i am incorporating ABB ACS M1 AC drive in a 7 blocker wire drawer machine, where for speed accuracy is the far most requirement, even though the ABB M1 drives are running in DTC mode. more accuracy is need as the wire drawer machine have to achieve the line speed of 15m/sec. for that purpose, I have used Pepprl+Fuchs GL5-U/43a/115 slot photo sensor against 36 slot holes metal wheel enclosed in a metal casing connected with the motor via shaft just like the normal encoder. but as per the sensor used (only 4-wire brown-+24V DC,Blue-COM,Black-SIG+ and White-SIG-) are there, so according to drives parameter no.-93 (pulse Encoder Configuration) Number of pulse per revolution PPR has to be 36 as the no. of holes in the wheel is 36 and single track is set. but the real problem is I am getting RPM feedback ( Encoder Feedback) only above 430rpm below that the module (FEN-01) sensing absurd values. even the feedback above 430rpm is not so accurate. kindly anyone suggest me what should i do...overcome this problem, one thing want to tell all of you that increasing the number of holes in the wheel is not possible as the our so called encoder module has been dispatched to client.