Triangulation based Fusion of Ultrasonic Sensor Data

نویسندگان

  • Olle Wijk
  • Patric Jensfelt
  • Henrik I. Christensen
چکیده

Ultrasonic sensors are still one of the most widely used sensors in mobile robotics A notorious problem in the use of sonar data is the lack of good spatial resolution which typically results in a high uncertainty in the resulting map of the environment In this paper a triangulation technique is used for ltering of data so as to obtain an improved grid map of the environment The basic technique is described and it is outlined how it can be used for identi cation of natural landmarks Introduction Almost all mobile platform are equipped with ultra sonic sensors sonars that are used for local mapping of the environment Examples of such work is illus trated in It is however characteristic for much of this work that the limited spatial resolution of the sonars results in fuzzy maps that require a large number of sonar readings or signi cant post processing In this paper a ltering technique that enable ro bust estimation of features in the environment is pre sented The technique relies on triangulation of sonar readings recorded from di erent positions for estima tion of the location of structures in the environment The triangulation technique provides a fast method for estimation of structures When combined with a voting method the result is an improved spatial reso lution and a more robust map of the environment Initially the physics of the sensors is brie y re viewed the underlying assumptions are explained to gether with the terminology used The basic technique is then developed in section and Section then describes how we can apply the method when building grid maps Finally it is explained how the technique can be used for robust estimation of features in sec tion The sensor The ultrasonic sensor transmits a short ultrasonic pulse and the time for return is then measured Based on knowledge of the transmission speed of sound in air m s and the time of ight it is trivial to com pute the distance to the target that re ected the pulse The sound pulse does not propagate along a straight line but is propagated within a cone The most fre quently used sonar sensor the Polaroid environmental transducer has a beam width of the half beam width This propagation pattern must be taken into account when the sensor is used for mapping of the environment as it is impossible from a single sensor reading to determine the position of the target within the beam Assumptions Throughout the rest of the paper the following assumptions will be made i when the robot is within a small radius the same structure in the environment will give rise to the re ection i e only a single major target returns the signal ii the size of the target is small i e it is the same point on the target that returns the signal iii the beam of the sonar is assumed to be a conic Only a two di mensional map of the environment is constructed and it is therefore assumed that the shape of the beam is a circular arc of Terminology It is assumed that the mobile plat form has m sonars that are distributed on the robot at known locations We use the following terminology in the algorithms and the derivation of the underlying equations T Estimated target data represented as T f xT yT nt xs ys g where xT yT is the es timated location of the target nt is the number of triangulations that have been done to obtain xT yT and xs ys is one of the nt sensors positions that have been used in the triangulation of the target Ri Reading from sonar i represented as Ri xsi ysi i ri where is the direction of the sonar with respect to the x axis of a local coordi nate system and r is the range reading Triangulation Given the assumption that the same target will re turn a signal if two sonars are red from almost the same position it is possible to estimate the target lo cation using simple triangulation Given two sensors readings recorded from di erent positions a situation as shown in gure arise r sensor 1 sensor 2 target

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تاریخ انتشار 1998