A Robotic Work Tool and a Method for Use in a Robotic Work Tool Comprising a Lift and Collision Detection

[0001] The present disclosure relates to a robotic work tool, such a lawn mower and to a method for improved lift and collision detection to be executed by a robotic work tool.

[0002] Automated or robotic power tools such as robotic lawnmowers are becoming increasingly more popular. In a typical deployment the robotic work tool may not be aware of objects in the work area, stationary or movable, that the robotic work tool may collide with. Therefore, collision detection is necessary in order to enable the robotic work tool to adapt its operation when a collision is detected, to avoid the robotic work tool from simply stopping in front of the object by trying to push through it.

[0003] Likewise, it is important also from a safety perspective to detect that a robotic work tool is lifted, so that the operating member or tool, typically a rotating knife of a robotic lawnmower may be turned off to prevent risk of injuring an operator. The lift and collision detection is usually achieved by arranging the cover of the robotic work tool from being movable in relation to the chassis or main body of the robotic work tool. Such arrangements usually comprise a movable or slidable member which movements are monitored and if a movement in an XY plane (the same as that of the surface being worked) is detected a collision is detected. If a movement in a Z direction (normal to the XY plane), a lift is detected.

[0004] However, in these type of conventional arrangements a collision with an object may also give rise to a movement in a Z direction, whereby a lift may be falsely detected.

[0005] As a lift detection usually turns off any active member or tool of the robotic work tool, a falsely detected lift will impair the operation of the robotic work tool, which is of course unwanted.

[0006] A further drawback of the conventional arrangements is that, in a collision situation, the robotic work tool may respond to a detected collision in a manner that does not improve the situation. Thus, there is a need for improved lift and collision detection for a robotic work tool.

[0007] It is therefore an object of the present disclosure to provide a robotic work tool that solves or at least mitigates one of the problems of the prior-art. In particular, it is an object of the present disclosure to provide a robotic work tool with accurate determination of lift and collision. A further object of the present disclosure is provide a robotic work tool with improved determination of the orientation of the robotic work tool relative a collision object. Yet a further object of the present disclosure is to provide a robotic work tool that may be realized with few parts and at low cost.

[0008] According to a first aspect of the present disclosure, at least one of these objects are met by a robotic work tool comprising a chassis, a body and a controller for controlling operation of the robotic work tool. The robotic work tool comprises at least one three-dimensional sensor arrangement for detecting relative movement of the body and the chassis. The sensor arrangement comprises a sensor element arranged in one of the body and the chassis and a detection element arranged in the other of the body and the chassis. Advantageously the controller is configured to receive sensor input indicating relative movement of the sensor element and the detection element and to determine, from the sensor input, whether a collision or a lift has been detected.

[0009] A three-dimensional sensor arrangement may differentiate a change in relative position of the detection element and the sensor element in lateral direction from a changes of their relative position in vertical direction. The use of a three-dimensional sensor in the robotic work tool according to the present disclosure therefore results in that the controller may be configured to accurately determine from the sensor output whether the robotic work tool has been subjected to a collision (lateral movement of the body) or a lift (vertical movement of the body). This in, turn results in effective operation of the robotic work tool since false alarms essentially are avoided. The robotic work tool may further be produced at a low cost since only one sensor arrangement is necessary for detection of collision or lift.

[0010] According to an embodiment, said sensor element may be configured to sense the position of the detection element in three dimensions.

[0011] According to an embodiment, the detection element may be a magnet and the sensor element may be a three-dimensional sensor configured to detect magnetic field in a plane, and in a direction which is normal to the plane.

[0012] According to an embodiment, said detection element may be a magnet and the sensor element may be configured to detect the magnitude and direction in three-dimensional space of the magnetic field of the magnet. By way of example, the sensor element may be a three-dimensional Hall sensor.

[0013] According to an embodiment, said the sensor element may be configured as an integrated unit, and preferably encapsulated in a single integrated circuit package. Such an integrated circuit package may be soldered to a printed circuit board.

[0014] According to an embodiment, the three-dimensional sensor arrangement may be positioned, as seen from above, at a distance from a geometrical centre of the robotic work tool, and preferably adjacent to a lateral side of the robotic tool. Thereby, a particularly efficient collision detection may be obtained in situations when the robotic work tool does not collide head-on.

