Position control of pistons with simple solenoids

[Kevin Sevcik]

I think it the main hurdle in frc is the fact that they used a linear slide with a pneumatic brake. I'm not certain the brake would be a legal pneumatic actuator. The other difficulty is the PWM period of 16ms. You'd have to get sub millisecond update rates out of the pneumatic control module, and I don't think that's possible. After that, it's sourcing a fast enough solenoid valve, and everything else is frc legal. If the pneumatic brake was legal, then I think you could have done this on a cRIO controller.

[BoilerMentor]

In 2013 1747 attempted to build a pneumatic climber. We used a fairly complex, but available set of manifold style solenoids from SMC. The climber ultimately did not work due to significant mechanical issues that were encountered, but the pneumatic end of the climber was successful. The climber was ultimately ditched after a single regional event spent playing defense the whole time and never hanging for a Frisbee shooter and a simple low bar pneumatic hang.

IIRC there was a double acting solenoid and two single acting solenoids per cylinder. The double acting valve controlled direction and the single acting valves were basically an on/off for air supply to the cylinder. What this resulted in was the ability to select a direction (extend or retract) and then either allow flow in to and out of the cylinder or cut that flow off at both ports which maintained the position of the pneumatic as long as the load on the cylinder didn't change. This wasn't instrumented with something like a string pot or linear pot, so I don't know what accuracy was, but I suspect with appropriate flow control it could be accomplished.

In hind sight, I'm wondering if this is/was legal considering the single-solenoid-output-per-cylinder rule, but maybe since they were in series it wasn't an issue. I know we had tons of questions, as you'd expect with a 5 gallon aluminum air tank on the robot.

I wish I had a video to share, but that robot, pre-Frisbee shooter, is our robot-which-shall-not-be-named.

[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.

[JCharlton]

A closed centre valve is a good solution to your problem, however beware R89 which requires a pressure vent plug to release all stored air. With a closed centre pressurised air would remain trapped in your cylinder.

In the past I think there have been rules saying something along the lines that if a mechanism could generate pressures higher than 60 psi (by an external force) you would have to have a way to relieve that pressure. I don't see that in the 2016 rules, but beware.