Patent Description:
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. <CIT> suggests a simple means of changing the cutting height of a hover mower, by repositioning the cutting blades on the upper or lower face of a substantially cone-shaped blade carrier disc. Another issue of some of today'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. <CIT> 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's rotation axis has a forward inclination of <NUM> 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.

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 according to claim <NUM>. 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 <CIT>, 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 <CIT>, 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.

Preferred embodiments of the invention are provided in the dependent claims.

The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and nonlimiting 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:.

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.

<FIG> schematically illustrates an overview of a robotic lawnmower system <NUM> configured to mow a lawn <NUM> within a predefined work area <NUM> delimited by a boundary wire <NUM> emitting a magnetic field in the manner known in the art. The robotic lawnmower system <NUM> comprises a self-propelled robotic lawnmower <NUM> and a charging station <NUM>. The robotic lawnmower is provided with wheels, such as a pair of front wheels <NUM> and a pair of rear wheels <NUM>, for moving within the work area <NUM>. Typically, at least one of the wheels <NUM>, <NUM> is connected to a motor, such an electric motor, either directly or via a transmission (not illustrated), for propelling the robotic lawnmower <NUM> across the lawn <NUM>.

<FIG> illustrates functional blocks of the robotic lawnmower <NUM>. In the example of <FIG>, each of the rear wheels <NUM> is connected to a respective electric propulsion motor <NUM>. This allows for driving the rear wheels <NUM> independently of one another, enabling e.g. sharp turning of the robotic lawnmower <NUM>. The robotic lawnmower <NUM> further comprises a controller <NUM>. The controller <NUM> 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 <NUM>. 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 <NUM> is configured to read instructions from the memory <NUM> and execute these instructions possibly in view of different sensor signals to control the operation of the robotic lawnmower <NUM>. Typically, the controller <NUM> 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 <NUM> also controls the operation of a cutter motor <NUM>, 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 <NUM> is connected to the controller <NUM>, and allows the controller <NUM> to communicate with the charging station <NUM> or any other device, such as a remote control or a smart phone (not shown).

The robotic lawnmower <NUM> further comprises a navigation system <NUM>. In the illustrated example, the navigation system <NUM> comprises an inertial navigation device <NUM>, such as an accelerometer or a gyroscope, and a magnetic field sensor <NUM> configured to detect a magnetic field emitted by the boundary wire <NUM> (<FIG>) on/in the ground. A boundary wire may be used for defining the boundaries of the area <NUM> to be treated, or to otherwise provide a reference to assist the robotic lawnmower <NUM> to navigate. The inertial navigation device <NUM> allows the robotic lawnmower <NUM> to keep track of its movement within the area <NUM> to be treated. The inertial navigation device <NUM> may be supplemented by a compass (not shown), to provide basic orientation information that may compensate for any drift of the inertial navigation device <NUM>.

The controller <NUM> also controls the propulsion motors <NUM>, thereby controlling the propulsion of the robotic lawnmower <NUM> within the area <NUM> to be treated. The propulsion motors <NUM> may be stepper motors, allowing the controller <NUM> to keep track of the respective numbers of turns of the motors <NUM>, and thereby also the distance travelled by the robotic lawnmower <NUM>, as well as any turning angle of the robotic lawnmower <NUM> when the motors <NUM> are operated at different speeds or in reverse directions. In this respect, the propulsion motors <NUM> may operate as odometers. Alternatively, the wheels <NUM> may be provided with odometer indexers configured to provide feedback to the controller <NUM> as regards the number of turns of each motor <NUM>. The navigation system <NUM> further comprises a GNSS (Global Navigation Satellite System) receiver <NUM> Navigation information from the navigation system <NUM> and the motors <NUM> is fused in the controller <NUM> to provide an accurate position indication, in order to enable e.g. a systematic movement pattern of the robotic lawnmower <NUM>, wherein the robotic lawnmower <NUM> traverses the lawn <NUM> along parallel, adjacent mowing tracks.

