Linear Actuators

[Jared]

Originally Posted by JamesTerm

So I have a question for everyone who has used the lead screw technique. How did you do a set point operation... like if you want it to go a certain distance? Did you:

1. Use an encoder?
2. Use a potentiometer?
3. Use a limit switch?
4. String potentiometer?
4. Something else?


If you used an encoder how do you ensure it remains calibrated (e.g. limit switch). I'd like to use this lead screw technique, but I haven't found reliable way to sense distance. (I have not tested the string potentiometer either).

Lead screws are super accurate for distance. The McMaster ones are rated per turn to within 0.009" per foot of travel. We used two end of travel limit switches and an encoder, coupled to a gearbox shaft with surgical tubing.

[electroken]

Originally Posted by JamesTerm

So I have a question for everyone who has used the lead screw technique. How did you do a set point operation... like if you want it to go a certain distance? Did you:

1. Use an encoder?
2. Use a potentiometer?
3. Use a limit switch?
4. String potentiometer?
4. Something else?


If you used an encoder how do you ensure it remains calibrated (e.g. limit switch). I'd like to use this lead screw technique, but I haven't found reliable way to sense distance. (I have not tested the string potentiometer either).

Our setup used a 10-turn potentiometer, belt driven off the lead screw, which was in turn direct driven from the window motor.

[zinthorne]

In 2013 we were a tower shooter. We used a "linear actuator" to move our shooter up and down for fin movements. We got our off of a car seat. (They are used to move seats forward and backwards in cars) We took the old motor off and then made a plastic converter on the lathe to the versa planetary gearbox we were using with a bag cim. Overall it took only a few hours to fully assemble.

[JamesTerm]

Originally Posted by zinthorne

In 2013 we were a tower shooter. We used a "linear actuator" to move our shooter up and down for fin movements. We got our off of a car seat. (They are used to move seats forward and backwards in cars) We took the old motor off and then made a plastic converter on the lathe to the versa planetary gearbox we were using with a bag cim. Overall it took only a few hours to fully assemble.

That is very cool! I googled around to see what they look like and they look like they are self contained when retracted (e.g. like a pneumatic cylinder). If you have any pictures of this setup... please share... thanks!

[Nyle]

Originally Posted by JamesTerm

So I have a question for everyone who has used the lead screw technique. How did you do a set point operation... like if you want it to go a certain distance? Did you:

1. Use an encoder?
2. Use a potentiometer?
3. Use a limit switch?
4. String potentiometer?
4. Something else?


If you used an encoder how do you ensure it remains calibrated (e.g. limit switch). I'd like to use this lead screw technique, but I haven't found reliable way to sense distance. (I have not tested the string potentiometer either).

For a while in 2013 our launcher was articulated using a lead screw (at first , we later replaced it with a piston) and we used a few magnetic reed switches (they sense the proximity of a magnet) to sense when the screw was close to either end, as well as a set-point in the middle.

If you just need to move to certain set-points, and are ok not knowing exactly where you are when between them, using switches that trigger at the important points can be both reliable and easy to set up.

[zinthorne]

Originally Posted by JamesTerm

That is very cool! I googled around to see what they look like and they look like they are self contained when retracted (e.g. like a pneumatic cylinder). If you have any pictures of this setup... please share... thanks!

This is the only picture I could dig up of the part. I am sorry that I could not find a better one. I can ask for a better one from someone else on the team.

http://www.idleloop.com/frctracker/p.../2013/3398.jpg

[Pault]

Originally Posted by JamesTerm

So I have a question for everyone who has used the lead screw technique. How did you do a set point operation... like if you want it to go a certain distance? Did you:

1. Use an encoder?
2. Use a potentiometer?
3. Use a limit switch?
4. String potentiometer?
4. Something else?


If you used an encoder how do you ensure it remains calibrated (e.g. limit switch). I'd like to use this lead screw technique, but I haven't found reliable way to sense distance. (I have not tested the string potentiometer either).

We prefer string potentiometers for our linear actuators. Quite simply, they are the easiest sensor to implement, and they are accurate enough for nearly every FRC application.

[JamesTerm]

A friend pointed me to this link last night: http://www.mcmaster.com/#ball-screws/=sy3xrc ... while the parts look great the prices are terrible... it makes me wonder if it would be easier to make your own using parts from say home depot or something. <I'm going to search around a bit more as well>.

[magnets]

Originally Posted by JamesTerm

A friend pointed me to this link last night: http://www.mcmaster.com/#ball-screws/=sy3xrc ... while the parts look great the prices are terrible... it makes me wonder if it would be easier to make your own using parts from say home depot or something. <I'm going to search around a bit more as well>.

