Reception of frequency spectra on the receiver side

A robotic lawnmower system comprising a charging station (210) and a robotic work tool (100), the charging station comprising a signal generator (240) to which a boundary cable (250) is to be connected, the signal generator (240) being configured to transmit a signal (245) through the boundary cable (250), and the robotic working tool (100) comprising a sensor (170) configured to pick up magnetic fields generated by the signal (245) in the boundary cable (250) thereby receiving the signal (245) being transmitted and a controller (110) configured to analyse the picked up signal, wherein the signal generator (240) is further configured to: prefilter the signal through an filter model; and transmit the prefiltered signal to through the boundary cable (250) by means of a current generator.

TECHNICAL FIELD

This application relates to automatic lawnmowers and in particular to a method for performing improved reception of a transmitted frequency spectra on the receiver side.

BACKGROUND

Automated or robotic power tools such as robotic lawnmowers are becoming increasingly more popular. In a typical deployment, a work area, such as a garden, is enclosed by a boundary cable with the purpose of keeping the robotic lawnmower inside the work area. The robotic lawnmower is typically also configured to communicate with a charging station located in the work area and connected to the boundary cable.

The boundary cable enclosing the work area may be modeled as a RL-circuit, which may act as a low pass filter. The characteristics of the work area, such as size and circumference will typically define the R- and L components of the RL-circuit.

When transmitting signals having several frequency components, such as in Code Division Multiple Access—CDMA—signaling, the signal is typically filtered through the RL-circuit of the work area leading to an attenuation of the higher frequency components.

The CDMA signals may thus not be correctly received on the receiving side. Since the R- and L-components differ from work area to work area, it is furthermore nearly impossible to compensate for all different values since it would require substantial signal processing.

Thus, there is a need for improved reception of frequency spectra on the receiver side.

SUMMARY

It is also an object of the teachings of this application to overcome the problems listed above by providing a robotic lawnmower system comprising a charging station and a robotic work tool, the charging station comprising a signal generator to which a boundary cable is to be connected, the signal generator being configured to transmit a signal through the boundary cable, and the robotic working tool comprising a sensor configured to pick up magnetic fields generated by the signal in the boundary cable thereby receiving the signal being transmitted and a controller configured to analyse the picked up signal, wherein the signal generator is further configured to: prefilter the signal through an filter model; and transmit the prefiltered signal to through the boundary cable by means of a current generator.

It is also an object of the teachings of this application to overcome the problems listed above by providing a method for controlling a robotic lawnmower system comprising a charging station and a robotic work tool, the charging station comprising a signal generator to which a boundary cable is to be connected, the signal generator being configured to transmit a signal through the boundary cable, and the robotic working tool comprising a sensor configured to pick up magnetic fields generated by the signal in the boundary cable thereby receiving the signal being transmitted and wherein said method comprises analysing the picked up signal, prefiltering the signal through an filter model; and transmitting the prefiltered signal through the boundary cable by means of a current generator.

Other features and advantages of the disclosed embodiments will appear from the following detailed disclosure, from the attached dependent claims as well as from the drawings.

DETAILED DESCRIPTION

It should be noted that all indications of rotational speeds, time durations, work loads, battery levels, operational levels etc. are given as examples and may be varied in many different ways as would be apparent to a skilled person. The variations may be for individual entities as well as for groups of entities and may be absolute or relative.

FIG.1Ashows a perspective view of a robotic working tool100, here exemplified by a robotic lawnmower100, having a body140and a plurality of wheels130(only one shown). As can be seen, the robotic lawnmower100may comprise charging skids132for contacting contact plates (not shown inFIG.1, but referenced230inFIG.2) when docking into a charging station (not shown inFIG.1, but referenced210inFIG.2) for receiving a charging current through, and possibly also for transferring information by means of electrical communication between the charging station and the robotic lawnmower100.

FIG.1Bshows a schematic overview of the robotic working tool100, also exemplified here by a robotic lawnmower100, having a body140and a plurality of wheels130.

It should be noted that even though the description given herein will be focused on robotic lawnmowers, the teachings herein may also be applied to robotic cleaners such as robotic vacuum cleaners and/or robotic floor cleaners, robotic ball collectors, robotic mine sweepers, robotic farming equipment, or other robotic work tools to be employed in a work area defined by a boundary cable.

