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| 4. |
- Makela, K. T., et al.
(författare)
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The benefits of collaboration: the Nordic Arthroplasty Register Association
- 2019
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Ingår i: Efort Open Reviews. - : Bioscientifica. - 2058-5241 .- 2396-7544. ; 4:6, s. 391-400
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Tidskriftsartikel (refereegranskat)abstract
- The Nordic Arthroplasty Register Association (NARA) was established in 2007 by arthroplasty register representatives from Sweden, Norway and Denmark with the overall aim to improve the quality of research and thereby enhance the possibility for quality improvement with arthroplasty surgery. Finland joined the NARA collaboration in 2010. NARA minimal hip, knee and shoulder datasets were created with variables that all countries can deliver. They are dynamic datasets, currently with 25 variables for hip arthroplasty, 20 for knee arthroplasty and 20 for shoulder arthroplasty. NARA has published statistical guidelines for the analysis of arthroplasty register data. The association is continuously working on the improvement of statistical methods and the application of new ones. There are 31 published peer-reviewed papers based on the NARA databases and 20 ongoing projects in different phases. Several NARA publications have significantly affected clinical practice. For example, metal-on-metal total hip arthroplasty and resurfacing arthroplasty have been abandoned due to increased revision risk based on i.a. NARA reports. Further, the use of uncemented total hip arthroplasty in elderly patients has decreased significantly, especially in Finland, based on the NARA data. The NARA collaboration has been successful because the countries were able to agree on a common dataset and variable definitions. The collaboration was also successful because the group was able to initiate a number of research projects and provide answers to clinically relevant questions. A number of specific goals, set up in 2007, have been achieved and new one has emerged in the process.
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| 5. |
- Skogar, O, et al.
(författare)
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"Effects of Tactile Touch on pain, sleep and health related quality of life in Parkinsons disease with chronic pain": A randomized, controlled and prospective study
- 2013
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Ingår i: European Journal of Integrative Medicine. - : Elsevier. - 1876-3820 .- 1876-3839. ; 5:2, s. 141-152
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Tidskriftsartikel (refereegranskat)abstract
- Introduction: Parkinsons disease (PD) is often associated with chronic PD related pain. Complementary medicine are widely used but randomized, controlled and prospective studies of the effects are sparse. less thanbrgreater than less thanbrgreater thanAims of the study: To compare the effects of Tactile Touch (TT) with Rest to Music (RTM) in PD patients with chronic pain and to describe effects within groups. less thanbrgreater than less thanbrgreater thanPatients and methods: A 34 week controlled randomized and prospective trial compared the effects of TT with RTM in 45 (29 TT and 16 RTM) patients with PD and chronic pain. The whole body tactile stimulation method was performed for each individual patient by the same therapist for 10 times during the first 8 weeks. The RTM group received the same therapy except for the tactile stimulation. Pharmacotherapy was kept unchanged. Participants were assessed at pre- and post-intervention for pain, sleep patterns and health related quality of life (HRQoL). less thanbrgreater than less thanbrgreater thanResults: Differences between TT and RTM groups were few. Total PDSS significantly improved within the TT but not in the RTM-group. No significant differences between groups were seen in pain parameters, although significant improvements were seen within the TT-group after the intervention period. There were significant improvements within both groups in HRQoL and between groups in the items physical role and social functioning 4 weeks after screening. less thanbrgreater than less thanbrgreater thanConclusions: No significant differences between the TT and RTM groups were seen. Only in single aspects did patients with PD and chronic pain have more benefit more from CAM therapy with TT in combination with RTM.
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| 6. |
- Steen, B., et al.
(författare)
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Body composition in the elderly estimated with an electrical impedance method.
