SE543247C2 - Robotic work tool system for detecting a lift event and method for use in a robotic work tool system - Google Patents

Abstract

A robotic work tool system (200) comprising a robotic work tool (100) comprising at least one radar device (190, 190’, 190”) and at least one corresponding reflector (195), the robotic work tool (100) being configured to determine a sensed distance (SD) to the at least one corresponding reflector (195) utilizing the at least one radar device (190, 190’, 190”); determine whether the sensed distance is greater than a threshold distance; and if so detect a lift event.

Description

ROBOTIC WORK TOOL SYSTEM FOR DETECTING A LIFT EVENT ANDMETHOD FOR USE IN A ROBOTIC WORK TOOL SYSTEM TECHNICAL FIELDThis application relates to robotic work tools and in particular to a systemand a method for perforrning improved lift detection to be performed by a robotic work tool, such as a lawnmower.

BACKGROUND Automated or robotic power tools such as robotic lawnmowers are becomingincreasingly more popular. In a typical deployment, a work area, such as a garden, isenclosed by a boundary cable with the purpose of keeping the robotic lawnmower insidethe work area. The robotic lawnmower is typically configured to work inside the workarea during a large portion of the year. As the robotic lawnmower is Working outside andwill be subj ected to weather, dirt, cut grass and other debris, it is important to protect thecomponents of a robotic lawnmower. However, as the safety of the user is paramount allrobotic lawnmowers are equipped with lift detection devices that ensures that a lift of therobotic lawnmower is detected within l0 mm lifting height and the cutters may be tumedoff to prevent or reduce the risk of damage.

Such lift detectors are commonly mechanical or electromechanical devices,where one part is attached to an upper part of the robotic lawnmower and a second part isattached to a lower part of the robotic lawnmower. A lift is detected when the upper partmoves in relation to the lower part, i.e. when the first part of the lift detector moves inrelation to the second part of the lift detector. The lift detection thus requires that therobotic lawnmower has two cover parts that are movable with regards to one another,which in tum renders the robotic lawnmower susceptible to being contaminated by water,dirt or other debris coming in between the two cover parts and risking to damage or causeincreased wear of components of the robotic lawnmower. This is also an expensive solution to implement.

An alternative is to have movable Wheels, Where one or several Wheels “falls”down When the robotic laWnmoWer is lifted. This is a cheaper solution to implement, butas the solution is oriented on movement of the Wheels, and the Wheels are being driventhrough the dirt, cut grass, mud and other debris, the solution is highly sensitive dirt andWater.

Thus, there is a need for improved deterrnining of a robotic laWnmoWer°s protection against dirt, Water and other debris.

SUMMARY As Will be disclosed in detail in the detailed description, the inventors haverealized that the use of high precision radar devices for deterrnining a distance Within aflexible body of the robotic laWnmoWer enables robotic Work tools to be made With onecoherent body part. It should be noted that the coherent body part may consist of severalbody parts, such as an outer cover attached to a chassis, but as the body does not need tobe movable With respect to itself (i.e. the body parts are not movable relative eachother), the body may be sealed in a more efficient manner, thereby being one coherentbody.

It is therefore an object of the teachings of this application to overcome orat least reduce those problems by providing a robotic Work tool system comprising arobotic Work tool comprising at least one radar device, the robotic Work tool beingconfigured to determine a sensed distance to the at least one corresponding reflectorutilizing the at least one radar device; determine Whether the sensed distance is greaterthan a threshold distance; and if so detect a lift event.

In one embodiment the body of the robotic Work tool is a coherent body. Inone such embodiment, the body comprises a chassis and an outer cover that are notmovable relative each other.

In one embodiment the reflector is comprised in the body of the roboticWork tool.

In one embodiment the robotic Work tool is further configured to determine the threshold distance over a time period.

In one en1bodin1ent the robotic lawnniower coniprises a first radar deviceand a second radar device, wherein the controller is further configured to deterrnine thesensed distance to the at least one corresponding reflector by: deterrnining a first senseddistance utilizing the first radar device; deterrnining a second sensed distance utilizingthe second radar device; and deterrnining the sensed distance based on the first senseddistance and the second sensed distance.

In one en1bodin1ent the robotic work tool is further conf1gured to receive aradar echo within a time window.

In one en1bodin1ent the robotic work tool is further conf1gured to recalibrateat least on tin1e Window.

In one en1bodin1ent the robotic work tool is further conf1gured to deterrninethe energy content of a received radar echo and deterrnine a reflection substrate basedon the deterrnined energy content of the received radar echo.

In one en1bodin1ent the robotic work tool is further configured to deterrninea distance to a reflection substrate based on the tin1e of reception of a received radarecho.

