Roelof Vos

Post-buckled precompressed elements: a new class of control actuators for morphing wing UAVs

This paper describes how post-buckled precompressed (PBP) piezoelectric bender actuators are employed in a deformable wing structure to manipulate its camber distribution and thereby induce roll control on a subscale UAV. By applying axial compression to piezoelectric bimorph bender actuators, significantly higher deflections can be achieved than for conventional piezoelectric bender actuators. Classical laminated plate theory is shown to capture the behavior of the unloaded elements. A Newtonian deflection model employing nonlinear structural relations is demonstrated to predict the behavior of the PBP elements accurately. A proof of concept 100 mm (3.94 �� ) span wing employing two outboard PBP actuator sets and a highly compliant latex skin was fabricated. Bench tests showed that, with a wing chord of 145 mm (5.8 �� ) and an axial compression of 70.7 gmf mm −1 , deflection levels increased by more than a factor of 2 to 15.25 ◦ peak-to-peak, with a corner frequency of 34 Hz (an order of magnitude higher than conventional subscale servoactuators). A 1.4 m span subscale UAV was equipped with two PBP morphing panels at the outboard stations, each measuring 230 mm

Post-buckled precompressed piezoelectric flight control actuator design, development and demonstration

This paper describes a new class of flight control actuators using post-buckled precompressed (PBP) piezoelectric elements. These actuators are designed to produce significantly higher deflection and force levels than conventional piezoelectric actuator elements. Classical laminate plate theory (CLPT) models are shown to work very well in capturing the behavior of the free, unloaded elements. A new high transverse deflection model which employs nonlinear structural relations is shown to successfully predict the performance of the PBP actuators as they are exposed to higher and higher levels of axial force, which induces post-buckling deflections. A proof-of-concept empennage assembly and actuator were fabricated using the principles of PBP actuation. A single grid-fin flight control effector was driven by a 3.5 (88.9 mm) long piezoceramic bimorph PBP actuator. By using the PBP configuration, deflections were controllably magnified 4.5-fold with excellent correlation between theory and experiment. Quasi-static bench testing showed deflection levels in excess of ± 6° at rates exceeding 15 Hz. The new solid state PBP actuator was shown to reduce the part count with respect to conventional servoactuators by an order of magnitude. Power consumption dropped from 24 W to 100 mW, weight was cut from 108 to 14 g, slop went from 1.6° to 0.02° and current draw went from 5 A to 1.4 mA. The result was that the XQ-138 subscale UAV family experienced nearly a 4% reduction in operating empty weight via the switch from conventional to PBP actuators, while in every other measure gross performance was significantly enhanced.

Magnification of Work Output in PBP Class Actuators Using Buckling/Converse Buckling Techniques

This paper explores the new fleld of active control of proverse (conventional) and converse buckling in piezoelectrically actuated beams using Post-Buckled Precompressed (PBP) elements. The paper shows that as a PBP bender element is axially loaded, proverse buckling imperfections induced by application of active moments from piezoelectric elements enhance beam deformations. As the sign of the active moments is switched, the PBP element enters a converse-buckled state. Experimental testing on 50cm long, 1cm wide curved aluminum strips shows that as the magnitude of the reversed active distributed moment is increased, the axial force required to hold a converse-buckled de∞ection level is also increased. Similarly, when the axial constraint distance of converse-buckled elements is increased, the axial force levels jump to nearly 4 times the perfect-column buckling load, which is typically followed by a higher-order snap-through to a proverse state. The paper captures this phenomenon with a simple analytical model which accurately predicts end rotations with external moments and axial forces. A second series of tests were conducted on an active piezoelectric PBP bender element with applied root moments. Both tests showed excellent correlation between theory and experiment and nearly a 3-fold increase in work output over conventional piezoelectric bender arrangements.

Pressure adaptive honeycomb : A new adaptive structure for aerospace applications

A new type of adaptive structure is presented that relies on pressurized honeycomb cells that extent a significant length with respect to the plane of the hexagons. By varying the pressure inside each of the cells, the stiffness can be altered. A variable stiffness in combination with an externally applied force field results in a fully embedded pressure adaptive actuator that can yield strains well beyond the state-of-the-art in adaptive materials. The stiffness change as a function of the pressure is modeled by assigning an equivalent material stiffness to the honeycomb walls that accounts for both the inherent material stiffness as the pressure-induced stiffness. A finite element analysis of a beam structure that relies on this model is shown to correlate well to experimental results of a three-point bend test. To demonstrate the concept of embedded pressure adaptive honeycomb, an wind tunnel test article with adaptive flap has been constructed and tested in a low speed wind tunnel. It has been proven that by varying the cell pressure the flap changed its geometry and subsequently altered the lift coefficient.

