TECHNICAL FIELD

The invention relates to a system of robotic cleaning devices and a method of a master robotic cleaning device of controlling at least one slave robotic cleaning device.

BACKGROUND

In many fields of technology, it is desirable to use robots with an autonomous behaviour such that they freely can move around a space without colliding with possible obstacles. Robotic vacuum cleaners are know in the art, which are equipped with drive means in the form of a motor for moving the cleaner across a surface to be cleaned. The robotic vacuum cleaners are further equipped with intelligence in the form of microprocessor(s) and navigation means for enabling an autonomous behaviour such that the robotic vacuum cleaners freely can move around and clean a space in the form of e.g. a room. Thus, these prior art robotic vacuum cleaners has the capability of more or less autonomously vacuum cleaning a room in which furniture such as tables and chairs and other obstacles such as walls and stairs are located.

In various environments where a surface is to be cleaned by a robotic cleaning device, situations may arise where the robotic cleaning device cannot reach for instance into a corner or under a sofa, and the user will manually have to remove the debris or dirt which is out of reach for the robot, or use another cleaning device suitable for cleaning the particular surface.

SUMMARY

An object of the present invention is to solve, or at least mitigate, this problem in the art and to provide a system of robotic cleaning devices.

This object is attained in a first aspect of the invention by a system of robotic cleaning devices. The system comprises a master robotic cleaning device and at least one slave robotic cleaning device. The master robotic cleaning device comprises a propulsion system arranged to move the master robotic cleaning device, a cleaning member arranged to remove debris from a surface to be cleaned, an obstacle detection device arranged to detect obstacles, a controller arranged to position the robotic cleaning device with respect to the detected obstacles from positional data derived from the obstacle detection device, and further to control the propulsion system to move the master robotic cleaning device based on the positional data, and a wireless

communication interface via which the master robotic cleaning device is arranged to submit commands the at least one slave robotic cleaning device to control a cleaning operation of the at least one slave robotic cleaning device, the commands being based on the derived positional data. The at least one slave robotic cleaning device comprises a propulsion system arranged to move the slave robotic cleaning device, a cleaning member arranged to remove debris from a surface to be cleaned, a wireless communication interface via which the slave robotic cleaning device is arranged to receive to commands submitted by the master robotic cleaning device, and a controller arranged to control the cleaning operation of the slave robotic cleaning device as indicated by the received commands.

This object is attained in a second aspect of the invention by a method performed by a master robotic cleaning device of controlling at least one slave robotic cleaning device. The method comprising detecting obstacles, deriving positional data from the detection of obstacles, positioning the master robotic cleaning device with respect to the detected obstacles from the derived positional data, controlling movement of the master robotic cleaning device based on the positional data, and submitting commands to the at least one slave robotic cleaning device to control a cleaning operation of said at least one slave robotic cleaning device, the commands being based on the derived positional data, wherein the cleaning operation of the slave robotic cleaning device is controlled as indicated by the submitted commands.

A system of robotic cleaning devices is provided comprising a master robotic cleaning device and at least on slave robotic cleaning device, where the master robotic cleaning device is capable of wirelessly controlling a cleaning operation of the slave robotic cleaning device.

A surface to be cleaned may for instance accommodate objects such as sofas, armchairs, bureaus, etc., under which the master robot cannot pass for performing a cleaning operation. Without a system according to the invention, the user would have to use for instance a broom or an ordinary vacuum cleaner to clean a surface below these objects.

Advantageously, the master robot instructs the slave robot, by submitting wireless control signals via its communication interface to a corresponding communication interface of the slave robot, to clean the surfaces that the master robot cannot reach based on the positional data derived by the master robot.

Further advantageous is that the slave robot is not required to be equipped with the same sophisticated obstacle detecting device as the master robot. Consequently, since the slave robot is instructed by the master robot how to navigate over the surface to be cleaned, based on positional data derived by the master robot, a less powerful slave robot controller can be used. Hence, the slave robot can advantageously be made much less complex than the master robot, with a correspondingly great reduction in cost. In an embodiment, the master robotic device advantageously communicates to the slave robotic device a command indicating a surface over which the at least one slave robotic cleaning device is instructed to move and clean, for instance a surface located under a sofa having a clearance height less than the height of the master robot. The command may e.g. indicate the coordinates delimiting the surface. Alternatively, the command may include control data for the slave robot propulsion system in order to guide the slave robotic device over the surface to be cleaned. Upon receiving the command from the master robot, the slave robot moves to, and cleans, the surface.

