Lawnmower robots – the end of VLF time signals? | Hacker News Reader

Lawnmower robots – the end of VLF time signals?

phk.freebsd.dk

96 pointsicehawk30 comments

The O.P. appears to live in Demark and is concerened about interference to the German DCF77 signal on 77.5kHz from a buried-loop lawnmower control signal on approx 77.5KHz.

His Spectrum Analyser pictures are not much help. About all they prove is that his local DCF77 signal is very, very weak.

In order to investigate the problem, he needs to accurately measure the modulation type, bandwidth and the frequency of the Loop TX, and then analyse exactly how the interference is occuring.

Is the Lawnmower system genuinely overlapping in frequency with DCF77? Or does the Clock receiver have insufficient Selectivity to separate the two? Or is the Clock receiver being overloaded? Or is the local DCF77 signal just too weak?

It is critical that he use a resonant and shielded magnetic antenna to measure the DCF77 signal, so as to eliminate the electric-field rubbish his Spec-An is picking up. If he has simply used a random wire antenna, his spectrum plots will be pretty much meaningless.

Note-1. Being a buried loop, the Lawnmower antenna is designed to radiate only a Magnetic field (eg the earth shielding and the loop balance should suppress any EM waves). Either the loop is incorrectly installed, or the transmitter and feed lines are physically close to his DCF77 Clock, and/or the DCF77 signal is very weak.

Note-2. In the USA, the equivalent time signal is WWVB on 60KHz. It is possible that the Lawnmower system he has installed was intended for North America only, and is not authorised for use in Europe.

Note-3. I don't know what the Service Area of DCF77 is, but it is likely that it's signals will be very weak in Denmark. It is possible that he is simply outside the DCF77 service area.

The modulation type for the lawn-mower is digital current pulses with no bandwidth limiting, as is obvious from the scope signal I conveniently included for your and everybody elses information.

As for what exactly goes wrong with DCF77, that is pretty obvious from the two spectrum-analyser traces provided by that signal: It gets overloaded with AM noise which the simple AM-discriminator receivers used in clocks cannot deal with.

The key is that the DCF77 signal is weak, and the lawnmower loop is very close. It's a classic example of poor signal-to-noise ratio, made worse by the DCF77 receiver being within the "near-field" of the lawnmower loop.

You could improve the S/N ratio by:

- separating the wiring for the two systems as much as possible, and fitting a mains noise filter to keep the loop signal out of the mains wiring.

- putting a remote (ferrite rod) antenna on the DCF77 receiver and experimenting to find a quiet location (possibly the interference can be nulled out by rotating the antenna).

- attending to the shielding, earthing and balance of the lawnmower loop (especially shielding and twisting the feed wires).

You could document the problem and write to your local radio licensing people. Unfortunately, I expect that they would consider that neither the DCF77 clock or the Lawnmower Loop come under their jurisdiction.

You could also write to the manufacturer of the loop and ask for their assistance. They may be surprisingly helpful.

Finally (if you haven't already) you could research the methods that Radio Amateurs have developed for receiving VLF signals in noisy domestic environments.

Bottom line: In my experience, correctly earthing the loop and shielding the feed wires should do a lot to eliminate the interference.

But as before, you need to understand the modulation characteristics of the lawnmower loop in detail. What bandwidth does the system actually need to operate? Can the interference be reduced by fitting a better low-pass-filter to the loop transmitter? Can you put a notch filter at 77.5 KHz?

Don't worry about what I can do to mitigate my problem, I have the electronic and RF skills+kit to deal with this.

My point is that John F. Consumer will likely conclude that his "radio controlled clock" is dying, throw it out, waste money on another one, and maybe never make the connection to his (or his neighbors) lawnmower robot.

And more importantly, that if popular enough, such (badly designed) signals will eventually make VLF timekeeping useless for everybody.

You're right of course, but this sort of thing happens whenever there is a poor signal-to-noise ratio.

Like any Broadcast service, VLF timekeeping relies on a saturation signal so it can override domestic electrical noise.

The level of Electrical Interference is steadily increasing world wide. Already services such as AM Broadcast Radio are unusable for many people.

The problem is caused by (1) the laws of physics, and (2) by our compromised EMC regulations.

At least your European EMC laws are much improved over the watered-down American ones.

Bottom line: The issue is much bigger than just VLF Time keeping.

I used to have an alarm clock, purchased in Germany, that would reset itself to German time every day. This was undesirable, since I lived in England (1 hour behind).

The diagram from Wikipedia suggests a very long range, Denmark should be no problem: http://en.wikipedia.org/wiki/DCF77#Reception_area

You should have bought a "Rugby" clock in England. Many of these clocks come in four different versions: Japan, Europe, UK and USA, depending on which signal they use (JFY, DCF77, Rugby, WWVB)

DCF77 works OK in Denmark, provided you don't have too much local noise (switch-modes, old televisions etc. etc.)

This would be considered "harmful interference" in my country and our communications authority would take a dim view of it. Amazingly they have way more power than the police in these matters - if they detect harmful interference they can enter property without a warrant, confiscate the offending equipment and impose hefty fines.

