How do i measure the frequency of a signal running round a wire loop.

Billericay-Boy

Hi, i have built a mobile platform with two powered wheels at the front and a caster type wheel at the back so that it will travel around. There are whiskers on the front so that when it hits an obsticle it will reverse, turn and then go forward again. I now want to contain it in an area without walls.

My mother has a Mowbot which picks up a sinusoidal signal from a burried wire in the lawn and I thought that I could find out the frequency her robot uses so that I could run my robot inside her perimiter. This way I can consentrate on building a receiver and get it working without having to build a transmitter at the same time.

My Question :-

How do i find out what the frequency of the signal running round her lawn wire is ?

Any help would be apresiated.

Mike Green

If you can't find it in the manual for the Mowbot, you could try to borrow a frequency counter, hook up a small loop (1 to 3 inches diameter, maybe 10 turns) and connect it to the input for the frequency counter and see if you can get a reading close to the buried loop.

Billericay-Boy

Thanks, i have borrowed a frequency counter and will make a simple coil and see what it picks up. I will post the results.

Billericay-Boy

Hi, i have been sitting on the lawn with the Frequency Counter and apart form a wet bottom i have the following readings.

Frequency Counter· Nikkai RP80
Coil· 5 turns of 2cm diameter

Orientation·· 300Mhz range···· 3GHz range
in line········· 17Mhz···············331Mhz
90%··········· 25Mhz·············· 300Mhz

I then turned OFF the power to the Mowbot system and the signals were a couple of Mhz lower on the 300Mhz range and 20Mhz lower on the 3Ghz setting.
but sitting at my·computer here,·i have turned it on and it reads

18MHz and 350Mhz

I am confused as to what the readings mean and wonder if all i picked up was background readings.

Any suggestions

Thanks

Richard


Post Edited (Billericay-Boy) : 3/22/2007 6:16:06 PM GMT

Mike Green

Hmmm. I was expecting something well under 1MHz, probably under 100KHz. What you got probably is indeed some kind of background, maybe even internal to the counter (although it should be somewhat shielded.)

Is there a lower range you could try? Maybe you could locate the generator for the Mowbot signal somewhere where it's dry and try measuring there?

Billericay-Boy

Unfortunately the 3ooMhz setting is as low as the borrowed counter goes. is there any other way to find the frequency. Would an oscilloscope find it ? assuming I can find one to borrow. or buy very cheap.

Any idea what sort of range ? i have seen plans for 50Khz to 70Khz and that is far lower than this is counter is reading.

Mike Green

An oscilloscope would be nice.

The range should be on the order of a foot or two. You don't want it to be so far that it would confuse the Mowbot, yet you want the wire to be able to be buried.

I can believe 50KHz to 70KHz.

Billericay-Boy

Thanks, my next quest is to find an oscilloscope.

I will post my results when i get one.

Any offers of a loan machine - Billericay - Essex - UK.

Richard

Billericay-Boy

Hi, i have just orderer the following so hope to receive it next week.

9999Hz LED DIGITAL frequency panel meter with 4-DIGIT RED led display

Display type :·· 4 Digits, Red LED Display
Operating Voltage :·· 5V DC / 500mA
Frequency Range :·· 0 ~ 9999Hz
Accuracy :·· ±1Hz·
Peak Voltage :·· <16V·
Input Waveform :·· Regular Periodic Waves
Installation Type :·· Flush Mounting
Dimensions :·· 79mm × 35mm × 27mm

so i should have some more readings next week.
Thanks
for the help.

Skywalker49

9999Hz range seems to be a bit low if previous messages are correct.
Did I miss something?

Mike writes :
The range should be on the order of a foot or two. You don't want it to be so far that it would confuse the Mowbot, yet you want the wire to be able to be buried.
I can believe 50KHz to 70KHz.

I don't understand what this foot or two etc. has to do the frequency of the signal.
Please Mike, can you explain.
Thanks,
Ed

Mike Green

Ed,
Higher frequencies would tend to be absorbed more by moist earth and would not be radiated well or evenly by wires on the order of several wavelengths. The lower frequencies would tend to be radiated more uniformly by a long wire and not be absorbed much by earth overlying the wire. There would also be a significant amount of inductive coupling to a small loop which would help with directionality (sensitive to small changes in direction and distance).

Skywalker49

Mike,

thank you for the explanation about behaviour at high / low frequencies but you lost me when you write that lower frequencies radiate
more uniformly by a long wire. Does this mean that EM radiation is depending on frequency (golf length) and wire length ? Makes
sense since similar ideas are behind antennas but .. the golf length of a 50KHz signal is 6000 meters and I have difficulty to
relate this length with the length of the wire for which this Mowbot is working fine.

Mike Green

When your radiator (the buried wire) is on the order of the wavelength of the signal, you'll get variation in the pickup along the wire as the electric field and magnetic field of the electromagnetic wave vary in relation to each other along the wire. When the wavelength of the signal is much longer than that of the wire, there's not much variation along its length.

Here's a starting point for information: en.wikipedia.org/wiki/Antenna_(radio)

Skywalker49

Mike,

thank you. That makes sense to me.

Regards, Ed

Billericay-Boy

Hi, i am back. i received my IBQ2006ST Frequency counter in the post this morning and took a reading of the signal coming from the perimeter wire.

the reading was fluctuating between 73Hz and 80Hz with the aerial in line with the wire and up to 83Hz with the aerial at 90 degrees to the wire.
Could there be 2 signals going down the wire one in the low 70's and one up at 80Hz ?

I suppose i will have to build a receiver that i can tune into the frequency between 70 & 80Hz.

Any suggestions or offers of help with PCBs would be accepted - i have plans from a guy's website (URL=http://www.betuwe.net/~mowbot/UDO5recvRcv.PDF] who has made one already.

Mike Green

The PCF8591 is an OK digital to analog converter, but is hard to interface to most Stamps and harder than needed for a Propeller. The ADC08034 is much easier to use, has its own voltage reference, and is very well documented for use with Parallax microcontrollers. You really don't need most of the circuit that you posted. The critical stuff is what's essentially a rectifier (diode) followed by a filter to change the AC from the detector loop into DC and smooth it out so it you get an average signal strength reading. The Stamp can actually read this itself without the ADC (see: www.emesystems.com where Tracy discusses measuring the battery voltage of a Stamp), but it's easier using the external ADC. Look at the tutorials in "What's a Microcontroller" and "Basic Analog and Digital" here: www.parallax.com/html_pages/edu/curriculum/sic_curriculum.asp.

Billericay-Boy

I have just obtained a broken Robomow which has a broken main board. i am going to try to understand how they have wired their wire sensors so i can try to make them work on my own lawn mower. if any one knows how they or their transmitter work then any information will be a help. It seems to have a coil (plastic casing labeled HANGBA...... shrink wrapped around it) then wires off to the main board. i think a magnifying glass and a note book for pcb tracing is required.