Robert D. Vocke

Development and Testing of a Span-Extending Morphing Wing

Recent developments in morphing aircraft research have motivated investigation into conformal morphing systems, that is, shape change without discrete moving parts or abrupt changes in the airfoil profile. In this study, implementation of a continuous span morphing wing is described. The system consists of two primary components: (1) zero-Poisson ratio morphing core and (2) fiber-reinforced elastomeric matrix composite skin with a nearly zero-Poisson ratio in-plane. The main goal for improved air vehicle efficiency was a nominal 100% change in area of the active wing section with less than 2.54 mm out-of-plane deflection under representative aerodynamic loading. Objectives of this study included exploring fabrication techniques for advanced morphing core shapes (i.e., having airfoil-shaped cross-section), exploiting customizable design parameters of in-house fabricated skin and core material, designing a prototype wing structure such that integration with a candidate UAV was feasible, and experimentally evaluating a laboratory prototype. As a result of this study, the ability to physically build and test a viable airfoil structure capable of increasing its planform area by 100% (doubling span with constant chord) was demonstrated on a prototype hardware demonstration article. Satisfying objectives of designing, fabricating, and testing a prototype morphing wing section capable of 100% span extension, while maintaining constant chord, a wind tunnel test highlighted the resulting viable aerodynamic surface in a wind tunnel test up to 130 km/h wind speeds. The prototype wing in its resting condition had a span of 61.0 cm, which could be extended to 122.0 cm, with less than 2.54 mm out-of-plane deflection in dynamic pressures consistent with the maximum speed, 130 km/h, of a candidate unmanned aerial vehicle platform. In meeting these goals, the morphing core was successfully transitioned from a simple 1D concept into a complex, cambered airfoil with sufficient free volume to house an actuation system. A refined elastomer matrix composite skin fabrication technique was also devised and experimentally validated on skins of various thicknesses and overall dimensions.

High Specific Power Actuators for Robotic Manipulators

Recent advancements in actuator technology suggest that the implementation of reliable, high power-to-weight ratio pneumatic actuation systems is now possible for robotic platforms. Existing robotic manipulator arms for casualty extraction and patient placement use hydraulic actuation, whereas related robotic prosthetic devices typically use heavy actuator motors. We have developed an alternative solution that employs pneumatic artificial muscles (PAMs). The goal of this study is to identify requirements for a lightweight, high-force robotic manipulator, design the system for heavy lifting capability, and assemble a prototype arm. Following characterization and comparison of different-sized PAM actuators, a proof-of-concept manipulator was constructed. A quasi-static model for the PAM actuators was applied to the system, which includes the Gaylord force, as well as non-linear elastic energy storage. Experimental testing was performed to measure the joint torque and dynamic response of the manipulator, and to validate the model.

Pneumatic Artificial Muscles for Aerospace Applications

Morphing aircraft reconfigure themselves to optimize flight performance for various and substantially different mission objectives. Research and simulation studies have shown the wide variety of performance benefits afforded by morphing technologies, but there have historically been barriers preventing the fabrication and testing of such vehicles. One of these barriers is a distributed actuation system having sufficiently high specific force or specific work capability. This paper reviews some of our development work on a pneumatic actuation technology aimed at overcoming many of the limitations experienced with more conventional actuation concepts for the broad-scale dimensional changes of morphing aircraft. In particular, the present actuation system focuses on employing pneumatic artificial muscles (PAMs). Various applications of pneumatic artificial muscles (PAMs) are presented for two actuations schemes: (1) a morphing cell for a wing section, (2) trailing edge flaps for wings or rotorcraft blades. Results of the prototype testing show that PAMs are capable of generating significant morphing or control authority at low frequency, as well as frequencies ranging up to 40 Hz, and can provide actuation for more than 120 million loading cycles without fatigue failure. These results establish the feasibility of PAMs for aerospace applications.

Design and testing of a high-specific work actuator using miniature pneumatic artificial muscles

Micro-air vehicle (MAV) development is moving toward smaller and more capable platforms to enable missions such as indoor reconnaissance. This miniaturization creates challenging constraints on volume and energy generation/storage for all systems onboard. Actuator technologies must also address these miniaturization goals. Much research has focused on active material systems, such as piezoelectric materials and synthetic jets, but these advanced technologies have specific, but limited, capability. Conventional servo technology has also encountered concerns over miniaturization. Motivation has thus been established to develop a small-scale actuation technology prototype based on pneumatic artificial muscles, which are known for their lightweight, high-output, and low-pressure operation. The miniature actuator provides bidirectional control capabilities for a range of angles, rates, and loading conditions. Problems addressed include the scaling of the pneumatic actuators and design of a mechanism to adjust the kinematic load-stroke profile to suit the pneumatic actuators. The kinematics of the actuation system was modeled, and a number of bench-top configurations were fabricated, assembled, and experimentally characterized. Angular deflection and angular rate output of the final bench-top prototype system are presented, showing an improvement over conventional servo motors used in similar applications, especially in static or low-frequency operation.

