Architecture of a UAV for volcanic gas sampling

Transcription

1 Architecture of a UAV for volcanic gas sampling Daniele Caltabiano, Giovanni Muscato and Angelo Orlando Dipartimento di Ingegneria Elettrica Elettronica e dei Sistemi Università degli Studi di Catania Viale A. Doria 6, 95125, Catania, Italy daniele.caltabiano@diees.unict.it, gmuscato@diees.unict.it, neworlando@hotmail.com Cinzia Federico, Gaetano Giudice and Sergio Guerrieri Istituto Nazionale di Geofisica e Vulcanologia Sezione di Palermo Via U. La Malfa 153, 90146, Palermo, Italy c.federico@pa.ingv.it, ggiudice@pa.ingv.it, s.gurrieri@pa.ingv.it Abstract This paper describes the architecture of a UAV designed to study the composition of gas inside volcanic plumes. The main aim of the system is that of flying inside the plume (volcanic cloud) to directly analyze the concentration of the main components of the fumes. The system must be capable of flying autonomously at up to 4000m altitude with a payload of 5Kg using electric motors, to avoid contaminations with the gas sampling system, at a cruise speed of 40km/h. The complete architecture is presented together with the HMI: this can be used both for path planning and for navigation. 1. Introduction The study of volcanic activity is of primary importance because of the huge impact that eruptions have on human activities. At this time 1,500 volcanoes on Earth are potentially active, approximately 500 of which have been active during the 20th century and about 70 are presently erupting. At the beginning of the third millennium, 10% of the world population lives in areas directly threatened by volcanoes, without considering the effects of eruptions on climate or air-traffic for example. About 30,000 people have died from volcanic eruptions in the past 50 years, and billions of euros of damage have been incurred. Significant advances in eruption prediction and forecasting have been made in recent years [3]. At DIEES University of Catania, Italy we are conducting the ROBOVOLC project with the aim to develop robotic systems for the exploration and analysis of volcanic phenomena [2]. The first system developed has been a six wheeled rover to minimize the risk for volcanologists and technicians involved in work close to volcanic vents during eruptive phenomena [3]. It should be noted that observations and measurement of the variables relating to volcanic activity are of greatest interest during paroxismal phases of eruptions, which unfortunately are also the periods of greatest risk to humans. Other projects regard the development of an AUV (Autonomous Underwater Vehicle) for underwater volcano exploration and the development of an UAV (Unmanned Aerial Vehicle) for volcanic gas sampling that is the subject of this paper. The composition analysis of gasses emitted by volcanoes is crucial for volcanologists to understand volcanoes behaviours. Volcanic gas is quickly contaminated by the atmosphere and is thus empirically worthless to collect for analysis far from the eruptive vent. Some indirect mechanisms to obtain these measures exist already; in fact the chemical composition of the gas can be estimated using spectrographs, observing the plume from a safe place through a telescope. Since this system is not enough reliable, an aerial vehicle should be used to collect direct measures of the most important variables of the plume (concentration of HCl, SO 2 and CO 2, pressure, temperature, wind velocity and direction). Data collected by the robot will be used primarily to enhance knowledge of volcanic processes. For example close field data collected by the robot during eruptions will be used as input data for computer simulation of volcanic activity, to improve forecasts for long-lived volcanic phenomena, such as lava flow eruptions. There are two main barriers that do not allow the use of a classic remote controlled vehicle. The first one is the long distance between a safe place and the volcanic plume. Volcanic terrains are usually very rough in proximity of the craters and it is not simple to carry directly the vehicle on top of the volcano and to find there a suitable place for take-off. So the required distance for the teleoperations makes very difficult the adoption of a remote controlled flying vehicle. The second obstacle is given from the fact that the gas within the plume is usually very dense and does not allow a visual recognition of the vehicle when is flying inside it X/05/$ IEEE

