An autonomous cleaning robot includes a drive system to maneuver the autonomous cleaning robot across a floor surface; a cleaning assembly for cleaning the floor surface; and a sensor system disposed at a forward portion of the autonomous cleaning robot. The sensor system includes a movable element having (i) a first configuration in which the movable element extends beyond a bottom surface of the autonomous cleaning robot by a first amount, and (ii) a second configuration in which the movable element extends beyond the bottom surface of the autonomous cleaning robot by a second amount less than the first amount; a spring mechanically coupled to the movable element, the spring being biased to hold the movable element in the first configuration; and a sensor assembly configured to generate a signal based on the configuration of the movable element.