In the exemplary embodiment ofFIG.1Bthe robotic lawnmower100has 4 wheels130, two front wheels130′ and the rear wheels130″. At least some of the wheels130are drivably connected to at least one electric motor150. It should be noted that even if the description herein is focused on electric motors, combustion engines may alternatively be used possibly in combination with an electric motor.

In the example ofFIG.1B, each of the rear wheels130″ is connected to a respective electric motor150. This allows for driving the rear wheels130″ independently of one another which, for example, enables steep turning.

The robotic lawnmower100also comprises a controller110. The controller110may be implemented using instructions that enable hardware functionality, for example, by using executable computer program instructions in a general-purpose or special-purpose processor that may be stored on a computer readable storage medium (disk, memory etc)120to be executed by such a processor. The controller110is configured to read instructions from the memory120and execute these instructions to control the operation of the robotic lawnmower100including, but not being limited to, the propulsion of the robotic lawnmower. The controller110may be implemented using any suitable, publically available processor or Programmable Logic Circuit (PLC). The memory120may be implemented using any commonly known technology for computer-readable memories such as ROM, RAM, SRAM, DRAM, FLASH, DDR, SDRAM or some other memory technology.

The robotic lawnmower100may further have at least one sensor170; in the example ofFIG.1there are four sensors divided into a first sensor pair170′ arranged at a front of the robotic lawnmower100and a second sensor pair170″, respectively arranged at the rear of the robotic lawnmower100to detect a magnetic field (not shown) and for detecting a boundary cable and/or for receiving (and possibly also sending) information from a signal generator (will be discussed with reference toFIG.2). The sensors170may thus be arranged as front sensors170′ and rear sensors170″.

In some embodiments, the sensors170may be connected to the controller110, and the controller110may be configured to process and evaluate any signals received from the sensor pairs170,170′. The sensor signals may be caused by the magnetic field being generated by a control signal being transmitted through a boundary cable. This enables the controller110to determine whether the robotic lawnmower100is close to or crossing a boundary cable, or inside or outside an area enclosed by the boundary cable. This also enables the robotic lawnmower100to receive (and possibly send) information from the control signal.

The robotic lawnmower100also comprises a grass cutting device160, such as a rotating blade160driven by a cutter motor165. The grass cutting device being an example of a work tool160for a robotic work tool100. The cutter motor165is connected to the controller110which enables the controller110to control the operation of the cutter motor165. The controller may also be configured to determine the load exerted on the rotating blade, by for example measure the power delivered to the cutter motor165or by measuring the axle torque exerted by the rotating blade. The robotic lawnmower100also has (at least) one battery180for providing power to the motors150and the cutter motor165.

The robotic lawnmower100may further comprise at least one supplemental navigation sensor190, such as a deduced reckoning navigation sensor for providing signals for deduced reckoning navigation, also referred to as dead reckoning. Examples of such deduced reckoning navigation sensor(s)190are odometers and compasses. The supplemental navigation sensor may also or alternatively be implemented as a vision navigation system, or Ultra Wide Band radio navigation system to mention a few examples. The supplemental sensor195will hereafter be exemplified through the deduced reckoning sensor.

The robotic lawnmower100may further be arranged with a wireless communication interface197for communicating with other devices, such as a server, a personal computer or smartphone, or the charging station. Examples of such wireless communication devices are Bluetooth™, Global System Mobile (GSM) and LTE (Long Term Evolution), to name a few.

In addition, the robotic lawnmower100may be arranged with collision sensor means for detecting when the robotic lawnmower100runs into an obstacle. The collision sensor means may be one or more separate sensors (such as accelerometers, pressure sensors or proximity sensors) arranged in or on the housing of the robotic lawnmower100and capable of detecting an impact caused by a collision between the robotic lawnmower100and an obstacle. Alternatively, the collision sensor means may be implemented as a program routine run by the controller110, being effective to detect a sudden decrease of the rotational speed of any of the drive wheels130″ and/or sudden increase in the drive current to the electric motor150.

FIG.2shows a schematic view of a robotic working tool system200in one embodiment. The schematic view is not to scale. The robotic working tool system200comprises a charging station210and a boundary cable250arranged to enclose a work area205, in which the robotic lawnmower100is supposed to serve. Adjacent to the work area205is another work area205′ enclosed by a boundary250′. Although not shown, in some embodiments, the other work area205′ may also comprise a charging station and robotic working tool deployed within.

