SE1650023A1 - A method and a system for navigating a self-propelled robotic tool - Google Patents

Abstract

The present disclosure relates to a method and a system e.g. for mowing grass using a self-propelled robotic tool (14). The robotic tool (14) receives a first signal from a beacon (40) and determines, based on this signal, a distance from the robotic tool to the beacon. The robotic tool moves, for instance based on odometry data generated by the robotic tool itself, along a circular arc (60) corresponding to a circle with a radius equal to the determined distance, and subsequently receives later signals from the beacon and determining the current distance from the beacon to the robotic tool to adjust its direction to maintain a is constant distance to the beacon over a period of time. This makes it possible to mow the grass in an area in a structured and efficient way.

Description

A |\/IETHOD AND A SYSTEM FOR NAVIGATING A SELF-PROPELLEDROBOTIC TOOL Field of the inventionThe present invention relates to a method of navigating a self-propelled robotictool, and a corresponding system.

Background of the invention Self-propelled robotic tools are widely used to perform maintenance opera-tions within a predetermined work area. By way of example, robotic lawn mowers areused for autonomously cutting lawns within a predetermined work area to be mowed.Boundary wires are typically used to delimit the work area, and the robotic lawnmower is typically arranged to move in a randomly generated irregular pattern toensure complete coverage of the working area. However, when the work area has anirregular or complex shape, some portions of the work area may be mowed lessfrequently than others, or not at all. Other exemplary considerations in lawnmowerdesign are cost, complexity, reliability, and ease of use.

US 2015/0234385 A1 discloses a systematic navigation method according towhich a lawn mower navigates within boundaries defining an area to be mowed, anduses grass length sensors to follow an edge between cut grass and un-cut grass.There is however a need for a less complicated and more reliable method of navigat- ing.

Summarv of the invention lt is an object of the present invention to solve, or at least mitigate, parts or allof the above mentioned problems. To this end, there is provided a method of navi-gating a self-propelled robotic tool, which method includes the robotic tool receiving afirst signal from a beacon; determining, based on the received signal, a distance fromthe robotic tool to the beacon; and moving, along a circular arc with a curvature cor-responding to a circle having a radius substantially equal to the determined distanceand with the beacon in its center.

Thanks to this method, the robotic tool can navigate over a work surface in asystematic way, e.g. in circles with increasing or decreasing radius, using relativelyuncomplicated navigating means and based on only the distance to the beacon.

The robotic tool may receive second and later signals from the beacon todetermine the current distance from the robotic tool to the beacon, and may adjust its 1 heading based on the determined current distance, such that the distance to thebeacon is kept substantially constant along the circular arc. This improves the navi-gating precision which means that the requirements on the robotic tool”s internaldead reckoning navigation means can be more relaxed. lf possible, the robotic tool may move along the circular arc until a completecircle around the beacon has been covered, however, the robotic tool may add anangular overlap to the complete circle to make sure that no area on the circle is leftunprocessed.

Once a circle is completed, with or without angular overlap, the robotic toolmay move towards or away from the beacon to increase or decrease the distance tothe beacon. lf the robotic tool is used to mow grass, the resulting increment or decre-ment may be the cutting width of the robotic tool minus a radial overlap. The movingof the robotic tool is then repeated along a circular arc corresponding to a circle hav-ing a radius equal to the new distance to the beacon.

The robotic tool may move along a circular arc until, when the robotic tool hasa first heading, a limitation is detected, which prevents the robotic tool from movingfurther. The robotic tool then increases or decreases the distance to the beacon,wherein the increment or decrement may be the cutting width of the robotic toolminus a radial overlap, turns to a second heading at 180 degrees from the first head-ing, and resumes the cutting in the second heading along a circular arc correspond-ing to a circle having a radius substantially equal to the new distance to the beacon.

The angular displacement with regard to the beacon may be determined usingdistance dead reckoning. The angle can be calculated from the covered circular arcand the distance to the beacon. This means that the robotic tool can establish acomplete outline of a work area despite that the only external input comes from thebeacon.

