Fred Nitzsche

Development of the Smart Spring for Active Vibration Control of Helicopter Blades

Significant structural vibration is an undesirable characteristic in helicopter flight that leads to structural fatigue, poor ride quality for passengers and high acoustic signature for the vehicle. Previous Individual Blade Control (IBC) techniques based on piezoelectric actuator schemes to reduce these effects have been hindered by electromechanical limitations of piezoelectric actuators. The Smart Spring is an active tunable vibration absorber using the IBC approach to adaptively alter the “structural impedance” at the blade root. In the paper, a mathematical model was developed to determine the response of the absorber under harmonic excitation. An adaptive notch algorithm using a DSP platform was developed to implement vibration control. Reference signal synthesis techniques were used to automatically track the shift in the fundamental vibratory frequency due to variations in flight conditions. Experiments using a mechanical shaker and wind tunnel tests conducted on the proof-of-concept hardware achieved significant vibration suppression at harmonic peaks. Investigation verified the capability of the Smart Spring to suppress multiple harmonic components in rotor vibration through active impedance control.

Application of Multi-Input Volterra Theory to Nonlinear Multi-Degree-of-Freedom Aerodynamic Systems

This paper presents a reduced-order-modeling approach for nonlinear, multi-degree-of-freedom aerodynamic systems using multi-input Volterra theory. The method is applied to a two-dimensional, 2 degree-of-freedom transonic airfoil undergoing simultaneous forced pitch and heave harmonic oscillations. The so-called Volterra cross kernels are identified and shown to successfully model the aerodynamic nonlinearities associated with the simultaneous pitch and heave motions. The improvements in accuracy over previous approaches that effectively ignored the cross kernels by using superposition are demonstrated.

Control laws for an active tunable vibration absorber designed for rotor blade damping augmentation

Most Individual Blade Control (IBC) approaches have attempted to suppress the rotor vibration by actively altering the varying aerodynamic loads on the blade using techniques such as trailing-edge servo-flaps or imbedded piezoelectric fibres to twist the blade. Unfortunately, successful implementation of these approaches has been hindered by electromechanical limitations of piezoelectric actuators. The Smart Spring is an unique approach that is designed to suppress the rotor vibration by actively altering the structural stiffness of the blade out of phase with the time varying aerodynamic forces. The Smart Spring system is able to adaptively alter the stiffness properties of the blade while requiring only small deformations of the actuator, which overcomes the major problems inherent in the former approaches. The theoretical characterisation of the Smart Spring system as a class of active Tunable Vibration Absorbers (TVA) is presented in the paper. A real-time adaptive control system was developed for a Smart Spring to suppress vibration. Initial aerodynamic wind tunnel test results using the proof-of-concept model of the device in a fixed blade indicate that the Smart Spring can evolve into a powerful approach for IBC.

Development of a maximum energy extraction control for the smart spring

Most active vibration suppression approaches have attempted to suppress structural vibrations through the use of active material actuators, such as piezoceramic, that are incorporated into a structure to act directly against vibratory loads. These approaches require the actuators to simultaneously supply significant force and deflection to effectively suppress vibration. Unfortunately, successful implementation of these approaches has been hindered by the electromechanical limitations of piezoceramic actuators due to high power requirements in active vibration control applications. The Smart Spring concept is a unique approach that is designed to actively control combinations of dynamic impedance characteristics of a structure, such as the stiffness, damping, and effective mass to suppress vibration. The Smart Spring does not use actuators to perform work directly against excitation loads, but rather adaptively varies the effective structural impedance properties. Therefore, the piezoceramic actuators in the Smart Spring are not required to simultaneously produce large forces and deflections. Thus, the concept requires considerably less power because it enables active vibration control in an indirect manner. This study demonstrates the ability of the Smart Spring to control dynamic impedance characteristics of a structure through numerical simulations and experimental investigations. In addition, the development of a feedback control system is demonstrated. According to the control strategy, the impedance characteristics of the Smart Spring are continuously changing in order to maximize the extraction of the mechanical energy of the system.

Reduced Order Modeling of Nonlinear Transonic Aerodynamics Using a Pruned Volterra Series

The following paper presents a reduced-order-modeling approach for nonlinear aerodynamic systems utilizing a pruned Volterra series. The method is applied to a two-dimensional transonic airfoil undergoing forced pitch oscillations. Pruned Volterra series reduced-order-models up to fourth-order are identified and compared against computational fluid dynamics models. Very favorable accuracies are attained over a wide range of Mach number, reduced frequency and oscillation amplitude. The computational resources associated with the pruned Volterra series are demonstrated to be several ordersof-magnitude lower compared to the standard Volterra series.

