Patent Publication Number: US-2021185911-A1

Title: Robotic Lawnmower Cutting Arrangement, Robotic Lawnmower, Cutting Blade, and Methods

Description:
FIELD OF THE INVENTION 
     The present invention relates to various aspects of a robotic lawnmower cutting arrangement to be carried by a robotic lawnmower. 
     BACKGROUND 
     Robotic lawnmowers, used for garden or lawn maintenance, are well known in the art. Typically, a robotic mower includes multiple cutting blades detachably connected to a rotating blade carrier disc, acting as blade or knife holder. The blade carrier, usually a circular disc, is arranged to rotate about a vertical axis, perpendicular to the grass surface, and is powered by an electric motor of the robotic lawnmower to rotate the cutting blades. 
     Generally, during cutting the robotic lawnmower uses energy for cutting grass, and to propel itself across the lawn. In a battery powered electric robotic lawnmower, the robotic lawnmower has to return to its charging station after its energy resources have been consumed. Besides performing the actual grass cutting, the robotic lawnmower also consumes energy through its control logic, sensors and communication system. Even the return trip to the charging station to recharge batteries contributes to energy being spent. Typical robotic lawnmowers navigate randomly, and thereby operate on already-cut grass multiple times. For some robotic mowers, several cuts on each particular area may anyhow be required before the desired cutting quality is reached, including aesthetical evenness of the grass surface. 
     Battery powered robotic mowers are limited by their battery capacity and energy efficiency for cutting grass. Energy efficiency depends on the power train efficiency of its electronic control logic, electric motors, wheels, transmission and cutting efficiency, besides environmental conditions such as grass properties, ground properties, obstacles, and proper maintenance of the robotic lawnmower. To cut grass at a high cutting height of the grass blades usually requires less energy than cutting deeper into the grass, with short cut grass length as a result. As a consequence, cutting very short grass is more challenging and energy demanding than cutting grass at a higher cutting height. Another issue of some of today&#39;s robotic lawnmowers is a tendency to get stuck, for example by beaching on bumps in uneven terrain. When this happens, energy such as battery power is also spent when the robotic lawnmower is trying to free itself. EP0808096 B1 suggests the use of a free-rotating shielding lower plate between the grass surface and the rotating blade carrier disc. In order to save energy, the blade carrier disc&#39;s rotation axis has a forward inclination of 7 degrees, such that the cutting blades will not touch again the grass at the rear of the cutting head. There is however still a need for a robotic lawnmower which is capable of covering a larger area, and/or provides improved cutting result, and/or is less prone to get stuck on terrain features. 
     SUMMARY 
     It is an object of the present invention to solve, or at least mitigate, parts or all of the above mentioned problems. To this end there is, according to a first aspect, provided a robotic lawnmower cutting arrangement comprising a blade carrier configured to be rotated by a cutting motor about a blade carrier rotation axis, the blade carrier extending radially away from the blade carrier rotation axis and comprising a blade attachment interface radially offset from the blade carrier rotation axis, the blade attachment interface being configured to pivotally hold a cutting blade; and a cutting blade comprising a substantially flat cutting portion provided with a cutting edge, and a blade carrier interface pivotally connected to the blade attachment interface of the blade carrier, the pivotal connection thereby allowing the cutting blade to pivot relative to the blade carrier about a blade pivot axis offset from the blade carrier rotation axis such that, when the blade carrier is rotated about the blade carrier rotation axis, the blade carrier interface of the cutting blade follows a circular path in a blade carrier interface rotation plane perpendicular to the blade carrier rotation axis, and the cutting portion follows a circular path in a cutting plane, wherein the cutting edge extends within the cutting plane and the cutting plane is axially, with respect to the carrier rotation axis, offset from the blade carrier interface rotation plane by a cutting plane offset distance. Such a robotic lawnmower cutting arrangement allows raising the blade carrier above the grass, which reduces friction against the grass and thereby reduces power consumption. Such a friction reduction may be obtained with a cutting arrangement which is free from, i.e. not provided with, a free-rotating lower plate, such as that of EP0808096B1, between the grass surface and the rotating blade carrier. At the same time, the blade design permits cutting the grass to a low cutting height. In particular, compared to the cutting arrangement of EP0808096B1, the friction between the lower plate and the grass may be avoided. The increased ground clearance thereby obtainable may also reduce the risk of getting stuck. The expression “axially, with respect to the carrier rotations axis, offset” is to be construed as offset in the direction of the carrier rotation axis. The blade pivot axis may be offset from the blade carrier rotation axis by a pivot axis radial offset defined as the radial distance from the blade carrier rotation axis to the point where the pivot axis passes through a plane, which plane passes through the blade carrier interface, and which plane is perpendicular to the blade carrier rotation axis. According to an embodiment, the cutting portion and the blade carrier interface may be interconnected by an offset portion extending in a direction transversal to a plane defined by the cutting portion. Preferably, the cutting arrangement is configured to be oriented, when in use, such that the cutting plane is below the blade carrier interface rotation plane. 
     According to embodiments, the cutting plane offset distance may exceed 5 mm, 10 mm, 15 mm, or 20 mm. Alternatively or additionally, the cutting plane offset distance may be less than 50 mm, less than 35 mm, or less than 25 mm. 
