Viresh Wickramasinghe

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.

Design and verification of a smart wing for an extreme-agility micro-air-vehicle

A special class of fixed-wing micro-air-vehicle (MAV) is currently being designed to fly and hover to provide range superiority as well as being able to hover through a flight maneuver known as prop-hanging to accomplish a variety of surveillance missions. The hover maneuver requires roll control of the wing through differential aileron deflection but a conventional system contributes significantly to the gross weight and complexity of a MAV. Therefore, it is advantageous to use smart structure approaches with active materials to design a lightweight, robust wing for the MAV. The proposed smart wing consists of an active trailing edge flap integrated with bimorph actuators with piezoceramic fibers. Actuation is enhanced by preloading the bimorph actuators with a compressive axial load. The preload is exerted on the actuators through a passive latex or electroactive polymer (EAP) skin that wraps around the airfoil. An EAP skin would further enhance the actuation by providing an electrostatic effect of the dielectric polymer to increase the deflection. Analytical modeling as well as finite element analysis show that the proposed concept could achieve the target bi-directional deflection of 30° in typical flight conditions. Several bimorph actuators were manufactured and an experimental setup was designed to measure the static and dynamic deflections. The experimental results validated the analytical technique and finite element models, which have been further used to predict the performance of the smart wing design for a MAV.

Material characterization of active fiber composites for integral twist-actuated rotor blade application

The primary objective of this work was to perform material characterization of the active fiber composite (AFC) actuator system for the Boeing active material rotor (AMR) blade application. The purpose of the AMR was to demonstrate active vibration control in helicopters through integral twist-actuation of the blade. The AFCs were a new structural actuator system consisting of piezoceramic fibers embedded in an epoxy matrix and sandwiched between interdigitated electrodes to enhance actuation performance. These conformable actuators were integrated directly into the blade spar laminate as active plies within the composite structure to perform structural control. Therefore, extensive electromechanical material characterization was required to evaluate AFCs both as actuators and as structural components of the blade. The characterization tests designed to extract important electromechanical properties under simulated blade operating conditions included nominal actuation tests, stress–strain tests and actuation under tensile load tests. This paper presents the test results as well as the comprehensive testing procedure developed to evaluate the relevant properties of the AFCs for structural application. The material characterization tests provided an invaluable insight into the behavior of the AFCs under various electromechanical conditions. The results from this comprehensive material characterization of the AFC actuator system supported the design and operation of the AMR blades scheduled for wind tunnel tests.

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.

Durability characterization of Active fiber composite actuators for helicopter Rotor blade applications

Theprimary objective of this work was to characterize the long-term durability performance of the Active Fiber Composite (AFC) actuator material system for the Boeing Active Material Rotor (AMR) blade application. The AFCs were a new structural actuator system consisting of piezoceramic fibers embedded in an epoxy matrix and sandwiched between interdigitated electrodes. These actuators were integrated directly into the blade spar as active plies within the composite structure to perform structural actuation for helicopter vibration control. Therefore, it was necessary to conduct extensive electromechanical material characterization to evaluate AFCs both as actuators and as structural components of the blade. The long-term durability characterization tests designed to extract important electromechanical properties were electrical fatigue tests and mechanical fatigue tests. This paper presents the test results as well as the comprehensive testing process developed to evaluate the relevant AFC durability properties. The durability tests conducted under simulated electromechanical loading conditions expected on AFCs during the blade operation provided an invaluable insight into the behavior of the AFCs under dynamic loading environment. The results from this comprehensive durability characterization of the AFC material system supported the design and operation of the Boeing AMR blade scheduled for wind tunnel tests.