[0015] According to an embodiment, the controller may be configured to receive sensor input indicating lateral movement of the sensor element and the detection element relative each other, and in response thereto, determine that a collision has been detected.

[0016] According to an embodiment, said at least one three-dimensional sensor arrangement may comprise a first and a second three-dimensional sensor arrangement, each of said sensor arrangements comprising a respective sensor element arranged in one of the body and the chassis and a respective detection element arranged in the other of the body and the chassis. Also in this case, a particularly efficient collision detection may be obtained in situations when the robotic work tool does not collide head-on. Each of said first and second three-dimensional sensor arrangements may optionally be configured in accordance with any of the embodiments hereinabove.

[0017] According to an embodiment, the controller may be configured to determine, from the sensor input, the direction of the collision.

[0018] According to an embodiment, the controller may be configured to receive sensor input indicating vertical movement of a sensor element and a detection element relative each other, and in response thereto, determine that a lift has been detected.

[0019] According to an embodiment, the controller may be configured to receive sensor input indicating lateral and vertical movement of a sensor element and a detection element relative each other and, in response thereto, determine whether collision or lift has been detected by comparing sensor input with a threshold value.

[0020] According to an embodiment, the robotic work tool may comprising at least one suspension device for movably supporting the body on the chassis. The suspension device may comprise an elongate, optionally rigid, support member having a first end that is supported on the chassis and a second end that supports the body. The elongate support member may be extendable and pivotal such that the body may move laterally and vertically relative the chassis.

[0021] According to an embodiment, the first end of the elongate support member may be configured to be pivotally supported on the chassis. The second end of the elongate support member may be joined to the body.

[0022] According to an embodiment, the elongate support member may comprise a base member and a lift member, respectively configured such that at least a portion of one of the base member and the lift member is slidably receivable in the other of the base member and the lift member, so that the elongate support member may be extended telescopically.

[0023] According to an embodiment, the elongate support member may comprise a first biasing element arranged to bias the lift member towards the base member.

[0024] According to an embodiment, the base member may comprise an elongate guide opening. The first biasing element may be arranged in the base member, and may comprise a guide element that extends through the elongate guide opening and is attached to the lift member.

[0025] According to an embodiment, the suspension device may comprise a second biasing element arranged to bias the elongate support member from a pivoted position to an upright position.

[0026] According to an embodiment, said three-dimensional sensor arrangement or arrangements, as the case may be, and the at least one suspension device, may be arranged laterally separated from each other.

[0027] According to an embodiment, the robotic work tool may comprise at least a first and a second suspension device.

[0028] According to an embodiment, the number of suspension devices may be greater than the number of three-dimensional sensor arrangements.

[0029] According to an embodiment, the robotic work tool may comprise at least a first and a second suspension device. A three dimensional sensor arrangement according to any of the embodiments defined hereinabove may be arranged between the first and the second suspension device.

[0030] According to an embodiment, the robotic work tool may be a robotic lawnmower.

[0031] According to a second aspect, there is provided a method for use in a robotic work tool comprising a chassis; a body; a controller for controlling operation of the robotic work tool and at least one three-dimensional sensor arrangement for detecting relative movement of the body and the chassis, wherein the sensor arrangement comprises a sensor element arranged in one of the body and the chassis and a detection element arranged in the other of the body and the chassis. The method may comprise:

[0032] receiving sensor input indicating relative movement of the sensor element and the detection element; and

[0033] determining, from the sensor input, that a collision or a lift has been detected.

[0034] Further embodiments of the method are provided by combining, wherever possible, the method with features of any of the embodiments defined hereinabove with reference to a robotic work tool.

[0043] The robotic work tool according to the present disclosure will now be described more fully hereinafter. The robotic work tool according to the present disclosure may however be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the present disclosure to those persons skilled in the art. Same reference numbers refer to same elements throughout the description.

[0044] It is appreciated that while the description given herein will be focused on robotic lawnmowers, the teachings herein may also be applied to robotic cleaners such as robotic vacuum cleaners and/or robotic floor cleaners, robotic ball collectors, robotic mine sweepers, robotic farming equipment, or other robotic work tools to be employed in a work area defined by a boundary cable.

