Optical Triggering Results

Coil Comparison

So, 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 -

1. 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.

2. 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.

3. 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.

This 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.

While 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.