Current and Past Research Activity
The current research activity in this area consists of externally funded research projects and ongoing research projects funded by the short course activity. The following represents a short summary of the topics that are currently being investigated by the members of the UC-SDRL activity:
Perturbed Boundary Condition Test Methods
A major application of experimentally estimated modal models is to update and correlate finite element models to allow accurate design modifications and decisions to be made. To meet this need modal models with large numbers of very accurately estimated modal parameters are required. The Pertubed Boundary Condition (PBC) test method being developed at UC-SDRL incorporates data from many test conditions into a single modal data base. This enables modal models of unprecedented fidelity and size to be estimated.
Graphical User Interface Based Modal Analysis Software
The X-Modal software package is the current generation of modal analysis software under development at UC-SDRL. X-Modal is a flexible, Unix Windows, graphical user interface based package which incorporates Matlab as a computational engine. Intuitive data handling, sorting and display is accomplished in a C language/Motiff based shell, while data processing is done in Matlab scripts. This allows custom algorithms to be implemented and evaluated efficiently by users with little programming background. X-Modal has been developed with the support and close cooperation of Boeing Aircraft Company, Structural Dynamics Laboratory (SDL).
Graphical User Interface Based Data Acquisition Software
The X-ACQuisition software package is currently under development as a VXI based data acquisition package at UC-SDRL. X-ACQuisition is a flexible, Unix Windows, graphical user interface based package which incorporates Matlab as a computational engine. Intuitive data handling, sorting and display is accomplished in a C language/Motiff based shell, while data processing is done in Matlab scripts. This allows custom algorithms to be implemented and evaluated efficiently by users with little programming background. X-ACQuisition is currently under development but is not yet available.
On-Line Parameter Estimation
On-Line Parameter Estimation with application to flight flutter testing and control of time varying systems is a current research area. Modal filtering and adaptive modal filtering in conjunction with recursive parameter estimation algorithms are being evaluated as means to obtain accurate estimates of frequency and damping from very short or time varying data records. This work is supported by NASA-Ames Flight Research Center and Boeing Aircraft Company Structural Dynamics Laboratory.
Active Vibration Control
UC-SDRL's activity in the active vibration control area was initiated with the participation in the NASA Control-Structure-Interaction program in 1989. Since that initial activity, modal filter based active control algorithms have been developed and implemented on laboratory structures (UC-SDRL CSI Testbed and other structures) and a full size highway bridge. Initial studies have been conducted for implementation on a 30,000 square foot ballroom floor. This work has been supported by the National Science Foundation and Nelson Industries.
Acoustic Noise Source Detection
Three current activities involve different methodologies of acoustic noise source detection. All methods take advantage of the simultaneaous multiple channel data acquisition capability of the UC-SDRL to utilize microphone arrays to detect noise sources. The first method is a collaborative effort with Purdue University involving Nearfield Acoustical Holography (NAH). NAH provides the capability of estimating acoustic intensity and/or surface velocity in both free-field and reverberant-field environments. The second method involves the use of a temporal array while the third method utilizes the inverse frequency response function matrix between potential noise source locations and microphones in an array.
Rotating Equipment Research
Recent research involves diagnostic techniques based on singular value decomposition of vibration signatures. Also, investigation of the properties of squeeze film dampers is of currrent interest. This research utilizes a specially designed test rig with the capability to test full size dampers under realistic operating conditions while recording all operating conditions and characteristics.
Spatial Filtering of Structural Response Data
Spatial Filtering or Modal Filtering of structural response data is a research activity which is an outgrowth of the CSI program and prior work in the area of spatial based modal parameter estimation algorithms. Modal filtering is a method of processing structural response data to isolate the response of a single mode, in real time. This research has applications to active control, flight flutter analysis, health monitoring of structures, and input source identification. A current project with NASA Ames Flight Research Facility is investigating applying modal filtering to flight flutter analysis.
Actuator Modeling and Design
The influence of actuator dynamics on experimental testing and active control has been investigated by modeling various actuators and simulating their interaction with structural systems. A low cost proof mass actuator with near ideal actuator characteristics has been designed and constructed utilizing disc drive actuator components.
Research in this area is concerned with utilizing the expertise of UC-SDRL in identifying modal models of structures to develop methods of identifying models of general actuator-structure-sensor systems for use in active control methodologies. UC-SDRL has participated in this research under the NASA Control-Structure-Interaction Guest Investigator (CSI-GI) program since 1989.This concept has particular application to space structures, robotics, vibration environmental testing, and active vibration control of any nature. This activity is being conducted in conjunction with personnel from the Department of Aerospace Engineering and Applied Mechanics within the College. Funding for future work in this area is potentially available from NASA-Langley Research Center and/or the USAF-Wright Patterson Air Force Base. An active control test bed consisting of a 5 meter space truss with 6 proof mass actuators has been constructed at UC-SDRL
Structural Analysis Hardware Development
In all of the UC-SDRL research areas, particular needs have been identified in terms of hardware or hardware systems which are not currently commercially available. Particularly in the area of transducers and analog to digital conversion equipment, a low cost, low frequency, limited capability approach to equipment design is increasingly needed as the multiple input approach to experimental analysis of structural systems begins to be used extensively. In these areas, as well as in the application of array processors to minicomputer systems, research is being pursued in order to facilitate the other research within the UC-SDRL activity. Several commercial products, including plastic accelerometers, enhanced excitation systems, and signal processing systems, are available that started as research projects within SDRL.
