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Registered Member #7669
Joined: Mon Oct 29 2012, 02:55PM
Location:
Posts: 19
Hi
This is my first post although I have been lurking for a while. I am building a coilgun for my mech.eng degree project. It will be a low voltage (50v) 3-stage model with optical triggering and optical speed measurement at the business end.
I am in the theoretical calculation stage at the moment, and I am doing all the calculations for inductance, capacitance, time constant etc, and I was wondering if anybody has tried to work out the current that goes through the coil as the cap discharges through it? Iassume that since the resistance of the coil is negligible, ohms law doesn't apply, so the current would depend on the magnetic fields, ESRs, etc, but I don't know where to start. Has anybody had any success calculating the current?
Registered Member #7669
Joined: Mon Oct 29 2012, 02:55PM
Location:
Posts: 19
Thanks for the quick response, I will give it a go when I get onto a computer with Java.
On a separate note, am I right in saying that; say the pulse starts when the 'slug' is flush with the back of the coil and ends when it reaches the middle of the coil; then it cannot travel any faster than the length of the coil divided by the pulse time? Say I have designed my LC circuit to have a time constant of 5ms, and the length of the coil is 25mm, would that mean the maximum speed achievable is (0.025/0.005) = 5m/s?
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
lauriethepigeon wrote ... ... Say I have designed my LC circuit to have a time constant of 5ms, and the length of the coil is 25mm, would that mean the maximum speed achievable is (0.025/0.005) = 5m/s?
Hi LTP. Are you still following this thread?
Be careful using the term "time constant". In LC circuits that usually means sqrt(LC). The time for a half cycle of oscillation is pi times that time constant.
As for the speed limit, it depends on the time, distance, and force profiles. Using values you gave, consider the simple case where force and acceleration are constant from x = 0 (entry to coil) until x = 25 mm (centered in coil), and zero elsewhere. Velocity is proportional to time, and kinetic energy is proportional to displacement. If the acceleration takes 5 ms, then average velocity is 5 m/s and final velocity is 10 m/s. Exercise: what is that acceleration in g's?
Any interest in seeing a simulation of that simple case, extended to 3 stages? Model has stages of identical length and diameter, capacitor energy, and energy added to projectile. Force vs distance curve, rectangular in this model, is the same for each coil. So the later stages need shorter current pulses. But same peak ampere-turns.
That would be tricky in practice, not even counting the rectangular force vs distance curve. To make the time scaling work, each stage would need a different value of L, C, and/or V. (L can be varied by winding different wire gauges on identical bobbins).
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