Category Archives: Work

Fun with FLIR

A little while back I posted about testing a Nichrome heater using a FLIR thermal camera. While I had the opportunity, I snapped a couple other pictures. First off, my laptop, a Lenovo Thinkpad T61p. As you may have guessed, the top image of the following composite was taken with the FLIR camera. The orange areas indicate high temperatures while the blue areas indicate lower temperatures.

Lenovo T61p Thermal Image

While it may look like flames are shooting out the left side of the laptop, that’s actually just the desk being heated by the CPU fan. And I’m sure you can guess where the processor is located – just under the caps lock. On the lower right part of the hand rest, you can also see the outline of my three fingers in blue. But those aren’t my fingers – what you’re seeing is merely a thermal “shadow” left by a brief touch that slightly cooled the plastic.

You’ll also notice that the screen is warmest right along the bottom edge. I’m guessing that’s where the back-light is located. And in case you’re wondering, the highest temperature in this image is approximately 95F.

Soldering Iron Thermal Image

This second and final image is, you guessed it, a soldering iron. And as you can see, the hottest part of this particular iron (indicated by white) isn’t the tip, but just above the tip. I also noticed that the iron’s temperature controller wasn’t all that accurate. The hottest point in this image is just over 600C, while the iron’s controller claimed 800C. Perhaps I hadn’t given it time to ready a steady-state temperature distribution?

LabVIEW: Managing Interactive SubVIs

I do a lot of LabVIEW programming. Quite a lot, in fact. Fortunately, I rather enjoy a good block diagram. Plus, the ability to work with virtual instruments (VIs) for control and data acquisition seems highly prized at RPI. Either that, or nobody else wants to do it.

When building programs with LabVIEW, I often find it convenient to group code into subVIs. This can really help clean up a messy block diagram. Occasionally I’ll employ a subVI for user interaction of some type – a sophisticated dialog box, or perhaps a sub-application of sorts. But just how is this done? By default, a when you drag a subVI onto your block diagram, its front panel won’t show when executed. To force the front panel to display when called, first right-click on your subVI, then select “SubVI Node Setup.” Then simply check the following:

SubVI Node Setup

If checked, the option to “Close afterwards if originally closed” will close your subVI once it’s finished executing, if it started closed. This is typically my preference, but there may be a time when you want your subVI to stay open after it stops.

Of course, there are a few other tricks you may wish to employ. Perhaps you’d like your application to be reset its subVI’s values to defaults. Or maybe you’d like to run the subVI, but only display its front panel when certain conditions are true. All this and more is possible with the “Static VI Reference” block, found under “Programming” → “Application Control” and shown here:

Static VI Reference

Place this block on your diagram. Now drag your subVI onto the “Static VI Reference” block you’ve just created. You’ll notice it takes on the same icon as your subVI. From here there are a world of possibilities. To see what I mean, move your cursor over the reference output of this block (as if you were wiring it) and right-click. Next, select “Create” and then “Method for VI Class.”

VI Methods

From here, you can choose to run the subVI, show or hide its front panel, reset its values to default and more. If you back up a step on this menu tree and select “Property for VI Class,” you can further alter the appearance of the subVI window, including hiding buttons (abort, run, etc.) from the toolbar and changing the window title.

Hopefully you’ve found these tricks helpful! I’ll try to post more from time to time.