In yet an embodiment, the slave robotic cleaning device is arranged to remove debris from a surface to be cleaned and gather the debris for subsequent pick-up by the master robotic cleaning device. Advantageously, there is no need to equip the slave robot with e.g. a dust container or suction fan, thereby facilitating an even less complex - and less noisy - slave robot. Further advantageous is that the slave robot becomes more or less

maintenance-free, as there is no need to empty a dust container.

In still a further embodiment, the commands submitted by the master robotic cleaning device comprises data indicating a time of day or night at which the slave robotic cleaning device is instructed to perform the cleaning operation. For instance, the slave robot may advantageously be instructed to work a particular surface during night time, when no obstacles in the form of humans and animals will impede the slave robot. This is particularly advantageous in case the slave robot is not equipped with components such as a suction fan and/or a rotatable brush, in which the case the slave robot is relatively silent. In the morning, when the slave robot has gathered debris, the master robot will vacuum clean the debris from the place where the slave robot did gather the debris.

In another embodiment, the commands submitted from the master robotic cleaning device to the slave robotic cleaning device comprises an instruction to the slave robotic cleaning device to return to its charger after the cleaning operation has been performed.

In a further embodiment, the communication between the master robotic cleaning device and the slave robotic cleaning device is not performed via line-of sight-communication, but advantageously via a network such as a Wireless Local Area Network (WLAN). Thus, the master robot and the slave robot connect to the WLAN via a so called Access Point (AP) such as e.g. a home router for wireless WiFi communication. Advantageously, by

communicating via a WLAN, the master robot and the slave robot can be located on a great distance from each other, such as on different floors in a building but still being capable of communicating with each other. In still another embodiment, the master robot is advantageously arranged to communicate wirelessly, for example via Bluetooth or WLAN, with a mobile terminal (such as a smart phone, a tablet, a laptop, etc.) of a user wishing to control the cleaning operation of the slave robot 30. The user may

communicate with the master robot via a particular app downloaded to the mobile terminal. Hence, the user may advantageously instruct the master robot regarding a desired cleaning operation to be performed by the slave robot, whereupon the master robot sends commands accordingly to the slave robot via its communication interface. In yet a further embodiment, in order to facilitate the detection of the slave robot for the master robot, the slave robot is advantageously equipped with one or more light sources, such as light emitting diodes (LEDs), and/or luminous reflectors detectable by the master robot.

In still another embodiment, in order to facilitate the detection of the slave robot for the master robot, the slave robot is advantageously equipped with an optical detector configured to detect light emitted by the master robot onto the slave robot. Upon detecting the laser light emitted by the master robot, the slave robot communicates via its communication interface to the master robot that the light is detected. In a further embodiment, by having the slave robot detected light emitted by the master robot, and instantly communicate that the light that has been detected at the optical detector, an operational clock of the master and the slave, respectively, can be synchronized to each other. Advantageously, any clock drift may be eliminated, which facilitates system navigation. It is noted that the master robotic cleaning device and the slave robotic cleaning device may perform different types of cleaning operations. For instance, the master robot may be a vacuum cleaner while the slave robot is a floor washer.

The invention relates to all possible combinations of features recited in the claims. Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. Those skilled in the art realize that different features of the present invention can be combined to create embodiments other than those described in the following. BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described, by way of example, with reference to the accompanying drawings, in which:

Figure ι shows a bottom view of a master robotic cleaning device according to an embodiment of the invention; Figure 2 shows a front view of the master robotic cleaning device of Figure l;

Figure 3 shows a front view of a system of robotic cleaning devices according to an embodiment of the invention;

Figure 4 shows a top view of a system of robotic cleaning devices according to an embodiment of the invention; Figure 5a illustrates detection of a slave robotic cleaning device according to an embodiment of the invention;

Figure 5b illustrates detection of a slave robotic cleaning device according to another embodiment of the invention;

Figure 6 shows a top view of a system of robotic cleaning devices according to another embodiment of the invention;

Figure 7 shows a top view of a system of robotic cleaning devices according to yet another embodiment of the invention;

Figure 8 illustrates a system of robotic cleaning devices communicating via a network according to an embodiment of the invention; and Figure 9 illustrates a system of robotic cleaning devices according to an embodiment of the invention where the master robotic cleaning device communicates with a mobile terminal.

