Dynamic wireless sensor networks for real time safeguardof workers exposed to physical agents in constructions sites

Elisa Pievanelli, Alina Plesca, Riccardo Stefanelli and Daniele Trinchero

iXem Labs, Politecnico di Torino, Torino, 10129, Italy

Abstract— The paper introduces a wireless sensor net-work platform specifically designed for the protection ofworkers employed in the building sector, exposed to criticalphysical agents, typical of their working scenario. The net-work configuration makes use of either a standard ZigBeecommunication scheme, or a more versatile ad-hoc set-up,which narrows the necessary transmission bandwidth andlowers the frequency of the carriers. The relevant part of theresearch activity has been concentrated on the design andrealization of a compact, wearable, washable, ergonomic, lowcost wireless sensor node, suitable to detect a huge variety ofphysical phenomena. The sample proposed in the paper hasbeen specifically developed to measure two different kinds ofexposure, UltraViolet rays and dust. Both these agents, fordifferent reasons, represent a critical factors and a certifiedsource of possible diseases. The sensor node is embedded onthe worker garment: the electronic components are sewed onthe cloth, and the antenna is integrated within the fabric,and, together with the fabric, forms a unique structure.Preliminary results, obtained with a non-optimized and non-compact node, testify the validity of the approach and itsapplicability to real cases.

Index Terms— Wireless Sensors, Wireless Sensor Net-works, Wireless Sensors in Mobility, Wireless Sensors inHarsh Environments.

I. INTRODUCTION

Wireless Sensor Networks (WSNs) are used in manytechnological areas, including environment monitoring,healthcare applications, home domotics, traffic control,logistic, industrial automation and management. They areparticularly efficient in situations when cables cannot beused or sensors are in mobility. Among all possible appli-cations, the monitoring of physical phenomena by meansof small, non invasive devices represents a challenge forthe next future.

In this paper a mobile wireless sensor network (MWSN)implementation is introduced, to monitor the safety con-ditions of workers employed in the building sector, inparticular the exposure to ultraviolet (UV) rays and mi-cro dust particles. These people are potentially at riskto develop different pathologies related to environmentalagents. Overexposition can cause DNA mutations thatcould result in a skin cancer or other cellular proliferativediseases, [1]. Also the exposition to dust particles leadsto "dangerous" diseases, as loss of lung function due tocumulative respirable dust exposure, [2], and autoimmune

diseases like scleroderma and rheumatoid arthritis relatedto silica dust exposure, [3].

II. MWSN ARCHITECTURE

The network architecture is composed of mobile sensingunits, gateways and a remote unit (see Fig. 1). The sensingunit, or sensor node (SN) must accompany the workerduring the whole day, and for this reason it must bewearable, light in weight, small, robust, as much washableas possible, but also extremely cheap. The SN is used todetect the physical phenomenon, process it numericallyand transmit it to the gateway (GW). The link amongthe SNs and the GW is realized with different methods,depending on the application.

SNs applied to workers employed in closed or indoorenvironments are connected through a 2.45 GHz ZigBeestandard. For outdoor large yard, an 868 MHz ZigBeestandard is preferred. For larger environments, ad-hocsolutions working in the 433 MHz unlicensed bandwidthare utilized. With the ZigBee, the system is robust butthe distance that it is possible to cover is not so largeand the power required for the transmission is too high;while using an ad-hoc standard it is possible to reach longdistances with less power. The drawback is that the antennais larger.

The GW receives the data from all the sensing units andthen retransmits them to a central server that collects allthe data related to all workers by using standard Internetconnections, as 3G, Hyperlan, WiFi.

Fig. 1. Wireless Sensor Network scheme: devices are worn bythe builders, independently on their working duty, local gatewayscollect the information and report to a remote station by standardconnection

978-1-4673-3105-0/13/$31.00 © 2013 IEEE WiSNet 201355

III. SENSOR NODE

The core of the MWSN is the SN, that must be designedaccording to the specifications reported in section II. It iscomposed of:

• a physical sensor used to detect the phenomenon,• a microcontroller and radio (hosted by one single

chip),• an antenna,• a battery,• a suitable energy harvester able to work both in indoor

and in outdoor locations,• a charging circuit.

In Fig. 2 is represented a possible solutions of our sensingunit.

Fig. 2. Sensor Node scheme. The device is composed of anharvesting part (left), which can be realized either with a micro-solar-panel (top left) or a thermoelectric generator (bottom left), amicrocontroller (top center), a textile antenna (bottom right), anda physical sensor, which can be either a UV sensor (top right)or a dust sensor (center right)

IV. PHYSICAL SENSOR

Among all the possible UV sensors we have chosen aSilicon Carbide (SiC) one, since this kind of sensor issmall, with a high speed and with a good spectral effi-ciency. The only drawback is that the output current is verysmall and if there is some interference it is not possibleto do measurement, this problem could be solved using apre-amplified sensor. We chose the SGLux TOCON-ABC1Nano UV sensor, this has a power consumption of 2.4 mW,the output voltage is proportional to the sun radiation andhas a sensitivity of 280 mV

nW cm2 .For the dust sensor we have decided to use an optical one

that is able to detect the reflected light of dust in air, thanksto an infrared emitting diode and a phototransistor insidethe sensor. We have chosen the Sharp GP2Y1010AU0Fthat is effective in detecting very fine particles, it has apower consumption of 60 mW and the sensitivity is 0.5

V0.1mgm3 .

