TECHNICAL FIELD

The present invention generally relates to robotic cleaning devices. In particular, a robotic cleaning device comprising at least one drive wheel and a first and second spring member associated with the at least one drive wheel is provided.

BACKGROUND

Some robotic cleaning devices, such as vacuum cleaning robots, use tension spring suspensions for the drive wheels. The spring forces facilitate travelling on thick carpets and climbing over thresholds, electrical cables and other objects.

Furthermore, some robotic cleaning devices rely partly or fully on odometry, i.e. the use of the wheel rotation as feedback to control the position of the robot. If a wheel slips on the travelling surface, the position control of the robot might be deteriorated.

WO 2014151501 Ai discloses a mobile surface cleaning robot where each drive wheel is rotatably supported by a drive wheel suspension arm having a first end pivotally coupled to the robot body and a second end rotatably supporting the drive wheel, and a drive wheel helical suspension spring biasing the drive wheel towards the floor surface. This helical suspension spring cannot provide the same force both at its minimum stretch and at its maximum stretch. In other words, when the robot adopts a low position, where the robot body is close to the ground surface, the suspension spring is in an extended state and thereby provides a relatively high force (according to Hooke's law). However, when the robot adopts a raised position, where the robot body is raised higher above the ground surface, the suspension spring is in a less extended state and thereby provides a relatively low force. Thus, the force generated by the suspension spring that pushes the drive wheel downwardly against the ground surface is rather low when the robot adopts the raised position. Thereby, there is an increased risk for the wheel to slip or spin and a consequential deterioration of the position control of the robot.

SUMMARY

One object of the present disclosure is to provide a robotic cleaning device with an improved travel performance.

A further object of the present disclosure is to provide a robotic cleaning device with an improved cleaning performance.

A still further object of the present disclosure is to provide a robotic cleaning device an improved grip between one or more drive wheels and a ground surface, in particular an improved grip between one or more drive wheels and a ground surface when the robotic cleaning device adopts a raised position.

A still further object of the present disclosure is to provide a robotic cleaning device having a compact and simple spring arrangement for one or more of its drive wheels. According to one aspect, there is provided a robotic cleaning device comprising a main body; at least one drive wheel for driving the robotic cleaning device on a horizontal ground surface; at least one linking member rotationally coupled to the main body about a suspension axis and

rotationally supporting the at least one drive wheel about a drive wheel axis such that by rotating the linking member about the suspension axis in a first direction, at least a section of the main body can be raised from a lowered position, closer to the ground surface, to a raised position, further away from the ground surface; and a first spring member and a second spring member each arranged to provide a moment on the linking member about the suspension axis in the first direction to press the at least one drive wheel towards the ground surface; wherein the moment provided by the first spring member is higher in the lowered position than in the raised position and the moment provided by the second spring member is higher in the raised position than in the lowered position. The first spring member may be arranged to provide a first, higher moment on the linking member about the suspension axis in the first direction when the main body is in the lowered position and to provide a second, lower moment on the linking member about the suspension axis in the first direction when the main body is in the raised position. As an alternative, the first spring member may be arranged to provide a moment on the linking member about the suspension axis in the first direction when the main body is in the lowered position and to provide no, or substantially no (e.g. less than 2% of the moment provided when the main body is in the lowered position), moment on the linking member about the suspension axis when the main body is in the raised position.

The second spring member may be arranged to provide no, or substantially no (e.g. less than 2% of the moment provided when the main body is in the raised position), moment on the linking member about the suspension axis when the main body is in the lowered position and to provide a moment on the linking member about the suspension axis in the first direction when the main body is in the raised position. As an alternative, the second spring member may be arranged to provide a first, lower moment on the linking member about the suspension axis in the first direction when the main body is in the lowered position and to provide a second, higher moment on the linking member about the suspension axis in the first direction when the main body is in the raised position.

The first spring member and the second spring member may be arranged such that the sum of the moments from the first spring member and the second spring member acting on the linking member about the suspension axis in the first direction when the main body is in the lowered position is the same, or substantially the same (e.g. less than 5% difference), as the sum of the moments in the raised position.

