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ramses

Mon Mar 22 2010, 12:22PM

Registered Member #1208 Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154

Long time, no post...

Anyway, I just won an all expense paid trip to the Intel ISEF, and I have a few weeks to do more research. I have a few specific questions.

First of all, I am researching electromagnetic induction, and I am using a ring launcher to do that. basically dumping a capacitor bank though a pancake coil. Google for more.

Anyway, I am going to get acceleration vs. distance from the coil. At the affiliated fair where I won the trip, I was talking to some engineers, and they suggested placing an LED on the projectile, blinking at a few khz, and taking a long-ish exposure image of it. I thought it was a great idea, and gave it some more thought. I am trying to decide whether to "measure" the length of each streak left from the LED, or the distance between streaks. How many velocity points do you think I need to generate a decent graph?

Also, I realized that whatever circuit I use will be exposed to the ridiculous magnetic pulse, and must be hardened against basically getting EMPed. Somehow, I don't see a Faraday cage working for this. Any help would be great. Estimated rise-time of the current pulse is 200 microseconds.

Also, since I will be testing during the day during spring break I need to do this in my basement (for darkness), I need to absorb around 200J of Kinetic energy from a projectile that is moving straight up, and not have it drop on my cap bank, or my head. Any suggestions?

EDIT: We have established on another forum that a steel Faraday cage will be my best bet for shielding.

The next issue is grounding for the oscilloscope. I My circuit is not grounded, how would I go about doing this? Should I use the wide "neutral" part of a polarized plug, or the safety ground?

Dave Marshall

Mon Mar 22 2010, 02:28PM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

I'm a little confused on what you're attempting to do, but it seems like a simple photo-gate based chronograph (Bjorn has a fantastic PIC based one that's very easy to build and costs next to nothing) would be far simpler and cheaper to implement. Using multiple gates at various points along the projectile's path would give you a good graph.

-Dave

ramses

Mon Mar 22 2010, 03:24PM

Registered Member #1208 Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154

Photo gates would be great, but the disk tumbles. Thanks, though!

Turkey9

Mon Mar 22 2010, 04:09PM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

If the disk tumbles then your led will also. This will make it just as accurate as a chronograph. I know they love new ways to measure stuff at ISEF but you need good reliable data from a device that won't give you problems.

As far as grounding your O-scope, if it has an earth ground on the plug it already is. If you want to measure an ungrounded circuit and want to keep the grounds isolated, use both o-scope channels. Clips the grounds together on your probes and use the tips to make the measurement. Set your scope to add the channels and invert channel 2. This will give you the differential reading.

P.S. see you at ISEF!

IntraWinding

Mon Mar 22 2010, 06:54PM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

Wouldn't it be easier to have a strobe illuminating the scene rather than attached to the disc?

ramses

Mon Mar 22 2010, 07:36PM

Registered Member #1208 Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154

I don't think a tumbling LED matters. I will be measuring the lateral length of the streak, parallel to the direction of travel, and It won't make a full rotation in the distance I am looking at. With the photo gates, you might get artifact pulses from the hole in the center and the changing apparent thickness of the disk.

As to the strobe, it would be easier if I had a ~3 khz strobe able to reflect enough light off the disk with each pulse The LED will be viewed directly by the camera, and I can faintly see other LED at f2.8, iso 1/40,000 of a second.

With the scope, that is a great idea! I would use both channels, but I really "need" one for current and one for voltage. So, any advice on grounding the circuit? Do I need to ground it to use a ghetto uncalibrated rogowski coil? I will mostly use either a 454 or a 565 scope. How much voltage do these tolerate on the inputs, without clipping (or blowing the transistors)

Thanks,
Chris

@turkey, what is your project?

