4 May 2005 Nic   » (Master)

April 30th, 2005

Today is Saturday. Our site visit is on Tuesday. I've been working non-stop on getting basic waypoint-following down, but it just won't work. I can't get rid of the weaving. The problem is that it doesn't ever know exactly which direction it's pointing. It's all a matter of heading. An artical from Circuit Cellar at http://www.circuitcellar.com/library/print/0404/Miller165/3. htm describes the problem:

"A single-antenna GPS can provide heading information, but it is not instantaneous. It inherently lags the movement of the robot or vehicle because the derived heading requires previous position data. A GPS could not tell you where you are heading if you were to stop and change directions. Like compasses, GPS receivers do not require external reference heading calibration. Once moving, the GPS heading update rate is a maximum of approximately 1 Hz, although some receivers add damping, which increases this time constant even more. A dual-antenna GPS receiver can provide instantaneous heading‧'or yaw‧'information, although the recommended distance between the two antennas is 1 m. This fact, along with its large price tag, can be a limiting factor for many mobile robot applications."

Because of this I have been using dead reckoning unless the Expeditor has been going straight for a few seconds, but it's still not that good. Today I bought a $400 GPS with a built in compass and switched it out, but the compass always thought the computer's harddrives were north. I raised it up on a PVC pipe a few feet, and it gave a good heading, but IT STILL LAGGED. It was better though. A little. It still works at speeds about 2 MPH but not higher. I did a lot of other work today also. I have a lot more work to do in the next two days.

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