- 1987
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Ingår i: Comprehensive gerontology. Section A, Clinical and laboratory sciences. - 0902-0071. ; 1:3, s. 102-105
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Tidskriftsartikel (refereegranskat)abstract
- The purpose of the present study was to test the reliability and validity of a new electrical impedance method for estimating body composition in 35 healthy persons and patients in different sex and age groups, and to obtain basal data on the size of different body composition compartments in elderly individuals in health and disease. The reliability studies showed no significant differences with the new method in intraindividual comparisons before and after changing the positions of the electrodes, drinking of water, and overnight fasting, as well as at a double estimation on 2 consecutive days. The results with this method are compared to results from measurements of total body potassium and total body water, and from a four-compartment model technique using total body potassium and total body water. We conclude that the method has an acceptable reliability and validity and should be of great value not only in field studies but also in clinical practice. Data on body composition with the electrical impedance method are obtained from 147 individuals in different groups, namely healthy females, 80-81-year-old males, female patients from a nursing home, and patients of both sexes from a day care unit.
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| 7. |
- Ackerman, I. N., et al.
(författare)
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Substantial rise in the lifetime risk of primary total knee replacement surgery for osteoarthritis from 2003 to 2013: an international, population-level analysis
- 2017
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Ingår i: Osteoarthritis and Cartilage. - : Elsevier BV. - 1063-4584. ; 25:4, s. 455-461
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Tidskriftsartikel (refereegranskat)abstract
- Objective: To estimate and compare the lifetime risk of total knee replacement surgery (TKR) for osteoarthritis (OA) between countries, and over time. Method: Data on primary TKR procedures performed for OA in 2003 and 2013 were extracted from national arthroplasty registries in Australia, Denmark, Finland, Norway and Sweden. Life tables and population data were also obtained for each country. Lifetime risk of TKR was calculated for 2003 and 2013 using registry, life table and population data. Results: Marked international variation in lifetime risk of TKR was evident, with females consistently demonstrating the greatest risk. In 2013, Finland had the highest lifetime risk for females (22.8%, 95% CI 22.5-23.1%) and Australia had the highest risk for males (15.4%, 95% CI 15.1-15.6%). Norway had the lowest lifetime risk for females (9.7%, 95% CI 9.5-9.9%) and males (5.8%, 95% CI 5.6-5.9%) in 2013. All countries showed a significant rise in lifetime risk of TKR for both sexes over the 10-year study period, with the largest increases observed in Australia (females: from 13.6% to 21.1%; males: from 9.8% to 15.4%). Conclusions: Using population-based data, this study identified significant increases in the lifetime risk of TKR in all five countries from 2003 to 2013. Lifetime risk of TKR was as high as 1 in 5 women in Finland, and 1 in 7 males in Australia. These risk estimates quantify the healthcare resource burden of knee OA at the population level, providing an important resource for public health policy development and healthcare planning. (C) 2016 Osteoarthritis Research Society International. Published by Elsevier Ltd. All rights reserved.
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| 8. |
- Andersson, Alina, et al.
(författare)
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Analytic Parameterization of Stabilizing Controllers for the Surge Subsystem of the Moore-Greitzer Compressor Model
- 2013
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Ingår i: Proc. 2013 American Control Conference (ACC2013). - 0743-1619. - 9781479901777 ; , s. 5257-5262
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Konferensbidrag (refereegranskat)abstract
- This paper is based on a new procedure for dynamic output feedback design for systems with nonlinearities satisfying quadratic constraints. The new procedure is motivated by the challenges of output feedback control design for the 3-state Moore-Greitzer compressor model. First, we use conditions for stability of a transformed system and the associated matching conditions to find the data of the stabilizing controllers for the surge subsystem. Second, using the set of stabilizing controllers satisfying the given constraints for the closed-loop system with the dynamic output feedback controller we made optimization over the parameter set. We present the data of the stabilizing controllers and the new constraints for the corresponding parameters. The contributions in this paper are simplified conditions for the synthesis and optimization over the control parameter set.
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| 9. |
- Andersson, Alina, et al.
