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Feb 14, 2009, 10:11 PM
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Going where no XBee has gone before. Clock synchronisation


The reflected sonar works about as badly on the tri rotor at 3.3V as it did on the T-Rex at 5V. It can probably be used below 6ft in reflected mode with a lot of massaging. Unfortunately, you'll have to focus on unidirectional sonar for the full sized airframes.

The answer is no. There's no way 2 crystal oscillators are going to stay synchronized enough for unidirectional sonar to work. Clocks R accurate enough for digital audio players to sound synchronized, but not enough for sonar. U need a way to synchronise the clocks.

The way U do it is by broadcasting simultaneous radio beacons to all the microprocessors. Each microprocessor sends back its clock value when it got the beacon. Since the radio beacons all have mostly the same latency, the broadcaster can then compute the drift. U don't need to calibrate the sonar anymore with this either.

Since the drift is linear & constant, U can use least squares to get really accurate values, but we're sticking with averaging to get sonar tested as fast as possible & keep U entertained. The XBees can get the drift between each microprocessor to within 10 microseconds or 1/4" of sonar. The actual sonar transducers R going to be much more unstable.
Last edited by Jack Crossfire; Feb 15, 2009 at 02:46 AM.
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