DETAILED DESCRIPTION

The invention will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout the description.

The invention relates to robotic cleaning devices, or in other words, to automatic, self-propelled machines 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 invention can be mains-operated and have a cord, be battery-operated or use any other kind of suitable energy source, for example solar energy.

Figure 1 shows a robotic cleaning device 10 according to embodiments of the present invention from below, i.e. the bottom side of the robotic cleaning device is shown. The arrow indicates the forward direction of the robotic cleaning device. The robotic cleaning device 10 comprises a main body 11 housing components such as a propulsion system comprising driving means in the form of two electric wheel motors 15, 16 for enabling movement of the driving wheels 12, 13 such that the cleaning device can be moved over a surface to be cleaned. Each wheel motor 15, 16 are capable of controlling the respective driving wheel 12, 13 to rotate independently of each other in order to move the robotic cleaning device 10 across the surface to be cleaned. A number of different driving wheel arrangements, as well as various wheel motor arrangements, can be envisaged. It should be noted that the robotic cleaning device may have any appropriate shape, such as a device having a more traditional circular-shaped main body, or a triangular-shaped main body. As an alternative, a track propulsion system may be used or even a hovercraft propulsion system. The propulsion system may further be arranged to cause the robotic cleaning device 10 to perform any one or more of a yaw, pitch, translation or roll movement.

A controller 22 such as a microprocessor controls the wheel motors 15, 16 to rotate the driving wheels 12, 13 as required in view of information received from an obstacle detecting device (not shown in Figure 1) for detecting obstacles in the form of walls, floor lamps, table legs, around which the robotic cleaning device must navigate. The obstacles detected may also be embodied in the form of landmarks, barcodes, signposts, etc. The obstacle detecting device may be embodied in the form of a 3D sensor system registering its surroundings, implemented by means of e.g. a 3D camera, a camera in combination with lasers, a laser scanner, etc. for detecting obstacles and communicating information about any detected obstacle to the microprocessor 22. The microprocessor 22 communicates with the wheel motors 15, 16 to control movement of the wheels 12, 13 in accordance with information provided by the obstacle detecting device such that the robotic cleaning device 10 can move as desired across the surface to be cleaned. This will be described in more detail with reference to subsequent drawings.

Further, the main body 11 may optionally be arranged with a cleaning member 17 for removing debris and dust from the surface to be cleaned in the form of a rotatable brush roll arranged in an opening 18 at the bottom of the robotic cleaner 10. Thus, the rotatable brush roll 17 is arranged along a horizontal axis in the opening 18 to enhance the dust and debris collecting properties of the cleaning device 10. In order to rotate the brush roll 17, a brush roll motor 19 is operatively coupled to the brush roll to control its rotation in line with instructions received from the controller 22.

Moreover, the main body 11 of the robotic cleaner 10 comprises a suction fan 20 creating an air flow for transporting debris to a dust bag or cyclone arrangement (not shown) housed in the main body via the opening 18 in the bottom side of the main body 11. The suction fan 20 is driven by a fan motor 21 communicatively connected to the controller 22 from which the fan motor 21 receives instructions for controlling the suction fan 20. It should be noted that a robotic cleaning device having either one of the rotatable brush roll 17 and the suction fan 20 for transporting debris to the dust bag can be envisaged. A combination of the two will however enhance the debris- removing capabilities of the robotic cleaning device 10.

The robotic cleaning device 10 may further be equipped with an inertia measurement unit (IMU) 24, such as e.g. a gyroscope and/or an

accelerometer and/or a magnetometer or any other appropriate device for measuring displacement of the robotic cleaning device 10 with respect to a reference position, in the form of e.g. orientation, rotational velocity, gravitational forces, etc. A three-axis gyroscope is capable of measuring rotational velocity in a roll, pitch and yaw movement of the robotic cleaning device 10. A three-axis accelerometer is capable of measuring acceleration in all directions, which is mainly used to determine whether the robotic cleaning device is bumped or lifted or if it is stuck (i.e. not moving even though the wheels are turning). The robotic cleaning device 10 further comprises encoders (not shown in Figure 1) on each drive wheel 12, 13 which generate pulses when the wheels turn. The encoders may for instance be magnetic or optical. By counting the pulses at the controller 22, the speed of each wheel 12, 13 can be determined. By combining wheel speed readings with gyroscope information, the controller 22 can perform so called dead reckoning to determine position and heading of the cleaning device 10. The controller 22 may employ e.g. the commonly used robotic localization method

Simultaneous Localization and Mapping (SLAM) to position the robotic cleaning device 10 with respect to its surroundings.

