Planning natural repointing manoeuvres for nano-spacecraft

In this paper the natural dynamics of a rigid body are exploited to plan attitude manoeuvres for a small spacecraft. By utilising the analytical solutions of the angular velocities and making use of Lax pair integration, the time evolution of the attitude of the spacecraft in a convenient quaternion form is derived. This enables repointing manoeuvres to be generated by optimising the free parameters of the analytical expressions, the initial angular velocities of the spacecraft, to match prescribed boundary conditions on the final attitude of the spacecraft. This produces reference motions which can be tracked using a simple proportional-derivative controller. The natural motions are compared in simulation to a conventional quaternion feedback controller and found to require lower accumulated torque. A simple obstacle avoidance algorithm, exploiting the analytic form of natural motions, is also described and implemented in simulation. The computational efficiency of the motion planning method is discussed

A new approach to the solution of free rigid body motion for attitude manoeuvers

A Hamiltonian formulation of free rigid body motion defined on the Special Unitary Group SU(2) is used to integrate the system to obtain a convenient quaternion representation for attitude engineering applications. Novel content of this paper concerns applying a modern approach, based on geometric control theory to obtain the kinematic solution in an elegant and compact form. Moreover, this integration leads to an attitude representation which is not Euler-angle-like, thus enhancing its applicability (e.g. to attitude motion design)

Planning natural repointing manoeuvres for nano-spacecraft

In this paper the natural dynamics of a rigid body are exploited to plan attitude manoeuvres for a small spacecraft. By utilising the analytical solutions of the angular velocities and making use of Lax pair integration, the time evolution of the attitude of the spacecraft in a convenient quaternion form is derived. This enables repointing manoeuvres to be generated by optimising the free parameters of the analytical expressions, the initial angular velocities of the spacecraft, to match prescribed boundary conditions on the final attitude of the spacecraft. This produces reference motions which can be tracked using a simple proportional-derivative controller. The natural motions are compared in simulation to a conventional quaternion feedback controller and found to require lower accumulated torque. A simple obstacle avoidance algorithm, exploiting the analytic form of natural motions, is also described and implemented in simulation. The computational efficiency of the motion planning method is discussed

Computationally light attitude controls for resource limited nano-spacecraft

Nano-spacecraft have emerged as practical alternatives to large conventional spacecraft for specific missions (e.g. as technology demonstrators) due to their low cost and short time to launch. However these spacecraft have a number of limitations compared to larger spacecraft: a tendency to tumble post-launch; lower computational power in relation to larger satellites and limited propulsion systems due to small payload capacity. As a result new methodologies for attitude control are required to meet the challenges associated with nano-spacecraft. This paper presents two novel attitude control methods to tackle two phases of a mission using zero-propellant (i) the detumbling post-launch and (ii) the repointing of nano-spacecraft. The first method consists of a time-delayed feedback control law which is applied to a magnetically actuated spacecraft and used for autonomous detumbling. The second uses geometric mechanics to construct zero propellant reference manoeuvres which are then tracked using quaternion feedback control. The problem of detumbling a magnetically actuated spacecraft in the first phase of a mission is conventionally tackled using BDOT control. This involves applying controls which are proportional to the rate of change of the magnetic field. However, real systems contain sensor noise which can lead to discontinuities in the signal and problems with computing the numerical derivative. This means that a noise filter must be used and this increases the computational overhead of the system. It is shown that a timedelayed feedback control law is advantageous as the use of a delayed signal rather than a derivative negates the need for such a filter, thus reducing computational overhead. The second phase of the mission is the repointing of the spacecraft to a desired target. Exploiting the analytic solutions of the angular velocities of a symmetric spacecraft and further using Lax pair integration it is possible to derive exact equations of the natural motions including the time evolution of the quaternions. It is shown that parametric optimisation of these solutions can be used to generate low torque reference motions that match prescribed boundary conditions on the initial and final configurations. Through numerical simulation it is shown that these references can be tracked using nanospacecraft reaction wheels while eigenaxis rotations, used for comparison, are more torque intensive. As the method requires parameter optimisation as opposed to optimisation methods that require numerical integration, the computational effort is reduced

