John P. Whitney

Aeromechanics of passive rotation in flapping flight

Flying insects and robots that mimic them flap and rotate (or ‘pitch’) their wings with large angular amplitudes. The reciprocating nature of flapping requires rotation of the wing at the end of each stroke. Insects or flapping-wing robots could achieve this by directly exerting moments about the axis of rotation using auxiliary muscles or actuators. However, completely passive rotational dynamics might be preferred for efficiency purposes, or, in the case of a robot, decreased mechanical complexity and reduced system mass. Herein, the detailed equations of motion are derived for wing rotational dynamics, and a blade-element model is used to supply aerodynamic force and moment estimates. Passive-rotation flapping experiments with insect-scale mechanically driven artificial wings are conducted to simultaneously measure aerodynamic forces and three-degree-of-freedom kinematics (flapping, rotation and out-of-plane deviation), allowing a detailed evaluation of the blade-element model and the derived equations of motion. Variations in flapping kinematics, wing-beat frequency, stroke amplitude and torsional compliance are made to test the generality of the model. All experiments showed strong agreement with predicted forces and kinematics, without variation or fitting of model parameters.

Pop-up book MEMS

We present a design methodology and manufacturing process for the construction of articulated three-dimensional microstructures with features on the micron to centimeter scale. Flexure mechanisms and assembly folds result from the bulk machining and lamination of alternating rigid and compliant layers, similar to rigid-flex printed circuit board construction. Pop-up books and other forms of paper engineering inspire designs consisting of one complex part with a single assembly degree of freedom. Like an unopened pop-up book, mechanism links reside on multiple interconnected layers, reducing interference and allowing folding mechanisms of greater complexity than achievable with a single folding layer. Machined layers are aligned using dowel pins and bonded in parallel. Using mechanical alignment that persists during bonding allows device layers to be anisotropically pre-strained, a feature we exploit to create self-assembling structures. These methods and three example devices are presented.

Monolithic fabrication of millimeter-scale machines

Silicon-based MEMS techniques dominate sub-millimeter scale manufacturing, while a myriad of conventional methods exist to produce larger machines measured in centimeters and beyond. So-called mesoscale devices, existing between these length scales, remain difficult to manufacture. We present a versatile fabrication process, loosely based on printed circuit board manufacturing techniques, for creating monolithic, topologically complex, three-dimensional machines in parallel at the millimeter to centimeter scales. The fabrication of a 90?mg flapping wing robotic insect demonstrates the sophistication attainable by these techniques, which are expected to support device manufacturing on an industrial scale.

Progress on 'pico' air vehicles

As the characteristic size of a flying robot decreases, the challenges for successful flight revert to basic questions of fabrication, actuation, fluid mechanics, stabilization, and power, whereas such questions have in general been answered for larger aircraft. When developing a flying robot on the scale of a common housefly, all hardware must be developed from scratch as there is nothing ‘off-the-shelf’ which can be used for mechanisms, sensors, or computation that would satisfy the extreme mass and power limitations. This technology void also applies to techniques available for fabrication and assembly of the aeromechanical components: the scale and complexity of the mechanical features requires new ways to design and prototype at scales between macro and microeletromechanical systems, but with rich topologies and material choices one would expect when designing human-scale vehicles. With these challenges in mind, we present progress in the essential technologies for insect-scale robots, or ‘pico’ air vehicles.

Effect of Flexural and Torsional Wing Flexibility on Lift Generation in Hoverfly Flight

The effect of wing flexibility in hoverflies was investigated using an at-scale mechanical model. Unlike dynamically-scaled models, an at-scale model can include all phenomena related to motion and deformation of the wing during flapping. For this purpose, an at-scale polymer wing mimicking a hoverfly was fabricated using a custom micromolding process. The wing has venation and corrugation profiles which mimic those of a hoverfly wing and the measured flexural stiffness of the artificial wing is comparable to that of the natural wing. To emulate the torsional flexibility at the wing-body joint, a discrete flexure hinge was created. A range of flexure stiffnesses was chosen to match the torsional stiffness of pronation and supination in a hoverfly wing. The polymer wing was compared with a rigid, flat, carbon-fiber wing using a flapping mechanism driven by a piezoelectric actuator. Both wings exhibited passive rotation around the wing hinge; however, these rotations were reduced in the case of the compliant polymer wing due to chordwise deformations during flapping which caused a reduced effective angle of attack. Maximum lift was achieved when the stiffness of the hinge was similar to that of a hoverfly in both wing cases and the magnitude of measured lift is sufficient for hovering; the maximum lift achieved by the single polymer and carbon-fiber wings was 5.9 × 10(2)( )μN and 6.9 × 10(2)( )μN, respectively. These results suggest that hoverflies could exploit intrinsic compliances to generate desired motions of the wing and that, for the same flapping motions, a rigid wing could be more suitable for producing large lift.

