Distance and Orientation Measurement of a Flat Surface by a Single Underwater Acoustic Transducer

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Distane and Orientation Measurement of a Flat Surfae by a Single Underwater Aousti Transduer Vinent Creuze To ite this version: Vinent Creuze.

EUSIPCO 11: 19th European Signal Proessing Conferene, Aug 11, Barelona, Spain. pp.179-179. lirmm-6177 HAL Id: lirmm-6177 https://hal-lirmm.sd.nrs.

they are published or not. The douments may ome from teahing and researh institutions in Frane or abroad, or from publi or private researh enters.

1 Distane and Orientation Measurement of a Flat Surfae by a Single Underwater Aousti Transduer Vinent Creuze To ite this version: Vinent Creuze. Distane and Orientation Measurement of a Flat Surfae by a Single Underwater Aousti Transduer. EUSIPCO 11: 19th European Signal Proessing Conferene, Aug 11, Barelona, Spain. pp lirmm-6177 HAL Id: lirmm Submitted on 9 Sep 11 HAL is a multi-disiplinary open aess arhive for the deposit and dissemination of sientifi researh douments, whether they are published or not. The douments may ome from teahing and researh institutions in Frane or abroad, or from publi or private researh enters. L arhive ouverte pluridisiplinaire HAL, est destinée au dépôt et à la diffusion de douments sientifiques de niveau reherhe, publiés ou non, émanant des établissements d enseignement et de reherhe français ou étrangers, des laboratoires publis ou privés.

2 DISTANCE AND ORIENTATION MEASUREMENT OF A FLAT SURFACE BY A SINGLE UNDERWATER ACOUSTIC TRANSDUCER Vinent Creuze LIRMM, CNRS / University Montpellier 161 rue Ada, 395, Montpellier, Frane phone: + (33) , fax: + (33) , reuze@lirmm.fr web: ABSTRACT In this paper, we detail a new method that allows measuring simultaneously distane and orientation of a flat horizontal or vertial surfae (e.g. wall, flat bottom) with a single aousti transduer. A single mono-frequeny ping is neessary. Then the analysis of the peaks in the reeived eho is suffiient to find out both distane and orientation. This method does not require any a priori knowledge of the target baksattering properties. Its advantages are: a very high sampling rate, the smallest and heapest sensor, and low aousti interferenes with other aousti devies (single frequeny, short ping). We present theoretial aspet, simulation results, and experiments. 1. INTRODUCTION Many appliations of underwater vehiles (remotely operated or autonomous) require the orientation and distane measurement of a flat surfae in order to ahieve partially / fully autonomous survey. For instane, video surveys of hydrauli dams by underwater vehiles allow raks or weaknesses detetion. Many interesting sensing approahes have been developed in the literature. Some rely on digital imaging assoiated with laser triangulation. However, this method suffers from range limitation due to the possible turbidity of the water. Other methods rely on aousti sensing. Then distane and orientation of the wall are obtained by ahieving segmentation of the image aquired by setor sanning sonars [1] [], or by using diretly the beams of a Doppler Veloity Log [3]. These approahes are very well suited to the underwater environment but need expensive and quite big sensors (more than 5k, and sizing at least m x 6m²). Other tehniques use differene of time-offlight with two or more transduers, and have been implemented on underwater vehiles as well as on terrestrial vehiles []. There also exist tehniques based on a single transduer. Some rely on template mathing or spetral analysis [5], but require a high omputational power. An original single transduer based tehnique uses the measurement of the time differene of the zero-rossing points of the eho of the surfae. It allows auray of the order of 1 within a range [6]. The new method that we propose in this paper is based on the diretivity pattern of a single transduer and allows orientation detetion within a range with the same auray. In addition, it requires only a short ping emission at a single frequeny. The proessing of the reeived eho allows us to find out both orientation and distane of a flat surfae without a priori knowledge of its baksattering properties. In omparison with other aousti methods, ours offers the following advantages: Highest sampling rate (only one ping is required to ompute both distane and orientation) and shortest aquisition time (in omparison with setor sanning sonar for instane). Single frequeny emission thus minimizing interferenes with other aousti devies (e.g. modem). Low power onsumption, small omputational ost. The sensor is basi, heap and small (in omparison with arrays of sensors). No rotating part (e.g. rotating antenna). First, we briefly remind the priniples of diretivity patterns and underwater sound baksattering. Then, we will detail the method and present an algorithm for automati omputation of the reeived eho. In the last part, simulation and experimental results will be detailed. Before being published, this work has led to the filing of the European Patent appliation n EP , on January 17th 11, entitled "Method and devie for measuring distane and orientation using a single eletro-aousti transduer".. PROBLEM STATEMENT.1 Problem statement We onsider the situation of an underwater Remotely Operated Vehile (ROV) aiming to navigate at onstant distane and orientation of a vertial wall (Fig. 1). d denotes the distane between the wall and the transduer along the u axis. ψ is the wall s orientation measured between the v axis and the wall. The orientation of the line of sight of the transduer (= axis of the main lobe) regarding the vehile axis u is denoted γ and is supposed to be onstant (mehanially fixed) and known. The transduer is dis-shaped but ould be replaed by any other type of transduer as long as its diretivity pattern ontains at least two lobes. The pithing angle of the ROV is supposed to be null, i.e. the bodyfixed axes u and v remain horizontal.

