MAV to Fly in Caves, Tunnels, and Forests
Backpackable, rapidly-deployable bird-sized aircraft, or Micro Air Vehicles (MAVs) can quickly and easily provide soldiers and command and control personnel with an ''over-the-hill'' or ''around-the-corner'' perspective from a remote location. Furthermore, MAVs can be designed to fly in caves, tunnels, and other tight, enclosed labyrinths which are densely populated with obstacles. Previous research yielded a MAV that was capable of acquiring aerial surveillance in a 10 x 10 m2 area (about 1/3 the size of a professional basketball court). However, this much space is not always available in labyrinths. As such, the prototype required modification in order to enable flight in an area one-tenth the size.
The two most important criteria for flight within labyrinths are (a) endurance so that the aircraft can survey a significant amount of ground before the mission's end, and (b) stationary flight to ensure the MAV's safety when flying through narrow passages. Rotary-wing aircraft are obviously capable of hovering, but sacrifice endurance for this ability. Fixed-wing platforms have the endurance advantage because the lift is provided primarily by the wings as opposed to electric motors, but are unable to hover. To meet both criteria, the two advantages of each aircraft configuration were married. The resulting prototype has the form of a fixed-wing aircraft, but with an additional flight mode for hovering. This is made possible by a high thrust-to-weight ratio (T/W > 1) which allows a quick transition from cruise flight (through the stall regime of conventional fixed-wing aircraft) to hovering mode.
Autonomous Hovering
Automating the hovering flight mode requires the aircraft attitude be measured. Microstrain's 30 gram 3DM-GX1 inertial measurement unit (IMU) outputs the MAV's orientation to an onboard processing and control unit every 10 ms (i.e. 100 Hz). After comparing the current orientation to the desired orientation to calculate the error, proportional-derivative (PD) control is implemented to yield the corresponding pulse-width modulated (PWM) elevator and rudder servo commands. A panoramic view of the flying area was desired and thus, the ailerons are not currently used to counter the effects of the motor torque. Furthermore, the throttle is controlled manually to allow for altitude adjustment. This is beneficial because surveillance at different heights (e.g. various floors, different perspectives, etc.) can be obtained.
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