Mounting an arm potentiometer

[Hazmatt]

Hello. I was wondering if anyone could give me any tips, pointers, or general help on mounting a potentiometer to an arm. Any pictures would be cool too! thanks.

[Manoel]

Hazmatt,

How does the arm work? Live or dead axle?
Knowing that we can help you decide the best way to mount it.

Either way, use a flexible coupling like surgical tubing to avoid side loading the potentiometer shaft, which will ruin it real quickly.

[Michael Corsetto]

A key thing to keep in mind when deciding where to mount a potentiometer is where will the potentiometer get the most resolution. Generally there are two types of potentiometers, 3/4 turn and 10 turn. You'll want to mount the potentiometer where it will take close to full advantage of number of turns, in order to increase the resolution and give your programmers better feedback data. For instance, if you have an arm that goes from pointing directly down to directly up, that is a 180 degree change, and a 3/4 turn pot mounted directly to the arm joint would work. However, if you wanted to have your potentiometer in a slightly less precarious position (ie. not on the top of your robot) you could attach a 10 turn potentiometer to one of the gear/chain reductions (if you have any) that would roughly translate 10 turns of the pot to half a revolution of the arm (a 20:1 ratio), given that the arm is still only traveling from down to up.

As far as actually mounting the pot, be sure not to mount it anywhere where significant side load will be applied, since more than a little side load can break the pot. A safe bet would be to mount the pot directly to the one of the arms drive shaft that is already supported on both sides, via some sort of coupler. We've used pots every year the last 4 years for various things from arms to shooters to crab drives, unfortunately I don't have any close-up pictures on how we mounted them. Hope this helps.

Mike C.

[AdamHeard]

Originally Posted by Rex114

A key thing to keep in mind when deciding where to mount a potentiometer is where will the potentiometer get the most resolution. Generally there are two types of potentiometers, 3/4 turn and 10 turn. You'll want to mount the potentiometer where it will take close to full advantage of number of turns, in order to increase the resolution and give your programmers better feedback data. For instance, if you have an arm that goes from pointing directly down to directly up, that is a 180 degree change, and a 3/4 turn pot mounted directly to the arm joint would work. However, if you wanted to have your potentiometer in a slightly less precarious position (ie. not on the top of your robot) you could attach a 10 turn potentiometer to one of the gear/chain reductions (if you have any) that would roughly translate 10 turns of the pot to half a revolution of the arm (a 20:1 ratio), given that the arm is still only traveling from down to up.

As far as actually mounting the pot, be sure not to mount it anywhere where significant side load will be applied, since more than a little side load can break the pot. A safe bet would be to mount the pot directly to the one of the arms drive shaft that is already supported on both sides, via some sort of coupler. We've used pots every year the last 4 years for various things from arms to shooters to crab drives, unfortunately I don't have any close-up pictures on how we mounted them. Hope this helps.

Mike C.

If you buy decent pots, (I'm very partial to the ETI MW22 series. ~$10ish on mouser)you can get 3, 5 and 10 turns.

Also, using surgical tubing as a flexible coupler idea. I first saw 330 do this last year, and have used it in several situations since. Very easy to do, and makes up for poor tolerances. Also, it will slip before breaking the POT if you have it go too far for some reason.

[lynca]

Originally Posted by AdamHeard

Also, using surgical tubing as a flexible coupler idea. I first saw 330 do this last year, and have used it in several situations since. Very easy to do, and makes up for poor tolerances. Also, it will slip before breaking the POT if you have it go too far for some reason.

Is there a picture of how surgical tubing and a pot would work ?

[Mike Hendricks]

Regarding flexible couplings ..

1388 has used Helicals in the last couple years for adding feedback to our arms. A former 1388 father/mentor is actually one of the engineers at the company.

http://www.heli-cal.com/

You can see it used in this picture on our 2007 bot:

[Manoel]

Originally Posted by lynca

Is there a picture of how surgical tubing and a pot would work ?

This is how we did it last year:

[Tapoore]

We have used vinyl tubing for mounting pots on several different mechanisms. It works very well, however, sometimes we have had problems with the shafts slipping inside the vinyl tubing. To fix this, we have usually drilled a hole through the shaft and through the tubing and pinned it with a cotter pin (a paper clip works just as well...)

[MrForbes]

Here is another method.

[Manoel]

Originally Posted by squirrel

Here is another method.

Dead axle, right? I'm looking at our options for this year (too much torque to use a live axle) and yours seems like a good one. Did you have problems side loading the potentiometer? Is the pot shaft pinned against the aluminum channel or is it a simple interference fit or what?

Thanks!

[MrForbes]

The little arm has a clamp screw, and a slot that you can't see that lets it compress and clamp onto the pot shaft. Very little side load, notice the slot where the nylon screw contacts the little arm. Worked fine all season, and we took it off and used it on a different type of prototype robot this year.

[Dan Richardson]

This is kind of a different way to do it, Its not suggested but it is a quick and easy and "relatively" reliable way to do a arm pot. I've used this method on encoders and pots on arms and drivetrains, all you need is a little wheel a bit of apoxy to mount the wheel to the shaft of the pot and a rounded area of whatever your measuring.

A problem with this method however is you have to constantly adjust your pot, every match you have to set it to make sure it didn't move and recalibrate.

However if you need a creative solution and can't drill into the end of the shaft or create a special coupler this is a solution we've used in the past and does work, and works well within the 2 minute First matches.

http://picasaweb.google.com/DanRRich...04765271712450

[Alex Cormier]

1126 has used the pneumatic tubing with a little heat to attach it to the pot, haven't had any issues with it.

[scottydoh]

we drive our pot with spur gears, (one on the pot and the other on the drive shaft for the arm), and have had no problems with side loads.

[lynca]

Thanks for all the helpful pictures !