3 ROV vehile C D top view v The baksattering strength S s for the surfae reverberation of the wall (or of the bottom in the ase of bottom traking) is given in db. m and is defined as the ratio, in deibel units, of the baksattered intensity produed by a unit area of the wall to the inident intensity. For inidene angle φ i beyond 5, a good approximation of the baksattering strength S S is given by Lambert's law [8]: B A max d u s O S S = S + log os φ i () where S is a onstant parameter depending on the wall harateristis, and φ i is the inidene angle. transduer For inidene angles φ i less than 5, the baksattering strength S S an be approximated by (linear approximation): S S = S N + φ i φ S S N + log os φ (3) Figure 1 Top view of the underwater vehile (ROV) navigating along the vertial wall. The transduer is plaed in O. (O,u,v) is the body-fixed frame of the ROV. Fig. - Diretivity pattern of a dis-shaped transduer (radius=13.5 mm, f= khz). The sidelobe attenuation is -17.7dB.. Diretivity pattern and baksattering It is well-known that the diretivity pattern b θ of a dis shaped-transduer is given by: b θ = J 1 πf r sin θ πf r sin θ where J 1 (. ) is the first order Bessel funtion, f is the aousti wave frequeny ( khz), r is the transduer radius, and is sound speed in water (15 m. s 1 ) [7]. A polar representation of the diretivity pattern in deibels highlights the main lobe and the sidelobes (Fig. ). In the following, sidelobe will designate only the first sidelobe (17.7dB attenuation). Others are weak enough to be negleted (their attenuation is larger than db). The sidelobe has usually no effet beause it is very weak and hits the target (e.g. seabed in the ase of eho-sounder) with large inidene angles. (1) where S N is the baksattering strength for normal inidene and φ is alled the transition diretion..3 Two lobes, two peaks If the transduer is tilted, the inidene angle of the main lobe is larger than the one of the sidelobe (Fig. 1). Then, the baksattering strength is lower for the main lobe signal than for the sidelobe. Moreover, due to the differene in length of the sound paths, transmission losses TL (mainly spherial spreading, as the distane is short) are lower for the sidelobe than for the main lobe. Consequently, in spite of diretivity attenuation, the global attenuation level H makes the sidelobe eho level signifiant in omparison with the main lobe eho level. Moreover, as the distane to the wall is smaller for the sidelobe than for the main lobe (Fig. 1), the eho of this latter will be reeived after the one of the sidelobe. Thus, the baksattered aousti eho onsists of two separated parts orresponding first to the sidelobe and seondly to the main lobe (Fig. 6). In the following, we will explain how the time of ourrenes of the maximum of both ehoes an lead to the omputation not only of the distane but also of the orientation. 3. PRINCIPLE OF DISTANCE AND ORIENTATION MEASUREMENT 3.1 Intuitive approahes In order to find out the orientation of the wall, some approahes onsist in analyzing the shape of the seond part of the eho (template mathing). However, the baksattering parameter S is a priori unknown. It depends on the refleting surfae (onrete wall, metalli hull, seabed ) and strongly affets the shape of the seond part of the eho. Thus, without a good knowledge of S, the shape analysis of the seond part of the eho will not lead to a preise estimation of the orientation of the wall. Another approah ould be to onsider that the maximum of intensity of the first part of the eho orresponds to the axis of the sidelobe, and the seond part of the eho orresponds to the axis of the main lobe. Thus, one ould easily

