Radio/magnetic field: detect direction of antenna

[petterg]

I'll try to answer all questions in one post.

I tried to narrow the subject for this thread down to the one particular issue. That turned out to be a mistake. Sorry about that.

The background for this project is that I bought a robot lawn mower from Bosch. It has an advantage over other similar robots in the fact that it's mapping the working area. It learns where obstacles are located, it knows where it has been working, and works systematically in the areas where it hasn't been for a long time.

That is in theory.

What Bosch has managed to do is the navigation. Some think it's using gps, but it's not. (It does have a built in gps chip, but it's not in use by the software for other than setting time/date.) It's navigating by following a wire, counting wheel rotations, and when it has moved past a couple of wire bends and counted the wheel rotations between the bends it's got a reference point. From this point it's navigating to where ever it needs to work. The navigation logic works well. Direction is also assisted from a compass - if you try to lift up one of the wheels so that it changes direction, it's instantly compensating for that by stopping the wheel still on the ground, and wait for the lifted wheel to recover grip so the course is correct. Error seems to be less than 1 degree.

Anything but the navigation logic is a total failure on this product. It's software is so full of bugs that it couldn't even be called a beta release. The mechanic is extremely poor. Even the razorblade-kind of knifes that is fitted to a spinning disc - the some samples of the product is shipped without tightening the screews holding the disc. (And such issues aren't related to a faulty batch. It occurs on a large number of samples sold i several countries, marked with different versions of the product.)
I can't mention all the faults here, I could write for days. (Really. I've probably spent more than 100 hours writing bug reports to bosch.)

I got my robot replaced by bosch service (after it had spent more time in the back of my car to/from service than it had spent on my lawn). The replacement with the newest updated firmware, fixed some bugs - and introduced just as many new ones.

Bosch seems to refuse that there are issues with their robot. Bosch is so hard to communicate with that loads of Homedepot-kind of stores stop selling them. The customers return the faulty products to the stores, but the stores only get to return a few of them to Bosch. One customer got his robot replaced 5 times in 8 weeks.

Then, 6 days after I got the replacement robot, it was stolen. (They even took the base station, the digged down wire and the neighbors garden furniture - while we were home!)

This left me with 4 choices: Not have a robot again, get a new Bosch, get a new robot with random mowing pattern, or make something better.

You might have guessed my choice is to make something better.

While commercial robots leave 10-30cm uncut grass around obstacles and edges, this new robot will have a edge-cutter function. That's why this robot will have 7 motors spinning, creating the "random" magnetic fields that makes navigation harder. It's quite important that the navigation can say that when a know obstacle is hit, it should know if this is an obstacle marked in the map, or an unknown obstacle. This will make the difference if the robot is cutting grass around a tree or the fur of a curious dog.

Most of the well known robot mowers recognizes which side of the wire they're on - and that's their only navigation. Also, if the robot is 20m from the wire and you cut the wire, it stops. As for Bosch there is a square wave of about 150mA in the wire.


I'd like the wire not to be an absolute boundary wire, as most others. I'd like to mark in the garden map that in some areas it's allowed to go outside the wire until it hits obstacles (or wire signal get below some threshold), while in other areas it's not allowed to cross the wire at all.


And, if it works, why not fit it with a leaf blower that also can be used on snow.

-----------------------

Are we permitted to know what the vehicle is for?
Yes

Is it armed and dangerous?
Yes. Hopefully within limits

Is it reasonable for us to assume it is a robot vacuum cleaner or lawn mower?
yes

What are the maximum dimensions of the field?
A square around the field will have sides of 28m. Although if the robot gets more than 20m from the wire the navigation has failed.

Is knowing position sufficient or is vehicle orientation also needed?
Both is needed. It will have reference points where it can go to calibrate it's sensors.

How accurately must position and orientation be known?
To make the wheel rotation counter work as part of the positioning the direction is required to be quite accurate. If position can be accurate with other kinds of sensors the direction doesn't have to be all that accurate.

How fast does the vehicle travel?
30cm/s is a goal.

Is it inside a building or outside?
Outside, or under the deck, or under trees.

Must it work in the dark and in bright sunlight, night, day or both?
Bright sun might introduce a risk for overheating. Other than that it should work under all kinds of light conditions.

What obstructions to visibility are there in the field?
A house, garage, trees, rocks, cars, poles, bushes, walking people. In worst case a dog or a thief. (Aim for the last one!)

