Post-buckled Precompressed Actuators Research Articles

Design, Manufacturing and Test of a High Lift Secondary Flight Control Surface with Shape Memory Alloy Post-Buckled Precompressed Actuators

The use of morphing components on aerospace structures can greatly increase the versatility of an aircraft. This paper presents the design, manufacturing and testing of a new kind of adaptive airfoil with actuation through Shape Memory Alloys (SMA). The developed adaptive flap system makes use of a novel actuator that employs SMA wires in an antagonistic arrangement with a Post-Buckled Precompressed (PBP) mechanism. SMA actuators are usually used in an antagonistic arrangement or are arranged to move structural components with linearly varying resistance levels similar to springs. Unfortunately, most of this strain energy is spent doing work on the passive structure rather than performing the task at hand, like moving a flight control surface or resisting air loads. A solution is the use of Post-Buckled Precompressed (PBP) actuators that are arranged so that the active elements do not waste energy fighting passive structural stiffnesses. One major problem with PBP actuators is that the low tensile strength of the piezoelectric elements can often result in tensile failure of the actuator on the convex face. A solution to this problem is the use of SMA as actuator material due to their tolerance of tensile stresses. The power consumption to hold deflections is reduced by approximately 20% with the Post-Buckled Precompressed mechanism. Conventional SMAs are essentially non-starters for many classes of aircraft due to the requirement of holding the flight control surfaces in a given position for extremely long times to trim the vehicle. For the reason that PBP actuators balance out air and structural loads, the steady-state load on the SMAs is essentially negligible, when properly designed. Simulations and experiments showed that the SMAPBP actuator shows tip rotations on the order of 45°, which is nearly triple the levels achieved by piezoelectric PBP actuators. The developed SMAPBP actuator was integrated in a NACA0012 airfoil with a flexible skin to carry out wind tunnel tests.

DYNAMIC PERFORMANCE OF POST-BUCKLED PRECOMPRESSED PIEZOELECTRIC ACTUATOR ELEMENTS

Post-buckled precompressed (PBP) piezoelectric elements have recently been used to enable a new class of actuators that are able to provide far higher deflections compared to the traditional bimorph piezoelectric actuators while maintaining full force and moment generating capabilities. Past research has proven that PBP actuators are capable of generating deflections three times higher than conventional bimorph actuators. In this paper, this work has been extended to the dynamic response realm and the performance of PBP actuators is investigated under various axial loads, at various actuation frequencies. Both analytical and finite element models have been developed in order to evaluate the performance of the actuator regarding the natural frequency shift under increased axial loads. Experimental verification has shown that the overall damping ratio of the structure is a function of the axial forces. Values derived from experiments have been used in the Finite Element model to predict the displacement output, phase angle shifting and end rotation. Numerical and analytical results correlate very well with the experiments and thus give credit to the formulation presented in this work.

Post-Buckled Precompressed Techniques in Adaptive Aerostructures: An Overview

An overview of the development and application of post-buckled precompressed (PBP) piezoelectric actuators is presented. It has been demonstrated that PBP actuators outperform conventional piezoelectric actuators by relying on axial compression to counter the inherent stiffness in the actuator element. In doing so, the mechanical work output has been shown to increase threefold compared with conventional bimorph actuators. Actuator stroke has been demonstrated to increase up to 300% without compromising the blocked force capability. This has resulted in an expansion of the design space of piezoelectric bender elements and has made them excellent candidates for potentially replacing certain classes of conventional electromechanical flight control actuators. The successful application of PBP elements can be found in unmanned aerospace systems ranging from subscale vertical-take-off-and-landing vehicles to supersonic missile fins. With respect to conventional electromechanical servoactuators, it is demonstrated that PBP actuator elements induce a lower systems weight fraction, a substantially higher bandwidth, and an order of magnitude lower power consumptions and part count.

