High Frequency beacon

Hi guys

I'm working on a lawn mower. I'd like it to precisely know its position in order to optimise its travel. As it is working outside and on a more than 500 square meters, the IR beacon (needs to remain in sight) , the GPS(not sufficient precision) won't fi my needs. Any idea like HF beacon ?

That is a lot of ground to

That is a lot of ground to cover! Especialy if you say you need better precision than the GPS can offer.

You could try to use a radio transmitter like the ones used for RC cars and planes. But then you’d still need to determine direction pretty accurately. If you could get a couple of those hooked up with a directional antenna and have the antenna on the beacon spin around broadcasting pulse signals; each signal telling the direction the beacon is pointed at at the current pulse.

The mower (hopefully) receives only the signals that are directed straight at it. Use two or three of those beacons and you can dertermine your position quite well. Each beacon telling the mower its bearing relative to the beacon.

Maybe this can be done the other way around by having a directional receiver on the bot spinning around and the beacons broadcasting their ID over the field. That way the bot gets a bearing on a couple of beacons and calculate its position.

I can’t think of any inexpensive easy solution.

Some vacuum cleaning robots and lawnmower robots use IR or laser fences. You just put them on the ground where you dont what the bot to go. Cross the IR or laser beam and either the fence signals to the mower it has crossed the line or the robot itself detects the fence.


500sq meters with IR? That
500sq meters with IR? That will need to be intensely bright. When I first read this post I thought it said "High Frequency Bacon"

500 sq m I think it’s too

500 sq m I think it’s too large for a positionning system based on IR, that 's why I’m thinkink of a radio system.

I was thinking of the mower sending a message " Clear to send beacon 1" then waiting for it’s response in order to calculate the distance between the mower and beacon1 and so on with beacon 2 and 3. At the end with these 3 circles, the mower could be able to compute it’s position. Not easy at first but has anyone ever compute a distance with such a system ?

I think the GPS precision won’t be enough as the mower requested precision has to be around 5 sq cm. The many best solution would be a kind of differential GPS with a beacon sending the error to the mower. not easier I think. But once more is there any one who has ever test this kind of solution ?



I think I’m wrong as the
I think I’m wrong as the time for the HF pulse would be too short to permit it’s simple computation : for 30m it would be only 0.1 micro second ! toos short formy PIC. Got to revert back to IR or US


I am thinking VOR. Ever played with (a) flightsimulator? Won’t be easy though.

Building a VOR station requires fancy electronics and/or a rotating directional aerial. For locational info you need two or three of them around the area. The geometry involved on your vehicle is probably too complex for a micro controller type brain.

As a pilot I should have
As a pilot I should have thought about it before ! In fact the distance measuring equipment is a DME but the precision won’t be enough as well. I’m thinking back to a I2C compass, US sensors and an “under ground” wire. So far I’m using only the wire and bumpers.

huh (that’s not an acronym, but an onomatopoeia) ?

Please update me on these acronyms: DME, US sensor.

I Get DME = distance measuring equipmen, but is that something that’s already on your mower, or already on your airplane?

(I am not a pilot.)

I don’t have any DME on my

I don’t have any DME on my mower : the DME is an UHF device that enables the aircraft to its distance to the beacon. most DME are collacated with a VOR which is a VHF device.

The ground base DME station is able to handle 100 aircrafts up to a range of around 200 Nm(Nautical Miles)

US stands for Ultra Sonic and IR for Infra Red


Ahh (another one)

I see. I remember reading about the DME when studying the principles of VOR. I did not pay attention because I always figured I would use multiple VORs and triangulate.


In fact a navigation system
In fact a navigation system prefers first the GPS and next come multiple DME and next only mutiple VOR. With the DME you get a circle from each DME station, the position is then at the intersection. When using the vor you get only a vector to the station, it seems easier but it’s not.

I agree to the solution of

I agree to the solution of the compass and the encoders on the wheels : the 2 things that are missing on my prototype !


beacon combos

I found this site today when googling for “robot beacon”. The site is by an active group in a Luxembourg school. It sports the Lego NXT throughout.

