Output Circuit of PI metal detector

[kazem2114]

Hi guys
I simulated the output of my "Luminous 15000 metal detector"
but I am a little bit confused of output waveforms.
my detector is a "PI detector" and it doesn't work and I wana fix it...
"IRF840" is on just for 200usec
Does it mean that my LOOP just search for only 200usec
What I expected to see was that "IRF840" be on for (1-td)
but now it's on for td=200usec
and approximately how much current according to ICs is going to coil(=LOOP)?

Please explain how this part works...
Thanks