Turkey9

Mon Mar 22 2010, 09:58PM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

I built a space propulsion system similar to the PIT that NASA did in the 90s.

klugesmith

Tue Mar 23 2010, 12:07AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

ramses wrote ...
As to the strobe, it would be easier if I had a ~3 khz strobe able to reflect enough light off the disk with each pulse The LED will be viewed directly by the camera, and I can faintly see other LED at f2.8, iso 1/40,000 of a second.

Congratulations on your progress in the tournament, Chris.

As for logging the motion of disk:

1) I think 10 time/position points is plenty to get the acceleration profile.

2) the time and space region of interest is either roughly the duration of current pulse (including ringing) or a displacement comparable to the diameter of disk and coil (whichever comes first). After that, there won't be much more acceleration.

3) an LED fixed to the disk is easy to see and photograph, but its weight & wiring perturb the free motion of disk. Wires should be thin & twisted together to minimize induced voltage in the LED circuit.

4) but photo showing whole disk illuminated by multiple flashes looks better & tells you more. The strobe light source can be an array of high-power LEDs, which can be substantially overdriven for a finite train of short pulses. Timing would be identical to that of the indicator LED, for example 100 microseconds ON, 300 microseconds OFF, 10 or 20 pulses starting with the launch event. (I have long wanted to experiment with LEDs as high-speed illluminating strobes).

ramses wrote ...
Also, since I will be testing during the day during spring break I need to do this in my basement (for darkness), I need to absorb around 200J of Kinetic energy from a projectile that is moving straight up, and not have it drop on my cap bank, or my head. Any suggestions?

What is special about straight up? Would you test a cannon by firing at that angle?

Also, re. EM interference with your instrumention: the H field amplitude drops off as the cube of distance. You can make a hand-held pickup coil, connect it to an oscilloscope using a 10-foot-long cable, and progressively approach the launcher. Find at what distance and orientation you get 3 A/m (I think that's a typical number for equipment susceptibility standards). For this you don't need full power discharges.

I hope your ISEF board gives credit to any ideas you get here.
-Rich

ramses

Tue Mar 23 2010, 03:10AM

Registered Member #1208 Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154

Thanks, and while illuminating the whole disk could potentially look better, I have a tight budget, and a huge time crunch. Also, I am closing the circuit with a mechanical switch, so there is no easy timing pulse to work with. I don't really have time to build a powerful driving circuit. My disks will approach at least 1/2" thick, and to record 10 images of them in a ~3 inches would tend to make a blurry mess, with at least 66% overlap. It will be rather hard to extract certain blurs out of this big blur. I have access to a high speed camera which is not fast enough to derive acceleration, but will suffice with the "looking nice" department. As to the weight, one of my variables may be "dead weight" added to the disk to simulate loading of induction motors.

Anyway, I expect that I will need to use full power pulses for the current data, because my switch arcs, and conductivity may vary strangely with voltage.

As to the straight up direction, it keeps the disk on the coil nicely, and dissipates all recoil into the ground effectively. I just have to make sure the disk doesn't land anywhere it isn't supposed to.

I have decided that I will probably not pursue either voltage trace, but I still need a common ground between the high and low side of my circuit for a safety interlock. Attached is a schematic. The interlock will dump the bank through a water resistor and disconnect the "primary" of the variac

I will definitely acknowledge this thread on my board. Lord knows I have to fill up the board somehow...

Thanks for all your help,
Chris

ramses

Wed Apr 28 2010, 11:06PM

Registered Member #1208 Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154

Okay, I just finished my tests and I found some surprising results. The Copper disks performed much worse than the aluminum ones, and the thick disks performed much worse than the thinner ones. I can honestly say I'm stumped.

I'll link and stuff to the spread sheet with my data and graphs.

here

Any Ideas? I'd compensate for conductivity, do a regression, and blame mass, but I can't find the relative conductivities. Anyone who has worked substantially with FEA software, feel free to find the hoop conductivity of copper and aluminum disks 6,8, and 12 mm thick, 72mm diameter, with no center hole, at 1785hz. The skin effect in the corners should be interesting.

thanks!

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