The controller <NUM>, navigation system <NUM>, transceiver <NUM>, and electric motors <NUM>, <NUM> are powered by a battery <NUM>. The robotic lawnmower <NUM> is configured to navigate to the charging station <NUM> on a regular basis, and/or whenever the battery charge is running low, in order to dock with the charging station <NUM> for recharging the battery <NUM>. The charging station <NUM> 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> shows an example of a robotic lawnmower cutting arrangement <NUM> of the robotic lawnmower <NUM> of <FIG>. The robotic lawnmower cutting arrangement <NUM> comprises a blade carrier <NUM> configured to be rotated by the cutting motor <NUM>, via a cutting motor shaft <NUM>, about a vertical blade carrier rotation axis <NUM>. The blade carrier <NUM> may be configured as an injection-molded plastic component. The blade carrier <NUM> may be rotation symmetric and, in the illustrated example, has a circular shape as seen along the blade carrier rotation axis <NUM>, which circular shape is concentric with the blade carrier rotation axis <NUM>. The blade carrier <NUM> comprises a set of blade attachment interfaces <NUM> radially offset from the blade carrier rotation axis <NUM>, each blade attachment interface <NUM> being configured to pivotally hold a respective cutting blade. In the illustration of <FIG>, for clarity of illustration, only one of the blade attachment interfaces <NUM> is connected to a respective cutting blade <NUM>.

The cutting blade <NUM> comprises, at a distal <NUM> end thereof, a substantially flat cutting portion <NUM> extending along a cutting portion plane <NUM> which is horizontal, i.e. parallel to the surface of the lawn to be cut. The cutting portion <NUM> is provided with a cutting edge <NUM> facing in a tangential direction with respect to the blade carrier rotation axis <NUM>. A blade carrier interface <NUM> is arranged at a proximal end <NUM> of the cutting blade <NUM>. The blade carrier interface <NUM> pivotally connects the cutting blade <NUM> to the blade attachment interface <NUM> of the blade carrier <NUM> via a cutting blade attachment screw <NUM>, which operates as a pivot pin and thereby defines the blade pivot axis <NUM>. The blade pivot axis <NUM> may be vertical, as is illustrated by the blade <NUM> attached to the blade carrier <NUM>. According to other embodiments, however, the blade pivot axis <NUM> may be inclined radially inwards by an inwards inclination angle <NUM>, or radially outwards by an outwards inclination angle <NUM>, as is illustrated by the alternative positions <NUM>', <NUM>" of the cutting blade <NUM> and cutting blade attachment screw <NUM> relative to respective blade pivot axes <NUM>', <NUM>". The blade pivot axis <NUM> is radially separated from the blade carrier rotation axis <NUM> by a pivot axis radial offset <NUM>, defined as the radial distance from the blade carrier rotation axis <NUM> to the point where the pivot axis <NUM> passes through a plane <NUM>, which plane <NUM> passes through the blade carrier interface <NUM>, and which plane <NUM> is perpendicular to the blade carrier rotation axis <NUM>. A typical pivot axis radial offset 102may be between <NUM> and <NUM>; and more typically, between <NUM> and <NUM>.

Between the cutting portion <NUM> and the blade carrier interface <NUM>, the cutting blade <NUM> has an offset portion <NUM> which extends in a direction transversal to the cutting portion plane <NUM>, and thereby defines an axial, with respect to the blade carrier rotation axis <NUM> as well as the blade pivot axis <NUM>, cutting plane offset <NUM> between the cutting portion <NUM> and the blade carrier interface <NUM>. The cutting plane offset distance <NUM> may be, by way of example, about <NUM>.

When operating the robotic lawnmower cutting arrangement <NUM>, the blade carrier <NUM> is rotated about the blade carrier rotation axis <NUM> such that the blade <NUM> orbits the blade carrier rotation axis <NUM>. Thereby, the blade carrier interface <NUM> of the cutting blade <NUM> follows a circular path in a blade carrier interface rotation plane <NUM> perpendicular to the blade carrier rotation axis <NUM>, and the cutting portion <NUM> follows a circular path in a cutting plane <NUM>, which planes <NUM>, <NUM> are vertically offset from each other by the cutting plane offset distance <NUM>.

