Monday R&D Fail
The joy of R&D is that moment when everything comes together and there’s a shiny bit of goodness in front of you, gleaming with potential. This is not one of those stories. 
One of my research goals is to ‘close the loop’ on a few of the MakerBot subsystems. This is tech-speak for adding feedback systems so that you can know if your motors (or whatever) are doing what you tell them to do. In particular, I want to close the loop on the XYZ motors so that we know the exact position of the bot at all times.
My first crack at this is based on a cool chip by Austria Microsystems called the AS5304/AS5306. These tiny little chips are magnetic linear encoders (measures movement on a line). I’ve used the Austria Microsystems chips before and they’re really nice. In particular our magnetic rotary encoder board (measures rotation) uses the AS5040. The linear encoder chip has some really nice specs: resolution down to 15microns (0.015mm or about 1600DPI) As you can see, this chip would be a really nice addition to our MakerBot arsenal. What that means is that you put down a strip of adhesive backed magnet and then run this chip over it. It will tell you your exact position over the strip down to 15 microns. Hawt.
So, I modified the rotary encoder design for this new chip, including creating a new part in Eagle. I looked over the datasheet and everything looked good. I placed the components, then slept on it. I routed it, then slept on it again. I saw a few flaws and re-placed and re-routed it and then pronounced it ‘good’. I sent it off to the fab and moved onto the next board. Well, unfortunately I made some mistakes. First, the AS5040 chip which I copied for the footprint is wider than this chip, even though they have the same pin spacing. CRAP! The pads dont line up. The chip definitely still shows great promise, so I’m going to fix this mistake and send it off to the fab soon.
Moral of the story: always print your design and test the footprints against the real chip. I usually do this, but I was in Portugal at the time and wanted to have a bunch of goodies when I came back. Guess I learned my lesson.
Oh, and if you’re interested in this design… we always do our dev out in the open and this one is located in subversion.
Wade
November 10th, 2009 at 10:58 pm #
Neat! Closed loop is great – I’ve been running your rotary encoders since day 2 – day 1 I tried to run my lumpy DC extruder open loop, with predictably bad results. The magnetic encoder should be fairly immune to the PLA dust that slowly builds up on my machine too, unlike an optical solution.
Charles Edward Pax
November 11th, 2009 at 8:29 pm #
NYC Resistor is now a place where you can do this prototyping. The printer we brought in is great and the board-making process works. I had to put a scaling factor in Eagle, so the prints come out the size they should. Always make measurements to ensure your print is the correct size. Grab me and I’ll walk you through the photoresist process.
And I’d be happy to take any boards you want to donate to my students for solder practice.
Oleksiy Pikalo
November 23rd, 2009 at 2:11 pm #
What is the production cost of the magnetic strips? Other than dust issues is there a good reason to use magnetic vs optical?