2 (Fig. 1.). Consequently the development of an UAV that could perform autonomously part of the mission was fundamental. The aim of the proposed UAV is then to measure directly the composition of the gas without any human interaction. For this reason a HMI (Human Machine Interface) allows to choose the desired path fixing waypoints on a map. The next two sections describe the importance of this work and the state of the art. A typical mission and the requirements of the designed UAV are presented in section four. The architecture of the system, its control system and the HMI are described in section five. In section six some preliminary tests are also presented. The last section contains the conclusions of the paper. Figure 1. Example of a Plume of Etna Volcano. 2. Motivations for Volcanic Gas Sampling Measuring the composition of volcanic plumes allows the computation of volcanogenic fluxes for other volatiles by scaling to SO 2 concentration [17], [19], and provides insights into volatile degassing mechanisms during magma ascent and eruption [16], [18], [8]. Significant pressureand solubility-dependent changes in the chemistry of the exsolving magmatic fluid phase are expected during magma ascent toward the surface [13], [12]. Among the various plume volatiles, sulfur and halogens have been the object of numerous investigations because of (i) accurate detection by both in-situ and remote techniques, (ii) significant concentration contrast between plume and background air, and (iii) conservative behavior during plume dispersal (at least on a short-term scale). Carbon dioxide and water, whose melt solubilities are lower than SO 2 and halogen acids, are probably responsible for vapor exsolution from most magma at depth. However, there is convincing evidence that sulfur plays a major role in shallow-depth degassing of basaltic melts, and its significant solubility contrast with halogens makes S/Cl and S/F plume ratios particularly useful in modeling shallow degassing processes [14], [9], [8] which are of major interest because they are closely linked with eruptive phenomena. Measurements on the concentration ratios between different acidic gas species during the recent Etna eruptions [6], [7], [8] provided insights onto the depth of volatile exsolution and the evolution of volcanic activity. Similar measurements performed on Stromboli volcano during the eruption evidenced very high S/Cl ratios just before the 5 th April paroxysm and a gradual decrease parallel to the exhausting of the eruption and the resuming Strombolian activity [7]. Plume monitoring is accomplished by an association of in situ and remote techniques [15]. Since the 1980s, direct sampling of acidic gases in volcanic plumes has been undertaken by active methods, i.e., pumping plume air at a constant flow rate through base-treated filters in series [10]. In situ air sampling poses several practical problems, mostly because of risks involved in sample collection and the consequent difficulty of maintaining a high frequency of sampling. In general, a few discrete measurements (e.g., a few short measurement surveys per month) can be carried out on volcanic craters, and one must accept that these discrete measurements are representative of the composition of the plume over the medium or long term, which is a rough assumption in the light of the fluctuating nature of plumes. FTIR spectroscopy was later proposed as an efficient method for remote sensing of acidic plume gases [11]. Thus, despite the increasing development of continuous remote techniques for volcano plume monitoring, one main open challenge of future research is to develop in situ methods which are safer for volcanologists and in which sampling frequency is adequate to the time-scale of variability of the processes involved. The UAV-Volcano project could satisfy the need of performing measurements within the plume and near eruptive vents, without particular risks for the operators. The vehicle would contain light IR-spectrometer for CO 2 measurements and electrochemical cells for SO 2 determinations. 3. The State of the Art The robotic literature is rich of UAV; they have been used for several applications concerning the measure of air pollution and weather variables [4], [5]. However we found only one experiment similar to ours in the literature. It was a 2002 project funded by NASA, a collaboration between the University of South Florida, Southwest Research Institute, and the University of Miami. An unmanned aerial vehicle was planned to be used to fly inside volcanic gas plumes and directly sample and analyze volcanic gases without the need for the human