[0045] FIG. 1 shows a schematic overview of the robotic working tool 100, which is exemplified by a robotic lawnmower 100, having a front carriage 101′ and a rear carriage 101″. The robotic lawnmower 100 comprises a chassis 110 which in the embodiment shown in FIG. 1 comprises a front chassis 110′ of the front carriage 101′ and a rear chassis 110″ of the rear carriage 101″. The robotic lawnmower 100 further comprises a body (not shown) which may comprise a front body (not shown) of the front carriage 101′ and a rear body (not shown) of the rear carriage 101″. The robotic lawnmower 100 comprises two pair of wheels 150. One pair of front wheels 150 is arranged in the front carriage 101′ and one pair of rear wheels 150 is arranged in the rear carriage 101″. At least some of the wheels 150 are drivably connected to at least one electric motor 450. It is appreciated that while the description herein is focused on electric motors, combustion engines may alternatively be used possibly in combination with an electric motor.

[0046] In the example of FIG. 1, each of the rear wheels 150 is connected to a respective electric motor 450. This allows for driving the rear wheels 150 independently of one another which, for example, enables steep turning.

[0047] The robotic lawnmower 100 also comprises a controller 400, which may be arranged in the rear carriage 101″ as shown in FIG. 1. The controller 400 may be implemented using instructions that enable hardware functionality, for example, by using executable computer program instructions in a general-purpose or special-purpose processor that may be stored on a computer readable storage medium (disk, memory etc.) 410 to be executed by such a processor. The controller 400 is configured to read instructions from the memory 410 and execute these instructions to control the operation of the robotic lawnmower 100 including, but not being limited to, the propulsion of the robotic lawnmower. The controller 400 may be implemented using any suitable, publically available processor or Programmable Logic Circuit (PLC). The memory 410 may be implemented using any commonly known technology for computer-readable memories such as ROM, RAM, SRAM, DRAM, FLASH, DDR, SDRAM or some other memory technology.

[0048] The robotic work tool 100 may further have at least one boundary sensor 470. In the example of FIG. 1 there are four boundary sensors 470. Two boundary sensors 470 are arranged in the front part of the front carriage 101′ and two boundary sensors 470 are arranged in the rear part of the rear carriage 101″. The boundary sensors 470 are configured to detect a magnetic field (not shown) and for detecting a boundary cable and/or for receiving (and possibly also sending) information from a signal generator (will be discussed with reference to FIG. 2).

[0049] In some embodiments, the boundary sensors 470 may be connected to the controller 400, and the controller 400 may be configured to process and evaluate any signals received from the boundary sensor 470. The sensor signals may be caused by the magnetic field being generated by a control signal being transmitted through a boundary cable. This enables the controller 400 to determine whether the robotic lawnmower 100 is close to or crossing a boundary cable, or inside or outside an area enclosed by the boundary cable. This also enables the robotic lawnmower 100 to receive (and possibly send) information from the control signal.

[0050] The robotic lawnmower 100, may comprise a grass cutting device 460, such as a rotating blade driven by a cutter motor 465. The grass cutting device 460 being an example of a work tool 460 for a robotic work tool 100. In the embodiment of FIG. 1 the grass cutting device 460 and the cutter motor 465 are arranged in the front carriage 101. The cutter motor 465 is connected to the controller 400 which enables the controller 400 to control the operation of the cutter motor 465. The controller 400 may also be configured to determine the load exerted on the rotating blade, by for example measure the power delivered to the cutter motor 465 or by measuring the axle torque exerted by the rotating blade. The robotic lawnmower 100 also has (at least) one battery 480 for providing power to the motors 450 and the cutter motor 465.

[0051] The robotic lawnmower 100 is also arranged with at least one suspension device 300 which will be described in greater detail with reference to FIGS. 7a-7c. In the example of FIG. 1, both the front carriage 101′ and the rear carriage 101″ are provided with suspension devices 300, e.g. four suspension devices 300 respectively. The robotic lawnmower 100 is also arranged with at least one three-dimensional sensor arrangement 200, which will be described in detail under FIG. 4. In FIG. 1 at least one three-dimensional sensor arrangement is provided in each of the front carriage 101 and the rear carriage 101′. The three-dimensional sensor arrangement 200 is connected to the controller 400, and the controller 400 may be configured to process and evaluate any signals received from the three-dimensional sensor arrangement 200.