Can there be several points in the area, three of which are always visible from the vehicle?
No. But there can be 8 points where at least 3 of them will be visible at all locations. Most locations will have visual to 4 of them.
There's already at least 4 wlans available at any location.

Will the vehicle be alone or with others, friends or enemies?
If it turns out well, I guess the neighbors would like to build them self a copy. If those turn out friendly or not depends on if the signals from ones area will disturb navigation for the neighboring areas.

 

[jim hardy]

original question was

Now the problem: How can we know if Xr=x1 or Xr=-x1 ?

and I'm guilty of getting distracted by the idea of a cool robot. Sorry about that.

Seems to me next step is to lay out some wire, take some measurements with coils near your wire and see what the signal looks like.

 

[sophiecentaur]

@petterg
I am still not clear whether you want a cure for the faulty device you already have, to reverse engineer it so you can find out how it works or to build another one with an entirely different system. Either way, you have managed to waste a lot of your time and PF's. The general rule on PF is that you attempt to write a clear question about a particular problem and you will invariably get some sense out of us. As it is, you have managed to get a load of idle ramblings, so far.

 

[petterg]

Yep. Components for generating signal (bunch of opamps, various conductors and resistors), an oscilloscope, and some arduino pro micros are on the way.

How do I find the ideal coil for picking up the wire signal? A large coil will give stronger signal, but might not cope with the frequency?
And for filtering the signal when the coil is the source of electric signal in the circuit; Should I consider the coil as part of the RLC-filter, or just as a source for a RC-filter?
(I can't figure how to simulate a coil as the source of signal.)

 

[sophiecentaur]

It would be normal to design a system before ordering the components. How is it going to operate? What principle and what values of current, frequency etc. etc. are you planning to use? Random assemblies of components tend to result in nothing happening - except, sometimes, burnt components.

 

[petterg]

The above post was for Jim

I am still not clear whether you want a cure for the faulty device you already have.

The two I had is gone. First one returned. Second one stolen. This will be a build from zero.

you have managed to waste a lot of your time and PF's. The general rule on PF is that you attempt to write a clear question about a particular problem and you will invariably get some sense out of us. As it is, you have managed to get a load of idle ramblings, so far.

I thought the problem was clear. Cause it was the remainding one problem I hadn't figured out to make all of the mentioned position calculations within the needs.
Allong the way someone came up with complex ideas that can potentially replace all the ideas I had for navigation in the first place.

I'm still at the side-of-wire-detector issue because the alternatives seems to complicated to me. (I understand the basic idea, but not well enough to implement it.)

 

[petterg]

It would be normal to design a system before ordering the components. How is it going to operate? What principle and what values of current, frequency etc. etc. are you planning to use? Random assemblies of components tend to result in nothing happening - except, sometimes, burnt components.

I have the sender planned. Opamps/conductors sending sawtooth at 4880Hz.
What I don't have is a plan for the receiver. It seems like I need to measure a coil around the field while the sender is sending and make the design based on those measurements.

 

[Baluncore]

As for Bosch there is a square wave of about 150mA in the wire.

What was the frequency of that square wave?

It may well be an RF system that satisfies the navigation requirements, but that is not certain. In principle, any AC modulation can operate with immunity from the Earth's magnetic field. I have built systems that use a buried wire that operate at 1kHz with a range of over 100m.

It is easy to fixate on the Hall Effect sensor as the solution to magnetic field detection problems. Unfortunately, Hall Effect sensors are not optimised for AC field detection as they are biased by the background field. A high gain Hall Effect sensor will saturate and so has a limited sensitivity when AC coupled. A pickup coil can be better optimised for small signal detection.

Most systems that measure phase have a very high conversion gain. Interference from power lines and electric motors is often overstated. It is not that much of a problem when making phase measurements because phase is not instant, it is temporal, it has the time to get it right. I suspect that a shaped pattern loop of wire is a local reference used to correct for slip / drift of the wheel navigation.

The GPS is almost certainly there to identify which lawn it is on. Since simple GPS is only accurate to about 2m it could only be used to detect extreme boundary excursion.

The wire loop does not have to be “ranged”, it is only required so the vehicle can recalibrate it's position by following the wire line around one bend.