3 operators to visit volcano craters. Unfortunately at present, due to some failure in the preliminary tests, they have abandoned this design. 4. A UAV to analyze volcano plumes As it was previously mentioned the main aim of this project is that of analyzing gas composition of the volcanic plumes. Our system has been designed mainly for Etna volcano (3300m high); hence the UAV must be capable of flying up to 4000m over the sea level. At this altitude the wing lift is lower due to the rarefaction of the air, hence the wing incidence must be emphasized and a powerful engine is needed. The project requires also electric propulsion, this feature is very important to avoid the contamination of the gas sampled with the emissions of an internal combustion motor. Gas analysis systems are, often very slow, since they have to collect a sample of air and then executing the chemical analysis on it. The system adopted for this project has a sampling frequency of 1 Hz: for this reason, in order to collect several points during the plume crossing, it is very important to keep a cruise speed very low. The project specifications requires a cruise speed of 40Km/h, in fact, flying at this (relative) speed the system will collect a sample every about 11m. Moreover the sensors adopted for the gas analysis have some mechanical parts (e.g. electro valves) with a significant weight and the batteries packs are heavy, because they have to power the electric motor for about thirty minutes; therefore, a payload of 5Kg has been estimated for the final UAV, including the control and measuring systems and the batteries packs. The UAV take off is a very important phase. Unfortunately near the craters the terrain is often very rough (Fig. 2.); for this reason, the base station must be positioned some kilometres away. In our case, for example, the base station is scheduled to be positioned at Piano delle Concazze area, at an altitude of 2900m, that is enough flat to allow the take off from the top of a car but is far (about 2km straight line) from the central crater of the volcano where we want to sample the plume. The UAV should fly autonomously to reach the plume with an altitude drop of about 500m, move with a predefined trajectory inside the plume and then come back to the base station. The total travel distance will be less than 6km. Fig. 3. shows a scheme of the mission divided into 5 main phases: 1) Take-off; 2) Reaching the plume; 3) Flying inside the plume (measurement phase); 4) Approaching the landing area; 5) Landing. Figure 2. Typical terrain in proximity of a crater. It must be observed that the main part of the mission (Phase 3) is performed inside the plume with a consequent scarce visibility of the vehicle from the operator. Consequently at least from phase 2 to phase 4 the mission must be performed autonomously by the UAV following a pre-planned trajectory. ETNA Central Crater 4 Figure 3. Example of a mission. Particularly important is the data storage of the sensors measures. These, in fact, are stored both on an on board PC and on the base station PC. Indeed the data are transmitted regularly to the base station using a pair of radio modems, but, since the radio link could fail for long distances, the acquired measures are stored also on board. The project specifications are hence: Adoption of Electric motors for propulsion Autonomy of 30 min Payload of 5 kg Cruise speed of 40 km/h Maximum altitude 4000 m Working range 3 km Local and remote data log of the measures Autonomous system (Except take-off and landing) Path planning through Way Points Real time visualization on a user friendly GUI As regard the last point it must be remarked that since the final system will be used by not skilled operators, the

4 design of a user friendly HMI to plan the mission and to operate the vehicle, is also very important. 