An ultrasonic sensor added to the the Tianchen TC-G158 robot mower (Arduino hack). For more details see http://grauonline.de/wordpress/?p=50.
Also, see http://www.ardumower.de for a complete open source robot mower hack.
, https://i.ytimg.com/vi/iHCZwA9AebY/hqdefault.jpg
source
Why the bumps taped to the wheels?
R2-D2 got shorter.
Also, there's an open source project now for these hacks (ardumower.de) !