The page I’m linking here proposes to combine Ultra sound emitters that fire simultaniously around the target area. The triggering is by one of the beacons that sends an Infra red broadcast. The robot receives the both the light and the sounds and deducts its relative position based on the time differences between the received signals. Therefor this system utilizes the Time-of-Flight of sound through the air for location determanation.

A few advantages:
system does not need a beacon to be “reflecting” the bots broadcast
the bot does not need to look in the direction of the beacon
geometry is relatively mild on the MCU

The maker has developed his ideas all the way. The idea, the mechanics, the electronics (complete with pcb layouts in cad software) and programming (assembler PIC). That might be an advantage. If you’re willing to follow his footsteps exactly. I am favouring an adaptation to personal preferences. If there exist any in your lab.

As stated in my blog today, I have an interest in outdoors navigation. But as it is today, I am nowhere near starting a project like this. Anxious to learn about your adventures Mike!


This is probably too small

This is probably too small for what your’re doing, but this is what I’m looking to base my beacon nav system around.




As far as knowing where I’m at, I’d be looking at a bit of trig to find that out based on 2 ref points and a known heading(compass bearing). As long as you have the beacons at set distances, you should be able to calculate where you are.

since my plan was to use it more as way points, I won’t be going into the detail you may need.

Since ir isn’t going to give you super range, why not make it a grid with intermidiate beacons? transmit id’s for each beacon so you know which is which and create a (hash)table or array that contains the list of all beacons so you know where each should be.

This would give you quite a bit of what you need and you’d be able to pinpoint your location.

This looks to be a cool project btw…I’d love to see how things go with it… :slight_smile:




Many thanks for all your

Many thanks for all your comments ! I think we 've already evaluated many solutions and I think that the best outdoor solution would be a differential GPS that is : 1 GPS on the mower, 1 GPS on the base with a known reference position. The base station is then able to compute the tiny difference between the known position and the position given by its GPS. Then this difference signal is sent by radio to the mower whitch is then able to compute its own very precise position .

Very smart solution and reliable but I think a bit too much for my mower ! So I’ve decided to choose the compass and the underground wire.


I would possibly try

I would possibly try something like this:

The robot has a fairly high power US transmitter, and you put receivers on three corners of your yard. The difference in receive time from the robots ping should allow you to estimate the position. Probably through some matrix equation for finding the closest approxmiate solution. (I forget what it’s called) This will of course have to be RF’ed to the robot.

Possible extensions to this include:

Adding a fourth receiver way out of the plane of the other three, which should allow positioning in 3-space. Three receivers will give two possible solutions - one above the plane and one below. This may not be a problem if you always know which solution to pick - if say you have mounted the receivers on small towers to ensure “line-of-hearing”, the robot will always be below.

Adding more receivers will cut down on error, and may allow the receivers to be self-calibrated - ping one of them in turn, and with same method vaguely outlined above, figure their relative positions.

I don’t know the power requirements of doing something like this with ultrasound on this scale, and I haven’t actualle solved the equations for it, but it “feels” computable.


edit: fudge, the math seems alot more involved than i had hoped for…

Well, as an RF link is already required, the ping could be initiated remotely. That would make the times absolute, and the prolbem simple - none of this finding, the intersection point of circles with unknown radii. Actually the intersections of several hyperbolas. I still think it should be solvable, but definately not a linear problem.

The poster has not been on
The poster has not been on for 2 months.

Heh, my how time flies -
Heh, my how time flies - what happened to November I ask?

no grass
Maybe hmn44 is just moping about the grass not growing in november. Now his mower has got nothing to do. Real robotocists would buy Astroturf!

HII’m back for a short while


I’m back for a short while not to say I’m not working any more on my mower (the grass is still growing even in winter) but I’m spending all my free time on designing the main board. I’m using KICAD and freerouter. Both are very good and complete but need a long time for a beginner.

I hope I will be able to show the result before spring and fast growing grass!

As I said before, I’ve decided to take the option of the magnetic sensor and optical sensors on wheels.