Ball screws are pretty overkill for FRC. McMaster's fast travel lead screws are more affordable, and are very smooth running. The brass nuts have very low friction.

http://www.mcmaster.com/#acme-precis...screws/=sy9ith

[pfreivald]

Originally Posted by magnets

Ball screws are pretty overkill for FRC. McMaster's fast travel lead screws are more affordable, and are very smooth running. The brass nuts have very low friction.

http://www.mcmaster.com/#acme-precis...screws/=sy9ith

This is what we used. Ball screws are just too expensive!

That said, you of course still want grease on those brass nuts.

[pribusin]

Originally Posted by Tom Line

I've been eyeing some of these models for some time:

http://www.servocity.com/html/12v_linear_actuators.html

I would love to have a drop-in electronic shifting option for gearboxes, and some of the lighter-duty short-throw actuators there have enough force to shift under load fairly quickly. They are pricey, however, and I haven't had time to find out what type of motor and shaft they are. I called servo city and they were unable to give me any more info than external dimensions, so someone would have to buy one to find out if any of our motors would mount in the same location.

Tom, We bought some similar to the ones you show in the link, but without the pot feedback. We modified the shaft of a mini CIM to accept the drive gear of the original motor. Works like a charm but still dead slow at 0.5"/sec. Would be ok for small adjustments that require precision and strength but if you wanted a 6" stroke for example, you'd wait 12 seconds - an eternity during a match. Still a very compact unit; easy to use.

[JamesTerm]

Originally Posted by pribusin

Tom, We bought some similar to the ones you show in the link, but without the pot feedback. We modified the shaft of a mini CIM to accept the drive gear of the original motor. Works like a charm but still dead slow at 0.5"/sec. Would be ok for small adjustments that require precision and strength but if you wanted a 6" stroke for example, you'd wait 12 seconds - an eternity during a match. Still a very compact unit; easy to use.

I also have been considering to use this model... Thanks for the feedback what would be great is to know how to make something similar to this buying separate parts that are affordable. From that one could buy a different lead screw like an 8 or 10 start. I'm having a hard time finding information about how to mount the motor onto the screw, and how this is self contained on the other end. I'd almost want to take one of these apart to see how they are doing it. (this is still research in progress)... Any insight to this would greatly be appreciated.

P.S. I was thinking of using this as a lead screw http://www.use-enco.com/CGI/INSRIT?P...&PMPXNO=942694

[Jared]

Originally Posted by JamesTerm

I also have been considering to use this model... Thanks for the feedback what would be great is to know how to make something similar to this buying separate parts that are affordable. From that one could buy a different lead screw like an 8 or 10 start. I'm having a hard time finding information about how to mount the motor onto the screw, and how this is self contained on the other end. I'd almost want to take one of these apart to see how they are doing it. (this is still research in progress)... Any insight to this would greatly be appreciated.

P.S. I was thinking of using this as a lead screw http://www.use-enco.com/CGI/INSRIT?P...&PMPXNO=942694

You don't have to drive the screw. If you drive the screw, you'll most likely need to support both ends with bearings and thrust bearings, and you'll need another bearing solution for the nut/carriage, which wants to twist. If you do drive the nut, you could probably mill a hex on the top/bottom and put a gear/versahub/sprocket on it.

You can also drive the nut, which is what we did on our climber. We found this to work well with the geometry of our climber, as the screws were non rotating and fixed to our climber arm.

We put the nut in an 1/8 inch wall aluminum box, drove it with a sprocket, and used two of these (http://www.mcmaster.com/#60715k11/=sysaez) bearings.

Multi start lead screws are awesome and can go pretty fast. We prototyped an 8 start climber that could lift the robot one level up in under 4 seconds, but were unable to use it due to other gearbox issues.

The material selection of both the nut and the screw are important too.

[JamesTerm]

Originally Posted by Jared

We put the nut in an 1/8 inch wall aluminum box, drove it with a sprocket, and used two of these (http://www.mcmaster.com/#60715k11/=sysaez) bearings.

I almost understand this and see the benefit of doing it this way... how exactly does the sprocket interact with the nut though? I know the vex #25 sprocket has a nice 1/2 inch Hex ID... does a sprocket like this mate with the driven nut in what you were saying?

[Jared]

Originally Posted by JamesTerm

I almost understand this and see the benefit of doing it this way... how exactly does the sprocket interact with the nut though? I know the vex #25 sprocket has a nice 1/2 inch Hex ID... does a sprocket like this mate with the driven nut in what you were saying?

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.

McMaster also sells flanges that can be screwed onto the end of the nuts too. You could also make your own if you have the ability to cut the threads. The 1/2" screw nuts have 15/16-16 threads on the outside.

If you aren't carrying a ton of weight, you could probably get away with using a few 8-32's to transfer the torque. You can buy "machinable" nuts that have room to add larger tapped holes to the end.