what can we say about optical triggering? It would seem to be the case that this
triggering method produces a slightly reduced (~3-5%) muzzle speed compared to
the optimal open loop method. This difference is quite small and it might not
be possible to attribute it to any one particular source. There are several possible
options as outlined below -
One reason could be due to a small timing differences. If we consider that the
muzzle speed is obtained by a tradeoff between an accelerating impulse and a retarding*
impulse then there will be an optimal turn off position when the difference between
the acceleration and retardation impulses is maximised. This peak is evident from
the open loop experiments where, as the pulse length was increased, the speed
was observed to build to a maximum and then start to decline. The turn off point
for the optical trigger might be slightly different than the optimal open loop
turn off point.
Another reason could be differences in the state of the battery source. The internal
resistance of the source varies according to its state of charge and the charging
history. A freshly charged source will give more 'punch' than one which has been
sitting for a while after charging. I must confess that this is something which
I didn't pay too much attention to. The open circuit voltage during the open loop
triggering was 'around' 32-33V but during the the optical triggering it was maintained
to 32.0 +/-0.1V. The small inconsistency between the open circuit voltage could
be the main reason for the difference.
A further possibility is that, even though the trigger pulses are quite similar,
the optical trigger pulse doesn't behave exactly like that of the open
loop trigger. This might influence the switching behaviour of the mosfet module.
is perhaps a good lesson in proper experimental conduct - trying to determine
very small differences between processes requires that careful attention be paid
to the experimental procedures and the state of the equipment. This is something
I'll give more attention to next time.
there are small experimental deficiencies I think we can say with some confidence
that optical triggering does provide almost ideal current timing.
In coilgun circles the retarding impulse is often referred to as the 'suckback'
effect. The term was coined by Barry of Barry's Coilgun Design Site.