The above-mentioned radial overlap may include a varying component, forinstance randomly varying. This means that systematic patterns resulting from therobotic tool operation can be blurred to some extent, for instance achieving a lawnwith a more natural appearance.

The robotic tool may determine whether a limitation is either detected at anangular difference exceeding a threshold angle from a previously detected limitation,or whether the preceding or succeeding arc is free from such limitations. lf either of those conditions apply, the robotic tool may initiate a separate area search operation.

Such an operation may involve following the limitation to determine whether a work2 area portion exists beyond the limitation, with the latest determined distance to thebeacon, i.e. when the limitation was found. Such a measure may allow the robotictool to discover the entire work area, even if the work area”s layout is complicated. lf a signal from a beacon temporarily cannot be received, the robotic tool maycontinue along its present circular arc using dead reckoning, and may correct anyerror in the distance to the beacon once reception is resumed. This allows the robotictool to navigate behind obstacles that prevent the reception of beacon signals.

The movement along the circular arc may be accomplished by driving rightand left wheels of the robotic device with different speeds or by turning repeatedly toaccomplish a polygon that provides an approximate circle.

The robotic tool may have a maximum and/or a minimum allowed distance tothe beacon and the robotic tool may be configured to turn when reaching the maxi-mum allowed distance. This allows the beacon to be used also to limit the work areaof the robotic tool.

The present disclosure also contemplates a system for grass mowing, includ-ing a self-propelled robotic tool for mowing grass and a beacon transmitting signalsto the robotic tool, where the robotic tool includes software and/or circuitry configuredto carry out the method defined above. lf the system comprises a charging station,the beacon may be arranged integrated with or wired to the charging station. How-ever, in many cases it may be advantageous to arrange the beacon remote from thecharging station.

Brief description of the drawinqs The above, as well as additional objects, features and advantages of thepresent invention, will be better understood through the following illustrative and non-limiting detailed description of preferred embodiments of the present invention, withreference to the appended drawings, where the same reference numerals will beused for similar elements, wherein: Fig 1 is a diagrammatic view in perspective of a robotic tool system.

Fig 2 illustrates a plan view of the underside of a robotic tool.

Fig 3 schematically illustrates functional blocks in a robotic tool.

Fig 4 shows the general layout of a work area where a robotic tool navigatesusing a beacon.

Figs 5-8 illustrate portions of the work area of fig 4 and different navigationscenarios.

Fig 9 illustrates a beacon where the work area is defined by different distancesto the beacon.

Fig 10 illustrates a basic flow chart of a navigation method.

Detailed description of the exemplarv embodiments Fig. 1 schematically illustrates an overview of an area treatment system 10configured to perform a task within a work area 12 such as a garden. The work areamay be limited by a guide wire 11. The area treatment system 10 comprises a self-propelled robotic tool 14 and a base station 16. As primarily described herein, therobotic tool 14 may be a robotic lawnmower working by cutting grass in a heading 15direction and having a cutting/processing width. However, the present disclosure mayalso be useful in connection with robotic tools configured as golf ball collecting toolsor any other type of robotic tool that is required to operate over a work area in amethodical and systematic or position oriented manner. ln particular, the teachingsherein may be of particular use in robotic tools configured to execute a task over anarea to be treated, wherein a full or at least predetermined coverage of the area to betreated is desired.

Fig. 2 illustrates the self-propelled robotic tool 14 as seen from below. Therobotic tool is provided with wheels 18, 20 for moving within the work area 12 to betreated. ln the example of Fig 2, the robotic tool 10 has two swivelling front wheels 18and two rear wheels 20. Typically, at least one of the wheels 18, 20 is connected to amotor, such an electric motor, either directly or via a transmission (not illustrated).The robotic tool 14 also comprises at least one tool element configured to performthe task on the area to be treated. The tool element may be a grass cutting device,such as a cutting blade 22, which may be rotatable about a vertical axis.