Dynamical analysis of an orbiting three-rigid-body system

The development of multi-joint-spacecraft mission concepts calls for a deeper understanding of their nonlinear dynamics to inform and enhance system design. This paper presents a study of a three-finite-shape rigid-body system under the action of an ideal central gravitational field. The aim of this paper is to gain an insight into the natural dynamics of this system. The Hamiltonian dynamics is derived and used to identify relative attitude equilibria of the system with respect to the orbital reference frame. Then a numerical investigation of the behaviour far from the equilibria is provided using tools from modern dynamical systems theory such as energy methods, phase portraits and Poincare maps. Results reveal a complex structure of the dynamics as well as the existence of connections between some of the equilibria. Stable equilibrium configurations appear to be surrounded by very narrow regions of regular and quasi-regular motions. Trajectories evolve on chaotic motions in the rest of the domain

On the dynamics of rigid multi-body systems in orbit

A critical factor for the success of space missions is the implementation of an appropriate spacecraft attitude control system. For multi-body space systems, the mechanical couplings have significant effects on the attitude dynamics. These effects, added to the high performance requirements and the number of strict constraints that, in general, space systems have to satisfy, make mission design very challenging. This calls for research to address these challenges. Dynamical systems analysis benefits system and control design by providing a quantity of information on the systems' natural behaviour. This thesis investigates the natural attitude dynamics of multi-rigid-body space systems and gains an insight into the nonlinear dynamics in order to develop efficient control techniques that exploit them. In addition this thesis aims to investigate the usefulness of dynamical systems tools in this area of application. To this end, the dynamics of the free single asymmetric rigid spacecraft, the two-body spacecraft in orbit, the threebody spacecraft in orbit and the generic N-body spacecraft are studied. Integrability of the single rigid body problem is used to derive a form of the well-known solution different from the classical and more suitable for aerospace applications. Hamiltonian and Lagrangian formalisms are used in the few-body problems where equilibria are identified and their nonlinear stability is addressed. Furthermore, the behaviour both in the neighborhood and far from the equilibria is examined, gaining an insight into the global nonlinear dynamics. Finally, the Newton-Euler formulation is employed to describe an N-body system and the problem of the dynamical coupling reduction, relevant for space manipulators, is addressed and a feedback controller is designed.A critical factor for the success of space missions is the implementation of an appropriate spacecraft attitude control system. For multi-body space systems, the mechanical couplings have significant effects on the attitude dynamics. These effects, added to the high performance requirements and the number of strict constraints that, in general, space systems have to satisfy, make mission design very challenging. This calls for research to address these challenges. Dynamical systems analysis benefits system and control design by providing a quantity of information on the systems' natural behaviour. This thesis investigates the natural attitude dynamics of multi-rigid-body space systems and gains an insight into the nonlinear dynamics in order to develop efficient control techniques that exploit them. In addition this thesis aims to investigate the usefulness of dynamical systems tools in this area of application. To this end, the dynamics of the free single asymmetric rigid spacecraft, the two-body spacecraft in orbit, the threebody spacecraft in orbit and the generic N-body spacecraft are studied. Integrability of the single rigid body problem is used to derive a form of the well-known solution different from the classical and more suitable for aerospace applications. Hamiltonian and Lagrangian formalisms are used in the few-body problems where equilibria are identified and their nonlinear stability is addressed. Furthermore, the behaviour both in the neighborhood and far from the equilibria is examined, gaining an insight into the global nonlinear dynamics. Finally, the Newton-Euler formulation is employed to describe an N-body system and the problem of the dynamical coupling reduction, relevant for space manipulators, is addressed and a feedback controller is designed

Humoral and cardiac effects of TIPS in cirrhotic patients with different "effective" blood volume

