A Biomimetic Quasi-Static Electric Field Physical Channel for Underwater Ocean Networks

A Biomimetic Quasi-static Electric Field Physical Channel for Underwater Ocean Networks Jonathan Friedman Dustin Torres Thomas Schmid Dept. of Electrical Engineering Dept. of Electrical Engineering Dept. of Electrical Engineering [email protected] [email protected] [email protected] Juyao Dong Mani B. Srivastava Dept. of Chemistry Dept. of Electrical Engineering [email protected] [email protected] Networked and Embedded Systems Laboratory University of California, Los Angeles ABSTRACT application of theory to explain the observed performance Nature has had millions of years to develop and optimize and predict future design improvements, (2) experimental life in the ocean. Nocturnal oceanic animals and those that proof of the existence and utility of the phenomenon, (3) live at depth cannot rely upon optical notions of vision to an engineering validation of the rationale for the naturally navigate, hunt, or avoid predators. Instead, many rely upon observed weak-electric fish waveforms, and (4) the design an electroreceptive capability achieved through a dense grid and implementation of a working short-range proximity sen- of electric field (Voltage) sensors. In this work, we develop sor for underwater wireless network neighborhood discovery and characterize an artificial system which seeks to mimic and station keeping. In the case of mobile network nodes, this capability. The detection range of our resulting proto- this sensor could assist in collision avoidance and formation type was ≈ 5cm. The position accuracy in the middle of the management. transmit axis was ±5cm after calibration. 2. PHYSICAL CHANNELS 1. INTRODUCTION A physical channel may be used for sensing, communication, or actuation. In this work we focus on the development The development and deployment of wireless underwater of an active sensor. Necessarily, this entails the transmission networks has been limited due to the lack of an accommo- of an encoded signal and the detection of environmental dis- dating physical channel. Acoustic pressure-wave systems turbances to that signal. Consequently communication or suffer from frequency-dependent bandwidth and attenua- actuation through the channel is also possible. Future work tion, time-varying multi-path, and the low speed of sound will explore these boundaries. in water, while optical and conventional radio systems suffer from severe attenuation. Nature, on the other hand, 2.1 Acoustics has had millions of years to develop and optimize life in the ocean. Many fish species are organized into social com- At a molecular-level, acoustic communication systems rely munities and society requires effective and reliable commu- on displacing the mass of the molecules in their way. Ac- nication to flourish. Survival itself dictates that jamming- cordingly, pressure waves can not exist without matter and avoidance (multiple access), self-recognition (modulation), propagate better (e.g. faster) as density increases, but at a localization, and all-weather availability (active) are requi- higher energy per distance cost. Physical displacement re- site qualities in a physical channel so evolution has naturally quires mechanical action and is limited in frequency and am- selected for them. plitude by the capabilities of mechanical excitation. Long- In this work we present a novel underwater proximity range (large amplitude) vibration can not be achieved at sensor that mimics the ability of some Teleost and Chon- frequencies in excess of the order of kHz and any vibrating drichthyes fish species [1] [12] [6] to detect and utilize quasi- sources in the environment will interfere (including reflec- static electric fields [3]. The contributions include (1) the tions of the transmitted signal from the surface, bottom, and suspended objects). 2.2 Electromagnetics Permission to make digital or hard copies of all or part of this work for In contrast, electromagnetic (radio) systems rely on aligned personal or classroom use is granted without fee provided that copies are charge movement releasing energy. As charges move, reverse not made or distributed for profit or commercial advantage and that copies direction, and move again the conservation of energy laws bear this notice and the full citation on the first page. To copy otherwise, to require that momentum dissipate prior to the reverse in di- republish, to post on servers or to redistribute to lists, requires prior specific rection. This extra energy is radiated outward in the form of permission and/or a fee. WUWNET ’10 Woods Hole, MA USA electromagnetic (EM) waves. EM waves propagate best in Copyright 2010 ACM X-XXXXX-XX-X/XX/XX ...