Opinions on using linear actuators?

Is your team using a linear actuator this year? Do you think it’s a viable plan for manipulating cargo (with a different mechanism for actually gripping the cones/cubes of course)?

Viable plan for …

Happy to give mechanical advice, just need to know about planned use cases.

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Post updated, thanks!

6045 will be attempting to use a custom linear actuator powered by a ball screw via a Max Planetary and a NEO.

cone_scoring_gif

This design will utilize a 16mm diameter ball screw from a CNC machine. They’re mass produced and therefore a lot cheaper than many other options. It’ll be direct driven via a hex shaft coupling on the Max Planetary side, and supported on the end via a bushing inside of a polycarbonate tube.

Math says the screw will need to output ~50 lbs for the 40" arm shown in the GIF above. Should be fine… but we’re testing it in the next week or so and I can report back results.

In general I’d be cautious of linear actuators. There’s a good chance ours doesn’t work… but that’s why we test early!

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My biggest concern is weight for an end effector, especially on the end of an arm. But that argument can be made for just about anything.

Some more hobby oriented linear servo options may not be up for the beating, so either designing in some rubber bushings for shock absorption or other ways to help isolate the actuator may be good for robustness.

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Looking into the array of actuators out there, I think it is hard to beat lead screws connected to motors from the FRC ecosystem for price, electrical compatibility, and mechanical compatibility. Even better get a wildly cheap ballscrew

Most ballscrews come with a bearing seat at each end. You will probably want to use that with thrust bearings (or combo) to keep the axial load off of your motor and coupler.

That goes for leadscrews too but you will likely need to put collars on or turn some bearing seats to take the axial load.

Linear actuators: good for linear things (especially where pneumatics don’t make sense). Not many teams design robots with non-pneumatic linear things, but they have a place.

Also linear actuators: absolutely terrible for 99% of arm designs; please I beg you resist the temptation of the linear actuator arm. I know it’s been a full student cycle since the last DART actuator arms but I promise they are a season-long ticket to depressionville.

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Besides the fact that gate swingers and desk lifters are not designed to be pistons, can you share the nature of the failures that were experienced?

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In a lot of cases it is easier to use a linkage to drive the linear movement. It’s amazing what a small polycarbonate linkage can do with a decently strong servo.

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Linear actuators can be very useful for moving arms etc. We have used them in the past but had to graft legal motors onto them. Also, if you search for them, there are some high speed units that are very interesting now that they are legal in stock configuration.

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We’ve got rotational binding at the upper and lower pivots (usually due to low overall rigidity, but it shouldn’t have been a problem), linear binding at the screw and guide/housing interface, and everything just exploding when they take a weird hit or end up being forcefully driven into an immovable object.

Chain drive arms solve all of these.
Why turn rotational motion into linear motion then back into rotational motion when it can just all stay rotational?

I am guessing chain phobia and backdrivability. I expect to steer my team toward a worm gear approach to backdrivability in our first arm pivot. Student made gear boxes don’t have the best track record though. Further down the arm I don’t know what they plan to do to remote the motors. I was trying to introduce capstans and push-pull boden pairs for the gripper and wrist. That is partly why I wrote

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Why not focus on reducing the loads on the arm with springs/gas shocks (i.e. make it “weightless”)? My experience with mechanisms that can’t back drive is one of annoyance. Should allow you to get away with smaller/lighter gearbox options too.

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Already there on the springs. I have limiting backdrivability in my back pocket if they end up burning some poor motor that is trying to hold position on a robot that is bouncing up and down as it drives.

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BTW, if you linked to the arm on the other side of the pivot the screw would be in tension instead of compression which would at least remove the risk of buckling.

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Oh the horror of seeing lead screws.

As a student on 1665 for the 2019 season, we decided not to go elevator (2018 was elevator, and we sucked) and went 4 bar. Lead screw on a redline with set screws to secure it. The part of the 4 bar which the lead screw was fixed to, weighed around 50lbs. 50lbs resting on the lead screw, on one motor. Needless to say, we were not careful and the 4 bar ended up going too far past the max limit and it resulted in a bent lead screw. We were able to replace it, but during our first competition, we were already seeing a slight bend in the screw, which was noticeable when the arm was operated.

Throughout the entire competition season, we could not get the arm to go to the exact height whenever we wanted. Getting it to go up meant it must go up “this high”, and you cannot tell it to come down just a tiny bit - it will kinda backdrive and hover up. Drive team was wrestling with the arm through all the matches we played. Not sure if even a slight bend in the screw directly caused in these problems, but i wouldnt doubt the possibility.

From my experience, i cannot recommend using a lead screw for heavy load bearing applications. There was a team that did use lead screws in 2020 (robot reveal, think there were 2 and used during climbing sequence) but as far as i can tell, it was not very heavy and i would not expect the screws to ever fail during the season.
If you or another team is able to use lead screws effectively, then good job. But please dont make the same mistake we did, and hinder the overall experience.

@Ryan_Swanson because i see that you’re attempting :slight_smile:

Edit: i suppose if you were able to counterbalance the arm, it would work better, not necessarily guaranteed. See Monkey Box #3 - Gravity Arm Counterbalancing - YouTube

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This is one of those cases when a few zipties and 5$ of latex surgical tubing could have offloaded that drive mechanism.

Edit: just saw your edit.

As others have said, linear actuators are not great for arms. When i was a student on 3452, we used 2 in 2016. To be fair, the arm was giant, wayyy too heavy, but any sort of shock impact resulted in having to rebuild the actuator, so nearly every match. We were using slightly modified (shorter length) DART actuators. I cant remember exactly what was happening, but I believe we were essentially destroying the bearing at the end, which caused it to bind. We fixed it by the time we got to worlds by making custom mounting plates that contained huge bearings.

BUT, in 2017, we used a custom linear actuator to open and close our claw for grabbing gears. That was a very beefy setup; 3/8 Acme lead screws i believe. We welded a left hand and right hand lead screw together, built up the center, and machined it to a 1/2" hex to be able to attach a pulley and spin it using a 775 pro through a versaplanetary. We absolutely mangled that arm multiple times, mainly the 1/2 hex that pivoted the entire arm, as well as the bottom stops for the gear that year, but we never had issues with that custom linear actuator setup.

Tl;dr: DARTs on big arm = bad. Custom, oversized setup for linear actuation = fantastic.

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Why physically resist impact with a stiff drive line when you could let the arm bounce and back drive the motor, and then spin the motor back to regain setpoint?