[0052] FIG. 2 shows a schematic view of a robotic work tool system 50 in one embodiment. The schematic view is not to scale. The robotic work tool system 50 comprises a charging station 51 and a boundary cable 56 arranged to enclose a work area 57, in which the robotic work tool e.g. a robotic lawnmower 100 is supposed to serve.

[0053] As in FIG. 1, the robotic work tool 100 is exemplified by a robotic lawnmower, but the teachings herein may also be applied to other robotic work tools adapted to operate within a work area.

[0054] The charging station 51 may have a base plate (not shown) for enabling the robotic lawnmower to enter the charging station 51 in a clean environment and for providing stability to the charging station 51.

[0055] The charging station 51 has a charger 52, that may be coupled to two charging plates 53. The charging plates 53 are arranged to co-operate with corresponding charging plates (not shown) of the robotic lawnmower 100 for charging the battery 480 of the robotic lawnmower 100.

[0056] The charging station 51 also has, or may be coupled to, a signal generator 54 for providing a control signal 55 to be transmitted through the boundary cable 56. The signal generator thus comprises a controller for generating the control signal. The control signal 55 comprises an alternating current, such as a continuously or regularly repeated current signal. The control signal may be a CDMA signal (CDMA—Code Division Multiple Access). The control signal may also or alternatively be a pulsed control signal, the control signal thus comprising one or more current pulses being transmitted periodically. The control signal may also or alternatively be a continuous sinusoidal wave. As is known in the art, the current signal will generate a magnetic field around the boundary cable 56 which the boundary sensors 470 of the robotic lawnmower 100 will detect. As the robotic lawnmower 100 (or more accurately, the boundary sensor 470) crosses the boundary cable 56 the direction of the magnetic field will change. The robotic lawnmower 100 will thus be able to determine that the boundary cable has been crossed, and take appropriate action by controlling the driving of the rear wheels 150 to cause the robotic lawnmower 100 to turn a certain angular amount and return into the work area 57. For its operation within the work area 57, in the embodiment of FIG. 2, the robotic lawnmower 100 may alternatively or additionally use the satellite navigation device 490, supported by the deduced reckoning navigation sensor 495 to navigate the work area 57.

[0057] As can be seen in FIG. 2, there is one example of an object exemplified as a tree (trunk) 58.

[0058] FIG. 3 shows a robotic work tool 100 according to an embodiment of the present disclosure in a perspective side view. The robotic work tool 100 comprises a front carriage 101′ and a rear carriage 101″ which are coupled by a shaft 140 that is rotationally attached to the rear carriage 101″ and fixedly attached to the front carriage 101′. The robotic work tool 100 further comprise a body 120 which in the described embodiment is embodied in a front body 120′ and a rear body 120″ of the respective front carriage 101′ and the rear carriage 101″. The body 120, which may be made of plastic or metal, forms a protective outer cover or housing of the robotic work tool 100 and protects components, such as the aforementioned motors and controller, that are located within the body 120 or on the chassis 110.

[0059] It is appreciated that the present disclosure is not limited to a robotic work tool 100 having separate front and rear carriages 101′, 101″ as described above. Rather, the robotic work tool 100 may also be of type that comprises one single integral chassis and one single integral body. Therefore, in the following description, when it is not necessary to differentiate between a front and rear carriage, reference will only be made to “chassis 110” or “body 120”.

[0060] FIG. 4 shows a view of a cross-section of the robotic work tool 100 along the line A-A in FIG. 3. The body 120 is movably supported on the chassis 110 by suspension devices 300 that will be described more in detail under FIGS. 7a-7c. The body 120 may therefore move laterally relative the chassis 110 when subjected to a collision with an object (not shown) in the surroundings of the robotic work tool. The body may 120 may also move vertically relative the chassis 110 subjected to lift. For example when a person grabs hold of the body 120 and lifts it upwards.