(författare)
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Sufficient Conditions for Dynamic Stabilization of 3-State Moore-Greitzer Compressor Model
- 2015
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Ingår i: 2015 IEEE 54th Annual Conference on Decision and Control (CDC). - 9781479978861 - 9781479978847 ; , s. 4394-4399
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Konferensbidrag (refereegranskat)abstract
- We consider the classical 3-state Moore-Greitzer model, which is commonly used for approximating dynamics of deviations of flow and pressure variables in an axial compressor from their nominal steady-state values. The linearization of the nonlinear system is not controllable and, therefore, even local asymptotic stability cannot be achieved using methods of linear control theory. We propose a family of parametrized partialstate feedback control laws and derive sufficient conditions to guarantee global asymptotic stability of the closed-loop system. The stability analysis uses the integral quadratic constraints technique for the case of non-strict frequency condition and novel arguments for characterization of omega-limit sets.
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| 10. |
- Badawy, Mona, et al.
(författare)
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Hospital volume and the risk of revision in Oxford unicompartmental knee arthroplasty in the Nordic countries -an observational study of 14,496 cases
- 2017
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Ingår i: BMC Musculoskeletal Disorders. - : Springer Science and Business Media LLC. - 1471-2474. ; 18:1
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Tidskriftsartikel (refereegranskat)abstract
- Background: High procedure volume and dedication to unicompartmental knee arthroplasty (UKA) has been suggested to improve revision rates. This study aimed to quantify the annual hospital volume effect on revision risk in Oxfordu? nicompartmental knee arthroplasty in the Nordic countries. Methods: 14,496 cases of cemented medial Oxford III UKA were identified in 126 hospitals in the four countries included in the Nordic Arthroplasty Register Association (NARA) database from 2000 to 2012. Hospitals were divided by quartiles into 4 annual procedure volume groups (≤11, 12-23, 24-43 and ≥44). The outcome was revision risk after 2 and 10 years calculated using Kaplan Meier method. Multivariate Cox regression analysis was used to assess the Hazard Ratio (HR) of any revision due to specific reasons with 95% confidence intervals (CI). Results: The implant survival was 80% at 10 years in the volume group ≤11 procedures per year compared to 83% in other volume groups. The HR adjusted for age category, sex, year of surgery and nation was 0.87 (95% CI: 0.76-0.99, p = 0.036) for the group 12-23 procedures per year, 0.78 (95% CI: 0.68-0.91, p = 0.002) for the group 24-43 procedures per year and 0.82 (95% CI: 0.70-0.94, p = 0.006) for the group ≥44 procedures per year compared to the low volume group. Log-rank test was p = 0.003. The risk of revision for unexplained pain was 40-50% higher in the low compared with other volume groups. Conclusion: Low volume hospitals performing ≤11 Oxford III UKAs per year were associated with an increased risk of revision compared to higher volume hospitals, and unexplained pain as revision cause was more common in low volume hospitals.
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| 11. |
- Bremander, Ann B, et al.
(författare)
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Revision in previously satisfied knee arthroplasty patients is the result of their call on the physician, not on pre-planned follow-up : a retrospective study of 181 patients who underwent revision within 2 years
- 2005
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Ingår i: Acta Orthopaedica. - : Medical Journals Sweden AB. - 1745-3674 .- 1745-3682. ; 76:6, s. 785-90
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Tidskriftsartikel (refereegranskat)abstract
- BACKGROUND: Degree of satisfaction with a knee arthroplasty is said to be correlated to reduced pain and better function. During a validation of the Swedish Knee Arthroplasty Register in 1997, previously operated patients were asked how satisfied they were with their knee. A subgroup of "satisfied" patients was identified who underwent revision within 2 years of having expressed satisfaction. Our aim was to study the revision diagnosis, to determine whether the problem leading to revision had been discovered as a result of routine follow-up, and also to find out when the symptoms leading to revision had started.METHODS: We retrospectively studied the medical records of 181 patients (181 knees), with a median age of 74 (31-88) years. 68% were women and the median time between primary operation and revision was 8 (3-21) years.RESULTS: Aseptic loosening (74/181) was the most common diagnosis. 2 cases were revised as a result of routine follow-up. 44% of the medical records included reports of pain in the replaced knee prior to answering the satisfaction questionnaire.INTERPRETATION: Few patients were admitted to knee revision surgery due to medical findings discovered during routine follow-up. The term "satisfaction" must be interpreted with care, as it seems to have a more complex meaning for the patients than absence of knee pain.
