TECHNICAL FIELD

The invention relates to a robotic cleaner, preferably configured for domestic use.

BACKGROUND

A robotic cleaner forms a self-propelling unit provided with a drive arrangement comprising a control arrangement configured to control a movement of the robotic cleaner along a surface to be cleaned. The control arrangement may comprise one or more sensors providing input to assist in controlling the movement of the robotic cleaner.

A robotic cleaner may comprise at least one brush arranged to rotate about a horizontal axis. The brush is configured to propel dust and debris towards and/or into an inlet of a housing of the robotic cleaner.

A robotic cleaner configured for domestic use is suitably sized such that it can travel underneath furniture, preferably even underneath low furniture such as underneath a sofa or a bed.

When the robotic cleaner is a robotic vacuum cleaner, a motor fan unit of the robotic vacuum cleaner is arranged in fluid communication with a nozzle inlet facing the surface to be cleaned. Dust and debris sucked or otherwise propelled into the nozzle inlet is directed into a debris receptacle of the robotic vacuum cleaner. The debris receptacle is emptied, or replaced, when filled with dust and debris to a certain degree.

A robotic cleaner, such as a robotic vacuum cleaner, configured to travel along a surface to be cleaned may be provided with a so-called side brush. Commonly, a side brush is rotatable about a substantially vertical axis and comprises bristles extending outwardly from the rotation axis. The side brush may sweep dust and debris from beside and/or a side portion of the robotic cleaner to underneath the robotic cleaner for further transportation into a debris receptacle inside the robotic cleaner. The side brush may be arranged to reach into narrow corners and crevices where an elongated brush roll underneath the robotic cleaner and/or the suction produced in the robotic cleaner does not reach.

The side brush comprises a number of bristles. As the side brush rotates about the substantially vertical axis, a large portion of each of the bristles extends in parallel to the surface to be cleaned. This constitutes a problem since a large portion of each bristle is subjected to wear, which weakens the bristles and/or causes the bristles to rupture over time.

WO 2019/035050 A1 discloses a robotic cleaning apparatus including a body and at least one antenna extending from a periphery of the body. The at least one antenna is configured to rotate about an axis that extends substantially parallel to a surface to be cleaned. The robotic cleaning apparatus is capable of cleaning beyond a periphery of the robotic cleaning apparatus due to the at least one antenna. The antenna may be an agitator such as a brush.

DE 1020154114775 discloses a robotic vacuum cleaner. A housing of the vacuum cleaner has an underside with a suction mouth. Arranged outside the suction mouth is a rotatable brush, the rotational axis of which forms an angle of less than 45 ° with the horizontal plane. The rotatable brush is arranged to contribute to improve cleaning of textile floor coverings. Dirt at the rotatable brush is not directly captured by a suction air stream. Instead, the dirt is picked up by the suction mouth, which is arranged separately at a spatial distance from the rotatable brush. According to one embodiment, in order to adjust a penetration level of the rotatable brush into a textile surface without overloading its drive arrangement, the vertical height or the angle of the rotational axis is adjustable. In order to clean solid or textile skirting boards, an angle of the rotational axis or an angle of the bristles is tilted in relation to the horizontal plane such that the bristles are moved in a circle which is tilted from the vertical plane.

SUMMARY

It would be advantageous to achieve a robotic cleaner overcoming, or at least alleviating, at least some of the above mentioned drawbacks. In particular, it would be desirable to enable a side brush of a robotic cleaner to reach surfaces laterally beside a housing of the robotic cleaner. To better address one or more of these concerns, a robotic cleaner having the features defined in the independent claim is provided.

