Andy von Flotow

Hood Tech

Blade-tip measurement systems are in development for monitoring and prognosis of rotor health and blade health in turbomachinery. The preliminary development of these systems suffers from lack of experience with failure signatures; technical staff are improvising with a combination of test and inspired guesswork. Perhaps mistakes are being made; it...

A Non-intrusive Stress Measurement (NSMS) was developed to measure coupled vibration on turbine blades. These systems can report the actual stresses seen on the blades; detect synchronous resonances that are the source of High Cycle Fatigue (HCF) in blades; measure individual blade mistuning and even help separate complicated, coupled resonances in...

A rotor blade for a turbine engine rotor assembly is provided comprising a root, an airfoil, a platform, and a means for damping vibrations in the airfoil. The airfoil includes a pocket formed into a chordwise surface. The damper is received into the pocket, forming a surface flush with the airfoil. Relative movement between the damper and the airf...

A method for reducing blade vibration in turbo-machinery is described. The method involves inserting flow obstructions and/or gas injections upstream of vibrating blades in such a manner that power flow into the blades is reduced by means of cancellation within a modal power integral.

Several types of Stewart platforms have been implemented by research groups to examine design and control issues in six-axis vibration isolation for space-based systems. Hood Technology Corporation and the University of Washington have taken the lessons learned from these various designs and developed a new hexapod that addresses the requirements o...

It is possible to detect passage of turbomachinery blades without penetrating the engine case. This "look through the wall" eddy current sensor eases the addition of a PHM system to an engine. This paper introduces this sensing concept, summarizes the physics, and presents results of preliminary tests. Blade material and geometry, case material, th...

Blade tip sensors embedded into the engine case have been used for decades to measure blade tip clearance and blade vibration. Many sensing technologies have been used; capacitive, inductive, optical, microwave, infra-red, eddy-current, pressure and acoustic. These sensors generate data streams far greater than have been historically used in engine...

Damage in rotor blades, including cracks, tend to shift the blades' resonant frequencies. Blade resonances have thus been envisioned as a damage indicator. This paper studies issues that arise in blade resonance identification using Non-contacting Stress Monitoring Systems (NSMS) when blade resonances have slight variation (mistuning) and are dynam...

Blade tip sensors embedded in the engine case have been used for decades to measure blade tip clearance and blade vibration. Many sensing technologies have been used: capacitive, inductive, optical, microwave, infrared, eddy-current, pressure and acoustic. This paper outlines the technology of blade and disk health monitoring with such sensors. The...

An aircraft engine is attached to the fuselage by a mounting structure that has a vibration absorbing system which includes first and second sensors to produce signals indicating vibration along two orthogonal axes. A third sensor, such as an accelerometer, is coupled to the mounting structure for sensing vibration of the engine. A tachometer circu...

An active vehicle suspension employs a multi-point support system between a vehicle's chassis and cab. Each support point has an isolator which includes a linear hydraulic actuator that is connected at its quiet end to the payload by a passive vibration isolator and is connected at its vibrating end to the chassis. Each isolator also has a support...

This paper attempts to confirm experimentally, an approach for sensing actual disturbance information which can propagate along one-dimensional structural waveguides. In this work, a polyvinylidene fluoride-PVDF distributed wave sensing scheme is reported which exploits the characterization of 1-dimensional structural motion in terms of traveling w...

A novel method for sensing actual disturbance information which can propagate along one-dimensional structural waveguides is developed. The motivation for this work arises from the inability to realize most active wave control designs using output information from physical measurements such as deflection, slope, curvature and internal shear force....

A simplified analytical structural acoustics model of a propeller aircraft is developed to study the potential of mechanical vibration absorbers for reducing interior noise. The results show that globally detuning the absorbers to minimize an interior acoustic cost function gives attenuations of the order of 6–10 dB of the blade passage frequency i...

This paper compares three actuation schemes to attenuate turbulent boundary layer sound radiated into the cabin through an aircraft fuselage. Two actuation schemes make use of piezoelectric materials and are compared with conventional electromagnetic voice coil actuation. Volume displacement of electromechanical actuators is typically proportional...