Fig. 3 illustrates functional blocks of the robotic tool 14. ln the example of Fig.3, each of the rear wheels 20 is connected to a respective electric propulsion motor24. This allows for driving the rear wheels 20 independently of one another, enablinge.g. sharp turning of the robotic tool 14. The robotic tool 14 further comprises a con-troller 26. The controller 26 may be connected to sensors, actuators, and com-munication interfaces of various kinds, and may be implemented using central processing unit executing instruction stored on a memory 28. Needless to say, 4 different combinations of general and application-specific integrated circuits may beused as well as different memory technologies. ln general, the controller 26 isconfigured to read instructions from the memory 28 and execute these instructionspossibly in view of different sensor signals to control the operation of the robotic tool14. Typically, the controller 26 is configured to, based on the instructions, control therobotic tool in an autonomous or semi-autonomous manner, i.e. with no, or onlyoccasional, instructions from a human operator. The controller 26 also controls theoperation of a cutter motor 30, which is configured to drive the cutting blade 22 (Fig2).

A wireless transceiver 32 may be connected to the controller 26, and allowsthe controller 26 to communicate with the base station 16 or any other device, suchas a remote control or a smart phone (not shown). ln the present disclosure, thetransceiver 32 may be used to receive signals from a beacon.

The robotic tool 14 further comprises a navigation system 34. ln the illustratedexample, the navigation system 34 comprises an inertial navigation device 36, suchas an accelerometer or a gyroscope, and a magnetic field sensor 38 configured todetect a magnetic field emitted by a guide wire on the ground. A guide wire 11 (cf. fig1) may be used for defining the boundaries of the area 12 to be treated, or to other-wise provide a reference to assist the robotic tool 14 to navigate. The inertial naviga-tion device 36 allows the robotic tool 14 to keep track of any movement within thearea 12 to be treated. The inertial navigation device may be supplemented by a com-pass (not shown), to provide basic orientation information that may compensate forany drift of the inertial navigation device 36.

The controller 26 also controls the propulsion motors 24, thereby controllingthe propulsion of the robotic tool 14 within the area 12 to be treated. The propulsionmotors 24 may be stepper motors, allowing the controller 26 to keep track of therespective number of turns of the motors 24, and thereby also the distance travelledby the robotic tool 14, as well as any turning angle of the robotic tool 14 when themotors 24 are operated at different speeds or in reverse directions. ln this respect,the propulsion motors 24 operate as odometers. Alternatively, the wheels 20 may beprovided with odometer indexers configured to provide feedback to the controller 26about the number of turns of each motor 24. Navigation information from the naviga-tion system 34 and the wheels/odometers 24 is fused in the controller 26 to providean accurate position indication.

The controller 26, navigation system 34, transceiver 32, and electric motors24, 30 are powered by a battery 40. The robotic tool 14 is configured to navigate tothe base station 16 on a regular basis, and/or whenever the battery charge is runninglow, in order to dock with the base station 16 for recharging the battery 40. The basestation 16 may be connected so as to receive power from the electric power grid.

The present disclosure relates to an improved method for navigating the ro-botic tool over the work area. As compared to a navigation method where the robotictool moves randomly over the work area, a systematic way of covering the work area is much more efficient, and therefore the robotic tool may handle a greater work area.

Fig 4 schematically illustrates the improved navigation method.

A beacon 40 is used to assist the robotic tool. As illustrated such a beacon 40”could be arranged integrated with or close to and wired to the base station 16. Thiscould be advantageous in some cases, as the base station 16 will be provided with apower supply (not shown) in order to be able to charge the robotic tool 14. However,as will be evident, it may be advantageous in other cases to locate the beacon 40more centrally in the work area 12. This may imply that the beacon 40 is providedwith its own power supply, or that a supply cable connects the beacon 40 with thebase station 16. ln the following example, the centrally located beacon 40 is used,the other beacon 40” being disregarded.