V. MICROCONTROLLER AND RADIO CARD

For the ZigBee there is an integrated transceiver module(Amber Wireless, AMB8420) that contains both the micro-controller and the radio card. Depending on the frequencythe requested power is different, e.g., at 868 MHz it islarger (135 mW) than the one available at 2.45 GHz(75 mW). For the ad-hoc transmission system we havechosen a system-on-chip module that includes both themicrocontroller and the radio card and it can be usedat different frequency, from 433 MHz up to 915 MHz(TI, CC1110F32). Obviously, the power required by themodule is different depending on the selected frequency,as an example at 868 MHz in transmission mode the powerconsumption is 108 mW.

VI. ANTENNA

Among all the SN components, the antenna represents amajor criticality, because of the need to combine compactsizes, folding adaptability, low manufacturing costs andmassif production with efficiency and washability. To ful-fill these objectives, a multiple layer configuration has beenadopted. At first, the antenna is realized with microstriptechnology. As a dielectric, aramid fabric (dielectric con-stant εr = 1.75, electrical conductivity σ = 1.3 10−3

S/m, thickness t = 2.2 mm) guarantees extreme flexibilityand durability. The ground plane and the microstrip con-ductor are realized by printing transparent polypropylenesheets with conductive paint (an extract of aluminum),εr = 2.2, electrical conductivity σ = 7.34 10−5 S/m,thickness t = 20µm. The two polypropylene sheets aresubsequently glowed on the two sides of the aramid, toform a compact sandwich, protected from damages andbreaks by the presence of the sheet (see Fig. 3).

Fig. 3. Realization scheme of the antenna sandwich, with thedimension of the final device

The thickness of the aramid fabric, as well as the dimen-sions of the patch, are obtained running an optimizationprocess that maximizes the efficiency and gain. As an-tenna configuration, we have chosen a standard microstriptrapezoidal design, optimize to work either at 868 MHz or433 MHz. Fig. 4 shows the outcome of the optimization,obtained running the commercial software HFSS, as afunction of the sandwich thickness. As a result, the antennahas been synthesized with the following dimensions: patch

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length 11.13 cm, patch width 12.68 cm, aramid thickness2.2 mm, polypropylene thickness 20 µm, ground plane size12.45 cm x 14 cm. Fig. 4 reports also a small box (bottomright) with the radiation pattern in the meridian plane.

Fig. 4. Optimization results for the definition of the sandwichdimensions. The results published here refer to an antennaworking at 868 MHz. Analogue results have been obtained forthe antenna working at 433 MHz. The small box on the bottomright shows the radiation diagram in the meridian plane.

The polypropylene offers strength, lightness, and trans-parency, and it is commonly used in industry, to manu-facture textiles, packaging, plastic parts, and automotivecomponents. Consequently, it is easily accessible and itscosts are affordable. In our specific case, the use of thepolypropylene simplifies the printing process. The use ofthe same sheet to cover all electronic devices, with the ex-ception of the sensor, provides an overall protection to thewhole system, which increases the usability of the deviceto more general cases and guarantees its washability. Themanufacturing process is very efficient, and based on theuse of rotating printing machines (see Fig. 5), normallyused to manufacture food packages. These machines arecharacterized by an optimum speed (up to 200 m perminute), excellent accuracy, and inexpensive assemblingcosts.

(a) (b)

Fig. 5. a) The cylinders used to print the patches and groundplanes on the polypropylene sheets. b) A fully metallized sheet

VII. ENERGY HARVERSTER

Since the SN has to be auto-rechargeable, one importantaspect to take into account is the power supply. AllSN components are chosen in order to minimize powerconsumption, but an energy harvester is added to extendthe operating life of the sensor.

Different type of energy harvesters have been ana-lyzed [4], and two were found suitable for the project:a thermoelectric one, applicable to indoor locations, and aphotovoltaic one, applicable outdoor.

The thermoelectric harvester (Micropelt, MPG D751) isbased on the Seebeck effect, with the hot side of the devicein contact with the human skin and the cold one exposedto the external environment. The difference of temperaturebetween the body, normally larger than 10◦C, is sufficientto generate electrical energy and charge the battery.

The photovoltaic harvester (Ixys, SolarMD600H10L) isoptimal in outdoor construction premises, as the situationis typically favorable to maximise the efficiency of thesolar modules.

The energy harvesters are interfaced with the batteries(Infinite Power Solution, Thinergy MEC 201) by meansof a dedicated charging circuit (Maxim, MAX17710) thatcontrols the state of the battery and that allows the currentflow when necessary.

VIII. CONCLUSION

The paper presents the configuration scheme and subse-quent realization of a wearable device suitable to monitorworking conditions of builders engaged in tough locations.The current prototype is not optimized in dimensions. Forthe moment it has been realized making use of develop-ment boards, instead of the single components. Neverthe-less, the preliminary tests demonstrate the applicability ofthe concept. In particular, the chosen manufacturing pro-cess is reliable, efficient, inexpensive, and allows a generalprotection to the device, allowing also its washability.

REFERENCES

[1] T. Takoro, N. Kobayashi, B. Zmudzka, S. Ito, K. Wakamatsu,Y. Yamaguchi, K. Korossy, S. Miller, J. Beer, and V. Hearing, “Uv-induced dna damage and melanin content in human skin differing inracial/ethnic origin,” The FASEB Journal, 2003.

[2] A. D. Oxman, D. C. Muir, H. S. Shannon, S. R. Stock, E. Hnizdo,and H. Lange, “Occupational dust exposure and chronic obstructivepulmonary disease: A systematic overview of the evidence,” Ameri-can journal of respiratory and critical care medicine, 1993.

[3] C. Parks, K. Conrad, and G. Cooper, “Occupational exposure tocrystalline silica and autoimmune disease,” Environmental HealthPerspectives, October 1999.

[4] H. Adnan, “Energy harvesting: State-of-the-art,” Renewable Energy,pp. 2641–2654, October 2011.

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