Microgravity vibration isolation: An optimal control law for the one-dimensional case

Certain experiments contemplated for space platforms must be isolated from the accelerations of the platforms. An optimal active control is developed for microgravity vibration isolation, using constant state feedback gains (identical to those obtained from the Linear Quadratic Regulator (LQR) approach) along with constant feedforward (preview) gains. The quadratic cost function for this control algorithm effectively weights external accelerations of the platform disturbances by a factor proportional to (1/omega)(exp 4). Low frequency accelerations (less than 50 Hz) are attenuated by greater than two orders of magnitude. The control relies on the absolute position and velocity feedback of the experiment and the absolute position and velocity feedforward of the platform, and generally derives the stability robustness characteristics guaranteed by the LQR approach to optimality. The method as derived is extendable to the case in which only the relative positions and velocities and the absolute accelerations of the experiment and space platform are available

Active Vibration Isolation of Microgravity Experiments with Spring Umbilicals Using an Electrodynamic Actuator

Microgravity experiments will require active vibration isolation in the low to mid frequency range of 0.1 Hz to 10 Hz. Approximately two orders of acceleration reduction (40 dB) will be required. Previous works have reported results for accelerations transmitted through the umbilical. This paper describes experimental and theoretical results for vibration isolation in one dimension (horizontal) where the simulated experiment is connected to the spacecraft by a spring umbilical. The experiment consisted of a spacecraft (shaker), experiment (mass), umbilical, accelerometer, control electronics, and Lorentz actuator. The experiment mass was supported in magnetic bearings to avoid any stiction problems. Acceleration feedback control was employed to obtain the vibration isolation. Three different spring umbilicals were employed. Acceleration reductions on the order of 40 dB were obtained over the frequency range of 0.1 Hz to 10 Hz. Good agreement was obtained between theory and experiment

Microgravity isolation system design: A modern control synthesis framework

Manned orbiters will require active vibration isolation for acceleration-sensitive microgravity science experiments. Since umbilicals are highly desirable or even indispensable for many experiments, and since their presence greatly affects the complexity of the isolation problem, they should be considered in control synthesis. In this paper a general framework is presented for applying extended H2 synthesis methods to the three-dimensional microgravity isolation problem. The methodology integrates control and state frequency weighting and input and output disturbance accommodation techniques into the basic H2 synthesis approach. The various system models needed for design and analysis are also presented. The paper concludes with a discussion of a general design philosophy for the microgravity vibration isolation problem

Microgravity isolation system design: A case study

Many acceleration-sensitive, microgravity science experiments will require active vibration isolation from manned orbiters on which they will be mounted. The isolation problem, especially in the case of a tethered payload, is a complex three-dimensional one that is best suited to modern-control design methods. In this paper, extended H(sub 2) synthesis is used to design an active isolator (i.e., controller) for a realistic single-input-multiple-output (SIMO) microgravity vibration isolation problem. Complex mu-analysis methods are used to analyze the isolation system with respect to sensor, actuator, and umbilical uncertainties. The paper fully discusses the design process employed and the insights gained. This design case study provides a practical approach for isolation problems of greater complexity. Issues addressed include a physically intuitive state-space description of the system, disturbance and noise filters, filters for frequency weighting, and uncertainty models. The controlled system satisfies all the performance specifications and is robust with respect to model uncertainties

An Integrated Musculoskeletal Countermeasure Battery for Long-Duration Lunar Missions

During extended periods of skeletal unloading, losses in strength and density of the proximal femur will occur. In long-duration spaceflight, resistive exercise is used to replace the normal loads exerted on the spine and hip. At the present time, there is no conclusive evidence that hip bone loss has been prevented in this scenario. Our group has recently developed and clinically evaluated a multifunctional exercise system, the Combined Countermeasure Device (CCD). The CCD comprises a low-footprint Stuart Platform for lower-body resistance exercise and balance training, and a cardiovascular exercise bicycle. A consideration for resistance exercise was targeting of the hip abductor and adductor muscles, which attach directly at the hip and which should subject it to the largest loads. In our training study, we found that CCD exercise increased hip adductor and abductor strength, and modeling results suggest that this exercise exerts forces on the hip of approx. 4-6 body weights at 1g, compared to forces of approx.2.5 body weight y squatting exercise. In our current study, we hypothesize that abductor and adductor exercise will increase the density and strength of the proximal femur

