Identification of flexible structures for robust control

Documentation is provided of the authors' experience with modeling and identification of an experimental flexible structure for the purpose of control design, with the primary aim being to motivate some important research directions in this area. A multi-input/multi-output (MIMO) model of the structure is generated using the finite element method. This model is inadequate for control design, due to its large variation from the experimental data. Chebyshev polynomials are employed to fit the data with single-input/multi-output (SIMO) transfer function models. Combining these SIMO models leads to a MIMO model with more modes than the original finite element model. To find a physically motivated model, an ad hoc model reduction technique which uses a priori knowledge of the structure is developed. The ad hoc approach is compared with balanced realization model reduction to determine its benefits. Descriptions of the errors between the model and experimental data are formulated for robust control design. Plots of select transfer function models and experimental data are included

Robustness and performance trade-offs in control design for flexible structures

Linear control design models for flexible structures are only an approximation to the “real” structural system. There are always modeling errors or uncertainty present. Descriptions of these uncertainties determine the trade-off between achievable performance and robustness of the control design. In this paper it is shown that a controller synthesized for a plant model which is not described accurately by the nominal and uncertainty models may be unstable or exhibit poor performance when implemented on the actual system. In contrast, accurate structured uncertainty descriptions lead to controllers which achieve high performance when implemented on the experimental facility. It is also shown that similar performance, theoretically and experimentally, is obtained for a surprisingly wide range of uncertain levels in the design model. This suggests that while it is important to have reasonable structured uncertainty models, it may not always be necessary to pin down precise levels (i.e., weights) of uncertainty. Experimental results are presented which substantiate these conclusions

Spirometry assessment of interstitial lung disease patients and correlation with its clinical and radiological profile

Background: Interstitial lung diseases (ILD) are a diverse set of lower respiratory tract illnesses that are defined by both abrupt and persistent inflammation as well as a largely irreversible and continuous progression of fibrosis in the interstitium and the walls of the alveoli. This study focuses on non-invasive techniques for clinically and radiographically confirmed situations. Since most patients refuse surgical or transbronchial lung biopsies and are in respiratory difficulty, an alternate method is preferable. Spirometry tests are often used as diagnostic aids. This study compares spirometry in ILD patients with their radiological and clinical features.Methods: In this prospective observational study, 50 ILD patients who were diagnosed on the clinical and radiological grounds included. A detailed history of illness was obtained and noted. All patients were examined clinically and underwent basic investigations. All patients were performed 6 MWT O2 saturation and spirometry. Correlation between spirometry findings and clinical and radiological profile was done.Results: The study group of 50 patients with ILD, idiopathic pulmonary fibrosis was the most common cause of ILD consists of 32 patients performing 64% of the study group. Average duration of symptoms in ILD patients in this study was 5.46±5.49 months. The mean age of the patients was 61.58±12.92 years ranging from 27 to 88 years with 27 (54%) male and 23 (46%) female. Cough and dyspnoea were the most common features at presentation in our study group, present in almost all the patients. Crepitations were present in 41 (82%) patients. Most common chest X-ray feature was reticular opacities which was present in 24 (48%) patients. Ground glass opacity in high-resolution computed tomography (HRCT) was seen in 33 (66%) patients. Most common spirometry pattern seen in our study was Restrictive pattern which was present in 42 patients. In ILD patients, mean values of FEV1% was 59.2±20.33, FVC% was 60.76±25.45, FEV1/FVC was 100.86±20.12.Conclusions: Idiopathic pulmonary fibrosis is the most common and chronic hypersensitivity pneumonitis along with cryptogenic organizing pneumonia are the second common ILD in our study. The underdiagnosis of interstitial lung disease is due to a lack of knowledge among doctors. Therefore, spirometry and 6MWT O2 saturation should be performed in all patients presented with complaints of chronic cough and breathlessness as screening tool and then HRCT chest and biopsy can be done for confirmation of diagnosis. So early diagnosis and treatment of ILD patients is possible with use of spirometry

Robustness and performance tradeoffs in control design for flexible structures

The design of control laws for the Caltech flexible structure experiment using a nominal design model with varying levels of uncertainty is considered. A brief overview of the structured singular value (µ) H∞ control design, and µ-synthesis design techniques is presented. Tradeoffs associated with uncertainty modeling of flexible structures are discussed. A series of controllers are synthesized based on different uncertainty descriptions. It is shown that an improper selection of nominal and uncertainty models may lead to unstable or poor-performing controllers on the actual system. In contrast, if descriptions of uncertainty are overly conservative, performance of the closed-loop system may be severely limited. Experimental results on control laws synthesized for different uncertainty levels on the Caltech structure are presented