As withFIG.1, the robotic working tool is exemplified by a robotic lawnmower, but the teachings herein may also be applied to other robotic working tools adapted to operate within a work area defined by a boundary cable. The work area is defined by a signal being transmitted through the boundary cable, which gives rise to a magnetic field that the sensor s170of the robotic lawnmower detects and based on this, the robotic lawnmower may determine whether it is inside or outside the work area and also when it crosses the boundary cable.

The charging station may have a base plate for enabling the robotic lawnmower to enter the charging station in a clean environment and for providing stability to the charging station210.

The charging station210has a charger220, in this embodiment coupled to two charging plates230. The charging plates230are arranged to co-operate with corresponding charging plates (not shown) of the robotic lawnmower100for charging a battery180of the robotic lawnmower100.

The charging station210also has, or may be coupled to, a signal generator240for providing a control signal245to be transmitted through the boundary cable250. The signal generator thus comprises a controller for generating the control signal. The control signal245comprises an alternating current, such as a continuously or regularly repeated current signal. The control signal is in one embodiment a CDMA signal (CDMA—Code Division Multiple Access). As is known in the art, the current signal will generate a magnetic field around the boundary cable250which the sensors170of the robotic lawnmower100will detect. As the robotic lawnmower100(or more accurately, the sensor170) crosses the boundary cable250the direction of the magnetic field will change. The robotic lawnmower100will thus be able to determine that the boundary cable has been crossed, and take appropriate action by controlling the driving of the rear wheels130″ to cause the robotic lawnmower100to turn a certain angular amount and return into the work area205. For its operation within the work area205, in the embodiment ofFIG.2, the robotic lawnmower100may use the satellite navigation device190, supported by the deduced reckoning navigation sensor195to navigate the work area205.

The use of more than one sensor170enables the controller110of the robotic lawnmower100to determine how the robotic lawnmower100is aligned with relation to the boundary cable250by comparing the sensor signals received from each sensor170. This enables the robotic lawnmower to follow the boundary cable250, for example when returning to the charging station210for charging. Optionally, the charging station210may have a guide cable260for enabling the robotic lawnmower to find the entrance of the charging station210. In some embodiments the guide cable260is formed by a loop of the boundary cable250. In some embodiments the guide wire260is used to generate a magnetic field for enabling the robotic lawnmower100to find the charging station without following a guide cable260.

Additionally, the robotic lawnmower100may use the satellite navigation device190to remain within and map the work area205by comparing the successive determined positions of the robotic lawnmower100against a set of geographical coordinates defining the boundary250, obstacles, keep-out areas etc of the work area205. This set of boundary defining positions may be stored in the memory120, and/or included in a digital (virtual) map of the work area205. The boundary250of the work area205may also be marked by a boundary cable supplementing the GNSS navigation to ensure that the robotic work tool stays within the work area, even when no satellite signals are received.

The charging station210may also be arranged (through the signal generator220) to emit a so-called F-field, referenced F inFIG.2. The F-field is a magnetic field generated around the charging station which enables a robotic lawnmower to navigate towards the charging station210without having to follow a guide or boundary cable, simply by navigating towards an increased field strength of the F-field.

The charging station210may also be arranged (through the signal generator220) to emit a so-called N-field, referenced N inFIG.2. The N-field is a magnetic field generated in the base plate215of the charging station which enables a robotic lawnmower to navigate correctly in the charging station for making contact with the charging plates230.

The control signal245may also be used to provide information I to the robotic lawnmower100from the charging station210through the boundary cable250and/or the guide wire260. The information may be transmitted as a coded message and may relate to an identity for the robotic lawnmower system200, such as an identity of the robotic lawnmower and/or an identity for the charging station210(or rather the signal generator), a command to be executed by the robotic lawnmower100and/or data that the robotic lawnmower may base an operating decision upon, such as weather data.

In one embodiment, the information I carried in the control signal245is coded using a CDMA (Code Division Multiple Access) coding scheme. In fact, the whole signal A is coded using CDMA, so the information I, and the portions relating to F, G and N signals are also coded using CDMA: As CDMA allows for more than one transmitter to transmit information simultaneously using the same frequency, and as the magnetic waves generated by a current signal in a cable all propagate through the same medium this allows for having more than one work area205,205′ being serviced by each a robotic lawnmower100and its corresponding signal generator240adjacent one another, while still being able to transmit a coded message that is directed at a specific robotic lawnmower100.