     According to a second aspect, parts or all of the above mentioned problems are solved, or at least mitigated, by a robotic lawnmower cutting arrangement comprising a blade carrier configured to be rotated by a cutting motor about a blade carrier rotation axis, the blade carrier extending radially away from the blade carrier rotation axis and comprising a blade attachment interface radially offset from the blade carrier rotation axis, the blade attachment interface being configured to pivotally hold a cutting blade; and a cutting blade comprising a substantially flat cutting portion extending along a cutting portion plane, the cutting portion being provided with a cutting edge, and a blade carrier interface pivotally connected to the blade attachment interface of the blade carrier, the pivotal connection thereby allowing the cutting blade to pivot relative to the blade carrier about a blade pivot axis, wherein the cutting blade further comprises an offset portion interconnecting the cutting portion and the blade carrier interface and extending in a direction transversal to the cutting portion plane, such that the cutting portion plane is axially, with respect to the carrier rotation axis, offset from the blade carrier interface, at least at the blade carrier interface. Such a robotic lawnmower cutting arrangement allows raising the blade carrier above the grass, which reduces friction against the grass and thereby reduces power consumption. At the same time, the blade design permits cutting the grass to a low cutting height. Optionally, also the offset portion may be provided with a cutting edge. 
     According to embodiments of any of the robotic lawnmower cutting arrangements defined above, the pivotal connection between the blade carrier interface of the cutting blade and the blade attachment interface of the blade carrier is configured to allow free, i.e. unbiased, pivoting of the cutting blade about the blade pivot axis. Thereby, the impact energy when striking an object will be low, which provides a long lifetime along with a high level of safety. According to examples, the cutting blade may be allowed to freely pivot about the blade pivot axis over a pivoting angle range of more than 90 degrees, over more than 120 degrees, over more than 150 degrees, over more than 180 degrees, or over more than 210 degrees. The cutting blade may also be allowed to rotate freely, to complete full turns about the blade pivot axis. 
     According to an embodiment, the blade pivot axis may be inclined in relation to the blade carrier rotation axis. The inclination may be oriented such that when the blade carrier is rotated about the blade carrier axis, the path of the blade pivot axis defines a cone having its apex pointing downwards. Thereby, when hitting an uncuttable object, the cutting blade may pivot about the pivot axis to a raised position, higher above the ground. By way of example, the blade pivot axis may form an angle of between 3 degrees and 20 degrees with the blade carrier rotation axis. Moreover, the blade pivot axis may lie in a plane along which the blade carrier rotation axis extends. Thereby, the same impact behaviour may be obtained regardless of in which direction the blade carrier is rotated. 
     According to embodiments of any of the robotic lawnmower cutting arrangements defined above, the blade carrier interface may be integrally formed with the cutting portion. This may result in a strong and inexpensive blade. According to embodiments, the cutting blade may be integrally formed of a sheet material. The sheet material may, by way of example, be sheet metal, such as steel sheet. A maximum thickness of the sheet material of the cutting portion may, for example, be less than 2 mm, or less than 1.25 mm. By using a thin, and thereby also low-weight blade, the power consumption may be even further reduced. At the same time, a sufficient strength for cutting the grass of a typical lawn is desired. A particularly suitable thickness of the sheet material may be, for example, between 0.4 mm and 0.8 mm. 
     According to some embodiments, the cutting portion plane may coincide with the cutting plane. According to other embodiments, the cutting portion plane may be inclined relative to the cutting plane. The cutting portion plane may, for example, be inclined relative to the cutting plane by an inclination angle of between one and five degrees, as seen in a vertical section comprising the blade carrier rotation axis and the blade carrier interface. 
     According to embodiments, the blade carrier interface may be made of a sheet material extending in a blade carrier interface plane, wherein the blade carrier interface plane is substantially parallel to at least one of the cutting plane and the cutting portion plane, and optionally, both. Such a cutting arrangement is particularly well suited for a situation wherein the blade pivot axis is parallel to the blade carrier rotation axis. According to embodiments, the blade carrier interface plane may form an angle with the at least one of the cutting plane and the cutting portion plane of less than ten degrees, or of less than five degrees. 
     According to embodiments, the cutting blade may be detachably connected to the blade carrier. In particular, the cutting blade may be tool-less detachable from the blade carrier. 
     According to embodiments, the engagement between the blade carrier interface of the cutting blade and the blade attachment interface of the blade carrier may have a radial play, with respect to the blade pivot axis, of at least 5 mm. Thereby, the blade may automatically be thrown radially inwards upon impact with an uncuttable object, which reduces damage to the blade and the uncuttable object. According to embodiments, the radial play may exceed 10 mm, 15 mm, or 20 mm. Alternatively or additionally, the radial play may be less than 50 mm, less than 35 mm, or less than 25 mm. The radial play may be configured to allow moving the cutting blade relative to the blade attachment interface in a direction along a longitudinal axis of the blade. In a transversal direction perpendicular to the longitudinal axis and the blade pivot axis, the engagement between the blade carrier interface of the cutting blade and the blade attachment interface of the blade carrier may have a tighter fit, such that the cutting blade is not allowed to substantially move relative to the blade attachment interface in said transversal direction. Such an arrangement may be obtained by, for example, configuring one of the blade carrier interface of the cutting blade and the blade attachment interface of the blade carrier as an elongate hole or slot having an axis of elongation extending, when in use, in a direction radially away from the blade carrier rotation axis, and pivotally and slidably connecting the other of the blade carrier interface of the cutting blade and the blade attachment interface of the blade carrier to the elongate hole, in a manner to allow it to slide along said elongate hole in said direction of elongation. 
