ALGORITHMS AND OPTIMAL CONTROL FOR SPACECRAFT MAGNETIC ATTITUDE MANEUVERS

This study focused on providing applicable control solutions for spacecraft magnetic attitude control system. Basically, two main lines are pursued; first, developing detumbling control laws and second, an improvement in the three-axis attitude control schemes by extending magnetic rods activation time. Spacecraft, after separation from the launching mechanism, experiences a tumbling phase due to an undesired angular momentum. In this study, we present a new efficient variant of the B-dot detumbling law by introducing a substitute of the spacecraft angular velocity, based on the ambient magnetic field data. This B-dot law preserves the orthogonality, among the applied torque, dipole moment and magnetic field vectors. Most of the existing variants of the B-dot law in the literature don\u27t preserve this orthogonality. Furthermore, the problem of minimum-time spacecraft magnetic detumbling is revisited within the context of optimal control theory. Two formulations are presented; the first one assumes the availability of the angular velocity measurements for feedback. The second formulation assumes the availability of only the ambient magnetic field measurements in the feedback; the latter is considered another optimal-based B-dot law. A reduction in detumbling time is fulfilled by the proposed laws along with less power consumption for the proposed B-dot laws. In magnetic attitude maneuvers, magnetic rods and magnetometers usually operate alternatively, to avoid the magnetic rods\u27 noise effect on magnetometers measurements. Because of that, there will be no control authority over the spacecraft during the magnetometer measurement period. Hence longer maneuver times are usually experienced. In this study, a control scheme that enables the extension of the magnetic rods’ activation time is developed, regardless of the attitude control law. The key concept is replacing the real magnetic field measurement by a pseudo measurement, which is computed based on other sensors measurements. By applying a known command to the spacecraft and measuring the spacecraft response, it is possible to compute the ambient magnetic field around the spacecraft. The system mathematical singularity is solved using the Tikhonov regularization approach. Another developed approach estimates the magnetic field, using a relatively simple and fast dynamic model inside a Multiplicative Extended Kalman Filter. A less maneuver time with less power consumption are fulfilled. These control approaches are further validated using real telemetry data from CASSIOPE mission. This dissertation develops a stability analysis for the spacecraft magnetic attitude control, taking into consideration the alternate operation between the magnetic rods and the magnetometers. It is shown that the system stability degrades because of this alternate operation, supporting the proposed approach of extending the operation time of the magnetic rods

A study illustrating the risk factors, clinical presentation and management of ectopic pregnancy in a secondary hospital in Bahrain

Background: Assess the risk factors, clinical presentation and type of intervention indicated in ectopic pregnancies at American mission hospital in Bahrain. Methods: This is a retrospective cohort study conducted from August 2021 to April 2022 at American mission hospital in Bahrain. All patients (n=205) who attended the hospital from January 2017 to June 2021 with a presentation suggestive of ectopic pregnancy were extracted from the electronic medical record database and reviewed. Data was recorded on Microsoft Excel version 16.61.1 and numerical data was extrapolated into graphs and tables via IBM SPSS Statistics version 28.0.0.0 (190). Data was subjected to descriptive and stratification analysis. Results: The leading risk factor for ectopic pregnancy was previous pelvic surgery (40.4%). The most reported symptom was amenorrhea (65.9%). Risk of rupture was tripled when serum B-hCG level was elevated, rendering it the strongest predictor of rupture according to our data (95% CI=1.25, 7.19; p-value 0.0139). Majority of patients (56.4%) were treated with methotrexate with a success rate of 88.7% and 8.5% of cases were managed expectantly. Those who failed medical management or presented with rupture were treated surgically.Conclusions: Our study found B-hCG levels to be the most sensitive predictor of rupture and need for surgical intervention. Therefore, management was largely, but not exclusively, based on B-hCG levels. Majority of cases were treated medically and patients undergoing expectant management experienced no complications.

