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Log 3 - Loom position encoder

A project log for Automate the art

An automated version of Petros Vrellis's knit art

shlonkinshlonkin 08/14/2016 at 11:520 Comments

It is rare for my initial ideas to work out just right. Often plan B doesn't work any better. This is such a story. Fear not, my friends, it has a happy ending.

Idea one - surface reflectance sensor and black stripes (fail)

In theory it should work. It seems perfectly reasonable that a surface reflectance sensor that is designed to detect the difference between dark and light surfaces should be able to look at a passing series of dark stripes and generate the desired signal. It probably does work in a professionally built machine with tight tolerances.

I found that it was extremely sensitive to distance from the surface, didn't give a very substantial current swing, and didn't have great resolution. Here's disappointment number one.

Idea two - Mechanical switches (fail)

This one actually worked a little, kind of, but between debouncing and the really tight tolerances required, it just wasn't a reliable way to do it. The idea was that the wires that cross between the sides of the rim push a sensitive microswitch as they pass by. To increase resolution I made a pair of switches that can be set for the desired interval. Although I was getting some almost acceptable results, I thought there has to be a better way. Here's disappointment number two.

Idea three - Photo interruptor (success)

Actually one of my first ideas, but I desired more resolution, so looked elsewhere. My good old friend the photo interruptor. I finally came to my senses and gave it a try. I have these convenient pokey out bits evenly spaced just where I need them, so I just needed to position the device right and bam! clean, reliable, fast signal. Actually it took a little more than that. The pins were not all exactly the same length, so I had to go around and snip all the long ones down a millimeter or two. Here it is just temporarily clamped for testing.

At this point I have a reliably rotating loom with working encoder. The next hardware step is the thread dispenser attached to a fast solenoid. I am waiting for a solenoid from China, but I'm not too confident that it will be the one. It has to move a very light piece of tube between pins flying by about every 30ms. That means it has to reliably move one way, wait a moment, and move back the other way all in 30ms. It's not impossible, but It's a challenge.

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