RESEARCH: Optical Flow Sensor
Optical Flow sensors provide reading of the movement by visually reading the surface movement. This is using a very similar technology like the optical mouse for detecting the direction of the movement. I think there are many excellent use cases for these sensors.
I saw an application of this sensor for a robot that had to push a box and call for help if it was too heavy to push on its own. If the robot is not detecting movement optically/visually despite of its wheels are turning, then we know the help is needed.
Some related videos
https://www.youtube.com/results?search_query=PMW3901+Optical+Flow+Sensor
https://www.youtube.com/watch?v=T1e3lTzjw90
https://www.youtube.com/watch?v=kmpxkKq3zNA
https://www.youtube.com/watch?v=6aQG-J4NzSE
https://www.youtube.com/watch?v=4UeJVA69frw
