Integration of imaging techniques for the quantitative characterization of pesticide sprays
Caractérisation quantitative de la pulvérisation de pesticides par imagerie
Résumé
In recent years, advances in plant protection have contributed considerably to increasing crop yields in a sustainable way. Easy to apply and rather inexpensive, pesticides have proven to be very efficient. However, when pesticides are applied to crops some of the spray may not reach the target, but move outside the intended spray area. This can cause serious economic and environmental problems. Most of the pesticides are applied using agricultural sprayers. These sprayers use hydraulic nozzles which break the liquid into droplets with a wide range of droplet sizes and velocities and determine the spray pattern. Small droplets are prone to wind drift, while large droplets can runoff from the target surface and deposit on the soil. Therefore, efforts are being undertaken to come to a more sustainable use of pesticides which is more and more regulated by international environmental laws. One of the main challenges is to reduce spray losses and maximize spray deposition and efficacy by improving the spray characteristics and the spray application process. Because mechanisms of droplets leaving a hydraulic spray nozzle are very complex and difficult to quantify or model, there is a need for accurate quantification techniques. The recent improvements in digital image processing, sensitivity of imaging systems and cost reduction have increased the interest in high-speed (HS) imaging techniques for agricultural applications in general and for pesticide applications in specific. This thesis focused on the development and application of high speed imaging techniques to measure micro (droplet size and velocity) and macro (spray angle and shape, liquid sheet length) spray characteristics.The general aim was to show that the spray characteristics from agricultural spray nozzles can be measured correctly with the developed imaging techniques in a non-intrusive way. After a review of the spray application process and techniques for spray characterization (Chapter 2), two image acquisition systems were developed in Chapter 3 based on single droplet experiments using a high speed camera and a piezoelectric droplet generator. 58 combinations of lenses, light sources, diffusers, and exposure times were tested using shadowgraph (background) imaging and evaluated based on image quality parameters (signal to noise rate, entropy ratio and contrast ratio), light stability and overexposure ratio and the accuracy of the droplet size measurement. These resulted into development of two image acquisition systems for measuring the macro and micro spray characteristics. The HS camera with a macro video zoom lens at a working distance of 143 mm with a larger field of view (FOV) of 88 mm x 110 mm in combination with a halogen spotlight and a diffuser was selected for measuring the macro spray characteristics (spray angle, spray shape and liquid sheet length). The optimal set-up for measuring micro spray characteristics (droplet size and velocity) consisted of a high speed camera with a 6 μs exposure time, a microscope lens at a working distance of 430 mm resulting in a FOV of 10.5 mm x 8.4 mm, and a xenon light source used as a backlight without diffuser. In Chapter 4 image analysis and processing algorithms were developed for measuring single droplet characteristics (size and velocity) and different approaches for image segmentation were presented. With the set-up for micro spray characterization and using these dedicated image analysis algorithms (Chapter 4), measurements using a single droplet generator in droplet on demand (DOD) and continuous mode were performed in Chapter 5. The effects of the operating parameters, including voltage pulse width and pulse amplitude with 4 nozzle orifice sizes (261 μm, 123 μm, 87 μm and 67 μm) on droplet diameter and droplet velocity have been characterized (...)
Dans les 50 dernières années, les avancées dans le domaine de la protection des plantes ont contribué à augmenter les rendements et à assurer une large production. Facile à utiliser et plutôt bon marché à l’époque, les pesticides ont prouvé leur efficacité. Cependant, quand ils sont appliqués aux cultures, une partie du produit n’atteint pas sa cible et est perdu dans l’air ou au sol. Par conséquent, des efforts ont été consentis pour améliorer leur efficacité et leur innocuité sanitaire, souvent grâce à des lois environnementales internationales. Les produits sont appliqués à partir de matériels combinant type de buse/pression induisant des gammes de vitesses et de tailles de gouttelettes très diverses (Chapitre 2). Une mesure simultanée de ces vitesses et tailles est ainsi d’une grande importance dans le processus de pulvérisation. Il existe de nombreuses méthodes pour la mesure des caractéristiques des gouttelettes qui peuvent être divisées en trois catégories: mécaniques, électriques et optiques. Ces dernières apparaissent comme les plus pertinentes puisqu’étant non invasives et en perturbant donc pas le processus de pulvérisation. Les améliorations récentes dans le domaine du traitement des images et la réduction du coût des systèmes d’imagerie ont ainsi accru l’intérêt des techniques d’imagerie rapide pour les applications agricoles telles que la pulvérisation de pesticides. Cette thèse s’est donc focalisée sur le développement d’une telle technique pour la caractérisation des sprays (micro et macro). Les travaux effectués ont permis de démontrer que les caractéristiques d’un jet de pesticides peuvent être correctement et précisément mesurées par des techniques d’imagerie non-invasives couplées à des traitements spécifiques. Les travaux à venir consisteraient notamment en l’amélioration de la précision des mesures effectuées: précision sub-pixellique, calcul des profondeurs de champ, mesure de particules non sphériques.
Origine : Version validée par le jury (STAR)