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Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
Now that I have 6x CM600 IGBT bricks, I am starting a 3 stage half bridge coilgun, as a filler project in between railgun shots. I set aside 3 capacitors from the railgun, each stage will use 312 joules, in a diagonal half bridge. Now the questions: On the few half bridge coilgun's I have seen, the pulse length is predetermined, and programmed to shut off the IGBT's. I would like to integrate a active sensing method to automatically turn of the IGBT's, when the current starts decaying. The best idea's I have had were magnetic B-dot probes to sense when the field is collapsing, or some sort of falling edge trigger. Are there any other ways, or are these ways good?
Why would you want to shut off the coils when the current starts decreasing? This may be too late and the projectile might be sucked back into the coil. I'd personally go with optical sensors.
Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
When the current starts decreasing, the magnetic field starts collapsing, so there is back EMF. After that, its a matter of tuning the coil. I will probably start with optical sensors, and move to both, the first one to trigger shuts it off. Optical sensors will probably work better, I want to try both though.
Registered Member #1513
Joined: Sat May 31 2008, 12:08PM
Location: Russia Klin
Posts: 48
rp181, you may turn on IGBTs when projectile is going into optical sensors, and turn off IGBTs when projectile is going out of optical sensors - it will be easy. Projectile lenght must be = lenght of coil
I use optital (IR) sensing to know when the projectile is approaching the next stage. I also use a 555 timer on each stage. This allows small incremental adjustment of each stage to give optimum acceleration as the projectile passes the IR sensor. The 555 is adjusted in increasingly small time increments as the projectile passes each stage.
It's tedious work work an o-scope and also a pair of IR sensors mounted approx at the end of the barrel and also 6" out. This tracks the time which can be converted to velocity in ft/sec. Usually you do the stage one first, then add stage two and do that one, then keep up this process until your projectile is really whizzing along.
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1kJ 3 stage half bridge coilgun
