SIGNAL INTERFERENCE REJECTION METHOD AND APPARATUS

A method controls an apparatus comprising at least two sensors, and an active device interfering with signals detectable by the at least two sensors. The method comprises: detecting, by means of the at least two sensors, signals at different points in time, the signals including burst signals from a signal source, storing data indicative for the detected signals in a memory, comparing signal values determined based on the data indicative for the signals stored in the memory of three different points in time with one another to differentiate the signal values into a low signal value, an intermediate signal value and a high signal value, subtracting the intermediate signal value from a current signal value detected by means of at least one of the at least two sensors to obtain at least one processed signal value and controlling operation of the apparatus based on the processed signal value.

TECHNICAL FIELD

The present invention generally relates to a method for controlling an apparatus using detected signals, to an apparatus, e.g. a robotic mower, and to a system comprising such an apparatus.

BACKGROUND ART

Various apparatuses, particularly robotic mowers, are operated using signals received by the apparatus from an external signal source. The apparatus may comprise an active device that interferes with the signals so that the received signals are altered. This may deteriorate the quality of the signal-dependent operation of the apparatus.

Robotic mowers, also called self-propelled lawnmowers, are generally known. These robotic mowers are provided with a rechargeable battery. An area of a lawn may be defined by a boundary wire. The robotic mower may use signals transmitted by means of the boundary wire to determine its position, e.g., to navigate and stay within the area. When the remaining power in the battery is below a certain level the robotic mower is programmed to return to the charging station to recharge the battery. There are different possibilities for returning the robotic mower to the charging station. One possible method is that the robotic mower, upon a command to return to the charging station, continues its movement until the boundary wire is detected nearby and then follows the boundary wire to the charging station that is provided somewhere along the boundary wire. Another option when returning to the charging station is to use a guide wire, which the robotic mower follows back to the charging station.

The active device of the apparatus, e.g., a cutting device comprising an electric motor and a blade of the robotic mower, interferes with the signals from the signal source. For example, the electric motor creates electromagnetic noise and/or rotation of the blade may effect a distortion of the signals from the signal source received by the apparatus.

WO 2020/148138 A1 describes to use a digital signal processor to provide an output corresponding to an average of a plurality of recorded transmitted signal bursts to deal with noisy conditions. This solution requires comparably complex processing hardware that finds signal bursts in the noisy signal and averages the bursts. Further, the averaging needs to record signals over a relatively long time span what leads to rather long response times. In addition, noise contributions may remain in the averaged signal.

US 2018/0199506 A1 proposes to reduce the voltage for a cutting blade motor of a grass mower when a magnetic field is detected by a guide wire sensor, and to restart the cutting blade motor after detection of the magnetic field has completed. This allows to avoid interferences generated by the cutting blade motor, but at the cost of a reduced cutting efficiency.

CN 103 941 600 B describes an automatic working system that includes a noise detection device which detects noise signals in a surrounding space and generates a detection signal accordingly, and a control device which is electrically connected to the noise detection device and that receives the detection signal, and filters out environmental signals according to the detection signal to attenuate a part of a processed signal corresponding to the noise signal. Such solutions generally require a complex hardware setup.

SUMMARY

An object of the present invention is to provide an improved signal interference rejection.

According to an aspect of the present invention this object is achieved by a method for controlling an apparatus comprising at least two sensors and an active device interfering with signals detectable by the at least two sensors. The method comprises detecting, by means of the at least two sensors, signals at different points in time t1-t3, the signals including burst signals from an external signal source, storing data indicative for the detected signals in a memory, comparing signal values D1-D3determined based on the data indicative for the signals stored in the memory of three different points in time t1-t3with one another to differentiate, e.g. sort, the signal values D1-D3into a low signal value, an intermediate signal value and a high signal value, subtracting the intermediate signal value from a current signal value detected by means of at least one of the at least two sensors to obtain at least one processed signal value, and controlling operation of the apparatus, e.g., navigating the apparatus, based on the processed signal value. These steps may be repeated iteratively, e.g., for each sample of a time-discrete, sampled sensor reading.