In some embodiments the robotic lawnmower100may also be arranged to send a signal to the charging station210to provide status updates, request data and/or for establishing a two-way communication. The robotic lawnmower may be arranged with a signal generator (not shown) and an antenna for transmitting electromagnetic signals (not shown). The electromagnetic signals may be transmitted directly to the charging station210or be picked up by the boundary cable250, the boundary cable then double acting as an antenna. In the following, focus will be on communication from the charging station210(via the signal generator240) to the robotic lawnmower100.

The robotic lawnmower listens or detects the signal by receiving the voltages generated by the sensor as it detects the magnetic fields generated by the signal. The received signal is then auto correlated with a reference signal. The autocorrelation may be shifted in time, i.e. synchronized, to provide as good a correlation as possible. This will enable the robotic lawnmower to compensate for any differences or drifts in the clock of the signal generator and the clock of the robotic lawnmower. In one embodiment the CDMA signal245is coded using Gold codes. In the field of robotic lawnmowers a normal frame length for Gold codes is in the order of 2047 bits, however to speed up the system, the robotic lawnmower system according to the teachings herein may be adapted to use a different frame length. The protocol used in one embodiment consists of two Gold frame lengths of each 2047 bits totalling 2X2047 bits, which frame is divided into smaller frames, for example 7 frames of each 584 bits, or 10 frames of each 409 bits. It should be noted that other number of frames may also be used.

Every other frame (1, 3, 5, 7 . . . , n) is used to send the control signal. Every other frame (2, 4, 6, 8, . . . , n+1) is used to send the F-field, the N-field and the guide signal(s), each being transmitted in a further sub frame. In one embodiment, time division is used for transmitting the F-field, the N-filed and any information I that may be needed to be transmitted to (from) the robotic lawnmower100. One example of such a protocol is shown inFIG.3where the upper figure shows the general format, and the lower figure shows an example. InFIG.3guide signal is abbreviated GS and there are three guide cables, each transmitting a guide signal; GS1, GS2, and GS3, and control signal is abbreviated CS. It should be noted that the time scales for the upper figure and the lower figure are not the same, the upper showing two frames and the lower showing8frames.

It should be noted that the control signal CS is transmitted through the boundary cable250, the guide signals are transmitted through their respective guide cables, the F field is transmitted through its respective cable and the N field is transmitted through its respective cable. The information bits I may be transmitted through any, some and/or all cables depending on the information and the design chosen.

It should also be noted that the number of sub frames my of course differ from system to system, depending on the systems capabilities. For example, in a robotic working tool system not having an F field, the F-field signal will not be transmitted, and in a robotic working tool system not having three guide cables, the number of sub frames used for the guide signals would also differ.

Using standardised coding such as Gold codes, has the obvious advantage that new coding schemes need not be invented. However, the inventors have realized that the frame length commonly used for CDMA coding, such as Gold coding, when used with technology commonly used for lawnmower systems leads to a transmission time for the entire frame that is in the order of seconds, such as 1 second, 0.5 seconds or up to 0.5 seconds. Such time spans may be unpractical in real life implementations as a robotic lawnmower operating using such time frames would move a distance that could not be neglected before being able to decode the entire frame. The robotic lawnmower may thus be rendered unable to detect whether it is still within the work area or not.

This would for practical reasons render gold coding inoperable for robotic lawnmower systems. To overcome this, the inventors realized that by dividing a frame as per above, shorter segments of the entire frame, i.e. sub frames, may be used to control the robotic lawnmower. By configuring the signal generator to transmit sub frames and by configuring the robotic lawnmower to operate according to sub frames, the gold coding of CDMA systems, may be used along with contemporary hardware technologies, commonly used in robotic lawnmower systems, such as transmitting a signal through a boundary cable, which signals is picked up by coil-based sensors in the robotic lawnmower.

By dividing the frame into several subframes, for example 10 or as in the detailed example given above, the robotic lawnmower is enabled to tune in and listen for shorter time spans since the control information is retransmitted more often, or at higher frequencies, and the robotic lawnmower does not need to receive and decode an entire frame before being able to make a control decisions, such as determining the crossing of a cable or if the robotic lawnmower is inside or outside a working area.

However, as a longer frame provides for a more robust system, that is less sensitive to interference, the inventors have realized that they may make use of the duality offered by utilizing subframes, namely that the robotic lawnmower is configured to decode and operate according to subframes under a first set of conditions, and to decode and operate according to complete frames under a second set of conditions, and even to decode and operate according to subframes and whole frames under a third set of conditions.

