Application of predictive control for manipulator mounted on a satellite

Specific conditions of on-orbit environment are taken into account in the design of all devices intended to be used in space. Despite this fact malfunctions of satellites occur and sometimes lead to shortening of the satellite operational lifetime. It is considered to use unmanned servicing satellite, that could perform repairs of other satellites. Such satellites equipped with a manipulator, could be used to capture and remove from orbit large space debris. The critical part of planned missions is the capture manoeuvre. In this paper a concept of the control system for the manipulator mounted on the satellite is presented. This control system is composed of the trajectory planning module and model predictive controller (the latter is responsible for ensuring precise realization of the planned trajectory). Numerical simulations performed for the simplified planar case with a 2 DoF manipulator show that the results obtained with the predictive control are better than the results obtained with adaptive control method

Liposomal cytarabine in the prophylaxis and treatment of CNS lymphoma : toxicity analysis in a retrospective case series study conducted at Polish Lymphoma Research Group Centers

Lymphomas with primary or secondary involvement of central nervous system (CNS) have poor prognosis despite specific treatment protocols which include whole brain radiotherapy and high-dose systemic and/or intrathecal chemotherapy. Toxicity of intrathecal liposomal cytarabine-based regimens collected between November 2006 and January 2012 was assessed retrospectively. Data from 120 adult lymphoma patients with, or at high risk of CNS involvement who received intrathecal liposomal cytarabine-based regimens at six Polish Lymphoma Research Group centres between November 2006 and January 2012 were assessed retrospectively. Patients were divided into three cohorts: A (high risk of CNS disease, n = 88), B (cerebrospinal fluid pleocytosis without neurological symptoms or pathological imaging findings, n = 7), and C (CNS disease/neurological involvement; n = 25). In all examined groups, toxicity of treatment was found to be acceptable (including the prophylactic setting). None of the patients in cohorts A or B who took intrathecal liposomal cytarabine 50 mg, repeated every 2–4 weeks (mean 3.8 doses) had experienced a CNS relapse at a median follow-up time of 3 years. Patients in cohort C had a 76 % overall neurological response rate (including a 40 % complete response rate) and median overall survival of 4.8 years. Regimens incorporating liposomal cytarabine seem to be safe and effective treatments for lymphomas with CNS involvement

The obstacle vector field (OVF) method for collision-free trajectory planning of free-floating space manipulator

Manipulators mounted on small satellites will be used to perform on-orbit servicing, removal of space debris, and assembly of large orbital structures. During such operations, the manipulator must avoid collisions with the target object or the elements of the assembled structure. Planning of the manipulator trajectory is one of the major challenges for the proposed missions because the motion of the manipulator influences the position and orientation of the satellite. Thus, the dynamic equations of motion must be used during trajectory planning. Methods developed for fixed-base manipulators working on Earth cannot be directly applied. In this paper, we propose a new obstacle vector field (OVF) method for collision-free trajectory planning of a manipulator mounted on a free-floating satellite. The OVF method is based on a vector field that surrounds the obstacles and generates virtual forces that drive the manipulator around the obstacles. The OVF method is compared with the classical artificial potential field (APF) method and the rapidly exploring random trees (RRT) method. In the presented examples the trajectory planning problem is solved for a planar case in which the satellite is equipped with a 2 DoF manipulator. It is shown that the OVF method is more efficient than the APF method, i.e., it allows us to solve the trajectory planning problem in some of the cases, in which the APF method is unsuccessful. The time required to find the solution with the use of the OVF method is shorter than the time needed by the APF and the RRT method

Impedance Control Using Selected Compliant Prismatic Joint in a Free-Floating Space Manipulator

The success of space missions like capture-and-deorbit or capture-and-service relies on the ability of the capturing satellite to establish a stable mechanical connection by its gripping tool with the object being intercepted. Most of the potential objects of capture missions are not equipped with dedicated docking ports; hence, the satellite robot intercepting them will have to provide the mechanical compliance necessary for the safe establishment of contact between the two structures. Articulated robotic arms with controlled mechanical impedance are one set of promising solutions for this challenge. In this study, the authors discuss how the mechanical impedance realized only along a single axis can be useful for facilitating the contact between the manipulator arm’s end effector of a free-floating robot and an uncooperative object in microgravity. By distinguishing a dominant direction in the final approach and contact establishment maneuver, the need for impedance control of six degrees of freedom may be relaxed, and a single prismatic joint with controlled impedance can be used at the end effector. Such architecture is simulated and compared with the full model-based six-degree-of-freedom Cartesian impedance control of a free-floating manipulator. Authors then discuss the limitations and possibilities of such architecture in a potential practical setting