On the Caltech Experimental Large Space Structure

This paper focuses on a large space structure experiment developed at the California Institute of Technology. The main thrust of the experiment is to address the identification and robust control issues associated with large space structures by capturing their characteristics in the laboratory. The design, modeling, identification and control objectives are discussed within the paper

Identification for Robust Control of Flexible Structures

An accurate multivariable transfer function model of an experimental structure is required for research involving robust control of flexible structures. Initially, a multi-input/multi-output model of the structure is generated using the finite element method. This model was insufficient due to its variation from the experimental data. Therefore, Chebyshev polynomials are employed to fit the data with a single-input/multi-output transfer function models. Combining these lead to a multivariable model with more modes than the original finite element model. To find a physically motivated model, as ad hoc model reduction technique which uses a priori knowledge of the structure is developed. The ad hoc approach is compared with balanced realisation model reduction to determine its benefits. Plots of select transfer function models and experimental data are included

Collocated versus Non-collocated Multivariable Control for Flexible Structure

Future space structures have many closely spaced, lightly damped natural frequencies throughout the frequency domain. To achieve desired performance objectives, a number of these modes must actively be controlled. For control, a combination of collocated and noncollocated sensors and actuators will be employed. The control designs will be formulated based on models which have inaccuracies due to unmodeled dynamics, and variations in damping levels, natural frequencies and mode shapes. Therefore, along with achieving the performance objectives, the control design must be robust to a variety of uncertainty. This paper focuses on the benefits and limitations associated with multivariable control design using noncollocated versus collocated sensors and actuators. We address the question of whether performance is restricted due to the noncollocation of the sensors and actuators or the uncertainty associated with modeling of the flexible structures. Control laws are formulated based on models of the system and evaluated analytically and experimentally. Results of implementation of these control laws on the Caltech flexible structure are presented

The Process of Control Design for the NASA Langley Minimast Structure

he NASA Langley Minimast Facility is an experimental flexible structure designed to emulate future large space structures. The Minimast system consists of a 18 bay, 20 meter-long truss beam structure which is cantilevered at its base from a rigid foundation. It is desired to use active control to attenuate the response of the structure at bay 10 and 18 due to impulse disturbances at bay 9 while minimizing actuator torque commanded from the torque wheel actuators. This paper details the design process used to select sensors for feedback and performance weights on the Minimast facility. Initially, a series of controllers are synthesized using H2 optimal control techniques for the given structural model, a variety of sensor locations and performance criteria to determine the "best" displacement sensor and/or accelerometers to be used for feedback. Upon selection of the sensors, controllers are formulated to determine the affect of using a reduced order model of the Minimast structure instead of the higher order structural analysis model for control design and the relationship between the actuator torque level and the closed-loop performance. Based on this information, controllers are designed using μ-synthesis techniques and implemented on the Minimast structure. Results of the implementation of these controllers on the Minimast experimental facility are presented

A sustainable method of effluent disposal: case study of Antalya sea outfall, Turkey

Antalya city, located along the Turkish Mediterranean coast, lacked a proper sanitation system till 1996. An integrated water & wastewater project has been implemented to protect groundwater resources used for drinking and seawater quality. The project involved collection, treatment and final disposal of effluents by a deep sea outfall system. A current research project has been realized to evaluate performance of Antalya Sea Outfall. Seasonal in-situ measurements and bacteriological monitoring studies have been realized. The discharged wastewater plume is observed to be submerged in summer and to reach sea surface in winter condition. The results of the monitoring program exhibit considerable spatial and temporal variations. The resultant total and fecal coliform numbers comply well with the Turkish Standards for the use of coastal and sea water for recreation

Vibration damping and robust control of the JPL/AFAL experiment using µ-synthesis

The technology for controlling elastic deformations of flexible structures is one of the key considerations for future space initiatives. A vital area needed to achieve this objective is the development of a control design methodology applicable to future structures. The mu -synthesis technique is employed to design a high-performance vibration attenuation controller for the JPL/AFAL experimental flexible antenna structure. The results presented deal primarily with the control of first two global flexible modes using only two hub actuators and two hub sensors. Implementation of the multivariable control laws based on a finite-element model is presented. All results are from actual implementation on the JPL/AFAL flexible structure testbed