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Re: Control systems and telemetry

Posted: Sun Jan 15, 2012 1:48 pm
by barbuduccle
Like so many others I grew up programming the ZX Spectrum and in latter years have taken to php/mysql programming and have dabbled a bit with PIC programming. Was very excited to hear of raspberry pi. I'm listing here my intentions in case others are interested in collaboration...

1) Greenhouse control system - monitoring temp/humidity etc and controlling watering and ventilation systems etc

2) Automated chicken house - when I get a chance I will try to get my chicken house automation web pages back online

3) Model rocketry - altitude/GPS

With each of these I'm interested in wireless telemetry system to receive data remotely as the projects won't be within wifi range of my main PC and there is no option for an ethernet cable. So some sort of radio comms will be necessary.

Re: Control systems and telemetry

Posted: Sun Jan 15, 2012 2:36 pm
by Ianw
Xbee Pro might be of use?

Can send serial up to 15 miles and should be pretty easy to interface on the RPi

It also depends on your budget, its also possible to send DTMF codes over small hand held radios using the sound port although the data-rate would be quite slow!

Or alternatively use a 3G/GPRS modem/dongle to send the data

Or use a Wifi router with a directional antenna – Using 2x Linksys WRT54GL and a RadioLabs backfire antenna it was possible to get 15Km line of sight across a valley on 802.10g

As for the rocket data, depending on the max relative velocity/altitude of the rocket, doppler shift and/or atmospheric effects may cause a problem so would be worth researching the device/frequencies as well, If your only talking a subsonic couple of hundred meters it shouldnt be a factor.

Any one else have some suggestions?

Re: Control systems and telemetry

Posted: Wed Jan 25, 2012 8:55 pm
by barbuduccle
Thanks for the suggestions Ian. I did wonder about wifi and directional antennae.   That's certainly an option for the for the static parts of the project but I was thinking it might work out more expensive. I was looking for a really low cost alternative. Likewise the xbee looks interesting but again seems rather expensive.

There are some rather cheeper looking data tranceivers such as this one for $7.90 which I shall investigate otherwise I have managed to find some dirt cheap transmitters/receivers which presumably would need some sort of data protocol to be worked out. These seem to be good for about 300 metres presumably line of sight.

The rocketry project is certainly low altitude (<1000ft) and there are some small specialised commercial units that can be added although they cost a bit. I usually strap a tiny half HD camera on the side of the rocket for some great launch videos. Would be nice to have a whole load of stuff operating off the raspberrypi pi during launch especially if it can beam it back to base. I like the idea of sending a raspi in to space!!

Re: Control systems and telemetry

Posted: Thu Jan 26, 2012 6:20 pm
by Ianw
For the radio protocol, if its not going to control alot and speed isnt an issue I would use DTMF pulses at X interval (X being a suitable time for the quality of the signal eg. 1 second or 10 ms spacings depending on the clarity/interference) and use the sound card to recieve the audio and a simple program to decode the audio DTMF.

As a suggestion I would make the first few pulses a timing signal so the PI can sync the pulses to a frame.

Basically using the DTMF tones its abit like a basic frequency division multiplex Vs. just a binary on/off meaning more data sent per audio pulse.

Im sure theres more complex protocols out there but if its just for turning something remote on/off etc then it would be a nice project to do.

I have seen a nice audio codec called Codec 2 used by Ham radio enthusiasts  perhaps it might be good to try that as well, especially if your looking at voice comms over serial:

Thats voice over 2500 Bits per second! would be nice to see if that could also be used for some kind of regular data transmission/compression over cheap radios!