Patent Publication Number: US-2022217904-A1

Title: Autonomous Robotic Lawnmower Comprising Suspension Means Progressively Limiting Pivotal Movement of a Cutting Unit

Description:
TECHNICAL FIELD 
     The present disclosure relates to an autonomous robotic lawnmower comprising a driving unit with one or more drive wheels and a cutting unit comprising one or more support wheels configured to support the cutting unit by abutting against a ground surface during operation of the lawnmower. 
     BACKGROUND 
     Autonomous robotic lawnmowers of different configurations are available on the market today which are capable of cutting grass in areas in an autonomous manner. Some robotic lawnmowers require a user to set up a border wire around a lawn that defines the area to be mowed. Such robotic lawnmowers use a sensor to locate the wire and thereby the boundary of the area to be trimmed. In addition to the wire, robotic lawnmowers may also comprise other types of positioning units and sensors, for example sensors for detecting an event, such as a collision with an object within the area. 
     A robotic lawnmower may comprise one or more batteries and one or more electric motors being powered by the one or more batteries. Some robotic lawnmowers comprise a photovoltaic module arranged to generate electricity from the sun&#39;s rays which may fully or partially provide an energy source for charging the one or more batteries. The robotic lawnmower may move in a systematic and/or random pattern to ensure that the area is completely covered. In some cases, the robotic lawnmower uses the wire to locate a recharging dock used to recharge the one or more batteries. 
     Generally, robotic lawnmowers operate unattended within the area in which they operate. Many areas comprise more or less slopes which may pose problems for the traction and navigability of the robotic lawnmower. Such problems may adversely affect the coverage of an area operated by a robotic lawnmower. Moreover, in general, an important aspect of robotic lawnmowers is the cutting result. 
     In addition, even though robotic lawnmowers are intended to operate unattended within an area, safety is a concern because people and animals may be present in the area during operation of the robotic lawnmower. 
     Furthermore, generally, on today&#39;s consumer market, it is an advantage if products, such as robotic lawnmowers, have conditions and/or characteristics suitable for being manufactured and assembled in a cost-efficient manner. 
     SUMMARY 
     It is an object of the present invention to overcome, or at least alleviate, at least some of the above-mentioned problems and drawbacks. 
     According to an aspect of the invention, the object is achieved by an autonomous robotic lawnmower comprising a driving unit comprising one or more drive wheels and a cutting unit configured to cut grass during operation of the lawnmower. The cutting unit comprises one or more support wheels configured to support the cutting unit by abutting against a ground surface during operation of the lawnmower. The cutting unit is movably arranged relative to the driving unit, and wherein the lawnmower comprises a suspension assembly configured to progressively limit movement between the cutting unit and the driving unit during operation of the lawnmower. 
     Irregularities in an area operated by a robotic lawnmower, such as bumps, slopes, undulations, and the like, are likely to adversely affect the cutting result. The cutting result can be subdivided into visual cutting result and uniformity of cutting. The visual cutting result can be defined as the visual cutting result determined by a person viewing a mowed lawn. The uniformity of the cutting can be defined as uniformity of a length of the grass of a mowed lawn, i.e. if straws of the grass in a lawn are cut to a uniform length. 
     Since the cutting unit is movably arranged relative to the driving unit and since the robotic lawnmower comprises the suspension assembly, a robotic lawnmower is provided capable of cutting an area with improved cutting result. This because when the lawnmower is operating on an area comprising irregularities, the cutting unit may move relative to the driving unit so as to follow the terrain of the area operated. In this manner, the cutting unit will obtain a more advantageous position relative to the lawn during operation of the robotic lawnmower which improves the cutting result. 
     In addition, since the robotic lawnmower comprises the suspension assembly, the cutting unit may move relative to the driving unit in a more controlled manner so as to follow the terrain of the area operated in a smoother and more controlled manner, which provides conditions for a further improved cutting result. 
     Furthermore, since the cutting unit is movably arranged relative to the driving unit and since the robotic lawnmower comprises the suspension assembly, a robotic lawnmower is provided having conditions for an improved terrain operating capability. This because when the lawnmower is operating on an area comprising irregularities, the cutting unit may move relative to the driving unit so as to follow the terrain of the area operated in a smooth and controlled manner. 