According to an aspect of the invention, there is provided a robotic cleaner comprising a housing, a drive arrangement being configured to drive the robotic cleaner along a surface to be cleaned, a control arrangement configured to control the drive arrangement to move the robotic cleaner along the surface to be cleaned in accordance with navigation information, a debris receptacle arranged in the housing, an inlet of the housing communicating with the debris receptacle, a first brush arranged to rotate about a first horizontal axis and configured to propel debris and dust towards or into the inlet, and a second brush arranged to rotate about a second horizontal axis and arranged to reach beyond the housing. The robotic cleaner has a maximum height extending from the surface to be cleaned to an uppermost surface of the housing. In a direction from the surface to be cleaned, the second horizontal axis is arranged at 1/3 of the maximum height or higher, or at 3/7 of the maximum height or higher, or at 1/2 of the maximum height or higher, or at 2/3 of the maximum height or higher, or within a range of 1/3 - 1/2 of the maximum height.

Since the second brush is arranged to rotate about a second horizontal axis and arranged to reach beyond the housing, and since the second horizontal axis is arranged at 1/3 of the maximum height or higher, or at 3/7 of the maximum height or higher, or at 1/2 of the maximum height or higher, or at 2/3 of the maximum height or higher, or within a range of 1/3 - 1/2 of the maximum height, the second brush is arrange d at a height above the surface to be cleaned such that surfaces laterally beside the housing of the robotic cleaner, also at a level above the surface to be cleaned, can be reached and cleaned by the second brush. In particular, the second brush may clean a horizontal surface at a level above the surface to be cleaned and/or a surface comprising a horizontal component at a level above the surface to be cleaned.

In this manner, for instance a skirting board extending along the surface to be cleaned may be cleaned by the second brush. More specifically, a horizontal surface of the skirting board and/or a surface comprising a horizontal component of the skirting board may be cleaned by the second brush.

This is not possible with the robotic cleaning apparatus disclosed in WO 2019/035050. In comparison with the robotic vacuum cleaner of DE 1020154114775, which utilises a brush mainly arranged underneath the robotic cleaner with a tilted axis or tilted bristles, in the present robotic cleaner skirting boards are reached due to the high arrangement of the second horizontal axis of the second brush at the housing of the robotic cleaner.

The inventors have realized that a second brush with a highly positioned horizontal rotational axis may be utilised for cleaning a horizontal surface at a level above the surface to be cleaned and that at least part of the second brush may require placement outside a periphery of the housing of the robotic cleaner. In doing so the inventors have accepted a horizontal axis at a high position, which means that the second brush cannot extend from underneath the robotic cleaner but must be arranged laterally at/of the housing. This is in sharp contrast with the traditional arrangement of side/additional brushes in the field of robotic cleaners, which side/additional brushes extend beyond the housing from underneath the robotic cleaner and close to the surface to be cleaned. The robotic cleaner may be a self-propelling unit configured to travel along a surface to be cleaned. The robotic cleaner may preferably be configured for domestic use, e.g. sized such that it may travel underneath at least some domestic furniture. The robotic cleaner may be controlled by the control arrangement to be guided along the surface to be cleaned. The navigation information utilised by the control arrangement may be provided e.g. by sensors, and/or stored and/or calculated position information, and/or e.g. beacons.

The robotic cleaner may be a robotic vacuum cleaner, which comprises a motor fan unit for producing an airflow through the inlet and towards the debris receptacle or through a separate nozzle inlet and towards a further debris receptacle.

The second brush may be referred to as a side brush. The second brush may be configured to sweep dust and debris from beside and/or a side portion of the housing of the robotic cleaner to underneath the robotic cleaner. The second brush may sweep dust and debris towards the first brush, for further transportation into the debris receptacle inside the robotic cleaner. The second brush may be arranged to reach into narrow corners and crevices where an elongated brush roll underneath the robotic cleaner and/or suction produced in the robotic cleaner does not reach.

According to embodiments, a coverage of the second brush may overlap or at least reach a coverage of the first brush. In this manner, it may be ensured that the entire portion of the surface to be cleaned covered by the first and second brushes is cleaned as the robotic cleaner travels along the surface to be cleaned. More specifically, there will be no strip of the surface to be cleaned between the first and second brushes that has not been touched by at least one of the brushes as the robotic cleaner travels along the surface to be cleaned.