A Shiny Box of Fuel Cells

This week I completed the last of what RPI requires for a Master of Science degree in Electrical Engineering. Since I’d finished my thesis in the spring, I only needed three more credits to graduate. So I opted for a summer independent study. And what did I independently study? Fuel cells! (From an educator’s perspective.) It just so happened that my supervisor had previously purchased a commercial 300W PEM fuel cell stack. He wanted to use it in some sort of educational demonstration, but wasn’t sure exactly how to make that happen. So my project for the summer was to design and build just such a system. Our goals were to make the system:

  • Instructional for students of all ages
  • Portable and self-contained (no need to ever plug into AC power)
  • Visible (both the components and their connections)
  • Interactive (something you’d like to play with)

With this in mind I put together what I felt was a solid design, then went to work constructing it. With the help of some of my lab mates, we built the following:

Fuel Cell Demo System FrontWhat you see here is a large 80/20 box measuring about 30″ x 20″ x 11″ and covered with acrylic panels. As you probably guessed, the large red tank is the hydrogen supply, pressurized to 2000psi when full. That pressure is regulated down to about 6psi for the fuel cell stack. The gas passes through a ball flow meter and solenoid valve before reaching the fuel cell stack – located just to the left of and behind the red LED panel voltmeter.

Our fuel cell stack is produced by Horizon Fuel Cell Technologies, model H-300 (more details here). It’s an air-breathing PEM-type stack, composed of 72 individual fuel cells strung together in series. The voltage produced by the stack varies from 40-60VDC depending on the amount of current drawn. Now this variation is unsuitable for most electronics, so it’s first passed through a DC-DC converter (the largest black box just to the left of center). The converter takes the varying input voltage and steps it down to about 13VDC for use in the rest of the system.

You may also notice a 12V lead-acid battery strapped into the middle of the demo box. This serves two purposes. First, and most importantly, it provides power to the stack’s control module during startup. This is necessary to open the solenoid valve and engage the three fans mounted to the side of the stack. Second, it provides a bit of buffering during transients (e.g. when all the light bulbs are flipped on). One problem with fuel cells is that they cannot respond quickly to changing loads. Batteries, however, can rapidly supply more or less current without significant changes in voltage (large capacitors also have the same effect).

The rest of the system consists of an array of ten 12V, 13W light bulbs, a 120VAC inverter, and equipment to monitor voltages and current at several points within the system. This equipment is mounted to the rear of the system, shown here:

Fuel Cell Demo System Rear DAQ HardwareThe data acquisition (DAQ) module shown above is produced by National Instruments, model USB-6009, and is capable of monitoring eight analog inputs at 14-bit resolution. These analog inputs are fed from a custom PCB I designed, mounted directly below the DAQ module. This PCB is responsible for measuring currents using ACS712 hall-effect sensor ICs. It also performs voltage division so that the system’s voltages are within the measurement range of the DAQ. Last but not least, the PCB allows for computer control of the ten light bulbs using MOSFETs controlled by the DAQ’s digital outputs.

From the start, I knew I wanted to use LabVIEW to monitor and control the system – it’s built for data acquisition and handles simple controls quite well. The only question was, what sort of hardware should I run it with? Since I didn’t need much horesepower and in fact was looking to minimize electrical power consumption, I went with the Asus Eee PC, model 1001PX:

Asus Eee PC 1001PXWith its dual-core Atom processor, the 1001PX actually performs quite well running Windows XP. Its 20-30W power consumption (when charging) is equally impressive. Running LabVIEW 2009 presented no performance problems whatsoever. My only qualm is the lack of screen resolution – 1024 x 600 is just a bit tight most of the time. However, space was no issue since all of my LabVIEW VIs were compiled into executables without scrollbars, menubars, etc. Here’s how the main panel turned out:

LabVIEW Front Panel

From this panel the user can monitor voltage, current, and power at different points throughout the system. The light bulbs can be turned on and off with a single mouse click. I’ve also created VIs for taking polarization curves (voltage vs. current density) and for monitoring the stack’s voltage at high speed (48kHz) during transients. To top it all off, the Eee is loaded with a sample presentation containing the principles of operation for fuel cells as well as diagrams for the demo system itself.

The system has yet to be tested in a real classroom environment. Sadly, I may not be around to see that happen. But I’m pretty confident that it’ll be put to good use. The grand total for all parts in the system? About $4000. Thanks for reading!