Zastosowanie sterowania impedancyjnego robotem kosmicznym typu free-floating w kontekście usuwania śmieci kosmicznych

A broad and significant class of space debris can be mitigated by means of a satellite, capable of capturing a large non-cooperating object by using a robotized arm with a gripper. The capture operation typically comprises of an approach, a close-on manoeuvre, establishing contact between the robotic grappler arm and a suitable feature on the target satellite, and finally it is concluded when a positive mechanical connection is achieved by the gripper closed on that feature. The phase of establishing contact poses a critical challenge in this scenario, since the target typically will be tumbling with respect to the chaser satellite causing high forces on the gripper and the robotic arm. A family of control methods known collectively as impedance control is typically employed in terrestrial robots for tasks involving an interaction with an environment, especially the dynamic contact. In this work, we present the model-based impedance control applied to a robotic manipulator on a free floating base. The derivation of impedance control law for a robotic manipulator on a free floating satellite, involving Generalized Jacobian Matrix (GJM), is presented, followed by simulation results comparing the loads in the manipulator joints against a classical GJM-based Cartesian controller. The simulation results show that the impedance controlled free floating robotic manipulator completes the task of trajectory following amid contact with unknown target with lower torques in the robot joints.Wiele obiektów orbitujących Ziemię stanowią wyeksploatowane lub nieczynne satelity i inne urządzenia kosmiczne oraz ich fragmenty. Poruszając się w sposób niekontrolowany po orbitach aktywnie wykorzystywanych stanowią zagrożenie dla czynnych satelitów, stacji kosmicznej, astronautów jak i również rakiet wynoszących w przestrzeń kosmiczną nowe satelity. Obiekty te uznawane są za śmieci kosmiczne. Zdolność chwycenia i manipulowania niewspółpracującym obiektem na orbicie Ziemi przez robota satelitarnego pozwoliła by na zmniejszenie liczby śmieci kosmicznych i zagrożeń z nimi związanych w dwojaki sposób: po pierwsze umożliwiła by chwycenie i usunięcie śmieci kosmicznych znacznej wielkości z orbity, po drugie dała by możliwość serwisowania i tym samym przedłużenia okresu eksploatacyjnego satelitów będących blisko końca swojej nominalnej misji, zapobiegając by stały się one śmieciami kosmicznymi. Oba te zastosowania wymagają fizycznego wejścia w kontakt pojazdu kosmicznego chwytającego oraz obiektu chwytanego. W naziemnych zastosowaniach robotów, w których dochodzi do kontaktu manipulatora robota z otoczeniem, powszechnie stosowane są metody sterowania impedancyjnego. W niniejszym tekście autorzy proponują wykorzystanie sterowania impedancyjnego w oparciu o model (model-based impedance control) do realizacji manewru wejścia w kontakt końcówki manipulatora robota satelitarnego z niewspółpracującym obiektem w stanie nieważkości. W pracy przedstawiono wyprowadzenie prawa sterowania impedancyjnego manipulatorem o swobodnej bazie w oparciu o model, z wykorzystaniem jakobianu uogólnionego (Generalized Jacobian Matrix, GJM), oraz rezultaty symulacji manewru wejścia końcówki roboczej manipulatora kosmicznego w kontakt z nieważkim obiektem. Wyniki symulacji pokazują, że zaproponowane prawo sterowania pozwala realizować zadanie śledzenia trajektorii zachowując momenty i obciążenia w przegubach robota na niskim poziomie

Control System for Free-Floating Space Manipulator Based on Nonlinear Model Predictive Control (NMPC)

Manipulator mounted on an unmanned satellite could be used for performing orbital capture maneuver in order to repair satellites or remove space debris from orbit. Use of manipulators for such purposes presents unique challenges, as high level of autonomy is required and the motion of the manipulator influences the position and orientation of the manipulator-equipped satellite. This paper presents a new control system that consists of two modules: trajectory planning module (based on trajectory optimization algorithm) and Model Predictive Controller. Both mod

Nonlinear model predictive control (NMPC) for free-floating space manipulator

Manipulators are widely used in orbital operations, for example, Mobile Servicing System on the International Space Station and Shuttle Remote Manipulator System are used for several years. Such manipulators are operated by astronauts and mounted on large platforms, thus influence of manipulator motion on the state of the platform is not significant. Application of manipulators for capture maneuvers in unmanned On-Orbit Servicing or Active Debris Removal missions requires reliable control algorithms that take into account the free-floating nature of the manipulator-equipped spacecraft. In this paper the possibility of using Nonlinear Model Predictive Control (NMPC) for controlling such manipulators is presented. Numerical simulations for a simplified planar case show effectiveness of the proposed controller