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Control Network for Modern Street Lighting Systems Gustavo W. Denardin, Carlos H. Barriquello, Alexandre Campos, Rafael A. Pinto, Marco A. Dalla Costa and Ricardo N. do Prado, Member, IEEE Electronic Ballast Researching Group (GEDRE) Federal University of Santa Maria (UFSM) Av. Roraima, 1000, CT, Room 171, Zipcode 97105-900, Santa Maria, RS, BRAZIL gustavogedre.ufsm.br, barriquellogedre.ufsm.br, ricardogedre.ufsm.br AbstractThis paper proposes a control network for a LED street lighting system. The use of LEDs is being considered a promising solution to modern street lighting systems, due to their longer lifetime, higher luminous effi ciency and higher CRI. The proposed control network enables disconnection of the street lighting system from the mains during peak load time, reducing its impact in the distributed power system automatically at overload conditions. It also allows to reduce the power consumption, decrease the management cost and monitor the status information of each street lighting unit. In order to meet the system requirements, a wireless sensor network based on IEEE 802.15.4TMstandard is employed. Its network layer is implemented using geographic routing strategy, which provides low overhead and high scalability features. However, due to well known drawbacks of the existing techniques, a novel routing algorithm is proposed. Simulations show that this algorithm leads to a signifi cant improvement of routing performance when applied to sparse large scale scenarios, which is the case of a street lighting system. Field tests have been performed on IEEE 802.15.4-compliant wireless control units. The obtained experimental results show that the proposed control network is able to meet the requirements of a LED street lighting system. Index TermsLED street lighting system, control network, wireless sensor networks. I. INTRODUCTION The street lighting system of a city plays an important role on its infrastructure. It is responsible of guaranteeing the society security and living during nighttime, traffi c safety, and city appearance. In the beginning, the control of the oil lamps was performed manually. With the invention of the electric lamps, the control of the street lighting started to be carried on automatically, by means of photocells, according to the light level. Light sources and their respective control gears are being constantly developed since their invention. On the other hand, photocells have been the adopted control system of public lighting for more than sixty years. Modern street lighting systems demand intelligent control networks, in order to reduce the power consumption, decrease the maintenance and management cost, monitor the status information of each device (lamp status, current and voltage measurements, etc.) and enable new control capabilities to the system (e.g.: dimming). The street lighting systems features a large number of independent devices, with geographic distribution depending on the city streets. Then, adding communication capabilities to these devices requires a complex network topology, as well as interoperability, scalability, security, robustness, low cost, ease to use and maintenance. A few contributions in the literature suggest some technologies that can be used to control a street lighting system. Some use the power lines for data transmission (PLC) 1, while others use wireless communication, such as cellular networks 2, wireless sensor networks 3 and both combined 4. Main drawbacks of the PLC technology are noisy medium, high signal attenuation, susceptibility to interference from nearby devices, high cost, high complexity and poor scalabil- ity. Similarly, the scalability and reliability of cellular networks are questionable, especially under high load traffi c. On the other hand, wireless sensor networks scalability is highly dependent on the routing algorithm performance. Therefore, designing a routing algorithm with high scalability and low overhead is a challenge. Regarding the lamp types used in street lighting, light emitting diodes (LEDs) are being presented as an alternative to replace the conventional lighting systems. Despite of their widespread use as signaling systems, LEDs are not commonly used as lighting systems. However, recent technology is im- proving gradually the LEDs effi ciency and color quality, which allows their application in lighting systems 5, 6. The use of LEDs in street lighting systems started to be reportedin the last few years, and it is being considered a very good solution to replace the conventional HID-based lighting systems, because of the longer lifetime, higher luminous effi ciency in mesopic visual conditions, and higher CRI 7. A. Problem Statement A problem faced by utilities is the peak load time, which is a short period of the day where the energy consumption is maximal. Due to this characteristic, the power generation plants and transmission lines must be able to supply the highest power to meet the demand. However, at off-peak periods, the system works with reduced power. The street lighting system is one of the responsible for overloading the power system during the peak load time, because the citys lights are turned on around this time. Consequently, the overload can be reduced if the street lighting ? system is supplied by an alternative energy source (such as a battery) during the peak load time. The battery can be charged by mains when the demand is lower (e.g. between 11 pm and 6 am). Therefore, this paper proposes a control network to manage a LED-based street lighting system, which is supplied by a battery during the peak load time. Such network enables real-time management of the proposed street lighting system, making possible to monitor the status of the LED array, battery and power circuitry; send control messages to a device, individually, or to a group of devices; keep local clocks synchronized; and so