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| 12. |
- Freidovich, Leonid B., 1973-, et al.
(författare)
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Shaping stable periodic motions of inertia wheel pendulum : theory and experiment
- 2009
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Ingår i: Asian Journal of Control. - : Wiley. - 1561-8625 .- 1934-6093. ; 11:5, s. 549-556
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Tidskriftsartikel (refereegranskat)abstract
- We consider an underactuated two-link robot called the inertia wheel pendulum. The system consists of a free planar rotational pendulum and a symmetric disk attached to its end, which is directly controlled by a DC-motor. The goal is to create stable oscillations of the pendulum, which is not directly actuated. We exploit a recently proposed feedback-control design strategy based on motion planning via virtual holonomic constraints. This strategy is shown to be useful for design of regulators for achieving orbitally exponentially stable oscillatory motions. The main contribution is a step-by-step procedure on how to achieve oscillations with pre-specified amplitude from a given range and an arbitrary independently chosen period. The theoretical results are verified via experiments with a real hardware setup.
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| 14. |
- Irmola, Tero, et al.
(författare)
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Association between fixation type and revision risk in total knee arthroplasty patients aged 65 years and older : a cohort study of 265,877 patients from the Nordic Arthroplasty Register Association 2000–2016
- 2021
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Ingår i: Acta Orthopaedica. - : Medical Journals Sweden AB. - 1745-3674 .- 1745-3682. ; 92:1, s. 91-96
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Tidskriftsartikel (refereegranskat)abstract
- Background and purpose — The population of the Nordic countries is aging and the number of elderly patients undergoing total knee arthroplasty (TKA) is also expected to increase. Reliable fixation methods are essential to avoid revisions. We compared the survival of different TKA fixation concepts with cemented fixation as the gold standard. Patients and methods — We used data from the Nordic Arthroplasty Register Association (NARA) database of 265,877 unconstrained TKAs performed for patients aged ≥ 65 years with primary knee osteoarthritis between 2000 and 2016. Kaplan–Meier (KM) survival analysis with 95% confidence intervals (CI) and the Cox multiple-regression model were used to compare the revision risk of the fixation methods. Results — Cemented fixation was used in 243,166 cases, uncemented in 8,000, hybrid (uncemented femur with cemented tibia) in 14,248, and inverse hybrid (cemented femur with uncemented tibia) fixation in 463 cases. The 10-year KM survivorship (95% CI) of cemented TKAs was 96% (96 − 97), uncemented 94% (94 − 95), hybrid 96% (96 − 96), and inverse hybrid 96% (94 − 99), respectively. Uncemented TKA was associated with increased risk of revision compared with the cemented TKA; the adjusted hazard ratio was 1.3 (95% CI 1.1 − 1.4). Interpretation — Cemented, hybrid, and inverse hybrid TKAs showed 10-year survival rates exceeding 95%. Uncemented fixation was associated with an increased risk of revision in comparison with cemented fixation. As both hybrid and inverse hybrid fixation were used in only a limited number of TKAs, indicating possibility of selection bias in their favor, cemented TKA still remains the gold standard, as it works reliably in the hands of many.
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| 15. |
- Irmola, Tero, et al.