When the main body is in the raised position or in the lowered position, also the linking member may be said to be in the respective raised position or lowered position. Throughout the present disclosure, a raised position of the linking member may be a maximally raised position, or any intermediate position between the lowered position and the maximally raised position. In the maximally raised position, the linking member may be inclined 30-60 ⁰ , such as 40-50 ⁰ , such as 45 ⁰ , with respect to the horizontal ground surface. The maximally raised position of the linking member may be mechanically defined by a protruding structure on the linking member that engages the main body (or vice versa) to stop further rotation of the linking member in the first direction about the suspension axis when the linking member has reached the maximally raised position. The robotic cleaning device may be constituted by an automatic, self- propelled machine for cleaning a surface, e.g. a robotic vacuum cleaner, a robotic sweeper or a robotic floor washer. The robotic cleaning device according to the present disclosure can be mains-operated and have a cord, be battery-operated or use any other kind of suitable energy source, for example solar energy.

The main body may be of various different designs, such as generally circular or generally triangular. The main body may have a flat appearance oriented substantially parallel with the ground surface. A dust collector bin, a battery, a suction fan, a suction nozzle and drive electronics etc. may be provided in the main body. Throughout the present disclosure, the main body may alternatively be referred to as a chassis. Although the robotic cleaning device is most typically commanded to travel on horizontal ground surfaces, it may also travel on uneven and/or slightly inclined surfaces.

As used herein, a vertical orientation is an orientation substantially

perpendicular to the ground surface on which the robotic cleaning device travels and a horizontal orientation is an orientation substantially parallel with the ground surface on which the robotic cleaning device travels. A substantially perpendicular/parallel relationship as used herein includes a perfectly perpendicular/parallel relationship as well as deviations from a perfectly perpendicular/parallel relationship with up to 5%, such as up to 2%. According to one realization, the robotic cleaning device comprises two drive wheels for driving the robotic cleaning device on the ground surface. The two drive wheels may be substantially concentrically arranged about concentric rotation axes substantially perpendicular to a forward travel direction of the robotic cleaning device. The drive wheels may comprise any suitable structure to increase the friction to the ground surface, such as rubber tires.

The linking member may be constituted by a suspension arm or swing arm, i.e. it may have an elongated appearance arranged in and operating in a substantially vertical plane. The linking member may be formed from one single piece of material (e.g. hard plastic) and/or may be rigid.

The suspension axis may for example comprise a pivot pin or hinge shaft connected to the main body in order to rotationally couple the linking member to the main body for rotation about the suspension axis. The suspension axis may be arranged substantially perpendicular to a forward travel direction of the robotic cleaning device.

Furthermore, the drive wheel axis may comprise a pivot pin or hinge shaft connected to the linking member in order to rotationally support the drive wheel about the drive wheel axis. Each drive wheel axis may be arranged substantially perpendicular to a forward travel direction of the robotic cleaning device.

The floor clearance control of the robotic cleaning device as described herein may be implemented entirely mechanically. For example, if the robotic cleaning device encounters an obstacle, the impact force from the obstacle (e.g. a carpet or a threshold) on the drive wheel together with the moment provided on the linking member about the suspension axis in the first direction by the first spring member (possibly also by the second spring member) may be sufficient to raise the main body from the lowered position to the raised position. Once the impact force from the obstacle is removed, the weight of the main body overcomes the moment provided on the linking member about the suspension axis in the first direction by the second spring member (possibly also by the first spring member) and the main body is allowed to again adopt the lowered position. When the main body is lowered from the raised position to the lowered position, the linking member rotates about the suspension axis in a second direction, opposite to the first direction.

The one or more drive wheels may be trailing with respect to the linking member, i.e. for each drive wheel, the suspension axis may be arranged in front of the drive wheel axis with respect to a forward travel direction of the robotic cleaning device. Throughout the present disclosure, the lowered position and the raised position may alternatively be referred to as a low clearance position or normal mode and a high clearance position or carpet mode, respectively.

