conveyor displacement

Hi

i have done a program to fill the oil drum the system consists of a conveyor , two filling station and one centring station

i give commands to the conveyor to move one or two steps to move the drum into or out the filling station


i used an encoder coulped to the the gearbox of the conveyor and counted the pulses coming out of it on a di and calculated the displacement from that.

The machine ran fine for about two months , then i found out that the displacemnt per count of the encoder changed and the a big shift occured in the displacement .

I reclaibrated the step but i want to know if u have an idea why this happened , is there any modifications that i should do to prevent that?

Has there been any slippage in the conveyor system or connection to the encoder? These things could cause an "error" in location.

You may want to add a photoeye and begin your counts when the drum breaks the beam.Good Luck.