(författare)
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Impact of the Nordic Arthroplasty Register Association (NARA) collaboration on demographics, methods and revision rates in knee arthroplasty : a register-based study from NARA 2000–2017
- 2022
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Ingår i: Acta Orthopaedica. - : Medical Journals Sweden AB. - 1745-3674 .- 1745-3682. ; 93, s. 866-873
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Tidskriftsartikel (refereegranskat)abstract
- Background and purpose — We have previously observed differences in treatment and outcome of knee arthroplasties in the Nordic countries. To evaluate the impact of Nordic collaboration in the last 15 years we aimed to compare patient demographics, methods, and revision rates in primary knee arthroplasties among the 4 Nordic countries. Patients and methods — We included 535,051 primary knee arthroplasties reported 2000–2017 from the Nordic Arthroplasty Register Association (NARA) database. Kaplan–Meier analysis (KM) and restricted mean survival time (RMST) analysis were used to evaluate the cumulative revision rate (CRR) and RMST estimates with 95% confidence intervals (CI) and to compare countries in relation to risk of revision for any reason. Results — After 2010, the increase in incidence of knee arthroplasty plateaued in Sweden and Denmark but contin-ued to increase in Finland and Norway. In 2017 the incidence was highest in Finland with 226 per 105 person-years, while it was less than 150 per 105 in the 3 other Nordic coun-tries. In total knee arthroplasties performed for osteoarthri-tis (OA), overall CRR at 15 years for revision due to any reason was higher in Denmark (CRR 9.6%, 95% CI 9.2−10), Norway (CRR 9.1%, CI 8.7−9.5), and Finland (CRR 7.0%, CI 6.8−7.3) compared with Sweden (CRR 6.6%, CI 6.4−6.8). There were differences among the countries in use of implant brand and type, fixation, patellar component, and use of unicompartmental knee arthroplasty. Interpretation — We evinced a slowing growth of incidence of knee arthroplasties in the Nordic countries after 2010 with Finland having the highest incidence. We also noted substantial differences among the 4 Nordic countries, with Sweden having a lower risk of revision than the other countries. No impact of NARA could be demonstrated and CRR did not improve over time.
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| 16. |
- Niburg, M., et al.
(författare)
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A complete sub-system of parallel optical interconnects for telecom applications
- 1996
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Ingår i: 1996 Proceedings 46th Electronic Components and Technology Conference. ; , s. 259-263
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Konferensbidrag (refereegranskat)abstract
- Increased throughput and flexibility requirements in large-scale electronic equipment create needs for increased interconnection density capacity and re-configurability. The growing demand for new broadband interactive telecom and computer services is expected to enforce this development, which presents a challenge to traditional electronic interconnection techniques and an opportunity for new optical solutions, provided that the cost of introduction of the optical technology is affordable for both system operators and service users. The authors describe a complete sub-system of parallel optical interconnects (i.e., a total solution including the link itself and the equipment practice supporting the link) which exhibits both cost and performance benefits compared to a pure electrical or a serial optical solution.
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| 17. |
- NiemeläInen, Mika J., et al.
(författare)
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Different incidences of knee arthroplasty in the Nordic countries : A population-based study from the Nordic Arthroplasty Register Association
- 2017
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Ingår i: Acta Orthopaedica. - : Medical Journals Sweden AB. - 1745-3674 .- 1745-3682. ; 88:2, s. 173-178
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Tidskriftsartikel (refereegranskat)abstract
- Background and purpose — The annual number of total knee arthroplasties (TKAs) has increased worldwide in recent years. To make projections regarding future needs for primaries and revisions, additional knowledge is important. We analyzed and compared the incidences among 4 Nordic countries Patients and methods — Using Nordic Arthroplasty Register Association (NARA) data from 4 countries, we analyzed differences between age and sex groups. We included patients over 30 years of age who were operated with TKA or unicompartmental knee arthroplasty (UKA) during the period 1997–2012. The negative binomial regression model was used to analyze changes in general trends and in sex and age groups. Results — The average annual increase in the incidence of TKA was statistically significant in all countries. The incidence of TKA was higher in women than in men in all 4 countries. It was highest in Finland in patients aged 65 years or more. At the end of the study period in 2012, Finland’s total incidence was double that of Norway, 1.3 times that of Sweden and 1.4 times that of Denmark. The incidence was lowest in the youngest age groups (< 65 years) in all 4 countries. The proportional increase in incidence was highest in patients who were younger than 65 years. Interpretation — The incidence of knee arthroplasty steadily increased in the 4 countries over the study period. The differences between the countries were considerable, with the highest incidence in Finland. Patients aged 65 years or more contributed to most of the total incidence of knee arthroplasty.
