After spending several months testing new robot lawn mowers, I feel pretty confident that robots are not going to take over the planet anytime soon. However, I’m also confident that many of these robots could take over one of summer’s persistent drudgeries: mowing the grass.Most of the robots we tested can reliably keep an average suburban lawn’s grass at a preferred height — with minimal human interference. But the good models stand out for different reasons, and that’s why we found four distinct options suited to different yards and the sentient beings who occupy them.