According to embodiments, the second horizontal axis may extend at an angle within a range of the 30 - 90 degrees of the first horizontal axis, or within a range of 45 - 90 degrees. In this manner, the second brush may be arranged to propel dust and debris in a direction such that it may be further propelled by the first brush towards or into the inlet.

According to embodiments, the drive arrangement may comprise a pair of drive means, such as a pair of drive wheels, and wherein the second brush is arranged outside the pair of drive means. In this manner, the second brush may reach outside a travelling path of the drive means. The term outside in this context means, seen along a travelling direction of the robotic cleaner, that the second brush is arranged beside the pair of drive means, i.e. that the second brush is not arranged between the pair of drive means.

As mentioned above, the pair of drive means may comprise a pair of drive wheels. An alternative example of a pair of drive means may be a pair of continuous tracks.

According to embodiments, the first brush may extend over a distance corresponding to at least 80 % of a distance between the pair of drive means, or corresponding to at least a distance between the pair of drive means, or reaching outside the drive means. In this manner, touching of the coverages or providing an overlap of the coverages of each of the first and second brushes may be easily achieved. More specifically, the first brush extending over a considerable distance across the pair of drive means, may facilitate for the second brush to be arranged with its coverage at least reaching the coverage of the first brush.

According to embodiments, the second brush may have a diameter within a range of 8 - 16 cm. In this manner, surfaces above the surface to be cleaned at a height within a range of 1 cm to approximately 14 cm may be cleaned by the second brush.

According to embodiments, a ratio between a diameter and a length of the second brush may be ³ 2:1. In this manner, a second brush having a shorter length than diameter may be provided. Such a short and large diameter second brush may fulfil the requirement of cleaning surfaces laterally beside the housing of the robotic cleaner and above the surface to be cleaned. Namely, it has been realised by the inventors that a broad momentary coverage of the second brush may not be required to clean such surfaces laterally beside the housing of the robotic cleaner and above the surface to be cleaned. Moreover, a short length and large diameter second brush arranged with its axis horizontally may require less precise navigation of the robotic cleaner, in a lateral direction thereof, then a robotic cleaner with a small diameter second brush. The short length and large diameter second brush may absorb errors in distance to objects laterally of the robotic cleaner to a greater extent.

According to embodiments, the second brush may have a far end positioned farthest from the housing, and the far end may be flexible. In this manner, the second brush may not damage vertical surface laterally beside the housing of the robotic cleaner.

These embodiments are not limit to any particular kind of flexible far end of the second brush. The flexible far end of second brush may be achieved in any suitable way, such as by soft bristles, or a sponge-like material. According to embodiments, the second brush may have a flexibility in a direction 90 degrees to a travelling direction of the robotic cleaner. In this manner, the second brush may not damage vertical surface laterally beside the housing of the robotic cleaner, e.g. as the robotic cleaner travels along the vertical surface.

These embodiments are not limit to the manner in which the flexibility in the direction 90 degrees to the travelling direction is achieved. The flexibility may be achieved in any suitable way, such as by soft bristles, a sponge-like material, or a flexible brush axis.

According to some embodiments, the flexibility in the direction 90 degrees to the traveling direction may be achieved by a flexible shaft of the second brush and/or by a flexible portion of the shaft of the second brush, such as a flexible portion of the shaft closest to a drive arrangement of the second brush, and/or a flexible suspension of the drive arrangement of the second brush. In this manner, the second brush may give way in case the robotic cleaner would drive the second brush into e.g., an electric socket or other similar wall-mounted device.

The second brush may be flexible in more directions than in the direction 90 degrees to the traveling direction of the robotic cleaner.

According to embodiments, a peripheral rotational speed of the second brush may be < 0.6 m/s, such as within a range of 0.1 - 0.4 m/s. In this manner, the peripheral speed of the second brush may be sufficiently low to prevent dust and debris from being flung too far away from the robotic cleaner. Accordingly, it may be ensured that the dust and debris cleaned from a surface by the second of brush is collected in the debris receptacle.