 

[sophiecentaur]

It is easy to fixate on the Hall Effect sensor as the solution to magnetic field detection problems. Unfortunately, Hall Effect sensors are not optimised for AC field detection as they are biased by the background field. A high gain Hall Effect sensor will saturate and so has a limited sensitivity when AC coupled. A pickup coil can be better optimised for small signal detection.

I agree. afaics, there is absolutely no need for a Hall sensor. Simple induction (to a tuned coil) would work just as well. I can see a problem with the (audio) field around a wire on a random route varying considerably as the return path is pretty well undefined. Hence my suggestion for using RF and a feeder, with its own, integral return conductor. I reckon that a piece of mains twin cable would probably serve as a good feeder with lots of leakage. Matching would hardly be a problem, either.

 

[petterg]

What was the frequency of that square wave?

A pattern that looks like -_-_--__-_-_--__
Where each - or _ lasting for about 100ns. (It would look better in text if - and _ had the same length.)

It may well be an RF system that satisfies the navigation requirements, but that is not certain. In principle, any AC modulation can operate with immunity from the Earth's magnetic field. I have built systems that use a buried wire that operate at 1kHz with a range of over 100m.

Sounds promising.
What kind of receiver did you use? Sender was a sin-wave?

It is easy to fixate on the Hall Effect sensor as the solution to magnetic field detection problems. Unfortunately, Hall Effect sensors are not optimised for AC field detection as they are biased by the background field. A high gain Hall Effect sensor will saturate and so has a limited sensitivity when AC coupled. A pickup coil can be better optimised for small signal detection.

Considered the motors noise I think a coil will perform better than a Hall sensor.

Most systems that measure phase have a very high conversion gain. Interference from power lines and electric motors is often overstated. It is not that much of a problem when making phase measurements because phase is not instant, it is temporal, it has the time to get it right.

It's a matter of filtering out the signals to compare, I guess.

I suspect that a shaped pattern loop of wire is a local reference used to correct for slip / drift of the wheel navigation.

The GPS is almost certainly there to identify which lawn it is on. Since simple GPS is only accurate to about 2m it could only be used to detect extreme boundary excursion.

When the bosch stops on the lawn and wait for a human to press a button (which it does hourly on a good day, every few minutes on a bad day) there is a choice for "continue mowing" or "go home". When I took it to service 105km away, and powered it up I got the same choice. I just had to try "go home" to see what it would do. It was thinking for minutes, then concluded no signal from cable.

When I got the replacement it was tested at the service location before I brought it home. A home it started out quite well before it figured that the cable boundaries did not fit with the stored map.

A guy reported that he'd tried the bosch in a concrete parking garage/basement. He concluded that it worked better there than in the yard. Probably because the wheels had better grip. Clock remained on 00:00 during his test, so no GPS coverage.

For those experienced I conclude that GPS is only used to setting the clock.

The wire loop does not have to be “ranged”, it is only required so the vehicle can recalibrate it's position by following the wire line around one bend.

It will need two bends and compass reading to have know which straight it just passed.
Bosch is following the line so much that all grass is gone and it's tracks turn into slippery mud. Hence the wheels lose traction and the wheel count along the wire fail.

Using phase timing, maybe it's possible for the robot to just stop over the wire and figure how far it is from the sender?

 

[petterg]

using RF and a feeder, with its own, integral return conductor. I reckon that a piece of mains twin cable would probably serve as a good feeder with lots of leakage. Matching would hardly be a problem, either.

Could you elaborate that, please?

 

[Baluncore]

Because the wire is near the surface it can be followed closely by the vehicle. The wire can be laid as a large irregular closed loop which defines the return current path in a useful way. There will be a very slight error in the sensed position of the wire due to the returning segment's far fields. That difference between the position of the physical wire and the virtual null will disappear in the calibration process.

 

[sophiecentaur]

Could you elaborate that, please?

This suggestion would involve feeding the rf along the two conductors (signal and earth), which acts as a transmission line laid out in a pattern just below the surface. Some of the signal (say 1%) gets to the far end and is absorbed in a load and there will be a loss of signal along the length of wire and the fields will be very local to the cable where the signal leaks out along the whole length of the cable. The best form of transmission line would depend upon the frequency you choose. You would need to experiment with a range of types - twisted telephone cable, mains cable, low quality TV feeder with a very open weave screen, etc.. Whatever you can get samples of. You only need a low level of signal in the vicinity of the wire. Some simple compass control could take care of the situation where the robot loses the cable and it would just track at right angles until it hits the cable and RF again. You could either get the robot to follow the cable , straddling the cable or use the cable as a 'fence' to limit where the robot goes. There are a number of algorithms to achieve this, using an array of, say four receivers under the mower to keep it tracking on the line. It's been done many times, optically, with 'line-following' robots on factory floors etc. and it would be straightforward to adapt for following a transmission line, leaking RF.

