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Rising numbers of aging spacecraft and new missions demand the development of novel approaches to perform various tasks in orbit. Complex servicing and assembly missions have been successfully completed by human astronauts in the past. However, currently available human-rated vehicles are not capable of accessing all relevant orbital locations, nor are there enough assets available for human operators to service all essential payloads directly. If sufficient capability to perform simple tasks remotely could be provided via a robotic manipulator, it may be possible to meet the servicing needs of a far greater number of missions at a lower program cost and without requiring risky extravehicular activities. The aim of this study is to develop a remote-operated robotic system capable of performing relevant tasks when mounted to a planar floating spacecraft simulator operating on an air bearing table. This system, known as PERSEUS, possesses three revolute joints to allow postural redundancy within a large workspace and a rapidly reconfigurable end-effector that enables simulation of various maneuvers for different planar orientations. When mounted to a simulated spacecraft, PERSEUS offers capability to simulate various grapple and hopping maneuvers representative of what may be required to inspect and service a host payload. Ensign, United States Navy Approved for public release. Distribution is unlimited.

The article of record as published may be found at http://dx.doi.org/10.3389/frobt.2019.00007 At very low orbital altitudes (?450 km) the aerodynamic forces can become major attitude disturbances. Certain missions that would benefit from a very low operational altitude require stable attitudes. The use of internal shifting masses, actively shifting the location of the spacecraft center-of-mass, thus modulating, in direction and magnitude, the aerodynamic torques, is here proposed as a method to reject these aerodynamic disturbances. A reduced one degree-of-freedom model is first used to evaluate the disturbance rejection capabilities of the method with respect to multiple system parameters (shifting mass, shifting range, vehicle size, and altitude). This analysis shows that small shifting masses and limited shifting ranges suffice if the nominal center-of-mass is relatively close to the estimated center-of-pressure. These results are confirmed when the analysis is extended to a full three rotational degrees-of-freedom model. The use of a quaternion feedback controller to detumble a spacecraft operating at very low altitudes is also explored. The analysis and numerical simulations are conducted using a nonlinear dynamic model that includes the full effects of the shifting masses, a realistic atmospheric model, and uncertain spacecraft aerodynamic properties. Finally, a practical implementation on a 3U CubeSat using commercial-off-the-shelf components is briefly presented, demonstrating the implementation feasibility of the proposed method.

CRUSER TechCon 2018 Research at NPS. Wednesday 1: Sensing Spacecraft equipped with robotic arms can fulfill a wide variety of space missions. For example, robotic spacecraft can service other vehicles, assemble large space structures, remove orbital debris, or even help astronaut crews with their tasks. In these missions, the servicing spacecraft needs to move on the surface of another space vehicle or structure in order to complete its missions. Two distinct approaches have been previously proposed to solve this mobility problem: a) propulsive free-flying b) zero-g climbing. In a propulsive free-flying approach, the servicing spacecraft uses its limited supply of propellant to free-fly from one location to another, whereas in a zero-g climbing approach the spacecraft uses its robotic arms to grasp onto the target vehicle or structure and traverse between locations. The NPS Spacecraft Robotics Laboratory is proposing an alternative mobility approach: hopping. In a hopping approach the servicing spacecraft uses its robotic arm to "jump" between two locations. The servicing spacecraft first uses its robotic arm to push itself off the target vehicle or structure, then, it performs a brief free-flying coast, and finally, it uses its robotic arm to soft-land on the target spacecraft or structure. The hopping mobility approach has the potential to be propellantless and it is simpler and potentially faster than zero-g climbing, which requires a long and time-consuming sequence of moves. Basic hopping maneuvers have already been demonstrated at the NPS POSEIDYN planar air bearing test bed. In this test bed, air bearings are used to provide a frictionless interface between the test vehicles and a smooth and horizontally leveled granite table. The test vehicles thus experience a weightless environment on the table plane, allowing to test spacecraft maneuvers on a dynamically representative environment. The demonstrations on the POSEIDYN test bed are inherently limited to a planar environment and the logical next step is to demonstrate hopping maneuvers in a six degree-of-freedom environment. In a partnership with NASA the NPS Spacecraft Robotics Laboratory is preparing a hopping maneuver demonstration onboard the International Space Station (ISS) using the Astrobee free-flyer platform. Scheduled for launch later this year, Astrobee is an assistive free-flyer with a propulsion system that allows it to autonomously fly inside the ISS. Additionally, Astrobee is equipped with a robotic arm that it can use to perch itself to the many handrails scattered throughout the ISS. The hopping maneuver demonstration will use the perching arm to push Astrobee off a handrail. The push maneuver will propel Astrobee in a specific direction and with a particular rotational state which after a brief coast period will place Astrobee in a position to capture another handrail. Astrobee will then use its perching arm to capture this handrail and complete the hop.