In many industrial and defense applications noise and vibration are important problems. The conventional method of treatment is to use passive damping techniques or to redesign the system. However, passive damping techniques are primarily effective at higher frequencies, and redesign is often costly and ineffective. In the last decade, active contr...

This article outlines various noise generation mechanisms in jet and turboprop aircraft and describes propagation paths into the cabin. Control techniques in current practice, both passive and active, are described.

Active control in the presence of resonant structural dynamics is challenging since such dynamics are both uncertain and easy to destabilize. This is particularly true when broadband control is required. Active vibration isolation is plagued by such difficulties. In spite of much development work, few systems are in service and these remain very se...

Theory and experiment are presented for a device which couples travelling waves in an electrical transmission line with flexural waves in an elastic beam. Coupling of the two wave types is achieved using a segmented piezoelectric layer adhered to the beam and connected to the transmission line. The key design parameter is the non-dimensional coupli...

The transfer function from gimbal torque to inertial pointing angle of a dynamic system composed of a rigid articulated body mounted through a frictionless gimbal on a flexible spacecraft is discussed. This transfer function is shown to be slightly modified by spacecraft flexibility. The results of the analysis are useful in the design process. The...

A hybrid active-passive vibration isolation system is considered for isolating sensitive payloads from seismic level base disturbances and payload induced force disturbances. Stability robustness requirements justify a detailed investigation of system dynamics, highlighting physical consequences of conventional actively enhanced soft mounting. The...

The response of a spatially continuous distributed sensor mounted on a vibrating structure is examined in this paper. The response of the distributed sensor to structural deformations is interpreted as a filtered version of the response of a point sensor to the same deformations, where the wavenumber filtering characteristics are determined by the...

This paper describes an active multi-axis vibration cancellation system designed for the expander of a Hughes split Stirling cryocooler. The axial force and the two lateral torques generated by the expander are cancelled by an active counterbalance driven by electrodynamic forcing elements. The counterbalance drive signals are generated by an narro...

This paper describes the results of current research being carried out at McDonnell Douglas Aerospace (MDA) to develop a flight-ready vibration isolation system for possible use on shuttle or space station. Measurements and predictions indicate that the natural vibration modes of these vehicles may be excited by on-board equipment and astronaut act...

The vibration produced by Stirling cryocoolers is incompatible with precision-pointing imaging instruments. This paper examines a vibration cancellation system that was designed for the expander of a Hughes tactical cooler. This system uses a custom-designed multi-axis counterbalance and an analogue implementation of an adaptive feedforward compens...

This paper describes the results of current research being carried out at McDonnell Douglas Aerospace (MDA) to develop a flight-ready vibration isolation system for possible use on shuttle or space station. Measurements and predictions indicate that the natural vibration modes of these vehicles may be excited by on-board equipment and astronaut act...

The vibration produced by Stirling cryocoolers is incompatible with precision-pointing imaging instruments. This paper reviews vibration reduction techniques and then examines a vibration cancellation system that was designed for the displacer of a Hughes tactical cooler. This system uses a custom-designed multi-axis counterbalance and an analog im...

Machinery noise and vibration is an important issue in many applications, including spacecraft, aircraft and naval vessels. A classic approach to ameliorating the effect of such disturbances is isolation at a vibration transmission bottleneck, the machinery mounts. Passive approaches are widespread, and introduce no risk of instability. Active tech...

Passive damping of structural dynamics using piezoceramic electromechanical energy conversion and passive electrical networks is a relatively recent concept with little implementation experience base. This paper describes an implementation case study, starting from conceptual design and technique selection, through detailed component design and tes...

The paper describes experimental research in the area of active vibration isolation. The objective of the research is to quantitatively assess the performance gained by augmenting a passive isolator with an active stage. Vibration isolation experiments were carried out on a flexible structure utilizing a proof-mass shaker as the disturbance source...