I must say, I don't have a clue why the robot needs GPS to tell it which garden it happens to be in - or indeed to tell it anything except, possibly what direction it's heading in.

The choice of line design is fairly critical. The loss per metre of the run needs to be low enough for the signal to survive to the end so the robot can still detect it. The receiver needs a dynamic range that's enough to cope with the high level signal at the start and the lowest level at the other end - but you could use a number of cable runs in parallel, perhaps the stripes could all be fed in parallel at one end in a 'star connection' (not a star shape - just a star topology)

 

[Baluncore]

Leaky feeder systems are no real problem these days, and the RF receiver chips with an RSSI output as used for FM reception are very common. The problem here is that they are optimised for extending communication systems in tunnels, not for navigation systems.

Leaky feeders are operated in air, not under a variable density layer of water. One problem is that the direction information sensed in a dry soil with a forward wave coupler, may become a backward wave coupler when the soil is wet. The dominance of Odd–Even coupling modes under different soil conditions would need to be modeled before burying the cable and waiting a couple of seasons for it to get lost. Replacing the feeder would temporarily fix the problem due to airing the soil. But then it would fail again.

As I understand it sophiecentaur is advocating the use of a leaky feeder because it has no return path while I am advocating a lower frequency loop with a usable return path.

I must say, I don't have a clue why the robot needs GPS to tell it which garden it happens to be in - or indeed to tell it anything except, possibly what direction it's heading in.

When the vehicle is used on several different lawns at a few different locations it would know which internal lawn map to use without needing to be reprogrammed when it was moved. It could be owned and operated by a contractor to service many customers. If it had some clever mobile phone ability it could let it's operator know it's immediate status and inform it's manufacturer where it was now. That anti-theft technique would be very useful.

 

[petterg]

When the vehicle is used on several different lawns at a few different locations it would know which internal lawn map to use without needing to be reprogrammed when it was moved. It could be owned and operated by a contractor to service many customers. If it had some clever mobile phone ability it could let it's operator know it's immediate status and inform it's manufacturer where it was now. That anti-theft technique would be very useful.

Considered that all the cheap robots are recommended to run 24 hours / day, only interrupted by charging batteries, to handle the given area I don't think moving it to other yards are very practical. Basically they mow so much that you can't use the garden for anything but watching the robot. (One was even recommended to run 27 hours/day.)
Bosch navigation keeps the mower within some smaller areas until (it believes) it's finished with the area. Then at least you can use parts of the yard safely for a couple of hours.

Using a robot in multiple gardens are unpractical for most robots, unless you have one dimensioned for a significantly larger yard than the sum of those you'll use it in. It's also impossible for Bosch, because it can only save one yard map.
So, that's surely not the reason of having a gps in it.

--------------

I'm starting to think of a cable loop around the area + a RF transmission line in the center. Does the transmission line need to be straight? (There's a house in the center.)

Sounds like some kind of automatic calibration will be required for RF. It shouldn't be too hard; follow the line end to end, count wheel rotations and record the strength.

 

[Baluncore]

The observation that the Bosch is not now capable of multiple maps, cannot preclude the possibility that the next version will have that capability. Indeed I would be very surprised if the original specification did not include a multiple maps option. That is why it was designed with an internal GPS.

Maybe the multiple map option has been disabled in the Bosch v1.0 release because it will be used to encourage users to buy v2.0

 

[the_emi_guy]

I may be wrong, but my impression is that these perimeter wire sensors do not, by themselves, make any distance measurements or detect which side of the wire we are on. They are very short range and simply tell the robot when we are on top of the wire. The robot interprets this as a hard boundary and turns around. The scheme assumes the mower is started inside the perimeter wire boundary.

In other words if you start the mower outside of your perimeter, won't the mower happily mow the entire neighborhood with the exception of your yard.

