Technology copies nature

24 May 2011

Suzanne Gill reports on developments in energy efficiency and lightweight design for automation that were manifested at the Hannover Trade Fair as SmartBird, an elegant project from Festo’s Bionic Learning Network, which in previous years has brought us the AirRay and AirPenguin

SmartBird was, for me at least, one of the highlights of the 2011 Hannover Trade Fair. SmartBird took off, flew, glided and sailed through the air in a realistic manner around the exhibition hall. Its wings not only beat up and down, but are also able to twist at specific angles thanks to the use of an active articulated torsional drive unit, in combination with a complex control system, which has enabled the company to attain an energy-efficient technical adaptation of bird flight.

An interesting feature of SmartBird is the active torsion of its wings and the fact that it does not require the use of any additional lift devices. The aim of the project was to achieve a structure that is efficient, in terms of resource and energy consumption, with minimal overall weight, in conjunction with functional integration of propulsion and lift in the wings and a flight control unit in the torso and tail regions.

The system is designed to operate in an energy-efficient manner. Propulsion and lift are achieved by the flapping of the wings and have a power requirement of around 23 watts. SmartBird weighs around 450 grams and has a 2m wingspan. Measurements have demonstrated an electromechanical efficiency factor of around 45% and an aerodynamic efficiency factor of up to 80%. The onboard electronics ensure precise wing control and the torsion control parameters can be optimised in real-time during flight. The wing flapping and twisting sequence is controlled to within a few milliseconds, resulting in optimum airflow around the wings.

Taking flight
Flapping-wing flight comprises two principal movements. First, the wings beat up and down, whereby a lever mechanism causes the degree of deflection to increase from the torso to the wing tip. Second, the wing twists in such a way that its leading edge is directed upwards during the upward stroke, so that the wing adopts a positive angle of attack.

Each wing consists of a two-part arm wing spar with an axle bearing located on the torso, a trapezoidal and a hand wing spar. The arm wing generates lift, while the hand wing provides propulsion. The active torsion is achieved by a servomotor at the end of the outer wing that twists the wing against the spar via the outmost rib of the wing.

When SmartBird lifts its wings, the servo motor for active torsion twists the tips of the hand wings to a positive angle of attack, which is then changed to a negative angle for a fraction of a wing beat period. The angle of torsion remains constant between these phases. This means that the airflow along the wing profile can be optimally used to generate thrust.

The battery, engine, transmission, crank mechanism and the control and regulation electronics are housed in the torso. By means of a two-stage helical transmission, the exterior rotor motor causes the wings to beat up and down with a reduction ratio of 1:45. This motor is fitted with three Hall sensors that register the wing’s position. Both the flapping and bending forces are conveyed from the transmission to the hand wing via a flexible link. The crank mechanism has no dead centre, running evenly with minimal peak loads, ensuring smooth flight.

The opposing movement of the head and torso sections in any spatial direction is synchronised by two electric motors and cables. The torso bends aerodynamically, with simultaneous weight displacement.

The tail also produces lift, functioning as both a pitch elevator and a yaw rudder. When the bird flies in a straight line, the V-position of its flapping wings stabilises it in a similar way to a vertical stabiliser of an aircraft. To initiate a turn to the left or right, the tail is tilted. When it is rotated about the longitudinal axis, a yaw moment about the vertical axis is produced.

On-board electronics allow precise and efficient control of wing torsion as a function of wing position. A microcontroller calculates the optimal setting of two servo motors, which adjust the torsion of each wing. The flapping movement and the torsion are synchronised by three Hall sensors, that determine the absolute position of the motor for the flapping movement.

The wing’s position and torsion are monitored by two-way radio communication using the ZigBee protocol (Use Linkcode 42144 for more information on ZigBee). Together with the electronic control system, intelligent monitoring enables the mechanism to adapt to new situations within milliseconds. This facilitates the simple, efficient and weight-optimised mechanical design of the bird model for optimised efficiency of the overall biomechatronic system in flight operation.

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