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[DPRG] encoder submission
Subject: [DPRG] encoder submission
From: Dan Miner
miner at centtech.com
Date: Tue Sep 25 19:34:56 CDT 2001
David,
The encoder article looks great! The custom encoder boards
>from Kip pushed me over the edge - I ordered 4 of the motors
today. :-) *If* we do a group build or buy of these boards,
put me down for 8 of them (if the price is low enough). I'll
use them on other motors too. I'm interested in fully built
boards OR just the blank boards.
A comment about how you measure speed. In section "C. Encoder
Segments", you discuss how you measure the number of encoder
counts per motor update period (100 msec or 50msec) and use
that to determine speed.
I do it a different way that you might benefit from. I use the
TIC (Timer Input Capture) function of my HC12 (also available
on HC11 and 68322 and many non-Motorola CPUs for those on the
list that don't know).
This gives me the number of CPU clocks (ECLKs) between encoder
signal edges. From that, I compute speed. My HC12 runs at
8MHz so an example is:
8000000 ECLKs/sec
----------------------- = 1190 encoder ticks/sec
6723 ECLKs/encoder tick
Actually, I keep a small circular buffer of these times and
compute a running average to determine average speed.
To be even more precise - I have a buffer of 4 encoder ticks.
When I get a new time, I subtract the oldest time in the buffer
>from the newest time. This automatically gives me the total
time of the 4 ticks (no summing required) and I just have to
shift right twice ( = divide by 4) to get the average time of
the 4 ticks. Then the newest time overwrites the oldest time
and I increment the pointer in the circular buffer.
The only "boundary case" is when the motor first starts moving
and the buffer isn't full yet. I handle this by looking at only
the most recent 2 edges. *IF* there is only 1 edge so far,
then I pick a default max_time of $00FFFFFF which translates
to roughly 0.5 encoder ticks per second.
This way, I still get a very accurate speed even at low speed.
- Dan Miner
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