[0061] According to the present disclosure, the robotic work tool 100 comprises at least one three-dimensional sensor arrangement 200 for detecting relative movement of the body 120 and the chassis 110. The three-dimensional sensor arrangement 200 comprises a sensor element 210 and a detection element 220. The sensor element 210 may be arranged on, or in, one of the body 120 and the chassis 110. The detection element 220 may be arranged in, or on, the other of the body 120 and the chassis 110. In the embodiment shown in FIG. 4, the detection element 220 is arranged in the body 120 and the sensor element 210 is arranged in the chassis 110. Preferably, the sensor element 210 is arranged in a water sealed space 112 of the chassis 110 so that it is protected from moisture and contamination. The sensor arrangement 200 is preferably arranged such that the detection element 220 and the sensor element 210 are facing each other in a predetermined default lateral position XY₀ (see FIG. 5a) when the robotic work tool 100 is not subjected to a collision. The sensor arrangement 200 is further arranged such that detection element 220 and the sensor element 210 are in the predetermined default vertical default position Z₀ (see FIG. 5b) when the robotic work tool 100 is not subjected to a lift. In the predetermined default vertical default position Z₀, the detection element 220 and the sensor element may be in contact with each other. However preferably, the detection element 220 and the sensor element 210 are spaced apart a predetermined distance from each other in order to reduce wear.

[0062] By “relative movement” between sensor element and detection element is meant that either the detection element 220 moves relative the sensor element 210 or that the sensor element 220 moves relative the detection element 210.

[0063] The sensor element 210 is configured to sense i.e. detect the position of the detection element 220 in three dimensions. That is, in three spatial dimensions. In detail, the sensor element 210 is configured to sense the lateral position of the detection element 220 relative the sensor element 210. That is, the position of the detection element 220 in a plane XY which is parallel to the sensor element 210. Additionally, the sensor element 210 is configured to sense the vertical position of the detection element 220 relative the sensor element 210. That is, the position of the detection element 220 along a normal between the detection element 220 and the sensor element 210. The sensor element 210 senses continuously or intermittently the position of detection element 220 during operation of the robotic work tool 100, and may thus detect any movement of the detection element 220 relative the sensor element 210.

[0064] It is appreciated that the three-dimensional sensor arrangement 200 is integrated. That is, the three-dimensional sensor arrangement 200 is an integrated unit comprising, or consisting of, one sensor element 210 and one detection element 220. It is further appreciated that the sensor element 210 is integrated. Thus, the sensor element 210 is an integrated, preferably single, physical unit configured to detect the position of the detection element 220 in three dimensions. Analogous, also the detection element 220 is an integrated, preferably single, physical unit.

[0065] FIG. 5a shows schematically the sensor arrangement 200 of FIG. 4 in view from above. The solid central circle 210 indicates the sensor element 210 and the solid dashed circle 210 indicates the detection element 220 in a lateral default position XY₀ in which the detection element 220 is centred over the sensor element 210. The dashed circle 220 indicates a situation in which the detection element 220 has moved laterally in a plane XY relative sensor element 210. The arrow B indicate the direction of the lateral movement of the detection element 220 relative the sensor element 210. Thus, FIG. 5a shows a situation in which the body 120 (not shown) of the robotic work tool 100 has been subjected to collision at location 130 on the body 120 and moved laterally in the XY-plane in direction of arrow B away from the location of the collision 130.

[0066] FIG. 5b shows schematically the sensor arrangement 220 of FIG. 4 in a side view. The solid box 220 indicates the detection element 220 in a vertical default position Z₀ relative the sensor element 210. The dashed box 220 indicates a situation in which the detection element 220 has moved vertically relative the sensor element 210 along the normal Z due to lifting of body 120 (not shown) of the robotic work tool 100.

[0067] The sensor element 210 is preferably a three-dimensional sensor that is configured to detect a magnetic field in a plane or in a direction (e.g. an axis) which is normal to the plane. The three-dimensional sensor element 210 may be a three-dimensional Hall-sensor. For example, the three-dimensional sensor 210 may be TLV493 three-dimensional sensor which is commercially available from the company Infineon Technologies AG. An example of a detection element 220 is a magnet, for example a permanent magnet.

[0068] Movement of the detection element 220 relative the sensor element 210 is detected by the three-dimensional sensor element 210 as a change in the magnetic field that is produced by the detection element 220.

[0069] The sensor element 210 may detect changes in the magnetic field when the detection element 220 moves laterally, i.e. in the XY-plane relative the sensor element 210.

[0070] The sensor element 210 may further detect changes in the magnetic field when the detection element 220 moves vertically relative the sensor element 210, i.e. along the normal Z.

