[Jeffrafa]

Multi-positioning is possible, as has been said. It can be done using two solenoids (controlling direction with one and exhaust with the other), or it can be done much more simply using a 3-position solenoid valve which springs to an all ports blocked position when neither solenoid valve is actuated. We have done this successfully and easily on both last year's and this year's robots. It can also then be done using only one spike instead of two.

I would defenitely not try to control this type of system with an analog imput from a joystick or whatever, but instead by digital imputs, since pneumatics are an on/off type device as has been mentioned earlier. The best way to control them is just with a three position switch (I'd tell you single pull double throw or whatever, but electronics and switches aren't my thing). Just have the switch actuate the cylinder each direction by pushing it one way or the other, then have the center positon leave the solenoid idle and stop the ram wherever it is. When done with a three position, dual solenoid valve wired to a single spike programming is easy - forward on the spike is one direction, reverse is the other, and idle is stopped. When done with two spikes and two solenoid valves things get a little more complicated because you need one spike to control the direction and you need the other to control whether or not the cylinder is actually moving.

One thing to note as far as multi-positioning with pneumatics is that the holding force is not all to impressive. When the cylinder is held in the center of its stroke it can be pushed in or pulled out rather easily, not to say it isn't a good way to have control over your device, just don't expect it to be rock solid when stopped anywhere along its travel.