Optical Encoding with Lightscribe

Remember Lightscribe? If you do, and you’re like me, you probably bought a stack or 2 of Lightscribe disks after Lightscribe was introduced, printed a few, and then put them on the shelf. It’s a pretty neat thing though and this post I’ll describe how you can do some optical encoding with Lightscribe by making a cheap and fairly precise optical encoder disk.

If you’re not familiar with Lightscribe, it’s a product/technology that allows you to “burn” your CD labels onto the back of specially treated CDs, DVDs, etc. I’ve been intrigued by thoughts of using it for other purposes, such as this. Now it’s true that you could also simply print a label and stick it on a CD and not bother with lightscribe, but there are certain advantages to Lightscribe. Labels can stain, be applied off center, peel off, possibly even cause balance problems at high speeds. Lightscribe will add no mass to the CD and allows precise printing. You can even print multiple passes with good registration between passes which is interesting in it’s own right.

What is an optical encoder disk you might ask? An optical encoder disk is a disk of material that is often made of glass for precision rotary position sensors for the purpose of precisely determining the angular position of a rotating shaft. It works by putting light and dark patterns or stripes, dots, etc. onto the disk in such a way that optical sensors can detect the light and dark stripes generate a binary number that directly report the position at a given time. Suffice it to say that the light and dark patterns don’t really even have to be visible to the naked eye, but adequate for an electronic sensor to assign a “0” or “1” logic levels to a particular color, shade, reflectivity, opaqueness, etc. The more bits you string together the more steps per revolution and precision you can get from your encoder.

Lot’s of information about rotary encoders is available on the web, such as this Wikipedia entry:http://en.wikipedia.org/wiki/Rotary_encoder

Many many many types of clever optical encoders have been devised, Some are linear, non linear, travel in straight lines, or complex motions. The image below shows a CD that I made using an encoder pattern that I modified to fit onto a CD. I started with the binary encoded disk image I found here: http://www.qsl.net/in3otd/roten.html There is also a Gray code encoder image available. There are certain advantages to using Gray code that are touched on in the Wikipedia article mentioned above. I’ve using the binary coded one here for illustration because it’s pretty straightforward to read visually. You can draw an imaginary line from the center to the edge and easily determine the “1” and “0” bits and directly read the position. Otherwise you’d need to convert the gray code to a number using a process that’s described in this presentation: http://www.wisc-online.com/Objects/ViewObject.aspx?ID=IAU8307

I had to open up the hole in the center to accommodate the printable area on a Lightscribe disk, otherwise and important part of the pattern would be left out. Towards the center of the disk are the “most significant bits” of the encoder and toward the edge of the disk lies the least significant bits. As you can see, when this disk spins, the transitions at the edge will happen more frequently than the transitions toward the center. Each successive “ring” toward the center will have transitions that are half the rate.

Here’s a picture of the CD I burned the encoder image onto. Perhaps in a future post we’ll rig up a little fixture to demonstrate this in operation.

EncoderPattern

I think a large share of CD writers being sold today still have Lightscribe capability. I wonder how long that will last, because I doubt many people use it. It is pretty neat though. You may think of other interesting things to do with this technology, other than simply printing a label.




Leave a Reply

You must be logged in to post a comment.