<FIG> illustrate the cutting blade <NUM> in greater detail, wherein the section of <FIG> also provides an exploded view of a fastener assembly for attaching the cutting blade <NUM> to the blade carrier <NUM>. Starting with the section view of <FIG>, the cutting blade <NUM> is integrally formed of steel sheet. The thickness <NUM> of the steel sheet may be, for example, somewhat less than <NUM>. The proximal end <NUM> with the blade carrier interface <NUM> is integrally formed with the rest of the blade <NUM>, and the blade carrier interface <NUM> extends in a blade carrier interface plane <NUM> coinciding with the blade carrier interface rotation plane <NUM>. Thereby, the blade carrier interface plane <NUM> is parallel to the cutting plane <NUM> as well as the cutting portion plane <NUM>. The fastener assembly comprises a cutting blade attachment screw <NUM> configured to penetrate the blade carrier attachment interface <NUM> and be secured in the blade attachment interface <NUM> of the blade carrier <NUM> (<FIG>), and a washer <NUM> configured to be positioned between the cutting blade <NUM> and the blade carrier <NUM> (<FIG>). The washer <NUM> is operative to reduce the friction in the pivotal engagement between the cutting blade <NUM> and the blade carrier <NUM>. The blade attachment screw <NUM> is configured to, when secured to the blade carrier <NUM>, offer the blade carrier interface <NUM> a slight axial play, such that the cutting blade <NUM> can pivot in, and slide along, the blade carrier interface rotation plane <NUM>.

As is illustrated in <FIG>, the blade carrier interface <NUM> is configured as an elongate through-hole. The elongate shape of the through-hole <NUM> offers the engagement between the blade carrier interface <NUM> and the blade attachment interface <NUM> (<FIG>) a radial play, with respect to the blade pivot axis <NUM>, corresponding to the length of the elongate through-hole along the carrier interface rotation plane <NUM>, minus the diameter of the blade attachment screw <NUM>. A typical radial play may be, for example, of the order <NUM>-<NUM>.

Turning now to <FIG>, the cutting blade <NUM> comprises a pair of straight cutting edges <NUM>, which extend along opposite side edges of the cutting blade <NUM>. Each of the edges <NUM> faces in a respective tangential direction, relative to the blade carrier rotation axis <NUM>. The cutting blade <NUM> is elongate, with its direction of elongation extending from the proximal end <NUM> to the distal end <NUM>, and has a cutting blade length L1 along its direction of elongation. The cutting portion <NUM> has a cutting portion length L2 along said direction of elongation of about half the cutting blade length L1. The blade carrier interface portion <NUM>, extending in the blade carrier interface plane <NUM> (<FIG>), has a blade carrier interface portion length L3 along said direction of elongation, which is also about half the cutting blade length L1. The proximal end <NUM> of the cutting blade has a pair of chamfers <NUM>, which facilitate the cutting blade's <NUM> pivoting motion about the blade pivot axis <NUM> (<FIG>.

<FIG> illustrates the robotic lawnmower cutting arrangement <NUM> in operation, and as seen from below, illustrating four cutting blades <NUM> pivotally attached to the blade carrier <NUM>. The freedom of each blade <NUM> to pivot about a respective pivot axis <NUM> (<FIG>), and the radial play of the cutting blade <NUM> in relation to its respective blade pivot axis <NUM> (<FIG>), allow the cutting blade <NUM> to freely rotate <NUM> about the pivot axis <NUM> (<FIG>), move radially outwards <NUM> from the blade carrier rotation axis <NUM> in response to centrifugal forces, move radially inwards <NUM> towards the blade carrier rotation axis <NUM> in response to a collision with an uncuttable object <NUM>, and also to pivot <NUM> about the pivot axis <NUM> in response to a collision with an uncuttable object <NUM>. Referring back to <FIG>, the controller <NUM> may be configured to operate the cutting motor <NUM> below a limit RPM adapted to give each respective cutting portion <NUM> a maximum impact energy of less than <NUM> 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 <NUM> be operated at an output power of less than <NUM> watt.