The work area 12 in the illustrated case is surrounded by a buried guide wire11 which is electrically connected to the base station 16 in order to produce a detect-able electric or electromagnetic field as is well known per se. However, limitations tothe work surface could be provided also for instance by stone walls, fences andbuildings that are optically detected by the robotic tool. Further, for illustration pur-poses, in the presented case an obstacle 42, such as a rock, is located inside thework area 12, and an indentation 44 provides a peninsular work area portion 46 thathas a relatively narrow connection to the rest of the work area 12. ln order to obtain a systematic and efficient covering of the work area 12, therobotic tool 14 moves, and in the illustrated example mows grass, in circles or cir-cular arcs, where the beacon 40 is in the centre of circles of which the circular arcsare segments. That is, the robotic tool 14 moves with constant distance to the be-acon 40 until a circle has been completed, or until the robotic tool encounters anobstacle.

The basic method 50 is illustrated in the flow chart of fig 10, describing me-asures carried out by the robotic tool 14, and with reference to fig 4. lnitially, a first 6 signal 58 is received 52 from a beacon 40. Based on the received signal the distancefrom the robotic tool 14 to the beacon 40 is determined. Finally, the robotic toolmoves along a circular arc 60 with a curvature corresponding to a circle having aradius substantially equal to the determined distance and with the beacon 40 in itscenter.

To start with, the reception 52 and the determining 54 can be carried out indifferent ways. For instance, the beacon 40 and the robotic tool may comprisesynchronized internal clocks. The beacon 40 transmits an ultrasound signal definingthe beacon”s time, or transmits an arbitrary signal at a predetermined time known tothe robotic tool 14. The robotic tool 14 picks up the transmitted signal and determinesthe distance by multiplying a detected time difference between transmission andreception with the speed of sound. As an alternative, the robotic tool 14 may transmitan ultrasound signal, and the beacon may immediately respond with a radio signalthat is picked up by the robotic tool 14. The processing time in the beacon 40 and theradio propagation time may be more or less negligible as compared to the propaga-tion time of the ultrasound signal, such that the robotic tool 14 can calculate thedistance in the same way as stated above. Needless to say, the skilled person realiz-es a number of other methods that can be used to detect the distance, includingradio-only transmissions e.g. detecting phase, infrared light and laser measuring, etc.High precision RTK (real time kinematic) systems could also be used in this context.Ultra wideband (UWB) radio transmissions may also be used.

Finally, the moving of the robotic tool 14, after determining the distance to thebeacon 40, can be carried out in different ways. lf the robotic tool is unaware of theazimuth to the beacon 40, it may begin to move while carrying out further measuringof the distance to the beacon 40 and adjust its heading 15 until it moves tangentiallyalong a circle with the beacon at the center of the circle. lf the robotic tool 14 is capable of detecting the azimuth of the beacon 40, itmay attain the correct orientation immediately, and begin moving along the circulararc 60. ln any case, once the correct orientation is achieved, the robotic tool 14 movesalong the circular arc by turning continuously or at short intervals while moving for-wards (in the latter case following a polygon approximating a circle). lf the rearwheels 20 are driven e.g. by stepper motors 24 (cf. fig 3) such turning may be achi- eved by applying different stepping frequencies on each wheel 20. These frequen-cies may for instance be obtained from a lookup table with the distance to the beaconasinput lf a robotic tool 14 is used to cut grass in this way, the procedure will in mostcases be much more effective than a random walk. ln gardens with some configura-tions, typically with high area in relation to circumference, this will be especially pro-nounced, in particular if the beacon 40 is located centrally in the work area 12.Further, this cutting scheme makes it possible for the robotic tool 14 to graduallyobtain a mapping of the work area 12, such that previously generated knowledge ofthe layout of the work area 12 can be used to increase efficiency even further.