The first set of conditions includes that the received quality signal level is high, whereby interference is assumingly low and shorter time frames may be sufficient and provide enough robustness.

The first set of conditions may alternatively or additionally include that the received signal power is high, whereby the robotic lawnmower is assumingly close to the boundary cable and also possibly that the interference is not strong enough to affect the reception, and whereby the shorter time frames may be needed to provide a fast enough control of the robotic lawnmower.

The second set of conditions includes that the received quality signal level is low, whereby interference is assumingly high and full frames are needed to provide sufficient robustness.

The second set of conditions may alternatively or additionally include that the received signal power is low, whereby the robotic lawnmower is assumingly far away from the boundary cable and also possibly that the interference is strong enough to affect the reception, and whereby the full frames may be needed to provide a robust enough control of the robotic lawnmower.

The third set of conditions may include that the signal quality level is low, but the signal strength level is high, indicating that the robotic lawnmower is close to the boundary cable but in a noisy environment whereby the robotic lawnmower may be configured to listen to the subframes to make fast control decisions, and listen to the full frames to confirm the control decisions made based on the subframes.

The inventors have also realized that by changing the order of the frames to be transmitted a more robust reception is provided. The table below shows a schematic view of a frame and how the frame is rearranged in order to provide the more robust reception. As can be seen, the frames are not transmitted in order. A Frame F comprising 5 subframes F1. . . F5will then be transmitted in the order F1F4F2F5F3, whereby the subframes and their corresponding bits (assuming 2000 bits) are given by:

In one embodiment, this is utilized for the boundary signal A. The other signals, (guide and so on) are generally too short to utilize from the benefits associated herewith, but may of course also be transmitted in a similar manner.

FIG.4shows a schematic flowchart for a general method according to the teachings herein. A signal generator transmits410a boundary signal utilizing CDMA coding having a frame length through a boundary cable. The signal generator transmits said boundary signal in subframes. A robotic lawnmower is configured to receive the boundary signal by detecting magnetic fields generated by the boundary signal. The robotic lawnmower determines a set of conditions420; and then determines430if said set of conditions correspond to a first set of conditions, and if so listen to the subframes435, and determining440if said set of conditions correspond to a second set of conditions, and if so listen to the full frames445.

It has been realized by the inventors that to simplify the installation process for a robotic lawnmower system, the user or installer may be given an option to install guide wires or not. Using guide wires has the benefit that the robotic lawnmower may be able to find its way back to the charging station more effectively than randomly searching for the charging station or F-field. Due to constrictions on allowed field strengths, the total field strength of the guide signal, including the F-field, must be kept below certain levels. This is a legal requirement to prevent a system from causing too much interference to its surroundings.

However, the inventors have realized the simple solution that by allotting the time slots for the guide cable(s) G to the F-field F, the field strength of the F-field may be increased without increasing the field strength for the whole guide signal, thereby allowing for a stronger F-field while staying within the legal requirements.

A controller of the charging station, the controller possibly being that of the signal generator240, may thus be configured to determine whether a guide cable is connected or not and if it is detected that the guide cable is not connected, allot or assign the corresponding or associated time slot to the F-field. Alternatively, the associated time slot may be assigned to another guide cable, thereby allowing the robotic lawnmower to find that guide cable more quickly.

Alternatively, a timeslot associated with a guide cable that has not been connected, may be (time) shared by the connected guide cables and the F-field.

In one embodiment the controller may be configured to detect that a first guide cable and a second guide cable are not connected and allot the time slot associated with the first guide cable to the F-field, and allot the time slot associated with the second guide cable to a third guide cable.

The controller of the charging station is thus also configured to communicate through the information field to the robotic lawnmower so that the controller of the robotic lawnmower may adapt its sensing of the (boundary) signal(s).

In one embodiment a user may provide user input to indicate which guide cables are connected or not and the controller may then determine the time slot to use for F-field transmission accordingly.

FIGS.5A,5B and5Care schematic views of a robotic lawnmower system where an F-field F and three guide cables G1, G2and G3are normally used, as inFIG.5A. A schematic view of the boundary signal245is also shown for each robotic lawnmower system, where inFIG.5A, the boundary signal has timeslots for the actual boundary signal A and time slots associated with the F-field F, a first time slot associated with a first guide cable G1, a second timeslot associated with a second guide cable G2and a third time slot associated with a third guide cable G3.