4 reover distane and orientation by simple trigonometri equations. This is wrong. In fat, the eho of sidelobe is so narrow that its maximum ould be onsidered to orrespond to its axis, but, due to spherial spreading and baksattering angle dependeny, the maximum of intensity of the seond part of the eho does not orrespond to the axis of the main lobe. In the following, we will explain how to determine in whih diretion of the beam the maximum of sound eho is reeived. This is also alled the highest eho diretion in Fig. 1. max (in degrees) Expressing the maximum of the eho as a funtion of orientation ψ We propose to study the eho intensity as a funtion of θ, where θ is an angle differene with the axis of the main lobe (Fig. 1). In the following, ψ is the wall orientation angle and d is the distane between the transduer and the wall along the u axis of the vehile. The inidene angle in the θ diretion is denoted φ i (θ), and the distane between the transduer and the wall along θ diretion is r θ. Thus, we have the following relationships: φ i = γ + θ ψ () and os (ψ) r(θ) = d. (5) os ( γ+θ ψ) Let us now determine H(θ, d, ψ), the funtion defining the eho attenuation in θ diretion. Absorption will be negleted within transmission losses as its effet is very small in omparison with spherial spreading (the measured distanes in this appliation are less than 15 meters). Thus, onsidering the two-way onfiguration of our transduer, the two-way spherial spreading losses, and the baksattering (angle of inidene of the main lobe is beyond 5 so Lambert s law applies), we have: H θ, d, ψ = J 1 πf r sin θ πf r sin θ 1 os (ψ) d os ( γ+θ ψ) where i f = S, with S defined in.. Then, one an see that the derivative i f os γ + θ ψ (6) dh θ,d,ψ dθ neither depends on the distane d, nor on the baksattering parameter S, whih means that the angle θ max orresponding to the maximum of H(θ, d, ψ) depends only on ψ (other parameters f, r,, and γ are known onstant values). Thus θ max an be expressed as θ max = g ψ, where g is a nonlinear fun- dh θ,d,ψ tion. Of ourse, = has no simple analytial solution, thus g annot be expressed analytially. However, it is dθ very easy to numerially ompute g and interpolate it by a seond order polynomial alled g (Fig.3). As it depends only on the measured diretivity pattern (it does not depend on d or S ), g has to be omputed only one time (off-line). One we know g, it beomes easy to estimate the wall orientation. The use of the interpolation g instead of g will allow easier implementation (on DSP) of the algorithm desribed in the following. - - wall orientation (in degrees) Figure 3 With the following set of parameters (f = khz, r = 1m, = 15 m. s 1, γ = 7. 8 ), funtion θ max = g ψ (blue dots) an be interpolated (solid red line) by g ψ =. 17ψ +. 13ψ It has also to be notied that the assumption of Lambert s law inidene of the main lobe is true only if the orientation of the wall with respet to the v axis of the ROV is greater than - (this mainly depends on the aperture of the main lobe and on the γ angle). As orientation and distane of the ROV with respet to the wall is lose-loop ontrolled, ψ an be assumed to be maintained lose to. Initialisation of this proess is out of the sope of this paper, but one an briefly say that this is ahieved by a 36 rotation of the ROV. In fat, when ψ is out of the range [- + ], there is either only one eho (superimposition) or no eho (sound is not emitted in the wall diretion). 3.3 Orientation and distane Angles shown on fig. 1 an be expressed as follows: ABO = π ψ + γ θ s (7) ADO = π ψ + γ + θ max (8) OAD = π + ψ (9) where θ s is the angle between the axis of the sidelobe and the axis of the main lobe (obtained from the diretivity pattern of the transduer). Then, we have the following equations: d sin ABO =.t s sin OAD d sin ADO =.t m sin OAD () (11) where t s and t m are respetively the times of ourrene of the maxima of the two parts of the reeived eho (Fig. ). As denotes the sound speed,. t denotes the distane between the soure and an impat point.

