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Unread 23-01-2005, 15:01
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Exclamation Potentiometer Trouble ??

For our robot we are using an arm that is in three sections. If you remember from nationals last year, it is similar to the "Hot" bot, team 67. We need to control this through a potentiometer. I need to know how to set up this system to control the arm.

Team 67, if you are reading this, can you please send me some of your code from last year.

Thanks
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Unread 23-01-2005, 15:06
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Re: Potentiometer Trouble ??

It seems that you are doing something very similar to what we did last year. If you can get in contact with me either through AIM/AOL or e-mail I am willing to share our setup and programming code with you. Visit our website www.raiderrobotix.org and see if our bot is similar to what you are thinking.
-Bharat
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Unread 24-01-2005, 20:06
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Talking Re: Potentiometer Trouble ??

Okay, i understand how to set the pot up now.

My next question is where can i get a 30K potentiometer that is legal in the first competitions?
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Unread 24-01-2005, 20:10
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Re: Potentiometer Trouble ??

Just about any decent electronics supply shop will have them. They're not a rare part. Radioshack might even have them
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Unread 24-01-2005, 23:24
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Re: Potentiometer Trouble ??

Quote:
Originally Posted by VideoMan053
where can i get a 30K potentiometer that is legal in the first competitions?
Why 30k? The manual suggests that you use 100k, which will also be easier to find.
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Unread 24-01-2005, 23:32
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Re: Potentiometer Trouble ??

Quote:
Originally Posted by VideoMan053
Okay, i understand how to set the pot up now.

My next question is where can i get a 30K potentiometer that is legal in the first competitions?
I think you are going to have a tough time finding a 30K pot. Standard values (i.e. stocked values) are most often either 25K or 50K. Some of the larger parts houses like Newark and Allied might have standard values but then 30K is still not a standard value.
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Unread 24-01-2005, 23:51
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Re: Potentiometer Trouble ??

A few other notes when you go looking for a pot. Theres a couple major things to think about when purchasing a potentiometer from anywhere. These include whether you want an audio taper or linear taper pot and also whether you want "10-turn" or single turn pot.

A linear taper pot has the same change in resistance for every degree it is turned, while an audio taper pot normally has different ranges of change of resistance. All the applications Ive used pots for in FIRST have primarily used linear taper pots.

Now onto the amount of turns. This is where it will normally come down to the application the pot is being used for to choose which type is best. If what you are measuring the amount of turn on goes less then 360 degrees a single turn pot normally is what you would use and would have more precision for small amounts of turn. A "10-turn" pot is like the name suggest a potentiometer that will turn more then one complete turn and is used normally on devices that can do more then one complete rotation.

[Al or anyone please correct me if im wrong and i searched how-stuff-works for more in depth explainations but it seems they dont haves pots described there.]
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Unread 25-01-2005, 09:32
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Re: Potentiometer Trouble ??

Quote:
Originally Posted by dez250
A few other notes when you go looking for a pot. Theres a couple major things to think about when purchasing a potentiometer from anywhere. These include whether you want an audio taper or linear taper pot and also whether you want "10-turn" or single turn pot.
Indeed. It's worth a bit of time getting a decent pot, since while you can prototype with an el-cheapo pot, once you've got the bot together, make sure you are using a nice pot.

Cheap pots have a lot of problems. The big ones are drift and non-linearity. Minor problems include poor resolution and directional hysteresis (giving a different value whether you turned the pot CW or CCW to get there).

I've usually used fairly high-end ($11 or so) 10K pots from digi-key (look up part number 6639S). Good linearity, little drift. They are single turn with no stop (so they wrap around from 10 kOhm back to 0 ohm at 0 degrees), but other than that they've been very, very reliable.
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Unread 25-01-2005, 09:53
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Re: Potentiometer Trouble ??

Keep in mind that single-turn pots often only have about 270 degrees of freedom while designing your hookup...
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Unread 25-01-2005, 21:43
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Re: Potentiometer Trouble ??

Quote:
Originally Posted by kaszeta
Cheap pots have a lot of problems. The big ones are drift and non-linearity. Minor problems include poor resolution and directional hysteresis (giving a different value whether you turned the pot CW or CCW to get there).

I've usually used fairly high-end ($11 or so) 10K pots from digi-key (look up part number 6639S). Good linearity, little drift. They are single turn with no stop (so they wrap around from 10 kOhm back to 0 ohm at 0 degrees), but other than that they've been very, very reliable.
I was looking for a pot like that, though, the price seems kinda steep... anyone have another recommendation?

exactly what I was looking for was a continuous pot, 100k (for Operator Interface), it would only have to be accurate to 10 degrees or so...

also, to achieve 360 degrees with only 320 or whatever of effective electrical angle you could use two pots offset and sort it out with some good old fashion programing.
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Unread 25-01-2005, 21:48
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Re: Potentiometer Trouble ??

I usually pick up my three-quarter-turn pots for about a dollar and the ten turn pots at about $6. How much is too much?
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Unread 25-01-2005, 23:21
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Re: Potentiometer Trouble ??

Quote:
Originally Posted by steven114
I usually pick up my three-quarter-turn pots for about a dollar and the ten turn pots at about $6. How much is too much?
Well if I wanted to use the ones kaszeta suggested I'd use two because I need 360 continuous rotation, and that would cost 20 bucks, which seems overkill for the lowness of the resolution I want. I guess 5 or so a piece would be ok...

(also: thanks for the reply)
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Unread 25-01-2005, 23:34
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Re: Potentiometer Trouble ??

If you need 360 degrees, either grab a three-turn pot (never bought one, but I would assume it would be a couplea bucks), and if you want continuous rotation go for some sort of {optical|mechanical} encoder.
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Unread 26-01-2005, 00:04
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Re: Potentiometer Trouble ??

Quote:
Originally Posted by steven114
if you want continuous rotation go for some sort of {optical|mechanical} encoder
That may not always be satisfactory. I have an encoder that clicks 32 times per revolution. This turns into 8 counts per revolution in code when using interrupts. With a continuous pot, you may get up to 1024 counts per revolution. If you were counting something that went fast and didn't need too much precision, like the drive train, then you would use an encoder, so you could monitor it with hardware interrupts. If you were using something that was slower and in need of more precision, but still made multiple rotations, like a winch or something, a continuous potentiometer would make more sense, because you wouldn't need interrupts; you could just poll it every 26.2ms without missing any data. While it is safe to say that it's always a good idea to use a potentiometer when you're making less than one complete revolution, encoders are not always the ideal solution for multiple revolutions.
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Unread 26-01-2005, 09:36
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Re: Potentiometer Trouble ??

Well, if you're willing to pay, you can get encoders which have multiple millions of stops (probably even more) per revolution. It is, however, by no means the be-all and end-all solution.
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