According to embodiments, the first and second brushes may be the only rotating brushes of the robotic cleaner.

According to embodiments, the first horizontal axis may extend in parallel with and at a distance from an axis of the pair of drive wheels. In this manner, the first brush may be arranged in the housing such that the first brush has the broadest possible coverage in the travelling direction of the robotic cleaner.

According to embodiments, the robotic cleaner may comprise a motor fan unit for producing an airflow through the inlet. Thus, the robotic cleaner may be a robotic vacuum cleaner utilising the airflow produced by the motor fan unit for transporting dust and debris into, or at least towards, the debris receptacle.

According to embodiments, a side of the housing configured to face the surface to be cleaned may be provided with a recess extending between the inlet and a periphery of the housing at the second brush. In this manner, during use of the robotic cleaner, a partial airflow of the airflow into the inlet may flow along the recess. Thus, dust and/or debris may be dragged in a direction from the second brush along the surface to be cleaned to the inlet and further towards the debris receptacle.

Further features of, and advantages with, the invention will become apparent when studying the appended claims and the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects and/or embodiments 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:

Figs. 1a -1e illustrate top, bottom, and side views of a robotic cleaner according to embodiments,

Fig. 2 shows a robotic cleaner in a top view,

Figs. 3a and 3b illustrate bottom views of robotic cleaners, and

Fig. 4 illustrates a shaft and a drive member of a second brush of a robotic cleaner.

DETAILED DESCRIPTION

Aspects and/or embodiments of the 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.

Figs. 1a, - 1e illustrate a top view, a bottom view, and three side views of a robotic cleaner 2 according to embodiments. In Figs. 1a and 1b a main travelling direction of the robotic cleaner 2 is downwards in the plane of the paper as indicated by arrows. In Fig. 1c and 1e the main travelling direction of the robotic cleaner 2 is to the right in the plane of the paper as indicated by an arrow. In Fig. 1d the main travelling direction of the robotic cleaner 2 is out of the plane of the paper, towards a viewer.

The robotic cleaner 2 comprises a housing 4, a drive arrangement 6 being configured to drive the robotic cleaner 2 along a surface 3 to be cleaned, a debris receptacle 8 arranged inside the housing 4, and an inlet 10 in the housing 4 facing the surface 3 to be cleaned. The inlet 10 communicates with the debris receptacle 8 such that dust and debris may be transported from the inlet 10 into the debris receptacle 8

The debris receptacle 8 may be removable in order to be emptied. The debris receptacle 8 may comprise e.g., a container or a disposable bag, inside which dust and debris are collected. Alternatively, the debris receptacle 8 may be connected to a cyclone for receiving dust and debris separated in the cyclone. A further alternative may be that the debris receptacle 8 forms a cyclone housing wherein dust and debris is collected.

The robotic cleaner 2 is a self-propelling unit. The drive arrangement 6 comprises a pair of drive means, such as a pair of drive wheels 12. The drive wheels 12 are directly or indirectly driven by one or more electric drive motors arranged inside the housing 4.

A control arrangement 14 is configured to control the drive arrangement 6 to move the robotic cleaner 2 along the surface 3 to be cleaned. The control arrangement 14 utilises navigation information to control the drive arrangement 6.

Various control arrangements are known in the art and the present invention is not limited to any particular type of control arrangement. For instance, the control arrangement 14 may comprise one or more sensors to provide input assisting in controlling the movement of the robotic cleaner 2. The at least one sensor may be of one or more different kinds, such as e.g. an infrared sensor, a laser sensor, an ultrasonic sensor, or a contact sensor. Additionally, or alternatively, the control arrangement 14 may utilise position information and/or one or more beacons.

