Data acquisition & sensors, Copper Corrosion, Failures, and ‘Name that device’

So my new recent project has been to develop a bunch of sensors and a data acquisition unit (DAQ), and I have already finished a few tiny boards with some more work to be done. Just as some eye candy, a voltage sensor (divider for a ADC on a microcontroller, selectable to 1:2, 1:3, or 1:5), and a current sensor (resistive 0.001ohm shunt with a current sense amplifier, MAX4372).

Another thing I thought I would bring up is the effect of silicone caulk on electronics. While on the ASV project, despite my protest that caulk would corrode circuit boards due to the release of acetic acid fumes, we ended up moving ahead with it anyways. Regardless, here are some photos of a circuit board exposed to some fumes from caulk, even at a far away distance.

Everyone has had some failure when trying to develop a new device, and as part of my line of sensors, I was going to use a strain gauge taken out of an electronic bathroom scale. Unfortunately I’ve noticed that the resultant sensitivity is far too low with a differential opamp, and that even 1% resistors are not good enough to get the desired value of 0V difference between the two ends of the wheatstone bridge. I’ll have to revisit it at some point, mainly because a strain gauge would be a useful piece of kit. I’m sure that if I get it trimmed out properly and maybe use a higher Vcc for more resolution that the strain gauge will be useful, but until then its a bit of a failure.

And now for a new thing, ‘name that device’. I don’t have anything to give away or award to whoever gets it right, but regardless here are some photos. If you have any idea of what this device is, leave a comment to this post. I’ll reveal its identity after a week or so.

2.5 kJ Capacitor Bank Complete!

After plenty of delays due to more important projects, I have finally completed the capacitor bank control system and construction.

The capacitor bank is a 22,500uF, 450V capacitor bank with a 10kA surge current capacity using solid state switching. A custom built charger was created so that I could use 12VDC (ie: car batteries) to charge the capacitor bank outdoors for more energetic tests. Along with the charger, a control system to monitor bank voltage, and provide safety interlocks to prevent discharge during firing, automatic voltage control, and discharge load control to abort high-current tests were key aspects of the charge controller.

DIY Robot-boat at AUVSI’s Autonomous Surface Vehicle Competition 2009

So the major project that I have been involved in recently was working on an autonomous surface vehicle (ASV) that was entered in AUVSI’s 2009 ASV competition. Given a small team (6 people, 2 builders), under $1000, and about 30 days we built what would be considered a feat of bodging and hacks based on extreme ingenuity and resourcefulness.

Note that of the four people in the photo above, it consists of (from left to right) a comp sci masters student on exchange, a political science student, a broadcast technician, and a chemistry/biology student.

Cap Bank – Rogowski Coil

So before I really get started with my capacitor bank, I decided I would design on paper all the instrumentation so I could characterize the capacitor bank discharges. In order to measure current over time, either current shunts (DC), current transformers (AC), or rogowski coils (AC, pulse) can be used. The advantage of a rogowski coil is that it is equivalent to a air-core current transformer allowing for much faster response to changes in current flow. Also, due to construction, a rogowski coil can be opened and closed for placement in temporary positions.

Yesterday I finished my constructing my rogowski coil, however it still requires an active integrator to make voltage proportional to current. I used RG6 coax cable, stripped the outer sheet, braid, and foil, then wound 30AWG magnet wire evenly around the dielectric. The magnet wire was attached to braid and core at either end, heat-shrinked, and luer-lock syringe fittings were placed on the ends to allow for the coil to be placed around an object.

A major key to being able to construct a accurate rogowski coil is that the windings must be absolutely even, and remain even as the coil is bent and closed. It is easier to wind a coil on a straight segment of dielectric, heatshrink, then bend, instead of attempting to precision-wind around a torroidal coilform.

The luer-lock fittings prior to attachment.

The completed rogowski coil, note the heatshrink around the coil and BNC connector on the end.

Closeup of the joint where the coil closes. I didn’t have small enough heat-shrink so I had to wedge some tiny pieces of balsa wood in to keep everything snug and secure.

Once I had the coil built I gave it a test, first using a small motor, and a second test using my old coilgun.

I used a induction motor fan wired up to 120VAC for the first test, it didn’t seem to be affected by an un-centered conductor, however I may still build a plexiglass support so that under higher-voltages there is no risk of arcing into the coil (the heatshrink + enamel can only stand so much).

The test setup and waveform (sine = 120VAC, other = current).

The second test was performed using my old 430J coilgun, I looped the coil around the heavy cable from the stud-SCR and tested a few voltages. I captured the waveforms using a soundcard oscilloscope program and got some data, however the higher powers generated voltages exceeding the max rating of the soundcard, causing clipping.

The 3 test waveforms captured at increasing capacitor voltages, note the last test at 430V caused significant clipping of the current waveform.

And also, 2 videos (the last 2 tests)

Mini project – Vacuum tube retro

So I was browsing through Active Surplus (downtown Toronto on Queen St.) and saw (like any other trip there) these gigantic cardboard boxes full of vacuum tubes. Apparently they were $1 a piece, but I never asked before. I came home with 5 tubes and 5 matching sockets and went to home depot to buy a prime (read $2) piece of wood.

I drilled evenly spaced 1″ holes (just a hair under the size of the tube socket base) and mounted the sockets, then added ‘feet’ to give it more presence.

I plan to look up the datasheets on the tubes and find out the fillament voltages, with any luck they will all have matching filament voltages so I can power them all off a 6.3V or 12.6V power supply (perhaps just a LM317 set for an appropriate voltage) to get warm glowing tube goodness. A bench test with 6V and 12V shows that this does actually work.

Note for photographers: this was a super low light photo, I had to take 2 separate 15 second exposures at both 200 and 400 ISO and composite them to get a decent looking photo…. in real life the tubes are no where near as bright.