     Moreover, since the cutting unit can follow the terrain of the area operated to obtain a more advantageous position relative to the lawn during operation of the robotic lawnmower, portions of the cutting unit are less likely to be exposed to the sides of the robotic lawnmower during operation of the lawnmower which normally would pose safety concerns in case people and/or animals are present in the area operated by robotic lawnmower. Accordingly, due to these features, a robotic lawnmower is provided capable of cutting an area in a safer manner. 
     Accordingly, an autonomous robotic lawnmower is provided overcoming, or at least alleviating, at least some of the above-mentioned problems and drawbacks. As a result, the above-mentioned object is achieved. 
     Optionally, the cutting unit is pivotally arranged to the driving unit. Thereby, during operation of the lawnmower, the cutting unit may pivot relative to the driving unit so as to follow the terrain of the area operated in a smooth and controlled manner. In this way, the cutting unit may obtain a more advantageous angle relative to the lawn during operation of the robotic lawnmower which can improve the cutting result as well as the safety during operation of the robotic lawnmower. 
     Optionally, the cutting unit is pivotally arranged to the driving unit to pivot around a pivot axis. Thereby, during operation of the lawnmower, the cutting unit may pivot relative to the driving unit around the pivot axis so as to follow the terrain of the area operated in a smooth and controlled manner. 
     Optionally, the pivot axis is substantially parallel to a forward direction of travel of the lawnmower. Thereby, during operation of the lawnmower, the cutting unit may pivot relative to the driving unit around the pivot axis so as to follow the terrain of the area operated in a smooth and controlled manner. In this way, the cutting unit may obtain a more advantageous angle relative to the lawn during operation of the robotic lawnmower which can improve the cutting result. Furthermore, since the pivot axis is substantially parallel to a forward direction of travel of the lawnmower, it can be ensured that the robotic lawnmower is rigid in the longitudinal direction thereof, i.e. that the cutting unit is not pivoted around an axis angled relative to the forward direction of travel of the lawnmower, such as an axis perpendicular to the forward direction of travel of the lawnmower. As a result, the cutting result can be improved and the risk that the cutting unit is bumping into objects protruding from an area operated is reduced. 
     Optionally, the lawnmower comprises a shaft, and wherein the cutting unit is pivotally arranged to the driving unit via the shaft. Thereby, a simple and reliable solution is provided for allowing the cutting unit to pivot relative to the driving unit so as to follow the terrain of the area operated in a smooth and controlled manner. As a further result, a robotic lawnmower is provided having conditions and characteristics suitable for being manufactured and assembled in a cost-efficient manner. 
     Optionally, the suspension assembly comprises one or more suspension units arranged at a distance from the pivot axis. Thereby, a simple and reliable solution is provided for progressively limit movement between the cutting unit and the driving unit during operation of the lawnmower in a smooth and controlled manner. Moreover, a robotic lawnmower is provided having conditions and characteristics suitable for being manufactured and assembled in a cost-efficient manner. 
     Optionally, the suspension assembly comprises a first suspension unit arranged on a first side of a vertical plane extending along the pivot axis and a second suspension unit arranged on a second side of the vertical plane extending along the pivot axis. Thereby, a simple and reliable solution is provided for progressively limit movement between the cutting unit and the driving unit during operation of the lawnmower in a further smoother and more controlled manner. Moreover, a robotic lawnmower is provided having conditions and characteristics suitable for being manufactured and assembled in a cost-efficient manner. 
     Optionally, the suspension assembly is configured to limit pivoting movement between the cutting unit and the driving unit. Thereby, when the lawnmower is operating on an area comprising irregularities, the cutting unit may pivot relative to the driving unit so as to follow the terrain of the area operated in a smoother and more controlled manner, which provides conditions for a further improved cutting result. 
     Optionally, the suspension assembly is configured to limit pivoting movement between the cutting unit and the driving unit to a maximum pivoting movement within the range of 7 degrees to 15 degrees, or within the range of 8 degrees to 12 degrees. Thereby, a robotic lawnmower is provided in which the cutting unit may move relative to the driving unit so as to follow the terrain of the area operated in a smooth and controlled manner, while it is ensured that the cutting unit does not become wobbly or unstable. As a further result thereof, a robotic lawnmower is provided having conditions for a further improved terrain operating capability. 
