Recent advances in contact dynamics and post-capture control for combined spacecraft

Abstract

The combined spacecraft is increasingly demanded to accomplish various types of space missions. Therefore, the dynamics and control technologies of combined spacecraft are significantly important to ensure the success of future space explorations. Several kinds of combined spacecraft can be obtained by different capturing approaches, and the dynamical property of combined spaceraft varies dramatically between different categories. Therefore, the control system should be designed according to the specific combined spacecraft category and mission requirement. Moreover, the contacting behavior between the end-effector and the target has been a complex problem that may influence significantly the performance of the combined spacecraft control system. Up to now, the complete review of the combined spacecraft contact dynamics and control technologies has rarely been reported. With these motivations, this paper presents the review and prospects of the combined spacecraft contact dynamics and control technologies including the capturing strategy analysis, contact dynamics modeling, inertia parameter estimation and post-capture controller design.

Introduction

When an active spacecraft captures another space object, a combined spacecraft is formed. As the modern space missions become more and more complex, the combined spacecraft is increasingly required to accomplish many specific tasks such as constructing space stations, building extra-large space facilities, capturing small asteroids, repairing non-functional satellites or removing orbit debris [1]. Depending on the mission scenario, various types of objects may be captured by the active spacecraft, such as a malfunctioning satellite, space debris, a module of spacecraft to be installed, or a small asteroid [2] These captured objects are often characterized as the ‘target’. Based on the different mission requirements and constraints, the combined spacecraft can be formed by different approaches including rigid docking and highly flexible tethered net capturing. Therefore, the dynamic property of the combined spacecraft varies dramatically between different capturing strategies. Furthermore, the target being captured may be non-cooperative, which implies that the target has no attitude and orbit maneuver abilities and its inertial parameters are completely unknown to the control system. With non-cooperative targets, the post-capture control process of the combined spacecraft becomes highly risky since the active spacecraft has limited control ability and the connection between the active spacecraft's end-effector and target can be fragile. Thus, the major research facilities around the world, including NASA [3], DLR [4], ESA [5] and JAXA [6], have made a significant amount of efforts to study the dynamics [7] and the post-capture control technologies of combined spacecraft. Since the 1960s, many theoretical research programs and engineering applications have been conducted in this research field [8]. Therefore, this paper presents a brief review of the current technologies and discusses the possible orientation for future works.

The first and most well-known mission scenario related to the combined spacecraft is the construction of space stations [9]. Since the space station is constructed by docking several spacecraft together, it should be considered as a combined spacecraft. It can be seen that the space station is usually formed by firmly docking two or more actively controlled spacecraft, which makes the space station a single rigid body system. Furthermore, the dynamical parameters of the space station are usually known to the control system, and the control of such combined spacecraft is relatively easy. Thanks to this property, many space docking missions have been performed, and the classic post-capture control method of combined spacecraft has been properly verified. During the early days of manned space exploration, the docking and module installation of space station relied mostly on the human-in-the-loop activities [10]. With the development of the automatics and Artificial Intelligence (AI), the next-generation space stations can be constructed in an unmanned process [11], which is much more efficient and less risky. The new construction concept for space stations will also reduce the budget and difficulty for astronaut training, which is favorable for massive human space explorations.

The second motivation for developing a combined spacecraft control technology is to realize the space debris removal operations [12]. Over the last few years, the number of space debris has significantly increased, and it will continue to rise in the future according to the model established in Ref. [13]. The space debris poses a direct threat to the operational spacecraft, and a huge amount of money has to be invested for space debris surveillance. Therefore, the idea of actively removing space debris has long been discussed. The mission scenario is to use an active spacecraft to capture the space debris by several optional methods and carry it away from the current orbit [14]. Unlike the traditional space rendezvous and docking missions, space debris is a non-cooperative target. Its geometrical shape and the inertial parameters are unknown, which means that the post-capture control becomes much more difficult and risky than a space station. Furthermore, the space debris removal mission may be realized by a space manipulator or a tethered spacecraft [15], which means that the combined spacecraft is a flexible multi-body system. If this kind of combined spacecraft is not properly controlled, its different moving parts may collide with each other and cause a catastrophic result. Due to this disadvantage, the orbit debris removal principle is not ready for engineering implementation yet.

The third category of mission which involves the combined spacecraft control is the On-Orbit Servicing (OOS) missions [1]. Unlike the space debris removal missions, the objective for the OOS mission is not to remove the target by orbit maneuver, but to carry out many complex actions such as constructing a large space structure or realizing maintenance service for mal-functioning satellites. The concept of extra-large space facilities has long been proposed, and the examples of such spacecraft can be given as the Space Solar Power Station (SSPS) [16,17], the large aperture space telescope [18] and large-scale truss-beam antenna structures [19]. This kind of spacecraft cannot be directly launched into space due to its large size and weight [20], and the space assembly process must be conducted [21]. In the meantime, the orbit maintenance mission has also been widely discussed. Instead of assembling an extremely large spacecraft, the orbit maintenance mission is more focused on building small satellites using existing modules [26], repairing mal-functioning satellites [33], refueling and module upgrading of existing spacecraft [34]. Among the targets being captured, it should be noticed that a malfunctioning satellite may be tumbling on its own, and a module to be installed may have no orbit or attitude control abilities. For this reason, the OOS mission can also be highly risky and difficult. Unlike the space debris removal concept, several OOS missions have been conducted in reality. One of the most well-known missions of orbit maintenance is the repairing mission of the Hubble space telescope [35] carried out by the space shuttle. Another example for orbit maintenance is the orbital express mission [36]. During this mission, a client satellite platform and the cargo carrier spacecraft were launched into space. A space manipulator took the modules onboard the cargo carrier then transported them to the client satellite and installed them in the correct position on the platform [37]. Thanks to the above-mentioned missions, the advanced docking and manipulator servicing technologies are properly validated [39].

By analyzing the above-mentioned mission scenarios related to the combined spacecraft, it can be seen that the combined spacecraft dynamics and control technology is a complex multi-disciplinary area [22]. To realize the safe and efficient control of the combined spacecraft, several different sub research areas must be taken care of including the multi-body dynamic modeling [23], inertial parameter estimation [24] and advanced controller design [25]. In the meantime, depending on the specific mission requirements, some auxiliary technologies should also be investigated. For instance, the space manipulator collision-free kinematics [26] and path planning [28] should be analyzed for space manipulator operation, while the robust docking and capturing mechanism design [29] must be profoundly studied for spacecraft docking missions. Many researchers around the world have achieved a considerable amount of research results in the past few decades to ensure the development of combined spacecraft control technology and the success of various space missions. However, a profound review of the combined spacecraft dynamics and control has rarely been reported. Therefore, in this paper, we present a brief review of this area and discuss the prospects for future research. The rest of the paper is organized as follows: Firstly, Sec. 2 presents the major methods to capture the target and to form a combined spacecraft. Then, Sec. 3 discusses the different dynamic models for the combined spacecraft created by the different capturing strategies, and Sec. 4 is focused on the inertial parameter estimation algorithms of the combined spacecraft. Finally, the controller design for different types of combined spacecraft is addressed in Sec. 5 before the conclusion and discussion are given in Sec. 6.