SE542098C2 - System method and a detecting device related to robotic working tools for detecting a break in a robotic work tool system boundary wire - Google Patents

Abstract

The present disclosure relates to a robotic work tool system where a base station 5 feeds a signal to a boundary wire 7, and a robotic tool 3 detects, by means of the signal, the boundary wire in order to stay within a work area 9 defined by the latter. The emitted signal has a high relative bandwidth. If there is a break in the wire, the location of this break can be detected by measuring the emitted signal’s absolute phase.

Description

SYSTEM METHOD AND A DETECTING DEVICE RELATED TO ROBOTIC WORKING TOOLS FOR DETECTING A BREAK IN A ROBOTIC WORK TOOL SYSTEM BOUNDARY WIRE Field of the invention The present disclosure relates to a system comprising a robotic work tool, a boundary wire loop, configured to define a work area, and a base station, feeding a signal to the boundary wire loop, the work tool being configured to detect the boundary wire and thereby boundaries of the work area by detecting said signal.

Technical background One example of such a system is illustrated in EP-1512053-A1. One problem associated with systems of this kind is how to make them more reliable and functional over time.

Summary of the invention One object of the present disclosure is therefore to obtain a robotic work tool which is more reliable and functional over time. This object is achieved by means of a system as defined in claim 1. More specifically, a system comprising a robotic work tool with a detecting device, a boundary wire loop, configured to define a work area, and a base station, feeding a signal to the boundary wire loop. The work tool is configured to detect the boundary wire and thereby boundaries of the work area by detecting said signal. The detecting device is configured to detect a break in the boundary wire loop by detecting an absolute phase of an electric field generated by the signal fed to the boundary wire loop, wherein the absolute phase of the electric field shifts at said break. With such a system, a break in the boundary wire can be quickly found and repaired, which provides for a low down-time and hence improved reliability over time.

The transmitted signal may have a relative bandwidth higher than 0.4, more preferred higher than 0.65. This makes it possible to detect the absolute phase of the signal with a relatively simple circuitry. The high relative bandwidth may be obtained by transmitting a pulsed signal, or e.g. by transmitting two signals with a frequency separation corresponding to the relative bandwidth. Another alternative would be a band spreading signal such as a CDMA-type signal.

It is also considered a method using a detecting device included in a robotic working tool for detecting a break in such a boundary wire loop, wherein a base station comprises a transmitter is configured to feed a boundary signal to a boundary wire loop. The method includes detecting a first signal on a first location along the boundary wire loop, detecting a second signal on a second location along the boundary wire loop, and determining that the break is located between the first and second locations if the first and second signals have different absolute phase.

It may be preferred to receive the first and second signals using a vertically oriented dipole antenna, especially if the boundary wire is buried in the ground.

Further, a detecting device is considered for detecting a break in a robotic work tool system boundary wire. The detecting device has a directional antenna, an amplifier, and a phase detecting block, wherein the phase detecting block is devised to detect an absolute phase of a received signal and to indicate a shift in said absolute phase.

An analogue to digital converter may be connected between the directional antenna and the phase detecting block, and the phase detecting block may be softwareimplemented. The detecting device may be included in a robotic working tool.

Further a base station for a robotic work tool is considered, where the base station comprises a transmitter connected to boundary wire terminals and being configured to feed a boundary signal to a boundary wire loop, such that the robotic work tool can determine a status of being inside or outside the boundary loop. The base station may be configured to feed a signal having a relative bandwidth higher than 0.4 to the boundary wire loop. More preferred, the signal may have a relative bandwidth higher than 0.65.

Brief description of the drawings Fig 1 shows a robotic tool system.

Fig 2 illustrates schematically a base station connected to a boundary wire with a break.

Fig 3 and 4 illustrates an electric equivalent of a boundary wire before and after a break.

Fig 5 illustrates schematically a detecting device used to detect a break.

Detailed description Fig 1 shows a robotic tool system 1, where the robotic tool 3 may for instance be a robotic lawnmower. The robotic tool 3 is intended to operate within a working area 9, which is defined by a boundary wire 7 which is typically buried. When reaching the boundary wire 7, the robotic tool is intended to detect that it is about to leave the defined working area 9 and change its direction in order to stay within the same. The boundary wire is supplied with a signal from a base station 5 which may also function as a charging station, that intermittently charges batteries of the robotic tool 3.

