Expanding pneumatic cylinders to specific points

[Pyrotechnics]

So I know there are flow regulators to make it go slowly, but is there a way to stop the movement of the cylinder at a certain point? Like I would want it to expand to about a quarter length. Would this be possible by making it expand slowly using a flow regulator and just add delays for when I want it to stop expanding? For example if i wanted it to move only a quarter out I would add a 30ms delay so it would close it at the correct spot.

[iseau395]

To my knowledge this isn’t possible to do. You can add in a physical limit for how far the pneumatic can move, though this would probably cause the mechanism to strain. The best option is to just build your mechanism so the pneumatic expands to it’s full length.

[Connor_Howard]

No, that’s now how cylinders work. If you want to do something like this, I would recommend putting spacers on the extending shaft of the cylinder, so it only closes about 1/4 of the way.

[karmanyaahm]

Adding a physical stop, a piece that prevents it from going any further, would probably be the best and most accurate idea. It also saves air. You could possibly use string to limit how far the piston can go. Of course, that doesn’t allow for dynamic positioning.

[nenik]

If you’re trying to have more than 2 controlled positions available, the answer is pretty much no. You have the 2 endpoints, achieved by pushing air to one side or the other of the piston inside the cylinder.

In theory, with a double-acting pneumatic cylinder, the meter slowing the movement of the piston and two single-acting solenoids, you could turn the air on momentarily, then let it escape on both sides, which may leave the piston somewhere in the middle. But it would provide no further force, so if it was moving some mechanism that applied force (gravity, rubber band, …), that external force would move it again to one of the extremes.

But: now that we have a system with 2 independently controllable solenoids, there are 4 possible states of those solenoids, which we can use to somewhat extend available actions:

• One solenoid on, the other off: business as usual - the rod moves to one of the extremes and applies full available force (up to 50N for extension, about 45N for contaction)
• With both solenoids off, no pressure is applied on either side of the piston and you’re free to move the rod. The rod, with no external force, will stay put. Imagine having a set of rubber bands centering the attached mechanism position somewhere in the middle, say applying 20N. That way, your mechanism would gain a middle position. Turning on one of the solenoids again, you’d have to fight the centering mechanism, so your available force would get reduced, but you’d still have control over that.
• With both solenoids on - pressure is applied on both sides of the piston, but the generated force is slightly larger on one side, since the other side has the piston area reduced by the diameter of the rod. That way, you’ll get a cylinder that pushes towards the extended state, but with much lower force, say 5N. With clever rubber-banding, it could again create a different state.

My middle school team did employ the first 3 modes very successfully on their semi-active mogo clamp. In the all-off mode, the clamp used rubber band for rapid passive action, then adding pressure to one side to lock the mogo in strongly. Pressure to the other side was used to unlock the mogo.
They have tried to use the 4th mode as the weak force for the passive action (instead of a rubber band), but it turned out the pneumatic cylinder in such a mode acts as a shock absorber, being too slow to move.