$10.00. a vacuum where there are no intermediate particles to col- lide with, which would result in absorption and scattering. Electromagnetic propagation through water is very differ- ent from propagation through air because of water's high permittivity and electrical conductivity. Plane wave attenuation is high compared to air and increases rapidly with frequency. The principal problem is that mobile charges respond to the incident EM wave, which by absorption increases their energy level. By acting to restore their former lower energy state, the charges radiate EM waves with opposite polarity (180deg out of phase). At some distance away (the far-field) the incident and re-radiated (scattered) waves appear near equal in amplitude and add destructively. 2.3 Electrostatics Figure 2: Current flow around the fish is disturbed by objects in the environment and detected through changes in the resulting transdermal potential [17]. receptive capability precludes the detection of passive tar- gets in the environment such as rocks and other navigational hazards. The electric organ (EO), as distinct from the electrosensory organ, is present in these fish to establish a Voltage gra- dient in the water. This is accomplished by the direct conduction of current generated in the EO into the surrounding environment during an event known as an Electric Organ Figure 1: The Ampulla of Lorenzini is a fundamental Discharge (EOD). component of the electric-field sensing organ in some In order to intuit how current flows around the fish in the Teleost and Chondrichthyes fish species. ocean consider: If two electrodes were placed at either end of a wide flat conductive plate and energized, the majority of the current in the plate would flow from the cathode directly 2.3.1 Electrosensory Organs in Fish to the anode along, or very close to, the direct path between In the eighteenth century, Italian biologist Stephan Loren- the electrodes. However, as the current in any direction is zini observed a peculiar pore and organ system in crampfish composed of like polarity charges there is a repulsive force [11] and theorized that they might be used for navigation between them. This causes some of the charges to take a and hunting. These structures were later named the Am- less direct, more circuitous, path. The current is tracing out pullae of Lorenzini in his honor. However, the purpose of the lines of force created by the potential difference between the ampullae was not clearly understood, and electrophysi- the electrodes as shown in figure 2. ological experiments suggested a sensibility to temperature, When an object less conductive than the ocean water mechanical pressure and possibly salinity [6]. It was not (rocks, plastics, bubbles, etc) is placed in the field (as is until 1960 that the ampullae were clearly identified as spe- the case in figure 2), the current, following the path of least cialized receptor organs for sensing electric fields [1] [9] [20]. resistance, will shunt around it. This redistribution of cur- Each ampulla is a bundle of sensory cells containing mul- rent spreads out the field lines changing the location of the tiple nerve fibers. These fibers are enclosed in a gel-filled isovoltaic line's intersection with the fish's body { effectively tubule which has a direct opening to the surface through a casting an electrical shadow by creating a region where the pore. The gel is a glycoprotein based substance with the Voltage is more constant per unit distance along the body. same resistivity as seawater [5]. Consequently, the ampullae Objects more conductive than the background water have can detect electric fields in the water through the Voltage the opposite effect, concentrating the field lines, creating an differential at the skin pore versus the base of the electrore- electrical bright spot { a region of rapid Voltage change per ceptor cells { that is, the difference in neurological activity unit distance along the body. Accordingly, when electrore- between the terminal axons at the pore and those in the ception is an active sensor, a mimetic system can not only interior vesicles of the ampulla [6] as shown in figure 2. detect and locate objects, but classify them as well. 2.3.2 How to see like a fish! 2.3.3 The physics of electrostatic fields Nocturnal oceanic animals and those that live at depth Electrostatics, as the name implies, involves the use of cannot rely upon optical notions of vision to navigate, hunt, electric (E) fields which are, traditionally, invariant with or avoid predators. Instead, many rely upon an electrore- time. For our purposes it is illustrative to consider a se- ceptive capability achieved through a dense grid of electric quence of time-invariant fields with each successive field hav- field (Voltage) sensors whose anatomy was just previously ing more (then less, then more, etc) strength than its pre- 1 summarized . However, relying exclusively upon a passive decessor. When this approximation is valid, the field is said 1There are actually several known types of electrotrecep- to be quasi-static [16]. tors in two broad categories: ampullary, i.e. figure 1, and In order to prove the validity of this assumption, consider tuburous [5] that when the electric field, and hence the current flowing in the field, changes with time two currents must be considered { the conduction current and the displacement current.

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