Pneumatic Linkage Calculator

Hi,
We are trying to implement a pneumatics system on our robot for retracting and deploying our intake mechanism. I don’t specialize in pneumatics however I was wondering how we can make this pneumatic cylinder work. We want the retracted (forward) to be 90 and the extended (forward) to be -18. I am not sure how the system works, however, any information would help our pneumatics lead. Thanks

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We have been racking our brains trying to figure out a similar mechanism on our robot. So I will be watching this thread intently. In the mean time, could you tell me where you found the pneumatic linkage calculator?

https://arimb.github.io/AMB_Design_Spreadsheet/

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This thread from earlier this year has a few good answers. I linked a video from the 973 RAMP series that is a big help in designing linkages like this.

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So it seems you’re off to a good start. You’ve input the extended and retracted angles (0° and -18°), the X and Y distances (the coordinates of the cylinder’s attachment to the intake relative to the intake pivot point), and the cylinder’s retracted length and stroke. The calculator now tells you the A and B distances (the coordinates of the cylinder’s pivot point relative to the intake’s pivot point. You can verify that this is a valid configuration in the drawing on the right, showing the cylinder and intake in it’s retracted and extended states. You can see all of the variable definitions graphically in the picture under the input/output cells.

Really, at this point you should be all set to take these values and put them into what ever CAD program or design method you’re using. Can you be a bit more specific in your question with what’s confusing you and I can try to give a more detailed answer?

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I haven’t used the calculators, but here’s how I have designed this sort of thing:

  • Create a drawing which shows the “arm” at both limits of motion on the robot chassis, view from along the arm’s axis of rotation. (the angular distance between these limits is referred to below as angle_range). CAD is probably best, but with use of the group function, PowerPoint is adequate to the task.
  • Determine the required work to be done by the cylinder. Calculate the difference in the height of the CoG of your arm from one end to the other and multiply by the weight to get the amount of work required. (my preferred units here are pound-inches because of what happens in the next step). I’m assuming your arm doesn’t go “over the top”, in which case things get a bit more interesting.
  • Divide by half your working pressure (if your working pressure is 60psi, divide by 30psi). The half is to get around inevitable losses. If you used pound-inches and pounds-per-square-inch, the answer is a volume in cubic inches which is the theoretical minimum volume cylinder which can do twice the required work.
  • Find the stoutest (short stroke, large bore) cylinder with that volume or a bit larger. Why stout? It stays away from the action, and in my experience will be more robust when FRC happens. FRC prohibits things designed to damage robots, but it is definitely a full-contact sport in terms of collisions. Stout cylinders take this abuse better than slender ones.
  • Figure out the pivot-to-pivot length of that cylinder both extended and retracted. Create circles with radii of those two lengths in your drawing tool.
  • The absolute best place to put your pivot point (assuming it isn’t under the carpet or otherwise impossible) is to center the small circle on the lower arm at a distance of stroke_length /(2*sin(angle_range/2)) from the pivot, and the large circle on the upper arm at the same distance. The point where the two circles are tangent is where you mount it to the robot. Another not-too-bad place is to reverse the circle centers and pivot above. This has the disadvantage of using the cylinder to pull rather than push, which is somewhat less force because the bore area of the cylinder is reduced by the cross sectional area of the rod.
  • If neither of those works, move the circle centers outward along the arms, centering both circles the same distance away. Where those two circles cross are viable locations for the fixed pivot.
  • If you significantly increased your distance from the arm pivot, go back and figure the angles and see if you really have enough force to lift the arm with at least 50% safety margin. If not, go up to a larger diameter cylinder until you do.
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Here are some pics of our current intake extension. It uses a double slider linkage to get the correct positioning for the wall we will vector our balls against and a virtual 4 bar linkage for the rollers. It mounts on top of our chassis rails. I don’t know how much help it will be, but I figure it will provide some inspiration.



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