The IDENTIFY study: the investigation and detection of urological neoplasia in patients referred with suspected urinary tract cancer - a multicentre observational study

Objective To evaluate the contemporary prevalence of urinary tract cancer (bladder cancer, upper tract urothelial cancer [UTUC] and renal cancer) in patients referred to secondary care with haematuria, adjusted for established patient risk markers and geographical variation. Patients and Methods This was an international multicentre prospective observational study. We included patients aged ≥16 years, referred to secondary care with suspected urinary tract cancer. Patients with a known or previous urological malignancy were excluded. We estimated the prevalence of bladder cancer, UTUC, renal cancer and prostate cancer; stratified by age, type of haematuria, sex, and smoking. We used a multivariable mixed-effects logistic regression to adjust cancer prevalence for age, type of haematuria, sex, smoking, hospitals, and countries. Results Of the 11 059 patients assessed for eligibility, 10 896 were included from 110 hospitals across 26 countries. The overall adjusted cancer prevalence (n = 2257) was 28.2% (95% confidence interval [CI] 22.3–34.1), bladder cancer (n = 1951) 24.7% (95% CI 19.1–30.2), UTUC (n = 128) 1.14% (95% CI 0.77–1.52), renal cancer (n = 107) 1.05% (95% CI 0.80–1.29), and prostate cancer (n = 124) 1.75% (95% CI 1.32–2.18). The odds ratios for patient risk markers in the model for all cancers were: age 1.04 (95% CI 1.03–1.05; P < 0.001), visible haematuria 3.47 (95% CI 2.90–4.15; P < 0.001), male sex 1.30 (95% CI 1.14–1.50; P < 0.001), and smoking 2.70 (95% CI 2.30–3.18; P < 0.001). Conclusions A better understanding of cancer prevalence across an international population is required to inform clinical guidelines. We are the first to report urinary tract cancer prevalence across an international population in patients referred to secondary care, adjusted for patient risk markers and geographical variation. Bladder cancer was the most prevalent disease. Visible haematuria was the strongest predictor for urinary tract cancer

Improved spacecraft magnetic attitude maneuvering

A magnetometer is essential in spacecraft magnetic attitude control due to the need for magnetic field information to compute the control command. The measurements of the magnetometer, however, are usually affected by other electric currents in the spacecraft, especially those of the magnetic coils when they are turned on during actuation. As a result, magnetic rods and magnetometers are usually turned on at alternate times, resulting in a reduced duty cycle of the magnetic rods, and hence longer maneuver times. This paper presents a magnetic attitude control system with extended duty cycle and low magnetometer measurements frequency. Instead of measurements, a computed magnetic field strength vector is used for updating the control command during the duty cycle. Using the measured spacecraft rotational motion, and knowing the control torque command, it is possible to compute the magnetic field strength vector. These computations are corrected using magnetometer measurements at a lower rate. The Tikhonov regularization approach is implemented to solve the singular magnetic torque system. This algorithm is demonstrated via Monte Carlo numerical simulations to have faster attitude maneuvers and lower power consumption by the magnetic rods. Real data obtained from the Cascade Smallsat and Ionospheric Polar Explorer spacecraft are used for validation of the proposed approach

A new variant of the B-dot control for spacecraft magnetic detumbling

With using the B-dot control law for magnetic detumbling, there will be no need for angular velocity measurements. The B-dot control law is a torque projection-based controller, however, it is not guaranteed to have the dipole moment orthogonal to the magnetic field, especially as the angular velocity gets smaller during detumbling, resulting in a suboptimal torque vector. This paper presents a new variant of the B-dot control. By computing an equivalent angular velocity, based on the magnetic field data, it is possible to develop a control law that guarantees the magnetic dipole moment to remain in the plane orthogonal to the ambient magnetic field. Using Monte Carlo simulations, the proposed B-dot control is compared to two other variants of B-dot control laws: a simple B-dot control and a recent variable gain B-dot control. The results show that the proposed B-dot control outperforms the other B-dot control laws in terms of the speed of detumbling and the power consumption