Different navigation scenarios are now discussed with reference to figs 5-8,where fig 5 depicts the area 62 in fig 4, while fig 6 depicts the area 64 in fig 4, fig 7depicts the area 66 in fig 4, and fig 8 depicts the area 68 in fig 4.

Fig 5 illustrates a case where there is a free area around a beacon 40, suchthat complete circles 70 around the beacon can be processed. This is highly efficientas a relatively long distance can be covered in a systematic manner without stoppingand turning. The robotic tool can keep track of how much of a circle has been cover-ed in different ways. lf the robotic tool is capable sensing with angular measurementmethods when a full turn has been completed, such methods may be used. A com-pass may also be useful to this end. Otherwise, the robotic tool may use dead rec-koning, i.e. simply measuring the distance it has travelled. For instance, the numberof turns of the wheels may be counted or accelerometers or odometers may be used.When 211 times the distance to the beacon 40 has been covered, the turn is com-plete. As indicated with dashed arrows in fig 5, it may be preferred to continuesomewhat longer, e.g. an additional 5% of the circumference, to produce an angularoverlap 71, making sure that the circle is fully completed, and that no uncut area isleft behind.

Once a turn is completed, with or without overlap, the robotic tool increases ordecreases its distance to the beacon 40. ln the illustrated case the robotic tool incre-ases the distance. This depends on whether the robotic tool operates inwards or out-wards with regard to the beacon 40. The increment, or decrement, may be the cuttingwidth, or more generally processing width, of the robotic tool minus a radial overlap,to avoid risking moving too far in the radial direction leaving an unprocessed areabehind in between circles. Thereafter, the robotic tool 14 navigates along a circular arc corresponding to a new, in the illustrated case greater, circle 73, with a radiusequal to the new distance to the beacon.

The aforementioned radial overlap may be varied, either from circle to circle orbetween subsequent working sessions (e.g. before and after charging the robotictool) to somewhat blur systematic patterns in a lawn, providing a less stripy appear-ance.

Further, the robotic tool may take into account changes in elevation resultingfrom the radial increment or decrement. For instance, if increasing of the radius to thebeacon takes place uphill, the radial increment to the beacon may not fully reflect thewidth of the surface patch in between the new path and the previous path. lf so, theincrement may be made smaller to ensure that no patch is left unprocessed in betwe-en the paths. The same applies if the increment is downhill. Changes in inclinationmay also be taken into account during processing along a circular arc. The robotictool may have dedicated inclination or elevation measuring sensors. However, it isalso possible to detect an inclination simply by measuring the load on the drivingwheel motors, as this load increases uphill and decreases downhill. Additionally, therobotic tool may have a general setting for hilly/rolling work areas that imply generallysmaller radial increments/decrements. lt should be noted that, as long as the lawn is cut in circles, the grass inadjacent circles may be cut in the same direction, i.e. with the robotic tool travelling inthe same heading (e.g. clockwise). This too may provide an improved lawn appear-ance.

Fig 6 illustrates a case from the area 64 in fig 4. The robotic tool 14 proceedsalong a circular arc and adjusts its steering based on information received from abeacon 4. When passing behind an obstacle 42, such as a rock or the like, the robo-tic tool may not receive the signal from the beacon 40 and loses this updated infor-mation as regards distance to beacon. lf a signal cannot be received when reachinga point 72 on the arc, the robotic tool 14 may continue along the circular arc usingdead reckoning, typically using the previously described right/left wheel turning speedratio. Once the robotic tool reaches a point 74 where the reception of the beaconsignal can be resumed, it may correct any error in the distance to the beacon 40, asindicated in fig 6, thereby regaining the lost arc and proceeding along the same.

Needless to say, most gardens are not circular, and the robotic tool may dealwith incomplete circles in different ways. Fig 7 illustrates a case from the area 66 infig 4. The robotic tool moves along a first circular arc 76 until a limitation is detected 9 at a point where the robotic tool has a first heading 78. ln the illustrated case, thelimitation is a guide wire 11, which indicates to the robotic tool that it reaches theborder of the work area 12, as is well known per se. ln other cases, the limitationcould be a wall, a fence, or the like which is for instance optically detected to therobotic tool. ln any case, the robotic tool 14 should not proceed further along the firstheading 78.

