Setting up modulo for servomotor

[Plc_User]

I have to commision a servosystem where one of the axes has a modulus : it is a chain that keeps on turning in the same direction, comes back to zero each turn at the same point.
Because even afther numerous rotations my position has to be repeatable, I should need to know the exact ratio of motor revolutions to revolutions of the chain. If it concerned a small chain, I could count the teeth of the chainwheel connected to the drive and count the segments of the chain.
But because the chain is really long, this is no solution.
I was thinking of rotating the chain for multiple turns and look how many turns the motor made, and then guess for the
ratio motor/chain.
This must be a situation many of you encountered.
How do you find the exact ration of motor revolutions to 'chain' revolutions?

 

[Peter Nachtwey]

Ask the mechanical designer. If they didn't kludge the system together they should know.

If the system is a kludge then your idea is the way to go. Mark the chain and the sprocket driving the chain. Turn the motor until the mark comes back to its starting point. You know how many motor turns or and therefore links have gone by.

Hopefully you don't have a gear box between the motor and the sprocket. If so then this is an irrational number and there will not be an exact answer. In this case you will need to have a metal flag on the conveyor that can be used to reset the counts.