single solenoid

[kmcclary]

Originally Posted by seanl

ok were having a problem with both our solenoids were using a festo and smc. the closest we got to get them to work was with the smc. when i turn the robot on the pump goes on and the pneumatic plugged in gets extended and when i flip the switch it goes down. we want the opposite to happen. we tried switching around evey thing the wires and the tubing and we cant figure out whats wrong. and the festo we have the input on 1 and the output on 2. the light turns "on" on the festo and the relay but nothing happens we tried switching stuff on this too and nothing. any ideas?

Be sure with the Festo that it is connected like this:

Festo Valve

Port # - Function
-----------------
Port 1 - Raw air input (35 PSI minimum to get the valve to work at all, 60 PSI maximum per FIRST "max working pressure" rule.)

Port 2 - "Normally Off" pressurized side - When off, this has pressure. Connect it to the port on the SHAFT end on the cylinder (through a flow control), and it will RETRACT when the robot is off, or the valve is unenergized. The flow control will control the EXTENSION SPEED (it is a "metered OUT" flow control). The input air pressure will control the MAX FORCE. Note that you need about 35 PSI to make it work at all.

Port 3 - AIR EXHAUST - leave disconnected

Port 4 - "Normally On" pressurized side - When ON, this has pressure. Connect it to the port on the PIVOT end of the cylinder (through another flow control), and it will EXTEND when the valve is energized. The flow control will control the RETRACTION SPEED (again, it is a "metered OUT" flow control). Also again, the input air pressure will control the MAX FORCE.
-----------------

To reverse the above behavior, swap the connections to the #2 and #4 ports.

Side note: You may notice that you will always have MORE force on extension, than on retraction. This is because the piston's disk is "shadowed" by the shaft on one side, but not on the other. This changes the "effective surface area" of the disk, changing the resulting force. (During our fall training, I like to make the calculation of this a student exercise.) To maximize force from a cylinder, always arrange your hardware to use the Extend function for the toughest job, not Retract.

NOTE: ALL valves in the FIRST KoP are "Piloted". You can look up the definition of that, but in practice this simply means they need a minimum pressure to make them work. Whenever the pressure drops enough (below the rated minimum pressure - typically somewhere about 30 PSI or so), then regardless of the coil's energization state all of the single coil valves (like the Festo) will switch off! The cylinders will then move to their "off state position".

These valves require pressure to operate the internal mechanism against the spring return. Trying to reduce the cylinder's force by dropping the regulator's pressure TOO low will cause the valves to "give up" and drop out, even though their indicator lights still say they're "on".

Having too low of a working pressure MAY explain why your cylinder's behavior didn't CHANGE when you turned the valve on (or off).

This behavior often surprises a lot of people that first experience these valves! I love to make a demo of this effect each fall, just to watch the new students' reactions.... (I first have them guess what'll happen when I turn the regulator's pressure down while the coil is ON, and then watch their faces when I do it. )

But, back to your problem...

The most common mistake with the Festo is flipping the thing over, causing you put the input air pressure in on the wrong port. Then, all bets are off! Make sure raw air is going into Port 1, and Ports 2 & 4 are connected to your cylinder.

If that is correct, then the most likely reason it's extending with the valve off is probably because you have the tubing to the cylinder ends reversed. Make sure that Port 2 is connected to the SHAFT end of the cylinder, and Port 4 is connedcted to the PIVOT end, and it'll retract when off, and extend when on. (<edit> These numbers are printed right on the valve's body... </edit>)

If THAT doesn't work, it's a switch wiring or programming logic problem.

Does this answer your question?

- Keith McClary
Chief Engineer, Team 1502 "Technical Difficulties" (AKA The Ec-Pneumatical Counsel"...)