5. The Architecture of the system In order to analyse the main problems of this project and to test the measurement system and the avionics a simpler preliminary prototype of UAV has been designed. This first prototype has been built in May It is a 2:3 scale version of the final planned system. The main difference is the propulsion system that is a 2-stroke engine instead of the 2000W electric motor planned for the final platform. The choice of using an internal combustion engine for the first prototype has been done to simplify the test phase, since no battery recharge is needed; in fact, while the batteries of the radio control and of the internal electronic can assure some hours of autonomy, the motor batteries should be recharged every thirty minutes. The prototype has been built using wood and carbon fibre: a wingspan of 2 metres allows it to carry a 2kg payload, moreover the absence of the motor batteries imply that this payload is sufficient to test the science system. The final platform (Fig. 4.), in fact, with a wingspan of 3m has a payload of 5kg including the motor batteries. An embedded PC Pentium 166 in standard PC-104 has been mounted on board the UAV in order to manage the scientific sensors, the navigation and stability system, the measurement database and the radio communication with the base station. Figure 4. A picture of the final platform. One of the two serial ports of the PC is used for the radio modem, the other for the navigation and stability system; the parallel port is used, instead, to connect the scientific data acquisition board. The chosen HCl, SO 2 and CO 2 sensors are in 4-20mA standard, so a terminator resistor is used to convert the output current in a 0-5V tension which can be measured by the 12bit DAC of the acquisition board. This board is also used to measure the pressure, the temperature and also to start and stop the pump used by the gas sampling system. The prototype will carry only one of the chemical sensors due to the limited payload. Fig. 5. shows the architecture of the system. RC Rx 5 channel CO 2 SO 2 HC L Tem p RC-Tx Ap 40 Pres Pump ADC(12bit) Ground Pc Base Station Servo 1 Servo 3 UAV Servo 2 Servo 4 Rs-232 Pc 104 Par - I 2 C Rs-232 M odem Modem Rs-232 Figure 5. The architecture of the system

5 The navigation and stability system is a commercial platform called AutoPilot AP40 by UAV Flight Systems [20]; this system, controlled through a serial port, is directly connected to the servos, the motors and the RC receiver: using the integrated GPS it is able to drive the UAV along a predefined path. A Specific RC channel is dedicated to change the AP40 modality from UAV to PIC, i.e. from autonomous flight to RC flight. This feature is particularly useful for preliminary tests and for safer take off and landing. The Base Station is composed by a PC connected to the radio modem and a RC transmitter. During the mission the robot position can be followed in real time on a MAP of the GUI (see Fig. 6.) installed in the Base Station PC. This interface is used at the beginning of the mission to choose the desired path of the mission via waypoints, while it is used to monitor all the UAV variables during the mission: the battery levels, the UAV position, speed and orientation, the sensors measurements (the chemical composition of the air), the servos position, the current waypoint and so on. 6. Real Test Some preliminary tests of the autonomous flight system have been performed with the preliminary platform but due to the payload limits the onboard data-logger and radio-modem could not be integrated. The first test of the final platform have been performed in June 2005, the take off procedure have been performed from the top of a car, as shown in Fig. 8, at a cruise speed of 50km/h, in PIC mode (Pilot In Command). Since the terrain morphology on the Etna volcano is not suitable for the UAV wheels, this seems to be the cheapest solution, given that a 150m flat surface is available near the volcano. A launching pad is also under development and would not require the use of an expert pilot for this crucial procedure. Figure 8. First test of the final platform. Figure 6. The base station GUI. The preliminary Ni/Mh battery pack has assured two flights of 8 minutes. For reason of clarity only the first test will be reported here (Fig. 9), with a total travelled length of 8km. A maximum speed of 120km/h permits to reach the desired place (the plume) very fast, moreover the use of the flap can reduce the cruise speed to 40km/h, as required for the gas analysis. Actually, the autonomy does not conform to the specifications but a more expensive lithium battery pack can increase the autonomy to the desired value without increasing the weight. The transmission of the navigation variables allowed us to follow the UAV in the GUI, while the on board PC recorded it for post-elaboration purpose. During the cruise, a small path has been set using the GUI and followed using autonomous flight system. RC Antenna PC-104 Ap-40 GPS Antenna Acquisition Board Radio Modem Power Supply Figure 7. The navigation system.

