Jean-Marc Pichon

Visual Guidance Of A Mobile Robot Equipped With A Network Of Self-Motion Sensors

This paper reports on the principles of an on-board electro-optical system for the guidance of an autonomous mobile robot. Some of the signal processing adopted here was directly inspired by natural visual systems, in particular by the compound eye of the fly. The visual system has compound optics with a panoramic field but relatively low spatial resolution. It makes use of elementary motion detectors (E.M.Ds) to estimate the distance to objects from the optic flow. Each E.M.D. constitutes one mesh of an analog network. It measures the relative angular velocity of any contrast point that passes across its receptive field as a result of the robots own motion and evaluates the radial distance to this contrast point from the motion parallax. For this purpose, the mobile makes translation steps at constant speed during each visual acquisition. An obstacle avoidance algorithm is implemented on a parallel, analog network. This network integrates the numerous data provided by the E.M.Ds and controls the drive motor and steering motor of the robot platform in real time. Other navigation modules may be added without altering the basic hardware architecture of the system. For example, a target detector has been associated with the system. No stringent hypothesis needs to be made as to the shape of objects in the environment. Both the visual processing principles and the obstacle avoidance strategy are described.

Real time visuomotor control: from flies to robots

Knowledge of the signal processing in the flys compound eye, using single neuron recording associated with optical microstimulation of the photoreceptor mosaic, led to the design and construction of a synthetic visual system dedicated to real time guidance of an autonomous mobile robot.<<ETX>>

Critical assessment of vegetation indices from AVHRR in a semi-arid environment

Abstract The most frequently used vegetation index (VI), the Normalized Difference Vegetation Index (NDVI) and its variants introduced recently to correct for atmospheric and soil optical response such as Global Environment Monitoring Index (GEMI) and Modified Soil-Adjusted Vegetation Index (MSAVI) are evaluated over a Sahelian region. The usefulness and limitations of the various vegetation indices are discussed, with special attention to cloud contamination and green vegetation detection from space. The HAPEX Sahel database is used as a test case to compare these indices in arid and semi-arid environments. Selected sites are characterized by sparse vegetation cover and day-to-day variability in atmospheric composition. Simulated indices values behaviour at the surface level shows that these VIs were all sensitive to the presence of green vegetation but were affected differently by changes in soil colour and brightness. We showed that GEMI is less sensitive to atmospheric variations than both NDVI and MS...

The Cloud Ice Mountain Experiment (CIME) 1998: experiment overview and modelling of the microphysical processes during the seeding by isentropic gas expansion

The second field campaign of the Cloud Ice Mountain Experiment (CIME) project took place in February 1998 on the mountain Puy de Dome in the centre of France. The content of residual aerosol particles, of H2O2 and NH3 in cloud droplets was evaluated by evaporating the drops larger than 5 μm in a Counterflow Virtual Impactor (CVI) and by measuring the residual particle concentration and the released gas content. The same trace species were studied behind a round jet impactor for the complementary interstitial aerosol particles smaller than 5 μm diameter. In a second step of experiments, the ambient supercooled cloud was converted to a mixed phase cloud by seeding the cloud with ice particles by the gas release from pressurised gas bottles. A comparison between the physical and chemical characteristics of liquid drops and ice particles allows a study of the fate of the trace constituents during the presence of ice crystals in the cloud. In the present paper, an overview is given of the CIME 98 experiment and the instrumentation deployed. The meteorological situation during the experiment was analysed with the help of a cloud scale model. The microphysics processes and the behaviour of the scavenged aerosol particles before and during seeding are analysed with the detailed microphysical model ExMix. The simulation results agreed well with the observations and confirmed the assumption that the Bergeron–Findeisen process was dominating during seeding and was influencing the partitioning of aerosol particles between drops and ice crystals. The results of the CIME 98 experiment give an insight on microphysical changes, redistribution of aerosol particles and cloud chemistry during the Bergeron–Findeisen process when acting also in natural clouds.

Changes of cloud microphysical properties during the transition from supercooled to mixed-phase conditions during CIME

A ground-based seeding experiment using carbon dioxide and propane sprayed from pressurized bottles was carried out under supercooled cloud conditions on a small spatial and short time scale. Water vapor deposition on the artificially generated dry ice and propane ice germs as the main ice formation process (nucleation and growth) is consistent with the experimental results. After nucleation, diffusional growth of the ice particles, partly at the expense of evaporating small droplets, was identified during the mixing of the seeding line with the ambient supercooled cloud. Within the seeding plume, ice water contents up to 80% of the total condensed water are observed, although the size of the formed ice particles did not exceed 25 μm. From the changes of the ice and supercooled liquid phase with time under mixed-phase conditions, liquid water content (LWC) evaporation, ice water content (IWC) formation, and ice crystal growth rates are estimated, which are not affected by the artificial nucleation process. Thus, these rates are assessed to be applicable for a growing ice phase of small ice particles in a young mixed-phase cloud, where other growth mechanisms, like riming or aggregation, are negligible.