The control arrangement 14 comprises a calculation unit which may take the form of substantially any suitable type of processor circuit or microcomputer, e.g. a circuit for digital signal processing (digital signal processor, DSP), a Central Processing Unit (CPU), a processing unit, a processing circuit, a processor, an Application Specific Integrated Circuit (ASIC), a microprocessor, or other processing logic that may interpret and execute instructions. The expression calculation unit may represent a processing circuitry comprising a plurality of processing circuits, such as, e.g., any, some or all of the ones mentioned above. The control arrangement may comprise a memory unit. The calculation unit is connected to the memory unit, which provides the calculation unit with, for example, the stored programme code and/or stored data which the calculation unit needs to enable it to do calculations. The calculation unit may also be adapted to storing partial or final results of calculations in the memory unit. The memory unit may comprise a physical device utilised to store data or programs, i.e., sequences of instructions, on a temporary or permanent basis. The control arrangement 14 is connected to or comprises sensors and electric motors in the housing 4, and/or other monitoring devices and/or controllable devices in order to control the robotic cleaner 2 during cleaning of the surface 3 to be cleaned.

The robotic cleaner 2 further comprises a first brush 16 arranged to rotate about a first horizontal axis 18. Accordingly, the first brush 16 extends in parallel with the surface 3 to be cleaned. The first brush may be a rotating brush roll. The first brush 16 is configured to propel debris and dust towards or into the inlet 10. In these embodiments, the first brush 16 is arranged partially inside the housing 4 at the inlet 10.

It may be mentioned that a distance between the surface 3 to be cleaned and a side 35 of the housing 4 facing the surface 3 to be cleaned has been exaggerated in Figs. 1c and 1d in order to clearly shown the drive wheels 12 and the first brush 16. In Fig. 1e the distance between the surface 3 to be cleaned and the side 35 of the housing 4 facing the surface 3 to be cleaned is shown more correctly.

Further, the robotic cleaner 2 comprises a second brush 20 arranged to rotate about a second horizontal axis 22. The second brush 20 is arranged to reach beyond the housing 4. The second brush 20 forms a so-called side brush of the robotic cleaner 2. The second brush 20 is configured to transport dust and debris towards the first brush 16 and/or the inlet 10.

The first and second brushes 16, 20 may be driven by one or more electric brush motors. Besides controlling the drive arrangement 6, the control arrangement 14 may also control the one or more electric brush motors.

The first horizontal axis 18 extends in parallel with and at a distance from an axis 19 of the pair of drive wheels 12. In this manner, the first brush 20 is arranged in the housing 4 such that the first brush 20 has the broadest possible coverage in the travelling direction of the robotic cleaner 2.

Although not limited thereto, in the shown embodiments, the robotic cleaner 2 comprises a motor fan unit 24 for producing an airflow through the inlet 10.

Accordingly, the robotic cleaner 2 may be a robotic vacuum cleaner utilising the airflow produced by the motor fan unit 24 for transporting dust and debris into, or at least towards, the debris receptacle 8. The control arrangement 14 may control the motor fan unit 24. The inlet 10 is arranged in fluid communication with the debris receptacle 8 via a debris conduit system. The motor fan unit 24 is arranged in fluid communication with the inlet 10 via the debris conduit system and optionally also via the debris receptacle 8. That is, the motor fan unit 24 in some embodiments may create a suction from the inlet 10 via the debris conduit system to the debris receptacle 8.

The robotic cleaner 2 comprises one or more rechargeable batteries configured to power the drive arrangement 6 including the control arrangement 14 and the various electric motors.

The robotic cleaner 2 has a maximum height H extending from the surface 3 to be cleaned to an uppermost surface of the housing 4. In a direction from the surface 3 to be cleaned, the second horizontal axis 22 is arranged at 1/3 of the maximum height H or higher. Put differently, the second horizontal axis 22 is arranged at a distance D of at least 1/3 of the maximum height H above the surface 3 to be cleaned.

Since the second horizontal axis 22 is arranged at 1/3 of the maximum height H or higher, the second brush 20 is arranged at a height above the surface 3 to be cleaned such that surfaces laterally beside the housing 4 of the robotic cleaner 2, also at a level above the surface 3 to be cleaned, can be reached and cleaned by the second brush 20.