The first spring member may be constituted by a tension spring, for example a coil spring. The tension spring may be extended a first, longer distance when the main body is in the lowered position and be extended a second, shorter distance when the main body is in the raised position. Thereby, the first spring member is arranged to provide a higher moment on the linking member about the suspension axis in the first direction in the lowered position of the main body than in the raised position of the main body. Alternatively, the first spring member may be constituted by a compression spring. The compression spring may be arranged to provide a higher moment on the linking member about the suspension axis in the first direction in the lowered position of the main body than in the raised position. That is, the compression spring may be compressed a first, longer distance (more compressed) when the main body is in the lowered position and be

compressed a second, shorter distance (less compressed) when the main body is in the raised position. The compression spring may for example be vertically arranged in front of the suspension axis, as seen in the forward travel direction of the robotic cleaning device. As a further alternative, the first spring member may be constituted by a torsion spring arranged concentric with the suspension axis. The torsion spring may be arranged to provide a higher moment on the linking member about the suspension axis in the first direction in the lowered position than in the raised position. It is also possible to implement the first spring member as a cantilever spring.

The second spring member may be constituted by a cantilever spring biased against the linking member. One example of a cantilever spring is a blade spring. The second spring member may comprise a fixed section and a free section, wherein the fixed section is fixed with respect to the main body and the free section is biased against the linking member. The second spring member may be substantially horizontal and may be arranged to exert a downward biasing force on the linking member. The linking member may comprise a cam profile engaged at a second spring engagement point by the free section of the second spring member. The cam profile may be designed such that the second spring engagement point along the second spring member is substantially maintained in a horizontal plane fixed with respect to the main body as the linking member rotates about the suspension axis.

The drive wheel axis may be positioned vertically between the second spring engagement point and the suspension axis in the lowered position and the suspension axis may be positioned vertically between the second spring engagement point and the drive wheel axis in the raised position. In the lowered position, the vertical distance between the suspension axis and the drive wheel axis may be 30-50%, such as 40%, of the vertical distance between the suspension axis and the second spring engagement point. In the raised position, the vertical distance between the drive wheel axis and the suspension axis may be 5-20%, such as 10%, of the vertical distance between the drive wheel axis and the second spring engagement point. The suspension axis and the second spring engagement point may be substantially horizontally aligned in the lowered position and the second spring engagement point may be positioned horizontally between the suspension axis and the drive wheel axis in the raised position. By positioning the second spring engagement point horizontally aligned or substantially horizontally aligned in the lowered position and by arranging the second spring member to provide a biasing force acting downwardly on the linking member, no or substantially no torque is generated about the suspension axis by the second spring member when the linking member is in the lowered position. In the raised position, the horizontal distance between the suspension axis and the second spring engagement point may be 20-40%, such as 30%, of the horizontal distance between the suspension axis and the drive wheel axis.

A moment arm of the free section of the second spring member biased against the linking member acting on the suspension axis may be

substantially zero when the main body is in the lowered position.

The first spring member and the second spring member may be substantially aligned in the lowered position and/or the raised position.

The first spring member and the second spring member may be substantially aligned (i.e. substantially flush) with an upper edge of the linking member in the lowered position. The upper edge of the linking member may be substantially horizontal when the linking member is in the lowered position.

In case the linking member has an elongated appearance, the upper edge of the linking member may be substantially parallel to a general extension direction of the linking member. The upper edge may thus be inclined, for example about 45 ⁰ , with respect to the horizontal ground surface when the linking member adopts the raised position.

The first spring member and the second spring member may be oriented substantially parallel with the ground surface in the lowered position and/or the raised position. For example, both the first spring member and the second spring member may be substantially horizontally aligned in the lowered position and in the raised position. Although this configuration may be preferable in terms of space limitations, other orientations of the first spring member and the second spring member, either in one or both of the lowered position and the raised position, are conceivable.

