How to measure wheel revolutions

What is an easy/reliable way to measure wheel rpms? using optical sensors? reed switches?

We have proximity sensors with a very short range (a few mm) pointed directly at the sproket turning our back wheel. The sensors are rigged to interrupts in the program (which can be either rising edge or change). From there it is pretty easy to go to RPM.

Rotary encoders work well, provided your robot doesn’t bounce too much.

There are “banner” sensors in the kit. White or Red reflective tape is legal for pulse generation. This years banners need to be adjusted as at full out they tend to false positive at close range. Search for sensors, or quadature, on this site for threads on coding.
Good luck.

Our team (663) used the banner sensors to measure drive sprocket rotation very successfully. Rather than use reflective tape, we used Microsoft Word to draw circles that were divided into 8 slices, with every other slice colored black. Printed this on sticky paper, and attached that to the sprockets. That allowed us to count 1/8 rotations, which given our wheel size meant accuracy to about 4 inches.

See this thread for an explanation of Team 30’s method:

It’s similar to the one used by 663, but a little more accurate.

Do you happen to know what kind of proximity sensors you’re using? Are they ultrasonic? magnetic? Maybe even a part number? Thanks in advance. :slight_smile:

Magnetic, and I’ll get you the part number in a few minutes…just gotta wait for Jay to get back…

we use optical encoders which provide input to a custom circuit which then converts that input into output like wheel direction speed and distance. everything you need for a decent autonomous mode. the shaft of the encoder is directly attatched to the output shaft of our transmission. its excellent and reliable. we may have replaced them once last year but overall i definately reccomend these encoders to measure wheel revs!