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MikeDubreuil
03-02-2003, 11:19 AM
I am looking for a 2 - 5 turn potentiometer. The style that has a shaft. Any help would be appreciated.
- Mike

D.J. Fluck
03-02-2003, 11:28 AM
We have tons of pots in a box in our shop. Im going there tomorrow for a competition readiness meeting, and Ill take a look for you to see if we have any.

MikeDubreuil
03-02-2003, 11:34 AM
Thanks a lot...

The pot is for our claw. We need more than a single turn. The only thing I can find in Digi-Key is a 10 turn. The device only rotates about 300 degrees which would kill our resolution.

Jnadke
03-02-2003, 11:46 AM
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=16944

Digikey does carry other multi-turn pots. Look under Potentiometers - Wirewound.

There are a few 300 degree (http://dkc3.digikey.com/PDF/T031/0762.pdf) potentiometers. You cannot use that one, though, because they only come in 25K Ohms. You need 100K Ohms. Search around the entire catalog and see if you can find another 300 or so degree pot.


The other option is to take a standard pot and stick a gear on it.


*edit* The second item on that link, 1/8Watt Carbon pot, is a 300 degree pot and comes in 100K Ohms. However, the wattage might be a bit too low (you'll find your pots burning up). The manual states 50mA max, which means you'd need 5*.05 = 1/4 Watt potentiometer.

MikeDubreuil
03-02-2003, 11:49 AM
There are a few 300 degree potentiometers. I know I found one, but it only comes in up to 25 KOhms. You need 100K Ohms.


I saw that one too. I don't mind going higher in resistance because I can just throw a resistor across it. Lower won't really help.

Jnadke
03-02-2003, 12:00 PM
There are some 300 degree pots listed under "Stock-pot" (http://dkc3.digikey.com/PDF/T031/0761.pdf) but again they are only rated for 1/3 Watt.

Andrew
03-02-2003, 12:55 PM
If you get a 100K pot, you are not limited to digikey or future.

The supplier that I like for things like this is mouser electronics (www.mouser.com). They have good selection, quick shipment, and no minimum order. Although, a quick look through their catalog did not disclose exactly the option you wanted.

Another approach might be to get a Grayhill gray-scale mechanical encoder from Digi-key. This can be connected to four digital inputs (16 bits per revolution resolution or one nibble), and gives you unlimited rotation. I don't know how fine a positioning you need to do. Of course, you could gear this and accumulate your value in software to improve your resolution.

I would definitely recommend against using a 300 degree (maximum) pot for 300 degree rotation. Your accuracy is very poor at the ends of the travel and if you do not position the pot exactly, you will break it quickly.

We normally use the 270 degree rotation pots for about 200 degrees of rotation.

MikeDubreuil
03-02-2003, 02:37 PM
We have the grayhill Encoder, we bought the 32 position though. Our device moves too fast for the serin to pick up every pulse.

Al Skierkiewicz
03-02-2003, 05:58 PM
You might try gearing the pot to the device you are trying to measure. That makes the pot an off the shelf item. The shaft of most pots are 1/4" so finding gears that mate with that shaft is pretty easy. Plastic or nylon gears are available from our list of suppliers cheap and they work well. You get to use maximum resolution from the pot without sacrifice. Just be sure to give yourself a little leeway. Pots are about 270 degrees but that is not guaranteed. Plan for no more than 250 and you should be fine.

Andrew
03-02-2003, 06:04 PM
Our device moves too fast for the serin to pick up every pulse.

Can you just look at the most significant bits and get a good reading. Or, do you need the 32 bit resolution?

MikeDubreuil
03-02-2003, 06:22 PM
You might try gearing the pot to the device you are trying to measure.
Yeah, I think that is what will have to be done. Right now the team is in a mindset where the control of mechanical parts are not part of their design. So the software guys are supposed to take the mechanicals final product and figure out ways to add sensors. Some devices are even given to us saying, "this can not run against the mechanical stop," without thought of where they could put a sensor. I'm sure all this is pretty typical.




Can you just look at the most significant bits and get a good reading.

We could, if we could even pick up the MSB ;o)

Lloyd Burns
03-02-2003, 07:00 PM
Do you suffer from the "we.ve finished building the robot, so let's add controls" syndrome ?

We have arms that rotate about 225 degrees, but they are mounted on a gear, so gearing it was easy. Unfortunately, the smallest meshing gear had too many teeth to allow more than about 4 turns of the ten-turn pot. Works ok though.

Al is right about leaving some leeway, though. When adjusting your system, there is nothing nicer than not having a motor drive the arm past the end of the pot's rotation, so the pot doesn't need emergancy replacement.:rolleyes:

Andrew
03-02-2003, 07:01 PM
Yeah, I think that is what will have to be done. Right now the team is in a mindset where the control of mechanical parts are not part of their design. So the software guys are supposed to take the mechanicals final product and figure out ways to add sensors. Some devices are even given to us saying, "this can not run against the mechanical stop," without thought of where they could put a sensor. I'm sure all this is pretty typical.

Sounds like your guys need to take a course or two in mechatronics. We require that all of our students undertake the "synergistic integration of mechanical design, sensors, and control" before we OK the design to be built.

Lloyd Burns
03-02-2003, 07:03 PM
You plan before you build ?!?!?!?!?

Sounds like something an engineer would do !

Radical !:D

Al Skierkiewicz
03-02-2003, 09:47 PM
Originally posted by MikeDubreuil
Yeah, I think that is what will have to be done. Right now the team is in a mindset where the control of mechanical parts are not part of their design. Some devices are even given to us saying, "this can not run against the mechanical stop," without thought of where they could put a sensor.
We have been using sensors, pots and microswitches to control mechanics for so long, the mechanical guys have just come to expect that it will be part of the design. We have a really good team effort in that the mech team knows that certain things can be taken care of with feedback and work with the electrical and software teams to make it happen. Anyone knows that some of the motors we have are strong enough to lift a full weight robot off the floor. If that power is applied without some form of feedback an arm could bend the frame or a pulley system could pull the motor out of it's mount before anyone could react.