This is not a question about expression pedals...

How would I perform an 'automated' volume increase of a looped arpeggio so that its volume gradually increases to max over a certain number of loops. I've created a arp that is ~7 seconds long. I turn it off after the 6th time through. I'd like the arp to come in a half volume and ramp up on its own over the duration of the first 4 loops, and play the final two loops at full volume. 

Has anyone managed to do this? And would you mind sharing how you did it? Thanks all!