     According to embodiments, the engagement between the blade carrier interface and the blade attachment interface may have an axial play, with respect to the blade pivot axis, of at least 0.2 mm or, according to other embodiments, of at least 0.5 mm. When combined with a cutting portion axially separated from the carrier interface, the axial play reduces the risk that the pivotal connection at the blade carrier interface jams during operation due to centrifugal forces striving to tilt the cutting blade. 
     According to embodiments, the engagement between the blade carrier interface and the blade attachment interface allows tilting the blade about a blade tilt axis extending through the blade attachment interface in the direction of travel of the blade attachment interface during rotation of the blade carrier, i.e. in a tangential direction with respect to the blade carrier rotation axis. Thereby, the blade may, upon rotation, automatically assume an optimal tilt for cutting. According to embodiments, the blade may be tiltable within a tilt play of between 1 degree and 30 degrees, or within a tilt play of between 5 degrees and 15 degrees. When combined with a cutting portion axially separated from the blade carrier interface, the tilt play reduces the risk that the pivotal connection at the blade carrier interface jams during operation due to centrifugal forces striving to tilt the cutting blade. 
     According to embodiments, the cutting edge may be substantially straight. The robotic lawnmower cutting arrangement may be devised in such a manner that, when the blade carrier is rotated, the cutting blade automatically, in response to centrifugal forces, assumes a position in which the cutting edge extends substantially radially away from the blade carrier rotation axis. According to embodiments, the cutting edge forms an angle of less than ten degrees, or of less than five degrees, with said radial direction. According to embodiments the blade may, in addition to the straight cutting edge of the cutting portion, comprise auxiliary cutting edges which may be non-straight, and which may optionally extend along e.g. any offset portion between the blade carrier interface and the cutting portion. 
     According to embodiments, the cutting blade may comprise a first cutting edge extending along a first side edge of the cutting blade, and a second cutting edge extending along a second side edge of the cutting blade, the second side edge being opposite to the first side edge. Thereby, the cutting arrangement may be operated by rotating in either rotation direction about the blade carrier rotation axis, or by rotating alternatingly in the two opposite rotation directions. In particular, the cutting blade may be symmetric about a symmetry axis passing through the blade carrier interface, such that the first and second cutting edges extend along the symmetry axis on either sides thereof. Thereby, the exact same behaviour may be obtained regardless of the direction of rotation of the cutting arrangement. 
     According to embodiments, the cutting blade may be elongate and have a cutting blade length along its direction of elongation, wherein the cutting portion has a cutting portion length along said direction of elongation of at least 20% of the cutting blade length. The cutting portion length may, according to embodiments, exceed 25%, 30%, or 35% of the cutting blade length. Alternatively or additionally, the cutting portion length may be shorter than 70%, or shorter than 50%, of the cutting blade length. 
     According to embodiments, the blade carrier interface may comprise a through-hole having an elongate shape along a plane perpendicular to the blade pivot axis. The elongate shape may have a direction of elongation extending, when in use, in the radial direction, with respect to the blade carrier rotation axis. According to an embodiment, the through-hole may be oval. 
     According to embodiments, the cutting blade may extend between a proximal end, which is provided with said blade carrier interface, and a distal end, which is provided with said cutting portion. The proximal end may be chamfered between short and long side edges of the cutting blade, in order to facilitate pivoting about the blade pivot axis. In an alternative configuration, the cutting blade may have both a cutting portion and a blade carrier interface at each longitudinal end. Such a cutting blade may be alternatingly connected to the blade carrier via either of the blade carrier interfaces for extended service life. 
     According to embodiments, the blade carrier may be made of plastic, and may optionally have the shape of a circular disc. 
     According to embodiments, the blade carrier may pivotally carry a plurality of cutting blades, in the manner defined hereinabove, distributed about its periphery. A suitable number of blades may be, for example, two, three, four, or five. According to a typical example, the cutting assembly comprises at least four cutting blades distributed about the periphery of the blade carrier. 
     According to a third aspect, parts or all of the above mentioned problems are solved, or at least mitigated, by a robotic lawnmower cutting blade comprising a substantially flat cutting portion extending along a cutting portion plane, the cutting portion being provided with a cutting edge; a blade carrier interface; and an offset portion interconnecting the cutting portion and the blade carrier interface and extending in a direction transversal to the cutting portion plane, thereby holding the blade carrier interface at a position offset from the cutting portion plane. Such a cutting blade may be used as the cutting blade in a robotic lawnmower cutting arrangement as defined above. Additional optional features of the robotic lawnmower cutting blade are apparent from the above defined exemplary robotic lawnmower cutting arrangements. 
     According to a fourth aspect, parts or all of the above mentioned problems are solved, or at least mitigated, by a robotic lawnmower comprising a robotic lawnmower cutting arrangement or a robotic lawnmower cutting blade as defined hereinabove. 
