Measuring the frequency of a signal in a wire

[billericayboy]

Thanks for the link. i have already seen this one but was told that inductive coils are not very reliable and that i should use a sine wave down a loop. i will give the inductive coil a go this weekend and see how i get on. i think it will be raining again so not planning to go out much.

Thanks for the help

BB

 

[Pommie]

In another thread someone (you?) stated that mowbot's wire could be canceled out by running it back the way it came. This suggests a magnetic signal. Mains wiring exhibits this behaviour if you separate the live (active) and neutral then you will get a magnetic field around the cables if a current is flowing. I would suggest that mowbot's signal is actually a short duration, large current pulse. This would tie in with the 80Hz reading you obtained. A way to verify this would be with a compass. The compass should line up perpendicular to the wire if it's pulsed DC or in line with it if it's AC. An electronic compass would also be a good way of detecting the wire.

HTH

Mike.

 

[billericayboy]

Hi Pommie, thanks for the insight.

Thinking back to when I asked the Mowbot people what the frequency of their signal was, they replied that they could not tell me as it was patented !!!

Thinking on what you have said I wonder if the Mowbot uses inductive guidance to stay within the boundary and to follow the wire home and if the special sine wave is just broadcast down the wire to ensure that the bot stops when power is lost or the wire breaks (and the magnetic field fails). If this is the case then it would make sense.
The one question that remains is how does the Mowbot know when it is outside the wire loop (as it does) or does it know that it has read an increasing then decreasing signal without a change of direction (perhaps using a accelerometer or some other system). If our Mowbot heads towards the wire on wet grass and carries on going forward when its wheels are in reverse due to the momentum it has it knows it has crossed the wire.

The Lego idea may be the way to go after all with a couple of other systems working in tandem. What do you think ?

BB

 

[Hero999]

If it was patented then there would be no reason why they shouldn't tell you.

They used the wrong words, they really meant that the frequency they use it's a trade secret.

 

[Pommie]

The fact that the bot knows which side of the wire it is suggests that it uses a pulsed DC current. Assuming that the current is flowing anticlockwise and we are inside the loop, as we (the bot) approach the wire we will see a pulsed north ahead and pointing upward. When we are directly over the wire, north will still be ahead but will now be level. Over the line, north will point down. If we approach from the other side then we will see south ahead.

A diagram may make it easier to understand.

**broken link removed**

Mike.

 

[mneary]

The fact that the bot knows which side of the wire it is suggests that it uses a pulsed DC current.

All the bot ever needs to know is never cross the wire. I'll bet if you place the bot on the outside of the perimiter it will stay outside.

 

[billericayboy]

The bot will not power up the drive or cutter motor outside of the wire loop and displays 'Perimeter error' so it knows it is outside of the wire.

i will check the wire with a compass and let you know what it does.

 

[Hero999]

Pulsed DC current is as good as AC as far as induction is concerned.

 

[tango190]

Guys - just remembered - a few years back (5+) Amateur Radio was allocated some experimental space on the 73KHz band.

They made up transmitters and receivers for experimental purposes, just wonder if anything published on the web would be of use.

Going for a Google ....... back later.

Bob

 

[tango190]

1. The fact that the bot knows which side of the wire it is suggests that it uses a pulsed DC current.
2.Assuming that the current is flowing anticlockwise and we are inside the loop, as we (the bot) approach the wire we will see a pulsed north ahead and pointing upward.
3. When we are directly over the wire, north will still be ahead but will now be level.
4. Over the line, north will point down.
5. If we approach from the other side then we will see south ahead.

A diagram may make it easier to understand. ......

**broken link removed**

Mike.

Mike if you have time could you expand on 3. and 4. above from your previous post.

I'm at a loss as to how the bot would detect the change in the mag field from ahead, level and down .....

I mean from the bot points of view it would be -
ok I see north ahead,
ok I see north ahead,
wow that norths getting strong ahead,
Strong - I'll say its strong - I'm going nowhere but North North North

Where'd it go ??? I lost North

Best I turn round a few degrees and see if I can pick up the trail ....

But in the middle of the lawn away from the edge poor old bot wouldn't be sniffing any North would he, so he then has to keep going until he picks up north again.

In trigger terms would this makes sense ....

1. Bot starts up from cold.
2. Bot says, I haven't had a sniff of north yet, best I keep moving forward.
3. Sniff Sniff - is that north I smell - I want move but I best save this sniff level.
4. Repeats 3. above storing the North "sniff" value until the last sniff was lower that the previous one - triggers a possible "wire detection" routine
5. Keeps sniffing and records values - if sniff level decreases and decreases then goes .... time to turn around.
6. Else .. sniff away

Mike, would my 5. above match up with your words on being level and pointing down regarding the "position" of North.

I'm nipping off to get a compass a battery and some wire .... happy days.

Bob

 

[Pommie]

If you place a compass on it's edge and approach the wire the compass will point up because the needle will try to be perpendicular to the wire. When we reach the wire the needle will be level (still perpendicular) and as we go away it will point down.

If you look at the above diagram and think that the compass needle will try and line up with the red lines, you should see what I mean.

Mike.

 

[billericayboy]

Having finally found my hiking compass i tested the perimeter wire and there was no disernable vaiation in the readings away from magnetic north or horizontal balance when holding it on it's side. I can only assume the field is too weak to pick up.