I have never used valves that work the way you describe, so I can't offer firsthand experience. What I meant by on/off is that you just turn a motor on and off and switch the polarity to slowly open or close the valve. If your flow rate changes gradually, this would actually be much easier to program. You just read the flow, say every second, and if flow is too high, turn valve on in closing direction. If flow is too low, reverse polarity. If flow is just right, turn valve off.
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