[0071] Thus, specific and different changes in the magnetic field produced by the detection element 220 may be detected by sensor element 210 when the detection element 220 moves laterally, respectively vertically, relative the sensor element 210. Specific changes in the magnetic field may for example be changes in the magnitude of the magnetic field. Other specific changes in the magnetic field that may be detected by the sensor element are for example changes in direction or orientation of the magnetic field.

[0072] In the following description, the three-dimensional sensor arrangement 200 is exemplified by a three-dimensional sensor 210 that is configured to detect a magnetic field and the detection element 220 is exemplified in the form of a magnet. However, the present disclosure is not limited to three-dimensional sensors that are configured to detect a magnetic field. Rather, other three-dimensional sensor element may be utilized, for example capacitive sensors or force resistance sensors (FSR).

[0073] The controller 400 (see FIG. 1) is connected to the sensor arrangement 200 and configured to receive sensor input indicating movement of the detection element 220 relative the sensor element 210. Thus, as described above, the sensor element 210 continuously or intermittently senses the magnetic field produced by the detection element 220 and outputs a corresponding sensor signal as sensor input to the controller 400.

[0074] The controller 400 is further configured to determine from the sensor input whether a collision or a lift has been detected. The controller 400 may thereby be configured to determine from specific changes in the magnetic field produced by the detection element 220 that a collision, respectively a lift, has been detected. The controller 400 may thereby be configured to determine that specific changes in the magnetic field indicates that the detection element 220 has moved laterally relative the sensor element 210 and based thereon determine that a collision is detected. The controller 400 may also be configured to determine that specific changes in the magnetic field indicates that the detection element 220 has moved vertically relative the sensor element 210 and based thereon determine that a lift is detected. The controller 400 may further be configured to operate the robotic work tool (not shown) in dependency of a detected collision or a detected lift. For example, if a collision has been detected, the controller may change the driving path of the robotic work tool. Alternatively, if a lift has been detected, the controller may be configured to switch off the work tool (e.g. the cutter) of the robotic work tool.

[0075] The controller 400 may further be configured to determine the direction of movement of the detection element 220 in the XY-plane relative the sensor element 210. In the described embodiment, the controller 400 may thereby be configured to determine that specific changes in the magnetic field indicate that the detection element 220 has moved laterally relative the sensor element 210 in a specific direction. This is advantageous since it allows the controller 400 to operate the robotic work tool very effectively in view of an object in the close surroundings of the robotic work tool.

[0076] Namely, the controller 400 may from the collision direction determine the position of the object with which the robotic work tool has collided and change the driving path of the robotic work tool so that the robotic work tool moves away or around the object. In a simple example, the controller 400 may be configured to, in response to a detected collision, steer the robotic work tool 200 in the direction of movement of the detection element 220 and thus away from the object.

[0077] The controller 400 may further be configured to determine the amount of lateral and vertical movement of the detection element 220 relative the sensor element 210. In the described embodiment the controller 400 may thereby be configured to determine that specific changes in the magnetic field indicates that the detection element 220 has moved a specific distance laterally away from the lateral default position or vertical away from the vertical default position.

[0078] This is advantageous in a situation where the body 120 of the robotic work tool 100 is subjected to simultaneous collision and lift since it makes possible for the controller to prioritize operation of the robotic tool with regards to e.g. a large vertical movement of the body and ignore a small lateral movement of the body. The controller 400 may thereby be configured to compare the amount of lateral and vertical movement of the detection element 220 relative the sensor element 210 with lateral and vertical movement threshold values. The controller 400 may further be configured to determine whether the movement of the detection element 220 relative the sensor element 210 is predominately vertical or predominately lateral. For example, the controller 400 may be configured to determine that movement of the detection element 220 is predominately lateral when the amount of lateral movement of the detection element 220 approximates or exceeds a lateral movement threshold value. Accordingly, the controller 400 may be configured to determine that movement of the detection element 220 is predominately vertical when the amount of lateral movement of the detection element 220 approximates or exceeds a vertical movement threshold value. The controller 400 may further be configured to determine, when the movement of the detection element 220 is predominately vertical, that a lift is detected. Accordingly, the controller 400 may be configured to determine, when the movement of the detection element 220 is predominately lateral, that a collision is detected.

