OVERVIEW

Locating gradient sources and tracking them over time has important applications to environmental monitoring and studies of the ecosystem. We present an approach, inspired by bacterial chemo-taxis, for robots to navigate to sources using gradient measurements and a simple actuation strategy (biasing a random walk). Extensive simulations show the efficacy of the approach in varied conditions including multiple sources, dissipative sources, and noisy sensors and actuators. We also show how such an approach could be used for boundary finding. We validate our approach by testing it on a small robot (the robomote) in a photo-taxis experiment. A comparison of our approach with gradient descent shows that while gradient descent is faster, our approach is better suited for boundary coverage, and performs better in the presence of multiple and dissipative sources. We have successfully developed a sensor actuated buoy-roboduck network which will now serve as the in-situ test bed for the evaluation of our work.

Bacterial motion towards increasing gradient concentration [Chemotaxis]

The ability to autonomously detect, locate and track such phenomena (the source of the induced gradient) would give scientists a tool to monitor and study ecosystems at an unprecedented level of detail. We are in particular motivated by the research goal to track the brown-tide algal blooms in nature and follow their migration over time. We plan to locate these algae and measure their concentrations using a chlorophyll sensor. Additional features which govern their abundance and survival in an area include temperature, nutrient concentrations etc. which we would monitor over time.

Some environmental phenomena we are interested in locating and tracking

Bacteria sense chemical concentration using receptors. They are able to detect temporal and spatial changes in chemical concentration based on the fraction of receptors occupied at successive time intervals. An increase in the fraction of occupied receptors is called a positive gradient while a decrease is called a negative gradient. A chemical whose concentration gradient attracts the bacterial cells is called a chemo-attractant.

Bacterial motion produced by flagellum motion

Bacterial motion alternates between two stages (run and tumble). The duration of the run (which is related to the mean free path) is dependent on the concentration gradient that is sensed in the vicinity of the bacterial cell. In the absence of a gradient, the run length is independent of the direction of motion and the bacterium executes a random walk. In the presence of a positive gradient, the frequency of tumbling is reduced resulting in a longer run length. The presence of a negative gradient does not have any effect on the tumbling frequency. This change of tumbling frequency in response to concentration gradient results in chemo-taxis, allowing bacteria to move preferentially towards nutrient sources.