Sclerosing Cholangitis: Surgical Significance

The authors\u27 experience with the rare entity of sclerosing cholangitis suggests that several anatomical patterns of involvement of the extrahepatic biliary tract may occur. The involvement may include the intraluminal ducts segmentally or diffusely, or the process may primarily be external to the ducts as pericholedochitis. It may or may not represent one component of a systemic disease, particulady autoimmune disease. Although involvement may be diffuse, progression may be slow and compatible with many years of life. Long time followup is essential before any value can be assigned to a given modality of treatment. Carcinoma of the ductal system may be found after a more exhaustive examination or at autopsy. All efforts should be provided operatively, however, to relieve biliary tract obstruction. If this is not possible or provides only incomplete relief, steroid therapy should be administered

Value of spleen stiffness measured by two-dimensional shear wave elastography combined with platelet count/spleen diameter ratio in evaluating moderate-to-severe gastroesophageal varices in patients with hepatitis B cirrhosis

ObjectiveTo establish a noninvasive diagnostic model for moderate-to-severe gastroesophageal varices (GEV) in patients with hepatitis B cirrhosis. MethodsThe patients with hepatitis B cirrhosis who attended The First Affiliated Hospital of China Medical University from October 2017 to December 2019 were enrolled, and with the results of gastroscopy as the gold standard, the patients were divided into none-to-mild GEV group and moderate-to-severe GEV group. The Mann-Whitney U test was used for comparison of non-normally distributed continuous data between groups, and the chi-square test was used for comparison of categorical data. A logistic regression analysis was used to investigate the association of moderate-to-severe GEV with spleen stiffness measured by two-dimensional shear wave elastography (2D-SWE) and platelet count (PLT)/spleen diameter ratio in patients with hepatitis B cirrhosis, and with the backward method for independent variable screening, a regression equation, i.e., a diagnostic model, was established and validated. The receiver operating characteristic (ROC) curve was plotted to investigate the diagnostic value of noninvasive examination indices including liver and spleen stiffness, PLT/spleen diameter ratio, and the above diagnostic model and obtain their cut-off values, the DeLong test was used to compare whether there is a statistical significance between the ROC curves of the above noninvasive indices. Results A total of 168 patients with hepatitis B cirrhosis were enrolled, among whom 67 were diagnosed with moderate-to-severe GEV. There were significant differences in PLT, alanine aminotransferase, albumin, white blood cell count, and international normalized ratio between the none-to-mild GEV group and the moderate-to-severe GEV group (Z=-6.508, -2.132, -2.470, -4.510, and -5.298, all P＜0.05). There were also significant differences in spleen stiffness measured by 2D-SWE, spleen diameter, and PLT/spleen diameter ratio between the two groups (Z=-7.264, -5.924, and -7.028, all P＜0.05). The PLT/spleen diameter ratio had an area under the ROC curve (AUC) of 0.821 (95% confidence interval ［CI］: 0.754-0.875) at the cut-off value of ≤6.7, with a sensitivity of 83.58% and a specificity of 74.26%; spleen stiffness had an AUC of 0.831 (95%CI: 0.766-0.885) at the cut-off value of ≥34.2 kPa, with a sensitivity of 8507% and a specificity of 73.27%; liver stiffness had an AUC of 0.557 (95%CI: 0.479-0.634) at the cut-off value of ≥10.8 kPa, with a specificity of 79.10% and a sensitivity of 40.59%. There was a significant difference in AUC between liver stiffness and spleen stiffness, as well as between liver stiffness and PLT/spleen diameter ratio (Z=4878 and 5536， P＜0.001). The model of Y=-0.682+0.068×spleen stiffness-0225 (PLT/spleen diameter ratio) was established for predicting moderate-to-severe GEV in patients with hepatitis B cirrhosis, which had an AUC of 0.860 (95%CI: 0.799-0.909), a sensitivity of 79.10%, a specificity of 81.19%, and an accuracy of 79.1%. ConclusionThe noninvasive diagnostic model based on spleen stiffness measured by 2D-SWE and PLT/spleen diameter ratio can be used to assist the judgment of moderate-to-severe GEV in patients with hepatitis B cirrhosis, with a higher accuracy than liver stiffness or spleen stiffness alone