In one en1bodin1ent the flexible joint joins an upper part of the body and alower part of the body whereby the upper part is flexibly attached to the lower partthrough the joint. In one en1bodin1ent the flexible joint is a rubber joint.

In one en1bodin1ent the flexible joint is of a bellow construction.

In one en1bodin1ent, the joint is a sealing.

In one en1bodin1ent the upper part is a cover and the lower part is a chassisarranged with wheels.

In one en1bodin1ent the radar device is arranged in the upper part of thebody and the reflector is arranged in the lower part of the body.

In one en1bodin1ent the reflector is arranged in the upper part of the bodyand the radar device is arranged in the lower part of the body.

In one en1bodin1ent the robotic work tool is a robotic lawnniower.

In one en1bodin1ent the robotic lawnniower is further conf1gured to deterrnine the height of grass based on the tin1e of reception of a received radar echo.

It is also an object of the teachings of this application to overcome theproblems by providing a method for use in a robotic work tool system comprising arobotic work tool comprising a radar device and at least one corresponding reflector, themethod comprising: deterrnining a sensed distance to the at least one correspondingreflector utilizing the at least one radar device; deterrnining whether the sensed distanceis greater than a threshold distance; and if so detecting a lift event.

Other features and advantages of the disclosed embodiments will appearfrom the following detailed disclosure, from the attached dependent claims as well asfrom the drawings. Generally, all terms used in the claims are to be interpretedaccording to their ordinary meaning in the technical field, unless explicitly definedotherwise herein. All references to "a/an/the [element, device, component, means, step,etc]" are to be interpreted openly as referring to at least one instance of the element,device, component, means, step, etc., unless explicitly stated otherwise. The steps ofany method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described in further detail under reference to theaccompanying drawings in which: Figure 1A shows an example of a robotic lawnmower according to oneembodiment of the teachings herein; Figure 1B shows a schematic view of the components of an example of arobotic work tool being a robotic lawnmower according to an example embodiment ofthe teachings herein; Figure 1C shows a schematic side view of a robotic lawnmower accordingto one example embodiment of the teachings herein.

Figure 2 shows an example of a robotic work tool system being a roboticlawnmower system according to an example embodiment of the teachings herein; Figures 3A, 3B and 3C each shows a schematic view of a robotic work toolbeing a robotic lawnmower for deterrnining a lift event according to an example embodiment of the teachings herein; Figures 4 shows a schematic view of a robotic work tool being a roboticlawnmower for deterrnining a lift event according to an example embodiment of theteachings herein; Figure 5A shows an example embodiment of how a radar device deterrninesthe reception of an echo according to an example embodiment of the teachings herein; Figure 5B shows an example illustrating how different echoes fromdifferent substrates are received at different times, and at different energy levels,according to an example embodiment of the teachings herein; and Figure 6 shows a corresponding flowchart for a method according to an example embodiment of the teachings herein.

DETAILED DESCRIPTION The disclosed embodiments will now be described more fully hereinafterwith reference to the accompanying drawings, in which certain embodiments of theinvention are shown. This invention may, however, be embodied in many differentforms and should not be construed as limited to the embodiments set forth herein.

Like reference numbers refer to like elements throughout.

It should be noted that even though the description given herein will befocused on robotic lawnmowers, the teachings herein may also be applied to, roboticball collectors, robotic mine sweepers, robotic farrning equipment, or other robotic worktools where lift detection is used and where the robotic work tool is susceptible to dust,dirt or other debris.

Figure 1A shows a perspective view of a robotic working tool 100, hereexemplif1ed by a robotic lawnmower 100, having a body 140 and a plurality of wheels130 (only one shown). The robotic lawnmower 100 may comprise charging skids forcontacting contact plates (not shown in figure 1) when docking into a charging station(not shown in figure 1, but referenced 210 in figure 2) for receiving a charging currentthrough, and possibly also for transferring information by means of electricalcommunication between the charging station and the robotic lawnmower 100.

Figure lB shows a schematic overview of the robotic working tool 100, also exemplif1ed here by a robotic lawnmower 100, having a body 140 and a plurality of wheels 130. In the exemplary embodiment of f1gure 1B the robotic lawnmower 100 hasfour wheels 130, two front wheels 130” and the rear wheels 130”. At least some of thewheels 130 are drivably connected to at least one electric motor 150. It should be notedthat even if the description herein is focused on electric motors, combustion enginesmay altematively be used, possibly in combination with an electric motor. In theexample of figure lB, each of the rear wheels 130” is connected to a respective electricmotor 150. This allows for driving the rear wheels 130” independently of one anotherwhich, for example, enables steep turning.