     According to embodiments, the blade carrier rotation axis may be substantially vertical, e.g. forming an angle of less than three degrees with a vertical line, when the robotic lawnmower is positioned in normal operating position on a horizontal surface. At least one of a cutting plane and a cutting portion plane may be substantially horizontal, e.g. forming a maximum angle with a horizontal plane of less than three degrees. By having a substantially horizontal cutting plane, the grass may be cut at any position around the periphery of the rotating carrier disc, and not only at the front position as suggested in EP0808096B1. Thereby, cutting efficiency and cutting quality is increased. By having a horizontal cutting portion plane, all grass leaves will receive a horizontal cut at the same height, even if the robotic lawnmower transits over the parcel at a relatively high speed. This also improves the visual quality of the cut. For example, a drawback of an inclined cutting arrangement such as that of EP0808096 B1 is that it tends to give a non-horizontal cut of the grass leaves and, when moving across the lawn at a high speed while cutting, also tends to result in a striped upper envelope of the grass leaves of the lawn. Thereby, multiple re-cuts of each spot of the lawn are required for obtaining an even cut, which consumes energy. As path planning and precision guiding of systematic cutting robotic mowers are reaching better precision, is particularly beneficial to just have to cut each spot once, with minimal overlap, to reach the desired lawn surface evenness quality expected. Hence, according to embodiments, the cutting arrangement is carried by a robotic lawnmower configured for systematic navigation, as opposed to the typical random navigation generally used in present-day robotic lawnmowers. 
     According to embodiments, the robotic lawnmower may further comprise a controller and a cutting motor, wherein the controller is configured to operate the cutting motor below a limit RPM adapted to give the cutting portion of said cutting blade a maximum impact energy of less than 5 joules, and preferably of less than 2 joules. Those limits may be safeguarded e.g. by operating the cutting motor at an RPM adapted to give said cutting portion a total kinetic energy of less than 5 joules or 2 joules, respectively. 
     According to embodiments, the cutting motor may be an electric motor configured to be operated at an output power of less than 200 watt. Such power levels are typical of robotic lawnmowers. According to embodiments, the cutting motor may be configured to be operated at an output power of less than 50 watt, or less than 20 watt. Such power levels may be particularly suitable for low-power robotic lawnmowers for domestic and garden use. 
     According to a fifth aspect, parts or all of the above mentioned problems are solved, or at least mitigated, by a method of operating a robotic lawnmower, the method comprising: rotating a blade carrier interface of a cutting blade about a blade carrier rotation axis in a blade carrier interface rotation plane; and rotating a cutting portion of said cutting blade, the cutting portion having a cutting edge facing in the tangential direction of travel of the cutting blade, wherein the cutting edge is rotated in a cutting plane below said blade carrier interface rotation plane. The cutting edge may extend within the cutting plane. By way of example, the cutting edge may extend in a direction which is radial to the blade carrier rotation axis. Moreover, the cutting edge may extend in a direction which is perpendicular to the blade carrier rotation axis. Additional optional features of the method are apparent from the above defined exemplary robotic lawnmower cutting arrangements. 
     According to a sixth aspect, parts or all of the above mentioned problems are solved, or at least mitigated, by the use of a robotic lawnmower cutting arrangement, robotic lawnmower cutting blade, robotic lawnmower, or method as defined herein for cutting the grass of a golf court fairway at a cutting height of less than 15 mm. The robotic lawnmower cutting arrangement, robotic lawnmower cutting blade, and robotic lawnmower as defined above are particularly well suited for cutting very short grass with very high accuracy, e.g. for golf courts and football fields. 
     According to a seventh aspect, parts or all of the above mentioned problems are solved, or at least mitigated, by the use of a robotic lawnmower cutting arrangement, robotic lawnmower cutting blade, robotic lawnmower, or method as defined herein for cutting the grass of a golf court green at a cutting height of less than 5 mm. According to embodiments, the cutting height may be lower than 4 mm, or even lower than 3 mm. The robotic lawnmower cutting arrangement, robotic lawnmower cutting blade, and robotic lawnmower as defined above may be suited for cutting grass at a cutting height as low as a couple of mm. For golf greens, it may be beneficial to provide the robotic lawnmower with a suction arrangement and grass collection chamber, for vacuuming grass clippings from the golf green. 
     According to an eighth aspect, parts or all of the above mentioned problems are solved, or at least mitigated, by a method of producing a robotic lawnmower cutting blade, the method comprising: bending a sheet metal blank to form a substantially flat cutting portion, a substantially flat blade carrier interface portion, and an offset portion separating the cutting portion from the blade carrier interface portion in a direction transversal to the plane of the cutting portion; and grinding a cutting edge; and optionally, after grinding the cutting edge, locally treating the cutting edge to change the material properties thereof. By locally treating only the cutting edge, and leaving other portions of the cutting blade untreated, the blade may remain sharp for a long time, while the untreated body portion of the cutting blade may allow it to withstand impact with uncuttable objects without breaking. Optionally, a through-hole may be punched in the blade carrier interface portion to form a blade carrier interface. 
     According to a ninth aspect, parts or all of the above mentioned problems are solved, or at least mitigated, by a method of producing a robotic lawnmower cutting blade, the method comprising: bending a sheet metal blank to form a substantially flat cutting portion, a substantially flat blade carrier interface portion, and an offset portion separating the cutting portion from the blade carrier interface portion in a direction transversal to the plane of the cutting portion; and attaching a cutting edge to the cutting portion. Using the method, a hardened cutting edge may be attached to an unhardened cutting portion. The cutting edge may be attached by e.g. soldering, welding, press-fitting or gluing. 