5 Equations () and (11) lead to: t s os ψ γ + θ s t m os γ ψ + θ max = (1) Replaing θ max by g ψ in (1) gives us the equation to solve to find out ψ. t s os ψ γ + θ s t m os γ ψ + g ψ = (13) Although there is no analytial solution, equation (13) an be easily numerially solved. One orientation ψ has been determined, the distane d is omputed by applying () or (11). 3. Ping duration The duration of the aousti ping to be emitted depends on the distane d. The loser from the surfae we are, the shorter must be the ping in order to keep the ability to distinguish both peaks. In fat the shape of the peaks t s and t m depends on the ping duration, and a too long ping would lead to superimposed ehoes of both lobes.. AUTOMATIC SIGNAL PROCESSING Automati proessing of the reeived aousti eho an be implemented by the following algorithm: Step 1 Low-pass filtering of the eho to extrat the envelope of the eho. Step Deteting the summits loations t s and t m, for instane by omputing the first derivative of the eho envelope. Step 3 Numerially solving (13), for instane by omputing all the values of the left part of the equation starting from ψ min (the smallest detetable value within the angular range of this system) until its sign hanges (of ourse more effiient methods an be applied depending of the omputing apabilities of the hosen devie). Step One orientation ψ has been omputed, it is easy to ompute the distane d by using () or (11). 5. SIMULATION RESULTS 5.1 Simulation environment Simulations have been onduted with Matlab TM environment. The refleting surfae is meshed. Eah ell of the grid is far smaller than the produt. τ, where τ is the ping duration and is the sound speed. All the ontributions of the mesh are weighted (depending on their attenuation), time delayed (depending on their position) and then added to ompute the reeived eho. Comparison of the simulation results with experimental aquisitions shows the ability of the simulator to reprodue the eho shape. As the eho intensity strongly depends on both sensor and environmental parameters, the eho intensity plotted on simulations has no unit (they should be in Pa or db). In fat, on the following plots, the intensity sale is relative and should be multiplied by an appropriate onstant to get the true intensity. As we only fous on the shape of the eho and the arrival time of its peaks, the depited intensity will be only relative. 5. Simulation results Simulations have shown that ψ an be properly estimated within the range [- + ]. Depending of baksattering properties of the refleting surfae and depending on the diretivity pattern of the transduer, this latter range value ould be extended. However for autonomous navigation purposes, the vehile orientation has to be kept onstant (with respet to the traked surfae). One the sensor is well oriented, the measurement range of our system is suffiient for servoing the vehile position. If a wider measurement range is neessary, the transduer ould be atuated by a servomotor. Figure shows simulation results with the following set of parameters: f= khz, r = 1m, = 15 m. s 1, γ = 37.8, d = m, ψ = + (first plot) or ψ = (seond plot). One an see the simulated reeived eho (in blue solid line) and the times of arrival of the two maxima (in red). Applying the automati signal proessing algorithm desribed in leads to the following results: d = 9.9m, ψ = +9.9 (first plot) and d =.8m, ψ = 9.7 (seond plot). For values smaller than -, ehoes of the two lobes our too losely or are superimposed (Fig. 5). For values bigger than +, the intensity of the sidelobe eho dereases and it beomes harder to detet its maximum. The errors measured during simulations (within the range - + ) are the following: distane error < %, orientation error < EXPERIMENTS We are urrently onduting experiments of the system (pool tests). First results onfirm the simulation results but the number of experiments is not yet suffiient to give a preise experimental estimate of the measurement errors. Figure 6 shows an aousti aquisition made under the following onditions: d =.m, ψ = +5. The values obtained by omputing the time of arrival of the two peaks are d =.18m, and ψ = +.9.

