Position control of pistons with simple solenoids

[viggy96]

Originally Posted by pribusin

There are 4-way 3-position solenoid valves available from SMC (and other valve suppliers) that should let you maintain any position of a pneumatic cylinder at other than end stops. Keep in mind that any load change on the cylinder rod will have a result on the cylinder rod position as this is not an active system but a passive one.

I say 'should maintain position' because we're about to test that with our first ever 3-position valves arriving today. The key is to get a 3-position valve with closed centers which effectively closes off the exhaust ports of both sides of the cylinder when the valve is in the OFF position. That way you would be working against 60psi of pressure on both sides giving you a significant amount of position hold force in both directions depending on your bore diameter.

We hope to assemble a test bed tonight to experiment with these valves and I'll report our findings.

That sounds perfect. Any idea if its any different to program one of those SMC valves versus normal solenoids? I really appreciate the information you've provided.

[pribusin]

The difference is only in our program logic. You have two solenoids that you need to activate alternately to make the piston move in either direction, just like with a normal 4-way double solenoid valve. But now you also have the option of stopping the piston mid-stroke by turning both solenoids off.

We didn't finish our valve test assembly last night and I have nothing to report today but tonight should give us results to report.

[FrankJ]

Interesting article. The IBM PC sort of dates it though. It would be a big consumer of air since the valves are constantly cycling. Pneumatic brakes are neither legal or illegal. You just need to use an legal pneumatic cylinder to actuate it. It can be an integral part of the brake as long as R77 J is met. The equivalent rule in past years was a lot tighter for passed for a pneumatic actuator.

A closed center valve works well for positioning assuming the dynamic forces aren't too high. The trouble is R89 requires the system to fully vent through one vent valve. R82 has an allowed working pressure of 60 PSI. A closed ended cylinder might exceed that. Since check valves are legal this year, you can comply with the rule by clever use of check valves to vent the closed ends of the cylinder back to the working pressure line. R90 will not be violated since you will be plumbing the outlet of the control valve to the inlet check valve.

[DonRotolo]

Just remember: Air is compressible. Put a load on your cylinder shaft and it will move.

In general, it is a best practice to use pneumatic cylinders for two positions only.

[pmangels17]

I don't know if you want to control the cylinder along a continuous length, or only have discrete points you want to reach. However, if you simply have more that two discrete points you want the cylinder to actuate to, there are a couple options. First, there are multi-position cylinders available, which may be a viable option. I have also seen teams successfully attach one cylinder to the end of another, such that two separate cylinders combine to be able to reach four different points. I would imagine that, given enough creativity, one could even stack two three-position cylinders together, or a few two-position cylinders, though this may require more custom parts, time, or weight. That being said, it is certainly a viable option if you don't need a continuously variable position.