Post-buckled precompressed (PBP) subsonic micro flight control actuators and surfaces

This paper describes a new class of flight control actuators using Post-Buckled Precompressed (PBP) piezoelectric elements to provide much improved actuator performance. These PBP actuator elements are modeled using basic large deflection Euler-beam estimations accounting for laminated plate effects. The deflection estimations are then coupled to a high rotation kinematic model which translates PBP beam bending to stabilator deflections. A test article using PZT-5H piezoceramic sheets built into an active bender element was fitted with an elastic band which induced much improved deflection levels. Statically the bender element was capable of producing unloaded end rotations on the order of ±2.6°. With axial compression, the end deflections were shown to increase nearly 4-fold. The PBP element was then fitted with a graphite-epoxy aeroshell which was designed to pitch around a tubular stainless steel main spar. Quasi-static bench testing showed excellent correlation between theory and experiment through ±25° of pitch deflection. Finally, wind tunnel testing was conducted at airspeeds up to 120kts (62m/s, 202ft/s). Testing showed that deflections up through ±20° could be maintained at even the highest flight speed. The stabilator showed no flutter or divergence tendencies at all flight speeds. At higher deflection levels, it was shown that a slight degradation deflection was induced by nose-down pitching moments generated by separated flow conditions induced by extremely high angles of attack.

Dynamic Elastic Axis Shifting: An Important Enhancement of Postbuckled Precompressed (PBP) Actuators

A new actuator arrangement which is designed to protect postbuckled precompressed (PBP) elements is presented. This actuator arrangement uses through-thickness dynamic elastic axis shifting (DEAS)to protect the convex face of PBP bending actuators at high curvatures. This innovation allows this new generation of high performance piezoelectric actuators to be more robust and less sensitive to tensile failure of the convex face. At a particular amount of curvature the elastic axis of the element is shifted dynamically from the center of the laminate through the thickness towards or beyond the convex face, thereby relieving the convex face from tensile loads and stifiening the entire laminate. This dynamic elastic axis shifting (DEAS) is achieved by adding a facing sheet to the laminate which only carries tensile loads at high curvatures. A silicone spacer between the facing sheet and the actuator element increases the moment of inertia of the facing sheet and increases the efiectiveness of the facing sheets. This paper presents an investigation into the efiects of the facing sheet/spacer arrangement on the PBP performance. Analytical models are presented that predict the end rotation at which facing sheet engagement occurs. Experimental tests were done on a variety of spacer and facing sheet geometries demonstrating the DEAS principle for each of the conflgurations. De∞ection testing of DEAS-modifled PBP beams was carried out through end rotations in excess of 10 ‐ with good correlation between theory and experiment. It is shown that with a weight penalty of only 12% with respect to the baseline actuator element the robustness of the PBP actuator elements can be signiflcantly increased.

Post-buckled precompressed subsonic micro-flight control actuators and surfaces

This paper describes a new class of flight control actuators using Post-Buckled Precompressed (PBP) piezoelectric elements to provide much improved actuator performance. These PBP actuator elements are modeled using basic large deflection Euler-beam estimations accounting for laminated plate effects. The deflection estimations are then coupled to a high rotation kinematic model which translates PBP beam bending to stabilator deflections. A test article using PZT-5H piezoceramic sheets built into an active bender element was fitted with an elastic band which induced much improved deflection levels. Statically the bender element was capable of producing unloaded end rotations on the order of ±2.6°. With axial compression, the end deflections were shown to increase nearly 4-fold. The PBP element was then fitted with a graphite-epoxy aeroshell which was designed to pitch around a tubular stainless steel main spar. Quasi-static bench testing showed excellent correlation between theory and experiment through ±25° of pitch deflection. Finally, wind tunnel testing was conducted at airspeeds up to 120kts (62m/s, 202ft/s). Testing showed that deflections up through ±20° could be maintained at even the highest flight speed. The stabilator showed no flutter or divergence tendencies at all flight speeds. At higher deflection levels, it was shown that a slight degradation deflection was induced by nose-down pitching moments generated by separated flow conditions induced by extremely high angles of attack.

