Crab drive without a pot
 

I was wondering if it would be possible to do a crab drive without using a pot to figure out what angle the wheel is it and if it is how difficult the programming would be. Thanks in advanced

Re: Crab drive without a pot
 

Trust me, you really want to put in a pot, it will only make your life easier during programming and then you can put wheels inexact positions and the robot will do exactly what u want it to do

Re: Crab drive without a pot
 

How expensive would a pot be and where could we get them?

Re: Crab drive without a pot
 

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

Re: Crab drive without a pot
 

get the linear-taper-potentiometer

Re: Crab drive without a pot
 

Thanks.

Re: Crab drive without a pot
 

anytime

Re: Crab drive without a pot
 

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

Re: Crab drive without a pot
 

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

Re: Crab drive without a pot
 

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.

Re: Crab drive without a pot
 

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.

Re: Crab drive without a pot
 

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.)

Re: Crab drive without a pot
 

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

Re: Crab drive without a pot
 

Did you try using a 10-turn pot?

Re: Crab drive without a 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).