[0079] FIG. 6 shows a flowchart for a general method according to the present disclosure where the controller 400 receives 610 sensor input from the three-dimensional sensor arrangement indicating relative movement of the sensor element 210 and the detection element 220 and; determines 620, from the control input, whether a collision or a lift has been detected.

[0080] Returning to FIG. 4. The robotic work tool 100 comprises at least one extendable and pivotal suspension device 300 for movably supporting the body 120 on the chassis 110 so that the body 120 may move laterally and vertically relative the chassis 110.

[0081] FIG. 7a shows the suspension device 300 in detail. Thus, the suspension device 300 comprises an elongate support member 310 which may have rotational symmetric form and that comprises a base member 320 and a lift member 330.

[0082] The base member 320 is elongate and has a first end 311, which forms the first end 311 of the elongate support member 310. The base member 320 has further a second end 323 and a stem portion 321 that extends from the second end 323 to a base portion 322 which extends to the first end 311 of the base member 320. The stem portion 321 may have generally uniform cross-section. In the embodiment of FIG. 7a, the stem portion 321 is hollow and of cylindrical cross-section. The base portion 322 may have a cross-section that widens towards the first end 311. The first end 311 is configured to be pivotally supported on a support surface 111 of the chassis 110. The first end 311 may thereby have a flat end surface which is supported on a corresponding flat support surface 111 on the chassis 110. It appreciated that the first end 311 of the elongated support member 310 is freely supported on the chassis 110. That is, the first end 311 is not permanently joined to the chassis 120. As shown in FIG. 7b, the flat end surface of the first end 311 allows the elongate support member 310 to pivot when the body 120 of the robotic work tool is subjected to a collision. Instead of a flat end surface, the first end 311 may have a concave or convex end surface which is configured to be pivotally supported on a corresponding convex or concave support surface 111 of the chassis (not shown). The stem portion 321 further comprises at least one elongate guide opening 324 for receiving a guide pin 341 of a biasing element 340 which will be described further hereinafter. In FIG. 7a only one elongate guide opening 324 is visible. However, two elongate guide openings 324 may be provided opposite to each other in the stem portion 323. The at least one elongate guide opening 324 extends in direction from the base portion 322 towards the second end 323 of the base member 320.

[0083] The lift member 330 is also elongate and hollow and comprises a first end 331 with an opening 332 for receiving the stem portion 321 of the base member 320. The lift member 330 further comprises a second end 312, forming the second end 312 of the elongate support member 310. The second end 312 may be supported on the inner surface of the body 120 of the robotic work tool. Preferably, the second end 312 is joined to the body 120. For example by formfitting.

[0084] The base member 320 and the lift member 330 are configured such that at least a portion of the base member 320 may be slidably received in the hollow lift member 330. Thus, at least a part of the stem portion 321 of the base member 320 may be received through the opening 332 in the first end 331 in the lift member 330 and thus extend within the hollow lift member 330. The elongate support member 310 may thereby be extended and retracted telescopically by sliding of the lift member 330 on the base member 320 in vertical direction towards or away from the chassis 110 (see FIG. 7c).

[0085] Configuration of the lift member 330 and the base member 320 may be achieved by appropriate selection of geometrical dimensions, such as shape, length and inner- and outer diameter of the base member 320 and the lift member 330. Preferably, at least a part of the stem portion 321 of the base member 320 is cylindrical and at least a portion of the hollow lift member 330 is of corresponding hollow cylindrical cross-section.

[0086] The suspension member 300 may further comprise a first biasing element 340 which is arranged to bias, i.e. force, the lift member 330 towards the base member 310. The first biasing element 340 is a resilient member for example a spring, such as helical coil spring, or a helical pressure coil spring. Alternatively, the first biasing element 340 is a resilient member manufactured by elastic material such as a rubber or elastomer. In the embodiment of FIG. 7a, the biasing element 340 is a helical coil pressure spring and is inserted into the stem portion 321 of the base member 320. A first end of the biasing member 340 is supported onto the second end 323 of the stem portion 321. The second end of the biasing member 340 comprises a guide element 341, for example in the form of a guide pin, that extends through the at least elongated guide opening 324 in the stem portion 321 of the base member 320 and into the lift member 330. The guide element 340 is further fixedly attached to the lift member 330. For example, the end of the guide element 341 (in the form of guide pin) is received in a circular opening in the base member 330 (not shown).