The aim of this study was to evaluate the cardiac effects of transjugular intrahepatic portosystemic shunts (TIPS) in cirrhotic patients with different effective blood volume. Two-dimensional echocardiography was performed before and 7 and 28 days after TIPS insertion in 7 cirrhotic patients with PRA 4 ng/mL/h (group B, reduced effective blood volume). Before TIPS, most cirrhotic patients showed diastolic dysfunction as indicated by reduced early maximal ventricular filling velocity (E)/late filling velocity (A) ratio. Patients of group B differed from patients of group A because of smaller left ventricular volumes and stroke volume, indicating central underfilling. After TIPS insertion, portal decompression was associated with a significant increase of cardiac output (CO) and a decrease of peripheral resistances. The most important changes were recorded in patients of group B, who showed a significant increase of both the end-diastolic left ventricular volumes and the E/A ratio and a significant decrease of PRA. In conclusion, these results show that the hemodynamic effects of TIPS differ according to the pre-TIPS effective blood volume. Furthermore, TIPS improves the diastolic cardiac function of cirrhotic patients with effective hypovolemia. This result is likely due to a TIPS-related improvement of the fullness of central blood volume

Diastolic dysfunction is associated with poor survival in patients with cirrhosis with transjugular intrahepatic portosystemic shunt

Background: Transjugular intrahepatic portosystemic shunt (TIPS) is a treatment for portal hypertension-related complications. Accurate prediction of the outcome of patients treated with TIPS is important, because some patients have very short survival. Diastolic dysfunction is frequently observed in patients with cirrhosis. Aim: To investigate whether or not diastolic dysfunction can predict the outcome after TIPS. Methods: Echocardiography with Doppler exploration was performed before and 28 days after TIPS insertion in 32 patients with cirrhosis. Several echocardiographic measures, including the early maximal ventricular filling velocity/late filling velocity (E/A) ratio as indicative of diastolic function, as well as laboratory, clinical and demographic variables were evaluated as predictors of survival. Results: Univariate analysis revealed that the presence of diastolic dysfunction observed 28 days after TIPS (E/A ratio 641) and baseline model of end-stage liver disease score were related to survival. Multivariate analysis identified diastolic dysfunction as an independent predictor of death (RR 8.9, 95% CI 1.9 to 41.5, p = 0.005). During the first year of follow-up, six out of 10 patients with an E/A ratio 641 died, whereas all 22 patients with E/A ratio >1 survived. Conclusions: Diastolic dysfunction estimated using E/A ratio is a promising predictor of death in patients with cirrhosis who are treated with TIPS

Surgical site infection after gastrointestinal surgery in children : an international, multicentre, prospective cohort study

Introduction Surgical site infection (SSI) is one of the most common healthcare-associated infections (HAIs). However, there is a lack of data available about SSI in children worldwide, especially from low-income and middle-income countries. This study aimed to estimate the incidence of SSI in children and associations between SSI and morbidity across human development settings. Methods A multicentre, international, prospective, validated cohort study of children aged under 16 years undergoing clean-contaminated, contaminated or dirty gastrointestinal surgery. Any hospital in the world providing paediatric surgery was eligible to contribute data between January and July 2016. The primary outcome was the incidence of SSI by 30 days. Relationships between explanatory variables and SSI were examined using multilevel logistic regression. Countries were stratified into high development, middle development and low development groups using the United Nations Human Development Index (HDI). Results Of 1159 children across 181 hospitals in 51 countries, 523 (45 center dot 1%) children were from high HDI, 397 (34 center dot 2%) from middle HDI and 239 (20 center dot 6%) from low HDI countries. The 30-day SSI rate was 6.3% (33/523) in high HDI, 12 center dot 8% (51/397) in middle HDI and 24 center dot 7% (59/239) in low HDI countries. SSI was associated with higher incidence of 30-day mortality, intervention, organ-space infection and other HAIs, with the highest rates seen in low HDI countries. Median length of stay in patients who had an SSI was longer (7.0 days), compared with 3.0 days in patients who did not have an SSI. Use of laparoscopy was associated with significantly lower SSI rates, even after accounting for HDI. Conclusion The odds of SSI in children is nearly four times greater in low HDI compared with high HDI countries. Policies to reduce SSI should be prioritised as part of the wider global agenda.Peer reviewe