Fig. 1d discloses an example by which the second brush 20 brushes an upper substantially horizontal surface of a skirting board 26 extending along the surface 3 to be cleaned.

Also, the second brush 20 is configured to touch, and accordingly, clean the surface 3 to be cleaned.

According to alternative embodiments, the second brush 20 may be arranged even higher above the surface 3 to be cleaned. Accordingly, in a direction from the surface 3 to be cleaned, the second horizontal axis 22 may be arranged at 3/7 of the maximum height H or higher, or at 1/2 of the maximum height H or higher, or at 2/3 of the maximum height H or higher. Put differently, the second horizontal axis 22 may be arranged at a distance D of at least 3/7 of the maximum height H, or the second horizontal axis 22 may be arranged at a distance D of at least 1/2 of the maximum height H, or at a distance D of at least 2/3 of the maximum height H, above the surface 3 to be cleaned. The higher the second brush 20 is arranged above the surface 3 to be cleaned, the higher horizontal surfaces, or surfaces having a horizontal component, may be cleaned by the second brush 20. Still, the second brush 20 is configured to touch and clean the surface 3 to be cleaned.

According to embodiments, the second brush 20 may be arranged within a range of 1/3 - 1/2 of the maximum height H. That is, the second horizontal axis 22 may be arranged at a distance D within a range of 1/3 - 1/2 of the maximum height H. Thus, the second brush 20 will not reach higher than the maximum height H above the surface 3 to be cleaned, and accordingly, will not extend above the housing 4 while still being positioned high enough to reach horizontal surfaces above the surface 3 to be cleaned.

A rotational direction of the first brush 16 is suitably such that the first brush 16 propels dust and debris towards or into the inlet 10. A rotational direction of the second brush 20 may be suitably such that the second brush 20 propels dust and debris in a direction of the first brush 16, or in a direction of an area covered by the first brush 16 as the robotic cleaner 2 travellers in its main travelling direction.

In embodiments wherein the robotic cleaner 2 comprises a motor fan unit 24 for producing an airflow through the inlet 10, and as shown in Figs. 1b and 1e, the side 35 of the housing 4 configured to face the surface 3 to be cleaned may be provided with a recess 36 extending between the inlet 10 and a periphery of the housing 4 at the second brush 20.

Thus, during use of the robotic cleaner 2, a partial airflow of the airflow into the inlet 10 may flow along the recess 36. That is, the recess 36 partially delimits a section of the space between the surface 3 to be cleaned and the side 35 of the housing 4 configured to face the surface 3, which section locally has a larger cross sectional area in a vertical plane than other sections of the space underneath the robotic cleaner 2. The section with the larger cross sectional area forms what may be referred to as a channel 37 extending between the periphery of the housing 4 and the inlet 10. Put differently, the recess 36 may partially delimit a channel 37 formed between the surface 3 to be cleaned and the side 35 of the housing 4 configured to face the surface to be cleaned 3, wherein the channel 37 may be configured for directing part of the airflow towards the inlet 10.

Accordingly, dust and/or debris may be drawn from the second brush 20 along the recess/channel 36 to the inlet 10 by the partial airflow and further towards the debris receptacle 8. In Fig. 1e the second brush is not shown for the sake of clarity, only the second horizontal axis 22 thereof is shown. As an alternative or in addition to the recess/channel 36, the housing 4 may be provided with a suction opening near the second brush 20. In such embodiments, the suction opening is arranged in fluid communication with the motor fan unit 24 and the debris receptable 8. In this manner an airflow drawing dust and debris from the second brush 20 through the suction opening may be transported to the debris receptacle 8.

According to embodiments, the maximum height H may be within a range of 6 - 12 cm. In this manner, the robotic cleaner 2 may be configured to travel underneath many different types of furniture placed on the surface 3 to be cleaned and thus, may be configured for domestic use.

