  Wings of flying animals can not only generate lift and control torques but
also can sense their surroundings. Such dual functions of sensing and actuation
coupled in one element are particularly useful for small sized bio-inspired
robotic flyers, whose weight, size, and power are under stringent constraint.
In this work, we present the first flapping-wing robot using its flapping wings
for environmental perception and navigation in tight space, without the need
for any visual feedback. As the test platform, we introduce the Purdue
Hummingbird, a flapping-wing robot with 17cm wingspan and 12 grams weight, with
a pair of 30-40Hz flapping wings driven by only two actuators. By interpreting
the wing loading feedback and its variations, the vehicle can detect the
presence of environmental changes such as grounds, walls, stairs, obstacles and
wind gust. The instantaneous wing loading can be obtained through the
measurements and interpretation of the current feedback by the motors that
actuate the wings. The effectiveness of the proposed approach is experimentally
demonstrated on several challenging flight tasks without vision: terrain
following, wall following and going through a narrow corridor. To ensure flight
stability, a robust controller was designed for handling unforeseen
disturbances during the flight. Sensing and navigating one's environment
through actuator loading is a promising method for mobile robots, and it can
serve as an alternative or complementary method to visual perception.
