mitsubishi alpha xl 2 tachometer

hi guys ,
im trying to get a rpm reading off a shaft of a low rpm rotor, the roter speed is well less than 200 rpm ,
how would i go about this with the alpha plc ? . im using a standerd proxmity for the pulse , do i need a magnetic pick up . i can use a counter and it counts most of the pulses of the proxy , but when it speeds up it looses some of the pulses , also tried the speed detect , but the same thing happens , im not really sure what i should be doing , is it a case of the proxmity just isnt up to the job ? i selected my input as a analoge dc input and used a smith trigger to fire into the speed detect over 1v , at higher speeds i dont reach 1v tho ,
what would the normally of mesureing rpm ?
thanks for any tips
stephen !

if alpha has trouble counting but has analog input, you should be able to add pulse to analog coverter.

it has an analoge input , i have selected the input as a analoge dc input , so are you saying try adding a one shot pluse to the signal ?


thanks for your post