Since I'm trying to use a PID controller to keep the chamber temperatures under control I need to tune the PID parameters. And since this is designed to run for hours at a time, and because I'm crap at PID tuning, getting it right it is very tedious. Also presumably I'll need to tune it differently for different chamber loads. A liter of water is going to behave differently from a 50mL of solvent, so I figure I may need to retune it several times for different uses. So I set it up to dump the parameters so I can visualize the process. Nothing fancy, just CSV out the serial port to a log file, then into Excel so I can see what's going on:

The code I've been using to run the temperature controller for the crystal chamber so far has been pretty bare, just the core PID loop. I don't have it tuned yet, but it's generally functional, so it's time to start getting it in shape to be an appliance. It's there at the bottom of the image. I used poly tube standoffs to keep the leads from contacting the aluminum tape and to provide a relatively stable mounting to the foam. The temp sensor is in the upper right-hand corner. Previously it was just laying on the floor of the chamber. Keeping it up in the warm air rising off the heater helps the response time, but also makes it clear that there's some pretty strong stratification going on in there. I'm pretty sure I'll need to add a tiny fan to keep the air mixed. I didn't want to do that because of the possibility for vibration to affect the crystals, but if it does end up being necessary I can probably isolation mount it somehow. Or maybe I should use an ionic wind type fan. Every project needs some nice high voltages, right? The next step will be to scare up a new power supply that'll fit into the old modem box and come up with a connector for attaching the chamber.

I cobbled together a simple temp controller to try to get a feel for what it'll take to make one that will work for my temp controlled crystal-growing chamber. For this first iteration I've just got a simple PID-controlled heater, LM75 I2C temp sensor, and a 25W heater element. For now I'm just running on an Arduino and a crappy bench supply (with a way-too-bright power indicator). For the finished project I'm thinking I will use a Teensy 3.0 as I've got some samples of them that I'd like to do a write-up about. To house the controller I'll be repurposing an old modem that has a front panel with a small LCD and a set of buttons that I can use for menu navigation. I can mount a power supply and all the electronics inside and put a connector on the back that plugs into different chambers. This big chamber will be nice for large beakers, but I'd like to have a smaller one for test tubes. I can put a resistor on the chamber side of the connector so the control box can identify which chamber is connected and select the appropriate PID tuning. I ran this for a few hours to see how it behaves. The 0.5C resolution on the sensor is pretty bad, but overall the results were encouraging. I don't really care much about how it handles large rates of change, since the hot solution will go into a preheated chamber. The main concern will be whether it can hold a stable downward ramp with minor fluctuations outside the chamber. I haven't tested that yet. Next step is to get the heating element mounted so it is stable and out of the way and then test with a beaker of hot water so I can see how well it do temperature ramping. I'm pretty sure I'll need to upgrade to a higher resolution sensor for really long ramps, but for a day or two this 0.5C should be ok if I screw with the PID tuning enough. The PID library has some kind of auto-tune that looks interesting, but I haven't figured out how to use it yet.

For my crystal-growing I'd like to have better control over temperature so that I can cool solutions very slowly. I've been using a heavy ceramic dish and a hot water bath for this, but I want to try cooling rates of days or weeks. To achieve that I think I'm going to want heavy insulation, radiant barrier, and probably a small computer-controlled heater element. As a start I'm using 2" pink foam with aluminum tape as radiant barrier on the inside surface:

Recently I've been playing around with growing crystals. It was always a fun and easy thing to do when I was a kid, so I thought I'd try some that are more interesting than the usual salt, sugar, alum, epsom, etc. This is recrystalized acetylsalicylic acid, asprin, from OTC tablets. 10g pulverized, washed with cold water, vacuum filtered, then dissolved in 250mL of 75C water and allowed to cool slowly. Crystals approximately 3-5 mm in length. I will chill this and allow crystal formation to finish, then wash the crystals and recrystallize at a slower rate. This is what I think is probably salicyclic acid. 1g of OTC tablets, pulverized, heated in 20mL water until dissolved. When cooled the entire 20mL formed one huge crystal block, which was amusing. Melted again, added 10mL water and heated until acetic acid formed. Cooling produced fine, hair-like crystals, which I suspect are salicyclic acid (more research to do). Washed and vacuum filtered, will recrystallize to see if more pure crystals look different.

A friend from the Omaha Maker Group gave me an interesting chip a while back. Apparently it is some kind of optically sensitive chip, it has a clear case.

Thanks again to everyone who helped make today’s booth at the Robotics Expo a huge success again this year.  Booth volunteers included Ben, Don, Mike, Michael, Brandon, Nick and Jared, with material support from Patrick, Sarah, Eric, Dave and Katlynn.  [Apologies in advance if I missed anyone].

I was curious why a large aperture on a lens reduces depth of field. To investigate this I set my camera up with a macro extension tube and a subject with lots of depth.


The subject is a potassium sodium tartrate (Rochelle salt) crystal mounted in a brass holder, with a copper contact wire for detecting the crystal's piezoelectric properties. From the perspective of the camera, this setup looks like this:

OMG’s innaugeral FirLumber Rally was a great success, with 17 cars competing.  Official results will be posted shortly, but in summary, Dave’s electric cheater was the fastest, Eric’s elephant was the most creative, and Nate’s car was the least effort to build.  Additionally, Dan’s kids’ cars were the fastest among the Squares.