Policy for planned placement of sensor nodes in large scale wireless sensor network

Abstract

Sensor node (SN) is a crucial part in any remote monitoring system. It is a device designed to monitor the particular changes taking place in its environs. Wireless sensor network (WSN) is a system formed by the set of wirelessly connected SNs placed at different geographical locations within a target region. Precise placement of SNs is appreciated, as it affects the efficiency and effectiveness of any WSN. The manual placement of SNs is only feasible for small scale regions. The task of SN placement becomes tedious, when the size of a target region is extremely large and manually unreachable. In this research article, an automated mechanism for fast and precise deployment of SNs in a large scale target region has been proposed. It uses an assembly of rotating cannons to launch the SNs from a moving carrier helicopter. The entire system is synchronized such that the launched SNs accurately land on the pre-computed desired locations (DLs). Simulation results show that the proposed model offers a simple, time efficient and effective technique to place SNs in a large scale target region.

1. Introduction

In the recent era of automation, Wireless Sensor Network (WSN) has emerged as a vital monitoring component for management of remote and unreachable environments [1][2][3][4][5]. Its prime application domains include- military, health monitoring, disaster management, surveillance, etc.[6][7][8][9][10][11][12]. A WSN consists of a large number of sensor nodes (SNs) deployed within a target region. A SN is a low cost device, which is formed by the fusion of two basic modules- a communication module and a sensor. The prime limitations of a SN are its limited battery, communication range (rc) and sensing range (rs) [13][14][15]. However, the workability of WSN is measured in terms of three basic parameters, i.e., coverage, connectivity and life. These three parameters further depend on the pattern of deployment of SNs within a target region. Based on the requirement and the domain of application, the deployment is classified as a blanket, barrier and target oriented [16].

In this paper a policy for planned placement of SNs in the large scale target region using an Enhanced Centrifugal Cannon based Sprinkler (ECCS) is presented. ECCS is an extension of Centrifugal Cannon based Sprinkler (CCS) model [17]. CCS is designed for large-scale, random scattering of SNs from deployment helicopter. Unlike CCS, ECCS is more complex and uses precise control mechanism for accurate positioning of SNs within a target region, thereby attaining the deployment pattern which is closer to optimal.

The rest of the paper is organized as follows. Section 2 summarizes review of the existing state of the art techniques used for the deployment of SNs. Section 3 describes the preliminary. The proposed model is presented in Section 4. Results of simulations are discussed in Section 5 and finally the work is concluded in Section 6.

 

2. Related Work

The deployment becomes a tedious task, when it is to be done within a large-scale target region. Various state of art models have been proposed by the researchers for large scale deployments.

Potential Field based method by A. Howard et al. [18], a Virtual Force based scheme proposed by Yi Zou et al. [19], Connectivity Preserved Virtual Force (CPVF) and FLOOR based methods by Guang Tan et al. [20], Distributed Deployment Scheme (DDS) by Ajay et al. [21] and Scalable energy efficient deployment scheme (SEEDS) proposed by Munish et al. [22] deploy the randomly scattered mobile sensor nodes (MSNs) by relocating them to the appropriate locations in order to enhance the coverage and connectivity of a WSN. The prime shortcoming of these models is that, they require a mobility module for each SN, which is costly and consumes more energy.

Authors in [23] [24] proposed the use of a robot helicopter to deploy the SNs in pre-computed locations. The model works effectively in the small scale region, but cannot be scaled to be used for deployment in large scale target regions, due to low carrying capacity and limited battery life of the robot helicopter. Yoshiaki et al. [25], proposed a uniform aerial deployment scheme (UAD) to uniformly scatter the SNs within a target region. The SNs are dropped with the help of parachute. After dropping, the SNs communicate with each other to determine their current density based on which they decide to fall vertically or to glide horizontally in order to attain the desired density. It is an effective model for the uniform dispersion of SNs from the sky, but it only works if the dropped SNs have similar altitude while falling.

Authors in [17] proposed a centrifugal sprinkler for random dispersal of SNs from the air. It consists of a set of cannons of different length, which rotate on an axis in order to stochastically launch the SNs within a target region. The model could not achieve the optimal coverage due to its stochastic nature, but presents an effective mechanism for quick dispersal of SNs in large-scale target regions.

