Limitless Potentiometers

[987HighRoller]

Hi,
I've been experimenting with limitless potentiometers....and have found them to be a little unreliable. Has anyone else used infinite rotation potentiometers?? If so, what kind and were they reliable? We are just trying to find another sensor other than a 10 rotation potentiometer for rotation measurement. This is because we had something like 8 potentiometers break on us last year, mainly because they get locked inside due to load on the shaft . We were also thinking of encoders but are afraid of using too many interrupts and messing with the interrupts already used by sensors like the camera and gyros. Any advise?
Thanks

[JesseK]

There are a couple of different ways to approach rotation sensing, and one you might not have thought about is a light sensor combined with a state machine. A couple of pictures of this are in the 2006 FIRST design book.

Take a light sensor that's not mounted on the rotation and have it reflect off a plate that is mounted on the rotation (or vice versa). Where you want the rotation to stop, mark it with a black sharpie. It's literally that simple, but at the same time it's not as robust as other sensing methods.

Another way to do this, if you're trying to measure drive train ticks, is to put a mark parallel to the axle shaft on a shiny part of one of your rotational pieces. This will give you only one interrupt per rotation. If you want it to be more accurate, put 2 marks to give you 2 interrupts per rotation.

The other thing you could do is ask for pictures of how others mounted their potentiometers. We only had 1 pot on our bot in 2007, but it never broke. It was mounted to the inside of 1/4 the length of a hinge, so the hinge bolt itself took most of the load.

[popo308]

We used encoders on are drive train for the wheel rotation. It worked fine and never broke. It was also very reliable. However we had a pot on the arm and that broke a lot we went through quite a few of those. Mainly because it was not protected very well though. When it was working it did not give very consistent readings though

[Leav]

Here are my tips regarding potentiometers:

• Use the best quality pots you can get - 0.25$ pots wont perform as well as the 3-4$ pots... it's a small amount that's worth it.
• mount the pots securely making sure they cannot move.
• pots should not be load bearing parts. use them in a place where they will not be damaged by the load applied to your robot.
• remeber that if you need accuracy it is best to use a small gearing system that should make the pot turn more, that way you are minimizing the effect of the looseness in the pot itself.

that's about it I guess.... good luck!

-Leav

[popo308]

Originally Posted by Leav

Here are my tips regarding potentiometers:

• Use the best quality pots you can get - 0.25$ pots wont perform as well as the 3-4$ pots... it's a small amount that's worth it.
• mount the pots securely making sure they cannot move.
• pots should not be load bearing parts. use them in a place where they will not be damaged by the load applied to your robot.
• remeber that if you need accuracy it is best to use a small gearing system that should make the pot turn more, that way you are minimizing the effect of the looseness in the pot itself.

I would have to agree! Get really good pots and be sure they are not bearing load!!!

Also a good pot mount with a protector always makes things nicer!

[Qbranch]

Uhm, can a moderator merge this thread into the other identically named Limitless Potentiometers thread?

-q

[BanksKid]

i must say that there cant possible be anything at all wrong with endless pot. ........(but seriously my big problem with them in the past is defining a definite start and end....i dont like them too much...)

[Bruce Newendorp]

We use Digi-Key part 6639S-1-103-ND for our arms and linkages. It is a Bourns Series 6639 potentiometer. It provides electrical output over 340 degrees of rotation but there are no mechanical stops in it. That means it is easy to install and calibrate since you do not need to worry about breaking the stops due to over travel. It is somewhat expensive at $11.

[PhilBot]

Hi

Team1629 uses this pot from Mouser.com (My favorite parts vendor)

http://www.mouser.com/search/refine....594-357-00-502

Datasheet here:

http://www.vishay.com/docs/57059/357.pdf

At $15 it's not cheap, but it gave us position feedback on our 2007 arm position all season and several off-season competions with no problems.

Phil.

[eugenebrooks]

Give Bournes 6639S-1-203 a try.

http://www.mouser.com/catalog/632/568.pdf

We have used it sucessfully when the mechanical
stops of a 270 degree pot posed a problem.

[Pavan Dave]

We have used 2 continuous potentiometers in the past. What we did was couple them together so we could account for 360 degrees. From there, all you have to do is work with the code a little bit to work between the two pots.

[MrForbes]

Originally Posted by 987HighRoller

This is because we had something like 8 potentiometers break on us last year, mainly because they get locked inside due to load on the shaft ...........Any advise?
Thanks

Sure, I'll offer some advice. Don't put any mechanical load on a potentiometer shaft.

Here's how we got around the problem.

And our shaft encoders were coupled to the transmission shafts with a piece of surgical tubing.

[popo308]

Originally Posted by squirrel

Sure, I'll offer some advice. Don't put any mechanical load on a potentiometer shaft.

Here's how we got around the problem.

And our shaft encoders were coupled to the transmission shafts with a piece of surgical tubing.

Very interesting... so how exactly did this work?? was the sergical tubing just attached to the output shaft the immediately attached to the sensor so there was very minimal drag??

I am very interested in how you did this?!?!

[MrForbes]

ok, I fibbed....we did the surgical tubing trick last year, this year we used a short piece of rubber automotive fuel line. The concept is the same.

The AM transmissions have a shaft with an end that is exposed, and turns with the output sprocket. We drilled and tapped the end of the shaft, then made a short piece of metal 1/4" diameter with a thread sticking out the end, which screws into the transmission shaft. Then just slipped the tubing over this short shaft and over the encoder shaft, and made a mounting bracket for the encoder. A lathe was used to drill the hole and make the adapter shaft.

The picture shows the mounting and protection for the encoder, and if you look close you can see a red tube clamp in there...the tube was not a very tight fit on the shafts.

[popo308]

Ahh.. interesting concept... seems like it would work well