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| 18. |
- Niemeläinen, Mika J., et al.
(författare)
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The effect of fixation type on the survivorship of contemporary total knee arthroplasty in patients younger than 65 years of age : a register-based study of 115,177 knees in the Nordic Arthroplasty Register Association (NARA) 2000–2016
- 2020
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Ingår i: Acta Orthopaedica. - : Medical Journals Sweden AB. - 1745-3674 .- 1745-3682. ; 91:2, s. 184-190
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Tidskriftsartikel (refereegranskat)abstract
- Background and purpose — Cemented fixation is regarded as the gold standard in total knee arthroplasty (TKA). Among working-age patients, there has been controversy regarding the optimal fixation method in TKA. To address this issue, we conducted a register-based study to assess the survivorship of cemented, uncemented, hybrid, and inverse hybrid TKAs in patients aged < 65 years. Patients and methods — We used the Nordic Arthroplasty Register Association data of 115,177 unconstrained TKAs performed for patients aged < 65 years with primary knee osteoarthritis over 2000–2016. Kaplan–Meier (KM) survival analysis with 95% confidence intervals (CI) and Cox multiple-regression model with adjustment for age, sex, and nation were used to compare fixation methods in relation to revision for any reason. Results — The 10-year KM survivorship of cemented TKAs was 93.6% (95% CI 93.4–93.8), uncemented 91.2% (CI 90.1–92.2), hybrid 93.0% (Cl 92.2–93.8), and inverse hybrid 96.0% (CI 94.1–98.1). In the Cox model, hybrid TKA showed decreased risk of revision after 6 years’ follow-up compared with the reference group (cemented) (hazard ratio [HR] 0.5 [CI 0.4–0.8]), while uncemented TKAs showed increased risk of revision both < 1 year (HR 1.4 [1.1–1.7]) and > 6 years’ (HR 1.3 [1.0–1.7]) follow-up compared to the reference. Interpretation — Both cemented and hybrid TKAs had 10-year survival rates exceeding 92–>93% in patients aged < 65 years. Cemented TKA, however, was used in the vast majority (89%) of the operations in the current study. As it performs reliably in the hands of many, it still deserves the status of gold standard for TKA in working-age patients.
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| 19. |
- Pchelkin, Stepan S., et al.
(författare)
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On Orbital Stabilization for Industrial Manipulators : Case Study in Evaluating Performances of Modified PD plus and Inverse Dynamics Controllers
- 2017
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Ingår i: IEEE Transactions on Control Systems Technology. - 1063-6536 .- 1558-0865. ; 25:1, s. 101-117
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Tidskriftsartikel (refereegranskat)abstract
- Orbital stabilization is one of the available alternatives to the classical asymptotic stabilization, known as the reference tracking control, which is typically considered and implemented for controlling motions of industrial robot manipulators. Since asymptotic orbital stability means convergence of solutions of a closed-loop system to an orbit of a reference trajectory, instead of tracking it as a function of time, new feedback designs can potentially improve performance with respect to several key criteria for industrial manipulators such as absolute path accuracy for tool's motions and robustness to uncertainties in the model. The main outcomes of this paper are a new class of controllers that achieve asymptotic orbital stabilization of motions and a novel analytical method for analysis and redesign of system's dynamics using an excessive set of easy-to-compute transverse coordinates. The contributions have been validated in a series of experimental studies performed on a standard industrial robot ABB IRB 140 with the IRC5-system extended with an open control interface. The outcomes of the tests show that the proposed redesign allows achieving significantly reduced deviations of the actual trajectories from the desired ones at different ranges of speeds and for several different paths, often outperforming the state-of-the-art commercial implementations. A comprehensive discussion of one of such experiments is given.