     It is noted that embodiments of the invention may be embodied by all possible combinations of features recited in the claims. Further, it will be appreciated that the various embodiments described for the devices according to some aspects are all combinable with the embodiments of the devices according to other aspects, as well as with the methods according to still other aspects, and vice versa. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of preferred embodiments of the present invention, with reference to the appended drawings, where the same reference numerals will be used for similar elements, wherein: 
         FIG. 1  is a view in perspective of a robotic lawnmower system; 
         FIG. 2  is a block diagram illustrating functional blocks of a robotic lawnmower; 
         FIG. 3  is a side view in section of a robotic lawnmower cutting arrangement, and illustrates three different configurations of a pivotal attachment of a cutting blade to a blade carrier; 
         FIG. 4A  illustrates a section of a cutting blade and blade fastener assembly of the robotic lawnmower cutting arrangement of  FIG. 3 ; 
         FIG. 4B  is a perspective view of the cutting blade of  FIG. 4A ; 
         FIG. 4C  is a top view of the cutting blade of  FIG. 4A ; 
         FIG. 5  is a top view of the robotic lawnmower cutting arrangement of  FIG. 3  when in operation; 
         FIG. 6  is a side view in section of the robotic lawnmower cutting arrangement of  FIG. 3  when mowing a lawn; 
         FIG. 7  is a side view in section of the robotic lawnmower cutting arrangement of  FIG. 3 , with and without axial and radial play of the cutting blade relative to the blade pivot axis; 
         FIG. 8  is a side view in section of an alternative embodiment of a robotic lawnmower cutting arrangement; 
         FIG. 9  is a side view in section of yet an alternative embodiment of a robotic lawnmower cutting arrangement; 
         FIG. 10  is a side view in section of still another alternative embodiment of a robotic lawnmower cutting arrangement; 
         FIG. 11  is a perspective view of an alternative embodiment of a cutting blade; 
         FIG. 12  is a perspective view of yet an alternative embodiment of a cutting blade; 
         FIG. 13  is a perspective view of yet an alternative embodiment of a cutting blade; and 
         FIG. 14  is a flow chart illustrating two alternative methods of manufacturing a robotic lawnmower cutting blade. 
     
    
    
     All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary in order to elucidate the embodiments, wherein other parts may be omitted. 
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
       FIG. 1  schematically illustrates an overview of a robotic lawnmower system  1  configured to mow a lawn  2  within a predefined work area  12  delimited by a boundary wire  11  emitting a magnetic field in the manner known in the art. The robotic lawnmower system  1  comprises a self-propelled robotic lawnmower  10  and a charging station  16 . The robotic lawnmower is provided with wheels, such as a pair of front wheels  18  and a pair of rear wheels  20 , for moving within the work area  12 . Typically, at least one of the wheels  18 ,  20  is connected to a motor, such an electric motor, either directly or via a transmission (not illustrated), for propelling the robotic lawnmower  10  across the lawn  2 . 
       FIG. 2  illustrates functional blocks of the robotic lawnmower  10 . In the example of  FIG. 2 , each of the rear wheels  20  is connected to a respective electric propulsion motor  24 . This allows for driving the rear wheels  20  independently of one another, enabling e.g. sharp turning of the robotic lawnmower  10 . The robotic lawnmower  10  further comprises a controller  26 . The controller  26  may be connected to sensors, actuators, and communication interfaces of various kinds, and may be implemented using a central processing unit executing instructions stored on a memory  28 . Needless to say, different combinations of general and application-specific integrated circuits may be used as well as different memory technologies. In general, the controller  26  is configured to read instructions from the memory  28  and execute these instructions possibly in view of different sensor signals to control the operation of the robotic lawnmower  10 . Typically, the controller  26  is configured to, based on the instructions, control the robotic lawnmower in an autonomous or semi-autonomous manner, i.e. with no, or only occasional, instructions from a human operator. The controller  26  also controls the operation of a cutter motor  300 , which is configured to drive a cutting arrangement comprising a blade carrier holding a set of cutting blades in a manner which will be elucidated further below. 
     A wireless data transceiver  32  is connected to the controller  26 , and allows the controller  26  to communicate with the charging station  16  or any other device, such as a remote control or a smart phone (not shown). 
     The robotic lawnmower  10  further comprises a navigation system  34 . In the illustrated example, the navigation system  34  comprises an inertial navigation device  36 , such as an accelerometer or a gyroscope, and a magnetic field sensor  38  configured to detect a magnetic field emitted by the boundary wire  11  ( FIG. 1 ) on/in the ground. A boundary wire may be used for defining the boundaries of the area  12  to be treated, or to otherwise provide a reference to assist the robotic lawnmower  10  to navigate. The inertial navigation device  36  allows the robotic lawnmower  10  to keep track of its movement within the area  12  to be treated. The inertial navigation device  36  may be supplemented by a compass (not shown), to provide basic orientation information that may compensate for any drift of the inertial navigation device  36 . 
