Microscale v2.5

I meant to be writing about this as it happened, but instead I will summarize. After the last attempt, we decided to make the scale out of metal, so it would be a bit more robust. The new version was designed in Solidworks in about a day, ported over to FlowMaster, and cut on a waterjet at work. The waterjet process took all of two hours, including polishing. The cut itself was around 7 minutes. Sadly, I didn't take any pictures of the individual pieces, but the final assembly photos are nice.

The scale is made out of 1/4" Aluminum. I hand drilled and tapped all the holes with bad equipment. The tap actually twisted if I applied too much torque. It's quite possibly the cheapest. tap. ever. At least it didn't break on me. I did break a screw that I was using to clean up the holes, but it wasn't strictly required, right? You can just make out the copper wire counterweight on the lower right side of the assembly. It's the hokiest part of the design, but it works.

Here's the best view you'll get of the new flag design.

The idea is that when it is centered, the IR beam is half blocked. When the arm is off balance, the changing radius of the flag blocks more or less than half of the beam. The advantage is that the arm can move up to 5 times further than it could before and still get a valid reading. The flag was drawn in Solidworks and cut out of 0.06" Aluminum on the waterjet. I then polished the cut face to make readings a little more consistent. There is an adjustable block above and below to limit the motion of the arm, and to protect the flag in general. Th flag hitting the lower stop actually makes me think of the big hammering guy sculpture out in front of SAM.

While assembling and tweaking the new flag design (using the capacitor as a simulated sample), I somehow damaged the galvanometer itself. It got really sticky, and would stay in the balanced position without being balanced. We ended up heading out to Radio Shack to get a new one (and another spare) for ~$16 each. While I was at it, I made the arm longer by about 50%, bringing the scale to about 150 counts per milligram. The longer arm and new flag allowed me to re-tune it to respond faster.

Here's a picture of the computer taking data via hyperterminal. The serial stram is simply recorded to a text file, and can be imported directly into Excel later.

Note: Remember to ask Erica for a picture of the scale in use, and a screenshot of the resulting graph.

Also, while trying to fix the balance arm, the screen went blank! It was fairly easy to trace. The scale still worked, but the negative voltage for LCD contrast went away. The only problem with having a donated collection of stashed parts is that the parts are old, and actually require some work to make them function. Current LCDs are fine with ground on the contrast pin, but older ones want -3 to -10 volts. Fortunately enough, the MAX233 creates -10 volts for itself to use for RS-232 communication. Since it's just a tiny load, I ran the LCD contrast to this supply. The problem turned out to be a fleck of loose solder shorting out the supply. Good thing the Max233 is resilient.

I'm calling it done at this point (Hence the completed tag). Erica's brought it to school, and she'll calibrate and tell me how it goes. It's version 2.5 because of all the mechanical upgrades, though the code is essentially the same as the v2.0.

Now for the next project...

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