[Leo M]

Against my better judgment, I am going to try to ride this horse one more time.

I have been thinking about this problem, and in experiments with the solenoids and trying out the ideas in this thread we have determined that – exactly as the manufacturer states – the valves will not change state reliably unless there is 30 psig at the ‘P’ port on the valve. In trying to make this happen with the ‘P’ port connected to the exhausting side of the cylinder, the 30 psig is not always there, and the valve will not shift reliably.

In addition, we want to avoid any rule-breaking involving going over the 60 psig limit for working air pressure, and not have to use a cumbersome solution involving regulators or relief valves in several different parts of the pneumatic circuit.

It is possible to purchase three –position solenoid valves with the center position having all ports blocked, or having the outlet ports pressurized. However, there is some question in my mind about the legality of these valves – it appears to me, at least, that the pneumatics rules and the flow chart are not in precise agreement. We would prefer to avoid the discussion with the judges if at all possible.

I have come up with a compromise solution shown in the accompanying file. Using two double solenoids, it is possible to supply air to the ‘P’ port of each valve, plug the ‘B’ outlets, direct the ‘A’ outlet of one valve to the ‘extend’ port, and the ‘A’ outlet of the other valve to the ‘retract’ port on the cylinder. To extend the piston, shift the valve on the extend port to supply air to the cylinder, and the valve on the retract port to exhaust. The 60 psig supply air at the ‘P’ ports will ensure that the valves shift when commanded. To retract, reverse the positions of both valves. And, to stop in mid-position, shift the valves so that air is supplied to both ports on the cylinder.

In the event that a force is applied to the device moved by the piston, any increase in air pressure above 60 psig will be relieved by the main regulator. Otherwise, the cylinder will be more-or-less locked in place by the 60 psig on both sides of the piston. Push hard enough and it will move, of course, but it will be possible to stop in mid-position as desired, using unquestionably legal components. The 60 psig working air pressure limit will not be exceeded, and the use of extra regulators or relief valves is avoided.