     Optionally, the suspension assembly comprises one or more suspension units each comprising a spring element. Thereby, a simple and reliable solution is provided for progressively limit movement between the cutting unit and the driving unit during operation of the lawnmower in a smooth and controlled manner. Moreover, a robotic lawnmower is provided having conditions and characteristics suitable for being manufactured and assembled in a cost-efficient manner. 
     Optionally, the driving unit comprises two or more drive wheels and the cutting unit comprises two or more support wheels. When a previously available robotic lawnmower having four or more wheels is operating an area having irregularities, at least one of the wheel will in many cases be lifted from the ground surface during operation of the lawnmower. If so, during movement of the robotic lawnmower, the lawnmower will probably tip at a certain position in which the lifted wheel engages the ground surface and another wheel is lifted from the ground surface. As a result thereof, the angle between the cutting unit of the robotic lawnmower and the ground surface is suddenly changed, which leads to an uneven cutting result. That is, upon such a tilting, the cutting unit will cut straws of the grass of the lawn to a non-uniform length. 
     Moreover, upon such a tilting of the robotic lawnmower, there is an increased risk of the cutting unit bumping into objects protruding from an area operated. Furthermore, upon such a tilting of the robotic lawnmower, portions of the cutting unit will be exposed to the sides of the robotic lawnmower during operation of the lawnmower which may pose safety concerns in case people and/or animals are present in the area operated by robotic lawnmower. 
     Accordingly, since the cutting unit according to these embodiments is movably arranged relative to the driving unit and since the robotic lawnmower comprises the suspension assembly, a robotic lawnmower is provided in which the two or more support wheels and the two or more drive wheels can follow the terrain of the ground surface in an improved manner with a higher probability of ground engaging contact of all wheels of the robotic lawnmower. As a result thereof, the cutting unit may move relative to the driving unit so as to follow the terrain of the area operated to obtain a more advantageous position relative to the lawn during operation of the robotic lawnmower, which provides conditions for an improved the cutting result, a reduced risk of the cutting unit bumping into objects protruding from an area operated, as well as an improved safety during operation of the robotic lawnmower. 
     Optionally, the suspension assembly is configured to bias the cutting unit towards a neutral position relative to the driving unit. Thereby, the cutting unit may move relative to the driving unit so as to follow the terrain of the area operated in a smooth and controlled manner while the suspension assembly biases the cutting unit towards the neutral position. As a result, conditions are provided for an improved the cutting result, improved safety during operation of the robotic lawnmower, as well as an improved terrain operating capability of the robotic lawnmower. 
     Optionally, the neutral position constitutes a position of the cutting unit relative to the driving unit in which each ground engaging portion of said drive wheels and said support wheels extend along a flat plane. Accordingly, due to these features, the suspension assembly will apply no biasing force to the cutting unit when the robotic lawnmower is positioned on a flat ground surface and will apply a biasing force to the cutting unit towards the neutral position when the cutting unit is moved therefrom. As a result, conditions are provided for an improved the cutting result, improved safety during operation of the robotic lawnmower, as well as an improved terrain operating capability of the robotic lawnmower. 
     Optionally, the suspension assembly is configured to bias the cutting unit towards the neutral position with a magnitude that increases with increasing offset of the cutting unit from the neutral position. Accordingly, due to these features, the suspension assembly will apply a greater biasing force to the cutting unit towards the neutral position when the cutting unit is moved or pivoted a greater distance from the neutral position than when the cutting unit is moved or pivoted a shorter distance from the neutral position. As a result, conditions are provided for an improved the cutting result, improved safety during operation of the robotic lawnmower, as well as an improved terrain operating capability of the robotic lawnmower. 
     Optionally, the suspension assembly is configured to progressively limit movement between the cutting unit and the driving unit with a limiting force that increases with increased rate of movement between the cutting unit and the driving unit. Thereby, conditions are provided for a further smoother and more controlled movement between the cutting unit and the driving unit. As a further result, conditions are provided for an improved the cutting result, as well as an improved terrain operating capability of the robotic lawnmower. 