The majority of earlier presented deployment schemes either stochastically disperses the SNs within a target region or uses a relocation mechanism to move the SNs to optimal positions after stochastic dispersion. Among all these schemes, the stochastic dispersion method is simpler, feasible, time efficient and simple, but cannot attain the optimal level of coverage. Although, the post-scattering relocation schemes can achieve an optimal level of coverage, but the MSNs used in these schemes are costly and suffer various mobility constraints in uneven terrains.

 

3. Preliminary

3.1 Carrier helicopter

It is equipped with ECCS and carries all the SNs to be deployed. It flies on a predefined path (traversal track) above the target region while the ECCS launches the SNs to reach their desired locations (DLs).

3.2 Traversal track

It is a predefined track followed by the carrier helicopter while aerially deploying the SNs.

3.3 Track width

It is the breadth of a rectangular area within which the SNs are deployed in a single scan of the target region. It is twice of the maximum distance to which the SN can be launched by ECCS.

 

4. Proposed model

4.1 Model assumptions

It is considered that all the SNs are encapsulated within a spherical capsule so as to standardize their shape and shield them from damage while landing. The spherical shield of SNs make them suitable to be used with ECCS. The carrier helicopter is outfitted with an accurate positioning device. The mass M of a SN and density of air ρ is considered to be 0.130 Kg and 1.250 Kg/m3, respectively.

4.2 Centrifugal cannon sprinkler (CCS)

CCS [17] was developed for stochastic dispersion of the SNs from a carrier helicopter. It constituted of the cannons of different lengths (see Fig 1), which rotate on a common axis to stochastically disperse the SNs in a target region. It is fixed on a carrier helicopter and the SNs are dispersed as the helicopter traverses the target region.

Fig 1.Centrifugal cannon sprinkler.

4.3 Enhanced centrifugal cannon based sprinkler (ECCS)

ECCS constitutes an assembly of rotating cannons and a governor software which synchronizes and regulates the working of all the hardware components in order to precisely launch of SNs. It is mounted on a carrier helicopter which scans the complete target region using a predefined traversal track.

Major components of ECCS are listed below (see Fig. 2):

Fig. 2.Enhanced Centrifugal Cannon based Sprinkler (ECCS).

Hopper: All the SNs are dumped within a hopper in order to feed them sequentially to the deployment machine.

Cannon: These are the pipes of varying lengths, which are rotated by motor in order to launch the SNs.

SN horizontal launch regulator (HLR): It consists of a series of actuators as shown in Fig. 3. It operates in a binary state, i.e., open or closed. Actuators dynamically switch their state between, closed → open → closed in order to load a SN into the cannon. An actuator remains in an open state for a time interval topen, which is a time required for a SN to shift into the cannon.

Fig. 3.SN horizontal launch regulator.

SN vertical launch regulator (VLR): It consists of a single actuator which governs the dropping of SNs on the DLs, which lies on the traversal track (i.e., just on the line above which the carrier helicopter moves).

SN capsule: Each SN is packed inside a spherical container. This is done to assure even and standard shape of every SN, which ease their usability with ECCS and shield them from any damage while landing (see Fig. 4).

Fig. 4.SN capsule.

SN container capsule comprises of two concentric spherical shells. The inner shell is made up of light-weight and porous substance (i.e., sponge) so as to cushion the SNs from excessive shock and the outer shell is made up of a skinny layer of brittle polymer substance (similar to the shell of an egg). The spherical container capsule is visualized as a fusion of two hemispheres (i.e., top and bottom) containing a space to hold the SN. A thick and sticky gel like substance is filled in the base of a bottom-hemisphere to keep it weighty. This is done to ascertain the landing posture of capsule and to damp its motion after hitting the ground. The base of the bottom-hemisphere fractures after hitting the ground. This absorbs the shock and releases the sticky gel which binds it to the surface of a target region.

Working of ECCS:

ECCS is made up of two sets of cannons, C = {C1, C2, C3,..., Cn} of variable lengths, L = {L1, L2, L3,..., Ln} such that, L1 < L2 < L3 < … < Ln.It also has a vertival cannon C0 to drop the SN on the traversal track (see Fig. 2).

The length Li of cannon starts from the axis of rotation of ECCS. The formulation of Li is given in Equation 1.

where B is the bore of a cannon (see Equation 2), Wc is the width of HLR (Wc = 0.02 m) and lvi is the changeable part of Li which determines the horizontal range of SN launched from Ci.

where Srad is the radius of spherical SN and dr a variable with very small magnitude whose value approaches to 0. The magnitude of cannon bore is slightly bigger than the diameter of SN, so that the SN can easily slide through it.

