Momentum Dumping for Space Robots

During the robotic capture of a target object on orbit, accidental contacts may happen. During contacts, momentum is transferred to the system, causing a drift of the space robot in the inertial space. When no remediation is taken, the arm might converge to singularity or workspace limit within seconds, compromising the capture operation. This article presents a method to control the end-effector while simultaneously extracting any accumulated momentum in the system to cancel the drift. A feature of the method is that external actuators are only used for the momentum extraction and not to counterbalance the manipulator control forces. The control is validated with experiments using a Hardware-In-the- Loop (HIL) robotic simulator composed of a 7DOF (Degrees Of Freedom) arm mounted on a 6DOF moving base

A Nonlinear Observer for Free-Floating Target Motion using only Pose Measurements

In this paper, we design a nonlinear observer to estimate the inertial pose and the velocity of a free-floating non-cooperative satellite (Target) using only relative pose measurements. In the context of control design for orbital robotic capture of such a non-cooperative Target, due to lack of navigational aids, only a relative pose estimate may be obtained from slow-sampled and noisy exteroceptive sensors. The velocity, however, cannot be measured directly. To address this problem, we develop a model-based observer which acts as an internal model for Target kinematics/dynamics and therefore, may act as a predictor during periods of no measurement. To this end, firstly, we formalize the estimation problem on the SE(3) Lie group with different state and measurement spaces. Secondly, we develop the kinematics and dynamics observer such that the overall observer error dynamics possesses a stability property. Finally, the proposed observer is validated through robust Monte-Carlo simulations and experiments on a robotic facility.Comment: 8 pages, 6 figure

Singularity Maps of Space Robots and their Application to Gradient-based Trajectory Planning

We present a numerical method to compute singularity sets in the configuration space of free-floating robots, comparing two different criteria based on formal methods. By exploiting specific properties of free-floating systems and an alternative formulation of the generalized Jacobian, the search space and computational complexity of the algorithm is reduced. It is shown that the resulting singularity maps can be applied in the context of trajectory planning to guarantee feasibility with respect to singularity avoidance. The proposed approach is validated on a space robot composed of a six degrees-of-freedom (DOF) arm mounted on a body with six DOF

Contact force observer for space robots

In this paper, the problem of estimating a contactwrench at the end-effector for a space robot is addressed. Tothis aim, a generalized force observer based on a base-jointsdynamics is first reviewed. Then, a different formulation isproposed, which is based on a centroidal-joints dynamics. Theproposed observer features interesting decoupling propertiesfrom the base linear velocity that lead to a more practicaland better-performing estimation when limitations in real spacescenarios are considered. The two observers are comparedand the advantage of the proposed one is shown through asimulation example featuring a free-floating robot composed ofa 7 degrees-of-freedom (DOF) arm mounted on a 6DOF movingbase

Systemic perfusion at peak incremental exercise in left ventricular assist device recipients: Partitioning pump and native left ventricle relative contribution

AbstractBackgroundIn continuous-flow left ventricular assist device (LVAD) recipients, little is known about the relative pump- and left ventricle-generated blood flow (PBF and LVBF, respectively) contribution to peak systemic perfusion during incremental exercise and about how PBF/LVBF interplay and exercise capacity may be affected by pump speed increase.MethodsTwenty-two LVAD recipients underwent ramp cardiopulmonary exercise tests at fixed and increasing pump speed (+ 1.5% of baseline speed/10 W workload increase), echocardiography and NT-proBNP dosage. Peak systemic perfusion was peak VO2/estimated peak arterio-venous O2 difference, and LVBF was systemic perfusion minus PBF provided by LVAD controller. A change of peak percentage of predicted VO2max (Δpeak%VO2) ≥ 3 in increasing- vs. fixed-speed test was considered significant.ResultsTricuspid annular plane systolic excursion (TAPSE) and NT-proBNP were significantly lower and higher, respectively, in Δpeak%VO2 13 mm as a predictor of Δpeak%VO2 ≥ 3.ConclusionsA significant LVBF contribution to peak systemic perfusion and pump speed increase-induced peak VO2 improvement was detectable only in patients with a more preserved right ventricular systolic function and stable hemodynamic picture. These findings should be taken into consideration when designing LVAD controllers aiming to increase pump speed according to increasing exercise demands

Operational Space Control for Planar PAN–1 Underactuated Manipulators Using Orthogonal Projection and Quadratic Programming

In this paper, we propose an operational space control formulation for a planar N-link underactuated manipulator (PA N–1 ) 1 with a passive first joint subject to actuator constraints (N ⩾ 3), covering both stabilization and tracking tasks. Such underactuated manipulators have an inherent first-order nonholonomic constraint, allowing us to project their dynamics to a space consistent with the nonholonomic constraint. Based on the constrained dynamics, we can design operational space controllers with respect to tasks assuming that all joints of the manipulator are active. Due to underactuation, we design a Quadratic Programming (QP) based controller to minimize the error between the desired torque commands and available motor torques in the null space of the constraint, as well as involve the constraint of motor outputs. The proposed control framework was demonstrated by stabilization and tracking tasks in simulations with both planar PA 2 and PA 3 manipulators. Furthermore, we verified the controller experimentally using a planar PA 2 robot

An Energy-Based Approach for the Multi-Rate Control of a Manipulator on an Actuated Base

In this paper we address the problem of controlling a robotic system mounted on an actuated floating base for space applications. In particular, we investigate the stability issues due to the low rate of the base control unit. We propose a passivity-based stabilizing controller based on the time domain passivity approach. The controller uses a variable damper regulated by a designed energy observer. The effectiveness of the proposed strategy is validated on a base-manipulator multibody simulation

Operando SXRD study of the structure and growth process of Cu2S ultra-thin films

International audienceElectrochemical Atomic Layer Deposition (E-ALD) technique has demonstrated to be a suitable process for growing compound semiconductors, by alternating the under-potential deposition (UPD) of the metallic element with the UPD of the non-metallic element. The cycle can be repeated several times to build up films with sub-micrometric thickness. We show that it is possible to grow, by E-ALD, Cu2S ultra-thin films on Ag(111) with high structural quality. They show a well ordered layered crystal structure made on alternating pseudohexagonal layers in lower coordination. As reported in literature for minerals in the Cu-S compositional field, these are based on CuS3 triangular groups, with layers occupied by highly mobile Cu ions. This structural model is closely related to the one of the low chalcocite. The domain size of such films is more than 1000 angstrom in lateral size and extends with a high crystallinity in the vertical growth direction up to more than 10 nm. E-ALD process results in the growth of highly ordered and almost unstrained ultra-thin films. This growth can lead to the design of semiconductors with optimal transport proprieties by an appropriate doping of the intra metallic layer. The present study enables E-ALD as an efficient synthetic route for the growth of semiconducting heterostructures with tailored propertie