There are 4-way 3-position solenoid valves available from SMC (and other valve suppliers) that should let you maintain any position of a pneumatic cylinder at other than end stops. Keep in mind that any load change on the cylinder rod will have a result on the cylinder rod position as this is not an active system but a passive one.
I say 'should maintain position' because we're about to test that with our first ever 3-position valves arriving today. The key is to get a 3-position valve with
closed centers which effectively closes off the exhaust ports of both sides of the cylinder when the valve is in the OFF position. That way you would be working against 60psi of pressure on both sides giving you a significant amount of position hold force in both directions depending on your bore diameter.
We hope to assemble a test bed tonight to experiment with these valves and I'll report our findings.
