The main problems witht his type of system is to ensure that the coin is rolling and not bouncing when it enters the coils,usually with a peiece of foam rubber just under the coin slot.
The the decision routine,you need to construct a two dimensional table,with damping on one axis,frequency shift on the other,by puttting lots of samples of coins through the system you will find that you get clusters for each coin which you can adjust statistically to get your desired rejection rates.If you need any other help mail me at jccs3000@yahoo.co.uk