     The controller  26  also controls the propulsion motors  24 , thereby controlling the propulsion of the robotic lawnmower  10  within the area  12  to be treated. The propulsion motors  24  may be stepper motors, allowing the controller  26  to keep track of the respective numbers of turns of the motors  24 , and thereby also the distance travelled by the robotic lawnmower  10 , as well as any turning angle of the robotic lawnmower  10  when the motors  24  are operated at different speeds or in reverse directions. In this respect, the propulsion motors  24  may operate as odometers. Alternatively, the wheels  20  may be provided with odometer indexers configured to provide feedback to the controller  26  as regards the number of turns of each motor  24 . The navigation system  34  further comprises a GNSS (Global Navigation Satellite System) receiver  42  Navigation information from the navigation system  34  and the motors  24  is fused in the controller  26  to provide an accurate position indication, in order to enable e.g. a systematic movement pattern of the robotic lawnmower  10 , wherein the robotic lawnmower  10  traverses the lawn  2  along parallel, adjacent mowing tracks. 
     The controller  26 , navigation system  34 , transceiver  32 , and electric motors  24 ,  30  are powered by a battery  40 . The robotic lawnmower  10  is configured to navigate to the charging station  16  on a regular basis, and/or whenever the battery charge is running low, in order to dock with the charging station  16  for recharging the battery  40 . The charging station  16  may be connected to receive power from the electric power grid. 
     Battery powered robotic mowers are limited by their battery capacity and energy efficiency for cutting grass. If energy consumption can be reduced, cutting coverage, cutting quality and cutting speed can be gained, while reducing wear and tear of the robotic mower machinery. 
       FIG. 3  shows an example of a robotic lawnmower cutting arrangement  100  of the robotic lawnmower  10  of  FIGS. 1-2 . The robotic lawnmower cutting arrangement  100  comprises a blade carrier  200  configured to be rotated by the cutting motor  300 , via a cutting motor shaft  301 , about a vertical blade carrier rotation axis  201 . The blade carrier  300  may be configured as an injection-molded plastic component. The blade carrier  200  may be rotation symmetric and, in the illustrated example, has a circular shape as seen along the blade carrier rotation axis  201 , which circular shape is concentric with the blade carrier rotation axis  201 . The blade carrier  200  comprises a set of blade attachment interfaces  202  radially offset from the blade carrier rotation axis  201 , each blade attachment interface  202  being configured to pivotally hold a respective cutting blade. In the illustration of  FIG. 3 , for clarity of illustration, only one of the blade attachment interfaces  202  is connected to a respective cutting blade  400 . 
     The cutting blade  400  comprises, at a distal  424  end thereof, a substantially flat cutting portion  401  extending along a cutting portion plane  410  which is horizontal, i.e. parallel to the surface of the lawn to be cut. The cutting portion  401  is provided with a cutting edge  403  facing in a tangential direction with respect to the blade carrier rotation axis  201 . A blade carrier interface  404  is arranged at a proximal end  422  of the cutting blade  400 . The blade carrier interface  404  pivotally connects the cutting blade  400  to the blade attachment interface  202  of the blade carrier  200  via a cutting blade attachment screw  418 , which operates as a pivot pin and thereby defines the blade pivot axis  405 . The blade pivot axis  405  may be vertical, as is illustrated by the blade  400  attached to the blade carrier  200 . According to other embodiments, however, the blade pivot axis  405  may be inclined radially inwards by an inwards inclination angle  407 , or radially outwards by an outwards inclination angle  409 , as is illustrated by the alternative positions  413 ′,  413 ″ of the cutting blade  400  and cutting blade attachment screw  418  relative to respective blade pivot axes  405 ′,  405 ″. The blade pivot axis  405  is radially separated from the blade carrier rotation axis  201  by a pivot axis radial offset  102 , defined as the radial distance from the blade carrier rotation axis  201  to the point where the pivot axis  405  passes through a plane  415 , which plane  415  passes through the blade carrier interface  404 , and which plane  415  is perpendicular to the blade carrier rotation axis  201 . A typical pivot axis radial offset  102  may be between 50 and 500 mm; and more typically, between 100 and 300 mm. 
     Between the cutting portion  401  and the blade carrier interface  404 , the cutting blade  400  has an offset portion  406  which extends in a direction transversal to the cutting portion plane  410 , and thereby defines an axial, with respect to the blade carrier rotation axis  201  as well as the blade pivot axis  405 , cutting plane offset  408  between the cutting portion  401  and the blade carrier interface  404 . The cutting plane offset distance  408  may be, by way of example, about 15 mm. 
     When operating the robotic lawnmower cutting arrangement  100 , the blade carrier  200  is rotated about the blade carrier rotation axis  201  such that the blade  400  orbits the blade carrier rotation axis  201 . Thereby, the blade carrier interface  404  of the cutting blade  400  follows a circular path in a blade carrier interface rotation plane  415  perpendicular to the blade carrier rotation axis  201 , and the cutting portion  401  follows a circular path in a cutting plane  402 , which planes  402 ,  415  are vertically offset from each other by the cutting plane offset distance  408 . 
