Tracking Control of Spacecraft by Dynamic Output Feedback - Passivity-Based Approach

In this study, we investigate the possibility of capturing an inoperative spacecraft using an orbital servicing vehicle or a space robot in future space infrastructure. These missions involve problems related to the tracking control of a target spacecraft; therefore, a control system design that takes into account the interference with the nonlinear motion of the spacecraft is required because the equations of motion of such a spacecraft are nonlinear system in which the six-degree-of-freedom (six-dof) translational motion and the rotational motion are coupled. They have been many studies on the six-dof tracking control problem related to spacecrafts (Ahmed et al., 1998; Terui, 1998; Dalsmo & Egeland, 1999; Boskovic et al., 2004; Ikeda et al., 2008; Seo & Akella, 2008). The control methods proposed by these researches are state feedback control methods and involve measurements of the linear and angular velocities of the spacecraft. It is necessary to develop an output feedback control method, which does not require velocity measurements in cases where a velocity sensor cannot be mounted on the spacecraft because of the limitations on the cost and weight of the spacecraft, or as a backup controller to ensure spacecraft stability when the velocity sensor breaks down. For the output feedback tracking control problem, a control method that eliminate the velocity measurement via the filtering of the position and attitude information (Costic et al., 2000; Costic et al., 2001; Pan et al., 2004) or the estimation of the velocity by the observer (McDuffie & Shtessel, 1997; Seo & Akella, 2007) has previously been proposed. However, these methods cannot be used for tracking a spacecraft with an arbitrary trajectory since the attitude controller has a singular point at which the control input diverges; another instance where the method cannot be used is when the initial state of the control system is restricted. In this paper, we propose a new passivity-based control method that involves the use of output feedback for solving the tracking control problem. Although the proposed method has a filter as well as (Costic et al., 2000), (Costic et al., 2001), and (Pan et al., 2004), and is implemented by using the conventional methods, it can track a spacecraft with an arbitrary trajectory because the controller does not have a singular point. Thus, the proposed method has characteristics that are better than those of conventional methods. This paper is organized as follows: Section 2 describes the tracking control problem and the derivation of the relative equation of motion; the equation is then used for transforming the tracking control problem to a regulation problem. In section 3, we construct the dynamic