The first spring member may be attached to the linking member at a first spring engagement point and the drive wheel axis may be positioned vertically between the first spring engagement point and the suspension axis in the lowered position and the suspension axis may be positioned vertically between the first spring engagement point and the drive wheel axis in the raised position. The first spring engagement point may be constituted by a protrusion, such as a hook, protruding upwardly (in the lowered position) from the linking member. The protrusion may be integrally formed with the linking member. The first spring member may also be attached to the main body in a corresponding manner, e.g. to a hook provided on the main body.

In the lowered position, the vertical distance between the suspension axis and the drive wheel axis may be 30-50%, such as 40%, of the vertical distance between the suspension axis and the first spring engagement point. In the raised position, the vertical distance between the drive wheel axis and the suspension axis may be 5-20%, such as 10%, of the vertical distance between the drive wheel axis and the first spring engagement point.

The first spring member may be attached to the linking member at a first spring engagement point and the suspension axis may be positioned horizontally between the first spring engagement point and the drive wheel axis in the lowered position and the first spring engagement point may be positioned horizontally between the suspension axis and the drive wheel axis in the raised position. For example, the horizontal distance between the first spring engagement point and the suspension axis may be 5-20%, such as 10%, of the horizontal distance between the first spring engagement point and the drive wheel axis in the lowered position. In the raised position, the horizontal distance between the suspension axis and the first spring engagement point may be 20-40%, such as 30%, of the horizontal distance between the suspension axis and the drive wheel axis.

The first spring member may be attached to the linking member at a first spring engagement point and the suspension axis and the first spring engagement point may be substantially horizontally aligned in the lowered position and the first spring engagement point may be positioned

horizontally between the suspension axis and the drive wheel axis in the raised position. For example, the horizontal distance between the suspension axis and the first spring engagement point may be 40-60%, such as 50%, of the horizontal distance between the suspension axis and the drive wheel axis in the raised position. As used herein, a horizontal distance and a vertical distance refer to the horizontal component and the vertical component, respectively, of the distance.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details, advantages and aspects of the present disclosure will become apparent from the following embodiments taken in conjunction with the drawings, wherein:

Fig. 1: schematically represents a front view of a robotic cleaning

device in a lowered position;

Fig. 2: schematically represents a bottom view of the robotic cleaning device;

Fig. 3: schematically represents a front perspective view of a drive

wheel assembly of the robotic cleaning device in the lowered position;

Fig. 4: schematically represents a rear perspective view of the drive

wheel assembly in the lowered position;

Fig. 5: schematically represents a front perspective view of the drive wheel assembly in a raised position;

Fig. 6: schematically represents a rear perspective view of the drive

wheel assembly in the raised position;

Fig. 7: schematically represents a side view of the drive wheel assembly in the lowered position; and

Fig. 8: schematically represents a side view of the drive wheel

assembly in the raised position.

DETAILED DESCRIPTION

In the following, a robotic cleaning device comprising at least one drive wheel and a first and second spring member associated with the at least one drive wheel will be described. The same reference numerals will be used to denote the same or similar structural features.

Fig. l schematically represents a front view of a robotic cleaning device 10 in a lowered position. The robotic cleaning device 10 comprises two drive wheels 12 for driving the robotic cleaning device 10 over a surface 14 to be cleaned and a main body 16. The clearance between the main body 16 and the surface 14 may be adjusted as will be described in the following.

The drive wheels 12 may be driven jointly to drive the robotic cleaning device 10 in a forward travel direction or in a backward direction, or independently to turn the robotic cleaning device 10. For example, one drive wheel 12 may be driven forwards and the other drive wheel 12 may be driven backwards in order to turn the robotic cleaning device 10 substantially on the spot or one drive wheel 12 may be driven forwards and the other drive wheel 12 may be locked in order to turn the robotic cleaning device 10 around the stationary drive wheel 12.

The robotic cleaning device 10 optionally comprises a rotatable brush roll 18 arranged horizontally at its front to enhance the dust and debris collecting properties of the robotic cleaning device 10. The robotic cleaning device 10 may further optionally comprise a 3D sensor system comprising a camera 20 and two line lasers 22, 24, which may be horizontally or vertically oriented line lasers.