The robotic lawnmower 100 also comprises a grass cutting device 160, suchas a rotating blade 160 driven by a cutter motor 165. The grass cutting device being anexample of a work tool 160 for a robotic work tool 100. The robotic lawnmower 100also has (at least) one battery 180 for providing power to the motors 150 and the cuttermotor 165.

The robotic lawnmower 100 also comprises a controller 110 and a computerreadable storage medium or memory 120. The controller 110 may be implementedusing instructions that enable hardware functionality, for example, by using executablecomputer program instructions in a general-purpose or special-purpose processor thatmay be stored on the memory 120 to be executed by such a processor. The controller110 is conf1gured to read instructions from the memory 120 and execute theseinstructions to control the operation of the robotic lawnmower 100 including, but notbeing limited to, the propulsion of the robotic lawnmower. The controller 110 may beimplemented using any suitable, available processor or Programmable Logic Circuit(PLC). The memory 120 may be implemented using any commonly known technologyfor computer-readable memories such as ROM, RAM, SRAM, DRAM, FLASH, DDR,SDRAM or some other memory technology.

The robotic lawnmower 100 may further be arranged with a wireless com-munication interface 115 for communicating with other devices, such as a server, apersonal computer or smartphone, or the charging station. Examples of such wirelesscommunication devices are Bluetooth®, Global System Mobile (GSM) and LTE (Long Term Evolution), to name a few.

The robotic lawnmower 100 may be further configured to have at least onemagnetic sensor 170 arranged to detect a magnetic field (not shown) and for detecting aboundary cable and/or for receiving (and possibly also sending) inforrnation from asignal generator (will be discussed with reference to figure 2). In some embodiments,the sensors 170 may be connected to the controller 110, and the controller 110 may beconfigured to process and evaluate any signals received from the sensors 170. Thesensor signals may be caused by the magnetic field being generated by a control signalbeing transmitted through a boundary cable. This enables the controller 110 todetermine whether the robotic lawnmower 100 is close to or crossing a boundary cable,or inside or outside an area enclosed by the boundary cable.

It should be noted that the magnetic sensor(s) 170 as well as the boundarycable (referenced 230 in figure 2) and any signal generator(s) are optional as is indicatedby the dashed line of the boundary cable (230) in figure 2. The boundary cable mayaltematively be used as the main and only perimeter marker. The boundary cable mayaltematively simply be used as an additional safety measure. The boundary cable mayaltematively be used as the main perimeter marker and other navigation sensors (seebelow) are used for more detailed or advanced operation.

The robotic lawnmower 100 may further comprise at least one beaconnavigation sensor 175, such as an Ultra Wide Band (UWB) sensor, configured toreceive signals from a Radio Frequency beacon, such as a UWB beacon.

The robotic lawnmower 100 may also comprise at least one satellitenavigation sensor, such as a Global Positioning System (GPS) device 185, or aGLONASS device.

The robotic lawnmower 100 also comprises at least one radar device 190.The radar device 190 may be arranged in font, behind or next to a handle 145 for liftingthe robotic lawnmower. It should also be noted that even if the example of figure 1Bshows one radar device arranged in the ceiling of the body, other numbers of radardevices 190 may be possible. Also, the radar device(s) may altematively be placed inthe bottom portion of the body facing upwards.

Radar is a detection system that uses radio waves to determine the range, angle, or velocity of objects. The radar operates by transmitting a short radio pulse in a direction. The direction is deterrnined by the directivity of an antenna of the radardevice. If there is an obstacle in the direction of the radio pulse, the energy of the radarpulse is scattered in all directions. A portion of the scattered energy is, however,reflected - or echoed - back to the radar. The radar pulse is thereby reflected by theobject. The reflected pulse is sometimes referred to as the echo or radar echo. As theradar receives the reflected pulse, a distance to the object can be deterrnined bymeasuring the time from transmission of the radar pulse to reception of the reflectedpulse. This may be done by knowing or noting the time of transmission and deterrniningthe time of reception. As the radar pulse travels at the speed of light, the deterrninationof the distance is straight forward knowing both the time t and the speed V bymultiplying the speed V with half the time t/2, as the time includes travel in bothdirections; distance = speed * time/2 = v*t/2.The inventors of the present application has realized that by utilizing a high precision radar device a sturdy and robust lift detection system may be provided. As aradar pulse will be reflected by the under part of the body - or opposite part of the bodydepending on the placement of the radar device - there is no need for the prior artsystems where one body part (for example the cover) was arranged movable relativeanother body part (for example the chassis). The body l40 can then be constructed to beone coherent piece. Using a coherent body l40 simplifies production and protects therobotic lawnmower from any debris, wet grass, dirt or other debris. The body l40 maystill be made from different body parts, but utilizing the teachings herein, the body partsmay be sealed in a more efficient manner as they do not need to be movable relativeeach other any longer. A coherent body is to be understood as a body without body partsbeing movably arranged relative one another. For example a body where the chassis,provided with the wheels, and the cover, especially an outer cover, are j oined togetherand are arranged to not be substantially movable relative one another. The joint may bea seal in which case, the chassis and the cover should not be movable in such a degreethat the sealing is compromised, possibly in that an opening in the sealing is revealedupon movement. A small flexibility of movement in the sealing is thus acceptable. Thesmall flexibility in the sealing is of a magnitude that may be undetectable by a person and in the order of less than 0.l mm, less than 0.5 mm or less than l mm.

