Popular Applications of Wireless Sensor Networks

by May 22, 2015 0 comments

– Arijit Roy, MS Scholar, Pushpendu Kar & Sumit Goswami, Ph.D Scholar, IIT Kharagpur

Wireless is an old technology, however, with the advancement of science and technology it is being used in communication devices like cell phones, cordless phone receiver, wacky talky, remotes of different devices, Wi-Fi, WiMAX, cordless mouse and keyboard.
‘Wireless’ system enables us to send some signal to one or more device without any tangible wire connections and reduces the complexity of physically wiring the connections. Rapid development of technology and increasing human need is leading to integration of different technologies into a single unit and Wireless Sensor Network is one such integration where sensors are integrated with wireless system to form a network. This network is used for a plethora of applications. Target tracking is an upcoming application of wireless sensor networks. Here, the term target signifies an object or multiple objects, which may be living or non-living. Depending on the size and speed of the target, the radius of the sensor network in operation may vary from a few meters to a few hundred thousand meters and the sensor nodes may be placed in a predefined or a symmetrical pattern or it may be placed randomly in the tracking zone. The type of sensor used in the network also changes accordingly with the type of target to be tracked. An example of a short range target tracking is movement of isotopic iodine in human body using body sensor network or even smaller network which is known as the Internalnet, i.e network of computer devices within a human body.


Sensor

A sensor is a device which can sense a phenomenon like temperature, pressure, humidity, vibration, etc. There are different types of sensors for sensing different phenomenon. The main components of sensor nodes are processor, transceiver, external memory, power source and sensing unit. The sensed information is converted digitally by ADC (Analog to Digital Converter) and thereafter this information is processed by the processing unit and sent by the transceiver to an external central unit (sink). The circuit is made as per the requirement. If it is required to sense humidity in the environment; the circuit is made like that only. Sometime the sensor may be used to sense more than one phenomenon and it can be programmed as per the needs. Each sensor has a certain sensing range within which it can sense the required environmental phenomena. Different types of sensors are available in market with varying cost. A few popular manufacturers of sensors are Crossbow, Motorola, MicroStrain, DeltaTRAcK, etc.

Wireless sensor networks & its applications
Wireless sensor networks play an important role in applications like knowing the physical characteristics of an object, its position, behavior or to track the object. Sensor nodes work in collaborating way to measure their surrounding (light, pressure, temperature, humidity, vibration etc.) because generally sensor nodes have short range of transmission and so intermediate nodes act as relay node to forward sensed information to the sink or base station. Sensors can be deployed in two ways — pre planned grid pattern or scattered/random deployment without any organized plan. Wireless sensor networks play an important role in wild life monitoring. The position of animals, their grazing habit, etc can be detected by the use of wireless sensor networks.
WSN can be stationary and mobile. Wild life tracking using sensors is the example of mobile WSN as the sensor nodes attached with animal body are mobile with the movement of the animal. Forming an efficient and good WSN is a challenging task in terms of sensor node’s power (as these are very low power), sensing range, misbehavior, fault etc. As sensor nodes have very low power, the routing for forwarding sensed data to sink or another node is also a challenge. Security is another important issue which must be considered. Sensor node may be affected by attacks, infiltration, eavesdropping or interference.


Target tracking

Target is the term used here for an object, which we want to track i.e. we want to know the current position or the trajectory (path) on which target has moved. Target may be static or mobile. For static object, we want to know the position or some characteristics of the object. In case of mobile object, it may require to track the path of the object like chasing the trajectory of an aircraft, following an animal for protecting it etc. The number of targets being tracked simultaneously may vary as it maybe more than one and even uncountable and it can be of similar or different types. The target tracking can be classified in two part -Real time target tracking is generally used for tracking high speed moving object, whereas in classical target tracking it may not track in a pre-defined fixed time, it may fail for high speed moving object.


Target tracking in wireless sensor networks

For tracking object/s, generally multiple sensors are required which can sense some specific characteristic of the object. All these sensors are deployed either in pre-planned basis or randomly. For establishing a good sensor network, the deployment of sensor nodes should be in such a way that there is no uncovered area from the range of at least one sensor. After sensing the object, a sensor sends some signal to its neighboring sensor node either for activating or for forwarding the information of the object. Each sensor node has some calculating or processing capabilities which may be required for different purposes. For saving energy of the sensor node to increase lifetime of the network, generally only one node is activated instead of redundant activation of nodes. Each sensor sends information to the sink. The sink finally decides the actual position of the object or trajectory of the object from the sensed information. A typical target tracking scenario is one in which the objects are of different type. In this type of tracking the sensor should have intelligence to differentiate between objects. Another way of tracking is ‘cluster based tracking’. Cluster represents a few sensors grouped to form a cluster. There is a leader node called cluster head. This cluster head may be elected by cluster members or it can be indicated in a pre-defined manner. A cluster head has some important responsibilities. For example, if an object is in the sensing range of a particular cluster then the cluster head activates only that sensor node which is nearest to the object. Apart from this, the sensed information is primarily sent to cluster head and cluster head further forwards it to the sink. Cluster heads of different clusters generally communicate with each other. The target tracking can be further strengthened by adding a robot chasing the moving object following the trajectory of the object. The concept of target tracking using sensor network becomes more challenging in mobile sensor network, in which sensor nodes are also mobile. Another important concern is routing.
In the figure above, a car is moving through a trajectory. Here trajectory of the moving car is tracked using sensor networks. Some sensor nodes are deployed over a region. Few numbers of sensors are in active state and maximum of them are in sleep mode. Red color nodes are in sleep mode and black color nodes are in active state. Only those nodes are activated, which are needed for tracking. In this figure car is the target and car is tracked by sensor nodes.


Faults during target tracking in WSNs

Different types of fault can occur during target tracking in wireless sensor networks. These faults generally occur due to the fault of the sensor nodes or from the external attacks. Fault may be due to hardware failure, energy depletion in sensor node, selfishness of the sensor node etc. A node may deplete due to some environmental disaster like flood, erosion of soil etc. Selfishness is the behavior of a node, when it drops received information for saving its own energy/resources.

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