I just happened upon this and I thought I'd share. Here's a kickstarter for a "Roomba" style weed wacker. I'd probably jump right on this if I didn't use Weed Proof plastic in my garden. Could be promising.
https://www.kickstarter.com/projects/rorymackean/tertill-the-solar-powered-weeding-robot-for-home-g
https://www.kickstarter.com/projects/rorymackean/tertill-the-solar-powered-weeding-robot-for-home-g