Looking for suggestions...

T. Pardy

Hi, I have started a new project and would be grateful for some suggestions.
My project:
A BS2 project which will track the amount of line passing over a pulley.
I am trying to decide on what type of sensor to use for tracking the rotations of the pulley.
For my prototyping I plan to use an IR transmitter/receiver with a pulley which has two holes. The final project will be used outside in the elements. Extreme temperatures, along with wind, dust, and rain. My thoughts so far is to maybe use some sort of proximity sensor mounted to the pulley, or a magnetic reed switch of some sort. Precision is key, as I would like to be able to measure the speed at which the pulley rotates.. Any ideas?

thanks,

travis

Leon

I'd use a shaft encoder on the pulley shaft. It will detect direction as well as position and speed.

T. Pardy

Thanks Leon for the reply. I looked over a few data-sheets for shaft encoders. It would be a great solution if I could find an encoder which could operate in the -40C to 40C temperature range. The minus temperature is particularly important. I'm also concerned with wear and tear as the solution would be used continuously in a heavy duty environment. Also, thanks for reminding me that direction sensing is also a must. Thanks again.

erco

Magnets on the pulley detected by reed switches or Hall effect sensors would be my first choice in that harsh environment. Reed switch activation is a bit funky in the magnet path and when it makes & breaks the connection, but they are good for millions of cycles (literally, they are very common in bicycle computers & wheel sensors). Experiment with the reed switch location, you can configure two reed switches as a quadrature encoder to get speed and direction.

erco

Useful info on reed switches: http://reed-switch-info.com/index.html (note funky butterfly trigger region 1/3 down)
also see:
http://www.reed-sensor.com/Notes/General_Reed_Switch_Theory.htm
http://www.hamlin.com/technical-detail-reed-switch.cfm
http://flo-products.com/floprod/switchmp.html
http://www.meder.com/reed-switches3.html

prof_braino

You might want to try hacking an optical mouse. This would give very accurate reading of the line passing under it, and would not be subject to the same error as in cases where the pully was frozen immobile and the line was slipping. Of course dust might be an issue.