7 Feb 2007 tbenedict   » (Master)

Two bits of news:

In the next ten days or so I'll be able to order an Orangutan-X2. My plan is to have Orangutan-lib ported to it as quickly as possible, and to make sure all future development on Orangutan-lib can run on all three Orangutan devices. This'll be fun!

The other bit of news is that after a lot of dinking around with some Parallax QTI sensors for my line follower, I finally figured out how the little beasties work. The datasheet for the QTI doesn't do a great job of explaining it. It took a fair bit of digging into the Parallax forums before I got a good handle on things:

Basically the QTI sensor is a photoreflector that feeds a charge integrator. The output pin of the QTI looks at the integrator, not the sensor itself. This is not typically how transistor-based sensors are wired up. I have my own theory as to why Parallax did things this way, but without tearing one apart I can't really test it.

Anyway, the post that unraveled this gave this advice for reading a QTI:

1 - Set your sensor's I/O line to output and bring it high.
2 - Wait 0.23ms for this to discharge the QTI's capacitor.
3 - Bring the sensor's I/O line low and switch to input. At this point the I/O line should be in tri-state mode, and present high impedence to the QTI. Essentially the QTI's capacitor is now isolated.
4 - Wait 0.23ms for the sensor to charge its capacitor.
5 - Read the I/O line through the ADC.

I really really wish Parallax had made this more clear in their documentation. I wouldn't have stumbled around as much, and I might not have even selected the QTI in the first place. I really was after a continuous analog device.

In any case my complaints have to end here. Parallax QTI sensors are in use on scads of robots, and they do a good job of keeping bots in the ring and following the line. I just wasn't using them correctly. And since I'm not using them on a Basic Stamp, I hadn't bothered to look at the sample code to see how they were reading them. My bad.

In any case Speedy is back on track, Orangutan-lib is still on track, and I might even be able to build out my 4WD mini sumo some day. Can't complain.


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