We have our intake that folds out with solenoids. are issue occurs when we retract we are worried that they are going to continuously retract until fully retracted. Is there a way to only make them retract so far or limit them?
You can add spacers onto the stroke, however that spaces your start position as well.
pneumatic cylinders will drive until a hard stop is reached.
That hard stop could either be the end of travel (of the cylinder) or a restricted range of motion for the mechanism.
If you incorporate hard stops into your mechanism design, you can restrict the range of travel. Does this make sense?
If you’re using a specific type of cylinder, you can employ a magnetic reed (limit) switch. Bimba makes these with a magnetic piston in the cylinder and supposedly they will allow you to stop the motion part way through. We have a couple of them around the shop (along with the reed switches) but have never gotten them to work successfully. As far as I know this is the only way to get a cylinder to stop itself other than at full extension or retraction.
In general, I’d say you were probably better off figuring out how to make a deployable hard stop that will let you drop your intake from start position and then only retract part-way. Use something like a spring-catch or pawl that will be held back by a part of the intake when it’s in start position but then springs/falls out to act as a stop once the intake is deployed for the first time.
It is just important to remember that pneumatics are not hydraulics. Similar on the surface? Yes. But compressible fluids… well 
You could have a spring loaded stop “lever”, attached to the yoke at one end through the pin, and drops down over the piston rod once it is extended, effectively limiting the stroke. Single deployment though.
Another option is to put 2 cylinders in line…
If you use 2 different lengths, you gain 4 position control depending if you activate Piston A, Piston B, Both, or None. You can also play games with the amount for force developed, (i.e. you are lifting a long lever arm (class 3), you need different amounts of force to lift it against gravity as its position changes along the sine curve. So in this case I would put a larger bore, short stroke (piston A), and a smaller bore, linger stroke piston (piston B) together. I world activate A B B A to lift then lower the arm.). You can also mess around with acceleration when you have multiple cylinders in a stack like that.
You can use a three position valve with a closed center. That will stop the cylinder at mid stroke. You will run into issues at inspection because the single point dump valve will not release the cylinder pressure. You can fix that with the appropriate use of check valves.
This would limit the effective (holding) force produced by the cylinder, no? I am not sure how “positively” this intake needs to be held out/in.
At the ends of the travel, the force is the same. At intermediate points, it somewhat squishy. Generally works well enough for a intake where the squishyness might be a feature. If you had feedback you could implement position control, but with pneumatics, that can be problematic.
That’s what I figured. If I ever do pneumatics again and need positional control I will just stack 2-3 cylinders of different lengths. Should be enough to get course positioning.
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