<FIG> illustrates the robotic lawnmower cutting arrangement <NUM> in operation, and as seen from the side, illustrating two of the four cutting blades <NUM> of <FIG>. While moving in a forward direction <NUM> across the lawn, the robotic lawnmower cutting arrangement <NUM> is rotated about the blade carrier rotation axis <NUM> by the cutting motor <NUM>. Grass blades <NUM> entering the front end <NUM> of the robotic lawnmower cutting arrangement <NUM> will be cut from the side, in a tangential direction relative to the blade carrier rotation axis <NUM>. When passing the trailing end <NUM> of the robotic lawnmower cutting arrangement <NUM>, 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 <NUM>, the grass blades <NUM> 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 <NUM>. The non-cutting parts <NUM>, <NUM> of the cutting blade <NUM> will rotate at a distance above the cut grass <NUM>, minimizing friction against the grass.

As is apparent from <FIG>, the robotic lawnmower <NUM> (<FIG>) is operated by rotating each blade carrier interface <NUM> about the blade carrier rotation axis <NUM> in the blade carrier interface rotation plane <NUM>; and rotating the cutting portions <NUM>, with the cutting edges <NUM> facing in the tangential direction of travel of the cutting blade <NUM>, wherein the cutting edges <NUM> are rotated in the cutting plane <NUM> below the blade carrier interface rotation plane <NUM>.

As is illustrated in <FIG> an axial play in the engagement between the blade carrier interface <NUM> and the blade attachment interface <NUM>, defined by an axial distance between the head of the blade attachment screw <NUM> and the blade carrier <NUM> exceeding the thickness of the washer <NUM> and the sheet material of the cutting blade <NUM>, allows tilting the blade <NUM> somewhat about a blade tilt axis extending through the blade carrier interface <NUM> in the direction of travel of the blade carrier interface <NUM> during rotation of the blade carrier <NUM>, i.e. in a tangential direction relative to the blade carrier rotation axis <NUM>. The tilt play <NUM> thus obtained may, for example, allow a tilt angle of between -<NUM> degrees and +<NUM> degrees relative to a horizontal plane. Upon rotation about the blade carrier rotation axis <NUM>, the cutting blade <NUM> will, in response to centrifugal forces acting thereupon, automatically assume a tilt position suitable for cutting.

<FIG> schematically illustrates an alternative embodiment of a robotic lawnmower cutting arrangement 100a. The robotic lawnmower cutting arrangement 100a comprises a flat blade carrier disc <NUM>, on which a cutting blade 400a is suspended in a non-illustrated manner. The cutting blade 400a is similar in most aspects to the cutting blade <NUM> described in detail hereinabove, but differs from the cutting blade <NUM> in that the blade carrier interface plane 412a is non-parallel to the blade carrier interface rotation plane <NUM> (<FIG>) and the cutting portion plane <NUM>, and the pivot axis <NUM> of the carrier interface is non-vertical. Still, the cutting plane <NUM> and the cutting portion plane <NUM> coincide, and are horizontal. Thanks to the blade pivot axis <NUM> being inclined radially outwards, when hitting an uncuttable object, the cutting blade 400a may pivot about the pivot axis <NUM> to a raised position, higher above the ground. The inclination is oriented such that the blade pivot axis <NUM> lies in the same plane as the blade carrier rotation axis <NUM>. When the blade carrier <NUM> is rotated about the blade carrier rotation axis <NUM>, the path of the blade pivot axis <NUM> defines a cone having its apex pointing downwards.