Actually it's probably not "harmful interference".

The German DCF77 signals may not be considered a protected broadcast in Denmark. It is possible that the DCF77 signals are just too weak.

But yes, the Radio Licensing Authorities have incredible powers in most countries, but these only apply in cases of extreme danger, eg someone is jamming an aircraft channel in an emergency. In normal situations, they must apply for a search warrant as would other civilian authorities.

DCF77 is indeed not a protected broadcast in denmark, despite the fact that an awfull lot of public infrastructure uses it for timekeeping.

Which country, if you don't mind me asking?

(the idea of the FCC breaking someone's door down and taking their equipment is pretty funny to me, although I can't say for sure it's never happened)

The FCC is pretty good about cracking down on illegal transmitters--I had a friend in high school who thought it would be a funny idea to set up an FM transmitter with one or more orders of magnitude Tx power than is allowed, and it only took the FCC a couple of days to come and seize his equipment, with a very stern warning. I would imagine if we had been older, he'd have been looking at a multi-thousand dollar fine, for good reason. The FCC really is not to be taken lightly.

The FCC does possess, and has exercised in the past, these powers.

The legal argument is that by using the public spectrum to transmit, you have agreed to FCC enforcement of such. Thus, you have tacitly agreed that the FCC in fact does not need a warrant to investigate, or seize your equipment, if they determine that that is an appropriate action.

I think this is beyond a flimsy argument, but it is the argument. It needs more looking over in courts, frankly. Seems like it could be easily abused.

It's happened, actually. Look at the history of pirate radio - there were armed raids done in cooperation and under the auspices of the FCC.

That was mostly because the were (perceived to be) an economic threat to some very big industries, it had very little to do with electro-magnetic radiation.

The man in the Ministry of Housinge cat detector van can pinpoint a purr at 400 yards, but he didn't have the right form. [1]

[1] https://www.youtube.com/watch?v=M5MnyRZLd8A

I don't know about the breaking down the door part, but the FCC does conduct spectrum surveillance and they do look for illegal stations and troublesome operations.

It is not obvious to me that emission limits are being exceeded here, and I don't have a calibrated antenna which would allow me to decide one way or another.

But even if emission limits are not exceeded VLF-based radio-clocks near the loop will still have much more noise to contend with.

Presumably he doesn't want someone to come along and take away his lawnmower.

How long do these mowers run for? Do they run continuously or for one hour every month?

They are very low (motor) power, so they need to run often, in order for the grass not to grow too tall.

Here in Denmark they usually run at least three times a week, but very often every day of the week.

How many hours they run each time depends mostly on the ratio of the lawn-size to the mower-width.

You should report this to the government agency in your region in charge of telecommunications signals. If it is anything like the situation in the United States, disrupting a licensed signal like this is probably not allowed.

I find it hard to believe that you can sell an electronic product that has that poor a behavior (it's emitting that much RF noise); this is the sort of hardware that really ought to be clamped down on, hard. RF Spectra is a shared resource, and a rather limited one, and it only takes one careless neighbor with poor electronic equipment to wreak havoc on other devices. If this lawnmower is getting used in the U.S. I would really like to see the FCC pay this manufacturer a visit.

If the image in your article is from the manufacturer, then they're based in Stockholm, Sweden. (http://www.autobotics.co.uk/automower-265acx.html)

If so, the Swedish equivalent to the FCC appears to be The National Post and Telecom Agency (PTS) (http://www.pts.se/).

While Husqvarna Group have offices in Stockholm, they're actually based in the small Swedish town of Huskvarna (see http://en.wikipedia.org/wiki/Husqvarna_Group).

Also, your first link is to a retailer, which seems to be a UK-based company called Autobotics. A bit confusing.

And yes, PTS is the proper agency to contact about radio interference issues. They're a government agency, they can prosecute offenders.

In California, at least, there are massive government credits for replacing lawn with other alternative landscaping choices. Perhaps replacing water-hungry landscaping with other lower-maintenance, water-efficient plants would be less net work, cost and environmental damage (e.g., smarter) than to engineer a solution to the wrong problem: continue enabling antiquated, excessive water consumption habits?

Consider that Denmark have so much rain that it's not a issue.

Is there any portion of the spectrum left over for them?

It reminds me of this recent article: http://www.wired.com/2015/04/irobot-lawnbot/

Discussed here: https://news.ycombinator.com/item?id=9388115

> I did what any sane person should do:

That actually would be to not get such a big lawn.

Or plant it as a wildflower meadow which encourages wildlife and plant species and is much less high maintenance.

Shameless plug for a mate http://www.daviddomoney.com/2015/03/13/how-to-grow-a-wildflo...

Yeah, my empathy meter registered at about the same level as when people reported their Apple watches malfunctioned when worn on their tattooed wrists.

I presume the yard is small enough that it does not need to be mowed 24x7x365. The simple solution is to unplug the guidance system when it is not guiding the lawnmower.

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