The main body 11 may further be arranged with a rotating side brush 14 adjacent to the opening 18, the rotation of which could be controlled by the drive motors 15, 16, the brush roll motor 19, or alternatively a separate side brush motor (not shown). Advantageously, the rotating side brush 14 sweeps debris and dust from the surface to be cleaned such that the debris ends up under the main body 11 at the opening 18 and thus can be transported to a dust chamber of the robotic cleaning device. Further advantageous is that the reach of the robotic cleaning device 10 will be improved, and e.g. corners and areas where a floor meets a wall are much more effectively cleaned. As is illustrated in Figure 1, the rotating side brush 14 rotates in a direction such that it sweeps debris towards the opening 18 such that the suction fan 20 can transport the debris to a dust chamber. The robotic cleaning device 10 may comprise two rotating side brushes arranged laterally on each side of, and adjacent to, the opening 18. The robotic cleaning device 10 further comprises a communication interface 29 comprising a wireless receiver and transmitter, typically embodied by a single unit known as a transceiver. The communication interface 29 communicates via e.g. infrared (IR), ultrasonic or radio-frequency (RF) signals with for instance a remote control utilizing line-of-sight

communication or a server using wireless local area network (WLAN) technology.

The communication interface may further be connected to a user interface (not shown) provided on the robotic cleaning device 10 via which a user can provide the robotic cleaner 10 with a particular type of instruction, such as "start", "stop", "return to charging station", etc. The user interface may be of touch-screen type or mechanically configured comprising physical buttons to be operated. Further, the user interface may comprise display means for visually indicating a user selection. It should be noted that the user not necessarily need to provide input to the user interface by physically touching it, but may alternatively communicate with the robotic cleaner by means of the previously mentioned remote control.

With further reference to Figure 1, the controller 22 embodied in the form of one or more microprocessors is arranged to execute a computer program 25 downloaded to a suitable storage medium 26 associated with the

microprocessor, such as a Random Access Memory (RAM), a Flash memory or a hard disk drive. The controller 22 is arranged to carry out a method according to embodiments of the present invention when the appropriate computer program 25 comprising computer-executable instructions is downloaded to the storage medium 26 and executed by the controller 22. The storage medium 26 may also be a computer program product comprising the computer program 25. Alternatively, the computer program 25 may be transferred to the storage medium 26 by means of a suitable computer program product, such as a digital versatile disc (DVD), compact disc (CD) or a memory stick. As a further alternative, the computer program 25 may be downloaded to the storage medium 26 over a wired or wireless network. The controller 22 may alternatively be embodied in the form of a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field- programmable gate array (FPGA), a complex programmable logic device (CPLD), etc.

Figure 2 shows a front view of the robotic cleaning device 10 of Figure 1 in an embodiment of the present invention illustrating the previously mentioned obstacle detecting device in the form of a 3D sensor system comprising at least a camera 23 and a first and a second line laser 27, 28, which may be horizontally or vertically oriented line lasers. Further shown is the controller 22, the main body 11, the driving wheels 12, 13, and the rotatable brush roll 17 previously discussed with reference to Figure la. The controller 22 is operatively coupled to the camera 23 for recording images of a vicinity of the robotic cleaning device 10. The first and second line lasers 27, 28 may preferably be vertical line lasers and are arranged lateral of the camera 23 and configured to illuminate a height and a width that is greater than the height and width of the robotic cleaning device 10. Further, the angle of the field of view of the camera 23 is preferably smaller than the space illuminated by the first and second line lasers 27, 28. The camera 23 is controlled by the controller 22 to capture and record a plurality of images per second. Data from the images is extracted by the controller 22 and the data is typically saved in the memory 26 along with the computer program 25.

The first and second line lasers 27, 28 are typically arranged on a respective side of the camera 23 along an axis being perpendicular to an optical axis of the camera. Further, the line lasers 27, 28 are directed such that their respective laser beams intersect within the field of view of the camera 23. Typically, the intersection coincides with the optical axis of the camera 23.

