Cycle hire rebalancing algorithm?

Hi - I’m interested in how the “rebalancing” of the bike hire stands is performed, i.e. the system that sends vans out to add/remove bikes so that the stands don’t fill up or empty. Is there any info about what method, what algorithm is used for this? Is there a formal objective specified (e.g. minimising the occurrence of empty stands)? I’d be grateful for any clues about the way it is kept ticking over.