How to move string (lengthwise)?

Hey guys, I know its a bit of a broad title.

I'm trying to build my first Arduino project, and I've decided on creating a motorized controller for my vertical blinds that will be driven over RF to open/close my blinds from a PC/smartphone/cron job etc.

The vertical blinds have a pull string that is a loop of string which pulling one strand opens, and pulling the other closes the blinds. I'm trying to figure out how to drive that push/pull interaction with the string.

Since I'm renting the apartment I don't necessarily want to mechanically alter the blind mechanism, so I'm trying to think of something to simulate what my hands do.

I am thinking of using a rubber roller attached to the shaft of a motor, and then 4 more rollers to clamp the string to the roller, something like this:

Has anyone tried something like this before and have any ideas on how to solve this problem?

Thanks! Matt

you might check out a multiturrn winch servo.

Id just wrap a few turns of the string around a pulley with a reasonably grippy surface, you shouldn't have any trouble getting enough force to operate the blinds as long as you keep a light tension on the pulley.