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[DPRG] Absolute reference for angular position
Subject: [DPRG] Absolute reference for angular position
From: Dale Wheat
dale at dalewheat.com
Date: Tue May 27 11:06:13 CDT 2008
I'm trying to calibrate Daphne's wheelbase dimension. Because the wheels are
not exactly round and the motor shafts are not exactly parallel, this is more of
an approximation than an actual measurement.
The method I'm using is to spin in place and measure the difference between
measured and actual rotation. To do this I need a very accurate method of
measuring the angular position. Daphne keeps up with what she thinks is her
current angular position using odometry. Comparing these two measurements will
help me determine the effective wheelbase dimension and allow me to increase the
accuracy of the odometry.
I'm thinking about stereo sonar as a reference point. Two identical sonar units
are aimed horizontally and diverge by 15 degrees or so. The sonar array should
return equal readings when the array is normal (i.e., 90 degrees) to the wall.
I believe dpa has used this technique to true up SR04 during long journeys such
as our Can-Can contest.
The only sonar module I've used before is the Devantech SRF04 module. It's a
simple trigger+echo interface. Unfortunately, I only have (or can only find)
one of them. I have several of the Mini-S modules but don't know how to use
them. They default to an automatic, fixed-distance operation. Also, they
require 12 volts and Daphne's battery is 7.4 volts.
I've also considered a small laser module and a pinpoint receiver.
Can anyone suggest an alternate method for measuring angular position with a
high degree of accuracy and repeatability? All I really need is an "index mark"
to tell me when the robot returns to a specific heading and not a continuous
readout of angular position.
Thanks,
Dale Wheat
(972) 486-1317
(877) DALE WHEAT
http://dalewheat.com
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