[0087] The biasing element 340 and the elongated guide opening 324 in the base member 320 as well as the position of attachment of the guide pin 341 to the lift member 330 are configured such that the biasing element 340 forces the lift member 330, in vertical direction, towards the base member 320 when the suspension device 300 is in a vertical default position. That is, when no lift force is exerted on the lift member 330. Thus, when the suspension device is in a vertical default position, as shown in FIG. 7a, the biasing element 340 (in the form of a coil pressure spring) is slightly compressed.

[0088] This configuration of the suspension device 300 is advantageous since the pre-biased biasing element 340 ensures that the body 120 of the robotic work tool does not move in vertical direction when the robotic work tool runs down a slope. This is further advantageous since any movement of the body 120 of the robotic work tool in vertical direction may be detected as a lift by the controller 400 and result in deactivation of the cutter. An advantage of the elongate guide opening 324 in the base member 320 is that the upper end of the elongated guide opening 324 provides a predetermined stop for movement of the guide element 341 and thus a predetermined stop for lifting of the body 120 of the robotic work tool.

[0089] Turning to FIG. 7c, which show a situation in which the body 120 (not shown) of the robotic work tool is subjected to a lift. This causes the lift member 330 to be pulled vertically upwards away from the base member 320. The guide element 341 thereby runs in the elongated guide opening 324 in the base member 320 and the biasing element 340 is compressed against the second end 323 of the base member 320. When the lifting force on the lift member 330 is released, the biasing element 340 expands, i.e. springs back and forces the lift member 330 in vertical direction towards the base member 320.

[0090] The suspension device 300 may further comprise a second biasing element 350 which is arranged to bias, i.e. force, the elongate support member 310 from a pivoted position (as shown in FIG. 7b) towards an upright position (as shown in FIG. 7a). The second biasing element 350 may also be a resilient element such as a helical coil spring or a helical pressure coil spring. The second biasing element 350, preferably in the form of a helical coil spring, may be arranged around the outer periphery of the base member 320 and attached by one end to the support surface 111 of the chassis 110.

[0091] FIG. 8 shows schematically an arrangement of three-dimensional sensor arrangement 200 and suspension devices 300 in a robotic work tool 100. Preferably, the three-dimensional sensor arrangements 200 are arranged laterally separated from the suspension devices 300. This makes possible to reduce the number of three-dimensional arrangements 200 in the robotic work tool. Typically, the suspension devices 300 are arranged in the corners of the robotic work tool 100 (as shown in FIG. 8). However, the corners of the robotic work tool 100, and thus the suspension devices 300, may be so far spaced apart that lifting of one corner of the robotic work tool not necessarily result in detectable vertical displacement of another corner of body of the robotic work tool. Therefore, if the sensor arrangements 200 are arranged within, or directly above, the suspension devices 300 it may be necessary to have one sensor arrangement in each suspension device 300 to ensure lift detection over the entire robotic work tool. Therefore, by arranging the sensor arrangements 200 laterally spaced apart from the suspension devices 300 it is possible to place at least one or at least two sensor arrangements 200 in positions on the robotic tool 100 that allows lift to be detected regardless which part of the robotic work tool that has been subjected to the lift.

[0092] In FIG. 8 the robotic work tool 100 one three-dimensional sensor arrangement 200 and a first and a second suspension device 300 are arranged in a row. Two such rows are arranged on opposite sides of the robotic work tool and in each row one three dimensional sensor arrangement 200 is arranged between the first and second suspension device 300. This arrangement is advantageous since it allows for a stable collision signal when the body 120 of the robotic work tool is subjected to a collision 130 in diagonal collision direction (as indicated by arrow B). When subjected to a diagonal collision, the body of the robotic work tool 100 may twist which in turn may result in only small movement of the body of the robotic work tool in the XY-plane. However, when the robotic work tool comprises at least two horizontally separated sensors 200, e.g. arranged as in FIG. 8, at least one sensor 200 will detect the collision even if the movement of the body is small. The arrangement of FIG. 8 also provides very reliable detection of lift over the entire body of the robotic work tool 100.