Inappropriate Uses for a Welder

Last week at work I was given the challenge of designing a custom heat sealing tool for one of our automated manufacturing systems. Since a lot of impulse heat sealers use strips of Nichrome foil as heating elements, we thought we’d start there. (As a side note, unless we were totally ripped off, Nichrome foil is apparently rather expensive; a 34″ x 6″ x 0.005″ sheet cost about $600.)

A few “back of the envelope” calculations indicated that we’d need to pump about 180 amps through the Nichrome; the resulting voltage would be about 12V. That’s quite a large power requirement – 2.16kW to be exact – enough to run a couple hair dryers on full blast. So the question was – how are we going to deliver this enormous amount of current? A 10:1 step-down transformer would do the trick, but finding one rated for more than 2kVA (at a reasonable price) was a bit of a challenge.

Lincoln Electric K2535Enter the Lincoln Electric 225A AC/DC (K2535) arc welder. Our machine shop picked one up about a year ago with year-end funds – hooray accounting! It’s not often used, so I decided to borrow it for a quick test. It’s really an impressive device – all you have to do is set AC/DC, then dial in your desired current (settable from 5 to 230A at 1A increments) and hit the foot pedal. There are a number of other settings available which I’m sure any legitimate operator would appreciate.

Anyway, my intention was to use this lovely device as a glorified power supply running in constant-current mode. To do so, we purchased a new plug and wire for the working electrode that would allow us to attach to the Nichrome via wire lugs. The ground electrode simply clamped onto the other end of our test apparatus. After frustratedly poring through a ton of Amphenol datasheets, I finally found the right connector for the foot pedal (parts MS3106A18-12P and MS3057-10A). With this connector I could wire up my own electronic control (a double-pole relay plus driver circuitry) to precisely set how long the welder should operate.

So down to testing! We cut ourselves a strip of Nichrome foil approximately 5″ x 0.5″ and strapped it down to a block of aluminum covered with an electrically insulating layer of Kapton. A couple thermocouples were stuck between the Kapton as well. With everything set, we clicked on the welder and hit the pedal. As it turns out, nothing exploded. In fact, set to DC, the welder made a very nice high-current PSU. Set to about 100A, we had our Nichrome glowing red-hot within a second or two (we calculated a strip resistance of about 0.1 ohms, so this means we were heating at 1kW).

A thermal camera indicated a maximum temperature of about 600C. The assembly, after quickly cooling, looked like this in the infrared:

Nichrome Strip Test ApparatusThe working (positive) electrode connects on the left and is held in place with a couple chunks of plastic (which are starting to get a little warm, as indicated by the orange/yellow false color) . The ground electrode is clamped to the right side of the bar.

It’s interesting to note that this welder begins each operation with about half a second of high-frequency (~2kHz) AC. My best guess is that this is designed to start an arc when one is actually welding:

Heater Strip VoltageThe scope screen-shot above shows an initial spike in current when the welder first kicks on, followed by a period of high-frequency pulsing. While this is happening you can actually hear a sort of “hiss” from the welder, which I imagine is some type of internal spark gap.

As far as the Nichrome is concerned, this is no problem. The strip heats just as well with AC as it does with DC. However, this 2kHz pulse can radiate and wreak havoc on nearby electronics. During testing I noticed that every time the welder started, an AMREL programmable load across the room would beep. It never actually produced an error message or malfunctioned, it just beeped (and beeped consistently). In addition, the first USB DAQ device I tried for controlling the welder would occasionally reset once the welder turned on. This was rather alarming the first time it happened, because the device failed in the “ON” position, leaving me scrambling to unplug the relay before the Nichrome melted. The machinist I work with also told me that “back in the day” he had a welder that would make his watch run faster…

In summary, we wound up with some interesting data on the heating rates of Nichrome. We may actually require a bit more than the 180A originally specified, but this may still be in range for our welder. It’ll be interesting to see how this plays with the rest of our equipment.

MS3106A18-12P