The base station 5 therefore has a transmitter connected to boundary wire 7 terminals and being configured to feed a boundary signal to the boundary wire loop. The robotic tool may pick up e.g. a magnetic field emitted by the base station such that the robotic work tool can determine a status of being inside or outside the boundary loop. This can be done by detecting the direction of a generated magnetic field that at any given time is directed upwards inside the boundary wire and downwards outside the boundary wire, or vice-versa. This may be detected with a coil, a Hall element or the like in the robotic tool.

Fig 2 illustrates schematically a base station 5 connected to a boundary wire 7 with a break 11. This is not an uncommon situation. It may often happen that a user cuts the wire 7, for instance by digging in the lawn. When this occurs, the electric properties of the cable changes abruptly such that the robotic tool 3 may no longer be able to detect the cable 7. Usually, the robotic tool is not allowed to operate under such circumstances, and is therefore disabled. In order to repair the wire 7, the position of the break 11 need be found, and this may prove difficult, as the cable most often is buried at a few centimeters depth in the ground.

A typical way to deal with this problem is to disconnect the base station 5 from the boundary wire 7, and to connect a dedicated kHz-frequency test signal transmitter to either of the leads. With a test signal receiver that lead can then be followed until the signal disappears, which most likely corresponds to the break location.

This is a complicated and cumbersome procedure. Further, it may be possible that the high-frequency signal bypasses the break location. This may for instance be the case where two pieces of the boundary wire run parallel to each other at some length. One example of this scenario is illustrated in fig 1, where the boundary wire at one location is led in from its general outer envelope to exclude an obstacle 13, e.g. a tree, from the work area 9. If a break would be located close to the obstacle, it is possible that one parallel piece could induce currents in the other, making it difficult to detect the break.

The present disclosure uses another approach. The base station 5 is configured to feed a signal whose absolute phase can be detected along the length of the boundary wire. Typically, a signal with a high relative bandwidth is fed to the boundary wire loop 7. The signal may have a relative bandwidth higher than 0.4, and more preferred higher than 0.65.

The term relative bandwidth as used herein is defined as the bandwidth of a signal divided by that signals center frequency, and that parameter has no unit.

By using such a signal, it is possible to detect the absolute phase of the signal emitted from the boundary wire. This can be done e.g. using a detecting device with a vertically oriented dipole antenna.

By absolute phase is here meant the phase of the detected signal compared with a same frequency internal reference, for instance. The detected phase corresponds to a direction clockwise or counter clockwise to the break in the loop.

The detected absolute phase of the electric field will remain roughly constant as the detection device is moved along the boundary wire but changes abruptly when passing the break. It is however not necessary to follow the boundary wire (which may be several hundred meters long) until the break is found. As an absolute phase will be detected, it may be possible to test a small number of locations along the boundary wire loop 7. In the case illustrated in fig 2, the user can test at location ‘a’ and get the result counter clockwise (CCW), ‘b’ clockwise (CW), ‘c’ - CCW, ‘d’ - CW, and ‘e’ - CCW, to quickly locate the fault 11.

A signal with a high relative bandwidth is preferred here. If a narrowband signal was used, which cannot easily be used to detect the absolute phase, it would be difficult for the receiver to distinguish between the phase shift at the break 11 and a drift of phase due to frequency errors between the receiver and sender. A user could for some installations keep the phase information by having the receiver close the boundary wire, but as soon as the receiver loses connection with the transmitter or boundary wire, the phase error between the receiver and transmitter will increase.

Fig 3 and 4 illustrates an electric equivalent of a boundary wire before and after a break. Before, as indicated in fig 3, the break 11 occurs, the boundary wire may be regarded as a transmission line model for instance with the parameters R=20 m?/m och L=2 ??/m. When a break 11 occurs as in fig 4, a significantly larger resistance is added to the circuit, and the transmission line is broken in two parts. The resistance of the break can vary from a few ? to a number of k ?, depending for instance of the water content in the soil where the wire is buried. In most cases, the change in the electric properties is so large that the base station 5 easily can detect that a break has occurred.