Pressure Adaptive Honeycomb: Mechanics, Modeling, and Experimental Investigation

A new type of adaptive structure is presented that relies on a pressure derential to perform gross structural deformations. This structure relies on highly compliant honey-comb cells that can be pressurized externally or can rely on a pressure differential that exists at elevated altitudes. By pressurizing this honeycomb, its stiffness can be altered and deformations can be controlled by means of a restoring force. The mechanics of this pressure-adaptive honeycomb is laid out in this paper. The concept of equivalent material stiffness is introduced that assigns a Youngs modulus to the honeycomb wall material that includes both the material-induced stiffness and the pressure-induced stiffness for a given cell differential pressure. The application of this model in a finite element analysis of a beam specimen is shown to correlate well to experimental results. In addition, the paper discusses possible applications for pressure adaptive honeycomb such as a Gurney flap and a solid-state flap. Wind tunnel test on a test article of a wing with pressure-adaptive flap demonstrates an increase in lift coefficient of 0.3 over a wide range of angles of attack. By increasing the pressure inside the flap to 40kPa its equivalent stiffness increases from 15kPa to 109kPa, thereby allowing the camber to decrease from 7.2% in deployed position to 2% in stowed position and shifting the point of maximum camber from 72% of the wing chord to 40%.

Design, development, and testing of a transonic missile fin employing PBP/DEAS actuators

This paper describes a new class of flight control actuators using Post-Buckled Precompressed (PBP) piezoelectric elements mounted within a transonic missile fin. These actuators are designed to produce significantly higher deflection and force levels than conventional piezoelectric actuator elements. Classical laminate plate theory (CLPT) models are shown to work very well in capturing the behavior of the free, unloaded elements. A new high transverse deflection model which employs nonlinear structural relations is shown to successfully predict the performance of the PBP actuators as they are exposed to higher and higher levels of axial force, which induces post buckling deflections. A 6 (15.2cm) square rounded diamond transonic fin was made with integral PBP actuator elements. Quasi-static bench testing showed deflection levels in excess of ±7° at rates exceeding 21 Hz. The new solid state PBP actuator was shown to reduce the part count with respect to conventional servoactuators by an order of magnitude. Power consumption dropped from 24W to 1.3W, slop went from 1.6° to 0.02° and peak current draw went from 5A to 18mA. The PBP actuator was wind tunnel tested and shown to possess no flutter, divergence or adverse aeroelastic coupling characteristics.

(Student paper) Nonlinear Semi-Analytical Modeling of Post-Buckled Precompressed (PBP) Piezoelectric Actuators for UAV Flight Control

Piezoceramic bimorph actuators are gaining popularity in uninhabited aerial vehicle (UAV) applications due to the small size of the aircraft, and hence small control forces. Conventional actuator architectures sufier from a trade ofi between force and displacement outputs. A novel architecture, the Post-Buckled Precompressed (PBP) actuator, has been developed to enhance the displacement output without reducing the blocked force by applying a compressive force on the actuator. If the applied compressive force approaches the buckling load, signiflcantly higher de∞ections and blocked force can be obtained. The inherent nonlinear behavior of the PBP actuator requires careful modeling. In this paper, a Raleigh-Ritz discretization technique is proposed to model the PBP actuator. The RaleighRitz approach bridges the gap between simple analytical treatment and general but often computationally demanding flnite element analysis. The feasibility of this technique with practical application to the static and dynamic response of two UAV ∞ight control systems is demonstrated; namely the Grid Fins of a VTOL UAV and a section of a UAV wing with substantial airfoil thickness able to morph its camber. It is shown for the flrst UAV that 15 shape functions are needed, while in the latter case 6 shape functions are required in the Rayleigh-Ritz method to get a 99 % convergence. Static results of the Grid Fin or wing de∞ection exposed qualitatively good results, but also the necessity to incorporate creep phenomena in the model. Dynamic results proved that the Rayleigh-Ritz is able to capture the flrst natural eigenfrequency very accurate.