Conceptual design of flapping-wing micro air vehicles

Traditional micro air vehicles (MAVs) are miniature versions of full-scale aircraft from which their design principles closely follow. The first step in aircraft design is the development of a conceptual design, where basic specifications and vehicle size are established. Conceptual design methods do not rely on specific knowledge of the propulsion system, vehicle layout and subsystems; these details are addressed later in the design process. Non-traditional MAV designs based on birds or insects are less common and without well-established conceptual design methods. This paper presents a conceptual design process for hovering flapping-wing vehicles. An energy-based accounting of propulsion and aerodynamics is combined with a one degree-of-freedom dynamic flapping model. Important results include simple analytical expressions for flight endurance and range, predictions for maximum feasible wing size and body mass, and critical design space restrictions resulting from finite wing inertia. A new figure-of-merit for wing structural-inertial efficiency is proposed and used to quantify the performance of real and artificial insect wings. The impact of these results on future flapping-wing MAV designs is discussed in detail.

Energetics of flapping-wing robotic insects: towards autonomous hovering flight

Flapping-wing mechanisms inspired by biological insects have the potential to enable a new class of small, highly maneuverable aerial robots with hovering capabilities. In order for such devices to operate without an external power source, it is necessary to address a complex system design challenge: the integration of all of the required components on board the robot. This paper discusses the flight energetics of flapping-wing robotic insects with the goal of selecting design parameters that enable power autonomy and maximize flight time. The subsystems of the robot are analyzed both from a broad perspective and using a detailed set of models for a piezoelectrically driven two-wing design. The models are used to perform a system-level optimization for the maximum flight time permitted by current technology, compare the resulting robot configurations to biological insects across several key metrics, and discuss the effect of performance gains in various subsystems of the robot.

Design and fabrication of ultralight high-voltage power circuits for flapping-wing robotic insects

Flapping-wing robotic insects are small, highly ma-neuverable flying robots inspired by biological insects and useful for a wide range of tasks, including exploration, environmental monitoring, search and rescue, and surveillance. Recently, robotic insects driven by piezoelectric actuators have achieved the important goal of taking off with external power; however, fully autonomous operation requires an ultralight power supply capable of generating high-voltage drive signals from low-voltage energy sources. This paper describes high-voltage switching circuit topologies and control methods suitable for driving piezoelectric actuators in flapping-wing robotic insects and discusses the physical implementation of these topologies, including the fabrication of custom magnetic components by laser micromachining and other weight minimization techniques. The performance of laser micromachined magnetics and custom-wound commercial magnetics is compared through the experimental realization of a tapped inductor boost converter capable of stepping up a 3.7V Li-poly cell input to 200V. The potential of laser micromachined magnetics is further shown by implementing a similar converter weighing 20mg (not including control functionality) and capable of up to 70mW output at 200V and up to 100mW at 100V.

Lift Force Control of Flapping-Wing Microrobots Using Adaptive Feedforward Schemes

This paper introduces a methodology for designing real-time controllers capable of enforcing desired trajectories on microrobotic insects in vertical flight and hovering. The main idea considered in this work is that altitude control can be translated into a problem of lift force control. Through analyses and experiments, we describe the proposed control strategy, which is fundamentally adaptive with some elements of model-based control. In order to test and explain the method for controller synthesis and tuning, a static single-wing flapping mechanism is employed in the collection of experimental data. The fundamental issues relating to the stability, performance, and stability robustness of the resulting controlled system are studied using the notion of an input-output linear time-invariant (LTI) equivalent system, which is a method for finding an internal model principle (IMP) based representation of the considered adaptive laws, using basic properties of the z-transform. Empirical results validate the suitability of the approach chosen for designing controllers and for analyzing their fundamental properties.

Lift force control of a flapping-wing microrobot

This paper introduces a methodology for designing real-time controllers capable of enforcing desired trajectories on microrobotic insects in vertical flight and hovering. The main idea considered in this work is that altitude control can be translated into a problem of lift force control. Through analyses and experiments, we describe the proposed control strategy, which is fundamentally adaptive with some elements of model-based control. In order to test and explain the method for controller synthesis and tuning, a static single-wing flapping mechanism is employed in the collection of experimental data. The empirical results validate the suitability of the chosen approach.