient optical flow computation applied to velocity estimation on pocket drones

Autonomous flight of pocket drones is challenging due to the severe limitations on on-board energy, sensing, and processing power. However, tiny drones have great potential as their small size allows maneuvering through narrow spaces while their small weight provides significant safety advantages. This paper presents a computationally efficient algorithm for determining optical flow, which can be run on an STM32F4 microprocessor (168 MHz) of a 4 gram stereo-camera. The optical flow algorithm is based on edge histograms. We propose a matching scheme to determine local optical flow. Moreover, the method allows for sub-pixel flow determination based on time horizon adaptation. We demonstrate velocity measurements in flight and use it within a velocity control-loop on a pocket drone.