Can pneumatic cylinders be retracted midway through their extension?
Not unless you use multiple solenoid valves in series. It is possible, but a big time sink to tune it for the right lengths.
Even for double-acting cylinders?
Are you asking if you need to wait for the cylinder to fully extend before you retract it? Or if you can hold the cylinder at a certain length between fully extended and retracted?
The answers to those questions are both no.
I wasn’t all to clear in my question. The piston starts retracted and I send a command for it to extend. Midway through its extension (as it is moving outward), I would like it to move back inward. I don’t need it to hold a certain position. Just go up and come back down on command.
Yes, they can do that. Just switch the valve to the other direction.
Silly question… Why not use a shorter cylinder? Or are you aiming for variable lengths?
I’m aiming for variable lengths.
This can work - see 2910’s level 2 climbs from 2019
It’s pretty hard to control the midpoint position of a pneumatic cylinder. You can connect two cylinders end to end and control them with two solenoid valves. Here’s an example. We were able to do something similar by connecting two Bimba cylinders with a threaded coupling.
regardless of if it’s possible or not, I’m pretty sure that it’s illegal to have pistons only partially extend. Don’t quote me on that, but i’m like 85% sure
I’m really not sure where you got that impression from. Can you provide a source from the manual? It’s really not a good idea to say something is illegal without providing a source, and especially if you’re not sure.
It is definitely not illegal - my team (2733), 2910, and 1540 all used partial extension for lvl 2 climbs in 2019
You should try a linear servo
Though I will note that the partial extension was a hack—we were never designed for level 2 climbs, and it was a lot less consistent than I would have liked. With a large cylinder, every time we toggled the solenoid back and forth we’d dump a massive amount of air, so we’d only have a second or two to actually get on the platform before we’d vented our entire system.