ACOUSTIC VALIDATION OF A NEW CODE USING PARTICLE WAKE AERODYNAMICS AND GEOMETRICALLY-EXACT BEAM STRUCTURAL DYNAMICS

This paper describes the validation of a new code for predicting both aeroacoustic and aeroelastic behaviour of hingeless rotors. The structural component was based a non-linear beam element model considering small strains and finite rotations, which uses a mixed variational intrinsic formulation. The aerodynamic component was built on a loworder panel method incorporating a vortex particle free-wake model. The aerodynamic and structural components were combined to form a closely coupled aeroelastic code that solves in the timedomain. The loading and thickness noise terms for the aeroacoustic calculations were calculated from the aerodynamic data using a formulation based on the Ffowcs Williams-Hawkings (FW-H) equation. The code was successfully validated for acoustic signature and BVI predictions using test cases from the HELINOISE program.

Smart Spring Control of Vibration on Helicopter Rotor Blades

c1, c2 = Smart Spring viscous damping coefficients associated with primary and secondary load paths F = external (input) force applied to the Smart Spring k = effective dynamic stiffness of the Smart Spring k1, k2 = Smart Spring constants associated with primary and secondary load paths m = effective inertia of the Smart Spring m1, m2 = mass of external and internal sleeves in the Smart Spring N = contact force applied by piezoelectric stack T = Smart Spring period of actuation t = time x = displacement (output) yielded by the Smart Spring y = displacement associated with the Smart Spring secondary load path = dynamic stiffness complex coefficients = dry friction coefficient = Smart Spring control frequency, 2 =T ! = Smart Spring frequency of excitation

Active Control of Aircraft Cabin Noise Using Collocated Structural Actuators and Sensors

This paper describes preliminary laboratory experiments conducted on a turboprop aircraft fuselage to reduce propeller-induced tonal cabin noise and vibration. Piezoelectric elements were grouped to construct a long one-dimensional array of actuators bonded to the fuselage in the main sound transmission path at the propeller footprint. Strain gauges and accelerometers were used as alternative sensor devices and were distributed along the actuator in a collocated arrangement. The array of actuators and sensors was designed to work in unison, generating a smart closed-loop array of control elements that possess wave-number filtering properties for the less critical acoustic modes of the cabin. The control system was tested in the laboratory using a simplified propeller pressure loading distribution. Promising results were obtained, as the closed-loop control system proved to be unconditionally stable and capable of significantly attenuating the fuselage vibration in the transmission path at the critical blade passage frequencies. Moreover, although only one array of control elements was used, interior noise reduction was also observed during the tests, proving the merit of the concept.

Smart spring: a novel adaptive impedance control approach for active vibration suppression applications

Most active vibration suppression approaches have attempted to suppress structural vibration by incorporating active material actuators, such as piezoceramic, within the structure to act directly against vibratory loads. These approaches require the piezoceramic actuators to generate significant force and deflection simultaneously to effectively suppress vibration. Unfortunately, successful implementation of these approaches has been hindered by the limited displacement capabilities of piezoceramic actuators. The Smart Spring concept is an unique approach to actively control combinations of dynamic impedance characteristics of a structure, such as the stiffness, damping, and effective mass to suppress vibration in an indirect manner. The piezoceramic actuators employed in the Smart Spring concept are not used to directly counteract excitation loads but rather adaptively vary the effective impedance properties of the structure. Therefore, the piezoceramic actuators in the Smart Spring are not required to produce large forces and deflections simultaneously. This paper demonstrates the ability of the Smart Spring concept to control dynamic impedance characteristics of a structure through numerical simulations and experimental investigations. Mechanical shaker tests using the proof-of-concept hardware verified the controllability of the impedance properties using the Smart Spring device and its ability to suppress vibration. More importantly, the tests conducted in a wind tunnel demonstrated the performance of the Smart Spring under highly varying unsteady excitation conditions. These tests confirmed that the Smart Spring system is able to actively suppress vibration through adaptive control of structural impedance properties.

Review of Active Rotor Control Research in Canada

The current status of Canadian research on rotor-based actively controlled technologies for helicopters is reviewed in this paper. First, worldwide research in this field is overviewed to put Canadian research into context. Then, the unique hybrid control concept of Carleton University is described, along with its key element, the “stiffness control” concept. Next, the smart hybrid active rotor control system (SHARCS) project`s history and organization is presented, which aims to demonstrate the hybrid control concept in a wind tunnel test campaign. To support the activities of SHARCS, unique computational tools, novel experimental facilities and new know-how had to be developed in Canada, among them the state-of-the-art Carleton Whirl Tower facility or the ability to design and manufacture aeroelastically scaled helicopter rotors for wind tunnel testing. In the second half of the paper, details are provided on the current status of development on the three subsystems of SHARCS, i.e. that of the actively controlled tip, the actively controlled flap and the unique stiffness-control device, the active pitch link.