Our under $150 Deebot 500 uses a bumper and sensors like gathermewool mentioned above. It random navigates pretty well though and rarely bumps into anything. As you say though, the size of our kitchen chairs are such that the vac can get caught dead center between the 4 chair legs. If this endless loop of gyrating due to being caught in there happens, it will eventually beep, send a message and shut down.
