RX Question

[talulah]

RX Question

Mark,

If you want to drive a 12V solenoid from 24V without it overheating, try just chopping the 24v supply to it at 50% duty cycle. The average voltage will then be 12V and the solenoid will not overheat. This assumes that the solenoid is already rated for continuous 12V operation (check that - some are rated only for a few seconds, or require a chopped/AC supply anyway).

The chopping frequency must be high enough so that the solenoid doesnt chatter on and off of course - a few kilohertz should be OK (less than 20kHz may give you problems with audible screaming of the solenoid - thats up to you!).

You can use a MOSFET or BJT to perform the chopping. If you need a circuit I can draw one, but you may be able to work that out yourself.

Congratulations on getting the range with a BEC - I consider you lucky with that!

[big_nipper]

RX Question

Paul, The solenoid is 24V but the compressor has a 12V motor. Same thing just need 10A at 12V but thats exactly what Ive now done. Got a PIC12C508A to get the signal from the RX and convert it to a PWM output through a MOSFET to the compressor motor. It also has a auto zero upon powerup for the RC input and a softstart function to reduce EMI from the motor. The pressure switch is then connected through an LED to another input of the PIC to light when upto pressure.

The PWM freq is only 2.08kHz but with 32 steps (only first 16 used upto 50% duty cycle) gives a smooth startup. This gives me 7uS for the PWM rountine and 8uS inbetween to read the Rx input with 16uS resolution. Now that was a challenge.

It seems to work ok so far.

[talulah]

RX Question

Mark, well done - that sounds good. Do you hear any audible noise from the 2kHz chopped drive?

[big_nipper]

RX Question

At low speeds yes. The output waveform shows clearly its in the discontinous mode but above around 10% the compressor is so noisy you cant tell anyway and it starts to enter the continuos mode. After its recieved 50 pulses within +/- 32uS it takes this as the zero position and theres a little subroutine that then sweeps from 200Hz to 1kHz and back with a 3% duty cycle to create an audiable tone to let me know its received this signal and is ready to turn on. To my suprise this actually moves the motor slightly but as its a compressor it doest matter.

If you would like a copy of the source code/circuit I can Email it to you.

[jim]

RX Question

Could I have a copy? Im just interested to see how you measure servo pulses. I did a similar thing a while ago, on the PIC16F871 but I should be able to follow the PIC12 code. EMail address is on my profile.

[kaideejee]

RX Question

Hehe I see a bandwagon approaching... one two three hup!

Could you also send me a copy please? Im currently thinking about how Im going to do this and other peoples ideas are always handy. Ill probably make my first version a direction controller rather than a speed controller (i.e. bang bang controls with mixing) but then Ill expand it into speed control too.

-- Kev

[kaideejee]

RX Question

Oh btw my email address is kevdgill at warm letters dot com. Thats my MSN ID as well if anyone wants to add me.

(Id like to see an email address spider interpret THAT one successfully.)

-- Kev

[big_nipper]

RX Question

I bet the spiders do have trouble with it. I cant send to it either. Try this instead:

ftp://81.106.67.7/Little%20Axe/Little%20Axe.zipftp://81.106.67.7/Little%20Axe/Little%20Axe.zip

Hope it works.

[kaideejee]

RX Question

Mark - Warm Letters = Hot Mail...? Sorry maybe a bit obscure.

-- Kev

[kaideejee]

RX Question

Ive managed to download it from the ftp link and Ill have a peruse at some point. Cheers.

[jim]

RX Question

That went right over my head too Kev! Anyway, got the file via email, thanks Mark. It look reasonably similar to PIC16F code, Ill have a proper look at it tonight.

[kaideejee]

RX Question

Hehe maybe I was in just too much of a weirdo mood (even for me) when I wrote that.

-- Kev

[fluppet]

RX Question

Speaking of embedded controllers (and since I cant think of a better place in the forum to mention this), Ive been browsing through my copy of the May 2004 edition of Doctor Dobbs Journal, and noted that it has a feature on using Game Boy Advances for nefarious purposes (Ive not yet read the article, but they seemed to be using one as an oscilloscope and a spectrum analyser).

As a complete system, they would seem like a cunning plan for a top-level receiver/control unit (assuming I stick some PICs in the way to handle the low-level stuff) - although I dont know to what extent a GBA would hold together in combat, even well padded and with the LCD (possibly) removed. I know my way round ARM processors, and using one in this form seems a reasonable alternative to soldering everything together from scratch.

Anyway, I just thought Id mention it, in case anyone found it interesting. (Im unsure whether its still on sale, but being a US mag its probably running behind in the UK.)

--
Fluppet

[talulah]

RX Question

Andrew, does the writer of the article use the existing operating system in the GBA, or is the existing software completely thrown away?

I dont suppose the existing operating system is particularly suited to real-time stuff, and the graphics drivers are probably a bit overkill for our requirements!

[fluppet]

RX Question

Sorry about the delay. Ive now read the article, and its not entirely clear - they use an Xport (which sits in the cartridge slot and adds an FPGA and a promising number of i/o lines), but other than saying they use some gcc libraries to do text its not clear what of the OS they use.

Chatting to a friend whos implemented an infocom interpreter for the GBA, he says what operating system? You can apparently set up interrupt handlers for some things; I dont know whether this means the bottom page of memory is in RAM and you can set the entire interrupt handler from scratch (as you can on Acorn boxes) or whether they go through a dispatch routine, which would add a bit of latency. Doesnt sound like theres context switching to worry about, though; Im not sure whether you can turn the screen off to reduce bandwidth requirements, but Id have thought you could (if it makes a noticable difference).

It would be an interesting thing with which to experiment, and probably less of a faff than getting the two SA1100 samples Ive got soldered onto something useful (theyre uBGA). Even a 15MHz ARM 7TDM variant ought to be plenty to control a robot, given some PIC assistance - although I may be pushing my luck with using 802.11 as a control protocol without some serious buffering.

--
Fluppet