This is based on the idea to provide an improved signal interference rejection for an apparatus, in particular for a robotic mower, by using a stored reference signal value as a baseline which has no extreme value in comparison with two other stored values, so that the reference signal value used as the baseline likely does not contain a burst signal. This allows to reject noise signal interference effectively and with very short response times. Further, it is not necessary to interrupt an operation of the apparatus for the signal detection. At least one of the low signal value, the intermediate signal value and the high signal value may be determined using a first one of the at least two sensors, and at another one of the low signal value, the intermediate signal value and the high signal value may be determined using a second one of the at least two sensors.

According to an embodiment the active device comprises a rotatable component being rotatable about a rotational axis. Optionally, the method further comprises determining a rotational speed of the rotatable component. The rotatable component may be a source of periodic noise for the signals detected by the at least two sensors. The periodic noise may interfere with the burst signals. By determining the rotational speed the selection of the three different points in time may be improved.

According to an embodiment the rotatable component is a blade and/or the active device comprises a motor, in particular an electric motor. For example, the motor is configured to rotate the blade. The apparatus may be a robotic mower. The motor and/or the blade may be sources of noise. By applying the method described herein it is possible to use a motor and/or a blade that create relatively strong noise, but subtract the noise components from the detected signals. This allows for a wider choice in the selection of the components used for manufacturing the apparatus and, in turn, reduced manufacturing effort and costs.

According to an embodiment the method further comprises determining a first time offset and a second time offset. Optionally, the second time offset is smaller than the first time offset. This allows a simple determination of the different points in time.

The active device may be designed such that it creates a periodic electromagnetic signal, particularly as an unwanted side effect. The periodic signal may interfere with the bust signals. The method and apparatus described herein allow to subtract this interference.

The at least two sensors comprise a first sensor and a second sensor. According to an embodiment the first time offset is the time of one, two or more complete revolution(s) of the rotatable component about the rotational axis, and/or the second time offset is the time to rotate a portion of the rotatable component from being closest to the first sensor to being closest to the second sensor. This allows for a particularly effective selection of a reference signal for noise subtraction. For example, the first time offset T1may be equal to the time for one revolution, e.g., of a motor, e.g. cutting motor, and/or disc, e.g. cutting disc. The second time offset T2can be equal to the time for a part of one revolution, e.g., depending on the number of blades on the cutting disc and/or the number of poles in the motor. For the example of a three-bladed cutting disc, the second time offset T2may be equal to the first time offset divided by three, T1/3.

According to an embodiment the three different points in time t1−t3comprise a, e.g. first, point in time t1at the first time offset T1before a current time T, t1=T−T1, a, e.g. second, point in time t2at the second time offset T2before the current time T, t2=T−T2, and/or a, e.g. third, point in time t3at the first time offset T1plus the second time offset T2before the current time T, t3=T−(T1+T2). For example, the blade rotates in the direction from the first sensor towards the second sensor. Alternatively it rotates in the opposite direction.

According to an embodiment the compared signal values D1-D3comprise a first signal value D1determined at the first point in time t1, a second signal value D2determined at the second point in time t2, and a third signal value D3determined at the third point in time t3. This allows to further improve the noise subtraction.

Optionally the first signal value D1is determined using the first sensor, the second signal value D2is determined using the second sensor, and/or the third signal value D3is determined using the second sensor.

According to an embodiment the signal values D1-D3are signal strengths of the corresponding signals at the respective points in time t1-t3.

According to an embodiment, an apparatus is provided. The apparatus comprises at least two sensors and an active device interfering with signals detectable by the at least two sensors. The apparatus is adapted to detect, by means of the at least two sensors, signals at different points in time t1-t3, the signals including burst signals from a signal source, store data indicative for the detected signals in a memory, compare signal values D1-D3determined based on the data indicative for the signals stored in the memory of three different points in time t1-t3with one another to differentiate, e.g. sort, the signal values D1-D3into a low signal value, an intermediate signal value and a high signal value, subtract the intermediate signal value from a current signal value detected by means of at least one of the at least two sensors to obtain at least one processed signal value and control its operation based on the processed signal value.

Regarding the advantages of the apparatus reference is made to the above description of the method for controlling the apparatus.

According to an embodiment the at least two sensors comprise a first sensor and a second sensor. Optionally, one or each of the first and second sensors, is or are adapted to sense a magnetic field.