The article of record as published may be found at http://dx.doi.org/10.3389/frobt.2018.00041 The paper provides a step-by-step tutorial on the Generalized Jacobian Matrix (GJM) approach for modeling and simulation of spacecraft-manipulator systems. The General Jacobian Matrix approach describes the motion of the end-effector of an underactuated manipulator system solely by the manipulator joint rotations, with the attitude and position of the base-spacecraft resulting from the manipulator motion. The coupling of the manipulator motion with the base-spacecraft are thus expressed in a generalized inertia matrix and a GJM. The focus of the paper lies on the complete analytic derivation of the generalized equations of motion of a free-floating spacecraft-manipulator system. This includes symbolic analytic expressions for all inertia property matrices of the system, including their time derivatives and joint-angle derivatives, as well as an expression for the generalized Jacobian of a generic point on any link of the spacecraft-manipulator system. The kinematics structure of the spacecraft-manipulator system is described both in terms of direction-cosine matrices and unit quaternions. An additional important contribution of this paper is to propose a new and more detailed definition for the modes of maneuvering of a spacecraft-manipulator. In particular, the two commonly used categories free-flying and free-floating are expanded by the introduction of five categories, namely floating, rotation-floating, rotation-flying, translation-flying, and flying. A fully-symbolic and a partially-symbolic option for the implementation of a numerical simulation model based on the proposed analytic approach are introduced and exemplary simulation results for a planar four-link spacecraft-manipulator system and a spatial six-link spacecraft manipulator system are presented.

The article of record as published may be found at http://dx.doi.org/10.1016/j.ast.2017.09.043 A set of laboratory experiments are conducted to demonstrate the autonomous capture of a simulated resident space object by a simulated spacecraft equipped with a robotic manipulator. A planar air-bearing test bed provides a quasi-weightless and drag-free dynamic environment on a plane. To control the chaser’s base, floating, flying, and rotation-flying control approaches are implemented and compared. A resolved-motion-rate controller is used to control the manipulator’s joints. Using these control methods a floating object at rest is successfully captured. Furthermore, the capture of a floating and rotating object is demonstrated using a flying base control approach. The originality of these experiments comes from the remarkably high dynamic coupling of the spacecraft-manipulator system used. Emphasis is given to the guidance and control problems, with the relative navigation problem being left outside the scope of this effort.

The article of record as published may be found at http://dx.doi.org/10.1007/s12567-017-0155-7 An experimental campaign has been conducted to evaluate the performance of two different guidance and control algorithms on a multi-constrained docking maneuver. The evaluated algorithms are model predictive control (MPC) and inverse dynamics in the virtual domain (IDVD). A linear–quadratic approach with a quadratic programming solver is used for the MPC approach. A nonconvex optimization problem results from the IDVD approach, and a nonlinear programming solver is used. The docking scenario is constrained by the presence of a keep-out zone, an entry cone, and by the chaser’s maximum actuation level. The performance metrics for the experiments and numerical simulations include the required control effort and time to dock. The experiments have been conducted in a groundbased air-bearing test bed, using spacecraft simulators that float over a granite table.

An onboard implementable optimization-based guidance approach for the capture of tumbling objects by spacecraft equipped with a robotic manipulator is demonstrated on a hardware-in-the-loop test bed. The experimental demonstration is conducted using spacecraft simulators operating on the reduced-gravity and drag-free dynamic environment provided by a planar air bearing test bed. The proposed approach uses a two-step sequential convex programming procedure introduced in an earlier work. The first step optimizes the center-of-mass translation while the second step optimizes the motion of the multibody system around its center-of-mass. A sequential convex programming procedure is used on both optimization steps, casting the original optimization problem into a collection of convex programming problems. A proof of convergence is introduced here for the system-wide translation in the presence of non-convex keep-out zone constraints. These experiments significantly advance the demonstrated state-of-the-art for robotic capture maneuvers.