 

[sophiecentaur]

It really would help to know whether this thread is discussing how this system is being achieved by Bosch or how to make a better one. If the latter, then it is necessary to state exactly what you want it to do. A specification is needed if you want to avoid even more wasted effort here. Inanimate circuit elements need to be told exactly what to do before you will have a working machine and design can only result from a properly stated requirement. What are your actual requirements for this, petter? You keep shifting the goal posts and randomly picking out ideas and systems.
Are you prepared for a network of buried cables? Do you want just cables around the perimeter? Is this machine just for your garden or do you want to create a design that can be used commercially for many different gardens? Will all these gardens have their own cable systems? If not, then why are we discussing a system with buried cables? Without them, you will need a map of every garden to be treated.
Please give us something concrete to address. (Actually, some green concrete could solve the problem of grass cutting for ever!)

 

[petterg]

Maybe the multiple map option has been disabled in the Bosch v1.0 release because it will be used to encourage users to buy v2.0

I think they intended to use gps for navigation, but failed to make the error within limits. So they left it there for possible use in later firmware.
Firmware update can be performed by inserting a bootable SD-card with software image on. Just that they refuse to make the updates available. Noone who have had a buggy v1 will put money in a v2 unless they get the money back for the v1.

 

[petterg]

I may be wrong, but my impression is that these perimeter wire sensors do not, by themselves, make any distance measurements or detect which side of the wire we are on. They are very short range and simply tell the robot when we are on top of the wire. The robot interprets this as a hard boundary and turns around. The scheme assumes the mower is started inside the perimeter wire boundary.

In other words if you start the mower outside of your perimeter, won't the mower happily mow the entire neighborhood with the exception of your yard.

That totally depends on which model you're talking about. What you say is true for the cheap brands, but not for the expensive ones.

 

[sophiecentaur]

So do you want to make one? If so, what specification are you aiming at? Time to get down to brass tacks, I think. If your not careful it will consist of 'perhapsatronic' circuitry and shave the pile of your living room carpet.

 

[petterg]

It really would help to know whether this thread is discussing how this system is being achieved by Bosch or how to make a better one. If the latter, then it is necessary to state exactly what you want it to do. A specification is needed if you want to avoid even more wasted effort here. Inanimate circuit elements need to be told exactly what to do before you will have a working machine and design can only result from a properly stated requirement. What are your actual requirements for this, petter? You keep shifting the goal posts and randomly picking out ideas and systems.
Are you prepared for a network of buried cables? Do you want just cables around the perimeter? Is this machine just for your garden or do you want to create a design that can be used commercially for many different gardens? Will all these gardens have their own cable systems? If not, then why are we discussing a system with buried cables? Without them, you will need a map of every garden to be treated.
Please give us something concrete to address. (Actually, some green concrete could solve the problem of grass cutting for ever!)

(I got interrupted by work, so the reply had to wait)

The way I work with projects is to look at what have been done with similar applications before - what have worked, what have not. That gives a lot of ideas for solutions. Some of which may conflict. Currently there seems to be three possible directions to go:
1) The border-loop-wire
2) The RF transmission line
3) The radio

Some ppl in the thread talk about one or two of these. Others talk of only one. The thread may seem a bit chaotic as subject is changing between those technologies. Maybe the technologies should be combined in the final system, or maybe we can exclude some?

Spec (ideally):
- Be able to calculate position (within 30cm error) and direction heading (within 3 degrees) for any location of the robot within an area of 20m from the cable, or a square of size 30x30m

- calculation of one of the two (position or direction) should be within limits even when obstacles like a house, a car, a person, are located between signal sender and receiver

- robot needs to know how accurate calculations a any given moment are. That way it can go for calibration and/or put a higher trust other methods like counting wheel rotations.

- total length of wire to dig down should not be more that 150m

- if system requires radio senders placed around or within the area, cables to these are included in the 150m limit.

- ground is not flat. There has to be taken in account that ground plane varies with +/- 50cm. robot height is max 40cm. Sensors can be placed high or low on the robot.

- there are 4 wlan senders around the area and 2 inside the area. At least 3 senders has coverage at any location. They can be used, but not modified.

- System should work independent on weather conditions and light conditions. Components that are not suitable for outdoors use need to work even when put in a box.

- System should not make sounds that can be heard by humans, or lights visible to humans.

- System should not disturb other electrical systems. Including an equal system installed next door.