A Comparison Study Between Acoustic Sensors for Bearing Fault Detection Under Different Speed and Load Using a Variety of Signal Processing Techniques

The use of ultrasonic sensor technology to detect incipient and evolving defects in rotating components such as bearings and gears is more desirable due to their high resolution. In a previous study, the sensitivity of a variety of sensors including an air-coupled ultrasound transducer to bearing faults was analyzed and thoroughly discussed. This article investigates the effectiveness of two ultrasonic sensors, namely, air-coupled and piezoelectric ultrasound transducers for rolling element bearings damage diagnostics. The former is a noncontact sensor and the latter is a contact sensor. An accelerometer was also used as the baseline sensor for comparison purposes. A series of tests was carried out on a laboratory test rig running with defective and undamaged healthy bearings under variable shaft speeds and several radial loads. The data were analyzed using selected signal processing techniques covering time, frequency, and advanced joint time–frequency domains. The results showed that certain acoustic features were responsive to the variation of operational condition and the damage; the detection capability of the sensors varied depending on the defect size, its location, as well as the applied signal analysis technique.

Development of Adaptive Seat Mounts for Helicopter Aircrew Body Vibration Reduction

Helicopter aircrew are exposed to high levels of vibration and noise during flight. This paper presents the investigation of adaptive seat mount approaches to reducing vibration on the helicopter seat. A flight test on a helicopter with typical pilot configurations showed that the vibration spectra on the pilot’s helmet not only included the dominant N/rev harmonic peaks of the rotor speed, but also consisted of a low-frequency resonant peak in the frequency range of human abdominal and spine resonant frequencies. Long-term exposure to this vibration may lead to occupational health issues such as damage to the pilot’s spine and neck. In order to address this issue, a novel adaptive seat mount concept was developed to mitigate the vibration levels transmitted to the aircrew. As a proof-of-concept demonstration, a miniature modal shaker was installed between the cabin floor and the seat bottom as an adaptive mount that provided the actuation authority. The objective was to reduce the vertical vibration transmitted to the aircrew helmet in order to decrease aircrew neck and spine injuries that are caused by the transmitted vibration. Extensive closed-loop control tests have been conducted on a full-scale helicopter seat and a mannequin with varying physical properties. A 10,000 lb(f) mechanical shaker was used to provide representative helicopter vibration profiles to the seat. Significant vibration reductions on the N/rev vibration peaks were achieved1 the low-frequency resonant peak was also suppressed simultaneously.

Smart Spring Impedance Control Algorithm for Helicopter Blade Harmonic Vibration Suppression

In this paper, an adaptive impedance control algorithm is developed for the Smart Spring to suppress helicopter rotor vibration through individual blade control. The harmonic frequencies of the blade response were estimated using parametric methods, which were used to synthesize the reference signal. Multiple equivalent notches were formed at corresponding frequencies to selectively suppress the blade vibration modes. To improve the adaptability of the algorithm, an on-line control path identification method is presented. The algorithm was implemented on a MATLAB xPC platform using the hardware-in-the-loop concept. Closed-loop experiments, conducted using a fixed helicopter blade section in both shaker and wind tunnel tests, have achieved significant vibration suppressions. Experimental results demonstrate that the algorithm, which is able to select the blade vibration modes and track the variations in vibration due to changes in flight condition, is promising for helicopter individual blade control applications.

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.

Performance characterization of active fiber-composite actuators for helicopter rotor blade applications

The primary objective of this work was to characterize the performance of the Active Fiber Composite (AFC) actuator material system for the Boeing Active Material Rotor (AMR) blade application. The AFCs were a new structural actuator system consisting of piezoceramic fibers embedded in an epoxy matrix and sandwiched between interdigitated electrodes to orient the driving electric field in the fiber direction to use the primary piezoelectric effect. These actuators were integrated directly into the blade spar laminate as active plies within the composite structure to perform structural actuation for vibration control in helicopters. Therefore, it was necessary to conduct extensive electromechanical material characterization to evaluate AFCs both as actuators and as structural components of the rotor blade. The characterization tests designed to extract important electromechanical properties under simulated blade operating conditions included stress-strain tests, free strain tests and actuation under tensile load tests. This paper presents the test results as well as the comprehensive testing process developed to evaluate the relevant AFC material properties. The results from this comprehensive performance characterization of the AFC material system supported the design and operation of the Boeing AMR blade scheduled for hover and forward flight wind tunnel tests.