Author Topic: RF Director  (Read 2306 times)

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Offline airman00Topic starter

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RF Director
« on: November 14, 2008, 09:18:31 AM »
I know this has been discussed in quite a few posts but the others are already too long or too much off topic. This should be a summary of those posts.

I have one RF transmitter and then maybe four RF receivers on the other end. Based on which RF receiver gets the best signal, I know in which general direction the RF transmitter is located . Will that work?

The intensity from the transmitter would be as follows. (Transmitter is on the center of the circle)
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Offline hudbrog

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Re: RF Director
« Reply #1 on: November 14, 2008, 11:02:35 AM »
Having omnidirectional transmitter antenna and narrow (like 90 degrees) receiver antenna (horn type for example) you will get what you want in open air. Indoors - lots of reflected signals, too low distances and stuff like that will decrease success rate drastically.
Plus it's going to be a pain to build proper receiver antennas.
I've been working on a project where we tried to determine distance and direction to "base station" using ISM band transceivers. We were able to get up to 70-80% success rate open air, 60-70% indoors. But there was serious science involved: each antenna was specifically designed, simulated, measured, redesigned... just a pain in the ass.

Best way for now - TI CC2431 with location engine, but still not as good as you would like it to be.

Offline airman00Topic starter

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Re: RF Director
« Reply #2 on: November 14, 2008, 02:55:13 PM »
All I need is direction. thats it.
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Offline HDL_CinC_Dragon

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Re: RF Director
« Reply #3 on: November 14, 2008, 10:03:39 PM »
triangulation is what youre going for, correct?
I think you need to plug all the transmission powers received into a little equation to get a direction in terms of degrees. Otherwise just going with whichever antenna has the most power will only give you 90 degrees of accuracy. But if you hooked 360 of them up in a circle, THEN you could take the one with the most power :P
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Offline ArcMan

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Re: RF Director
« Reply #4 on: November 15, 2008, 10:42:46 AM »
Having omnidirectional transmitter antenna and narrow (like 90 degrees) receiver antenna (horn type for example) you will get what you want in open air. Indoors - lots of reflected signals, too low distances and stuff like that will decrease success rate drastically.
Plus it's going to be a pain to build proper receiver antennas.
I've been working on a project where we tried to determine distance and direction to "base station" using ISM band transceivers. We were able to get up to 70-80% success rate open air, 60-70% indoors. But there was serious science involved: each antenna was specifically designed, simulated, measured, redesigned... just a pain in the ass.

Best way for now - TI CC2431 with location engine, but still not as good as you would like it to be.
That's good information.  I'm glad airman started this post.  It turns out I was thinking about the same thing for my yard robot so it could follow me around the yard.  It doesn't sound like you were very successful with the RF method.  Did you end up going with some other homing method that worked better (like IR).  I was thinking about IR, but I would be working in the yard turning, bending over, etc., and I didn't think IR would work very well.  I would have to have IR transmitters all over my body so that the robot could always "see" me.


Offline HDL_CinC_Dragon

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Re: RF Director
« Reply #5 on: November 15, 2008, 11:33:45 AM »
you could always carry a nuclear fuel rod in your back pocket, stick a geiger counter on the front of the bot, and tell it to look for the radiation on your rear end :P

I think the biggest problem you would have with IR is naturally occurring IR such as the sun. Thats why IR range finders arent as reliable outdoors. I havnt done anything with auto locating/following but perhaps something along the lines of sonar triangulation could work?
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Offline ArcMan

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Re: RF Director
« Reply #6 on: November 15, 2008, 04:42:15 PM »
Well, in my case, I'm not interested in triangulating my position or my robot's position.  I only want the robot to be able to pinpoint my direction.  Once it's done that, I could use sonar ranging to have it keep to the correct distance away.

Offline airman00Topic starter

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Re: RF Director
« Reply #7 on: November 15, 2008, 07:10:15 PM »
I want to stick with RF because you don't have to be within "view" of the source.

We were able to get up to 70-80% success rate open air, 60-70% indoors.
Also the success rate is based on what? How many successful packet transmissions in a second? What kind of test of success rate did you do?
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Offline hudbrog

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Re: RF Director
« Reply #8 on: November 16, 2008, 11:26:15 PM »
That's good information.  I'm glad airman started this post.  It turns out I was thinking about the same thing for my yard robot so it could follow me around the yard.  It doesn't sound like you were very successful with the RF method.  Did you end up going with some other homing method that worked better (like IR).  I was thinking about IR, but I would be working in the yard turning, bending over, etc., and I didn't think IR would work very well.  I would have to have IR transmitters all over my body so that the robot could always "see" me.
Well, after that project I've told myself that RF is not the thing to use for positioning of any kind, at least indoors. For homing it might just work, but still - to much work for not so much result. If I was doing some kind of homing application now.. well, solution would really depend on conditions it should work in. For indoors - IR or maybe CV+IR. Outdoors... doh.. RF =(
On that project RF was the only possible way.. and achieved success rate was enough for task. Success rate was kind of aggregated parameter based on several factors. Like difference between actual distance and detected, correct timing, passing security checks... well, it was pretty strange project.. closest thing you could imagine - car alarm and anti-theft system.. with much more security stuff =)
Anyway, transmission speed in terms of packets/second was not applicable.

airman00, you may actually try RF thing.. with rotating antenna for example(and good saw filters at least).. "base station" chip with omnidirectional antenna transmits any packets every millisecond or so.. you receive them, read RSSI, create a diagram of signal levels based on antenna angle.. if distance is too low, you will get maximum RSSI on any angle - that is the problem. To solve it you should choose RF chip with software programmable signal gain both for transmitter and receiver, and when you get plain maximum - you reduce gain on reciever (or send a command to transmitter to reduce gain).. and then try again. Still wont work on really low distances. I guess about 0.5-0.3m minimum.

 


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