Advantages of a four bar mechanism vs a wrist?

Quick preface; I have experience designing robotic arms so the design and analysis of a wrist are pretty familiar to me.

We’re currently weighing the benefits between attaching our intake mechanism to a short four bar mechanism or having a wrist. We’re using a cascading elevator to help get to the positions we want.

The four bar seems nice in that it will allow us to mechanically control the angle of our intake mechanism, even allowing us to keep it parallel to all scoring locations if we so desire. I am worried about what will likely be a constant power draw from the motor supporting it though. I think the possibility may exist with a wrist to have the mechanism perfectly vertical and thusly draw very little to no power to hold position, but I am not sure it’s a guarantee. Both mechanisms seem like they can assist in moving game pieces to the proper heights, allowing us to lower the overall height of the elevator mechanism.

Is there some clear advantages and disadvantages that I’m missing? For what reasons should a team pursue a four bar linkage to actuate an intake?

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You raise a good point…we would need to make sure the end positions were where we needed them to be, but with the elevator that seems like we could make that work pretty easily.

You can use surgical tubing or some other sort of spring to reduce the load on the 4-bar.

You can use pneumatics to actuate the 4-bar.

You can put a disc brake on the 4-bar.

A 4-bar on top of an elevator seems silly to me. It’s been done before but to me, it seems like you’re putting a lot of cantilevered weight out on the elevator; not the best structural solution. Plus you’re just throwing more motors or pneumatics at the same goal- lift things.

A behavior (could be a pro, could be a con) of a 4-bar is that it will move the end outwards-and-then-inwards (movement in an arc) as it lifts.

The main difference between those two systems would be the angle of your intake system down vs up. The 4-bar system will keep your intake at the same angle, while the wrist will rotate it. This could be advantageous or a hindrance depending on your situation.

A 4-bar might work well for Cargo intakes this year as you want to place the Cargo horizontally; the same way you would intake from the floor. However, any Hatch Panel floor intake would hold it in the same orientation, which would need an extra sub-system to rotate it 90deg to be scored.

A wrist would work great with Hatch Panel floor intakes, but might not do so well with Cargo. You would either need to keep it down to score the Cargo (leaving it open to defensive hits) or raise it to bring it mostly back in your frame and have to score shooting the Cargo upwards.

Now you can compromise and do any combination of the two, or develop your sub-systems to work with whichever you go with.

I think the bigger question is… what problem are you trying to solve with these two options? Depending on the problem, one solution is likely “better” than the other.

While true in most applications, 4-bar systems do not necessitate parallel movement. That only holds true when the two opposite sets of arms are both equal in length.

There are some really, really interesting applications for non-parallel 4-bar linkages.


True, it’s another variable to play with.

You hit the nail on the head with what I was going to reply with next.

I should have included these details in my initial post; we are not intaking hatches from the ground. We will be collecting cargo both from the ground and the feeder stations, and hatches only from the feeder station. Therefore, we need some efficient extension and retraction in and out of our frame perimeter.

With our current climbing design, we have a limited amount of space for the elevator and intake mechanism on one side of the robot. Our current intake approach has both the cargo and hatches being manipulated in the same orientation.

Depending on your design of the four-bar (and your overall weight), you can do a counter-weight on the other side. If you’re worried about the extra weight, use a constant-force spring.

we’re working on a similar design, and looking at how to play with the 4 bar lengths to get the motion we want. We also have an almost working prototype of a 4 bar climber, using unequal length bars.

I don’t know if we’ll get the elevator done, though, so it might just be a low level 4 bar robot when we’re done. Hopefully one that climbs for an RP at the end of the match.

Then it probably depends on the exact design of your intake. If the intake is relatively short but sticks out a fair amount, rotating it up would get it out of the way easier. If it doesn’t stick out very much, but is taller, then rotating it may not work out, as you would run into your elevator. In that case, a 4 bar could be used to bring it up and inside the frame perimeter, or horizontal and sticking out further.