This paper investigates the benefits of passive damping in single-input single-output (SISO) and multi-input multi-output (MIMO) feedback controlled structures. Theoretical formulations are derived verifying improved stability robustness characteristics for simple controlled structures on phase margin, gain margin, and root locus properties of a SI...

The design of a microgravity isolation system must take into account predicted vibration levels, payload requirements, and logistic considerations. This paper describes the results of current research being carried out at MDSSC to produce a vibration attenuation system for possible use on Space Station Freedom (SSF). The system is a very-low-freque...

This study presents a control scheme to retrieve a Tethered Sub-satellite System (TSS) consisting of the Space Shuttle and a tethered sub-satellite. The paper focuses upon the final 2 km of the retrieval. A nominal retrieval trajectory is computed by determining the tether length history which minimizes a combination of final pitch angle, pitch rat...

The benefits and recent advances in the field of structural acoustics are briefly discussed. The objectives of structural acoustics are summarized. Directions for future research are suggested.

Techniques for designing and implementing algorithms for the control of periodic narrowband disturbances are discussed, and the strong similarities between the different methodologies are shown. The analysis of linear time invariant feedback systems results in a suggestion of how to extend two of the LQ-based multiple-input, multiple-output methodo...

A number of experiments to be carried out on flights of the Space Shuttle as well as others planned for execution on Space Station Freedom require extremely low levels of acceleration. In order to achieve the desired low acceleration levels, it will be necessary to curtail crew activities and operation of machinery, or at least schedule them for 'n...

This paper considers the problem of high-bandwidth positioning and pointing of a small mass mounted on a flexible structure. A potential application would be the positioning of mirrors in a space-based interferometer. For high-performance control it is a general rule that a sufficiently accurate model of the plant is needed in the control bandwidth...

This paper describes the development and manufacture of distributed piezoelectric film strain sensors. The sensors are spatially shaped such that, when bonded to a vibrating structure, their output is selectively proportional to a particular deformation pattern of the flexible structure. In this paper, the selectivity is based on the spatial orthog...

This paper presents a trajectory design for a solar sail journey from LEO to an intercept location behind the moon. The very weak thrust level induces extremely slow variations of the orbital parameters near the Earth. In a first part, it is intended to leave the Earth's vicinity where the solar thrust/Earth attraction ratio is very small. Averagin...

Two approaches to eliminating nonlinear effects that would otherwise render the linear controllers designed for a plant ineffective are shown to work well experimentally. A neural network compensation scheme assumes no a priori knowledge as to the structure of the nonlinearity and, with suitable computational capability and sufficient training and...

The linear quadratic regulator control design methodology using frequency shaping of cost functionals (FSCF) is used to address narrowband, linear time invariant disturbance rejection problems. Frequency dependent state weighting matrices are incorporated into an augmented linear quadratic cost functional and result in a dynamic control law with hi...

The possibility of dissipating mechanical energy with piezoelectric material shunted with passive electrical circuits is investigated. The effective mechanical impedance for the piezoelectric element shunted by an arbitrary circuit is derived. The shunted piezoelectric is shown to possess frequency dependent stiffness and loss factor which are also...

Equations of motion for an autonomous unicycle are given as derived using Kane's formalism, and the unstable nonminimum-phase behavior of the linearized system is illustrated. Lateral and longitudinal dynamics nominally decouple for low yaw rates but couple nonlinearly for large yaw rates and yaw accelerations. Time scale separation is employed to...

This paper investigates the filtering of wave propagation dynamics at a cross-section along one-dimensional structures using point and distributed measurements. This work is motivated by the desire to determine the magnitude and direction of wave propagation in a member and to implement previously developed wave control designs. In this paper the s...

While such passive approaches to helicopter sound and vibration control as vibration isolation/acoustic blocking, damping/acoustic absorption, and tuned absorbers, can be actively implemented and enhanced, active control methods transcend these possibilities. A survey is presently conducted of the literature generated by active-control research in...

This paper describes an aeroelastic analysis performed on a human-powered aircraft. The structural characteristics of this aircraft (namely, very flexible wings, high aspect ratio, and low wing loadings) are typical of proposed high-altitude long-endurance aircraft. Because of these unique features, the aircraft exhibits uncommon aeroelastic charac...

