Perching Landing Gear Systems
Mechanics of Aerospace Structures II
The Air Force Research Laboratory (AFRL) has begun efforts in development of technologies for bird and insect-sized micro air vehicles (MAVs). In order to meet its goals for urban missions, the MAVs need to be able to perch for either recharging or for intelligence, surveillance and reconnaissance of stationary targets. The landing environment might be on a branch, but more likely would be on a ledge or other horizontal platform. Either way, the landing will not resemble a roll-out landing, but a perching maneuver much like a bird. Concepts for a landing gear are to be developed for this platform. The landing gear must enable the landing itself, limited ground mobility for repositioning and other maneuverability at the perching site, and possibly incorporate energy harvesting or other functionalities needed for completion of the mission. The requirements of this SBIR grant were distilled to 8 major points based on feasibility, tangible and intangible value to the objective, and cost vs. reward. A summary of the major system requirements decided upon is presented below. 1. The landing gear design selected must be able to interface and attach to a quadcopter. 2. The system must be capable of flat surfaces down to a width of 500mm and up to a tilt of 30 degrees. This is based upon an assumption of window ledges and roofing angles. 3. The design must be capable of landing on rigid branches down to 50mm diameter given expected branch diameters. 4. The landing gear must have some mechanism which provides take-off assist for energy efficiency and operational purposes. 5. This device is to be designed for prolonged field use and as such must be capable of multiple take-off and landings without manual reset 6. Another function of the environmental conditions expected for the MAV and pilot error, the design must be able to withstand a drop from 2m to concrete 7. For optimal surveillance the quadcopter should be able to reposition on the landing surface/ground. 8. The landing gear must be able to provide stabilizing forces to the quadcopter in winds up to 5kt, such that sustained perching on the landing surface without motors can be achieved. This design was a small and stable system, low to the ground, with four points of contact for horizontal surfaces and 8 for branches. It used cable driven linkages operated via high-torque servo-motor to actuate the appendages. The device works via combination of motor-driven linkage actuation, and torsion springs. When the motor turns on the cables in the linkages pull tight, creating a compressive force around whatever object is within the range of the arms. When the motor is turned off the linkages snap out due to the torsional spring force. Mitre gears make up the power transmission and transfer power from the motor to the cable spool. In the images above the motor has tensioned all the linkages to a folded state. In this state a ratchet and pawl in the power transmission locks the system in the state above. When the motor is back-driven slightly the pawl releases and the 8 torsion springs, 2 on each leg, provide a strong torque to each leg, which then pushes off the ground and ‘launches’ the system into the air. In the grasping phase the motor applies a tension force to the cable linkages within each arm which causes the linkages to close on one another. There is a small ratcheting mechanism within the geartrain that locks the position of the legs once the motor tension is no longer applied. Small micro-spines on each leg provide additional stabilizing force given surface asperities 5nm or greater. This allows the mechanism to grasp on to any surface. Micro-spines can be fabricated by a rapid prototyping process; shape deposition manufacturing, which permits hard and soft materials to be combined into a single structure.