Definition Of Sensors Computer Science Essay

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Imagine being in a battlefield with the orders to hold the position until further assistance arrives. The year is 1943, Nazi troops are marching towards your location and the best method that could be used to judge the direction of the attack is the deployment of land mines. Scarce number of mines is placed in the surrounding area. The patience is put to the test as you wait without any true idea where and how the attack would begin. Now imagine the same scenario but with the technological advancements of modern warfare. The deployment of land mines is replaced by heat sensors that send real time data of enemy movement in the neighbouring area. Each and every soldier on your side has access to handheld GPS receiving that imperative information. Being well aware of your surroundings you foresee the attack and plan your defensive manoeuvre accordingly. The bravery of the soldiers in the world war is unmatched but it is quite easily seen how the use of heat sensors will give the latter better chance of survival.

As time is passing the enhancement in technology is leading us to unbound complexities, obliging us to be synced with the surrounding environment accurately. This project is about the use of sensor networks to localize heat-radiating point sources that are present in a specific noise-less 2-D field of concern, both in time and space. These sources induce diffusive fields and hence our true aim shall be to come up with a well enough reconstruction of the fields, leading us to locate the sources.

Mainly based on simulation this project will emphasize on the software side, though some background of hardware necessities of sensor networks would also be given. Different available algorithms will be studied and the chosen one put to test. The simulations will be carried out in the Matrix Laboratory (i.e. MATLAB) and the results analysed.

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BACKGROUND-GLOBAL

DEFINITION OF SENSORS

A basic sensor is a transducer, a battery powered device which is able to take physical measurements of any type of energy and convert them into another. Mostly transducers are used to measure the quantity of interest and give from them respective electric signals that can be analysed. As the processors used to analyse the data implement signal processing the usual output of such transducers is voltage or current. The following are some of the types of sensors that are used.

As the forms of energies are not limited to a small number so are the types of sensors that could be used. Sensors belonging to the Mechanical Sensor class need to have a physical contact to take measurements. For example a capacitive sensor (i.e. a Mechanical Sensor) could be used to measure force exertion. This is done by the increase and decrease in the capacitance of a two plate parallel capacitor with one fixed and the other movable plate. The movable plate is displaced due to the force hence changing the measurable capacitance. Another form is the class of Electromagnetic Sensors which is used to study the proximity effects in circuits and do not require physical contact to take the measurements. ADDDDDDDDDDDD EXAMPLE

The type of sensor that could be used in conjunction with locating heat sources belong to the class of Thermal Sensors. Thermal sensors are transducers that take as input heat energy or heat flux to give out electrical signals. One of the most accurate Thermal Sensor available today is called the Resonant Temperature Sensor. The main reason for the greater accuracy of a Resonant Temperature Sensor is the fact that it uses SiO2. Temperature changes around SiO2 alter the resonant frequency of the silicon-oxide. As the measurements depend upon the resonant frequency the precision is greatly improved.

A complex sensor may consist of a transducer accompanied by a processing unit (discussed later), a storage unit and a transceiver for communication. The communication may be between sensors or with a single base station (also discussed in the next section). The signals attained by the transducers can be used in two different forms by the processing unit to apply the algorithms; Analog or Digital. If the signals are firstly converted to binary (1’s and 0’s) this ensures less noise, fading and throughput. Analog on the other hand ensures much better resolution as there is no quantization error.

Sensors come in different forms depending upon the guidelines set and the use they are going to be put to . For example a weather station that takes measurements of the surrounding temperature may have a size comparable to a shoebox while a sensor to be used by the military should be miniscule in comparison. Though the thought of a tiny sensor does seem good it comes with a drastic increase in cost and decrease in lifetime of the sensor. In other cases the size is not the factor and the sensor needs to be robust against the harsh environment (e.g. the monitoring of the penguins in the North Pole).

The battery life and power consumption by the sensor is directly governed by the required cost/ size of the device and is inversely proportional to the complexity of the processing/ storage unit. The more processing that needs to be done by the sensor itself the more power consuming it will be, leading to the deployment of such sensors less cost and energy efficient. Although a sensor does allow us to study physical quantities by the conversion into electric signals, a standalone sensor is not always very useful.

