Potentiometer issues

We currently bought 2 which had similar problem like:

https://vexforum.com/showthread.php?t=78781

asked.

I’m not sure if this is either of your guys problem. But we had a potentiometer that had some dust or something inside of it and our coach cleaned it out for us by rotations a shaft back and forth a ton of times.

Did the potentiometers come in standard packaging, or those cardboard packages?

Brand new and packed in standard package.

Have you checked multiple ports of the cortex for the issue? I know that often there can be an issue with the port. We recently also had potentiometer issues which turned out to be caused by a faulty 3-wire extension cable, though there were slightly different symptoms. That sort of thing can also be a programming issue such as bad variable types, but I think you are more experienced with robot programming than I :).

We did changed port , but the ports were fine. the interesting is our ones were also bounced to 250 range

We have found that if we over tighten ours they end up bouncing in the 250 range. This is why you see our potentiometers mounted with multiple zip ties. We have noticed this on more than one potentiometer and more than one robot. They are still our favorite sensor for arm or lift control, it’s been a matter of learning how to get along with them.

I will try to see if it is the problem. Thanks

Just for reference, if the issue is between 200-300, that’s usually because the cortex isn’t detecting anything for some reason or another. You can see this effect by unplugging the pot altogether and seeing the value. Perhaps over-tightening it causes connection issues, I’m not sure.

I thought it was a cortex issue since I have noticed it a little more on lower battery.
E.G.<7.7v.

Thank you! I am going to try loosening the arm pot.

It sounds like tightening the pot wrong causes the contacting points inside the pot to be flaky.

The pot housing should have no trouble being securely connected to structure, however, if the axle running through the center of the pot can twist it away from parallel (does that make sense?) rather than just turning it you may get bad values.

See this post Potentiometer repair #1 and this one Potentiometer repair #7 to get an idea of what is inside.

This is a bump… I’m having similar issues with our pots aswell. We’ve gone through 4 different ones already and they are all exhibiting similar issues.

The lowest value the arm should be able to reach is 1350.

Over a course of a few hours of programming, the arm suddenly jumps to values of 800 when the arm is near the bottom. With our loop, this causes the cortex to compensate and run the motors up causing the arm to bounce up and down.

Would over tightening as mentioned above be a problem? How do you attach the pot securely so the arm values do not change over time?

A bent axle could be a possibility, however would that cause identical problems at the same point in the arm, after switching out different pots
? The arm still goes up and down and there is no visible bending.

Thank you,
2K Robosavages

We had a robot suddenly start doing this at PA states. So they turned it a bit and the range of 1200+ was OK. Luckily they did not need the full 3/4 turn.

Not sure of the cause as it was a quick fix.

We use 4 zip ties to hold our potentiometers. Besides a possible bend, wear to the bearing blocks and metal can cause a little run-out in the shaft causing some play in it. as long as our potentiometer has a little “Wiggle” room we get good readings. If we use a potentiometer where there is little chance of the shaft developing bearing wear or run-out we are able to mount our potentiometers rigid with screws and nuts without issue. The potentiometer is an awesome sensor to have access to once you figure out which variables effect it’s performance. We have a whole handful we thought were bad, once we realized how they needed to be mounted in our specific application they all worked perfectly. The item below works perfectly for mounting potentiometers with nuts.

http://www.vexrobotics.com/media/catalog/product/cache/11/image/296x/5e06319eda06f020e43594a9c230972d/2/7/275-1029-rubber-link.jpg

the rubber joints can aid in that? Wow, I guess you do learn something every day! If you don’t mind me asking, how does that particular part help with the mounting process? I’m curious because we had a potentiometer come loose and it sort of shifted are numbers in the program by about -200 which caused us to have to rewrite some of our autonomi… Also, I’d like to see the rubber links put to use instead of sitting in the bins gaining dust hahaha

Using the rubber couplers allows the potentiometer to have some slight side to side movement to accommodate any slop or run-out in the shaft. It is the fact that the potentiometer is mounted rigidly and the shaft going through the center of it probably has a few thousandths of an inch or more slop in them. The shaft moving from side to side is what is causing the potentiometers to fail. As I have said in the past, they are great sensors, you just need to learn how to get along with them.

Do you happen to have a picture of how you mount your potentiometers with the rubber links?

Ok, I think I get what you’re saying. Thanks for your time :slight_smile: You’re right, they are indeed great sensors. They can save time while programming and can get more accurate measurements. I may have to suggest that to my team since the bump can cause quite a shockwave throughout the robot.

Correct me if I am wrong, I believe you would attach them so that the rubber is hitting the pot. while the other side of the rubber is connected to the metal by a locking nut (or regular nut). So in other words, put the rubber pieces in-between the metal and the pot so it has a little cushion room. Also, being the visual learner I am, I wouldn’t mind a picture to see.

We are moving to Rockwall next week so almost everything is already packed leaving me with no robots or potentiometers to use for illustration. I did find a rubber mount and a short stand off to show what we use to mount with. The threaded part of the rubber mount is too short to go through the potentiometer so we use a short stand off and then cut the head off of a longer screw to make a stud that sticks through the potentiometer. We get a little variance in our readings, but not enough to worry about. If we want to raise our lift to say 1850 we will end up around 1835 to 1865ish which is usually well within our acceptable limits.

We had battled for hours with trying to get our potentiometers to work right when our mentor looked at the robot for about 2 minutes and suggested this idea. If there are still questions next week when I unpack I will get a picture of a potentiometer mounted on a robot using this design.
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uploadfromtaptalk1394297065885.jpg

That’s weird. I have never had a problem with potentiometers being over tightened. I attach it and it works. The only problem I have had is that for some reason the potentiometer would keep spinning without stopping. I use that as an autonomous selector, so it is not a big deal.

You gave a range where it could be “30” off. Our 45 point programming skills only scored 2 points at the last tournament because of a “28” difference in our pot when it got slightly shifted.