[FTC]: Motor encoders Love'em? or leave'em?

[Ether]

Originally Posted by MattRain

Ok, yes there is that fact of that robot could have gone a little under or over that number [ed: 0.001 inch]

As a pre-kickoff project, you might want to try this:

1) Command your robot to move exactly 20 feet forward and stop.

2) Using a tape measure, determine how far it actually traveled.

3) I think you may find the results surprising.

[MattRain]

I will have to students try that out today. I shall post the results tonight.

[Ether]

Originally Posted by MattRain 15Nov2013 11:00

I will have to students try that out today. I shall post the results tonight.

Did you forget?

[aakerberg]

Well, accurate distance isn't all that important. Presumably you should have some opportunity to tune your program to account for any inaccuracy in the encoder. However, consistent results ARE important. A better experiment would be:

1. Mark a very specific start point
2. Program the robot to go any distance
3. Mark where it finishes moving.
4. Repeat several times - preferably with different loads (add/subtract weight) and/or different levels of battery charge.

How close does it get to your original finish mark each time? if the encoders are reasonably consistent, you should end up in the same spot (or very close) each time, regardless of load or battery condition.

The problem with using encoders for measuring robot position in autonomous mode is they offer no defense against the things that are most likely to trip you up - getting bumped off course by another robot, wheels slipping on the edge of the board, etc.. I would argue that provided you don't make radical changes to the robot during the course of a single competition and manage your battery pack properly, you can get equally effective results using time for judging distances driven.

If they are consistent, I can see them being more useful for measuring things like precision arm movements, or maybe feedback for a sophisticated driving control system...

We have been also been unhappy with they way they are mounted to the outside of the motor. We are experimenting this year with other ways to give us the information we need.

[Ether]

Originally Posted by aakerberg

Well, accurate distance isn't all that important.

That may well be true in many cases, but the specific point of posts 7, 11, 12, 13, 14, and 15 was to think through the statement in post 2.

[Sasha]

We do use encoders for autonomous. They work fine for straight motion (of course, not to 0.01 inch - but it is not necessary). However, we found them rather unreliable for measuring turns (when turning, both wheels slip), so we use gyro sensor for this. Works much better for us. Your mileage may vary.

[Ether]

Originally Posted by MattRain 15 Nov 2013, 11:00 AM

I will have to students try that out today. I shall post the results tonight.

Still interested in your test results.