Experimental Modal Analysis Methods
Ongoing research in this area is focusing on a technique that can be considered a hybrid technique, combining benefits of phase resonance and phase separation technologies. This method is a multiple input, stepped sine method that allows random magnitudes and phases between the inputs. This research has been supported by NASA-Marshall Space Flight Center and PCB, Piezotronics, Inc.
Modal Parameter Identification
This research area is concerned with developing modal parameter identification algorithms that compute a single set of frequency, damping, and modal vector estimates from a set of data containing more than one row or column of the frequency response function matrix. This approach reduces the variation in the estimation process and allows for the prediction of repeated roots or apparent repeated roots. With the increased capability of exciting structural systems with more than one input, this type of parameter estimation is becoming a requirement. Current research is concentrating on methods that can utilize frequency data with unequal frequency spacing and on methods that take advantage of the spatial domain nature of the data as well as the time or frequency domain nature of the data.
Multiple Input Estimation of Frequency Response Functions
This effort was pioneered at the University in 1980 for the case of two inputs. Research has continued for cases up to six inputs as well as for different excitation signal types. The importance of this research is that the time required for the multiple input case is essentially the same for the single input case, for a given level of quality in the resulting frequency response functions. This means that the time saving potential is very attractive. Additionally, the potential for improved quality of the data and the fact that the frequency response functions are acquired simultaneously, are two concepts that are important to the success of multiple reference modal parameter estimation algorithms. One further area of investigation involves the detection of nonlinearities during the acquisition of frequency response function data.
General Multiple Input/Output Problem
Research into the solution of the general multiple input/output problem, where neither the source nor the characteristics of the input is known, is needed in both the vibrations and acoustic areas to determine the source of disturbances in operating systems. While this problem is difficult, the ability to simultaneously measure responses and the experience gained in the multiple input frequency response function research are directly applicable to the study of a practical approach to this concept. Application of this research is particularly appropriate to signature analysis and health monitoring of systems.
Experimental Impedance/Modal Modeling Methods
Research into the current impedance and modal modeling techniques is continuing and is directed toward the problem of quantifying the effect that test parameters have on the prediction of changes in structural dynamics by these algorithms. Different groups currently using these methods have documented varying success in the ability to predict the accurate modal parameter changes due to structural alteration using these experimental models. For each of these methods, the research centers on understanding the sensitivity of the accuracy of the modal parameter predictions to each experimental test parameter. A related effort in this area involves the prediction of motion at any point on a rigid body based upon a set of measured degrees of freedom. This technique assumes that the rigid body motion is composed of six rigid body modes of vibration that can be determined from the measured degrees of freedom. With the six rigid body modes, the response can be estimated at any location on the rigid body. This technique is particularly useful for systems, such as car/truck engines that act as essentially rigid bodies but are difficult to measure at desired locations, such as the engine mount locations.
Optimization of Dynamics Design
Research into a new approach to modification of impedance models based upon optimization and perturbation theory is being explored. This approach allows magnitude and phase limits to be established over specific frequency ranges of specific impeadance measurements. Optimization theory, using the MATLAB Optimization Toolbox, is used to find the mimimum mass changes to the structure required to achieve the desired specification(s). This technique utilizes either measured or synthsized frequency response functions as the basis for the optimization model and does not require a modal model.
Finite Element Modeling
Development work in the area of finite element modeling of welded joints is an area of ongoing concern. Welded joints are not ideal, are difficult to approximate, and are often the source of considerable error in the finite element model. Primarily, an empirical technique, to be used as a pre-processor to MSC-NASTRAN, is being studied. This capability is being developed to support research work in the area of experimental correlation and correction of finite element models.
Finite Element Correlation/Correction Technique
Research has been initiated on the study of methods of correcting finite element mass, stiffness, and damping matrices on the basis of experimental data. This capability would provide a clear link between the ability to model structural systems and to test the actual structural system. Currently, this type of correlation and correction is based on the experience of the modeling and test engineers. This activity includes supported research on the development of a finite element for welded joints based upon experimental definition of welded joint properties.
Acoustic Intensity Technique
Research into the use of three dimensional acoustic intensity methods in fluids other than air. Research has been ongoing in the area of correlation of acoustic intensity patterns to the modal vector patterns, both determined experimentally. The goal of this effort is to provide an ability to predict noise characteristics from modal vector patterns.