Satellites are uniquely alone in their environment. They are launched with everything they need for their entire mission, from initial operational capability (IOC) to end-of- life. Government and commercial satellite operators are beginning to recognize that their current fleets last longer than their anticipated “design life.” With some minimum exceptions, the ability to physically upgrade, refuel, or repair satellites once they are on-orbit does not currently exist; little manufacturing is done in space as well. On-Orbit Servicing, Assembly, and Manufacturing (OSAM) activities – up-close inspection, intentional and beneficial changes to resident space objects, and manufacturing on orbit – can have significant implications on many aspects of current satellites and the way future satellites are designed and operated.
Dexterous On-Orbit Maintenance utilizes innovative concepts from robotic motion planning and control to perform mechanical repair, replacements and enhancements for on-orbit vehicles (ie client satellites), whose on board enabling functions – propellants, pressurants or coolants, etc. – naturally deplete or whose subcomponents fail or require upgrade. Performing these operations in space requires safe rendezvous and precise docking of a mobile robot platform onto a client satellite without disturbing its operations. Therefore, a key requirement is minimizing the imparted contact force on the client satellite during repair operations. However, estimating the pose of the client satellite during repair is made challenging when visual feeds are occluded and poor lighting conditions coupled with delayed measurements exacerbates the problem. We draw parallels between the satellite rendezvous problem and classical manipulation problem where we extend existing techniques to on-orbit conditions. Our goal is to develop a multi-modal control approach where control of the mobile robot platform is tightly coupled with estimation from visual and force sensory feedback that enables successful servicing of satellites on-orbit.

In order to enable our dextrous in-orbit maintenance missions, we have implemented a state-of-the-art feedback control system specifically designed for floating base robotic systems. In the literature, our controller is described as an ‘operational space controller’, as our controller commands joint torques to achieve an objective in an operational (or task) space. We can achieve our control objectives in the operational space by applying the following controller to our dynamical system:
Equation (1) represents the dynamic equations of motion of our mobile space robot platform. Equation (2) is our operational space controller. We derive our controller by transforming the dynamics in equation (1) into the operational space, and then performing inverse dynamics to cancel out nonlinear terms and produce motion in a desired direction based on equation (3).