When I use a standard manual vacuum, I often notice that I have to pass over a spot several times because a single pass does not necessarily catch all the dirt. My eyes/brain can easily perceive this information visually, but I don't know how an autonomous robot vacuum can detect whether a pass over a patch of dirt was successful or not. What kind of sensor/action can I use to determine if the robot vacuum successfully picked up the dirt from a particular patch?
I would prefer to avoid a visual camera if at all possible because it would necessarily have to be mounted above the robot and thereby limit the range of reachable locations. Is there some other low-cost sensor that can accomplish the same task that can be placed low to the ground?