Sensors 2002, 2, 455-472 sensors ISSN 1424-8220 © 2002 by MDPI http://www.mdpi.net/sensors Design of a Wireless Sensor Network for Long-term, In-Situ Monitoring of an Aqueous Environment Xiping Yang1, Keat G. Ong2, William R. Dreschel2, Kefeng Zeng3, Casey S. Mungle3 and Craig A. Grimes3,* 1 Department of Electrical Engineering and Materials Research Institute, The Pennsylvania State University, University Park, PA 16802 USA 2 SenTech Corporation, 200 Innovation Boulevard Suite 236, State College, PA 16803 USA 3 Dept. of Electrical Engineering and Materials Research Institute, The PennState University, 217 Materials Research Lab, University Park, PA 16802 USA. * Author to whom correspondence should be addressed. E-mail: [email protected] Received: 24 November 2002 / Accepted: 26 November 2002 / Published: 30 November 2002 Abstract: An aqueous sensor network is described consisting of an array of sensor nodes that can be randomly distributed throughout a lake or drinking water reservoir. The data of an individual node is transmitted to the host node via acoustic waves using intermediate nodes as relays. Each node of the sensor network is a data router, and contains sensors capable of measuring environmental parameters of interest. Depending upon the required application, each sensor node can be equipped with different types of physical, biological or chemical sensors, allowing long-term, wide area, in situ multi-parameter monitoring. In this work the aqueous sensor network is described, with application to pH measurement using magnetoelastic sensors. Beyond ensuring drinking water safety, possible applications for the aqueous sensor network include advanced industrial process control, monitoring of aquatic biological communities, and monitoring of waste-stream effluents. Keywords: Sensor network, sensor node, aqueous environment, underwater, sensor array. Introduction While considerable effort has recently been focused on development of networked sensors for operation in air [1–9], sensor network technology has not been developed for application to liquidSensors 2002, 2 456 environments. The importance of developing a network sensor technology for operation in aqueous environments has recently been highlighted in reports [e.g. Science, vol. 295, pg. 2209, 22 March 2002] detailing the chemical slurry of antibiotics, estrogen-type hormones, insecticides, PPCPs, nicotine, etc. in the rivers of industrialized countries. While water quality is of the utmost importance to our future, analysis is still primarily conducted in a laborious manner by physical collection of a sample that is analyzed back in the laboratory. Consequently liquid analysis, be it of river water down stream from a sewage treatment plant, the water supply of a large city, or the physical and chemical composition of a local pond, generally requires a sample to be physically collected and brought back to the laboratory. Such sampling is expensive, time consuming, in many instances dangerous and prone to miss short lived events such as the periodic release of toxins or pollutants. We describe a significant advancement in liquid analysis technology, an aqueous sensor network able to autonomously, continuously, in-situ and in real-time monitor streams, lakes, ocean bays, liquid streams in processing plants, and other bodies of water. Beyond helping to ensure drinking water quality, the aqueous sensor network would, for example, be a tremendous tool for biologists seeking to monitor the temperature, flow characteristics, and chemical environment of aquatic communities. The operation of the aqueous sensor network within a liquid medium, such as a reservoir, is depicted in Fig. 1. Since electromagnetic waves rapidly attenuate in water the nodes (black spheres) communicate with each other acoustically (illustrated by red connecting lines). Each node is a data router, and contains environmental sensors as desired by the user. Node to node communication enables wide-area coverage using modest node power levels making practical long-term monitoring. All nodes are identical in design except two nodes: the host node and the uplink node. The host node, which is placed on land, is physically connected to a computer and uses a RF transceiver for wireless communication. The uplink node (floating green sphere) transfers information across the water/air boundary, using a RF transceiver to communicate with the host node, and acoustic transducer to communicate with the submerged nodes. Figure 1. An illustrative drawing of the aqueous sensor network. The nodes (blue spheres) are scattered throughout the lake and communicate acoustically (red lines). The uplink node (green sphere) contains both an acoustic transducer, and a RF transceiver through which it communicates to the host node (black sphere) that is directly attached to a computer.Sensors 2002, 2 457 Fig. 2 is a block diagram illustrating the components of an aqueous sensor network node. The critical components are the main controller module that oversees the sensor node operation, the acoustic transducer interface circuitry that amplifies and modulates the output and input signal to/from the transducer, the sensor interface circuitry that converts the raw sensor signals to digital information, and the power supply. Although the nodes can be equipped with different types of sensors in this work they are equipped with magnetoelastic sensor arrays [10-12] to monitor ambient pH. The host node contains only a RF transceiver, and communicates with the computer via RS232 protocol. The uplink node contains both RF and acoustic transducers, while the rest of the nodes contain only acoustic transducers. The following sections detail the function and design of these components. Sensor Interface CircuitrySensorMain Controller ModuleAcoustic Transducer Interface CircuitryRF Transceiver CircuitryPower SupplyAcoustic TransducerRF AntennaComputer Serial Interface (RS232) Figure 2. Block diagram showing the components of an aqueous sensor node. The host node connects directly to a computer, and contains a RF transceiver for communication with the network uplink node. The uplink node has both a RF transceiver and acoustic transducers, while the rest of the nodes have only acoustic transducers. A sensor node is shown in Fig. 3a. The circuit modules and batteries are mounted on a frame that slides into the protective frame made of PVC pipe having a
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