Measuring Distance of a string that is wound

Community Projects
1

For science!

I would like to measure distance of a string the gets wound around a small spool. I would like to do this and log the distance of the string every minute or so. The string is immersed in water so I wanna avoid lidar and ultrasonic distance measurement devices. I would be cool with a thing that measured distance as it wound on the spool.

Help me out with ideas. :smiley:

2

Find the circumference of the spool, then count revolutions the spool makes. Multiply revolutions by circumference and you have the length of the string.

3

Better to let the string pass over a wheel of known radius, and count the revolutions. The spool will have a variable radius as it unwinds, or winds up. The only thing you have to worry about is the string slipping if it is wet. And need to apply the right amount of tensioning.

4

Thanks for all your ideas. I think the best way to go will be to get a Hobby Motor and Encoder Kit (ROB-13260) and wind the string around the wheel/pulley and have it log the distance. Now the fun thought experiment is how to code the motor to stop and return the pulley to its start position AND record how much distance of the string it wound. I think magnets need to be involved…