Pre-deployment arrangements

The whole of the target region is disintegrated into hexagonal cells with each side = rs and the central point of these cells are considered as the DLs for optimal deployment of SNs (see Fig. 5).

Fig. 5.(a). Optimal placement pattern of SNs. (b). Relation between rc and rs.

In order to facilitate the SN deployment using ECCS, the target region is labeled by a grid made up of lines connecting the adjacent DLs (vertically and horizontally) as shown in Fig. 6 (a).

Fig. 6.(a). Grid formation of target region. (b). Traversal track

The horizontal lines of a grid are represented as follows:

where k is the number of scans required to traverse the complete target region. The horizontal lines of a grid are separated by a distance hd (given in Equation 3). Few of the horizontal lines are selected to form a traversal track. Selection is done in a manner such that the adjacent lines of traversal track are separated by a distance dtr (given in Equation 4). However, the total number of horizontal lines involved in traversal path is given in Equation 5. Among all the horizontal lines, the traversal traversal track is labeled by Fig. 6 (b) presents a clip of traversal track, such a track is laid in the pattern of parallel lines to cover the entire target region. The carrier helicopter follows the traversal track to scan the entire target region while the ECCS precisely launches the SNs to place them on their DLs as shown in Fig. 7 .

where w denotes the width of a target region. The vertical lines of a grid are marked as P1, P2, …, Pm, where m denotes the total count of vertical lines within a target region. The vertical lines are separated by a distance

Fig. 7.Trajectory formed by the SNs fired from the cannons of variable lengths.

In absence of air

The launch velocity Vl is a speed with which a SN-capsule is released from ECCS. It is directly proportional to the speed of rotation of a cannon Ci and its length li. (See Equation 6).

Where, ω denotes the angular velocity. The time tf taken by a SN launched from cannon Ci to reach the ground is given by Equation 7.

Where, ψ is the altitude of dropping and g denotes the gravitational acceleration (g = 9.80 m/s2). The horizontal span Hd(li) traveled by any SN launched from Ci is given in Equation 8.

The horizontal distance traveled by a SN is directly proportional to the length and speed of rotation of the cannon from which it is launched (see Fig. 8).

Fig. 8.Relationship between the length of the cannon, rotation speed and horizontal displacement.

In presence of air

In actual conditions, the air acts as a vital component to define the trajectory of a SN. The air offers a resistive force on the falling SN, due to which it stops accelerating after attaining its terminal velocity Vt (given in Equation 9). The magnitude of terminal velocity depends on the weight and size of a dropped SN (see Fig. 9).

where M and A denotes the mass and cross-sectional area of a spherical SN, respectively. ρ denotes the density of the air (ρ = 1.250 Kg/m3 ) and Dc denotes the coefficient of drag of a spherical SN (Dc = 0.5 ). Every SN launched from ECCS encounters an opposite force Fo (given in Equation 10) due to air friction, which decreases the horizontal span covered by them.

Fig. 9.Relation between radius, weight and terminal velocity of SN.

where v denotes the present velocity of a SN. The trajectories of SN launched from ECCS in the presence and absence of air are shown in Fig. 10 (a) and (b), respectively.

Fig. 10.(a). The trajectory of SN launched from ECCS (in the presence of air). (b). The trajectory of SN launched from ECCS (in the absence of air).

The method to compute the horizontal span crossed by a SN launched from ECCS in the presence of air is given in Algorithm 1.

SN launch regulation

Launch regulation of SN is an essential task carried out by ECCS. It is concerned with the computation of angle of loading (AOL) for each cannon. AOL is an angle at which the SN is inserted into the cannon. It depends on the length of a cannon Ci and the angular velocity of ECCS. Each loaded SN undergoes acceleration Ca (given in Equation 11) due to centrifugal force exerted on it.

Launch velocity is the velocity with which a SN is released from the cannon. It is given by Equation 6. Each cannon Ci consumes a specific time Ti to launch a SN, called dispense time (given in Equation 12).

where TL is the time taken by the actuator to load the SN into the cannon and ti (given by function T(t, β, Li) in Equation 13) is the time taken by the SN to pass through the cannon Ci (see Fig. 11).

where β is the distance of SN from the axis of rotation of ECCS at an instant when it is loaded by HLR into the cannon Ci (β = B). The value of time t = 0 at this instant.

