# Question about encoders and degree measurements?

Our team needs to use encoders to get degree measurements out of a wheel. Only problem is, the encoders are mounted on the motor, and we cannot re-mount them anywhere else. The gear ratio is 1 motor rotation is 1.2 wheel rotations, and 497 encoder pulses is 1 motor revolution. How would we go about getting the wheel degree measurements from the encoder on the motor?

1 motor rev per 497 encoder pulses
times
1.2 wheel revs per motor rev
times
360 degrees per wheel rev

(1/497)(1.2/1)(360/1) = 0.869 degrees per encoder pulse

One motor rotation is 1.2 wheel rotations, so you geared up by 20%?

Anyway, 497 encoder pulses = 1 motor rotation = 1.2 rotation = 436 degrees.

Therefore, one pulse = 432 degrees / 497 = 0.869 degrees.

The way I showed the solution above is known as the Factor-Label Method and is an extremely useful skill to learn because one mastered it makes it easy to solve any conversion problem.

And as GeeTwo mentioned, make sure that 1.2 number is correct.

*

:eek:

If you get confused when using this method, you could include the units as well. This makes it easier to verify that you’ve properly canceled units. This one would be:

(1 motor revolution / 497 pulses) * (1.2 wheel revolution / 1 motor revolution) * (360 wheel degrees / 1 wheel revolution)

The revolutions all cancel each other, leaving the units as wheel degrees / pulse.

OOPS - didn’t see the picture there:eek:

What sort of encoder produces something odd like that?

If you are using the same definition of “pulses” that US Digital uses, for a quadrature encoder you cannot have an odd number of encoder pulses per motor rev if the encoder is mounted directly on the motor shaft.

Are you sure the encoder is mounted directly on the motor?

AndyMark sells a 7 “PPR” encoder, but that spec probably means 7 CPR (cycles per rev).

Interestingly, 497 = 71*7. Coincidence?

Probably not. http://www.andymark.com/product-p/am-2971.htm

That would explain why you might want to gear up a bit. I’m guessing that the SpinBox drives the wheel.

Alright, now that first competition is over, team secrecy is over. We’re using these things: http://www.andymark.com/product-p/am-3009.htm. We need to get rotation correct by mapping joystick degrees to wheel rotation. We got everything else working, but the wraparound is a major problem. Thanks for all the help so far, everyone!