     Optionally, the lawnmower comprises a driving unit chassis and a cutting unit chassis, and wherein the suspension assembly is configured to progressively limit movement between the cutting unit chassis and the driving unit chassis. Thereby, a simple and reliable solution is provided for improving the cutting result, improving the safety during operation of the robotic lawnmower, as well as an improving the terrain operating capability of the robotic lawnmower. 
     Optionally, the driving unit comprises one or more electrical motors configured to rotate said one or more drive wheels to provide motive power to the lawnmower. Thereby, a simple, reliable, and environmentally friendly propulsion of the robotic lawnmower is provided. 
     Optionally, the lawnmower comprises a control unit configured to propel the lawnmower in an autonomous manner. Thereby, a robotic lawnmower is provided capable of navigating and cutting grass in an area without the intervention of a user. 
     Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects of the invention, including its particular features and advantages, will be readily understood from the example embodiments discussed in the following detailed description and the accompanying drawings, in which: 
         FIG. 1  illustrates an autonomous robotic lawnmower, according to some embodiments, 
         FIG. 2  illustrates a lawnmower chassis of the autonomous robotic lawnmower illustrated in  FIG. 1 , 
         FIG. 3  illustrates a top view of the lawnmower chassis illustrated in  FIG. 2 , and 
         FIG. 4  illustrates a cross section of the lawnmower chassis illustrated in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     Aspects of the present invention will now be described more fully. Like numbers refer to like elements throughout. Well-known functions or constructions will not necessarily be described in detail for brevity and/or clarity. 
       FIG. 1  illustrates an autonomous robotic lawnmower  1 , according to some embodiments. As is further explained herein, the autonomous robotic lawnmower  1  is a self-propelled autonomous robotic lawnmower  1  capable of navigating and cutting grass in an area without the intervention of a user. For the reason of brevity and clarity, the autonomous robotic lawnmower  1  is in some places herein referred to as the robotic lawnmower  1  or simply the lawnmower  1 . The robotic lawnmower  1  comprises a driving unit  3  comprising drive wheels  5 . According to the illustrated embodiments, the robotic lawnmower  1  comprises two drive wheels  5 . According to further embodiments, the robotic lawnmower  1  may comprise another number of drive wheels  5 , such as one, three, or the like. 
     Moreover, the robotic lawnmower  1  comprises a cutting unit  7  configured to cut grass during operation of the robotic lawnmower  1 . According to the illustrated embodiments, the cutting unit  7  comprises two support wheels  9  configured to support the cutting unit  7  by abutting against a ground surface  11  during operation of the robotic lawnmower  1 . According to further embodiments, the robotic lawnmower  1  may comprise another number of support wheels  9 , such as one, three, four, or the like. 
     The robotic lawnmower  1  comprises a control unit  23 . The control unit  23  is configured to propel and navigate the robotic lawnmower  1  in an autonomous manner without the intervention of a user by controlling electrical motors configured to rotate the driving wheels, using input from a sensor  25 . The control unit  23  may be configured to control propulsion of the robotic lawnmower  1 , and steer the robotic lawnmower  1 , so as to navigate the robotic lawnmower  1  in an area to be operated. The sensor  25  may comprise one or more sensors arranged to sense a magnetic field of a wire, and/or one or more positioning units, and/or one or more sensors arranged to detect an impending or ongoing collision event with an object. In addition, the robotic lawnmower  1  may comprise a communication unit connected to the control unit  23 . The communication unit may be configured to communicate with a remote communication unit to receive instructions therefrom and/or to send information thereto. The communication may be performed wirelessly over a wireless connection such as the internet, or a wireless local area network (WLAN), or a wireless connection for exchanging data over short distances using short-wavelength, i.e. ultra-high frequency (UHF) radio waves in the industrial, scientific, and medical (ISM) band from 2.4 to 2.485 GHz. 
     The control unit  23  may be configured to control propulsion of the robotic lawnmower  1 , and steer the robotic lawnmower  1 , so as to navigate the robotic lawnmower  1  in a systematic and/or random pattern to ensure that an area is completely covered, using input from one or more of the above described sensors and/or units. Furthermore, the robotic lawnmower  1  may comprise one or more batteries arranged to supply electricity to components of the robotic lawnmower  1 . As an example, the one or more batteries may be arranged to supply electricity to electrical motors of the robotic lawnmower  1  by an amount controlled by the control unit  23 . 