on. This paper is organized as follows: Section II presents the main features of the proposed LED street lighting system. In section III, an outline of the control network proposed to manage such system is presented. In Section IV, the proposed algorithm responsible to route packets in the control network is depicted. The analysis of the control network performance, based on simulations and experimental results, is presented in Section V. Finally, the paper is concluded in Section VI. II. PROPOSEDSTREETLIGHTINGSYSTEM Outdoor lighting applications such as parks, roads, parking lots and street lighting, usually use High-Intensity Discharge Lamps (HID). The High-Pressure Sodium Lamps (HPS) is the most effi cient lamp, among the high pressure discharge types, and its lifetime is greater than incandescent and fl uorescent ones. The effi ciency of this lamp reaches 150 lm/W, and the lifetime can reach up to 32,000 hours. Its main disadvantage is the low Color Rendering Index (CRI). So, the colors of the illuminated objects appear different from the real ones 8. Light emitting diode has features like high effi ciency (120 lm/W), high useful life (50,000 hours) and CRI higher than HPS lamps 9. Besides, it also provides a reduction in the amount of garbage due to the small size and long useful life. Furthermore, light emitting diodes do not use fi lament or gas for lighting generation. So, they do not require a high voltage pulse for ignition. Thus, they are compact light sources and their material is more resistant to shocks and vibrations. HPS lamps are not recommended to operate supplied by batteries, because they work with alternate voltage and current, and also require a high voltage level to their ignition. In counterpart, the LEDs are supplied by DC current, have small forward voltage, between 2.5 V and 4 V, and do not require ignition, different from the discharge lamps. These characteristics become advantageous when it is necessary to supply them by battery. Some street lighting systems using LEDs have been de- veloped and can be found in 10 and 7. These systems usually use converters that are well known in literature and sometimes used in electronic ballast for discharge lamps, like Buck, Boost, Buck-Boost, and Flyback converters. The proposed street lighting system based on LEDs provides high power factor, high operating effi ciency, and regulates the output current to control the light. The power circuitry of the proposed circuit for a street lighting system is presented in Fig. 1. Such circuitry is composed of a Power Factor Correction (PFC) stage, a DC-to-DC power stage (Driver) used to control the LEDs current when supplied by mains or by battery, and a bi-directional converter to charge the battery. A microcontroller could be used to select if the circuit will operate by mains or by battery according to the peak load time. Also, the same microcontroller could be used to control the battery charge and to drive the high-frequency switches, as well as to control the current in the LEDs. However, the system control performed by a microcontroller has some limitations as described below. A. Synchronization In order to switch from mains to battery supply during peak load time, the system must be able to know the right time to do it. If it is supposed that peak load times occur in a daily basis with a periodical frequency, each subsystem should at least keep a local clock synchronized with good accuracy. On the contrary, if the system can not be sure of peak load start time, such a system would be worthless. To cope with this requirement, there is a need of a synchronization method that keeps each local clock running in a deterministic way, making each one capable to correctly decide whether or not a peak load is to occur. Usually, a digital system clock (as used on microcontrollers) is fed through a crystal with frequency precision in the range of 10 to 100 ppm, which becomes even worse with temperature variation. So, one can easily check that such a clock will be running out of synchronization in a matter of few months (e.g 5min/month considering a crystal with +/ 100 ppm frequency accuracy) 11. This poor performance implies in a need of periodical synchronization. Despite of a higher cost, GPS based clocks have much better performance (e.g +/ 100 ns accuracy). A system equipped with a GPS receiver can be able to maintain a local clock with enough precision to know whether a peak load is to occur, if one considers such events will ever occur in a periodical fashion with frequency and duration known a priori. Unfor- tunately, peak load times present a not so constant behavior. Peak load frequency and duration change not only day-to-day or week-to-week, but also at different seasons. Several factors like daily temperature, holidays, seasonal effects and human behavior infl uence the behavior of peak load times. Clearly, maintaining a local clock with good accuracy is important, but it is worthless if the system does not have a way to know when a peak load will occur and how long it will be. In order to fulfi ll such requirement, it is proposed to add communication capability to each street lighting unit, connecting all units into a control network. If peak load time can be detect and its occurrence reported through the network, each unit of the system will be able to correctly decide whether to switch from mains to battery supply (or vice-versa). B. Lighting System Monitoring The failure of one LED can result in an open circuit or a short circuit. For the series connection, the short circuit in ? Fig. 1.Block diagram of the proposed street lighting system unit. one LED just causes the no-operation of this semiconductor, keeping the other LEDs in operation. But in the case of an open-circuit, the entire group is disconnected because the current path is interrupted. However, an abrupt change in converter output voltage can be detected, indicating a LED fault. If the system is