According to embodiments, the second horizontal axis 22 may be arranged at a height above the surface to be cleaned within a range of 4 - 8 cm. In this manner, the second brush 20 may be arranged to clean surfaces having a horizontal component positioned within a range of approximately 1 - 14 cm above the surface 3 to be cleaned. For instance, the second bush 20 may thus, reach on top of most common types of skirting boards 26, while also being able to clean the surface 3 to be cleaned.

Accordingly, the second brush 20 may have a diameter d within a range of 8 - 16 cm, see

Fig. 1c.

The second horizontal axis 22 may be arranged at a height above the surface to be cleaned within a range of 4 - 12 cm, but preferably not above the maximum height H of the robotic cleaner 2. The closer the second horizontal axis 22 may be arrange to the surface having a horizontal component which is to be cleaned by the second brush 20, the more efficient the cleaning of the surface having a horizontal component.

A ratio between a diameter d and a length L of the second brush 20 may be ³ 2:1 , see Fig. 1c. Thus, the second brush 20 may have a shorter length L than diameter d. A short length L and large diameter d second brush 20 may reach surfaces laterally beside the housing 4 and above the surface 3 to be cleaned and provide a satisfactorily cleaning result with low energy consumption.

The length of the second brush 20 is measured along the second horizontal axis 22. The diameter of the second brush 20 is measured perpendicularly to the second horizontal axis 22. According to embodiments, a peripheral rotational speed of the second brush 20 may be < 0.6 m/s, such as within a range of 0.1 - 0.4 m/s. In this manner, the peripheral speed of the second brush 20 may be sufficiently low to prevent dust and debris from being flung too far away from the robotic cleaner 2 by the second brush 20, such that the dust and debris will not be collected by the robotic cleaner 2. Accordingly, it may be ensured that the dust and debris cleaned from a surface by the second of brush 20 ends up in the debris receptacle 8.

In order to prevent the second brush 20 from damaging surfaces laterally of the housing 4, the second brush 20 may be flexible. Herein the term flexible encompasses such properties as resilient, soft, giving, bendable, et cetera, i.e. any property that will lend the second brush 22 to give if/when it is moved against a surface positioned laterally of the housing 4 of the robotic cleaner 2. For instance, a far end 30 of the second of brush 20 positioned farthest from the housing 4 may be flexible. Alternatively, the second brush 20 may be flexible in a direction 90 degrees to a travelling direction of the robotic cleaner 2.

Fig. 4 illustrates a shaft 38 and a drive member 44 of a second brush of a robotic cleaner, such as the robotic cleaner 2 as discussed with reference to Figs. 1a - 3b.

As mentioned above, the second brush may be flexible in a direction 90 degrees to a travelling direction of the robotic cleaner. Thus, the second brush may give way in case the robotic cleaner 12 would drive the second brush into an obstacle. The second brush may be flexible in more directions than in the direction 90 degrees to the traveling direction of the robotic cleaner.

In Fig. 4 different means for achieving the flexibility in the direction 90 degrees to the traveling direction are shown. One or more of these means may be provided:

- A flexible shaft 38 of the second brush 20. The shaft 38 may be made of a flexible material such as rubber or a slender metal rod.

- A flexible portion 40 of the shaft 38 of the second brush 20. Only a portion of the shaft 38, such as portion 40 closest to the drive member 44 may be flexible. In the illustrated embodiments, the flexible portion comprises a helical spring.

- A flexible suspension 42 of the drive member 44 of the second brush. The drive member 44 may be mounted in the housing 4 of the robotic cleaner via a flexible suspension to permit at least partial pivoting of the drive member 44.

- Soft bristles 46 arranged at an outer end of the second brush. Only a few bristles have been schematically indicated in Fig. 4.

- A spongelike material 48 arranged at an outer end of the second brush. The drive member 44 may include an electric brush motor 45 and a transmission 47, such as a planetary gear.

