Hi Patrick:

In my opinion, polling and separate MCUs would be the most practical
based on the info provided. I do not know the various PICs but would be
looking the lowest cost with a UART capable of interrupts.

With nothing else to do, I would manually (in line) measure periods
between teeth to allow elimination of false signals (Capture does not
care about duration of controlling signals). At a min of 20mS per tooth
I would not even worry about compensating for PI servicing of UART - or
working around it.

Physical implementation would be a big issue - must be industrial and
more. For starters, probably RJ45 with boots into individual enclosures
per shaft to distribute power alongside RS422 (for easy isolation of bad
nodes) or RS485.

The operator interface would ideally be graphical - never seen one of
these machines but can imagine the need for an instant but rough picture
of what is going on.

David Lawrence
Rhombus

----- Original Message -----
From: Patrick B. Murphy <luke631@MTS.NET>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Tuesday, September 02, 2003 2:59 PM
Subject: [PIC:] How to monitor a dozen shafts?


> 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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