R89
The outputs from multiple valves may not be plumbed together.
Controlling them, do you need reed switches? Do you need to track what position the cylinder is in?
Finally, can you share some neat implementations of 3 position cylinders on FRC robots?
Ive never used one but im nearly certain they are used in power take offs to give your shifter a neutral allowing you to switch drive train power to another mechanism (ie climber)
here is a picture of 254s 2010 pto they used a three position cylinder i believe http://www.chiefdelphi.com/forums/showthread.php?t=88333&highlight=254+2010+gearbox
I made a cut-away of 3-position cylinder (attached) so you can see the plumbing. This is a bimba 3-position cyl.
If you pressurize the front (left in picture) port, the cylinder retracts all the way. If you pressurize the rear one, it goes to the middle position (because the rear rod has a shorter range of motion). It pushes the forward rod out, but not as far as it could. If you pressurize the middle port, the rod comes out all the way.
If you want to keep track of the position, Bimba sells an option to have a magnetic piston.
Looked at Bimba’s site today, and the random tech tip of the day was…
I don’t think your cutaway did it’s thing right. Because the way it looks, two of the ports would send air right into the rod, and not extend or retract anything.
I’m not sure how it is possible so send air “into” a cylinder rod. It looks like the the darkest gray in the picture (gradient) is the cavity in which the rods can slide back and forth, and the lightest gray is where air can enter, but not a rod. I think air can flow freely between the light gray and dark gray sections, and the boundary simply indicates where the rod stops.
You would plumb it just like any other actuator. What the rule is stating is that two solenoids cannot have their outputs merged to feed one actuator port. The simplest explanation I can think of at this time is to use 3 single-acting solenoids on a common manifold. You can use double-acting solenoids as well.
[deleted original comment because the next one below is WAY better and contains a graphic.]
True three positions cylinders like the one shown above have a few downsides that should be noted.
If you used two stacked equal-stroke double-acting cylinders, you would get three unique positions with two solenoids. Each solenoid has two positions, which really gives you four positions. When stacked, two of those positions are essentially duplicates of one another. One retracted, one extended, or one extended, one retracted. Basically the same thing relative to where they mount depending on stroke lengths. Obviously with different stroke lengths you could get 4 positions.
With the three position cylinder, you run in to an issue where the outer most cylinder rod cannot be held at nominal force in all three positions when loaded while both extending and retracting. See below:
https://dl.dropboxusercontent.com/u/30166861/three%20position%20cylinder.jpg
With some tricky software, you can make this work for most applications that do not need full force in both extension and retraction (such as a neutral position in a gearbox where minimal force is needed to hold position). If you need full force in extension and retraction in the middle position, you should use a different type of cylinder, or two double-acting cylinders stacked as described above.
Hope this helps.
Yeah, I did a pretty poor job describing what I meant.
The solid light grey areas (where you can see the cutaways of the threaded ports), don’t seem to allow the air from the ports to actually go anywhere (except for the port on the blind end of the cylinder).
If the cylinder is built like the diagram in Travis’ post shows, then there is clearance around the rod to allow airflow, but as it is, I’m not seeing it.
I’ve seen mention of tying two double-acting cylinders back to back a number of times. Does anyone have any photos or CAD of how they’ve actually safely implemented this (mechanically)?
Hope this is related enough to be on topic: Has anyone found a way other than a 3 position cylinder or two back to back cylinders to get 3+ stroke positions while still satisfying the rule about releasing all stored pressure with a single vent? Life would be so much easier with closed-center solenoids, but they trap air… Anyone try to pass inspection with multiple vent valves tied to one lever?
we used a 3 position cylinder on our 2011 robot to control the rotation of the arm that held tubes. it gave us one angle for the middle pegs and a different angle for the high pegs.
we used 2 double acting solenoids with the pressure port of one of them blocked. i don’t remember how we plumbed them, but i assume it was like the Bimba schematic shows.
The one drawback, as noted above, is that you just have 3 positions - fully closed, fully open, and one intermediate step. So with a 4+6, you would have 0, 4" and 10" extensions. you cannot do a 6" extension.
This is the inventor version of the robot, but you can see the cylinders.
Thanks Chris. How did you attach the rod pin of one to the head pin of the other, and prevent rotation / “buckling”?
1986 did this to change the angle of their shooter.