     According to the illustrated embodiments, the control unit  23  is configured to steer the robotic lawnmower  1  by controlling drive wheels  5  on opposite sides of the driving unit  3  to rotate at different speeds. In  FIG. 1 , a forward direction fd of travel of the robotic lawnmower  1  is indicated. According to the illustrated embodiments, the forward direction fd of travel is a direction obtained when the drive wheels  5  of the driving unit are rotating at the same rotational speed in a forward rotational direction and the robotic lawnmower  1  is propelled on a flat horizontal surface with no wheel slip. According to the illustrated embodiments, the cutting unit  7  is arranged in front of the driving unit  3  seen in the forward direction fd. Moreover, according to the illustrated embodiments, the driving unit  3  is arranged behind the cutting unit  7  seen in the forward direction fd and the drive wheels  5  can, according to the illustrated embodiments, be referred to as rear wheels. 
     According to the illustrated embodiments, the robotic lawnmower  1  is configured to be used to cut grass in areas used for aesthetic and recreational purposes, such as gardens, parks, city parks, sports fields, lawns around houses, apartments, commercial buildings, offices, and the like. The sports fields may include soccer fields, golf courses, and the like. According to some embodiments of the present disclosure, the weight of the robotic lawnmower is less than 100 kg, or less than 75 kg. Moreover, according to some embodiments of the present disclosure, the length of the robotic lawnmower  1 , measured in the forward direction fd, is less than 1.5 metres, and the width of the robotic lawnmower  1 , measured in a direction perpendicular to the forward direction fd, is less than 1.5 metres. 
       FIG. 2  illustrates a lawnmower chassis  1 ′ of the robotic lawnmower  1  illustrated in  FIG. 1 . Below, simultaneous reference is made to  FIG. 1  and  FIG. 2 . As is evident from  FIG. 2 , the robotic lawnmower chassis  1 ′ of the robotic lawnmower  1  illustrated in  FIG. 1  comprises a driving unit chassis  3 ′ and a cutting unit chassis  7 ′. The driving unit chassis  3 ′ form part of the driving unit  3  and is arranged to support components of the driving unit  3 . The cutting unit chassis  7 ′ form part of the cutting unit  7  and is arranged to support components of the cutting unit  7 . 
     Moreover, in  FIG. 2 , the support wheels  9  of the cutting unit  7 , as well as the drive wheels  5  of the driving unit  3 , are fully visible. Moreover, as is evident from  FIG. 2 , according to the illustrated embodiments, the robotic lawnmower  1  comprises one electrical motor  21  per drive wheel  5 , wherein each electrical motor  21  is configured to rotate one drive wheel  5  to steer and propel, i.e. provide motive power to, the robotic lawnmower  1 . According to further embodiments, the robotic lawnmower  1  may comprise another number of electrical motors  21 , such as one electrical motor arranged to rotate more than one drive wheel to provide motive power to the robotic lawnmower  1 . 
     Moreover, as can be seen in  FIG. 2 , the cutting unit  7  comprises a number of cutting members  12  arranged to cut grass during operation of the robotic lawnmower  1 . According to the illustrated embodiments, the cutting unit  7  comprises three cutting members  12  in the form of cutting discs. According to further embodiments, the cutting unit  7  may comprise another number of cutting members  12 , and another type of cutting members  12 , such as one or more cutting arms. 
     According to the embodiments explained herein, the cutting unit  7  is movably arranged relative to the driving unit  3 . According to the illustrated embodiments, the cutting unit  7  is pivotally arranged to the driving unit  3  to pivot around a pivot axis ax. As can be seen in  FIG. 1 , the robotic lawnmower  1  comprises a shaft  15 , and wherein the cutting unit  7  is pivotally arranged relative to the driving unit  3  via the shaft  15 . The direction of elongation of the shaft  15  extends in a direction substantially coinciding with the forward direction fd of travel of the robotic lawnmower  1 . Thus, due to these features, the cutting unit  7  is pivotally arranged to the driving unit  3  around a pivot axis ax being substantially parallel to a forward direction fd of travel of the robotic lawnmower  1 , as well as being substantially parallel to a horizontal flat plane p 2  onto which the robotic lawnmower  1  is positioned. 