SENSOR NETWORKS, NETWORK TOPOLOGIES AND NETWORK ALGORITHMS

A network that consists of tens to thousands of sensors (often referred to network nodes) deployed in an area to take some physical phenomenon into consideration is called a sensor network. The deployment of such a network could be done once or could be a continuous process. If the deployment is to be a one-time activity, the installing phase and the usage of the network are two separate entities. This type of deployment is only helpful in less robust environment where the network is usually going to be used for a rather small amount of time. The continuous deployment is useful if the environment is robust and the data collection is going to span over a long time, as in this case it is more likely to face sensor failure or the requirement of battery replacement.

The nodes in the network are usually connected using wires but due to advancements in optical links and RF transceivers, efficient wireless sensor networks (WSN) have also become practical. Though sensors are usually placed manually at preferred locations, they could also be deployed randomly (e.g. deployment by aircraft). The latter has only been plausible after the success of WSN and use of algorithms that allow self-organization of the network.

Many types of sensor network topologies exist which are merely altered versions of the two main topologies; Ad hoc network (also known as Mesh Network) and infrastructure network (also known as Star network):

In an Ad hoc network all of the nodes of the network are able to communicate with each other. All of the sensor nodes have been preloaded with processors which can process the measurements attained by the transducers and can apply the algorithms required themselves. A single node may be able to communicate with all of the other nodes in the network, or if this is not the case it could communicate with the ones residing in some particular radius.

The authors of [][] have discussed some model algorithms that could be employed in an ad hoc network. Two of major ones are known as Distributed Algorithm and Localized Algorithm. In the case of the Distributed Algorithm the nodes run their own algorithms separately and a priori a single node has no information about the states of all the other nodes. Hence learning of the network by each node is done through repeated exchange of messages with the neighbouring nodes. The Localized algorithm is just a new generation of the Distributed Algorithm which takes into account the energy consumption of repetitive sending and receiving of messages. In a k-localized algorithm a single node behaves just like the way in the Distributed one, except that it is only allowed k number of message exchanges. However the node is given the option to wait between message exchanges as it sees fit.

In the infrastructure network all the sensor networks are directly linked to a single major processing unit which has the sole duty to apply the algorithms based on the measurements taken from all the nodes. Processing unit is often referred to as the base station or the central hub. The base station also has a large storage unit which can hold the moment by moment data received from all the nodes in the network. The nodes are only there to take measurements and cannot communicate with each other except through the base station. Out of the many algorithm models shown by [][] have shown Global Algorithm is the one that would be employed as that would give the base station total command on top of all the nodes.

The ad hoc network is more robust, for example a failure of a set of nodes would not mean the total breakdown of the network. This is not the same case for the infrastructure network as if for example the base station breaks down the whole network would be in stand-by until working base station has been installed. Ad hoc network requires more computations by sensors (i.e. use of complex sensors) while the infrastructure network uses the sensors just to accumulate the measurements. This means that nodes of an ad hoc network are more power consuming. In order to conserve the most energy in both the topologies, the best way is to use the least complex algorithm as that requires less power consuming processors leading to a more energy efficient(hence cost efficient) long lasting networks.

The power consumption of the nodes can be decreased in a number of methods. Authors of {}{} have suggested the use of softwares in the node that allow ‘Sleep Mode’ operation. Nodes consume the most power when they are required to transmit a signal, but as this is not frequent an impressive amount of energy is also lost during the idle period where a node is waiting or receiving the required information. This waste is taken care of by synchronising the nodes such that all of the nodes are able to send and receive data only at specific intervals, while during the remaining times they are unable to do so. Another fact that affects the power consumption is the distance the signals sent by the nodes have to travel to reach the receiving end. To counter this effect the Tree network (i.e. a hybrid of the two discussed topologies) or use of multi-hop messages could be used. In the multi-hop system the messages sent from a node are not received directly by the recipient but takes a course of small steps from the neighbouring nodes. The tree network is a set of infrastructure networks which are interconnected by a single root node which aids in the communication between all the other nodes. Implementation of these two can be seen below.

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