Author Topic: Axon Mote + Sharp IR... And Project Designer problem  (Read 587 times)

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Offline GertlexTopic starter

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Axon Mote + Sharp IR... And Project Designer problem
« on: December 02, 2011, 05:39:58 PM »
I decided to test my Sharp IR range finders with my Axon mote... Two issues so far in getting set up.

1.  Per the documentation, Sparkfun, and Project Designer, I should be supplying Vcc = 4.5 to 5 V.  Lower voltage should work though, and indeed, Admin includes this exact setup in his getting started tutorials for the Mote.  I'm guessing I can use 3.3V and the signals will just be proportionally lower voltage... If I used USB power, could I power them off the unregulated rail, instead?

2.  I can't generate an Axon Mote project with Sharp IR sensors because I can't tell Project Designer a 5V rail to use. (Webbot's the only one that can help here, really x) )
This is the same problem I was having with Mote + Motor drivers in Project designer a couple of months ago: http://www.societyofrobots.com/robotforum/index.php?topic=14465.msg105991#msg105991

Edit: I got around the second problem by modifying the Axon_Mote.brd file in ProjectDesigner.jar and adding it back in as Axon_MoteX.brd :)
« Last Edit: December 02, 2011, 06:32:08 PM by Gertlex »
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Offline knossos

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Re: Axon Mote + Sharp IR... And Project Designer problem
« Reply #1 on: December 02, 2011, 08:01:23 PM »
I have not personally tested the Sharp at 3.3v but it should work of the 3.3v rails.  Also it would work off the supply bus if powered by USB.  Don't forget this warning from the Mote getting started tutorial.
Quote
warning: Applying more than ~2V to the ADC will cause the Rx of the wireless transmitter to stop working. Some SharpIR rangefinders will output higher than 2V, so a voltage divider must be added to the sensor output.
"Never regret thy fall,
O Icarus of the fearless flight
For the greatest tragedy of them all
Is never to feel the burning light."
 
Oscar Wilde

Offline Admin

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Re: Axon Mote + Sharp IR... And Project Designer problem
« Reply #2 on: December 02, 2011, 08:34:47 PM »
Here is the thing . . . the Sharp IR's, when powered at 3.3V, give out voltages up to about 3V or so.

That's a problem, as the Axon Mote ADC cannot accept more than ~1.8V. That was a decision made by Atmel, not me. A very bad decision in my opinion. There are advantages to having 1.8V for other sensors, but that's for another day . . .

To get around this, you need to make your own voltage divider using two resistors. Since WebbotLib Project Designer doesn't know what resistors you are using, the calibration that's programmed into WebbotLib becomes useless. So Webbot and I decided not to support 3.3V Sharp IR's with WebbotLib.

See here for details:
http://www.societyofrobots.com/axon_mote/axon_mote_datasheet.shtml#voltage

Offline GertlexTopic starter

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Re: Axon Mote + Sharp IR... And Project Designer problem
« Reply #3 on: December 02, 2011, 08:38:23 PM »
Thanks for that reminder, as I would have made that mistake otherwise :)  Guess I'll start off with half voltage via voltage divider. (once I get my Pololu programmer working)
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Re: Axon Mote + Sharp IR... And Project Designer problem
« Reply #4 on: December 02, 2011, 09:11:40 PM »
A 1/2 divider might not be enough.

Power up your Sharp IR at 3.3V (using jumper wire) and determine the maximum output voltage. Then design your divider to make sure it doesn't exceed 1.6V. Above that and the accuracy gets a bit flaky. In software, you'll have to write your own calibration equation by experimenting and using Excel to give you a trend line equation.

What I did was make a little special reusable adapters that I can just plug in between the Sharp IR and the male headers of the Mote. I can't seem to find pictures of it, but I think you get the gist . . .

 


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