     Moreover, according to the embodiments explained herein, the robotic lawnmower  1  comprises a suspension assembly  13 ,  13 ′. The suspension assembly  13 ,  13 ′ is configured to progressively limit movement between the cutting unit  7  and the driving unit  3  during operation of the robotic lawnmower  1 . As is evident from  FIG. 2 , according to the illustrated embodiments, this is achieved by the suspension assembly  13 ,  13 ′ being configured to progressively limit movement between the cutting unit chassis  7 ′ and the driving unit chassis  3 ′. Due to these features, when the robotic lawnmower  1  is travelling/moving over a ground surface  11  having irregularities, the suspension assembly  13 ,  13 ′ will to progressively limit movement between the cutting unit  7  and the driving unit  3 . As a result, the cutting unit  7  may move relative to the driving unit  3  in a more controlled manner so as to follow the terrain of the ground surface  11  in a smoother and more controlled manner, which provides conditions for a further improved cutting result. As understood from the herein described, the feature that the suspension assembly  13 ,  13 ′ is configured to progressively limit movement between the cutting unit  7  and the driving unit  3  during operation of the robotic lawnmower  1  means that the suspension assembly  13 ,  13 ′ is configured to progressively limit movement between the cutting unit  7  and the driving unit  3  during travel/movement of the robotic lawnmower  1  over a ground surface  11 . The cutting unit  7  may be active or inactive during such travel/movement of the robotic lawnmower  1 . 
       FIG. 3  illustrates a top view of the lawnmower chassis  1 ′ illustrated in  FIG. 2 . As best seen in  FIG. 3 , the suspension assembly  13 ,  13 ′ comprises a first suspension unit  13  arranged on a first side s 1  of a vertical plane p 1  extending along the pivot axis ax and a second suspension unit  13 ′ arranged on a second side s 2  of the vertical plane p 1  extending along the pivot axis ax. The vertical plane p 1  is a plane perpendicular to a horizontal plane onto which the robotic lawnmower  1  is positioned. Such a horizontal plane p 2  is indicated in  FIG. 2 . Moreover, in  FIG. 3 , the electrical motors  21  of the respective drive wheel  5  are fully visible. 
       FIG. 4  illustrates a cross section of the lawnmower chassis  1 ′ illustrated in  FIG. 3 . Below, simultaneous reference is made to  FIG. 1 - FIG. 4 . The cross section of  FIG. 4  is made in a vertical plane perpendicular to the pivot axis ax at a portion of the lawnmower chassis  1 ′ comprising the suspension assembly  13 ,  13 ′. As can be seen in  FIG. 4 , each suspension unit  13 ,  13 ′ is arranged at a distance d from the pivot axis ax. Moreover, each suspension unit  13 ,  13 ′ comprises a spring element  15 ,  15 ′. 
     According to the illustrated embodiments, the suspension assembly  13 ,  13 ′ is configured to bias the cutting unit  7  towards a neutral position relative to the driving unit  3 . In  FIG. 1 - FIG. 4 , the cutting unit  7  is illustrated in the neutral position. As indicated in  FIG. 2 , the neutral position constitutes a position of the cutting unit  7  relative to the driving unit  3  in which each ground engaging portion  19  of said drive wheels  5  and said support wheels  9  extend along a flat plane p 2 . 
     As understood from the herein described, according to the illustrated embodiments, the suspension assembly  13 ,  13 ′ is configured to limit pivoting movement between the cutting unit  7  and the driving unit  3 . If the cutting unit  7  is pivoted counterclockwise around the pivot axis ax in  FIG. 4 , from the neutral position to an offset position, a first spring element  15  of the first suspension unit  13  will be compressed and a second spring element  15 ′ of the second suspension unit  13 ′ will be expanded. Similarly, if the cutting unit  7  is pivoted clockwise around the pivot axis ax in  FIG. 4 , from the neutral position to an offset position, a first spring element  15  of the first suspension unit  13  will be expanded and a second spring element  15 ′ of the second suspension unit  13 ′ will be compressed. The distanced between each suspension unit  13 ,  13 ′ and the pivot axis ax and the biasing force of each suspension unit  13 ,  13 ′ may be adapted to provide a certain returning torque of the cutting unit  7  towards the neutral position. 