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| 20. |
- Robertsson, Anders, et al.
(författare)
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Implementation of Industrial Robot Force Control Case Study: High Power Stub Grinding and Deburring
- 2006
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Ingår i: 2006 IEEE/RSJ International Conference on Intelligent Robots and Systems (IEEE Cat. No. 06CH37780D). - 1424402581 ; , s. 2743-2748
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Konferensbidrag (refereegranskat)abstract
- In this paper, the results from a joint industry-academia project in industrial robotic force control are presented. The extension and implementation of an external sensor system for an industrial robot system, which can be used for high-bandwidth force control, are described. Results from two industrial applications using the system are presented, a stub grinding application using a new compliant grinding end-effector integrated with the robot control system, and a deburring application with a stiff tool requiring high-bandwidth force control in six degrees of freedom. Using the system an easily reconfigurable control structure was achieved, which was able to control contact forces with a sampling bandwidth of an order of magnitude higher than for conventional robot controllers
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| 21. |
- Shiriaev, Anton S., et al.
(författare)
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Criteria for Global Stability of Coupled Systems with Application to Robust Output Feedback Design for Active Surge Control
- 2009
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Ingår i: Proc. 2009 IEEE Multi-conference on Systems and Control (MSC 2009). ; , s. 1021-1026
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Konferensbidrag (refereegranskat)abstract
- The well-known and commonly accepted finite dimensional model qualitatively describing surge instabilities in centrifugal (and axial) compressors is considered. The problem of global output feedback stabilization for it is solved. The solution relies on two new criteria for global stability proposed for a class of nonlinear systems exploiting quadratic constraints for infinite sector nonlinearities. Two families of robust output feedback controllers are proposed. Controllers from the first family ensure global exponential stabilization. The ones from the second family provide integral action but only ensure local exponential and global asymptotic stability. Performance is verified by simulations.
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| 22. |
- Shiriaev, Anton S., et al.
(författare)
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IQC Arguments for Analysis of the 3-State Moore-Greitzer Compressor System
- 2015
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Ingår i: IFAC-PapersOnLine. - : Elsevier BV. - 2405-8963. ; 28:11, s. 252-257
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Tidskriftsartikel (refereegranskat)abstract
- The Integral Quadratic Constraint (IQC) framework developed by Professor Yakubovich and his co-workers, see Yakubovich et.al. (2004), is one of few available constructive tools for establishing robust stability of nonlinear systems. An explicit format of stability conditions, procedures for computing a Lyapunov function and developed libraries IQCs for common nonlinearities in dynamics, all together have made the approach unique and at the same time so to say automatic for recovering stability conditions for many applications: in the process of analyzing a dynamical system, an engineer is just required to search for a sufficiently rich set IQCs describing nonlinearities in the dynamics so that such nonlinearities can be substituted in analysis by quadratic constraints, which they satisfy. The power of the methodology becomes also its weak part in an analysis of concrete systems. Searching IQCs is the difficult task in new examples, where a lack of a rich set of IQCs for concrete nonlinearities makes the method inconclusive or too rough to detect (in)-stability. The paper is aimed at a discussion of such an example of a nonlinear dynamical system (the classical 3-state Moore-Greitzer compressor model) augmented with the dynamical feedback controller, whose parameters should be adjusted to meet a stability condition. The closed-loop system has several nonlinearities and searching the corresponding IQCs to meet the stability conditions for this example is rather involved. To overcome the problem, we have previously described by different methods a set of parameters for which any solution of the closed loop system, if bounded, will converge to the origin and that the origin is locally asymptotically stable. However, the proof is incomplete without showing a boundedness of all solutions. To solve the task we have re-used some of the IQC framework ideas, where the method has been utilized and the corresponding IQCs have been found only for unbounded trajectories, if they would be present in closed loop system. The arguments have allowed completing the proof of stability and illustrating deliberate use of the IQC framework aimed at analysis of behavior specific trajectories.
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