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[DPRG] Why high resolution encoders are desirable

Subject: [DPRG] Why high resolution encoders are desirable
From: Clay Timmons ctimmons at cadence.com
Date: Fri Aug 24 11:11:00 CDT 2001

Some discussion has come up about why so many encoder tics are needed.
The robot has to go 20 feet,  Do you really need 100 tics per inch?

Suppose you only have 1 tic per inch.  Assume your robot
can stop within 1 tic of deisired position.   You program
it to go 20 feet = 240 tics.   It goes 241 tics  (my algorithm
only overshoots, never undershoots).   Still the audience is
impressed since the average spectator's eye cannot
discern a 1 out of 240 error   (0.4 % error).

1 tic per inch seems to be plenty of accuracy.
Now lets talk about turns.   Say you program your
robot to do a 90 degree right turn.  Assume your
robot has a 12 inch wheel base.    Each wheel
must go (12 * pi)  /  4  = 9.42459 inches.
This would be 9 tics.   First of all you have
already introduced error by rounding to an
integer number of tics!   Next lets say you
robot overshoots to 10 tics.   10 tics =  95.49 degrees
9 tics = 85.95.   Either way your off by  5 out of 90 degrees
or 5.5% error!   Still the audience is probably impressed
untill your robot starts driving 10 feet.   Starting 5 degrees
off your robot will be 120 sin 5 = 10 inches off !
Make one or two more turns and your robot will
be hitting the walls!

Bottom line.  More accuracy is better!
Turns need much more accuracy than straight moves.


-Clay Timmons-

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