     Moreover, as understood from the above described, according to the illustrated embodiments, the suspension assembly  13 ,  13 ′ is configured to bias the cutting unit  7  towards the neutral position with a magnitude that increases with increasing offset of the cutting unit  7  from the neutral position. The first and second spring elements  15 ,  15 ′ may each provide a small biasing force, i.e. a small pre-tension, when the cutting unit  7  is in the neutral position. 
     According to further embodiments, the suspension assembly, as referred to herein, may comprise a torsion spring configured to limit pivoting movement between the cutting unit  7  and the driving unit  3 . 
     According to some embodiments of the present disclosure, the suspension assembly  13 ,  13 ′ may be configured to progressively limit movement between the cutting unit  7  and the driving unit  3  with a limiting force that increases with increased rate of movement between the cutting unit  7  and the driving unit  3 . According to such embodiments, the robotic lawnmower  1  may comprise one or more dampers, such as one or more mechanical or hydraulic shock absorbers, each being configured to limit movement between the cutting unit  7  and the driving unit  3  with a limiting force that increases with increased rate of movement between the cutting unit  7  and the driving unit  3 . Such a damper or dampers are not illustrated in the figures for the reason of brevity and clarity. Alternatively, the suspension assembly  13 ,  13 ′ and the robotic lawnmower  1  may be free from such a damper or such dampers. 
     According to the illustrated embodiments, the suspension assembly  13 ,  13 ′ is configured to limit pivoting movement between the cutting unit  7  and the driving unit  3  to a maximum pivoting movement of approximately 10 degrees. According to further embodiments, the suspension assembly  13 ,  13 ′ may be configured to limit pivoting movement between the cutting unit  7  and the driving unit  3  to a maximum pivoting movement within the range of 7 degrees to 15 degrees, or within the range of 8 degrees to 12 degrees. 
     According to the illustrated embodiments, the first suspension unit  13  comprises a pair of stop surfaces  18  arranged to abut against each other when the first spring element  15  has been compressed a compression distance d 2 , to hinder further compression of the first spring element  15 . According to the illustrated embodiments, compression distance d 2  and distance d between the first suspension unit  13  and the pivot axis ax are arranged such that the pair of stop surfaces  18  abut against each other when the cutting unit  7  is pivoted approximately 5 degrees counterclockwise around the pivot axis ax from the neutral position illustrated in  FIG. 4  to a first stop position. Likewise, the second suspension unit  13 ′ comprises a pair of stop surfaces  18 ′ arranged to abut against each other when the second spring element  15 ′ has been compressed a compression distance d 2 , to hinder further compression of the second spring element  15 ′. According to the illustrated embodiments, compression distance d 2  and distance d between the second suspension unit  13 ′ and the pivot axis ax are arranged such that the pair of stop surfaces  18 ′ abut against each other when the cutting unit  7  is pivoted approximately 5 degrees clockwise around the pivot axis ax from the neutral position illustrated in  FIG. 4  to a second stop position. In this manner, the stop surfaces  18 ,  18 ′ of the suspension assembly  13 ,  13 ′ will limit pivoting movement between the cutting unit  7  and the driving unit  3  to a maximum pivoting movement of approximately 10 degrees. As understood from the above, the wording maximum pivoting movement, as used herein, refers to a pivoting movement of the cutting unit  7  from the first stop position, past the neutral position, to the second stop position. 
     It is to be understood that the foregoing is illustrative of various example embodiments and that the invention is defined only by the appended claims. A person skilled in the art will realize that the example embodiments may be modified, and that different features of the example embodiments may be combined to create embodiments other than those described herein, without departing from the scope of the present invention, as defined by the appended claims. 
     As used herein, the term “comprising” or “comprises” is open-ended, and includes one or more stated features, elements, steps, components, or functions but does not preclude the presence or addition of one or more other features, elements, steps, components, functions or groups thereof.