This paper presents an analysis of the dynamics and control of the pitch and roll motion during retrieval of a Tethered Satellite System, consisting of the Space Shuttle Orbiter and the tethered satellite. Two nonlinear control designs are investigated and compared. The first approach uses visual observation of the line-of-sight angle and phase-pla...

This report summarizes 16 months of research into active control of elastic wave propagation in flexible structures. The report format is that of a brief executive summary supported by an extensive appendix containing the research publications generated in the course of this research. The research performed can be broken into two major fields: (1)...

Research into dynamics and control of large flexible spacecraft has a history of several decades. More recently, application of these techniques to naval structural acoustics has received attention. The dynamic similarity of these two applications is uncanny. This presentation will offer a broad overview of several years of work performed in the De...

This paper describes the techniques developed for a flight test program of a human powered aircraft, the application of these techniques, and the results of the flight testing. A system of sensors, signal conditioning, and data recording equipment was developed and installed on the aircraft. Flight test maneuvers that do not exceed the aircraft's l...

The paper models the effects of dynamic coupling between a structure and an electrical network through the piezoelectric effect. The coupled equations of motion of an arbitrary elastic structure with piezoelectric elements and passive electronics are derived. State space models are developed for three important cases: direct voltage driven electrod...

When the disturbances acting on a system reflect the dynamics of the process generating them (i.e. disturbances other than white noise), this information can be exploited for the design of disturbance rejection controllers. Two modern control approaches for designing linear disturbance rejection controllers are discussed: disturbance modeling and f...

In each of the two presented approaches to cases of locally observed wave components propagating along one-dimensional structural components, the solution of the PDE which characterizes the dynamics of the latter is expressed in terms of travelling waves. The first approach uses this form of the solution to combine a sequence of spatially discrete...

The report lists in detail the equipment purchased with this instrumentation grant (granted under the DoD-URIP program). Also summarized is the impact this equipment has had upon the work and the reputation of the affected group at MIT. The appendix includes numerous theses and papers which have benefited from the availability of this equipment.

This paper describes several techniques for deriving optimal feedback compensators for structural waveguides at junctions. A frequency dependent cost functional composed of power flow and control effort, is minimized. Control of power flow, by modifying junction reflection and transmission properties, enables selective absorption of incoming vibrat...

This paper investigates, analytically and experimentally, the possibility of actively blocking the propagation of bending waves along a uniform beam. The work takes the form of a case study; a thin brass plate-beam is used, it is excited with a short duration impulse, and the resulting disturbance spreads dispersively as it travels along the beam....

This paper considers the problem of actively isolating a vibrating machine from a flexible structure. The structural dynamic response is assumed to be known with too little fidelity to permit model-based control design. The discussion is specialized to narrow-band disturbances, and to single-axis mounting. Narrow-band compensation, exploiting knowl...

When an engine is mounted on a structurally flexible vehicle, it is important to inhibit the transmission of vibrations of the engine into the structure. In the past, vibration isolation has been achieved using passive schemes. Active control at the mount should permit a marked improvement in its vibration isolation characteristics. Three control s...

The author considers the isolation of vibration originating in a comparatively compact, rigid machine. The machine can be an internal combustion engine, a turbine, compressor, etc. The machine is mounted in a structurally flexible vehicle using several machinery mounts. A machinery mount should be capable of transmitting low-frequency forces and su...

A structural network is an assemblage of slender one-dimensional members. Each member is demarcated, by definition, by two junctions. A junction may involve only one member (a termination) or may be an interconnection of many members. In this paper, computationally and theoretically, the dynamics of such networks are investigated. The focus of the...

The dynamics of chains of flexible bodies undergoing large rigid body motions and small elastic deflections is investigated, with emphasis on the role of nonlinear strain-displacement relations in the development of the motion equations for the deflections of these systems. Numerical results are presented for a two-link chain constrained to move in...

