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I want to use hydraulic flow to proportionally vary the resistance of a rpi controlled exercise machine. The victim would rotate a positive displacement pump where the inlet and outlet ports are connected through one or more valves.

I would prefer to use a servo to vary the flow through a single valve. Integrated servo control valves on ebay are rather expensive. How practical would it be to attach a servo, say, to a plumbing shutoff valve?

Solenoid vales are cheap by comparison. I'm considering using only one as a pulse width modulator to vary the flow. I think it would need to be banged open and shut between 5-10 hz for a smooth flow. Would it likely hold up under that?

Alternatively, a couple could be ganged with harmonically arranged open flows between an outlet and an inlet manifold:

schematic

simulate this circuit – Schematic created using CircuitLab

Appreciate feedback/recommendations!

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    That seems pretty fast for a solenoid. Have you estimated the required stroke length? If is short it could go faster. And the return is via a spring typically.
    – IanJ
    Mar 9 at 1:59
  • About a year ago I played with solenoid valve and found them interesting. Then I wrote some 30+ posts to share my experience. You might be interested to skim through them: (1) Solenod Valve How To - tlfong01 rpi-org-forums: raspberrypi.org/forums/…, (2) Searched query: solenoid valve tlfong01, found 35 matches - rpi.org.ofrums: raspberrypi.org/forums/…. Cheers.
    – tlfong01
    Mar 9 at 3:43
  • You might like to watch my solenoid valve in action: Puroxd12 solenoid valve 2019jan1901:youtube.com/watch?v=h3DAcJ07mo4. Cheers.
    – tlfong01
    Mar 9 at 3:53

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