Fig. 2 schematically represents a bottom view of the robotic cleaning device 10. As can be seen in Fig. 2, the main body 16 has a substantially triangular appearance parallel with the horizontal ground surface 14 and has a substantially straight side facing in a forward travel direction 26 of the robotic cleaning device 10. At the rear portion of the main body 16, a caster wheel 28 is disposed to support a rearward portion of the main body 16. In this implementation, the caster wheel 28 is arranged to swivel about a vertical axis.

The robotic cleaning device 10 further comprises two wheel motors 30, one associated with each drive wheel 12, to rotationally drive the respective drive wheel 12 and a control unit 32 to control the drive of the respective wheel motor 30. Various different types of transmissions may be used in order to transmit a driving force from the wheel motor 30 to the drive wheel 12, such as a gear transmission or a belt transmission.

Fig. 2 further shows that the robotic cleaning device 10 may comprise, a rotatable side brush 34, a suction fan 36 drivable by a fan motor 38 communicatively connected to the control unit 32 from which the fan motor 38 receives instructions for controlling the suction fan 36 and a brush roll motor 40 operatively coupled to the brush roll 18 to control its rotation in line with instructions received from the control unit 32.

Figs. 3 and 4 schematically represent a front perspective view and a rear perspective view, respectively, of one of two drive wheel assemblies 42 of the robotic cleaning device 10 in the lowered position. The lowered position may for example be adopted when cleaning a hard floor (e.g. parquet) and there are no obstacles to be climbed. In addition to the previously mentioned drive wheel 12 and wheel motor 30, the drive wheel assembly 42 comprises a linking member 44, a first spring member 46 and a second spring member 48. The linking member 44 is pivotally connected to the main body 16 and rotationally supports the drive wheel 12.

In the following, the first spring member 46 is exemplified as a tension spring and the second spring member 48 is exemplified as a cantilever spring in the form of a blade spring. However, these types of springs are not essential for the general function to provide a pressing force on the drive wheel 12 in both the lowered position and in the raised position.

The first spring member 46 is connected between the main body 16 and the linking member 44. The attachment point between the first spring member 46 and the linking member 44 is referred to as a first spring engagement point 50. The second spring member 48 comprises one section fixed with respect to the main body 16 and an opposing free section 52. In the illustrated lowered position, the first spring member 46 is in an extended state to pull the first spring engagement point 50 and the second spring member 48 provides a downwardly acting force on the linking member 44.

Both the first spring member 46 and the second spring member 48 are substantially horizontally aligned and arranged parallel to each other. In the illustrated implementation, both the first spring member 46 and the second spring member 48 are flush with an upper edge of the linking member 44. As can be seen in Figs. 3 and 4, the first spring member 46 and the second spring member 48 are aligned in a compact arrangement in the lowered position.

Figs. 5 and 6 schematically represent a front perspective view and a rear perspective view, respectively, of the drive wheel assembly 42 in the raised position. The raised position may be adopted when the robotic cleaning device 10 travels on a thick carpet and/or when climbing an obstacle. In the raised position, the drive wheels 12 of the robotic cleaning device 10 are moved out from the main body 16 and downwards towards the ground surface 14 (e.g. floor). In this state, the first spring member 46 still pulls the linking member 44 at the first spring engagement point 50. However, since the first spring member 46 is in a less extended state in the illustrated raised position, the force by the first spring member 46 is lower in raised position as compared to the lowered position. The second spring member 48 also provides a downwardly acting force on the linking member 44 in the raised position. Also in the raised position, the first spring member 46 and the second spring member 48 are aligned in a compact arrangement.

Fig. 7 schematically represents a side view of the drive wheel assembly 42 in the lowered position and Fig. 8 schematically represents a side view of the drive wheel assembly 42 in the raised position.

The linking member 44 is rotationally coupled to the main body 16 about a suspension axis 54. The linking member 44 is further arranged to rotationally support the associated drive wheel 12 about a drive wheel axis 56. Both the suspension axis 54 and the drive wheel axis 56 are oriented substantially perpendicular to the forward travel direction 26 of the robotic cleaning device 10. As can be seen in Figs. 7 and 8, the suspension axis 54 is arranged in front of the drive wheel axis 56, as seen in the forward travel direction 26, and the linking member 44 may therefore be said to constitute a trailing suspension. In the lowered position, a general extension direction of the linking member 44 is substantially parallel with the forward travel direction 26 of the robotic cleaning device 10.

