Gas-Drone: portable Gas Sensing System on UAVsfor Gas Leakage localization

Maurizio Rossi and Davide BrunelliDep. of Industrial Engineering

University of Trentovia Sommarive 9, Trento, Italy{name.surname}@unitn.it

Andrea Adami and Leandro LorenzelliBioMems unit

Bruno Kessler Foundation (FBK)via Sommarive 18, Trento, Italy{andadami,lorenzel}@fbk.eu

Fabio Menna and Fabio Remondino3DOM unit

Bruno Kessler Foundation (FBK)via Sommarive 18, Trento, Italy{fmenna,remondino}@fbk.eu

Abstract—Volatile chemical concentration and gas leakagerecognition can be crucial in environmental monitoring for riskassessment. The use of Unmanned Aerial Vehicles (UAVs) tomeasure spatially distributed gas concentration is of great interestbecause it allows a Simultaneous Localization And Mapping(SLAM) of the volatiles. This field is quite recent and, so far, fewefforts have been dedicated to the design of integrated sensinginstruments that focus on the optimization of crucial featuresas weight, dimension and energy autonomy, as important asselectivity and sensitivity of sensors on board UAVs. The proposedGas Sensing System (GSS) is a fully autonomous board based ona 32bit MCU with 30min autonomy (on its own battery), datastoring, wireless connectivity for real-time feedback and embeds acustom micro-machined MOX (Metal Oxide) sensor. This systemcan be mounted on any UAV thanks to its small dimensionsand light weight. Experiments demonstrate that the sensingperformance is not impaired by the air flow during the flightand we are able to spatially describe the volatile concentration.

I. INTRODUCTION

Unmanned Aerial Vehicles (UAV) or Remotely PilotedAircraft Systems (RPAS) or simply Drones are nowadaysused for a large variety of applications, from mapping tosurveillance, from reconnaissance to inspection [1]. Flightsare normally done with single platforms although fleets ofmicro-drones that act as a swarm are demonstrated in literature.Among the applications, robot-based automatically generatedgas concentration map is considered a recent research field [2]both for environmental monitoring and for surveillance ofcritical buildings [3], [4].

The Gas Sensing System (GSS) proposed in this workaims at joining two fields of research: (i) drone mappingand (ii) gas concentration analysis. A drone or swarm ofmicro-drones can greatly speed up the environmental sensingprocess and the measurement of volatiles diffused in a largearea. Unfortunately, the weight they can bear and the energyautonomy, affected by the presence of energy hungry deviceslike MOX gas sensors, are the major challenging constraints.

Commercial gas sensors are usually based on alumina plat-forms which usually lead to high power consumption and theydo not consider the integration into other devices. For thesereasons, a custom MOX gas sensor has been implemented tobe installed on a sensing module of the UAV platform.

This class of sensors is particularly suitable for gas sourcelocalization where the sensitivity to gases is the main require-ment. Moreover this technology provides for the followingadvantages: (i) sensors are developed with micromachining(MEMS) techniques, thus providing multi-parametric systems

Fig. 1. Multiparametric MEMS sensor bonded to TO package (left). Detail ofthe MOX sensor (the greenish area is the active nanoporous layer) (right) [5].

equipped with transducer for air temperature, air velocity, andgas detection; (ii) reduction of the payload; (iii) minimizedpower requirements; (iv) possibility to implement differentsensing layers for gases like NO, NO2, CO, and VOCs(Volatile, Organic Compounds).

The target applications are twofold: (a) we can achieve het-erogeneous environmental sampling by using sensors targetedto multiple chemical compounds each mounted on a singlemicro-drone tailored for this application; (b) it is possible toadd chemical sensing capability to pre-existing drones thanksto the proposed system.

This work is organized as follows, Section II presents anoverview of the state of the art in the field of volatile mappingusing drones. Section III introduces and describes the proposedembedded system for air quality mapping with UAVs while inSection IV we discuss the experimental results achieved witha prototype version. Finally Section V concludes the paper.

II. RELATED WORKS

In the last decade, the diffusion of low-cost UAVs origi-nated an increasing interest in the scientific community towardsthe use of these devices in very different scenarios. Amongthe others, localization and mapping of geographical areasby means of advanced 3D imaging techniques have beendemonstrated [6]. Simultaneously, drones have been studiedfor pollution and gas concentration mapping in geographicalareas where environmental concern is a hot topic. The mainproblems of all the current implementation are (i) the limitedflight autonomy and (ii) the size-to-payload ratio.

