A Zigbee Based Wireless Sensor Network for Sewerage Monitoring #1
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C.H. See , K.V. Horoshenkov , S.J. Tait , R.A. Abd-Alhameed , Y.F. Hu , E.A.Elkhazmi#2 and J.G.Gardiner#1 #
School of Engineering, Design and Technology University of Bradford Richmond Road, Bradford, West Yorkshire, UK, BD7 1DP 1
[email protected] # The Higher Institute Of Electronics, Bani Walid-Libya 2
[email protected]
Abstract — Blockages in sewers are major causes of both sewer flooding and pollution. Water companies which fail to tackle this problem face hefty fines and high operational costs if they unsuccessful to provide a practical solution to prevent flooding. As a result, the detection of sewer condition is routinely required to inform on the best course of action to eliminate this critical problem. This paper presents a novel low cost wireless sensor technology to detect blockages proactively, and feed these event data back to a central control room. The practical deployment of the proposed WSN in an urban area will be demonstrated. In addition, the challenges of this technology in a field trial and the recorded data in terms of the sensor and communication reliability will be addressed.
This paper describes a practical implementation of a low cost wireless WSN using Zigbee communication and acoustic sensor technologies to monitor the water level of the gullies in a residential urban area. The purpose of this field trial was to evaluate the preliminary design of the proposed system in terms of durability of sensors, sensor nodes and gateways and reliability of communication under real operational conditions and within a typical urban environment.
Index Terms — WSN, Wireless sensor
I. INTRODUCTION Sewer flooding (DG5 Other Causes) and pollution incidents are significant issues in the wastewater business process in the water industry. One of the most pressing issues for prevention of sewer flooding and pollution is sewer and gully blockages [1]. Monitoring and maintenance are important part of the many water companies business to prevent catastrophic failures that can shut down a facility which may cost several millions pounds per day. On top of that, water industries regulated by several UK’s government agencies, such as the OFWAT and EA, may be required to pay hefty fines for not meeting the basic standard performance of their services. Currently, many water companies have deployed telemetry systems to replace some of the manual operations, running costs remain expensive. Low cost wireless sensors may be the only cost-efficient option to replace traditional visual inspection which is extremely inefficient and costly. Moreover, existing telemetry systems require extensive cabling for (Public Switch Telephone Network) PSTN and power and cannot be deployed over a large catchment area because of the cost. Low cost and low power sensors could be deployed over an extensive footprint network and provide early warning of impending failure offering time for maintenance teams to prevent service or regulatory compliance failure.
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Fig.1: Wireless Sensor Network- System Architecture
II. SYSTEM ARCHITECTURE In this section, the architecture of the proposed low power mesh network wireless sensor system will be discussed. Inherent power limitations of radio communication devices might introduce hard restrictions on the coverage of the WSN [2]. Due to this limitation, direct communication between sensor and base station is not always possible especially over a difficult radio environment with strong attenuation. In order to overcome this difficulty and to extend the communication distance, an obvious way forward is to use multi-distributed nodes that use multi-hops to send the data over these nodes
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Fig.3: Wireless Sensors Distribution on field trial
(sensors) on the way to the base station. As can be illustrated in Fig.1, by implementing a mesh network communication configuration, this WSN allows for continuous connections and reconfigurations around blocked paths. This might results in hopping from sensor node to node until a connection can be established with the base station [3-5]. It should be noted that the mesh networks posses the selfhealing capability that will operate even when a node breaks down or a connection fails. As a result, it forms a very reliable network. Once the data is received by the data gatherer/hub, it will be stored and published on a webpage via connection to the internet. By accessing the internet, remote mobile devices can request the recorded data with the right user name and password. A Zigbee based short range WSN was selected for this application due to its attractive features such as low data rate, low power consumption, simple communication infrastructure, low latency and capability to support one master and up to 65000 slave control units. In general, the proposed system has led to the development of knowledge and expertise in four areas of research: a) Embedded Antenna design; b) Sensors and Instrumentation for use in water industry assets; c) Wireless communications and distributed wireless sensor networking; d) Remote monitoring of water related assets.
There are two ways of using a sonic transmission to detect the water level, i.e. (i) By measuring the time of flight from transmitter to receiver – it will be faster in water, (ii) By using the level of the received signal – the received signal will be louder if the transmitter and receiver are both under water. Both of these methods of detection will require a driver for the acoustic transmitter and an amplifier for the received signal. Method (i) suffers from the leaves or debris and most likely to cause a false alarm. Therefore, method (ii) was adopted in this project. A novel acoustic probe [7] and data acquisition circuit (DAC) board were developed to identify three status of water level, i.e., Low, Normal and High. The DAC board was designed to operate in low power consumption mode and considerably extend the life time of the sensor.
