Encoder Teardown

By | 06/12/2020

I’ve been working on getting the wheel encoders working today and I’ve got a circuit that works. It turned out that it uses a photodiode rather than a light dependant resistor. An LDR changes resistance depending on how much light hits it, they’re often used in night light sensors and in phones to dim the screen at night. Photodiodes work very differently.

A photodiode generates a small amount of voltage when light hits it, kinda like a solar panel, so you need to wire it up like a switch to measure on or off.

Inside the wheel housing is a wheel with cutouts that sit between the light source, a red LED, and the photodiode. This wheel has 32 cutouts, knowing this we’ll be able to count how many times the input shaft has rotated.

The wheel has another gearbox internally, not sure what the reason is at the minute, but we should get at least a few hundred pulses per revolution of the wheel itself which should be ideal for odometry.  It was designed for this after all but still nice to have confirmation.

For reasons unknown I can count the movement of one wheel or the other at the minute, not both at the same time, which is incredibly annoying and I can’t see why!

No doubt it’ll take hours and be obvious once I find it but that’s par for the course… 

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