It’s not entirely clear what’s giving rise to that behavior; might be something with the electronics, might be some characteristic of your valve. I’m still working completely from imagination as to the nature of your system, so it’s difficult to say what might or might not work with confidence. (did you ever get a datasheet for that valve? if so, try sharing again…)
In the interest of gaining more info, I’d suggest grabbing a multimeter and checking a few things;
-
disconnect the coil and record output voltage from the driver circuit as a function of pot position. if you don’t get a linear relationship (e. g. 1/4 output at 1/4 turn, 1/2 @ 1/2, etc) between the two, there’s a good chance that either I’m imagining your circuit incorrectly or that something’s amok with the wiring.
-
Re-connect the coil and repeat the above measurement process. Note the point at which motion of the cylinders begins. If the output voltage of the circuit becomes grossly non-linear with a load (coil) attached, that’s instructive also.
-
If it’s not too much trouble (you’d need two meters…) record the current through the coil as a function of the applied voltage, again noting the point where motion begins.
If your valve has significant stiction or other non-linear behavior, it’ll be difficult to get a nice control behavior if it doesn’t provide you with some sort of feedback mechanism. But again, I’m still doing a lot of guessing here. The above measurements should help get a better handle on what’s going on, and hopefully avoid a few cycles of the buy-it/try-it cycle.