Fig 5 illustrates schematically a detecting device 16 used to detect a break. As shown, at the location of the break 11, there will be a shifting electric field which indicates a phase change that can be sensed by an antenna 15 in a vertical direction (or more generally in a direction of a normal to a plane in which the boundary wire runs). When moving from one side of the break to the other, the absolute phase of the received signal changes abruptly about 180°.

As mentioned, in order to produce a signal where the absolute phase can be detected, a significant relative bandwidth may be needed. This can be achieved in different ways.

In a first example a non-sinusoid signal such as a square wave or pulsed signal is used to produce a number of harmonics. The frequency shift between a fundamental tone and its harmonics may be sufficient to detect an absolute phase.

In a second example, two superimposed tones are used where the frequency shift between those tones should be large enough to allow detection of absolute phase.

In a third example, a band spreading technology similar to what is used in CDMA-(code division multiple access)-systems could be considered, which provides a significant relative bandwidth as well.

As the skilled person realizes, there are different ways to devise a receiver which is able to detect the absolute phase of an electric field. This can be achieved providing a broadband amplifier 17 between the antenna 15 and an analogue to digital converter, A/D 19. Using a hardware mixer may add an unknown offset to the phase and is less useful. Instead a low frequency amplifier with high input impedance may be preferred.

Further signal processing to determine the absolute phase and indicate a direction to a break may be software-implemented in a software block SW, 21.

The software block may thus detect the phase. For a two-tone system this can be implemented simply by comparing the phases of the two tones using a software routine. If instead a e.g. a square wave arrangement or CDMA signal is used, the software unit can detect phase using matched filters tuned to the harmonics used. The phase shift may differ over frequency and position, and may not necessarily be a true 180° shift. This may however be handled by thereceiver software.

The detecting of absolute phase may also be hardware-implemented.

The detecting device may be provided as a separate unit, or may be included in a robotic lawnmower that is devised to find a break.

The present disclosure is not restricted to the described examples, and may be varied and altered in different ways within the scope of the appended claims.

Claims (9)

1. A system (16) comprising a robotic work tool (3) with a detecting device (16), a boundary wire loop (7), configured to define a work area (9), and a base station (5), feeding a signal to the boundary wire loop (7), the work tool (3) being configured to detect the boundary wire and thereby boundaries of the work area (9) by detecting said signal, characterized by the detecting device (16) being configured to detect a break (11) in the boundary wire loop (7) by detecting an absolute phase of an electric field generated by the signal fed to the boundary wire loop, wherein the absolute phase of the electric field shifts at said break (11).

2. A system according to claim 1, wherein said signal has a relative bandwidth higher than 0.4, more preferred higher than 0.65.

3. A system according to claim 2, wherein the relative bandwidth is obtained by transmitting a pulsed signal.

4. A system according to claim 2, wherein the relative bandwidth is obtained by transmitting a band spreading signal such as a CDMA-signal.

5. A system according to claim 2, wherein the relative bandwidth is obtained by transmitting two signals with a frequency separation corresponding to the relative bandwidth.

6. A method using a detecting device included in a robotic working tool for detecting a break in a boundary wire loop, wherein a base station comprising a transmitter is configured to feed a boundary signal to a boundary wire loop, characterized by the method including detecting a first signal on a first location along the boundary wire loop, detecting a second signal on a second location along the boundary wire loop, and determining that the break is located between the first and second locations if the first and second signals have different absolute phase.

7. Method according to claim 6, wherein the boundary wire is buried in the ground, and the first and second signals are received using a vertically oriented dipole antenna.

8. A method according to any of claims 6 or 7, wherein said signal has a relative bandwidth higher than 0.4, more preferred higher than 0.65.

9. A detecting device, which is included in a robotic working tool, for detecting a break in a robotic work tool system boundary wire, characterized by a directional antenna (15), an amplifier (17), and a phase detecting block (21), wherein the phase detecting block is devised to detect an absolute phase of a received signal and to indicate a shift in said absolute phase, wherein an analogue to digital converter (19) is connected between the directional antenna and the phase detecting block (21), and wherein the phase detecting block is software-implemented.