Three main volatile gas sensing technologies exist that aresuitable for light-weight portable electronic devices: chemo-

978-1-4799-0162-3/14/$31.00 ©2014 IEEE

Fig. 2. Microfabricated sensor array that allows the concurrent evaluation of8 different compounds.

optical [7], chemoresistive [8] and electrochemical [9]. Chemo-optical devices are the most expensive and bulky and elec-trochemical sensors need periodic refill and maintenance ofreagent substances. Chemoresistive technology, such as MOXsensors, offers fast response and long term stability withoutmaintenance even if their power consumption is not negligible.To overcome the power consumption limit of MOXs, duty-cycle based strategies have been demonstrated in literatureand allow to greatly increase the lifetime of battery poweredembedded electronic devices [10].

III. PROPOSED SYSTEM

Even if the micro-machined gas sensors implementation issupported by several examples, reports on airborne applicationsof these sensors are very limited. In particular, MOX sensorscan detect events that are not revealed by standard instrumen-tation commonly adopted for air quality measurement.

The Gas Sensing System (GSS), mounted on our Gas-Drone, is based on a MEMS metal-oxide sensor (Fig. 1), thepower and signal conditioning circuit to interface it with apowerful 32bit microcontroller (MCU) running at 32MHz, arechargeable battery and an integrated GSM/GPS modem.

After a demonstration phase in which we used commercialMOX sensor like USM-VOC 3.0 from Unitronic1, a MEMSbased gas sensor array, already developed in the past, is inte-grated into the modular system for improving the performanceof Gas-Drone.

A. Sensing Element

The application of metal oxide sensors in an array con-figuration has been evaluated in the past [11] with goodresults for identification of different gases in a mixture andalso for a fast measurement of concentrations. Due to thehigh level of integration and low power consumption up toa few milliwatts, we propose the same technological approacha suitable candidate for sensing tasks in air monitoring withUAVs.

The main advantage for adopting this configuration ismainly related to the contemporary signal acquisition fromeight different metal oxide sensors working at different tem-peratures, with total power consumption below 130mW. Theperformance of this array as quality air module, in relation to

1http://en.unitronic.de/unitronic-sensor-module/voc.html

this specific application, was demonstrated in [12] for differentgases like CO2, NO2 and SO2 at different concentrations.

A preliminary activity has been addressed to evaluate thecompatibility of the sensor technology with the present casestudy and to test a monolithic single-sensor chip, developedwith the same technological process packed on a TO-12 socket,of the signals coming from a WO3 thin film structure. Withrespect this preliminary evaluation study, the advantage of theproposed array approach is to make available a sensitive areaheated in a linear temperature gradient mode, allowing anoverall evaluation of gas sensing properties of the materialin a 100◦C-wide window.

The fabrication technology consists of a bulk micromachin-ing process implemented for the realization of a thin dielectricmembrane where the sensing layer will be deposited. Thestarting material was a (1 0 0) 4 in., p-type, silicon wafer,double side polished. The membrane structure was realizedonto the substrate by a 300 nm-thick layer of silicon oxidegrown by thermal wet oxidation, followed by a 150 nm-thickstoichiometric Si3N4 film deposited by low pressure chemicalvapor deposition (LPCVD). Onto the backside of the wafer300nm of LTO (low temperature oxide) has been deposited,used as a protection for the silicon nitride layer. The heatingelement and the temperature sensors have been realized by a450 nm-thick, in-situ p-doped polysilicon layer deposited byLPCVD from silane. The polysilicon has been subsequentlypatterned to define resistors for the heater element and temper-ature sensors. Finally, a further insulating multilayer have beendeposited, to electrically insulate heaters and thermometersfrom gas sensitive films deposed on the membrane surfaceand to obtain a planarized membrane surface. Platinum inter-digitated contacts to the sensitive layer have been realized byevaporation of a Cr/Pt (5/50 nm) and patterned by a lift offtechnique. The sensitive layer, consisted of a tungsten oxide(WO3) film, deposited by RF sputtering starting from a puretungsten oxide 99.99% 4 in. target at fixed oxygen partialpressure and power density directly on a suspended hot platewafer [11].

Fig. 2 shows a photograph of the microfabricated deviceswithout package; the central area of figure represents thesuspended thin dielectric membrane where the thin WO3metal oxide semiconductor (MOX) layer (with dimensions1140µm×500µm) was deposited as a gas sensitive layer;working temperature along the MOX rectangular area was setto 300−400◦C range, reported in the literature as the suitabletemperature range for these materials [13] and also experi-mentally found as a better choice. Concerning gas sensingproperties and lowest detection level, tungsten oxide depositedat 30% of oxygen partial pressure has demonstrated to be thebest solution in terms of sensor response and readiness [14],[15].

As already reported, the VOCs were considered as targetgases for this kind of application, but the experimental workwas focused on typical gases easy to find in a common urbanenvironment. In our application scenario, different gases werechosen, in particular carbon monoxide (CO), the nitrogendioxide (NO2) and the sulphur dioxide (SO2).