When this condition is present, the robotic tool increases or decreases itsdistance to the beacon, depending on if it works inwards or outwards with regard tothe beacon, and preferably as previously described with an overlap. The robotic toolturns 180 degrees from the first heading 78 to a second heading 80. Finally, therobotic tool resumes processing/cutting in the second heading along a circular arc 81corresponding to a circle having a radius substantially equal to the new distance tothe beacon. lf the robotic tool operates outwards with respect to the beacon 40 in a workarea 12 as outlined in fig 4, the circles will at some point split into separate arcsegments which have the same distance to the beacon 40 without being connectedto each other. This applies to most non-circular work areas. The most pronouncedexample is given in the lower part of fig 4 where an indent 44 in the work area 12forms a peninsular work area portion 46. lf the robotic tool 14 continues to processthe area to the right of the indent 44 without noticing this, the peninsular portion maybe left unprocessed. Fig 8 illustrates a case from the area 68 in fig 4 where this isdealt with. When the limitation is detected at point 82, the robotic tool checks itsmemory for the limitation 84 (cf. fig 4) received in a previous circle. lf the robotic tooldetermines that the limitation is detected at an angular difference cp exceeding athreshold angle from a previously detected limitation a separate area search may beinitiated. ln the illustrated example, the angle cp between points 82 and 84 is about 90degrees which is a strong indication that there may be an unexplored area in betwe-en those points. The threshold value may vary with the distance to the beacon. lnanother example at the lower right portion of the work area in fig 4, the angulardistance between points 86 and 88 is very small, which implies that there is probablyno separate area in between those points. The threshold may be set in different waysdepending on the type of work area and a tradeoff between processing speed andallowed risk of leaving an area unprocessed. The separate area search may also beinitiated if the limitation is the first limitation encountered (moving outwards) or the last limitation detected (the robotic tool moving inwards and processing completecircles thereafter). Several search area operations may be nested. ln any case, if a separate area search operation is initiated, the robotic toolmay follow the limitation from the point 82 to determine whether there is an area withthe same radius as the previous circular arc on the other side of the limitation. As analternative, the robotic tool may first finish processing the area it was operating inbefore detecting the need for a separate area search, and may return to the point 82to carry out the search at a later stage, and finish the peninsular area 46 at that time.lt should be noted that once the robotic tool has covered the entire work area it mayhave created a map thereof in its memory that may simplify subsequent processingof the work area.

Fig 9 illustrates schematically a beacon 40 where the work area is defined bydifferent distances to the beacon. A minimum distance 90 to the beacon 40 may bedefined, and the robotic tool 14 may turn around when reaching this distance, or mayfollow the defined border. This may be useful, for instance by placing the beacon inthe middle of a patch of sensitive vegetation which becomes protected from beingprocessed by the robotic tool. ln the same way, a maximum distance 92 to thebeacon may be defined, and the robotic tool turns when reaching the maximumallowed distance. This may be useful e.g. to replace a guide wire and create acircular work area or, when a guide wire is used, as an outer backup. lf the guidewire malfunctions, the outer limitation prevents that the robotic tool moves too faraway. lt may also be combined with guide wires and other limitations such asbuildings, fences, etc. to form closed areas.

This feature may also be useful even without operating in circular arcs. Forinstance, a case could be considered where a robotic tool operates randomly within amaximum distance and optionally outside a minimum distance from a beacon. lt has thus been considered a method of navigating a self-propelled robotictool, where the robotic tool receives a first signal from a beacon and determines itsdistance to the beacon using the signal, wherein the robotic tool has a maximumallowed distance to the beacon and the robotic tool turns when reaching the maxi-mum allowed distance.

