Find location of wire break in low voltage wire buried in garden grass

Question

I have a Flymo 1200R robotic grass lawn mower.

The auto mower detects the boundary position of the edge of the lawn through a low voltage electrical wire that is buried close to the lawn edge which emits some sort of electrical field that the auto mower detects.

This boundary wire runs around the edge of the lawn to contain the mower within the desired grass cutting area.

Boundary wire is about 210 meters long.

Wire breaks in this boundary wire stop the system working.

One way the wire can break is if a rabbit, for example, digs a hole over the boundary wire and then chews through the wire. The mower will then stop working.

Finding the location of a break can be a long, long, task involving lots of digging in different places where the wire is buried.

This boundary wire emits some sort of electrical field or radio signal while the auto mower is mowing. I am unable to ascertain what frequency is emitted after a fairly extensive google search and youtube search.

I have tried moving round the perimiter of the boundary wire, with an am radio, set to a few different frequencies, while one end of the boundary wire is disconnected from the mower docking station, and the other is still connected and transmitting, but this did not pick up any signal, from the wire at any location. This method seems to work for some people as a youtube search can show.

I can disconnect both ends of this boundary wire from the mower docking station and pulse my own signal if need be to aid detection.

Can anyone recommend a low cost signal generator and detector kit, or other method to easily detect the location of the break in the wire.

Answer 1

There are dedicated devices for that purpose.

I own a Fluke 2042 :

The right device is an RF emitter that you connects to your buried cable.

The left one is a detector that displays the strength of the detected signal.

It needs a little practice, but I am very confident that you could easily follow the buried cable and find out when the signal disappears. As shown in a picture of the user manual :

I found mine second hand on a well known auction site.

Answer 2

Can anyone recommend a low cost signal generator and detector kit,

Well if you want a cheap function generator, you can make your own for very low of a price, since you are in the UK ordering the required components from RS is a good option since they deliever most of their stuff within a day!

There are countless designs online, but try to look into oscillators and find which one suits your application best. Alternatively, you could just buy a cheap microcontroller (like the STM32 blue pill for as low as 2£) to generate your own pwm at your frequency and this does not require that much of a programming knowledge, generating PWM is usually the first example when going thorugh the MCU's documentation. However, if you are familiar with fourier transform, you would know a sqaure wave has different frequency components, but sine is only one peak, and makes for a easier system. For this reason, I think a Phase Shift Oscillator best matches your need since they don't need gain controll.

I must admit I am not aware if there are any standard ways of doing what you mentioned, but what I would personally do if I am done wtih digging, I would make the oscillator as mentioned, and a tuned reciever (This would be your antenna, connected to your notch filter, fixed at your generator's frequency), and walk along the path of the wire, seeing where the picked up signal has the lowest peak.

Alternatevely, you could purchase all the aformentioned devices and do the same thing.

I don't know how fimiliar you are with these stuff, but being able to carry them out does require some level of knwoeldge, but it won't be rocket science.

Answer 3

Easy and cheap way: I have used this method a couple of times, use a portable AM radio (an FM-band only radio can not be used)- You will hear a humming, when you are above the cable, listen for variations in the humming.... That is where your cable is broken/cut. no need for digging up the whole lawn..

There is also cheaper alternative to the Fluke instrument, have one for arround 10$.