The radar device is one example of a non-contact distance sensor that maybe used to detect the distance to the reflector. Other examples include optical sensorssuch as infrared (IR) sensors, Laser or Lidar sensors to mention a few. Otherelectromagnetic sensors operating by transmitting a pulse and receiving the reflection ofthe pulse are also possible.

Figure lC shows a schematic side view of a robotic lawnmower accordingto one example of the teachings herein. The radar device l90 is arranged in the ceilingof the body l40. It should be noted though that the radar device may be placed at otherpositions as well and/or facing other directions. For example, the radar device l90 couldbe placed at the floor of the body l40 facing upwards. Regardless of arrangement, theradar device l90 should be placed so that the transmitted radar pulse may be reflectedback to the radar device l90 from within the body l40. The reflection may be caused bya wall of the body l40, the reflector 195 then being comprised in the body l40, or by aspecific reflector 195 placed opposite the radar device. In the example shown in figurelC, the radar device l90 is arranged close to a handle l45 used for lifting the roboticlawnmower.

The body l40 is arranged to be flexible. A flexible body l40 may comprisedifferent body parts, but as they do not necessarily need to be substantially movable toone another, an improved or enforced sealing can be achieved compared to prior artmodels utilizing mechanical or electromechanical lift detectors.

The body l40 may be arranged to be flexible in the choice of materials forthe body l40, such that when the robotic lawnmower is lifted, for example by thehandle l45, the actual body l40 will be deforrned and stretched. The body may bearranged to be flexible by choosing cover portions that are used for lifting to be made ofa softer, more flexible material.

Additionally or altematively, the body l40 may be arranged with a flexiblejoint portion l40-l. Such a flexible joint portion l40-l may be arranged between twobody parts being attached to one another, possibly through the flexible joint portionl40-l. The flexible joint l40-l may be a rubber seal or seal of other flexible material.

The flexible joint l40-l may be of a bellow construction.

The flexible joint 140-1 may thus join an upper part 140-2 of the body 140and a lower part 140-3 of the body 140 and the upper part 140-2 is flexibly attached tothe lower part 140-3 through the sealing 140-2.

In one embodiment, the joint is a sealing thereby effectively sealing thecoherent body by joining the upper part 140-2 to the lower part 140-3 in a sealedmanner through the sealing joint 140-1.

The upper part 140-2 may be a cover and the lower part 140-3 may be achassis. In one embodiment the lower part 140-3 is arranged with the wheels 130 of therobotic lawnmower 100.

In one embodiment, the radar device 190 is arranged in the upper part 140-2of the body 140 and the reflector 195 is arranged in the lower part 140-3 of the body140.

In one embodiment, the reflector 195 is arranged in the upper part 140-2 ofthe body 140 and the radar device 190 is arranged in the lower part 140-3 of the body140.

Figure 2 shows a schematic view of a robotic working tool system 200 inone embodiment. The schematic view is not to scale. The robotic working tool system200 comprises a charging station 210 and a robotic working tool 100. As with figures1A and 1B, the robotic working tool is exemplified by a robotic lawnmower, wherebythe robotic work tool system may be a robotic lawnmower system or a systemcomprising a combinations of robotic work tools, one being a robotic lawnmower, butthe teachings herein may also be applied to other robotic working tools adapted tooperate within a work area.

The robotic working tool system 220 may also optionally comprise aboundary cable 230 arranged to enclose a work area 205, in which the roboticlawnmower 100 is supposed to serve.

The robotic working tool system 220 may also optionally comprise at leastone beacon 220 to enable the robotic lawnmower to navigate the work area using thebeacon navigation sensor 175.

Additionally or altematively, for its operation within the work area 205, in the embodiment of figure 2, the robotic lawnmower 100 may use the satellite navigation ll device 185, possibly supported by a deduced reckoning navigation sensor (not shown)to navigate the work area 205.

The work area 205 is in this application exemplif1ed as a garden, but canalso be other work areas as would be understood. The garden contains a number ofobstacles (O), exemplif1ed herein by a number (3) of trees (T). The trees are markedboth with respect to their trunks (filled lines) and the extension of their foliage (dashedlines).