The first and second line laser 27, 28 are configured to scan, preferably in a vertical orientation, the vicinity of the robotic cleaning device 10, normally in the direction of movement of the robotic cleaning device 10. The first and second line lasers 27, 28 are configured to send out laser beams, which illuminate furniture, walls and other objects of e.g. a room to be cleaned. The camera 23 is controlled by the controller 22 to capture and record images from which the controller 22 creates a representation or layout of the surroundings that the robotic cleaning device 10 is operating in, by extracting features from the images and by measuring the distance covered by the robotic cleaning device 10, while the robotic cleaning device 10 is moving across the surface to be cleaned. Thus, the controller 22 derives positional data of the robotic cleaning device 10 with respect to the surface to be cleaned from the recorded images, generates a 3D representation of the surroundings from the derived positional data and controls the driving motors 15, 16 to move the robotic cleaning device across the surface to be cleaned in accordance with the generated 3D representation and navigation information supplied to the robotic cleaning device 10 such that the surface to be cleaned can be navigated by taking into account the generated 3D representation. Since the derived positional data will serve as a foundation for the navigation of the robotic cleaning device, it is important that the positioning is correct; the robotic device will otherwise navigate according to a "map" of its surroundings that is misleading.

The 3D representation generated from the images recorded by the 3D sensor system thus facilitates detection of obstacles in the form of walls, floor lamps, table legs, around which the robotic cleaning device must navigate as well as rugs, carpets, doorsteps, etc., that the robotic cleaning device 10 must traverse. The robotic cleaning device 10 is hence configured to learn about its environment or surroundings by operating/cleaning. Hence, the 3D sensor system comprising the camera 23 and the first and second vertical line lasers 27, 28 is arranged to record images of a vicinity of the robotic cleaning from which objects/obstacles may be detected. The controller 22 is capable of positioning the robotic cleaning device 10 with respect to the detected obstacles and hence a surface to be cleaned by deriving positional data from the recorded images. From the positioning, the controller 22 controls movement of the robotic cleaning device 10 by means of controlling the wheels 12, 13 via the wheel drive motors 15, 16, across the surface to be cleaned. The derived positional data facilitates control of the movement of the robotic cleaning device 10 such that cleaning device can be navigated to move very close to an object, and to move closely around the object to remove debris from the surface on which the object is located. Hence, the derived positional data is utilized to move flush against the object, being e.g. a thick rug or a wall. Typically, the controller 22 continuously generates and transfers control signals to the drive wheels 12, 13 via the drive motors 15, 16 such that the robotic cleaning device 10 is navigated close to the object.

Now, with reference to Figures 1 and 2, it can be deducted that the

autonomously operating robotic cleaning device 10 is a highly sophisticated and complex device, requiring an advanced navigation system for moving over the surface to be cleaned. As previously has been discussed, it may be desirable to complement the robotic cleaning device 10, in the following being referred to as the "master" robotic cleaning device, with one or more less complex supporting or assisting robotic cleaning devices, in the following referred to as "slave" robotic cleaning devices.

With reference to Figure 3, in an embodiment of the invention, a system of robotic cleaning devices is thus provided comprising the master robotic cleaning device 10 as described with reference to Figures 1 and 2 and at least one slave robotic cleaning device 30, where the master robotic cleaning device is capable of wirelessly controlling a cleaning operation of the slave robotic cleaning device 30. In Figure 3, it is illustrated that the master robot 10 has a height h ₂ while the slave robot 30 has a height h ₂ . Assuming for instance that the surface to be cleaned accommodates objects such as sofas, armchairs, bureaus, etc., having a clearance height h (i.e. distance from the floor up to an under side of the object) being less than hi but greater than h ₂ .

Thus, for objects having a clearance height less than hi, the master robot cannot pass under for performing a cleaning operation. Without a system such as that shown in Figure 3 illustrating an embodiment of the invention, the user will have to use for instance a broom or an ordinary vacuum cleaner to clean a surface below these objects.

Advantageously, in the embodiment illustrated with reference to Figure 3, the master robot 10 instructs the slave robot 30, by submitting wireless control signals via the communication interface 29 to a corresponding

communication interface 31 of the slave robot 30, to clean the surfaces under the objects having a clearance height less than hi, as detected by the obstacle detecting device of the master robotic cleaning device 10. As previously has been discussed with reference to Figure 2, the obstacle detection device (exemplified by the 3D sensor system comprising the camera 23 and the first and second line laser 27, 28 in Figure 2) is arranged to detect obstacles surrounding the master robot 10, and the controller of the master robot 10 uses positional data derived from the obstacle detection device to position itself with respect to the surroundings, which also includes positioning the master robotic device 10 in relation to the slave robotic device 30 (being an "obstacle" in the surroundings of the master robot 10). As in the case of the master robot 10, the slave robot 30 comprises a controller 30 configured to control a propulsion system comprising driving means in the form of e.g. two electric wheel motors 33, 34 for enabling movement of the driving wheels 35, 36 (or any other appropriate movement means) such that the slave cleaning device 30 can be moved over a surface to be cleaned, such as the surfaces under the furniture which the master robot 10 cannot reach. In this particular example, the slave robot 30 further comprises a cleaning member in the form of a rotatable brush roll 37 for more effectively removing debris from the surface to be cleaned.

