Ultrasonic carpet sensor

Robots that vacuum and mop in one pass need to know what surface they are on. Many use an ultrasonic sensor: it emits a sound pulse you cannot hear and reads how the floor bounces it back. Soft pile absorbs differently than tile or wood.

iRobot told Gizmodo this is how the Roomba Max 775 decides when to cover its mop roller. It is not foolproof – the company said very low-pile carpet can be harder to detect, and a clear plastic chair mat may have read as hard floor when the robot mopped a corner of a rug.

If yours gets it wrong, the fix is usually a no-mop zone drawn on the map in the app.