Returning to Figs. 1a - 1e, according to embodiments, the first and second brushes 16, 20 may be the only rotating brushes of the robotic cleaner 2.

According to alternative embodiments, the robotic cleaner 2 may comprise one or more further brushes in addition to the first and second brushes 16, 20.

As mentioned above, the drive arrangement 6 comprise a pair of drive means, such as the pair of drive wheels 12. The second brush 20 is arranged outside the pair of drive means. In this manner, the second brush 20 may reach outside a travelling path of the pair of drive means, i.e. a portion of a travelling path of the robotic cleaner 2 where the first brush 16 does not have any coverage, or only has limited coverage.

Fig. 2 shows a robotic cleaner 2 in a top view. The robotic cleaner 2 may be a robotic cleaner 2 as discussed above with reference to Figs. 1a - 1e.

In Fig. 2 a coverage 32 of the first brush 16 and a coverage 34 of the second brush 20 are illustrated. More specifically, the coverages 32, 34 of the first and second brushes 16, 20 of the surface 3 cleaned by the robotic cleaner 2 as it travels along the surface 3 to be cleaned are illustrated. Each of the coverages 32, 34 are shown with hatchings. The momentary coverages of each of the first and second brushes 16, 20 of the surface 3 being cleaned are shown with cross hatchings.

The coverage 34 of the second brush 16 may overlap or at least reach the coverage 32 of the first brush 16. Thus, it may be ensured that the entire portion of the surface 3 to be cleaned covered by the first and second brushes 16, 20 is cleaned as the robotic cleaner travels along the surface 3 to be cleaned.

In the illustrated embodiments, the second brush 20 is arranged on the robotic cleaner 2 such that it travels in front of the first brush 16 as the robotic cleaner 2 travels in its main direction of travel as indicated by the arrow. In alternative embodiments, the second of brush 20 may be arranged on the robotic cleaner 2 such that it travels beside or after the first brush 20 as the robotic cleaner 2 travels in its main direction of travel. Irrespective of where the second brush 20 is arranged in relation to the first brush 16, the coverage 34 of the second brush 16 may overlap or at least reach the coverage 32 of the first brush 16.

The first brush 16 may extend over a distance corresponding to at least 80 % of a distance between the pair of drive means, or corresponding to at least a distance between the pair of drive means, or reaching outside the drive means. In this manner, touching of the coverages or providing an overlap of the coverages of each of the first and second brushes 16, 20 may be easily achieved. More specifically, the first brush 16 extending over a considerable distance across the pair of drive means, may facilitate for the second brush 20 to be arranged with its coverage at least reaching the coverage of the first brush 16.

Figs. 3a and 3b illustrate bottom views of robotic cleaners 2. Each of the robotic cleaners 2 may be a robotic cleaner 2 as discussed above with reference to Figs. 1a - 2.

In Figs. 3a and 3b different embodiments of robotic cleaners 2 with different arrangements of the second brush 20 in relation to the first brush 16 are illustrated.

The second horizontal axis 22 may extend at an angle a within a range of the 30 - 90 degrees to the first horizontal axis 16. Alternatively, the second horizontal axis 22 may extend at an angle a within a range of the 45 - 90 degrees to the first horizontal axis 16.

Thus, the second brush 20 may propel dust and debris in a direction towards the first brush 16 or an area covered by the first brush 16 as the robotic cleaner 2 travellers in its main travelling direction. Accordingly, the first brush 16 may further propel the dust and debris towards or into the inlet 10.

In the example of Fig. 3a, the second horizontal axis 22 extends at an angle a of 45 degrees to the first horizontal axis 16. In the example of Fig. 3b, the second horizontal axis 22 extends at an angle a of 90 degrees to the first horizontal axis 16.

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 invention, as defined by the appended claims. For instance, a shield may be provided partially around the second brush 20. A portion of the second brush 20 may extend outside the shield. The shield may be flexible to prevent damage in case the robotic cleaner should drive the shield into an obstacle lateral of the robotic cleaner 2.