One Dimensional Morphing Structures for Advanced Aircraft

Since the Wright Brothers’ first flight, the idea of “morphing” an airplane’s characteristics through continuous, rather than discrete, movable aerodynamic surfaces has held the promise of more efficient flight control. While the Wrights used a technique known as wing warping, or twisting the wings to control the roll of the aircraft (Wright and Wright, 1906), any number of possible morphological changes could be undertaken to modify an aircraft’s flight path or overall performance. Some notable examples include the Parker Variable Camber Wing used for increased forward speed (Parker, 1920), the impact of a variable dihedral wing on aircraft stability (Munk, 1924), the high speed dash/low speed cruise abilities associated with wings of varying sweep (Buseman, 1935), and the multiple benefits of cruise/dash performance and efficient roll control gained through telescopic wingspan changes (Sarh, 1991; Gevers, 1997; Samuel and Pines, 2007).

Development of a Quasi-Static Span-Extending Blade Tip for a Morphing Helicopter Rotor

This work presents the design and validation results for a quasi-static morphing helicopter rotor blade with an adaptive tip. The adaptive tip can increase its local span by 100% while maintaining a constant chord, effectively doubling the airfoil area of the morphing section leading to an overall increase in the full rotor radius. The technology components include a morphing honeycomb-like core structure that has a Poisson’s ratio of zero as it extends and an elastomer-matrix-composite skin that is bonded to the core structure. Design analyses along with experimental validation on hardware specimens are presented. The validated model is used in a numerical optimization to design a system that will self-actuate under a rotor’s normal operating conditions. Feasibility of the design is demonstrated in terms of hardware fabrication methods and overall performance to satisfy system design goals.

Development of a Span-Extending Blade Tip System for a Reconfigurable Helicopter Rotor

†‡ This work, conducted under the System Concept Definition phase of the Mission Adaptive Rotor Program, presents the design and validation results for a morphing helicopter rotor blade with an active blade tip, which can increase its active span by 100%, while maintaining a constant chord, effectively doubling the active airfoil area. The technology components include a morphing honeycomb-like structure that has a Poisson’s ratio of zero as it extends and an elastomer-matrix-composite skin that is bonded to the core structure. Design analyses are presented, along with experimental validation testing on hardware specimens. Feasibility of the design has been demonstrated in terms of hardware fabrication methods and overall performance to satisfy system requirements.

Mechanism and bias considerations for design of a bi-directional pneumatic artificial muscle actuator

Pneumatic artificial muscles (PAMs), or McKibben actuators, have received considerable attention for robotic manipulators and in aerospace applications due to their similarity to natural muscles. Like natural muscles, PAMs are a purely contractile actuator, so that, in order to produce bi-directional or rotational motion, they must be arranged in an agonist/antagonist pair, which inherently limits the deflection of the system due to the high parasitic stiffness of the antagonistic PAM. This study presents two methods for increasing the performance of an antagonistic PAM system by decreasing the passive parasitic torque, rather than increasing the active torque. The first involves selection of the kinematic mechanism geometry, and the second involves the introduction of bias into the system, both in terms of PAM contraction and passive (antagonistic) PAM pressure. It was found with the proper selection of design parameters, including mechanism geometry, PAM geometry, and bias conditions, that an ideal actuator configuration can be chosen that maximizes deflection for a given arbitrary loading. When comparing a baseline design to an improved design for a simplified case, a nearly 50% increase in maximum deflection was predicted simply by optimizing mechanism geometry and bias contraction. These results were experimentally verified with quasi-static testing that showed a 300% increase in actuator deflection over the baseline design.

Mechanism and Bias Considerations for Design of a Bi-Directional Artificial Muscle Actuator

Pneumatic artificial muscles (PAMs), or McKibben actuators, have received considerable attention for robotic manipulators and in aerospace applications due to their similarity to natural muscles. Like natural muscles, PAMs are a purely contractile actuator, so that, in order to produce bi-directional or rotational motion, they must be arranged in an agonist/antagonist pair, which inherently limits the deflection of the system due to the high parasitic stiffness of the antagonistic PAM. This study presents two methods for increasing the performance of an antagonistic PAM system by decreasing the passive parasitic torque, rather than increasing the active torque. The first involves selection of the kinematic mechanism geometry, and the second involves the introduction of bias into the system, both in terms of PAM contraction and passive (antagonistic) PAM pressure. It was found with the proper selection of design parameters, including mechanism geometry, PAM geometry, and bias conditions, that an ideal actuator configuration can be chosen that maximizes deflection for a given arbitrary loading. When comparing a baseline design to an improved design for a simplified case, a nearly 50% increase in maximum deflection was predicted simply by optimizing mechanism geometry and bias contraction. These results were experimentally verified with quasi-static testing that showed a 300% increase in actuator deflection over the baseline design.