> 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.

What response time to you need to shaft speed changes?  How often do you
need to send out updated shaft speeds?

200RPM with 16 counts/revolution is only one pulse about every 19mS.  This
is so slow a PIC should be able to handle 12 of these inputs at once.  A
20MHz PIC with enough input pins could easily read all the inputs once
every 200uS, which is once every 1000 instructions.  This allows you to
measure the period between teeth to 200uS resolution.  A little low pass
filtering and you get a sortof average of the recent few pulses.  I would
keep these 12 filtered shaft period values live with a background process.
The foreground code just considers these the official shaft periods
whenever it needs them.  At regular intervals, it grabs the appropriate
value, inverts it to make speed, then passes it on.  You could
automatically send the latest shaft speeds every 250mS or so to keep a
display live.  Depending on where these values are going, it may be
acceptable to send period instead of speed.

What are these values going to get used for that the existing display
isn't sufficient for?


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