B. Gas Sensing System (GSS)

The proposed embedded system is based on a commercialboard that combines a powerful MCU with an integratedGSM/GPS modem, depicted in Fig. 3.

Fig. 3. Picture of the acquisition board, MCU on the top side (left), combinedGPS-GSM modem on the bottom side (right).

Fig. 4. Block scheme of the embedded acquisition system.

The MCU is the JN5148 from NXP-Jennic2. It has inte-grated radio transceiver and antenna; a micro SD-card is usedfor data logging, both controller and card holder are realized inthe same embedded system. The integrated GSM/GPS modemis the SIM908, provided by Simcom3. It is an integratedmodule that needs few external components to operate, a simcard holder and the antennas.

The block scheme of Fig. 4 presents in details the intercon-nection between the two modules, highlighting the peripheralsused in the proposed design. The MCU offers both digitaland analog channels that allow the integration of differentperipherals. We interfaced commercial sensors (USM-VOC3.0) to evaluate the sensing performance of the GSS mod-ule during flights, using the digital communication interface.Moreover the analog channels allowed to interface with thecustom multiparametric MOX just introduced, solution that iscurrently under characterization.

The integrated voltage regulator allows a wide range ofinput voltage supply to be used with the board. For the testspresented we selected two 9 V - 900 mAh commercial batteriesarranged in parallel for a total 1800 mAh capacity whichresulted in 30 min. autonomy with continuous sensor supply.

2http://www.jennic.com/products/wireless microcontrollers/jn51483http://wm.sim.com/producten.aspx?id=1024

Fig. 5. Pictures of experimental setup: (left) the prototype board attachedbelow the drone, (center) the drone and the source of volatile plume (isopropylalcohol used in first experiment) and (right) the drone used for tests.

IV. RESULTS

A DJI hexacopter4 was used for running two experiments.With an estimated autonomy of 15 min. and a maximumpayload of 300 gr. (thanks to its 80 cm diameter) this droneis suitable for limited region gas distribution reconstruction.

The purpose of the real field experiments were to evaluatethe sensitivity of MOX sensors in presence of turbulent airflow due to the propellers and with respect to the placement ofthe embedded system right below the main body of the UAV.The first one (Fig. 6) employed a MOX targeted to VOCs,testing the system’s sensitivity during the flight with isopropylalcohol (Fig. 5). In the second experiment the UAV surveyedthe kitchens of the FBK center (smaller spot in the rectangle ofFig. 6 marked with high concentration) and of the Universitycanteen (bigger spot in the same rectangle).

Detailed results of the previous experiments are presentedin Figures 7 and 8. In both pictures, the decreasing spikesmarked with arrows and boxes, highlight the ability of the sys-tem to discriminate volatile substances in air during flights. Thesecond experiment, that covered a wider spatial range, demon-strated the capability of the system to localize gas sourcesduring movements. These results confirms that environmentalconditions and cross correlation of volatile chemicals stronglyaffect MOX’s response, moreover the characterization of thisresponse in open environments is a challenging task, to thisreason we are currently testing multiparametric custom MOXto eventually extract volatile discrimination.

With the proposed setup, we achieved ≈30min auton-omy for continuous gas sampling, data logging and wirelesscommunication which is longer than the autonomy of theemployed UAV. We expect to further increase the energyautonomy by introducing clever sampling strategies based onduty-cycled sensor usage and aggressive data compressionusing compressive sensing techniques [16].

V. CONCLUSION

We presented a modular design based on a MEMS metal-oxide sensor, to realize a gas concentration measurementdevice suitable for UAVs employment. MOXs sensors candetect events that are not revealed by standard instrumentationcommonly adopted for air quality measurement. In this workwe used a custom sensor developed with micromachining(MEMS) techniques, thus providing multi-parametric systems

4http://www.dji.com/product/spreading-wings-s800/feature

Fig. 6. Map of two flight experiments performed near the premises of ourinstitutes. The circle highlights the first monitored area, whereas the rectangledepicts the area of the second flight. The base station is in the circle.

Fig. 7. Detail of the sensor’s response during the first flight experiment.

equipped with transducer for air temperature, air velocity, andgas detection. The embedded system includes the power andsignal conditioning circuit to interface the MOX with the pow-erful 32bit microcontroller (MCU) running on a rechargeablebattery. We achieved 30min autonomy for continuous gassampling, data logging and wireless communication whichis longer than the autonomy of the UAV employed. Futurework will be the implementation of algorithms for automaticcontrol and routing toward the gas source, and methods whichextend remarkably the autonomy, such as energy harvestingtechniques [17] or alternative energy storage devices [18].

Fig. 8. Detail of the sensor’s response during the second flight experiment.

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