Figure 3A shows a schematic view of an example embodiment of a roboticwork tool, being a robotic lawnmower 100, according to the teachings herein.

The robotic lawnmower 100 is traversing or travelling over a surfacereferenced G. For the example of a robotic lawnmower 100, the surface is most likelythe ground. A radar device 190 is arranged within the robotic lawnmower 100. Eventhough the radar device 190 is shown as being arranged on the upper part or ceiling ofthe body 140 of the robotic lawnmower 100, it should be noted that this is only forillustrative purposes enabling details to be shown without cluttering smaller areas of thefigure. It should also be noted that the example of figure 3A only relates to one radardevice 190, but the teachings may also be used for embodiments utilizing or comprisingseveral radar devices. The radar device 190 is arranged opposite the reflector 195,which may be a portion of the body itself, or a specific reflector installed to increase thereflection, providing a stronger echo. In one embodiment, the reflector 195 is a metalreflector. In one embodiment, the reflector is plastic. In one such embodiment, thereflector has a metallic coating.

In one embodiment the reflector is a flat reflector 195. In one embodiment, the reflector is a concave reflector. In one embodiment the reflector is a comer reflector.

In figure 3A, the radar device 190 is at a first distance from the reflector195. The first distance is referenced D in figure 3A and indicated by the arrow. In thisexample, the distance D is taken to be a norrnative intemal distance. As the surfacetravelled, in this example the ground G, may be irregular causing vibrations in the body140 of the robotic lawnmower 100 the norrnative distance D may be an interval of distances, or it may be a deterrnined average distance. 12 In one embodiment, the norrnative distance may be deterrnined by thecontroller upon start-up, in one such embodiment before the robotic lawnmower leavesthe charging station.

In one embodiment, the norrnative distance may be deterrnined by thecontroller when the controller deterrnines that the robotic lawnmower is travelling overa smooth surface, for example a garden path. The controller may be conf1gured todeterrnine that the surface being travelled is smooth by deterrnining that vibrations arebelow a threshold level. The vibrations may be measured through the accelerometer1 95 .

In one embodiment, the norrnative distance is updated continuously.

In one embodiment, the norrnative distance is updated at intervals. In onesuch embodiment, the intervals are regular. In an altemative or additional suchembodiment, the intervals are dependent on an action, such as start-up.

Figure SB shows a schematic view of an example embodiment of a roboticwork tool, being a robotic lawnmower 100, according to the teachings herein. In figureSB, the robotic lawnmower 100 has been lifted off the ground G.

In this example embodiment, the body 140 of the robotic lawnmower 100,being flexible, has been slightly deforrned thereby increasing the distance between theradar device 190 and the reflector 195. In this situation the robotic lawnmower 100deterrnines the distance to the reflector 195 by deterrnining a sensed distance SD. Therobotic lawnmower 100 is conf1gured to deterrnine or measure this sensed distance SDutilizing the radar device 190 by transmitting radar pulses and detecting echoes, orreflected pulses, and based on the time difference, deterrnine the sensed distance SD.

As the sensed distance is deterrnined, the robotic lawnmower is configuredto compare the sensed distance to a threshold distance. The threshold distance is basedon the norrnative distance. In one embodiment, threshold distance is the norrnativedistance. In one embodiment, to account for variations and reduce the risk of false liftdetections, the threshold distance is the norrnative distance multiplied by a factor.Examples ofthe factor are 1.001, 1.002, 1.003, 1.004 and 1.005.

In one embodiment, the threshold distance is deterrnined based on a rate of change of the sensed distance. If the sensed distance is increasing at a too high rate (i.e. 13 the rate of change is higher than a change threshold) indicating a lift, the thresholddistance will be set to a value below the current sensed distance, thereby detecting thelift. In one such embodiment the deterrnination of rate of change is perforrned incombination with a deterrnination of data received from an accelerometer. In one suchembodiment, if the accelerometer data does not indicate a vertical movement of therobotic lawnmower, the rate of change may be caused by a hole or such being traversedand a lift is not detected. The controller is thus configured to determine that in additionto the rate of change for the sensed distance is exceeding a threshold, the accelerometerdata also indicates a vertical movement. In one embodiment, the controller is configuredto base the change threshold on the accelerometer data, wherein the change threshold isincreased if the accelerometer data indicates no vertical movement.

The threshold distance may also be based on a time from when the senseddistance exceeded the norrnative distance. In such an embodiment, the thresholddistance may be set to a value below the current sensed distance, thereby detecting a lift,if the time from exceeding the norrnative distance exceeds a time threshold. Thisenables for detecting a lift even if the lift does not deforrn the coherent body.

