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Subject: [DPRG] Propeller Reading Ecoders
From: John Abshier jabshier at kc.rr.com
Date: Tue May 13 08:22:36 CDT 2008

At the outdoor competition, someone asked me about the Propeller's ability
to read encoders.  The quadrature encoder object uses one cog (of 8).  From
the documentation:

 

REQUIRED CYCLES AND MAXIMUM RPM:

 

Encoder Reading Cog requires 144 + 50*(TotEnc-1) cycles per sample.  That
is: 144 for 1 encoder, 194 for 2 encoders, 894 for 16 encoders.

 

Conservative Maximum RPM of Highest Resolution Encoder = XINFreq *
PLLMultiplier / EncReaderCogCycles / 2 / MaxEncPulsesPerRevolution * 60

 

Example 1: Using a 4 MHz crystal, 8x internal multiplier, 16 encoders where
the highest resolution encoders is 1024 pulses per revolution:

           Max RPM = 4,000,000 * 8 / 894 / 2 / 1024 * 60 = 1,048 RPM

 

Example 2: Using same example above, but with only 2 encoders of 128 pulses
per revolution:

           Max RPM = 4,000,000 * 8 / 194 / 2 / 128 * 60 = 38,659 RPM

 

Note that most people run the propeller at 5 MHz with a 16x internal
multiplier.  Therefore, the numbers above are conservative by a factor of
over 2.

 

Thanks to David Anderson and everyone else for a great contest.

 

John Abshier

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