Fig. 11.The relation between cannon length and the time taken by a SN to pass through it.

The vertical lines of the grid are visualized as real (RPj) and virtual (VPj) vertical lines. RPj are the definite lines marked on the target region based on DLs. However, the VPj are the replica of RPj over the target region at an altitude ψ. These are shifted by a distance Ds in the direction inverse to the movement of a carrier helicopter (see Fig. 12). The computation of Ds is given in Equation 14. VPj are shifted in order to neutralize the expected dislocation introduced because of inertia within a SN launched from a moving helicopter.

where VH denotes the velocity of a carrier helicopter.

Fig. 12.(a). left to right movement of a carrier helicopter. (b). Right to left movement of a carrier helicopter.

ECCS uses a parallel path scanning method [17] to scan the entire region using deployment helicopter. AOL is important to precisely launch the SNs when cannons are aligned parallel to the vertical lines. AOLi for any cannon Ci with length Li is given in Equation 15.

The errors introduced while deployment of SNs can be broadly classified as uncertainty-error EU and calibration error Ec. Uncertainty error is due to uncontrollable environmental conditions (i.e., wind, humidity, temperature, etc.) that affect the precise placement of SNs. It mainly depends on the launch height of a SN. However, the calibration error is due to the time taken by ECCS to rotate to AOLn from current position before loading the cannons. The value of Ec is given by Equation 16.

The value of Ec can be minimized by setting high angular velocity of ECCS and lower speed of carrier helicopter.

Length of various cannons in a set is computed using Algorithm 1. It is determined on the basis of launch-height of the SN and angular velocity of ECCS. Fig. 13 represents the horizontal distance covered by the SNs launched from the cannons of variable length for specific dropping height and angular velocity.

Fig. 13.Relation between cannon length and horizontal distance covered by the SN.

Algorithm 2 governs and synchronizes the operation of various components of ECCS. Thread-1 maintains the rotation speed of ECCS while moving the carrier helicopter at a specified height above the traversal track. Thread-2 is executed for every SN launch regulator. It precisely computes the time to load the SN into the cannon and accordingly instructs the regulators.

 

5. Simulation results and discussion

The proposed scheme is simulated in Quorum Comm (a Java based simulator developed by us) Table 1 represents the value of various parameters used in the simulation.

Table 1.Simulation variables.

Fig 14 (a) and (b) represents the coverage achieved by ECCS and CCS, respectively. It is observed that ECCS can achieve about 95% of coverage, which far better than 70% coverage achieved by CCS for the same number of SN. Fig. 15 shows the relationship between the number of cannons and the time required for deployment.

Fig 14.(a) Coverage by ECCS (b) Coverage by CCS.

Fig. 15.Relation between number of cannons and deployment time.

A drastic enhancement in the performance of relocation based deployment models is observed when used with ECCS. Fig. 16 shows that the average movement of SEEDS is reduced from 95m to 8m when the MSNs are dropped using ECCS.

Fig. 16.Enhanced performance of SEEDS with ECCS.

PTP based aerial dropping is a method to drop the SNs just above the DL using a deployment helicopter. It is considered as the best method to achieve optimal coverage, but it requires a large amount of time for deployment, which makes it impractical to be used for large scale target regions. Fig. 17 presents the comparison of percentage coverage achieved by different schemes used deploy SNs from the sky. It is observed that the coverage achieved by ECCS is approximately equal to that of PTP for the same number of SNs.

Fig. 17.Coverage achieved by various deployment models.

 

6. Conclusion and future work

In this research article, an automated mechanism for fast and precise deployment of SNs in a large scale target region has been proposed. It uses an assembly of rotating cannons to launch the SNs from a moving carrier helicopter. The entire system is synchronized, such that the launched SNs accurately land on the pre-computed desired locations (DLs). The simulation results show that the coverage achieved by the proposed model is very close to optimal. It is very time efficient and can be used for quick establishment of WSN in large scale target regions. The proposed model works efficiently for plain surfaces but, the performance may degrade for hilly regions as vertical and horizontal lines of the reference grid may not remain parallel throughout the target region. We are in the process of extending the model for uneven terrains and designing a hardware model for real time testing, analysis and implementation.