Olfaction is a long distance sense, which is widely used by animals for foraging or reproductive activities (Dusenbery, 1992; Vickers, 2000; Zimmer & Butman, 2000): homing by Pacific salmon (Hassler & Scholz, 1983), homing by green sea turtles (Lohmann, 1992), foraging by Antarctic procellariiform seabirds (Nevitt, 2000), foraging by lobsters (Basil, 1994), foraging by blue crabs (Wiesburg & Zimmer-Faust, 1994), mating and foraging by insects (Cardé & Mafra-Neto, 1997). Olfaction plays a significant role in natural life of most animals. For some animals, olfactory cues are far more effective than visual or auditory cues in search for objects such as foods and nests (Bell & Tobin, 1982). Although odor sensing is far simpler than vision or hearing, navigation in a chemical diffusion field is still not well understood (Lytridis et al., 2006). Therefore, this powerful primary sense has rarely been used inside the robotics community.

For many military and civilian applications in turbulent fluid flow environment, it would be useful to detect and track a chemical plume to its source. Chemical Plume Tracing (CPT) program, which is sponsored by US Office of Naval Research (ONR), seeks to learn how animals successfully accomplish similar tasks, and to develop algorithms for plume tracing using Autonomous Underwater Vehicles (AUVs). AUVs capable of such chemical plume tracing feats would be of great significance for many applications, e.g., the detection of chemical leaks, locating unexploded ordnance, and locating biologically interesting phenomenon such as thermal vents.

This chapter describes the development and field test of a chemical signal guided REMUS AUV system to find a chemical plume, trace the chemical plume to its source, declare reliably the source location, and map the plume source area after source declaration. The basic idea of the chemical signal guided AUV system is illustrated in Fig. 1. An AUV is constrained to maneuver within a region referred to as the OpArea. Within the OpArea the AUV should search for a specified chemical, for which a binary sensor is available. The mission starts with the AUV searching the OpArea for the chemical plume. A binary sensor outputs 1.0 if the chemical concentration is above threshold or 0.0 if the chemical concentration is below threshold. If above threshold chemical is detected, the AUV should trace the chemical plume to its source and accurately declare the source location. Following the source declaration, additional AUV maneuvers might be desired to acquire additional data, possibly using auxiliary sensors. The plume depicted in Fig. 1 is greatly simplified.

Realistic plumes may meander, are intermittent or patchy distributions of chemical, and do not have a uniformly increasing width as a function of the distance from the chemical

Fig. 1. A prototype CPT mission with post-declaration maneuvering. The depicted plume is a rendition that does not attempt to include intermittency or meander.

A typical vehicle hardware, control, guidance, mapping, and planning architecture for chemical plume tracing are shown in Fig. 2. The figure shows that the assumed inputs to the on-line mapping system are sensed concentration c(pv(ti)), vehicle location pv(k), and flow velocity u(ti) = (ux(pv(ti)); uy(pv(ti))) at time U The online planner would optimize a desired vehicle trajectory based the online map. The guidance system outputs heading, speed and depth commands to the controller to achieve the planner's desired trajectory without violating the heading and velocity constraints.

Fig. 2. AUV based Chemical Plume Tracing Architecture.
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