According to an embodiment the apparatus comprises a main body, and the first sensor and the second sensor are arranged at the main body displaced with respect to one another. By this arrangement the interference of the active component at a certain point in time with the signals detected by the first and second sensors may be different.

According to an embodiment the active device comprises a component being rotatable about a rotational axis. Optionally, the first and second sensors are arranged so that the rotational axis and the first and second sensors define a quarter circle or less than a quarter circle of a circle around the rotational axis as the center of the circle. More generally, together, the first and second sensors may describe an angle α with respect to the rotational axis. The ratio of the second time span divided by the first time span, T2/T1, may be equal to the ratio of this angle α and a full circle, α/360 degrees. This allows to filter out noise regular noise components particularly effectively.

For example, the active device comprises a motor and a blade rotatable by the motor.

Optionally, the blade comprises a magnetic material. Using the above-described method it is possible to easily filter out signal components induced by such a magnetic blade. Blades may be magnetic due to the alloy used and/or due to manufacturing conditions. Avoiding blades to be magnetic may significantly increase the manufacturing complexity and limit the choice of the material. Filtering out the interference by means of software may thus simplify the manufacturing and allow a wider choice in the materials for the blades.

According to an embodiment the apparatus is a robotic mower.

Optionally, the apparatus of any embodiment described herein is adapted to perform the method of any embodiment described herein. In turn, the method of any embodiment described herein may use the apparatus of any embodiment described herein.

According to an aspect, a system comprising the apparatus according to any embodiment described herein and a wire as the signal source is provided. Therein, the wire may be a guide wire arranged within an area delimited by a boundary wire, or the boundary wire.

The system may further comprise a signal generator adapted to send burst signals through the signal source.

By this, a method, apparatus and system are provided that allow an improved signal interference rejection.

DESCRIPTION OF EMBODIMENTS

In the following, a detailed description of exemplary embodiments for controlling an apparatus in the form of a robotic mower2using wires4,8as signal source according to the present disclosure will be presented.

FIG.1shows a schematic overview of a system controlling the robotic mower2by means of a guide wire8and/or by means of a boundary wire4. The robotic mower2, or as it also may be called a self-propelling lawnmower, is battery powered and needs to be recharged at regular intervals. The robotic mower2is during operation configured to move across an area A surrounded by the boundary wire4. As is obvious the robotic mower2is depicted somewhat enlarged for the sake of clarity. The boundary wire4may be configured in many different ways, such that it delimits the area A within which the robotic mower2is allowed to move. The boundary wire4is preferably provided under the ground in the lawn, such that is not visible, but may also be provided on or above the ground. The boundary wire4could be an ordinary copper wire of single-core type. There are of course also other options, which are well-known by a person skilled in the art, such as multi stranded wire types. As may be seen inFIG.1the boundary wire4makes a loop4ain the charging station11. This loop4ais used to guide the robotic mower2into charging contact with the charging station11.

The system also comprises the charging station11mentioned above. The charging station itself11may be seen as the place where the charging of the robotic mower2takes place, and could for an example be provided with a charging station plate24onto which the robotic mower2is guided when performing docketing. Further, there is provided a charging station loop10at the charging station11. The charging station loop10is entirely arranged at the charging station11, more specifically, mounted on the charging station plate24.

A system according to the present disclosure may also comprise one or more guide wires8. A guide wire8is a wire that the robotic mower2may follow when returning to the charging station11, when exiting the charging station11to start a mowing cycle and/or to move along a way that is otherwise difficult to find. The robotic mower2may also be adapted to follow the boundary wire4back to the charging station11and/or to exit the charging station11to start a mowing cycle.

The boundary wire4, the charging station loop10and the one or more guide wires8are all connected to a signal generator which feeds each wire and loop with a, particularly wire-specific, current signal, in particular an Alternating Current, AC, signal, such that the robotic mower2may recognize which wire or loop it is detecting when it is within sensing distance. In general, the robotic mower2may be adapted to detect magnetic fields of the different signal wires.