connected to a network, the circuitry can be monitored and the luminary can be quickly replaced. In the same way, the battery can be monitored, which allows report- ing the occurrence of any problem as well as the estimated battery life span. Thus, not only a preventive maintenance of the system can be easily programmed, but also software updates can be done (which allows future enhancements and new functions addition). Then, a street lighting system with communication capa- bility is justifi ed by its advantages. The complete proposed street lighting unit is shown in Fig. 1. The system consists of a power circuitry and a network control circuit. In the next section, a control network to monitor and control these street lighting units is proposed. III. PROPOSEDCONTROLNETWORK The street lighting system features a large number of independent devices, with geographic distribution depending on the city streets. Clearly, each device of this system must be able to communicate at least with a utility control center in order to receive control commands, send status and metering information as well to maintain its local clock synchronized. Note that such system is similar to an automatic meter reading (AMR) system, which is one of the main components of a Smart Grid 12. Recently, AMR technology has evolved to a wireless AMR, which employs two-way communications between utilities and the metering devices. There are several wireless technologies that can be used to provide the required network infrastructure, such as cellular networks and short range radio 13. Among those, wireless sensor networks (WSN) based on short range radio have been widely rec- ognized as a promising technology that can enable WAMR systems 14. Wireless sensor networks presents a collaborative operation that brings signifi cant advantages over traditional communi- cation technologies, including rapid deployment, low cost, fl exibility, and aggregated intelligence via parallel processing 14. Its recent advances have made feasible to deploy Smart Utility Networks (SUN). SUNs are intended to enhance the public utilities services, such as: monitoring and controlling the electricity, water and gas supply 15. The network model adopted by most of the wireless sensor networks is based on the OSI reference model. Usually, in this model, only the physical, data link, network and application layers are implemented. The IEEE 802.15.4TMstandard is commonly employed to perform the physical and data link layers for WSNs. While other wireless network standards aim to achieve large throughput and high quality of service, the IEEE 802.15.4TMis designed to provide simple wireless communicationswith relatively short range, low power, limited data throughput, low cost, and small size. Despite of these features, such standard is suffi cient to satisfy the requirements of most remote monitoring system 16. Although the IEEE 802.15.4TMis a well-established standard, it does not include network and application layers, which should be properly specifi ed. In this paper, a possible approach to a network layer is proposed, which is applied into the control network for the LED street lighting system. For the sake of simplicity and convenience, a single unit of the proposed lighting system will be called node or router and a utility control center will be called base station in the next sections. At fi rst, the network formation procedure as well as the most employed routing techniques applied to WSNs are presented. Following section describes the proposed algorithm used to improve the performance of the network layer when applied to large scale outdoor scenarios. A. Network Formation The network formation starts with a neighbor discovery procedure. The most widely used technique for neighbor discovery in WSNs is to send periodic activity messages, also known as beacons or “hello messages”. As asymmetric data links affect this technique performance, found neighbors might not be feasible paths for routing packets. Therefore, Symmetric Geographic Forwarding (SGF) technique, proposed in 17, is recommended to minimize these effects. To improve even further the delivery success rate, it is recommended the modifi cation proposed in 18, which considers the use of a link quality metric (for example, Received Signal Strength Indicator and Link Quality Indicator) for selecting neighboring nodes when forwarding packets. B. Routing in WSNs Currently, WSN physical and data link layers are based on well-established standards. Nevertheless, network formation and multi-hop routing is still a challenging problem. A variety of protocols were developed in order to properly perform these routing tasks. However, those existing protocols may ? not satisfy the requirements for large-scale applications 19. Geographic routing approach is one of the most suitable routing strategies for such scenario due to its low overhead and high scalability features 20, 21. Geographic routing mainly relies on a really simple geo- graphic greedy-forwarding strategy, where each router node must select a locally optimal neighbor with a positive progress towards the data packet destination 22. In this strategy, the destination address is the position of the destination node, instead of a topological address. To discover a node position in the network, each router node must implement a location service. This service is responsible to determine its own position and the destination position. A node position could be obtained by preconfi guration if the node is stationary, or by a GPS receiver. As network nodes are static in the street lighting case, the location information of the nodes is stored during the installation process. When considering realistic outdoor scenarios, such as a city, any block could turn into a void. A void occurs when all neighbors of a router node are farther away from the destination node than the router itself. In this case, the router fails to locate a next h