When the linking member 44 is rotated about the suspension axis 54 in a first direction 58, the linking member 44 can be moved from the lowered position, as illustrated in Fig. 7, to the raised position, as illustrated in Fig. 8. The raised position is here constituted by a maximally raised position where the linking member 44 is inclined approximately 45 ⁰ with respect to the horizontal ground surface 14, but may also be constituted by an intermediate position. Since the suspension axis 54 is raised higher above the horizontal ground surface 14 in the raised position in Fig. 8 than in the lowered position in Fig. 7, also a section of the main body 16, to which the linking member 44 is attached, is raised higher above the horizontal ground surface 14 in the raised position than in the lowered position.

This clearance control may be entirely independent between the two drive wheel assemblies 42 of the robotic cleaning device 10. For example, one linking member 44 may adopt the lowered position while the other linking member 44 adopts the raised position, and vice versa. Of course, both linking members 44 may also simultaneously adopt the lowered position or the raised position.

Since the first spring member 46 is extended in the lowered position in Fig. 7, it generates a force on the first spring engagement point 50, here

implemented as an upwardly protruding hook, to which the first spring member 46 is attached. This force acting on the first spring engagement point 50 in turn generates a moment on the linking member 44 about the suspension axis 54 in the first direction 58. Thereby, the first spring member 46 is arranged to provide a moment on the linking member 44 about the suspension axis 54 in the first direction 58 to press the drive wheel 12 downwardly towards the ground surface 14.

In the raised position in Fig. 8 however, the first spring member 46 is less extended in comparison with Fig. 7. As a result, in the raised position, the force acting on the first spring engagement point 50 and the consequential moment acting on the linking member 44 about the suspension axis 54 in the first direction 58 are lower in comparison with the lowered position. The first spring member 46 is thereby arranged to provide a higher moment in the lowered position than in the raised position. More specifically, the first spring member 46 is thereby arranged to provide a first, higher moment on the linking member 44 about the suspension axis 54 in the first direction 58 when the main body 16 is in the lowered position and to provide a second, lower moment on the linking member 44 about the suspension axis 54 in the first direction 58 when the main body 16 is in the raised position. The second spring member 48 comprises a fixed section 60 that is fixed with respect to the main body 16 and a free section 52 that is biased against the linking member 44. The second spring member 48 is biased downwardly and provides a downward force 62 on a cam profile 64 of the linking member 44. The contact point between the second spring member 48 and the linking member 44 is referred to as a second spring engagement point 66. As illustrated by a vertical line 68 in Fig. 7, the force 62 by the second spring member 48 acting on the linking member 44 is directed towards the suspension axis 54. As a consequence, in the lowered position, the second spring member 48 does not generate any moment on the linking member 44 about the suspension axis 54.

When the linking member 44 starts to rotate about the suspension axis 54 in the first direction 58, for example if the robotic cleaning device 10 encounters an obstacle so that the impact force from the obstacle on the drive wheel 12 together with the moment provided on the linking member 44 about the suspension axis 54 in the first direction 58 by the first spring member 46 overcomes the gravital force from the main body 16 acting on the drive wheel assembly 42, the second spring engagement point 66 is horizontally displaced (in a backward direction, opposite to the forward travel direction 26) with respect to the suspension axis 54. As a consequence, the downward force 62 from the second spring member 48 acting on the linking member 44 starts to generate a moment on the suspension axis 54 in the first direction 58. The moment arm of this moment is illustrated by the line 70.

