Crab drive without a pot

[eshteyn]

it would be about 2 dollars and 50 cents from radio shack

http://www.radioshack.com/search/ind...otenti ometer

but if you want it in large quantities u can get it cheaper at

www.digikey.com

[eshteyn]

get the linear-taper-potentiometer

[dpick1055]

Thanks.

[eshteyn]

anytime

[Daru]

I have a question about using pots on crab drives.

It seems to me like you could have a 359 degree rotation using a pot, but then to go from, say, rightish forward to leftish forward wouldnt you need to rotate the wheel all the way around the long way? Assuming 0 on the pot is straight ahead.

Are there pots that give you a range of values up to one turn and if you turn past that they would reset?

We were considering using encoders on these shafts... but I'm not the best programmer in the world

Any advice is greatly appreciated.
Thank you,
Dave

[Lil' Lavery]

You can buy pots that are set to a different amount of rotations, or you could use (like you suggested) shaft encoders.

[AdamHeard]

You could also use endless pots, it would be slightly harder to program than normal (you have to anticipate the "wrap around", still not to hard though) and you still get endless rotation as you desire.

[Dave Scheck]

Originally Posted by Daru

We were considering using encoders on these shafts...

I would highly recommend not using encoders to determine crab position. Yes they can work, but they require you to precisely position the wheel at startup. Since the encoder doesn't tell you where you are, you could have the wheel turned 90 degrees and the RC has no way of knowing that it's not centered. Pots are far superior in this aspect because they give you an absolute position at startup and the necessary corrections can be made.

[Ian Mackenzie]

Originally Posted by Dave Scheck

I would highly recommend not using encoders to determine crab position. Yes they can work, but they require you to precisely position the wheel at startup. Since the encoder doesn't tell you where you are, you could have the wheel turned 90 degrees and the RC has no way of knowing that it's not centered. Pots are far superior in this aspect because they give you an absolute position at startup and the necessary corrections can be made.

True, but normal potentiometers have the nasty disadvantage of having a hard stop. When we did swerve drive in 2004, we broke a lot of potentiometers (especially in testing) by turning them too far. In hindsight, it might have been a better idea to use encoders, set the wheel to a known position at the start of the match, and probably have a limit switch as a backup that would be triggered (e.g.) when the wheel was straight ahead - so if anything happened, you could have a reset button that would spin the wheel until it triggered the limit switch and then would reset the encoder count. (You could also do this on the fly - every time that switch happens to be triggered, have an interrupt that corrects the encoder count.)

[meatmanek]

A couple of years ago our crazy electrical guy made a full rotation potentiometer by cracking it open and cutting off the stop. (Not recommended, you can screw things up by putting voltage across a shorted potentiometer)

There are ways to use optical sensors to encode position rather than motion.
http://http://en.wikipedia.org/wiki/Gray_code

[Bharat Nain]

Originally Posted by Ian Mackenzie

True, but normal potentiometers have the nasty disadvantage of having a hard stop. When we did swerve drive in 2004, we broke a lot of potentiometers (especially in testing) by turning them too far. In hindsight, it might have been a better idea to use encoders, set the wheel to a known position at the start of the match, and probably have a limit switch as a backup that would be triggered (e.g.) when the wheel was straight ahead - so if anything happened, you could have a reset button that would spin the wheel until it triggered the limit switch and then would reset the encoder count. (You could also do this on the fly - every time that switch happens to be triggered, have an interrupt that corrects the encoder count.)

Did you try using a 10-turn pot?

[Ian Mackenzie]

Originally Posted by Bharat Nain

Did you try using a 10-turn pot?

We had 5-turn pots, so 2.5 turns in either direction. 10 turns might have been better, but I still think we would have broken a few (and, of course, we would have had lower resolution - but I'm not sure that would have been significant).

[Al Skierkiewicz]

There are a couple of things that teams can do to use a standard 270 degree pot without breaking it. You can easily use a couple of nylon gears (from Small Parts) to couple the pot to your steering mechanism. You can mechanically limit the travel of your crab so that you stop turning before the pot is broken or you can spend some money and buy a continuous pot.
But dpick, as you can see, no one is telling you to do crab without a pot. There is no way for the robot or the driver to know that the crab system has actually gone where the driver has told it to go or for the software to get it back to driving forward.

[Tristan Lall]

Why not an absolute encoder? (Like this.)

[fledman]

Originally Posted by Tristan Lall

Why not an absolute encoder? (Like this.)

Is there any functional difference between a pot and an absolute encoder? (I realize that they are different internally.) Why would someone use one over the other (for low speed applications, eg arms, steering)? Is there a reason teams use encoders that give counts instead of absolute encoders on higher speed applications (wheels)?