As the threshold distance is based on the norrnative distance, the thresholddistance is also deterrnined over a period of time in embodiments where the norrnativedistance is deterrnined over a period of time. The deterrnination over time may beperformed as a weighted average, favouring current measurements over pastmeasurements.

If the robotic lawnmower 100 deterrnines that the sensed distance exceedsthe threshold distance, the robotic lawnmower deterrnines that a lift event has beendetected.

Figure 3C shows a schematic side view of an example embodiment of arobotic work tool, being a robotic lawnmower 100, according to the teachings herein. Inthis example embodiment, the body 140 of the robotic lawnmower 100, being arrangedwith a flexible sealing 140-1, has been slightly deforrned by the sealing 140-1 beingstretched thereby increasing the distance between the radar device 190 and the reflector 195. 14 The distance between the radar device 190 and the reflector may bedeterrnined in the same manner as for the example embodiment of figure 3B.

It should be noted that all distances, differences in distances and angles infigures 3A, 3B and 3C have been exaggerated for illustrative purposes.

As an altemative or in addition, the robotic lawnmower is, in oneembodiment configured to prevent false lift event detections and/or ensure that a lift isdetected by utilizing more than one radar device 190. The robotic lawnmower 100 maybe arranged with two or more radar devices, wherein each may be placed adj acent a liftposition or a position that will be stretched due to a lift.

Figure 4 shows a schematic side/view of a robotic work tool 100 accordingto one example embodiment of the teachings herein, where a plurality of radar devices,in this example two, are arranged.

A first radar device 190” is arranged at one end of the robotic lawnmower100, adjacent a first lifting point 145”, here indicated by a first handle. The first radardevice 190” is arranged opposite a first reflector 195°to determine a first sensed distanceSD 1 .

A second radar device 190” is arranged at another end of the roboticlawnmower 100, adjacent a second lifting point 145”, here indicated by a secondhandle. The second radar device 190” is arranged opposite a second reflector 195”todetermine a second sensed distance SD2.Although the radar devices are shown as beingarranged at each an end of the robotic lawnmower, it should be understood that otherarrangements are possible, and depend on the actual design of the robotic lawnmower.The radar devices are placed at locations where the cover 140 is flexible, to allow for amovement of the cover so that a changed distance may be deterrnined.

Arrangements with more than one radar device 190 are thus configured toprovide a plurality of sensed distances, one from each radar device. In one exampleembodiment, the robotic lawnmower 100 is configured to determine the sensed distanceSD to the reflector by deterrnining a first sensed distance SD1 utilizing a first radardevice 190. The robotic lawnmower is also configured to determine a second sensed distance SD2 utilizing a second radar device 190. The robotic lawnmower 100 is configured to determine the sensed distance SD based on the first sensed distance SDland the second sensed distance SD2.

In one embodiment, the robotic lawnmower 100 is configured to deterrnineif a lift event is detected by comparing each sensed distance SD to the reflector to each athreshold distance. Several situations may apply.

If two sensed distances exceeds the threshold distance, a lift event isdetected.

If only one sensed distance exceeds the threshold distance, a lift event isdetected.

In an embodiment with more than two radar devices, if sensed distances forradar devices arranged at diagonally opposite corners exceed the threshold distance, alift event is not detected.

In one embodiment, the sensed distance is deterrnined as the average of theplurality of sensed distances, i.e. for an embodiment with two radar devices the averageof the first and the second sensed distance. This enables for reducing the effect ofuneven surfaces.

In one embodiment, the sensed distance is deterrnined to be the minimum ofthe plurality of sensed distances, the lower of the first and the second sensed distances.This enables for reducing the risk of a false lift event.

In one embodiment the sensed distances are f1ltered before deterrnining a liftdetection.

In one embodiment a fusion algorithm is used to determine a lift detectionbased on the plurality of radar devices.

Returning to the functionality of the radar device 190. As the emitted ortransmitted radar pulse intercepts an object or surface, from here on a substrate, aportion of the energy of the radar pulse will be reflected in every direction, including thedirection back to the radar device. The reflected pulse or echo will thus be received aftera time t. As has been discussed in the above, this time corresponds to double thedistance travelled. As most substrates that reflect the pulse or give rise to an echo are irregular, the echo will in most cases not be a short pulse, but rather an extended wave 16 form. Using a reflector 195 provides a narrower echo, which is easier to detect andidentify.

To reduce the power, both electrical and processing power, required orconsumed by the radar device, the radar device 190 is, in one embodiment, configuredto deterrnine if a pulse has been received at given times or time intervals. The times (ortime intervals) correspond to expected distances to substrates. By only performing thepulse reception analysis at given times, the processing power can be reducedsignificantly.