With the system of cleaning robots according to embodiments of the present invention, the slave robot 30 is advantageously not required to be equipped with the same sophisticated obstacle detecting device (embodied in Figure 3 by the 3D sensor system comprising the camera 23 and the two line lasers 27, 28) as the master robot 10. Consequently, since the slave robot 30 is instructed by the master robot 10 how to navigate over the surface to be cleaned (based on positional data derived by the controller 22 of the master robot 10 from the obstacle detection device as previously described), a less powerful controller 32 can be used. Hence, the slave robot 30 can

advantageously be made much less complex than the master robot 10, with a correspondingly great reduction in cost.

Figure 4 illustrates, in a top view, the cleaning operation of the slave robotic device 30 as discussed with reference to the embodiment illustrated in Figure 3. Hence, the master robotic device 10 communicates to the slave robotic device 30 a command indicating a surface 40 over which the at least one slave robotic cleaning device 30 is instructed to move and clean, for instance a surface located under a sofa having a clearance height less than hi. The command may e.g. indicate the coordinates delimiting the surface 40; in

Figure 4 denoted (Xi, Yi), (X ₂ , Y ₂ ), (X3, Yi). Alternatively, the command may include control data for the slave robot propulsion system in order to guide the slave robotic device 30 over the surface to be cleaned. Upon receiving the command from the master robot 10, the slave robot 30 moves to, and cleans, the surface 40. The slave robot 40 works the surface 40 in a pattern as indicated by the arrows, thus cleaning the surface 40. The slave robot is indicated in Figure 4 to have a cylindrically shaped main body; this is exemplifying only, and the slave robot 30 may have any appropriate shape.

Figure 5a illustrates an embodiment of the invention where the master robotic cleaning device 10 positions itself in relation to the slave robotic cleaning device 30. Now, in order to facilitate the detection of the slave robot 30 for the master robot 10, the slave robot is equipped with one or more light sources 38, such as light emitting diodes (LEDs), and/or luminous reflectors detectable by the master robot 10.

Thus, the master robot 10 emits light by means of its laser light sources 27, 28 onto the slave robot 30 and the camera 23 records images of a vicinity of the master robotic cleaning device 10 from which the slave robot 10 may be detected. Thereafter, the master robot 10 derives positional data of the detected objects from the recorded images, and positons itself in relation to the objects, including the slave robot 30. Figure 5b illustrates a further embodiment of the invention where the master robotic cleaning device 10 positions itself in relation to the slave robotic cleaning device 30. Now, in order to facilitate the detection of the slave robot 30 for the master robot 10, the slave robot is in this embodiment equipped with an optical detector 39 configured to detect light emitted by the laser light sources 27, 28 of the master robot 10 onto the slave robot 30.

Upon detecting the laser light emitted by the master robot 10, the slave robot 30 communicates via its communication interface to the master robot 10 that the laser light is detected. As previously has been discussed, the interface may communicate via e.g. IR, ultrasonic or RF signals (possibly utilizing WLAN technology).

In this way, the master robot 10 is able of detecting - and positioning itself in relation to - the slave robot 30 using for instance SLAM.

Further advantageous is that, by having the slave robot 30 detected light emitted by the line lasers 27, 28 of the master robot 10, and instantly communicate that the light that has been detected at the optical detector 39, an operational clock of the master and the slave, respectively, can be synchronized to each other. Hence, any clock drift may be eliminated, which facilitates system navigation. It should be noted that the embodiments of the invention illustrated in Figures 5a and b can be combined; the slave robot 10 may thus comprise both LEDs 38 and an optical detector 39. Figure 6 illustrates, again in a top view, a further embodiment of the invention where, the master robotic device 10 communicates to the slave robotic device 30 a command indicating a surface 40 over which the at least one slave robotic cleaning device 30 is instructed to move. Again, the command may e.g. indicate the coordinates delimiting the surface 40; in Figure 6 denoted (Xi, Yi), (X ₂ , Y ₂ ), (X3, Yi). Upon receiving the command from the master robot 10, the slave robot 30 moves to the surface 40.