<FIG> schematically illustrates yet an alternative embodiment of a robotic lawnmower cutting arrangement 100b. The robotic lawnmower cutting arrangement 100b is similar in most aspects to the robotic lawnmower cutting arrangement 100a described with reference to <FIG>, but differs from the robotic lawnmower cutting arrangement 100a of <FIG> in that the carrier interface plane 412b is parallel to the cutting portion plane 410b, and both planes are inclined relative to a horizontal plane. Still, the carrier disc rotation axis <NUM> is vertical such that the cutting plane <NUM>, 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 <NUM>, is horizontal. The cutting portion plane 410b is inclined relative to the cutting plane <NUM>, as seen in the illustrated vertical section comprising the blade carrier rotation axis <NUM> and the blade carrier interface.

<FIG> illustrates still another alternative embodiment of a robotic lawnmower cutting arrangement 100c. The robotic lawnmower cutting arrangement 100c is similar in most aspects to the robotic lawnmower cutting arrangement 100a described with reference to <FIG>, but differs from the robotic lawnmower cutting arrangement 100a of <FIG> in that the carrier interface <NUM> extends along a carrier interface plane 412c which coincides with the offset portion <NUM>.

<FIG> illustrates still another alternative embodiment of a robotic lawnmower cutting blade 400d. The cutting blade 400d is provided with two blade carrier interfaces <NUM> and two cutting portions <NUM>, and is functionally symmetric about its offset portion <NUM>, such that it can be attached to the blade carrier <NUM> (<FIG>) at either end.

<FIG> illustrates still another alternative embodiment of a robotic lawnmower cutting blade 400e. The cutting blade 400e is integrally formed with a threaded rod <NUM>, which doubles as both offset portion <NUM> and blade carrier interface <NUM>, as well as defines the blade pivot axis <NUM>.

<FIG> illustrates still another alternative embodiment of a robotic lawnmower cutting blade 400f, which has its cutting portion plane <NUM> inclined relative to the cutting plane <NUM>, as seen in a radial direction from the blade pivot axis <NUM>.

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>, the method comprising the steps.

In an alternative embodiment, steps <NUM> and <NUM> may be replaced by the step <NUM>: attaching a cutting edge <NUM> to the cutting portion <NUM>. Also, the order of steps <NUM> and <NUM> may be reversed.

Claim 1:
A robotic lawnmower cutting arrangement (<NUM>; 100a; 100c) comprising
a blade carrier (<NUM>; 200a) configured to be rotated by a cutting motor (<NUM>) about a blade carrier rotation axis (<NUM>), the blade carrier extending radially away from the blade carrier rotation axis (<NUM>) and comprising a blade attachment interface (<NUM>) radially offset from the blade carrier rotation axis (<NUM>), the blade attachment interface (<NUM>) being configured to pivotally hold a cutting blade (<NUM>; 400a; 400c; 400d; 400e); and
a cutting blade (<NUM>; 400a; 400c; 400d; 400e) comprising a substantially flat cutting portion (<NUM>) provided with a cutting edge (<NUM>), and a blade carrier interface (<NUM>) pivotally connected to the blade attachment interface (<NUM>) of the blade carrier (<NUM>; 200a), the pivotal connection thereby allowing the cutting blade (<NUM>; 400a; 400c; 400d; 400e) to pivot relative to the blade carrier (<NUM>; 200a) about a blade pivot axis (<NUM>) offset from the blade carrier rotation axis (<NUM>) such that, when the blade carrier (<NUM>; 200a) is rotated about the blade carrier rotation axis (<NUM>), the blade carrier interface (<NUM>) of the cutting blade (<NUM>; 400a; 400c; 400d; 400e) follows a circular path in a blade carrier interface rotation plane (<NUM>) perpendicular to the blade carrier rotation axis (<NUM>), and the cutting portion (<NUM>) follows a circular path in a cutting plane (<NUM>),
characterized in that the cutting edge extends within the cutting plane, and the cutting plane (<NUM>) is axially, with respect to the carrier rotation axis (<NUM>), offset from the blade carrier interface rotation plane (<NUM>) by a cutting plane offset distance (<NUM>).