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

This paper describes a new class of flight control actuators using post-buckledprecompressed (PBP) piezoelectric elements to provide much improved actuatorperformance. These PBP actuator elements are modeled using basic largedeflection Euler-beam estimations accounting for laminated plate effects. Thedeflection estimations are then coupled to a high rotation kinematic model whichtranslates PBP beam bending to stabilator deflections. A test article usingPZT-5H piezoceramic sheets built into an active bender element was fitted withan elastic band which induced much improved deflection levels. Statically thebender 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 four-fold.The PBP element was then fitted with a graphite–epoxy aeroshell which wasdesigned to pitch around a tubular stainless steel main spar. Quasi-static benchtesting showed excellent correlation between theory and experiment through ± 25° of pitch deflection. Finally, wind tunnel testing was conducted at airspeeds up to 120 kts(62 m s−1,202 ft s−1). Testing showedthat deflections up to ± 20° could be maintained at even the highest flight speed. The stabilator showed no flutter ordivergence tendencies at all flight speeds. At higher deflection levels, it was shownthat a slight degradation deflection was induced by nose-down pitching momentsgenerated by separated flow conditions induced by extremely high angles of attack.

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

This paper describes how post-buckled precompressed (PBP) piezoelectric benderactuators are employed in a deformable wing structure to manipulate its camberdistribution and thereby induce roll control on a subscale UAV. By applying axialcompression to piezoelectric bimorph bender actuators, significantly higher deflectionscan be achieved than for conventional piezoelectric bender actuators. Classicallaminated plate theory is shown to capture the behavior of the unloaded elements. ANewtonian deflection model employing nonlinear structural relations is demonstrated topredict 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 compliantlatex skin was fabricated. Bench tests showed that, with a wing chord of 145 mm(5.8′′) and an axialcompression of 70.7 gmf mm−1, deflection levels increased by more than a factor of 2 to15.25° peak-to-peak, with a corner frequency of 34 Hz (an order of magnitude higher thanconventional subscale servoactuators). A 1.4 m span subscale UAV was equippedwith two PBP morphing panels at the outboard stations, each measuring 230 mm(9.1′′) in span. Flight testing was carried out, showing a 38% increase in roll control authorityand 3.7 times greater control derivatives compared to conventional ailerons. The solid statePBP actuator in the morphing wing reduced the part count from 56 down to only 6,with respect to a conventional servoactuated aileron wing. Furthermore, powerwas reduced from 24 W to 100 mW, current draw was cut from 5 A to 1.4 mA,and the actuator weight increment dropped dramatically from 59 g down to 3 g.

Post-buckled precompressed piezoelectric flight control actuator design, development anddemonstration

This paper describes a new class of flight control actuators using post-buckledprecompressed (PBP) piezoelectric elements. These actuators are designed to producesignificantly higher deflection and force levels than conventional piezoelectric actuatorelements. Classical laminate plate theory (CLPT) models are shown to workvery well in capturing the behavior of the free, unloaded elements. A new hightransverse deflection model which employs nonlinear structural relations is shown tosuccessfully predict the performance of the PBP actuators as they are exposed tohigher and higher levels of axial force, which induces post-buckling deflections. Aproof-of-concept empennage assembly and actuator were fabricated using theprinciples of PBP actuation. A single grid-fin flight control effector was driven by a3.5′′ (88.9 mm) long piezoceramic bimorph PBP actuator. By using the PBP configuration,deflections were controllably magnified 4.5-fold with excellent correlation between theoryand 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 partcount with respect to conventional servoactuators by an order of magnitude. Powerconsumption dropped from 24 W to 100 mW, weight was cut from 108 to 14 g, slop went from1.6° to0.02° and current draw went from 5 A to 1.4 mA. The result was that the XQ-138 subscale UAVfamily experienced nearly a 4% reduction in operating empty weight via the switch fromconventional to PBP actuators, while in every other measure gross performance wassignificantly enhanced.