Whisker Sensor Directionality

While on land and in the air, image processing, pathfinding, and laser and camera based technologies have largely solved the challenge of navigation sensing, these techniques do not work underwater due to the low visibility, turbidity, high RF absorption, and constantly changing environment. Current methods for underwater navigation, such as acoustic and low-dirft inertial sensing, are expensive and tend to be active sensors, so they require lots of power. Hence, it is a hassle for humans to navigate underwater.However, marine life thrives in aquatic environments; harbor seals have wavy whiskers that allow them to detect the smallest disturbances in the flow even when blindfolded and earmuffed [1]. Thus, our group takes inspiration from seals to develop a low cost, low power whisker sensor that will help AUVs detect changes in flow in the environment to help with navigation. Based on Heather Beem’s research on the hydrodynamics of harbor seal whiskers, many scientists have developed whisker sensors utilizing piezoelectric, capacitive, thermal, optic, and magnetic effect approaches [2] [3] [4]. Professor Pablo Valdivia y Alvarado of the Singapore University of Technology and Design developed a collection of piezoresistive sensors, and he sent them to MIT Sea Grant Laboratory for us to experiment on and use. Through a calibration experiment of his sensors (details outlined in our paper Whisker Sensor Calibration and Replication), we explored how the sensor works, determined how it should be used in sensing, and saw that a larger difference between the stiffness of the base and whisker materials seemed to lead to more precise displacements [5].Based on this information, we are developing our own whisker sensors, aiming for something that is easily manufacturable, inexpensive, and behaves much like a seal whisker. As a first attempt, we created 3D printable molds and a casting process to make our own sensors described in Whisker Sensor Calibration and Replication [5]. Further, to facilitate whisker production at scale, we have selected EcoFlex-30 as the base material and conductive carbon grease as the resistive element.With designing the whisker base, a question that arose was, "How does the placement of the whisker and corresponding material distortion in relation to the grease affect sensing?," which led to the question of whether we could determine which direction a disturbance comes from with the whisker sensor.Sensor directionality would be beneficial as the AUV could identify the relative direction of disturbances of interest with this information. Currently, to get directionality with current seal sensors, multiple sensors would be necessary (other whisker sensors or different sensors entirely to gather other types of information), which requires more space and power on the vehicle and adds cost. Hence, with directionality as part of the whisker, we would have a more compact sensor system as well as more opportunities for additional data collection. Currently, other whisker sensors with this feature use more complex manufacturing techniques (e.g. narrower and longer channels encasing conductive liquid or capacitive technology) [6] [7]. We believe that our sensor would provide more flexibility as it is easily manufacturable, can be optimized for each application, and cheaper.We approached the directionality design from a few perspectives including experimenting with whisker placement relative to the conductive grease and changing grease channel geometry. These changes did not change our overall manufacturing process, which is a straightforward casting process outlined in Whisker Sensor Calibration and Replication [5]. Based on these experiments, we observed that grease compression leads to the greatest change in voltage, so we developed a four pronged sensor that has four strips of grease around one whisker. It gathers four voltage readings from one whisker displacement and is able to determine the direction the signal comes from. In-situ experimental results and further research are discussed in the following paper.