In the example ofFIG.5A, all three guide cables are used and the associated time slots are allotted accordingly.

In the example ofFIG.5B, only one guide cable G3is used. As the controller of the charging station detects this, the controller allots the time slots associated with the unconnected guide cables G1, G2are allotted to the F-field, and in this instance also to the third guide cable. As a skilled reader would realize, other allotments would also be possible within the teachings of this document. Both the F-field and the third guide cable are thus provided with a higher signal level and may thus be found more easily by the robotic lawnmower and thereby simplify the installation of the robotic lawnmower system, while enabling the robotic lawnmower to find the charging station easily without randomly searching for it through most of the working area, all while staying within the legal requirements.

In the example ofFIG.5C, no guide cable is used. As the controller of the charging station detects this, the controller allots the time slots associated with the unconnected guide cables G1, G2, G3are allotted to the F-field, which then receives four time slots, thereby significantly increasing its field strength enabling the robotic lawnmower to more easily find the F-field thereby simplifying the installation of the robotic lawnmower system, while enabling the robotic lawnmower to still find the charging station easily without randomly searching for it through most of the working area, all while staying within the legal requirements. In one embodiment the time slots for F-signals replacing guide wire signals are time synchronized to match a corresponding sequential time slot number.

FIG.6shows a flowchart for a general method according to herein where a controller of a charging station detects610whether a guide cable is connected or not, and if the guide cable is not connected, the controller allots an associated time slot to another use620.

In one embodiment, the other use is to allot the associated time slot to the F-field F630.

In one embodiment, the other use is to allot the associated time slot to another guide cable640.

In one embodiment, the other use is to allot the associated time slot to be time shared between the F-field F and at least one other guide cable650.

As a robotic lawnmower100picks up a signal, through the sensors170, the picked up signal is analyzed by the controller110. It could be noted that the picked up signal differs somewhat to the transmitted signal as if the sensors are based on coils detecting magnetic field changes, the sensors will only be able to detect changes in the signal(s), that is, only the derivate of the signal(s) is picked up. This would be understood by a skilled person and in the remainder of this description no explicit difference will be made between the transmitted signal and the picked up signal unless specifically specified.

As the controller receives a (picked up) signal, the signal is analyzed, which analysis comprises correlating the (picked up) signal to a stored (or calculated) library or referenced signal to determine if the (picked up) signal originates from the signal generator220of the robotic lawnmower system200—or from another source. The time slots in the analyzed signal serve to identify if the (picked up) signal is transmitted through the boundary cable or a guide cable.

To establish a synchronization between the signal generator220and the controller110of the robotic lawnmower100, the controller is configured to convolute the (picked up) signal and correlated to the library signal for the same time, that is S(0)=L(0), where S is the picked up signal and L is the library signal. The (picked up) signal may also be correlated to previous or subsequent times t=+/−1, +/−2, etc, i.e. the signal is shifted in time.

As has been discussed above, the robotic lawnmower100may comprise at least one front sensor170′ and at least one rear sensor170″. During operation, the robotic lawnmower100will distance itself from the boundary cable250i.e. to operate within the inner portions of the work area254. At such distances, the received signal may be received at a lower amplitude or signal strength. The reception will then be more susceptible to noise and interference. One of the major interference sources are actually the electric motors and the control signals for these. Traditionally, the robotic lawnmower is then configured to synchronize to the received signal using the front sensor(s)170′ as they are spaced further apart from the electric motors.

However, during operation, the robotic lawnmower may also operate in close vicinity of the border cable250. At such close distances, the predominant interference is from neighbouring systems. Traditionally, the robotic lawnmower is then configured to synchronize on the rear sensor(s)170″ as they would be further away from the neighbouring systems.

The traditional method thus detected an amplitude of the received signal, and adapted which sensor(s) to synchronize on accordingly. However, the inventors have realized that this manner of selecting which sensor to synchronize on does not provide the optimum synchronization when the work area comprises overlapping or stacked (adjacent) work areas. The inventors have also realized that this synchronization scheme did not perform optimally when following a boundary cable.

The inventors have therefore devised an improved manner of selecting which sensor(s) to synchronize on. In this improved manner, the controller110of the robotic lawnmower100is configured to compare the signal (possibly obtained through autocorrelation) of one sensor170to a received reference sensor level. The received reference sensor level may be the total signal level, or the received reference sensor level may also or alternatively be the average signal level of all sensors170.

