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Design >> RF Design >> Mono-directional DF loop for 'homing' a robot car
https://designers-guide.org/forum/YaBB.pl?num=1444354744

Message started by Old Steve on Oct 8^th, 2015, 6:39pm

Title: Mono-directional DF loop for 'homing' a robot car
Post by Old Steve on Oct 8^th, 2015, 6:39pm

Hi, this will be a little long-winded, but I'll keep it as brief as possible:-

I'm making a small Arduino-controlled robot car, and want to be able to make it home in on it's charger from a distance. Once close, I can use an ultrasonic sensor, then IR LED / sensor pair to get the car onto the charger 'ramp', but want to try to use the RSSI output from a 433MHz RF receiver to get into the general vicinity of the charger from further away, out of line-of-sight.

I'll use a 2kHz square-wave to modulate the transmitter carrier, so that it will be relatively easy to get a constant RSSI value at the receiver, independant of the actual data.

My plan, (so far), is to use a small DF loop attached to the receiver, but as I understand DF loops the car won't know if it's pointing directly towards or directly away from the transmitter.

My thought was to shield one side of the loop with a grounded tin shield, to provide a null area in that direction, so that the antenna is truly mono-directional.
I don't know a lot about RF, so don't even know if this is a feasible idea.

Can anyone provide advice on this? It would be very much appreciated.

Edit: And if there's a better way to go about this, I'm all ears....

... Old Steve.

Title: Re: Mono-directional DF loop for 'homing' a robot car
Post by loose-electron on Oct 9^th, 2015, 5:13pm

This is more of an antenna question - You may get better answers on an Amateur Radio forum or a hobby robotics site.

Suggest that you research "directional antenna arrays"

The ARRL (see arrl.org) may have something of use.

Title: Re: Mono-directional DF loop for 'homing' a robot car
Post by Old Steve on Oct 9^th, 2015, 5:23pm

Thank you for the reply, loose-electron. I'll check out arrl.org. I've checked out a couple of hobbyist robotics sites, but couldn't find anything suitable.
(I noticed that my question was comparatively low-tech when I looked at some of the other posts, but thought I'd try anyway. :) )

... Steve

Title: Re: Mono-directional DF loop for 'homing' a robot car
Post by loose-electron on Oct 14^th, 2015, 10:00am

also research "radio direction finder" - it is an older form of navigation that has largely been supplanted by GPS methods.

Title: Re: Mono-directional DF loop for 'homing' a robot car
Post by Old Steve on Oct 14^th, 2015, 10:47am

loose-electron wrote on Oct 14^th, 2015, 10:00am:

also research "radio direction finder" - it is an older form of navigation that has largely been supplanted by GPS methods.

I've done a fair bit of research and some testing since posting here. RF 'fox-hunting' provided some good leads, but I came up against a basic problem that I'd overlooked - noise. There's so much noise in the 433MHz band that the RSSI signal is always very strong whether or not I'm transmitting. Maybe someone more knowledgable in RF could get around this, but I couldn't.
In desperation, I dropped the RF idea in favour of 25kHz ultrasound. (I'm already using 40kHz for ranging, so had to go to 25kHz for homing.) A lot trickier for echoes, but far less background noise than the 433MHz RF that I was using. I even tried putting the whole receiver in a large, earthed biscuit tin and still got heaps of noise.
Thanks for your help anyway. I appreciate the fact that you went to the trouble to post again.