Time-optimal magnetic attitude Detumbling

The problem of minimum-time spacecraft attitude detumbling using magnetic rods is revisited in this paper within the context of optimal control theory. Two formulations are presented; the first one assumes the availability of the angular velocity measurements for feedback. The second formulation assumes the availability of only the ambient magnetic field measurements in the feedback. In both formulations, the constraint in this optimal control problem is a limit on the maximum magnetic dipole moment of the magnetic rods. It is shown that the time optimality will be achieved if the triple orthogonality condition between the torque, the dipole moment, and the magnetic field vector is fulfilled. This triple orthogonality is considered as a condition of optimality, which is often neglected in the existing B-dot law and its variants in the literature. The Pontryagin minimum principle is used to derive analytically the control logic, for each formulation, in this nonautonomous system, under the assumption of high angular velocity. The second formulation is shown to yield a new variant of the B-dot law. The Monte Carlo simulation results presented in this paper confirm that the two controls found in both formulations outperformmost existing algorithms. In addition, the results show lower power consumption by the magnetic rods when using the proposed variant of the B-dot law as compared to existing B-dot laws

Improved Magnetic Attitude Control

© 2019 IEEE. Magnetometer measurements periods are usually different from magnetic rods activation periods, in order to lessen the effect of the rods\u27 generated magnetic field on the measurements. Magnetic field estimation methods are proposed to be utilized in the control feedback, to estimate the magnetic field, at times, instead of real measurement. This proposed approach results in reducing the maneuver time and the power consumption

Wave prediction using wave rider position measurements and NARX network in wave energy conversion

© 2018 Elsevier Ltd Several control methods of wave energy converters (WECs) need prediction in the future of wave surface elevation. Prediction of wave surface elevation can be performed using measurements of surface elevation at a location ahead of the controlled WEC in the upcoming wave. Artificial neural network (ANN) is a robust data-learning tool, and is proposed in this study to predict the surface elevation at the WEC location using measurements of wave elevation at ahead located sensor (a wave rider buoy). The nonlinear autoregressive with exogenous input network (NARX NN) is utilized in this study as the prediction method. Simulations show promising results for predicting the wave surface elevation. Challenges of using real measurements data are also discussed in this paper

A Recursive Approach for Magnetic Field Estimation in Spacecraft Magnetic Attitude Control

This paper is concerned with magnetic attitude control of spacecraft. The operation of the magnetic actuators is usually on a duty cycle; during the off times in this duty cycle the magnetometers are used to measure the magnetic field around the spacecraft. This alternate operation of magnetic actuators and sensors avoids the noise effect on the magnetometers coming from the magnetic actuators. This alternate operation results in longer maneuver times. This paper presents an estimation approach for the magnetic field, as well as the spacecraft attitude, that increases the duty cycle of the magnetic rods while reducing the rate of collecting the magnetometer data. A modified Multiplicative Extended Kalman Filter (MEKF) is used in the proposed approach. A relatively simple and fast dynamic model is developed for use in the MEKF. Monte Carlo simulations presented in this paper show that the proposed approach results in less maneuver time, and less power consumption by the magnetic rods when compared to a standard magnetic control approach. The magnetic field estimation process is verified using data collected from the CASSIOPE spacecraft using its telemetry system and the results are presented

Factors affecting the decision of moving ERP to the clouds - from customer perspective

Nowadays cloud computing is becoming very popular and it has started to evolve in the ERP arena as well. Known as cloud based ERP it is a paradigm which is in an infancy stage and growing steadily. There is a lot research about the cloud computing as a technology but, it is still foggy in organizations perceptions of what this technology really means for their ERP. In the scientific studies there are a limited number of research concerning the cloud computing and ERP as well. The purpose of this study is to identify the factors which are affecting the movement of ERP to the clouds and how these factors affect the decision to adopt this technology. In order to investigate these factors we made a descriptive study and developed a theoretical framework. The framework was used for guiding our research through our interviews, which were conducted within five organizations. The findings are followed by comprehensive analysis where a deep discussion is taking place over the different factors in each organization. There is no strong inference about the importance of the factors, though it's a set of factors combined together affecting the decision of moving ERP to the cloud