6 eho intensity eho intensity eho intensity measured eho intensity 3.5 ψ = t m t s Figure Simulations of aousti ehos (f= khz, r = 1m, = 15 m. s 1, γ = 37.8, d = m, ψ = or ). The first small peak orresponds to the eho of the sidelobe, while the seond peak orresponds to the eho of the main lobe ψ = ψ = Figure 5 Simulation of an aousti eho outside the range (f= khz, r = 1m, = 15 m. s 1, γ = 37.8, d = m, ψ = 5 ). Both peaks are superimposed and thus annot be distinguished. t s t m 1 3 x -3 Figure 6 Experimental aquisition of an aousti eho (in a pool). Peaks an easily be deteted and lead to an appropriate estimate of distane and orientation of the bottom of the pool. 7. CONCLUSION More experiments will be onduted in the next months in order to verify the experimental auray of the system. This work has been presented for the D geometry of a vertial wall. The ase of non vertial surfaes (3D) is under study. However, as the above-desribed priniples also apply in the air, many appliations exist for terrestrial (e.g. wall following) or aerial robotis (floor or eiling following). REFERENCES [1] Y. Petillot, I. Tena Ruiz, and D. M. Lane, Underwater Vehile Obstale Avoidane Using a Multi-Beam Forward Looking Sonar, IEEE Journal of Oeani Engineering, Vol. 6, no., pp. -51, April 1. [] J.J. Leonard and H.F. Durrant-Whyte, Direted Sonar Sensing for Mobile Robot Navigation, ser. The Kluwer International Series in Engineering and Computer Siene, Norwell, MA: Kluwer, 199. [3] R. Damus, S. Desset, J. Morash, V. Polidoro, F. Hover, C. Chryssostomidis, J. Vaganay and S. Willox, A new paradigm for ship hull inspetion using a holonomi hoverapable AUV, Informatis in ontrol, automation and robotis I, Springer Netherlands, 3, pp. 195-, 6. [] T. Yata, A. Ohya, S. Yuta, A fast and aurate sonar ring sensor for a mobile robot, in Pro. IEEE Int. Conf. Robot. Autom., pp , [5] T. Yata, S. Yuta, Measurement of refleting diretion from the enter frequeny of the eho in ultrasoni sensing, Advaned Robotis: International Journal of Robotis Soiety of Japan, Vol. 11, n 3, pp 69-83, [6] T. Yata, L. Kleeman, and S. Yuta, Wall Following Using Angle Information Measured by a Single Ultrasoni Transduer, in Pro. of the 1998 IEEE International Conferene on Robotis and Automation, pp , [7] R.J.Urik, Priniples of Underwater Sound, Peninsula Pub, [8] X. Lurton, An introdution to Underwater Aoustis Priniples and Appliations, Springer Praxis Books, 3.

A Spatiotemporal Approach to Passive Sound Source Localization

A Spatiotemporal Approach to Passive Sound Source Localization A Spatiotemporal Approah Passive Sound Soure Loalization Pasi Pertilä, Mikko Parviainen, Teemu Korhonen and Ari Visa Institute of Signal Proessing Tampere University of Tehnology, P.O.Box 553, FIN-330,