Linear Actuators

[JamesTerm]

Originally Posted by Jared

http://www.mcmaster.com/#fast-travel-acme-nuts/=syswre
We used nuts identical to the ones at the top of that page. All the half inch multi start lead screws and their nuts can be purchased from amazon.com for a lot less. IIRC, they were from nook industries. To connect the nut to the sprocket, used a 14 tooth sprocket from McMaster, removed the hub and put a clearance hole for the lead screw.

To transfer the torque, we cut a small (0.1 thick?) groove about .25" deep on the diameter of the nut. We left behind part of the sprocket's hub, and it fit into this groove. There were also 2 4-40's that went into holes we tapped in the nut.

Thanks for reply as it has inspired my to try an experiment with 3D printing just to see how far I can push the envelope in that direction using nothing more than the VEX motor, which has about 9 inch-pounds of torque on the higher setting. In this picture I want to create a lead screw which encases itself within a cylinder casing, and the motor can drive a printed nut that is fused with a gear that is compatible with Vex gears. The pitch on the screw is going to be very large like one revolution per inch, and I may be able to have multiple helix (e.g. 2 or 4) cut grooves for the screw. I'm not sure yet of the optimal geometry of the groove profile, but I'll study the acme angles... This will end up being able to push weak load (probably less than 9 pounds) that has a quick time probably 2-4 seconds for full range motion. Since the Vex motor turns less than 2 revolutions per second the friction should be minimal... although ABS material does have a high melting point. This is probably a few months out to complete, but I'd like to share this experiment to see what people think about it and any advise on how to make it actually work.

Thanks again... wish me luck!

[BBray_T1296]

Originally Posted by JamesTerm

What internally is preventing the (what I presume to be) threaded rod from rotating inside the casing? It would have to be held radially stationary by some sort of slot it rides in which, if there, can't be seen.

[JamesTerm]

Originally Posted by BBray_T1296

What internally is preventing the (what I presume to be) threaded rod from rotating inside the casing? It would have to be held radially stationary by some sort of slot it rides in which, if there, can't be seen.

Yes... let me try to explain the proposed parts not yet posted:


Thruster Bearing | Geared nut | Thruster bearing All within the same inch where the vex gear would be. The thing I do not know yet is how far the screw can go before it needs support bearings when the rod gets retracted. I was thinking maybe 6" (halfway) of placing a bearing there.

I've been trying to learn from this video:

I know this video is not driving a geared nut, but it does give me some direction as to what has worked before.

Thanks for the feedback... any ideas about how to solve that would greatly be appreciated.

[JamesTerm]

I know it's been quite a while since I've last posted but finally I have something that works well:



Here you can see it in action:
https://www.dropbox.com/s/y49mpgqedi...84622.mp4?dl=0

and Here is where it is pushing the battery:
https://www.dropbox.com/s/h8bvnyu8ui...85045.mp4?dl=0

More media... shared here via ftp:
ftp://ftp.termstech.com/LeadScrewPics/
username sharedmedia@termstech.com
password SharedMedia!!!!

It is unfortunate that the 393 is no longer a legal motor for FRC, but we still intend to use it for another project, and intend to have it work fully with the roboRIO... there are 3 solutions we are currently exploring with that:

1. Find out what the real-world current draw of the motor is actually using and see if this is more than what the roboRIO can provide using a direct connection. I suspect it's under in most cases as it was on the Digital Side Car... but I also believe is issue is why they removed it from the list and probably won't put it back.

2. Use external battery... Vex Support has helped us find a solution that is sound. Doing this, and some simple wiring that will get the 7.2 volts we need (and correct current)... of course this would not be allowed in FRC

3. Use a custom voltage regulator with the PDP... this one seems to be the most robust assuming that we may pull more current than what the roboRIO can provide. We're looking to several possible voltage regulators and may try 2 parallel circuits of 2amp with 7 volts since these are common to find... still work-in-progress.


That said... it's back to the drawing board for FRC... we'll have to pick a motor that is on the list if we wish to have something... the problem there is the speed of the motor... so if you want to help us out... please reply with which motor you think would be ideal replacement. The ideal case is slower speed motors that have a lot of torque like the 393... from that we can try to figure out how to make it work.

This linear actuator shown here is the first revision... we intend to add the 10 turn potentiometer, and limit switches in the future revisions. Also the back half of actuator it is not shown in the picture this one spans 7 inches, but can go up to 12, by screwing on the back end of the appropriate size. It's lightweight because it is 3D printed, and very cost-effective solution. Will be nice to have something for FRC though.

[AllenGregoryIV]

This is a very cool little project, I like it a lot.

Originally Posted by JamesTerm

It is unfortunate that the 393 is no longer a legal motor for FRC, but we still intend to use it for another project, and intend to have it work fully with the roboRIO... there are 3 solutions we are currently exploring with that:

We ran the 393s off TalonSRs in previous years, you can probably just use a small speed controller like an RC one if you want to. We actually overvolted the motors to about 10 or 11 volts without issues but we were only having them turn about 100 degrees.

[JamesTerm]

Originally Posted by AllenGregoryIV

This is a very cool little project, I like it a lot.

We ran the 393s off TalonSRs in previous years, you can probably just use a small speed controller like an RC one if you want to. We actually overvolted the motors to about 10 or 11 volts without issues but we were only having them turn about 100 degrees.

Thanks for the feedback,
That is interesting to run 10-11 volts... but this will be turning up to 10 revolutions. We tested using a voltage regulator that shows how much current is being drawn (as well as set it for ~=7.2 volts)... using the higher voltage gives more torque, so it may be worth it to use... it really depends on the load requirements of the manipulator. If we can get by with 6 volts for lighter loads (checking the current as well) then a direct connection may be fine (will need to research that a bit more though).

[Ryan_Todd]

I can't believe that nobody has yet chimed in with the following information:

As of the 2015 season, there IS an FRC-legal COTS electric lead-screw linear actuator available via FIRST Choice!

http://firstchoicebyandymark.com/fc15-037



As an FRC alumnus & mentor working at Bosch, I actually helped to coordinate the first donation of these motors by making sure that the documentation was detailed enough for the FRC crowd. That first donation was somewhat smallish by motor count, so they went pretty quickly; given the positive reaction to this first-year offering, however, I wouldn't be surprised to see the donation increase for the 2016 season.

[AllenGregoryIV]

Originally Posted by Skywalkar

I can't believe that nobody has yet chimed in with the following information:

As of the 2015 season, there IS an FRC-legal COTS electric lead-screw linear actuator available via FIRST Choice!

http://firstchoicebyandymark.com/fc15-037

1/3" per second is pretty slow and there isn't an easy way to change that gear ratio, stroke, or mounting options like there would be with a custom printed one.

[Ryan_Todd]

Agreed! There are many benefits to designing an actuator specifically for the application at hand, and I have nothing but respect for the innovative approaches described above. I mean seriously, James' 3D-printed actuator is crazy awesome!

I just put two and two together, however, that the OP was actually a Bosch intern scouting out the potential demand for the self-same linear actuator that I posted above. In other words, the Bosch linear actuator was only donated as a FIRST Choice option-- and only became an allowable COTS motor-- because of the initial positive response in this thread that showed that FRC teams would indeed be interested in having that option available.

Pretty cool to see this come full circle like that.