A. Crossbow Mica2 Sensor Node The Crossbow Mica2 [6] node is an advanced tiny wireless platform for smart sensors, which is constituted by CC1000 radio, Atmega 128L processor, 128kB Flash, 4kB RAM and 10 bits ADC. By enabling the Low Power Listening (LPL) mode of this platform, it will perform the operation in the lowest duty cycle mode. Hence, this wireless module can prolong the battery life significantly. An important part of this system is the water level sensor element. There are a number of water level sensors available in the market. However, many commercial low cost water level sensors are unreliable and too delicate to be used in the hostile gully environment. In order to reliably and effectively measure the water level of a gully, a novel and low cost acoustic sensor was designed and developed.
Fig.2: Deployment of wireless sensors in gullies and data gatherer on lamppost.
B. Antenna design Unforeseen RF transmission disruption can occur when deployment of wireless sensors is implemented over a large area. Multipath reflections occur when an RF signal takes different paths on the propagation channel between the source
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nodes (e.g., a radio NIC) to a destination node (e.g., access point). In this scenario the received signal can be routed along different paths, which causes the signal to bounce in different directions. As a result, some of the radio signals arrive with a delay as they travel longer paths to the receiver. This is likely to corrupt the information contained in the broadcast message and cause significant delay to the whole communication system. To alleviate this problem in mesh radio network, hop-by-hop algorithm can be adopted. Conventional low power mesh network of wireless communications sensors suffers from limited communication range. In order to achieve an optimum reasonable communication distance with minimum power consumption, the antenna plays an important role in implementation of the wireless sensor network (WSN). The objective of this project is to design, develop and implement a gully pots wireless monitoring system. Therefore, the proposed wireless sensors have to be able to operate in a harsh environment, with high radio signal attenuation (lossy and watery) surroundings and invisible to third party. As a result, the commercial off the shelf (COTS) type aerial which is a monopole, is not suitable for this application. This requires redesigning an aerial to satisfy the demands of the proposed WSN. Ideally, the proposed antenna has to provide the following characteristics: (1) able to operate from 902MHz to 926 MHz frequency band, (2) have omni- directional radiation patterns, (3) high gain, (4) robustness, (5) low cost and profile and (6) ease of installation and maintenance. For the proposed system, an embedded microstrip antenna was designed, tested and implemented within a waterproof enclosure. It is believed to have a high potential and feasibility to be adopted in underground infrastructure monitoring. The description of this antenna is provided in [8].
full of fat. ie a high risk wastewater disposal system, and (ii) the topology of the site allowed the proposed Zigbee mesh network wireless sensor system to be tested easily. The site was a row of residential houses, sensors could relay information between its neighbour to reach the data gatherer. Fig.3 shows the distribution of the Zigbee-based communication system which currently includes eight gully monitoring units and a stargate. As can be observed, the shortest and longest distances between the sensor to sensor (STS) and sensor to hub(STH) are 5.5m and 38.5m, and 12.3m and 66.5m respectively. As can be noticed, the red triangle spot and the white ring symbols in Fig.3, indicate the location of the data gatherer and sensors respectively. For the purpose of extending the battery life of the Zigbee flooding monitor up to a long as possible (ideally two years), the proposed Zigbee monitor was programmed to operate in the following modes: 1. A two way low power mesh network communication was designed and implemented into the proposed wireless system. In order to keep the minimal power consumption in the sensor nodes, a low power listening (LPL) mode was enable on the radio module (CC1000) of the Crossbow transceiver. The LPL mode enabled the radio module to go into sleep i.e. extreme LPL mode, instead of turn off the radio completely. By implementing this mode, the sensor and base station were in a 1% duty cycle, maximum 0.89 packet transmission/reception per second and 0.258kbps effective throughput operation mode [11]. 2.The sensor was programmed to wake up from sleep mode to measure the data in every five minutes. If it detected five consecutive flooding/leaking alarms, then it transmitted the radio packet to either nearby sensor node or directly to data gatherer/hub depend on the detected receive signal strength indication (RSSI) level, otherwise, it operated in sleep mode to keep the current consumption to a few microamperes. In general, there were three operation modes in this application, which were sleep mode, sensing mode and radio broadcast mode, these modes were corresponding to average 20uA, 9mA and 18mA current consumption, respectively.
C. Stargate Platform Stargate platform is a linux operating system based on a mini-computer [9-10]. In this present application, it is acted as a data gatherer/hub which collects the data from all the wireless sensors. As can be seen in Fig. 2, it was mounted on a lamppost for optimum coverage of the monitored area. Stargate is programmed to operate into sleep mode when it is idle to conserve the battery power. In addition, GPRS was used to transfer the received data from Stargate to the remote server. Constantly turning on the GPRS connection will flatten the battery within days so in order to maintain the battery life for months; Stargate sends the recorded data back to the remote server once a day based on GPRS connection.
3.Once it detected a low/high alarm, it sensed the RSSI, from the hub as well as its neighbour sensor’s node. Then, it compared the RSSI and find out the best route to relay the data back to the GPRS server. It should be noted that the open source Xmesh reliable route protocol [12] was employed for this application. However, for the health check purpose, the sensor broadcasted a health condition packet back to the hub to indicate its battery level and water level condition on a daily basis.