If it works out in the end, I guess this may turn into some kind of DIY-kit-list that can be distributed.

(Green concrete solves the grass cutting, but not leaf blowing or snow blowing.)

 

[sophiecentaur]

Leaky feeder systems are no real problem these days, and the RF receiver chips with an RSSI output as used for FM reception are very common. The problem here is that they are optimised for extending communication systems in tunnels, not for navigation systems.

Leaky feeders are operated in air, not under a variable density layer of water. One problem is that the direction information sensed in a dry soil with a forward wave coupler, may become a backward wave coupler when the soil is wet. The dominance of Odd–Even coupling modes under different soil conditions would need to be modeled before burying the cable and waiting a couple of seasons for it to get lost. Replacing the feeder would temporarily fix the problem due to airing the soil. But then it would fail again.

As I understand it sophiecentaur is advocating the use of a leaky feeder because it has no return path while I am advocating a lower frequency loop with a usable return path.

I'm only proposing the leaky feeder to be operated in very near field - not necessarily for direction finding - so variation of field along the feeder would not matter. It would be a relatively short run and not necessarily involve any modulation (perhaps an audio tone, to make the detection more rugged). Any standing wave, due to mismatches / reflections, would not be relevant in this context. It would be a matter of locating a maximum as the robot crosses the line and then sticking to the line of maximum field strength, just above the line.
I would not imagine that the stuff that's used for communication in mines would be a reasonable price (high spec, for sure) and the requirement is for a usable field strength a a distance of several metres at least in mines; in this case, you'd be talking in terms of 0.5m maximum and a minimum carrier to noise ratio of no more than 20dB. Trivial requirement, I sure and there's bound to be a very cheap solution (sticking my neck out but with some certainty about it). I would even think that twisted bell wire would be a suitable candidate.
The only downside would be that the cable runs would need to be installed initially - but a simple mole plough works fine for drainage of lawns and the routing is fairly non-critical (just how perfect do your lawn stripes have to be?

 

[the_emi_guy]

That totally depends on which model you're talking about. What you say is true for the cheap brands, but not for the expensive ones.

Just curious how you know this.

 

[petterg]

I'm only proposing the leaky feeder to be operated in very near field - not necessarily for direction finding - so variation of field along the feeder would not matter. It would be a relatively short run and not necessarily involve any modulation (perhaps an audio tone, to make the detection more rugged). Any standing wave, due to mismatches / reflections, would not be relevant in this context. It would be a matter of locating a maximum as the robot crosses the line and then sticking to the line of maximum field strength, just above the line.
I would not imagine that the stuff that's used for communication in mines would be a reasonable price (high spec, for sure) and the requirement is for a usable field strength a a distance of several metres at least in mines; in this case, you'd be talking in terms of 0.5m maximum and a minimum carrier to noise ratio of no more than 20dB. Trivial requirement, I sure and there's bound to be a very cheap solution (sticking my neck out but with some certainty about it). I would even think that twisted bell wire would be a suitable candidate.
The only downside would be that the cable runs would need to be installed initially - but a simple mole plough works fine for drainage of lawns and the routing is fairly non-critical (just how perfect do your lawn stripes have to be?

How would I start creating a test rig for this?
Is the sender just a regular sin-wave generator?
Transmission is done be grounding one pole, and connect the other to a wire (experiment with several types of wire). For testing I suppose it's ok to not dig the wire down. Should the far end of wire have some kind of terminator? Should it be grounded in the far end?

Receiver can use any kind of coil for sensing the signal?
How do I select the best coil for the purpose? When designing a filter at the receiver, do I consider the pickup coil as a part of the filter or as a signalsource?

 

[petterg]

Just curious how you know this.

By communicating with lots of robot owners. "Bwt, what happens with your robot if the power goes out while it's mowing? Does it keep going across the wire for greener grass other places in the neighborhood?"
Those who tested to pull the power while the robot was mowing could report that:
Bosch, Husquarna, Gardena and Robomow stops within a couple of seconds.
Worx and "the china robots" (those who sell under various names and colors depending on which store you get them from, but are essentially just 3 different models) keeps going random directions until battery runs out.
All though, the best story so far came from a owner of a bosch; He had his mower in the garage over night. When he got out next morning there was an other bosch docked in his charger. I don't think that was a power out though. Probably just another example of when the software tell the robot to turn the wrong way at the cable.