Hi all,

I am a hobbyist who is trying to help a friend with a project. He
wants to monitor the speed of about a dozen shafts on his potato
digger. The shafts normally run around 200 rpm, but each shaft may
run at a different rpm. One or more shafts may even stop if the
digger is overloaded. I am wondering which approach you might
consider most feasible to monitor so many shafts. On each shaft is a
16 tooth sprocket and a hall effect sensor. I already have a 5.7"
LCD display from Amulet Technologies to display the speeds.

One idea I have is to use one PIC16F877 and a 16 to 1 multiplexer
(or two 8 to 1 multiplexers if a 16 to 1 doesn't exist) and measure
the pulse period of each shaft in turn. The trouble with this is if
a shaft is turning at, say, 30 rpm, it may take a long time to get a
reading. If I am correct, with a 4MHz clock and Capture Mode set to
every fourth rising edge, it would take a half second; 15 rpm would
take a full second. If more than one shaft is turning so slowly, I
think the time it would take to update all the shaft speeds would be
unacceptably long. If I set Capture Mode to occur on every rising
edge instead, this situation improves but I'm guessing my RPM's may
show a lot of fluctuation as the input frequency may not be stable.

Another idea is to dedicate a PIC that has a UART to each shaft and
poll each of them in turn. Those PIC's could either measure the
pulse periods or count the pulses. I have some PIC16F628's available
to try.

Any other ideas or advice? I've enjoyed reading the posts for a few
years now, and finally am finding the time to try a few projects.

Thanks in advance,
Patrick Murphy
James Valley Colony

P.S. I have put together an Excel spreadsheet showing Hertz to RPM
and pulse periods using the various Capture Modes and a 4MHz (or
other) clock. If anyone is interested in it, let me know off list
and I would be glad to e-mail a copy to you.

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