Figure 5A shows an example embodiment of how the radar device 190deterrnines the reception of an echo. Figure 5A shows a graph illustrating the receivedenergy E over time t. Three time windows wl , w2 and w3 are also illustrated. Asdiscussed above, the radar device 190 is, in one embodiment, configured to deterrninethat an echo has been received within at least one time window wl, w2, w3. Thedifferent time windows correspond to expected distances that an echo could be expectedto be reflected from. As also discussed above, the time of reception ti corresponds to adistance Di. One example of a distance Di that an echo is expected to be received fromis the norrnative distance. In figure 5A this would correspond to the time window wl.Another example of a distance Di that an echo is expected to be received from is aseries of distances around the threshold distance, or from the norrnative distance to thethreshold distance, possibly going beyond the threshold distance. In figure 5A thiswould correspond to the time window w2.

Another distance that an echo could be expected to be received from is theheight of the grass being cut (or other substrate being serviced) or from the surfacebeing travelled. By measuring the height of the grass, the operation of the roboticlawnmower may be optimized as relates to scheduling, such as cutting time, and cuttingefficiency, such as cutting height and/or cutting power. In figure 5A the echo from thegrass would correspond to the time window w3.

To deterrnine which time windows to be used, the robotic lawnmower 100 isconfigured to calibrate the radar device by receiving pulses in wider or more time windows and deterrnine over time the most likely or rather most frequently times where 17 an echo is received. The time window(s) can then be set to be closer to or narroweraround the expected times.

In order for allowing for shifts of distances, such as when changing surfacesor when changing cutting heights, the robotic lawnmower 100 is conf1gured in oneembodiment, to repeatedly calibrate the time windows to accommodate any shifts. Forshifts occurring slowly, the robotic lawnmower 100 may be conf1gured to move a timewindow if it is deterrnined that the expected echo is received closer to an edge of thetime window, whereby the time window is moved so that the echo is received in acenter area of the time window. For shifts occurring suddenly, the robotic lawnmower100 may be conf1gured to expand a time window or introduce more time windowsaround the time window if it is deterrnined that an expected echo is not received.Especially if the echo is not received for a given number of times to allow for variouserrors and irregularities. The expanded or added time window(s) will then enable therobotic lawnmower 100 to recalibrate the time window(s) to the new expected timeposition of the expected echo.

This allows the robotic lawnmower 100 to continuously or repeatedlycalibrate the norrnative distance, the threshold distance and any other distance that is tobe monitored (such as the grass height).

The robotic lawnmower 100 may also be calibrated in how often and forhow long to calibrate the time windows. For a surface with many irregularities thenorrnative distance may vary greatly and a more frequent calibration may be needed.Likewise a longer time window over which the norrnative distance is deterrnined maybe required. For smooth surfaces, a less frequent calibration will be needed. Likewise ashorter time window over which the norrnative distance is deterrnined may be required.

As mentioned above, a portion of a radar pulse is reflected upon beingintercepted by a substrate. How large this portion is depends on a number of factorssuch as the electrical properties, density and composition of the substrate. Differentsubstrates will thus give rise to echoes having different energy content, i.e. they will beof different strength or amplitude. A substrate may thus be identified based on the amplitude of the echo, i.e. the energy content of the echo. 18 Figure 5B shows an example illustrating how different echoes fromdifferent substrates (at different distances to the radar device) are received at differenttimes, and at different energy levels. In figure 5B two pulses are shown to be received ineach a time window, a first echo el is received in a first time window wl and a secondecho e2 is expected to be received in a second time window. The first echo elcorresponds to the norrnative distance, i.e. the echo from the reflector 195. And thesecond echo e2 corresponds to a distance exceeding the threshold thereby indicating alift event. As the two echoes el and e2 will not be received from the same reflector 195at the same times, the second echo e2 has been dashed in figure 5B.

A further echo e3 may also be received in one or more time windows,corresponding to the grass height, which will be more or less constant for a gardenbefore being serviced, and at another constant level after having been serviced.

Also shown is an echo e4 without a corresponding time window. Such anecho may be the result of intemal structures, such as the reflection from a component ofthe robotic lawnmower 100, the reflection from the cutting tool, the reflection from theground or other surfaces or substrates. Even though the fourth echo is shown to appearlast, it may appear any time before, in between or after the other echoes, depending onfrom where it is reflected. Through the use of time windows such echoes may be filteredout or ignored by not assigning a time window to such echoes appearing at unexpectedlocations. In the example of figure 5B, showing four echoes, this would entail a savingin processing power of 25%. It will also render the radar device less susceptible to noiseand other interferences.

By using a reflector of a highly reflective material, echoes from unwantedsources/reflectors may also be filtered out as the reflector made of highly reflectivematerial increases (at least relative to other reflected echoes) the amplitude of thewanted reflected echoes.