In other words, the second spring member 48 is arranged to provide a higher moment on the linking member 44 in the raised position than in the lowered position. More specifically, the second spring member 48 is thereby arranged to provide no moment on the linking member 44 about the suspension axis 54 when the main body 16 is in the lowered position and to provide a moment on the linking member 44 about the suspension axis 54 in the first direction 58 when the main body 16 is in the raised position. As the linking member 44 rotates about the suspension axis 54 from the lowered position to the raised position, the second spring engagement point 66 travels along the cam profile 64 of the linking member 44. As can be gathered from Figs. 7 and 8, the cam profile 64 is designed such that the second spring engagement point 66 is substantially maintained in the same horizontal plane with respect to the main body 16 as the linking member 44 rotates about the suspension axis 54. In other words, the second spring member 48 is maintained substantially horizontal and is lifted together with the main body 16 as the main body 16 moves from the lowered position to the raised position, and vice versa.

Fig. 7 shows that the drive wheel axis 56 is positioned vertically between the second spring engagement point 66 and the suspension axis 54 in the lowered position. More specifically, a vertical distance between the

suspension axis 54 and drive wheel axis 56 is approximately 40% of the vertical distance between the suspension axis 54 and the second spring engagement point 66 when the linking member 44 adopts the lowered position.

Fig. 8 further shows that the suspension axis 54 is positioned slightly above and vertically between the second spring engagement point 66 and the drive wheel axis 56 in the raised position. More specifically, the vertical distance between the drive wheel axis 56 and the suspension axis 54 is approximately 10% of the vertical distance between the drive wheel axis 56 and the second spring engagement point 66.

Fig. 7 further shows that the suspension axis 54 and the second spring engagement point 66 are horizontally aligned in the lowered position such that no torque is generated about the suspension axis 54 by the second spring member 48 when the linking member 44 is in the lowered position. In other words, the moment arm 70 of the force 62 from the second spring member 48 acting downwardly on the linking member 44, as illustrated in raised position of Fig. 8, is zero, or substantially zero, in the lowered position of Fig.

7· Fig. 8 further shows that the second spring engagement point 66 is positioned horizontally between the suspension axis 54 and the drive wheel axis 56 in the raised position of the linking member 44. More specifically, the horizontal distance between the suspension axis 54 and the second spring engagement point 66 is approximately 30% of the horizontal distance between the suspension axis 54 and the drive wheel axis 56. l8

Fig. 7 further shows that the drive wheel axis 56 is positioned vertically between the first spring engagement point 50 and the suspension axis 54 in the lowered position. More specifically, the vertical distance between the suspension axis 54 and the drive wheel axis 56 is approximately 40% of the vertical distance between the suspension axis 54 and the first spring engagement point 50.

Fig. 8 further shows that the suspension axis 54 is positioned vertically between the first spring engagement point 50 and the drive wheel axis 56 in the raised position. More specifically, the vertical distance between the drive wheel axis 56 and the suspension axis 54 is approximately 10% of the vertical distance between the drive wheel axis 56 and the first spring engagement point 50.

Fig. 7 further shows that the suspension axis 54 and the first spring engagement point 50 are substantially horizontally aligned in the lowered position. Fig. 8 further shows that the first spring engagement point 50 is positioned horizontally between the suspension axis 54 and the drive wheel axis 56 in the raised position. More specifically, the horizontal distance between the suspension axis 54 and the first spring engagement point 50 is approximately 50% of the horizontal distance between the suspension axis 54 and the drive wheel axis 56 in the raised position.

The second spring member 48 thus ensures that the drive wheel 12 is pressed downwards against the ground surface 14, with a sufficient force to prevent slippage, also in the raised position where the force generated by the first spring member 46 is reduced. Due to the stronger contact between the drive wheel 12 and the ground surface 14, any navigation by the robotic cleaning device 10 based entirely or partly on odometry is improved. The robotic cleaning device 10 is thus less likely to lose track of its position.

The increased downward force on the drive wheel 12 in the raised position also gives a stronger force to a suction nozzle in the raised position and the robotic cleaning device 10 is thereby less prone to stick to, for example, a carpet.

While the present disclosure has been described with reference to exemplary embodiments, it will be appreciated that the present invention is not limited to what has been described above. For example, it will be appreciated that the dimensions of the parts may be varied as needed. Accordingly, it is intended that the present invention may be limited only by the scope of the claims appended hereto.