Quote:
Originally Posted by ebarker
fundamentals, fundamentals, fundamentals,
When using a Festo valve on a cylinder you can do a double acting cylinder super easy. solenoid on or solenoid off.
Question:
when using the SMC valves how do I do a single acting and a double acting cylinder arrangement ?
the documentation says you can 'pulse' the solenoid to make it actuate. Okay, so how do you de-actuate it ?
on a double deal, if you pulse both coils then both ends of the cylinder are connected to the hp air and the piston finds its 'home'. how do you de-actuate this ?
It is the pulsing that I don't understand. If it is pulse to open, what to close ?
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I can see how the word "pulsing" might cause confusion. First of all note that there is a slight difference between a double-acting valve and a double-acting cylinder. In the valves the SMC with "one end and two wires" and the Festo are single acting. That means you can hook the ground to the M- side of the spike and the red wire to the M+ side. When you make the spike go "forward" the valve activates. When you stop (or when you disable the robot or power down...) the single acting cylinder activates in the other direction. It requires power to stay in position and must always stay "on" in order to keep the cylinder locked in place. With the double acting solenoid valves (the SMC's with two ends and four wires) you have to actively signal the valve to change position. You can hook the two black wires to ground (not the spike, but direct to ground) then hook one red wire up to M+ and the other to M-. Now when you tell the spike to go "forward" the valve will activate in one direction. When you turn the spike off, or deactivate or power down, nothing will happen. You will have to tell the spike to go "reverse" in order to force the double-acting solenoid activate the cylinder in the opposite direction. This can be handy if you want an arm to stay in the position it is in at the end of the match, while the automatic "off" of the single acting valves allows us a margin of safety by automatically venting our pneumatic launchers for us at the end of a match.
I suspect the word "pulse" was used for the double acting valves because in comparison to the single-acting valves you only need to apply power briefly in one direction to get them to hold the cylinder in that position, while with the single-acting valves you require a continuous voltage.
A single acting cylinder is one that returns by itself to it's start position when pressure is relased. In commercial single acting cylinders this is achieved by placing a spring inside the cylinder. Apply air to one end, the cylinder extends. Vent that end and it retracts. Although we are restricted to double-acting cylinders in FIRST, by leaving one end of the cylinder open to atmosphere and providing an external force to return the cylinder to a start position you can achieve a similar effect. In this robot, for instance, retracting the cylinder rod will cause the arm to lift. It is not necessary to use air to extend the cylinder... gravity will do that nicely. So in this case just plug one of the outlets from your valve (you can either choose the one that is pressurized at power-down or when power is applied, if using a single-acting valve) and leave one end of the cylinder open to atmosphere.
If gravity won't do the trick of returning your cylinder for you... perhaps a nice bungee cord or surgical tubing will.
Jason
P.S. I just noted a typo in my previous post... when I said "33psi consumes the same amount of air and produces the same amount of force as a 1.5" cylinder at 33psi" I meant a 1.5" cylinder at 60 psi.