However, in this particular embodiment, the slave robotic cleaning device 30 will itself not vacuum clean the surface 40. Instead, the slave robotic cleaning device 30 works the surface 40 in the pattern as indicated by the arrows by pushing the debris in from of it, for instance with the aid of a cleaning member in the form of a brush. The slave robot 30 will at the end of the cleaning operation leave any collected debris 50 outside of the surface 40 where the master robot 10 can reach and remove the debris 50. This embodiment is advantageous since there is no need to equip the slave robot 30 with a dust container or suction fan, thereby facilitating an even less complex - and less noisy - slave robot 30. Further, with this embodiment, the slave robot is advantageously more or less maintenance-free, as there is no need to empty a dust container. A user will only occasionally have to remove debris that is stuck to the cleaning member of the slave robot 10.

Again with reference to Figure 6, in yet another embodiment, the commands submitted by the master robotic cleaning device 10 comprises data indicating a time of day or night at which the slave robotic cleaning device 30 is instructed to perform the cleaning operation. For instance, the slave robot may advantageously be instructed to work a particular surface during night time, when no obstacles in the form of humans and animals will impede the slave robot 30. This is particularly advantageous in case the slave robot is not equipped with components such as a suction fan and/or a rotatable brush, in which the case the slave robot is relatively silent. In the morning, when the slave robot 30 has gathered debris, the master robot 10 will vacuum clean the debris from the place where the slave robot 30 did gather the debris. l8

Figure 7 illustrates yet a further embodiment of the present invention, where the master robotic cleaning device 10 and the slave robotic cleaning device 30 perform different types of cleaning operations. In this particular

embodiment, the master robot 10 is a vacuum cleaner while the slave robot 30 is a floor washer. Thus, as the master robotic vacuum cleaner 10 sets out to vacuum clean the surface 40, it instructs the slave robotic floor sweeper 30 to perform its cleaning operation by sending commands indicating the position of the surface 40 to be cleaned as previously discussed. The slave robotic floor washer 30 can thus be operated to follow the master robotic vacuum cleaner 10 to advantageously perform the complementing cleaning operation of washing the floor as represented by the surface 40.

In the exemplifying embodiment of Figure 7, the main body of the slave robotic floor washer 30 has the same shape as that of the master robotic vacuum cleaner 10. However, as previously discussed, in terms of

intelligence, it can advantageously be made substantially less complex than the master robot 10. In case the slave robot 10 is embodied in the form of a robotic washer, its cleaning member typically comprises a swab component such as a mop.

In an embodiment of the invention, the commands submitted from the master robotic cleaning device 10 to the slave robotic cleaning device 30 comprises an instruction to the slave robotic cleaning device 30 to return to its charger (not shown) after the cleaning operation has been performed.

Figure 8 shows yet a further embodiment of the invention, where the communication between the master robotic cleaning device 10 and the slave robotic cleaning device 30 is not performed via line-of sight-communication, but via a network such as a Wireless Local Area Network 60 (WLAN), commonly referred to as "WiFi". Thus, the master robot 10 and the slave robot 30 connects to the WLAN 60 via a so called Access Point (AP) 31 such as e.g. a home router for wireless WiFi communication, whereupon instructions can be submitted from the master robotic cleaning device 10 to the slave robotic cleaning device 30, in order for the slave robot to perform a desired cleaning operation.

Advantageously, by communicating via a WLAN, the master robot 10 and the slave robot 30 can be located on a great distance from each other, such as on different floors in a building but still being capable of communicating with each other.

Figure 9 illustrates still a further embodiment of the present invention, where the master robot 10 is arranged to communicate wirelessly, for example via Bluetooth or the WLAN 60, with a mobile terminal 80 (such as a smart phone, a tablet, a laptop, etc.) of a user wishing to control the cleaning operation of the slave robot 30. The user may communicate with the master robot 10 via a particular app downloaded to the mobile terminal 80. Hence, the user may advantageously instruct the master robot 10 regarding a desired cleaning operation to be performed by the slave robot 30, whereupon the master robot 10 sends commands accordingly to the slave robot 10 via its communication interface.

The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.