       FIGS. 4A, 4B and 4C  illustrate the cutting blade  400  in greater detail, wherein the section of  FIG. 4A  also provides an exploded view of a fastener assembly for attaching the cutting blade  400  to the blade carrier  200 . Starting with the section view of  FIG. 4A , the cutting blade  400  is integrally formed of steel sheet. The thickness  419  of the steel sheet may be, for example, somewhat less than 1 mm. The proximal end  422  with the blade carrier interface  404  is integrally formed with the rest of the blade  400 , and the blade carrier interface  404  extends in a blade carrier interface plane  412  coinciding with the blade carrier interface rotation plane  415 . Thereby, the blade carrier interface plane  412  is parallel to the cutting plane  402  as well as the cutting portion plane  410 . The fastener assembly comprises a cutting blade attachment screw  418  configured to penetrate the blade carrier attachment interface  404  and be secured in the blade attachment interface  202  of the blade carrier  200  ( FIG. 3 ), and a washer  411  configured to be positioned between the cutting blade  400  and the blade carrier  200  ( FIG. 3 ). The washer  411  is operative to reduce the friction in the pivotal engagement between the cutting blade  400  and the blade carrier  200 . The blade attachment screw  418  is configured to, when secured to the blade carrier  200 , offer the blade carrier interface  404  a slight axial play, such that the cutting blade  400  can pivot in, and slide along, the blade carrier interface rotation plane  415 . 
     As is illustrated in  FIG. 4B , the blade carrier interface  404  is configured as an elongate through-hole. The elongate shape of the through-hole  404  offers the engagement between the blade carrier interface  404  and the blade attachment interface  202  ( FIG. 3 ) a radial play, with respect to the blade pivot axis  405 , corresponding to the length of the elongate through-hole along the carrier interface rotation plane  415 , minus the diameter of the blade attachment screw  418 . A typical radial play may be, for example, of the order 10-50 mm. 
     Turning now to  FIG. 4C , the cutting blade  400  comprises a pair of straight cutting edges  403 , which extend along opposite side edges of the cutting blade  400 . Each of the edges  403  faces in a respective tangential direction, relative to the blade carrier rotation axis  201 . The cutting blade  400  is elongate, with its direction of elongation extending from the proximal end  422  to the distal end  424 , and has a cutting blade length L 1  along its direction of elongation. The cutting portion  401  has a cutting portion length L 2  along said direction of elongation of about half the cutting blade length L 1 . The blade carrier interface portion  422 , extending in the blade carrier interface plane  412  ( FIG. 4A ), has a blade carrier interface portion length L 3  along said direction of elongation, which is also about half the cutting blade length L 1 . The proximal end  420  of the cutting blade has a pair of chamfers  430 , which facilitate the cutting blade&#39;s  400  pivoting motion about the blade pivot axis  405  ( FIG. 4B . 
       FIG. 5  illustrates the robotic lawnmower cutting arrangement  100  in operation, and as seen from below, illustrating four cutting blades  400  pivotally attached to the blade carrier  200 . The freedom of each blade  400  to pivot about a respective pivot axis  405  ( FIG. 4B ), and the radial play of the cutting blade  400  in relation to its respective blade pivot axis  405  ( FIG. 4B ), allow the cutting blade  400  to freely rotate  504  about the pivot axis  405  ( FIG. 3 ), move radially outwards  503  from the blade carrier rotation axis  201  in response to centrifugal forces, move radially inwards  501  towards the blade carrier rotation axis  201  in response to a collision with an uncuttable object  500 , and also to pivot  502  about the pivot axis  405  in response to a collision with an uncuttable object  500 . Referring back to  FIG. 2 , the controller  26  may be configured to operate the cutting motor  300  below a limit RPM adapted to give each respective cutting portion  401  a maximum impact energy of less than 2 joules, thereby resulting in a robotic lawnmower cutting arrangement which is relatively safe to people. For such a situation, it may be sufficient that the cutting motor  300  be operated at an output power of less than 20 watt. 
       FIG. 6  illustrates the robotic lawnmower cutting arrangement  100  in operation, and as seen from the side, illustrating two of the four cutting blades  400  of  FIG. 5 . While moving in a forward direction  600  across the lawn, the robotic lawnmower cutting arrangement  100  is rotated about the blade carrier rotation axis  201  by the cutting motor  300 . Grass blades  501  entering the front end  610  of the robotic lawnmower cutting arrangement  100  will be cut from the side, in a tangential direction relative to the blade carrier rotation axis  201 . When passing the trailing end  620  of the robotic lawnmower cutting arrangement  100 , any remaining, un-cut grass blades will be given a second chance to be cut at the same cutting height. When passing the trailing end  620 , the grass blades  501  will again be cut from the side, in an opposite tangential direction, thus leaving an even quality cut lawn after just one passage of the robotic lawnmower cutting arrangement  100 . The non-cutting parts  404 ,  406  of the cutting blade  400  will rotate at a distance above the cut grass  503 , minimizing friction against the grass. 
     As is apparent from  FIG. 6 , the robotic lawnmower  10  ( FIG. 1 ) is operated by rotating each blade carrier interface  404  about the blade carrier rotation axis  201  in the blade carrier interface rotation plane  415 ; and rotating the cutting portions  401 , with the cutting edges  403  facing in the tangential direction of travel of the cutting blade  400 , wherein the cutting edges  403  are rotated in the cutting plane  402  below the blade carrier interface rotation plane  415 . 
     As is illustrated in  FIG. 7  an axial play in the engagement between the blade carrier interface  404  and the blade attachment interface  202 , defined by an axial distance between the head of the blade attachment screw  418  and the blade carrier  200  exceeding the thickness of the washer  411  and the sheet material of the cutting blade  400 , allows tilting the blade  400  somewhat about a blade tilt axis extending through the blade carrier interface  404  in the direction of travel of the blade carrier interface  404  during rotation of the blade carrier  200 , i.e. in a tangential direction relative to the blade carrier rotation axis  201 . The tilt play  450  thus obtained may, for example, allow a tilt angle of between −5 degrees and +5 degrees relative to a horizontal plane. Upon rotation about the blade carrier rotation axis  201 , the cutting blade  400  will, in response to centrifugal forces acting thereupon, automatically assume a tilt position suitable for cutting. 