A signal qualifier may thus be used to determine which sensor to use.

The signal qualifier may be related to a quality of the received signal and/or a correlation value of the received signal when correlated to a reference signal.

The sensor which has the highest signal qualifier, will be the sensor used for synchronization. This ensures that the best synchronisation is achieved irrespective of the current situation or location.

It also enables the robotic lawnmower to synchronize to which ever cable or field that happens to provide the best signal level, be it the guide cable, the boundary cable, the F-field or the N-field.

This provides for a more robust synchronization that can adapt to changing environments. It also provides a synchronisation that is more beneficial for use with overlapping or adjacent work areas.

The controller is thus configured to receive a signal through a first sensor170′ and to determine a first signal level for the first sensor170′.

In one embodiment the signal qualifier is determined based on the average of received signal qualifiers from signals received through all sensors170.

In one embodiment the signal qualifier is determined based on the total of received signal qualifiers from signals received through all sensors170.

The controller is also configured to receive a signal through a second sensor170″ and to determine a second signal qualifier for the second sensor170″.

The sensor170providing the highest signal qualifier is then selected for synchronisation.

In one embodiment the signal level is determined through auto correlating with a library signal. In one such embodiment, the quality of the received signal is determined using the formula below:

signal  quality  level=∑1N⁢ci*xiN∑1N⁢xi2N

where

ci=value at time i of the library signal

xi=value at time i of the received or picked up signal

The flowchart ofFIG.7shows a method according to the teachings herein, where an robotic lawnmower receives710a signal through a first sensor170′ and determines720a first signal qualifier for the first sensor170′.

In one embodiment the signal quality level is determined based on the average of received signal quality levels from signals received through all sensors170.

In one embodiment the signal quality level is determined based on the total of received signal quality levels from signals received through all sensors170.

The robotic lawnmower also receives730a signal through a second sensor170″ and determines740a second signal quality level (or quality) for the second sensor170″.

The robotic lawnmower then compares the first and second signal quality levels750and the sensor170providing the highest signal quality level is then selected for synchronisation760.

As has been discussed in the background section, the work area and the boundary cable of a robotic lawnmower system may be modelled as an RL-circuit, where the R and L components (resistance R and inductance L) depend on the physical characteristics of the work area, including the characteristics of the ground and the size of the area. As such, and as the inventors have realized, this makes it difficult when transmitting CDMA signals, as the decoding on the receiver side need to be highly advanced for correct decoding of the received signals, as the received signal will differ depending on which work area it is used in. As the inventors have realized, an RL-circuit will act as a low-pass band filter when transmitting signals with varying or several frequency components. This will, as is also mentioned in the background section, attenuate the higher frequency components, which may lead to that signals having many frequency components, such as CDMA signals, may be incorrectly received or decoded.

Assuming that all frequencies are transmitted at the same amplitude, the RL-circuit (composed by the work area) will filter the frequencies differently, and the received signal will be difficult to decode correctly. The amount of filtration will depend on the RL-components modelling the actual work area.

Instead of simply and straightforwardly increasing the voltage level for the higher frequency components, which would also enable the CDMA signals to be correctly received, the inventors have proposed a more clever and ingenious manner of providing the boundary signals. The inventors firstly propose to use a current generator instead of a voltage generator traditionally used. Secondly, the inventors propose to prefilter the transmitted signal corresponding to a model of the work area. The prefiltering basically means that the higher frequencies should be transmitted at lower amplitudes, as inFIG.8showing a frequency-amplitude diagram for a transmitted signal It.

This enables the CDMA signal to be correctly received by the receiver without increasing the overall field strength too much.

As the inventors have realized, this is made possible by the fact that the receiver side reacts to the derivate of the signal (i.e. to the changes in the signal). As the higher frequency components change more rapidly, the sensors will also pick them up more easily. The received signal will thus look as in the schematic view ofFIG.9.

Thus by modelling the work area on an RL-circuit and adapting a current generator to transmit a signal accordingly, an improved reception of the CDMA signal is provided. And, this is achieved without increasing the power at which the high frequency components/signals are transmitted.

By transmitting the signal in the manner shown inFIG.8, the reception becomes substantially indifferent to the RL-modelling of the work area.

In one embodiment, the controller of the robotic lawnmower may be configured to determine a signal level for a low frequency component and to determine a signal level for a high frequency component and based on this, the RL-model used by the current generator may be adapted accordingly, whereby the output for the current generator is adapted to fit the RL model as based on the measurements made by the robotic lawnmower. Even though only two signal levels are mentioned herein, the skilled person would realize that to provide an accurate model, several signal levels could be needed.

The measurements made by the robotic lawnmower may be communicated wirelessly through an RF interface (if such is comprised in the robotic lawnmower), or through the charging plates upon docking in the charging station.

The signal to be transmitted through the boundary cable may thus be pre-filtered according to the actual RL-model of the work area, before being transmitted by the current generator.

Although the description herein have been focussed on an RL-model for the prefiltering representing the work area, it should be understood that the RL-model may also comprise other components, such as capacitance.

In one example the resistance, that is the R component, of the filter is 20 Ohm, and the inductance, that is the L component, of the filter is 20 mH.

FIG.10shows a schematic flowchart for a general method according to the teachings herein, where a signal generator is configured to prefilter a signal to be transmitted through a model filter1010, and then transmit the signal using a current generator1020.

As the inventors have also realized, in order to improve the reception of higher frequency components, the current generator may be arranged to operate by transmitting the signal at a constant current level. This will enable the high frequency components to be received at a higher signal level, and therefore be less susceptible to noise and other interference, which is preferable when using CDMA signalling. This is also achieved by not transmitting at higher power levels compared to contemporary systems, thereby reducing or at least not increasing the interference of other, neighbouring systems and devices.

The realization of using a current generator to transmit a prefiltered signal provides for an improved manner of transmitting CDMA signals that reduces or at least not increases the interference of other, neighbouring systems and devices.

As the signal is transmitted through the boundary cable, there might be a leak of current through stray capacitances that occur between the boundary cable and the surrounding environment. This can lead to that currents are flowing through the boundary cable affecting the quality of the signal being transmitted there through. The leaking current may also create a negative magnetic field that may fool the robotic lawnmower to believe it is operating outside the boundary cable250, when in fact it is still inside the work area205. This may result in that the operation of the robotic lawnmower is hindered or cancelled. The inventors have realized this problem and have, after insightful reasoning, come up with a simple solution that does not require any change or addition to the boundary cable, which would increase the cost of the robotic lawnmower system and also complicate the installation of the robotic lawnmower system. The inventors are proposing to adapt the synchronization done by the robotic lawnmower so that as it detects that it is sufficiently close to a guide cable G/260, sufficiently meaning being able to receive a signal with good signal quality (or at least better than the quality at which the control signal being transmitted through the boundary cable is received) and far from the boundary cable (i.e. at a distance where the signal amplitude level is below a threshold value or lower than that of the guide cable and/or where the signal quality level is below a threshold value or lower than that of the guide cable), the robotic lawnmower is configured to start synchronizing on the guide signal instead of the boundary signal. This enables the robotic lawnmower to safeguard against being fooled by the stray capacitances and leaking currents to believe it is outside the area when in fact it is not.

In one embodiment, the robotic lawnmower is configured to only listen to the low frequencies of the control signal transmitted through the boundary cable. As mostly the higher frequencies are affected by these leaking currents, only listening to the lower frequencies reduces the risk of being fooled. The robotic lawnmower will thus still be able to determine that the boundary cable is still present, even when synchronizing on the guide signal. To listen to both the guide signal and to the lower frequencies of the boundary signal also allows for determining the correct position of the robotic lawnmower, even in situations such as when the guide cable is actually outside the work area. It also allows the robotic lawnmower to determine that the robotic lawnmower is closer to the boundary cable, where the effects of the stray capacitances are smaller and then switch over or back to synchronize on the boundary signal again. Examples of such low frequency ranges are 0.5 kHz to 2 kHz in a signal having a frequency range of for example 0.5 kHz to 7 kHz, Other examples is that the low frequencies correspond to the lower 15% of the frequencies in the signal, the lower 20% of the frequencies in the signal, the lower 25% of the frequencies in the signal, the lower 30% of the frequencies in the signal, the lower 35% of the frequencies in the signal or the lower 50% of the frequencies in the signal.

FIG.11shows a flowchart for a general method according to the teachings herein where a robotic lawnmower is configured for receiving1110the control signal and determining1120a signal level for the control signal; and if the signal level for the control signal is below1130a threshold value, the robotic lawnmower receives said guide signal and synchronizes1140on said guide signal.