Turning now toFIG.2, an exemplary embodiment of the robotic mower2will be closer described. The robotic mower2comprises a control unit22, wheels20, at least two sensors12,14, in particular two sensors12,14, optionally three or four sensors, and a battery18. The sensors12,14each are adapted to sense magnetic fields. Optionally, the robotic mower2comprises exactly two sensors12,14. The control unit22, which will be closer described in conjunction withFIG.4, comprises a processor80for controlling the movement of the robotic mower2. When the robotic mower2is in operation, the sensors12,14can sense a magnetic field that is generated in the boundary wire4, the charging station loop10and/or the one or several guide wires8. The signals of the different wires4,8and wire loop10may be encoded differently. The sensed magnetic field, i.e. signal, is decoded in the control unit22to determine from which loop or wire it was received. The robotic mower2further comprises charging connectors16.

It is worth noting that the robotic mower2has a forward-rearward axis along which the robotic mower2moves when it drives straight ahead or straight backwards. In the present example, the robotic mower2has a longitudinal extension in accordance with the forward-rearward axis. The two sensors12,14are arranged at, e.g. fixed with respect to, a main body34of the robotic mower2displaced to one another in a direction orthogonal to the forward-rearward axis. In this example, the sensors12,14are arranged in a front region of the robotic mower2and could be referred to as front sensors12,14. Two rear sensors may optionally be provided at the rear of the robotic mower2and arranged displaced to one another in a direction orthogonal to the forward-rearward axis.

FIG.3shows an exemplary embodiment of the charging station11. The charging station11comprises the charging station plate24at which the charging station loop10, which can also be referred to as far-field loop, and the boundary wire loop4a, which may also be referred to as near-field loop, are arranged. The charging station11further comprises the signal generator6. As shown inFIG.3, the charging station11comprises charging connectors26which are arranged so as to be contacted by the charging connectors16of the robotic mower2when docking into the charging station11. The charging connectors26are mounted on a tower28of the charging station11.

With reference toFIG.4, the control unit22of the robotic mower2will be closer described. The control unit22comprises, as mentioned above, the processor80and a memory82. The memory82may comprise a computer program84comprising computer program code, i.e. instructions. The computer program code is adapted to implement method steps performed by the robotic mower2when the code is executed on the processor80. The control unit22further comprises an interface86for communication with the sensors12,14, and one or more motors that operate(s) the robotic mower2, in particular a motor for driving a blade which will be described below with reference toFIG.6. The control unit22is adapted to receive, store and process signals from the sensors12,14.

The processor80may comprise a single Central Processing Unit (CPU), or could comprise two or more processing units. For example, the processor80may include general purpose microprocessors, instruction set processors and/or related chips sets and/or special purpose microprocessors such as Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs) or Complex Programmable Logic Devices (CPLDs). The processor80may also comprise a storage for caching purposes.

FIG.5depicts the signal generator6, which also comprises a processor60and a memory62. The memory62may comprise a computer program64comprising computer program code, i.e. instructions. The computer program code is adapted to implement method steps performed by the signal generator6when the code is executed on the processor60. The signal generator6further comprises an interface66for transmitting the generated, e.g. AC, signals to the boundary wire4, charging station loop10and guide wire or wires8, particularly as burst signals. Correspondingly, at least one wire or wire loop4,8,10, or all of the described wires4,8and loop10, may be fed with burst signals by the signal generator6, wherein two consecutive bursts are spaced in time.

The processor60may comprise a single Central Processing Unit (CPU), or could comprise two or more processing units. For example, the processor60may include general purpose microprocessors, instruction set processors and/or related chips sets and/or special purpose microprocessors such as Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs) or Complex Programmable Logic Devices (CPLDs). The processor60may also comprise a storage for caching purposes.

As shown inFIG.6, the robotic mower2comprises at least one blade32to cut the lawn, e.g., two, three or four blades32. The, or each, blade32may be elongate, have the shape of a disc or other shapes. In the example ofFIG.6, the robotic mower2comprises a cutting disc on which a plurality of blades, namely four, alternatively three or two, blades32are mounted. The blade(s)32is/are rotatable around a rotational axis R with respect to a main body34of the robotic mower2. The blade32is rotatable with respect to the main body34of the robotic mower2by means of a motor30, which is an electric motor in this example and supplied with energy from the battery18. In the example ofFIG.6, the cutting disc is rotatable by means of the motor30so as to rotate the blades32mounted to the cutting disc. The first and second sensors12,14are arranged in the same distance to the rotational axis R. The first and second sensors12,14are arranged in the same plane. Optionally, an angle formed by the two sensors12,14and the rotational axis R is 90 degrees, or 75 degrees, or 100 degrees, or within the range of 45 degrees to 150 degrees, particularly within the range of 75 to 100 degrees.