       FIG. 8  schematically illustrates an alternative embodiment of a robotic lawnmower cutting arrangement  100   a . The robotic lawnmower cutting arrangement  100   a  comprises a flat blade carrier disc  200 , on which a cutting blade  400   a  is suspended in a non-illustrated manner. The cutting blade  400   a  is similar in most aspects to the cutting blade  400  described in detail hereinabove, but differs from the cutting blade  400  in that the blade carrier interface plane  412   a  is non-parallel to the blade carrier interface rotation plane  415  ( FIG. 3 ) and the cutting portion plane  410 , and the pivot axis  405  of the carrier interface is non-vertical. Still, the cutting plane  402  and the cutting portion plane  410  coincide, and are horizontal. Thanks to the blade pivot axis  405  being inclined radially outwards, when hitting an uncuttable object, the cutting blade  400   a  may pivot about the pivot axis  405  to a raised position, higher above the ground. The inclination is oriented such that the blade pivot axis  405  lies in the same plane as the blade carrier rotation axis  201 . When the blade carrier  200  is rotated about the blade carrier rotation axis  201 , the path of the blade pivot axis  405  defines a cone having its apex pointing downwards. 
       FIG. 9  schematically illustrates yet an alternative embodiment of a robotic lawnmower cutting arrangement  100   b . The robotic lawnmower cutting arrangement  100   b  is similar in most aspects to the robotic lawnmower cutting arrangement  100   a  described with reference to  FIG. 8 , but differs from the robotic lawnmower cutting arrangement  100   a  of  FIG. 8  in that the carrier interface plane  412   b  is parallel to the cutting portion plane  410   b , and both planes are inclined relative to a horizontal plane. Still, the carrier disc rotation axis  201  is vertical such that the cutting plane  402 , i.e. the plane along which the grass tips are being cut, defined by the circle followed by the lowermost ends of the cutting edges of the cutting blades  400 , is horizontal. The cutting portion plane  410   b  is inclined relative to the cutting plane  402 , as seen in the illustrated vertical section comprising the blade carrier rotation axis  201  and the blade carrier interface. 
       FIG. 10  illustrates still another alternative embodiment of a robotic lawnmower cutting arrangement  100   c . The robotic lawnmower cutting arrangement  100   c  is similar in most aspects to the robotic lawnmower cutting arrangement  100   a  described with reference to  FIG. 8 , but differs from the robotic lawnmower cutting arrangement  100   a  of  FIG. 8  in that the carrier interface  404  extends along a carrier interface plane  412   c  which coincides with the offset portion  406 . 
       FIG. 11  illustrates still another alternative embodiment of a robotic lawnmower cutting blade  400   d . The cutting blade  400   d  is provided with two blade carrier interfaces  404  and two cutting portions  201 , and is functionally symmetric about its offset portion  406 , such that it can be attached to the blade carrier  200  ( FIG. 3 ) at either end. 
       FIG. 12  illustrates still another alternative embodiment of a robotic lawnmower cutting blade  400   e . The cutting blade  400   e  is integrally formed with a threaded rod  1200 , which doubles as both offset portion  406  and blade carrier interface  404 , as well as defines the blade pivot axis  405 . 
       FIG. 13  illustrates still another alternative embodiment of a robotic lawnmower cutting blade  400   f , which has its cutting portion plane  410  inclined relative to the cutting plane  402 , as seen in a radial direction from the blade pivot axis  405 . 
     Thanks to the ability of reaching very close to the ground with high precision and low friction, the methods and devices herein are suitable for cutting very short grass, in applications such as golf court fairways and golf court greens. 
     The cutting blade described above may be produced following a production method illustrated in the flow chart of  FIG. 14 , the method comprising the steps 
       1101 : providing a sheet metal blank; 
       1102 : bending the sheet metal blank to form a substantially flat cutting portion  401  ( FIG. 4B ), a substantially flat blade carrier interface portion  420 , and an offset portion  406  separating the cutting portion  401  from the blade carrier interface portion  420  in a direction transversal to the plane of the cutting portion; 
       1103 : grinding a cutting edge  403 ; and 
       1104 : after grinding the cutting edge, locally treating the cutting edge to change the material properties thereof, for example to change its hardness. 
     In an alternative embodiment, steps  1103  and  1104  may be replaced by the step  1105 : attaching a cutting edge  403  to the cutting portion  401 . Also, the order of steps  1102  and  1105  may be reversed. 
     The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims. For example, a vertical a blade carrier rotation axis has been illustrated. However, also a robotic lawnmower cutting arrangement having a blade carrier rotation axis which is tilted relative to a vertical axis may benefit from the teachings disclosed herein. Hence, even though less preferred for cutting very short grass, such an arrangement is also intended to be within the scope of the claims. Similarly, a blade carrier without any free-rotating lower protection plate, such as that of EP0808096 B1, has been illustrated. However, also a robotic lawnmower cutting arrangement having a free-rotating lower protection plate may benefit from the teachings disclosed herein. Hence, even though less preferred for cutting very short grass, such an arrangement is also intended to be within the scope of the claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality.