Trying to determine wheel location

[eneubec1]

We are experimenting with an idea on our robot where we need to know the position of a wheel to make a decision. We have very little experience with sensors but learning quickly this year (2nd yr).

We are not trying to measure wheel speed, only location. The wheel we are trying to watch will be moving between 0-100RPM.

Is this a good use of an encoder, optical sensor, hall affect, or something else?

Where would I purchase one of these sensors?

[rich2202]

There are encoders that come in the kit of parts. Use those until you figure out what you really need.

Do you mean "wheel" location, or "robot" location?

Because wheels slip, counting encoder ticks gets you close to robot location, but not precise. All an encoder can tell you about wheel position is how many rotations, not location. that might be helpful if you are using a winch and wanting to estimate where the object is that you are lifting.

[Alan Anderson]

How precisely do you want to know its position? Do you need to know its angle to the nearest degree? Or would it be sufficient to get a signal telling you it is whether it's within a few degrees of a particular "special" angle? Knowing more about the application will help determine an appropriate sensor for it.

[eneubec1]

To clarify....
We need wheel location and don't care about robot location.

Our challenge is ....we are using omni wheels and the middle strafing wheel does get enough traction to push our robot sideways. Our experiment is to put a 8" rubber traction wheel in the middle and remove the tread between every other spoke (the wheel would have ~4" of tread then 4" of no tread). We would use the traction wheel to move sideways, but then need to "park" the wheel in the nearest location that doesn't have tread. (We understand the "parking" action will potentially alter our robot location by up to 2" when we need to "park" the wheel.) Determining the wheel location on the drive axle would be ideal for our application. This would allow us to rotate the wheel to the nearest "park zone" when needed. We would start the wheel in the same location every time when powering up the robot. (unless there is a better idea for identifying "home" on our sensor design)

We are using this wheel (https://www.andymark.com/product-p/am-0420.htm)
Hope this helps.

Pulling this all together is a real stretch for our young team but will be a GREAT learning experience. We have a student that is really inspired to pull this all together and this will be a great knowledge to have on our team.

[Ben Wolsieffer]

In that case it might be best to use an absolute encoder, so you wouldn't even have to worry about starting the wheel in a certain orientation. I don't really have much experience with them so I can't recommend any specific ones.

[rich2202]

Try the encoder that comes with the KOP. If that doesn't work for you, we joined the group buy for:

http://www.chiefdelphi.com/forums/showthread.php?t=133142&highlight=encoder