The operation of the motor30creates time-varying electromagnetic fields that interfere with the signals emitted by a wire4,8or loop10being in sensing distance. Further, also the blade(s)32may interfere with these signals due to its permittivity. The blade(s)32may optionally be made of a magnetic or magnetized material and interfere with said signals for this reason when rotating.

FIG.7illustrates the amplitude of different signals S1, S2, S1′, S2′ versus time t and thus for a plurality of points in time. A first signal S1detected by the first sensor12is shown as the uppermost signal. The second signal S2from above is detected using the second sensor14. Approximately in the middle of the time span shown for the illustration there is a burst signal from a wire4,8or loop11, e.g., from the boundary wire4. The burst signal is detected both by the first and second sensors12,14at the same time.

Both sensors12,14, however, also capture other interfering signal components, particularly from the motor30and/or the blade(s)32. The motor30and blade(s)32can also be referred to as active device interfering with the signals S1, S2detectable by the sensors12,14. By rotating with a certain rotational speed, the motor30and/or blade(s)32are the source for a periodic signal component in the signals S1, S2detected by the sensors12,14. By this, the signal bursts are altered and the quality of their reception is degraded.

In order to reject the interfering signal components, the robotic mower2is adapted to detect, by means of the sensors12,14, the signals S1, S2including the burst signals from, e.g., the boundary wire8as signal source at different points in time, particularly continuously. The robotic mower2is further adapted to store data indicative for the detected signals S1, S2in the memory82, to compare signal amplitude values determined based on the data indicative for the signals S1, S2stored in the memory84of three different points in time t1, t2, t3with one another to differentiate the signal values by sorting them into a low signal value, an intermediate signal value and a high signal value, to subtract the intermediate signal value from a current signal value detected by means of the sensors12,14to obtain at least one processed signal S1′, S2′ value and to control its operation based on the processed signal S1′, S2′ value.

The amplitude values of the processed first signal S1′ and the processed second signal S2′ versus time is shown in the third and fourth row ofFIG.7. It can be directly seen that the interfering signal components have been effectively subtracted.

A particularly effective implementation of the interference rejection is described in the following.

The control unit22determines the rotational speed of the blade(s)32, e.g., as rotations per minute, RPM. By this, the interfering signal repetition rate is determined by the control unit22and known to the control unit22.

Next, the control unit22calculates a first time offset T1as the time for one complete revolution of the blade(s)32, and a second time offset T2as the time difference between the arrival of one blade32at the first sensor12and at the second sensor14. In one embodiment, there are four blades located on the cutting disc, the second time offset T2may be approximately quarter revolution of the blades32.

Using the signals S1, S2from both sensors stored in the memory84as corresponding data, three signal values D1, D2, D3are extracted by the control unit22from this data from the following points in time before a current time T: a first point in time t1is at the first time offset T1before the current time, t1=T−T1; a second point in time t2is at the second time offset T2before the current time T, t2=T−T2; and a third point in time t3is at the first time offset T1plus the second time offset T2before the current time T, t3=T−T1−T2. A first signal value D1is determined as the amplitude of the first signal S1at the first point in time t1, D1=S1(t1). A second signal value D2is determined as the amplitude of the second signal S2at the second point in time t2, D2=S2(t2). A third signal value D3is determined as the amplitude of the second signal S2at the third point in time t3, D3=S2(t3). The three signal values D1, D2, D3are sorted by the control unit22with respect to their value. The intermediate value M among these is selected as being most likely free of any burst signal, so that it is suited as a baseline. Using this intermediate value M the induced interference component can be removed by subtracting the intermediate value from the first signal S1value at the current time T to obtain a processed signal S1′ value, S1′(T)=S1(T)−M. A corresponding calculation may be performed using the second signal S2. The control unit22samples the sensor12,14readings with a given sample rate, and over the time the above calculation of the processed signal S1′ value is repeated for each sample at the respective current time T. For example, when the latest signal samples are stored in the memory84, the oldest samples are removed from the memory84. The control unit22may be adapted to store the signal data within a certain period of time. The result are the first and second processed signals S1′, S2′ shown inFIG.7. Using these (or one thereof), the robotic mower2is operated, particularly navigated. This allows a strongly improved precision of the operation. Further, it is possible to arrange the sensors12,14closer to the motor30and blade32. By this it is also possible to reduce the size of the robotic mower2.

FIG.7also shows the width of the first time offset T1and the second time offset T2. It can be seen that the interfering signal component is periodic and repeats after one revolution of the blade32corresponding to the first time offset T1. Further, due to their displaced arrangement, the signals recorded by the two sensors12,14are offset by the second time offset T2. In the example ofFIG.7the second signal S2is inverted with respect to the first signal S1due to an inverted orientation of the second sensor14with respect to the first sensor12. In this case the second signal S2values may be multiplied by −1 in the above calculation. Alternatively, both sensors12,14are arranged with the same orientation. The time offset between two consecutive burst signals from the boundary wire4(or other wire8or wire loop10) is greater than the first time offset T1. That is, the wires4,8and wire loop11are only active at a portion of the time, e.g., between 1% to 10% of the time, e.g., at about 4% of the time. The noise signals, however, are continuously active while the motor30is active.

Thus, it can be seen that at any current time T only the signal value D1, D2, D3of one of the three points in time t1, t2, t3may comprise a contribution of a burst signal. This one of the three signal values D1, D2, D3most likely has the highest or lowest amplitude while the intermediate one of the three signal values D1, D2, D3most likely has no contribution of a burst signal and therefore qualifies to remove the interference signal component.

To provide an illustrative example, assume the three signal values are as follows: D1=45, D2=33, D3=34. The intermediate signal value sample is determined to be the third signal value D3=34.

It is worth noting that in general the first time offset T1may be the time of an integer number of complete revolutions of the blade, e.g., 1, 2, 3 or more. T1can generally be determined based the RPM of the blade32and T2can generally be determined from the fixed relative arrangement of the sensors12,14with respect to the rotational axis R. For example, a signal of the first sensor12is cleaned up, e.g., an interference is removed, by finding a time-delayed copy of an interference on the signal of the second sensor14, and subtracting it from the signal of the first sensor12. Alternatively or in addition, a signal of the second sensor14may be cleaned up, e.g., an interference is removed, by finding a time-delayed copy of an interference on the signal of the first sensor12, and subtracting it from the signal of the second sensor. The interference may be the same every revolution of the blade32or cutting disc.

FIG.8shows a method for controlling an apparatus, e.g., the robotic mower2described above, comprising at least two sensors12,14, and an active device30,32interfering with signals detectable by the at least two sensors12,14. The method comprises the following steps.

Step S101: Detecting, by means of the at least two sensors12,14, signals S1, S2at different points in time, the signals S1, S2including burst signals from a signal source, e.g., the boundary wire4.

Step S102: Storing data indicative for the detected signals S1, S2in a memory82. For this, a FIFO may be used.

The active device30,32may comprise a rotatable component, e.g., the blade32, being rotatable about the rotational axis R, and the method optionally comprises step S103: Determining a rotational speed of the rotatable component. This may be done, e.g., using the detected signals S1, S2and/or based on set points defined in the control unit22.

Step S104: Comparing signal values determined based on the data indicative for the signals S1, S2stored in the memory84of three different points in time t1, t2, t3with one another to sort the signal values into a low signal value, an intermediate signal value and a high signal value. The three different points in time t1, t2, t3may be calculated based on the determined rotational speed and/or using the first time offset T1and the second time offset T2.

Step S105: Subtracting the intermediate signal value from a current signal value detected by means of at least one of the at least two sensors12,14to obtain at least one processed signal S1′, S2′ value.

Step S106: Controlling operation of the apparatus, e.g., the robotic mower2, based on the processed signal S1′, S2′ value.

Some or all of these steps may be iteratively repeated, e.g., at a given sample rate, in particular steps S104and S105.

For example, the computer program84of the control unit22may comprise instructions that, when executed by the processor80cause the robotic mower2to perform the above method.

Although the present invention has been described above with reference to specific embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the invention is limited only by the accompanying claims.

LIST OF REFERENCE NUMERALS