The invention has mainly been described above with reference to a few em-bodiments. However, as is readily appreciated by a person skilled in the art, otherembodiments than the ones disclosed above are equally possible within the scope ofthe invention, as defined by the appended patent claims. 11

Claims (18)

CLAIIVIS

1. A method of navigating a self-propelled robotic tool (14), the methodcharacterized by the robotic tool: receiving (52) a first signal from a beacon (40); determining (54), based on the received signal, a distance from the robotic toolto the beacon; and moving (56) along a circular arc (60) with a curvature corresponding to a circlehaving a radius substantially equal to the determined distance and with the beacon inits center.

2. The method according to claim 1, further comprising the robotic tool: receiving second and later signals from the beacon and determining the currentdistance from the robotic tool to the beacon; and adjusting its heading based on the determined current distance, such that thedistance to the beacon is kept substantially constant along the circular arc.

3. The method according to claim 1 or 2, wherein the robotic tool moves alongthe circular arc until a complete circle (70) around the beacon has been covered.

4. The method according to claim 1 or 2, wherein the robotic tool moves alongthe circular arc until a complete circle around the beacon plus an angular overlap(71) has been covered.

5. The method according to claim 3 or 4, further comprising subsequentlymoving the robotic tool towards or away from the beacon to increase or decrease thedistance to the beacon, and repeating the moving of the robotic tool along a circulararc corresponding to a circle (73) having a radius substantially equal to the newdistance to the beacon.

6. The method according to claim 1 or 2, further comprising the robotic tool:moving along the circular arc (78) until, when the robotic tool has a first heading (78),a limitation (11) is detected which prevents the robotic tool from moving further; increasing or decreasing the distance to the beacon; turning the robotic tool to a second heading (80), 180 degrees from the firstheading, and resuming the cutting in the second heading along a circular arc (81)corresponding to a circle having a radius substantially equal to the new distance tothe beacon. 12

7. The method according to claim 5 or 6, wherein the increase or decrease ofthe distance to the beacon results in an increment or decrement which is theprocessing width of the robotic tool minus a radia| overlap.

8. The method according to claim 7, wherein the radia| overlap includes avarying component.

9. The method according to any of the preceding claims, wherein an angulardisplacement with regard to the beacon is determined using dead reckoning.

10. The method according to claim 6, further comprising the robotic tool: determining whether the limitation (11) is either detected at an angle with adifference (cp) exceeding a threshold angle from a previously detected limitation, orwhether the preceding or succeeding arc is free from limitation; and if either of these conditions applies, initiating a separate area search operation.

11. The method according to claim 10, wherein the separate area searchoperation involves following the limitation to determine whether an area with the distance to the beacon where the limitation was detected exists beyond the limitation.

12. The method according to any of the preceding claims, wherein, if a signalfrom a beacon temporarily cannot be received, the robotic tool continues along thecircular arc using dead reckoning, and corrects any error in the distance to thebeacon once reception is resumed.

13. The method according to any of the preceding claims, wherein themovement along the circular arc is accomplished by driving right and left wheels (20)of the robotic device with different speeds.

14. The method according to any of the preceding claims, wherein the robotictool has a maximum allowed distance (92) to the beacon and the robotic tool turnswhen reaching the maximum allowed distance.

15. The method according to any of the preceding claims, wherein the robotictool has a minimum allowed distance (90) to the beacon and the robotic tool turnswhen reaching the minimum allowed distance.

16. A system for processing a work area (12), the system including a self-propelled robotic tool (14) for processing the area and a beacon (40) configured totransmit signals to the robotic tool, wherein the robotic tool includes software and/orcircuitry configured to carry out the method of any of claims 1-15. 13

17. The system according to claim 16, wherein the system further comprises acharging station (16), and the beacon (40”) is arranged integrated with or wired to thecharging station.

18. The system according to claim 16, wherein the system further comprises acharging station (16), and the beacon (40) is arranged remote from the chargingstation. 14