6 Altitude [m] Long [m] + 520km UAV Path, System Test UAV Path Start Point End Point 0 Lat [m] km Figure 9. UAV path of the first test. The gas sampling system has been tested only off board, it will be integrated soon on the final platform. 7. Conclusion In this paper a new project for the automatic sampling and analysis of gas inside volcanic plumes has been presented. The UAV designed must be partially autonomous to perform the required missions. An analytical model of the vehicle and of the control system has been derived and tested in simulation. A first prototype of the system has been already realized and successfully tested while the final platform is under testing. The electronic control system and the measurement systems have been designed and built and are currently under test on the final system. The next phases of the project are the test of the sampling system on the final vehicle, the development of the launching pad and the autonomous flight tests with the end user (not expert pilot). The final tests on the volcanic area are planned for the beginning of the autumn References [1] D. Caltabiano, S. Guccione, D. Longo, G. Muscato, M. Coltelli, E. Pecora, A. Cristaldi, G.S. Virk, P. Sim, V. Sacco, P. Briole, A. Semerano, T. White, ROBOVOLC: A Robot for Volcano Exploration Result of first test campaign, Industrial Robots, Vol. 30, N.3, May [2] The ROBOVOLC project homepage [3] S. Alwyn, "La Catastrophe: The Eruption of Mount Pelee, the Worst Volcanic Disaster of the 20th Century", Oxford University Press, [4] [5] [6] Aiuppa, A., Federico, C., Giudice, G., Gurrieri, S., Paonita, A., Valenza, M. Plume chemistry provides insights into the mechanisms of sulfur and halogen degassing at basaltic volcanoes. Earth Planet. Sci. Lett. 222(2) (2004) [7] Aiuppa, A., Federico, C. Anomalous magmatic degassing prior to the 5th April 2003 paroxysm on Stromboli. Geophys. Res. Lett., 31, (2004) L14607, doi: /2004gl020458, 2004 [8] Aiuppa, C. Federico, A. Paonita, G. Pecoraino, M. Valenza, S, Cl and F degassing as an indicator of volcanic dynamics: the 2001 eruption of Mount Etna, Geophys. Res. Lett (2002) /2002GL [9] M. Edmonds, D. Pyle, C. Oppenheimer, A model for degassing at the Soufrière Hills volcano, Montserrat, West Indies, based on geochemical data, Earth. Planet. Sci. Lett. 186 (2001) [10] Finnegan, D.L., J.P. Kotra, D.M. Hermann, and W.H. Zoeller, The use of 7 LiOH-impregnated filters for the collection of acidic gases and analysis by instrumental neutron activation analysis, Bull. Volcanol., 51, 83-87, [11] Francis, P., M.R. Burton, and C. Oppenheimer, Remote measurements of volcanic gas compositions by solar occultation spectroscopy, Nature, 396, , [12] W.F. Giggenbach, Chemical composition of volcanic gases, in: R. Scarpa and R.I. Tilling (Eds.), Monitoring and Mitigation of Volcanic Hazards, Springer Verlag, Berlin-Heidelberg, [13] P. M. Nuccio, A. Paonita, Magmatic degassing of multicomponent vapors and assessment of magma depth: application to Vulcano Island (Italy), Earth Planet. Sci. Lett. 193 (2001) [14] M. Pennisi, M.F. Le-Cloarec, Variations of Cl, F, and S in Mount Etna s plume, Italy, between 1992 and 1995, Geophys. Res. Lett. 103 (1998) [15] Stix J., and Gaonac h, H., Gas, plume and thermal monitoring, in Encyclopædia of volcanoes, edited by H. Sigurdsson, Academic Press, , [16] R.E. Stoiber, S.N. Williams, B. Huebert, Sulphur and halogen gases at Masaya caldera complex, Nicaragua: total flux and variations with time, J. Geophys. Res. 91 (1986) 12,215-12,231. [17] R. Symonds, W.I. Rose, M.H. Reed, Contribution of Cl- and F-bearing gases to the atmosphere by volcanoes, Nature 334 (1988) [18] R. Symonds, W.I. Rose, G.J.S. Bluth, T.M. Gerlach, Volcanic-gas studies: methods, results and applications, in: M.R. Carroll, J.R. Halloway (Eds.), Volatiles in Magmas, Rev. in Mineralogy, 30, 1994, pp [19] S.N. Williams, S.J. Schaefer, V. Calvache, D. Lopez, Global carbon dioxide emission to the atmosphere by volcanoes, Geochim. Cosmochim. Acta 56 (1992) [20]