Roland Behrens, a German scientist, wants to teach robots how not to kill humans. But first, he needs to know how hard they can hit before causing any serious damage. According to a Bloomberg report, a team of researchers at the Fraunhofer IFF Institute in Germany is focused on finding a way for machines to coexist with humans, without hurting them. They've devised a machine that hits a group of volunteers on the arm or the shoulder so that they can study the impact at the highest mass and the lowest velocity. After the pendulum-like robot arm strikes, the volunteer rates the pain on a scale of one to ten. The team stops testing when the pain reaches five and the subjects are then put through an ultrasound to check for bruises from the impact.