As the radar device will receive an echo from each substrate that causes areflection, the radar device 190 may be utilized to determine the height of the grass. Asthe ground will provide a stronger echo, i.e. a reflected pulse having a higher energycontent or signal strength, than for example grass, utilizing the radar device 190 enables the robotic lawnmower 100 to also determine the height of the grass. 19 Figure 6 shows a flowchart of a general method according to the teachingsherein. The robotic lawnniower 100 deterrnines 610 a sensed distance (SD) to a reflector195 utilizing a radar device. The robotic lawnniower 100 then deterrnines 620 Whetherthe sensed distance is greater than a thresho1d distance; and if so detects 630 a lift event.

As has been discussed above, the robotic lawnniower 100 may a1so be configured to deterrnine the height of the grass by utilizing the radar device 190.

Claims (19)

1. A robotic Work tool system (200) comprising a robotic Work tool (100)comprising at least one radar device (190, 190°, 190”) and at least one correspondingreflector (195), the robotic Work tool (100) being configured to determine a sensed distance (SD) to the at least one corresponding reflector(195) utilizing the at least one radar device (190, 190°, 190”); deterrnine Whether the sensed distance is greater than a threshold distance;and if so detect a lift event.

2. The robotic Work tool system according to claim Wherein the reflector (195) is comprised in the body (140) of the robotic Work tool (100).

3. The robotic Work tool system according to any preceding claim, Whereinthe robotic Work tool is further configured to deterrnine the threshold distance over a time period.

4. The robotic Work tool system according to any preceding claim, Whereinthe robotic laWnmoWer (100) comprises a first radar device (190”) and a second radardevice (190”), Wherein the controller (110) is further configured to deterrnine the senseddistance (SD) to the at least one corresponding reflector (195) by: deterrnining a first sensed distance (SD1) utilizing the first radar device(1 90 °); deterrnining a second sensed distance (SD2) utilizing the second radar device (190”); and deterrnining the sensed distance (SD) based on the first sensed distance

5. (SDl) and the second sensed distance (SD2).

6. The robotic work tool system according to any preceding claim,wherein the robotic work tool is further configured to receive a radar echo within a time window (wl, w2, w3).

7. The robotic work tool system according to claim 5, wherein the robotic work tool is further configured to recalibrate at least on time window (wl , w2, w3).

8. The robotic work tool system according to any preceding claim, whereinthe robotic work tool is further configured to determine the energy content of a receivedradar echo and determine a reflection substrate based on the deterrnined energy content of the received radar echo.

9. The robotic work tool system according to any preceding claim, whereinthe robotic work tool is further configured to determine a distance to a reflection substrate based on the time of reception of a received radar echo. q _.\ _-\ v'\^\ (lv VU . ' \ t. ~ .Fc ,~ts“=~\§\f<š>àt\? i

10. The robotic work tool system according to any ”pg ' ~ , wherein a flexible joint (140-1) joining an upper part (140-2) of the body (140) and 7 a lower part (140-3) of the body (140) whereby the upper part (140-2) is flexiblyattached to the lower part (140-3) through the joint (140-1).

11. 1 1. The robotic work tool system according to claim o. wherein the upper part (140-2) is a cover and the lower part (140-3) is a chassis arranged with wheels(130). 21 *\

12. The robotic Work tool system according to claim or liflïs, Wherein the flexible joint (140-1) is a rubber joint.

13. The robotic Work tool system according to claim or Wherein the flexible joint (140-1) is of a be11oW construction.

14. The robotic Work tool system according to claim -?{.~*~_;--10, 1 or lß-Iš, Wherein the flexible joint (140-1) is a seal.

15. The robotic Work tool system according to any of claims to 1¿~§É§;Åš,Wherein the radar device (190) is arranged in the upper part (140-2) of the body (140)and the reflector (195) is arranged in the lower part (140-3) of the body (140).

16. The robotic Work tool system according to any of claims to lfåïgj-š,Wherein the reflector (195) is arranged in the upper part (140-2) of the body (140) andthe radar device (190) is arranged in the lower part (140-3) of the body (140).

17. The robotic Work tool system according to any preceding claim, Wherein the robotic Work tool is a robotic laWnmoWer (100).

18. The robotic Work tool system according to claim being dependenton claim 8, Wherein the robotic laWnmoWer is further configured to determine the height of grass based on the time of reception of a received radar echo.

19. A method for use in a robotic Work tool system (200) comprising arobotic Work tool (100) comprising a radar device (190) and at least one correspondingreflector (195), the method comprising: deterrnining a sensed distance (SD) to the at least one corresponding reflector (195) utilizing the at least one radar device (190, 190°, 190”); 22 deterrnining Whether the sensed distance is greater than a threshold distance;and if so detecting a lift event.