III. FIELD TRIALS A dense residential area in Bradford, UK, was chosen to carry out the field trial. There were two main reasons for the selection of this site: (i) In this area, every single house only has one gully which collected all the wastewater, such as hot water, cold water, bath water full of hair, rain water, kitchen sink water
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IV.
RESULTS AND DISCUSSION
From the field trial, according the daily data received from the server, it was observed that the reliability of the proposed Zigbee system can reach up to 80%. Fig.4 interprets the reliability of the proposed wireless communication system over each stage of the field trial. In general, this field trial has been divided into four stages. In the first stage, eight sensors and one data gatherer (i.e. stargate) were installed. It should be noted that after the installation there are 2 sensors (ID 4520 and 4521) working reliably which operate under LOS condition. By adding relay points (i.e. repeaters) which was about 40m to 70m away from the hub and changing the orientation of the aerial of the hub, in addition of installation a high gain aerial of the hub, which is corresponding to stage 2,3 and 4 in Fig.4, the efficiency of the proposed system can be improved considerably. As a result, 5 out the 8 of the sensors were working reliably over trial period. However, this communication reliability can be improved in future and applied to large scale deployment by increasing the frequency of the transmission of the Zigbee sensor, installing additional low cost relay points and improve the aerial design on the hub. This can be done by introducing multiple aerials on the hub to mitigate the fading effects of a multipath environment. Moreover, adding amplifier on the reception side of the Zigbee radio module will improve the receive sensitivity of the hub. As for the sensors that did not work with a high reliability, this was due to unforeseen man-made issues, such as covering the sensors with rubbish and concrete blocks. These activities impaired the radio communication significantly and invalidated the communication reliability of the proposed system. An investigation has been carried out to pinpoint the reasons on what possibly blocked the radio communication on those gully monitoring units (ID: 4510, 4512 and 4513). According to findings from the investigation, it seemed that failure of Sensors ID 4510, 4512 and 4513 can be attributed to covering the gully.
Fig.4: Communication Reliability of the proposed system
V.
CONCLUSIONS AND RECOMMENDATIONS
This paper presents a practical implementation of the Zigbee based wireless communication system on an urban residential area. These proposed technologies will enable to transfer effectively and process data from large numbers of potentially diverse sensors distributed within a sewer infrastructure network to achieve potentially zero pollution and DG5 other causes. Outcomes from the field trial enabled researcher in this research area to gain expertise in the issues associated with the practical monitoring of the performance of elements of the urban sewerage infrastructure managed in the UK water industry. ACKNOWLEDGMENT The authors acknowledge the support provided by funding from Yorkshire Water Services and the Technology Strategy Board, via the KTP Project “The development of a new sewerage telemetry system” REFERENCES [1] OFWAT, “Service and Delivery- performance of the Water Companies in England and Wales 2007-08 Report”,http://www.ofwat.gov.uk/regulating/reporting/rpt_los_ 2007-08.pdf [2] Cauligi S. Raghavendra, Krishna M. Sivalingam, and Taieb Znati, “Wireless Sensor Networks”, Springer-Verlag New York Inc.; Sep 2006. [3] D. Geer, "User make a Beeline for ZigBee sensor Technology," Computer, vol. 33, pp. 16-19, Dec.2005. [4] H.-C. Huang, Y.-M. Huang, and J.-W. Ding, "An Implementation of Battery-aware Wireless Sensor Network Using ZigBee for Multimedia Service," International Conference on Consumer Electronics (ICCE '06) Digest of Technical Papers, pp. 369-370, Jan. 2006. [5] C.Evans-Pughe, "Bzzzz zzz [ZigBee Wireless Standard]," IEE Review, vol. 49, pp. 28-31, March 2003. [6] Crossbow, Inc., Mica2 wireless module, http://www.xbow.com/Products/productdetails.aspx?sid=174 [7] A.Tolstoy, K.V.Horoshenkov and MT Bin Ali, “Detecting pipe changes via acoustic matched field processing”, Applied Acoustics, Vol.70, May 2009, pp.695-702 [8] C.H.See, R.A.A.Alhameed, D.Zhou, Y.F.Hu and K.V.Horoshenkov, “Measure the Range of Sensor Networks”, Microwave and RF, vol.47, Oct. 2008, pp. 69-76. [9] Crossbow, Inc., Stargate gateway, http://www.xbow.com/Products/Product_pdf_files/Wireless_pd f/Stargate_Datasheet.pdf [10] I.Stoianov, L.Nacham, S.Madde and T.Tokmouline, “PIPENET: A Wireless Sensor Network for Pipeline Monitoring”, IPSN’07, pp.264-273 [11] TinyOS, http://www.tinyos.net/tinyos-1.x/doc/mica2radio/ CC1000.html [12] A.Woo, T.Tong and D.Culler, “Taming the underlying challenges of reliable multhop routing in sensor networks, In proceedings of the 1st International conference on Embedded networked sensor systems (SenSys 2003), pp. 14-27. Los Angeles, California:ACM Press.
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