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Re: Fertilizer pump fix...

>Thanks for the insight...  here is some more information if it would help.
>Its a FlexiFlow III Digital Peristaltic Pump used in hospitals for IV
>pumping.  It has an adjustable range from 0-300ml/hr as well as a fixed
>dose (0-999ml) setting.  The flow meter sensors seem to be contained in a
>black box shaped clip.  Perhaps it is meant to contain a disposable in-line
>plastic container, which makes me think further that its optical and >sensed
>fluid rather than shaft rotation.  I have already tried simply blocking >the
>sensors, but this didn't seem to work.  I'll bet your idea that its an
>infrared sensor is spot on.  I've got all the parts at home to build a
>555 based flasher, sans the infrared led (I'll bet rat shack has that).
>test your theory with my vcr remote and if that doesn't work, some
>mirroring with foil.  If its as simple as a flashing infrared led, I'm set.

There are simple flow indicators that use a little ball in a donut of
tubing to show flow. The ball goes zooming around in the donut when a fluid
goes through. While I doubt these are very precise, perhaps your pump is
expecting to see the pulsing an indicator like that would create and
interpret the pulse frequency as a flow rate. Precision systems use
feedback loops like this all the time to ensure that they are really doing
what they are expected to be doing. If that is the case, the frequency
would be crictical, since if it is set to be pumping 100 ml/hour and it
sees a frequency from the sensor that indicates it is doing 200 ml/hour it
would probably shut down.

>As far as it not being continuous duty, this is counter to what I >expected.
>I honestly expected that a hospital IV pump would be continuous duty,
>considering where and how its used.  Any idea what kind of duty cycle I
>should expect?  What it looks like to me is a circuit controlling a stepper
>motor connected to the pump head.  So, at 1ml/hr, the stepper motor duty
>would be VERY low, and I would expect that the circuit itself would have no
>duty-cycle problems.  OTOH, if I were to run it at 300ml/hr, the duty cycle
>would be very high, and that might be a problem.  I'm assuming the stepper
>motor duty cycle varies with its stepping rate, unless current is >continuous
>and used as a brake against reverse rotation.  Am I misunderstanding

Stepper motors can brake by shorting a winding or running steady current to
a winding. They function by activating windings in sequence to rotate the
shaft. Most of the metering pumps I've seen use synchronous motors that
have rotation rates that are linked to the line frequency of their power
supply. The synchronous motors usually can't stand to be running for long
periods due to heat, and thus the duty cycle limit. A unit built around a
stepper would probably be better able to run continuously.

BTW, typically with steppers the overall duty cycle of the motor as a whole
doesn't change with the speed, but the duty cycle for each winding does
(lower speed = higher duty cycle for each winding). Thinking about it
though, I would suspect a hospital grade device like you describe wouldn't
have a problem with running all the time since I would suspect that long
runtimes would be common in the device's intended operation. The whole duty
cyle / runtime thing is based on heat in the device though, so if your unit
doesn't get too hot in your application you should be fine.


Waveform Technology
UNIX Systems Administrator