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
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
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?
Just about any decent electronics supply shop will have them. They’re not a rare part. Radioshack might even have them 
Why 30k? The manual suggests that you use 100k, which will also be easier to find.
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.
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.]
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.
Keep in mind that single-turn pots often only have about 270 degrees of freedom while designing your hookup…
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.
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)
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.
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.
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.
Would I be able to use an encoder on the operator interface
note: though accuracy isn’t necessary for my idea, drift would be catastrophic…
Alex,
My interpretation of the rules has centered around the power used for the device.
<R69> All equipment connected to the Joystick Ports of the Operator Interface must be powered solely through the power available through the port. External power sources of any type are not permitted on any equipment connected to the Joystick Ports. Portable computing devices may not be connected to Joystick input ports on the Operator Interface.
Additionally, the LED outputs have current limited 5 volts for tally lights on the human interface. If you can interface an encoder under these conditions then there is no other rule I am aware of that would prevent an encoder. A pot is just an infinitely switched resistance, so it can be replaced with a switch and a resistor string if you only need 1 of 4 selections. Check the parts sources for a 1 or 2 pole, four throw (position) switch.
Besides getting the encoder to work electrically on the OI, you need to make sure you can reliably read the data.
If you need approximately 10 degrees accuracy, you’ll want a 32 ticks/rev encoder. To accurately read the encoder, you’ll need at a minimum of 1 logic 1 reading followed by a logic 0. The RC receives data at 40 times/sec, so at the very best you’ll be able to read 20 ticks/sec. However, you will have a human turning the encoder, and they definetly won’t do it at a defined rate. To give yourself margin, figure on 5-10 ticks/sec. This means your maximum turn rate is (5/32 or 10/32) revolutions per second.
First off, thanks for all the help I am getting on the board.
well, for what I am looking for… is something that could get me the position even while experiencing 3 rev/sec of speed, so I am startin’ ta think that an encoder might not be the solution for me…
and
to make it simple, here what I am looking for:
Resistance: 100k
Mechanical angle: continuous
and 10 degree of accuracy
thanks for anymore help.