Wave propagation in a periodic truss-work beam is investigated computationally. The analysis is based upon the transfer matrix of a single bay of the structure. The results, with members modeled as rods with pinned joints, agree well with results obtained from an equivalent continuum model of the same structure. The inclusion of bending in member m...

A distributed parameter model of the lateral dynamic behavior of a moving web in an n -roller system is proposed. A span of web moving longitudinally and under tension is modeled as a beam with shear flexibility (Timoshenko beam). A quasistatic simplification of the model is made on the basis of spectral separation. Boundary conditions at the web-r...

Active techniques are beginning to be considered for the control of structure‐borne sound. Expertise from many fields is being brought to bear upon this problem, including experience in active control of the acoustic response of ducts and air volumes, and the active control of flexible spacecraft. High‐fidelity modeling is a necessary prerequisite...

The accuracy of component mode synthesis is investigated experimentally for substructures coupled by nonideal joints. The work is based upon a segmented experimental beam for which free-interface frequency response matrices are measured for each segment. These measurements are used directly in component mode synthesis to predict the behavior of the...

This report summaries 20 months of research into Travelling Wave Concepts for the Modelling and Control of Space Structures. A good portion of the research has focused upon the development of techniques for the modelling of structural response in terms of disturbance propagation. Such models are of interest for several reasons: 1. Understanding the...

This paper investigates the acoustic limit of active control of structural dynamics; the limit as the control bandwidth includes a very large number of natural modes of the structure. The point is made that in this limit modal analysis cannot provide reasonably accurate models of the structural dynamics, and that control design with respect to moda...

A theory for active control of elastic wave propagation in structures is developed. Attention is focused on active modification of the scattering behavior of discrete locations in a structural network. The wave mode input/output relation at a structural junction containing control actuators can be altered in two ways. First, the closed loop reflect...

This paper attempts a critical discussion of selected physical effects in the dynamics of spacecraft tethers, pointing out that for many purposes the simplest models are adequate. Spectral separation is invoked to decouple the motion into a slow libration akin to that of a rigid body upon which fast oscillations involving tether deformation and end...

Three distributed parameter models for the lateral dynamic behavior of a moving web in an n-roller system are investigated. A span of web, moving longitudinally and under tension, was modeled as either a string, a Bernoulli-Euler beam, or a Timoshenko beam. A quasi-static simplification based upon spectral separation is made to simplify the model....

This paper describes theoretical and experimental work performed on the modeling and vibration control of a hanging flexible beam. The synthesis and laboratory implementation of low-authority controllers based upon feedback of local velocity to actuator force has been the subject of previous studies. This paper extends this work with the design and...

This paper reviews past approaches to control-motivated dynamic tailoring and their impact upon the problem of active control of structural dynamics. The costs and benefits of passive damping augmentation are estimated; the conclusion is drawn that the optimum structural design will be in the "left-half .+planen. New results on dynamic tailoring of...

This paper introduces the point of view that elastic deformations in large spacecraft structures may be aptly viewed in terms of propagating disturbances. Since the main topic of this paper is the control concepts, which result from such a viewpoint, the required structural dynamic description in terms of traveling disturbances is described only br...

A structural network is taken to be an assemblage of slender structural members (beams, cables, rods) connected to each other at structural junctions. The junctions may include flexible bodies which, in this work, are restricted to those whose dynamics are described by a finite set of ordinary differential equations. Elastic disturbances in such a...

This paper presents a new family of deployment trajectories for a pair of tethered satellites. Deployment begins in orbit with the two satellites in physical contact, and progresses to the final state with the tethered pair stabilized along the local vertical. The entire deployment is accomplished in less than one orbit, using only gravity gradient...

This paper investigates the dynamic analysis of certain large space structures via travelling wave mathematics. It is assumed that large space structures may be modelled as networks of interconnected one-dimensional structural members. Bodies with a finite number of internal dynamic degrees of freedom may be scattered throughout the network. The wa...

University Microfilms order no. 84-20632. Thesis (Ph. D.)--Stanford University, 1984. Includes bibliography.