Jian Guo

Adaptive Postcapture Backstepping Control for Tumbling Tethered Space Robot–Target Combination

a = positive parameter atx aty atz T = linear acceleration due to the tether tension, m · s−2 ax ay az T = linear acceleration of the gripper’s thruster force, m · s−2 C k = matrix in coordinated desaturation controller d = position vector of the capture position, m Fl = tether tension, N I = inertia matrix of the combination, kg · m I0 = nominal value of combination’s inertia matrix, kg · m Kξ = positive-definite design matrix k2 = positive-definite design matrix m = mass of tethered space robot–target combination, kg Oxlylzl = space tether frame Oxpypzp = space platform orbital frame Oxtytzt = combination orbital frame Ox 0 t y 0 t z 0 t = combination body frame P = positive-definite design matrix R = transformationmatrix from platform orbital frame to combination body frame S k = constant positive weighting matrix Tl = tether control torque, Nm x y z T = centroid position of the combination in the platform orbital frame, m ΔI = inertia matrix uncertainty, kg · m e = positive parameter λ k = Lagrange multiplier λL = upper bound of disturbance λL = estimation values of disturbance μ = positive design parameter ξ = state of the auxiliary design system σ = modified Rodrigues parameters σd = desired modified Rodrigues parameters τ k = vector of optimal thruster force and tether tension τc = control torque of the combination, N · m τd = disturbing torques, N · m τt = control torque of the thruster, N · m τl = control torque of the tether, N · m ω = absolute angular velocity of the combination, rad · s−1 ωd = desired angular velocity of the combination, rad · s−1

Dexterous Tethered Space Robot: Design, Measurement, Control, and Experiment

In this paper, we systematically introduce a novel geostationary orbit (GEO) space debris removal system called dexterous tethered space robot (DTSR). The DTSR has three notable characteristics: dexterity, lightweight, and cost effectiveness. We first describe the systems design and a typical mission scenario, and then present its two core technologies (vision-based pose measurement and coordinated controller design) in detail. Finally, we present the extensive simulations and ground semiphysical experiments to verify that the DTSR is a feasible solution to effectively remove the GEO space debris.

Dynamics Modeling of the Space Tether

This chapter is devoted to the dynamics modeling of the TSRS. The dynamics modeling is always the steadfast foundation of any research, and this holds true for the research of the space tethered system. At this point, because space experiments and ground tests are expensive and physically challenging, theoretical research has taken on an important role. Take, for instance, the flexible and elastic tether. It is a difficult feature to study while, on the ground; and it is even more complicated to study in space. Researchers have been dedicated to finding a perfect dynamics model for space tether for decades. However, they still have not come up with a perfect result in their research. If the model is very close to the real physical model, the analysis and numerical computation will be very complex; if the computation efficiency is on the top of the list, the dynamics model cannot describe all the specifics of the physical model. For this reason, there needs to be a trade-off between the reality and the feasibility of the space tethered system. Therefore, we introduce five different dynamics models of the TSRS.

Approaching Control Based on Mobile Tether Attachment Points

The orbit control during the approaching target phase is one of the key missions for the TSR and has been studied by many researchers. A new method is proposed by S. Pradeep to determine the tension control law, which is designed based on theorems in analytical mechanics. Nakamura discussed the collaborative control of the tension (controlled by the service satellite) and thruster (controlled by tethered robot) in approaching the target of the tethered retriever but the attitude is not considered. A boundary control was proposed for station keeping of a tethered satellite system in the literature, but the proposed boundary controller does not constrain the tension in the tether to be positive. V.J. Modi investigated an off-set control strategy that uses a manipulator mounted on the platform to regulate the tether swing. The corresponding state feedback controller is designed using a graph theoretic approach and the tethered system is successfully regulated by the controller. Some particular laws of deployment/retrieval leading to analytical solutions for the small in-plane and out-of-plane motions of the system are obtained and then these results are extended to a massive tether. To fulfill the station-keep control of the rotating TSS along halo orbits, a nonlinear output tracking control scheme based on the h-D technique is proposed. This approach overcomes some limitations such as on-line computations of the algebraic Riccati equation and is easy to implement.

Impact Dynamic Modeling and Adaptive Target Capture Control

The control of the tethered space system has received extensive attention with several articles published on the subject in recent years. Nakamura proposed a collaborative control method of the tension and thruster primarily during in the approach phase to the target. V.J. Modi designed an off-set control strategy that is implemented using a manipulator mounted on the platform to regulate the tether swing. A boundary control was proposed for station keeping of a tethered satellite system in the literature, but the proposed boundary controller does not constrain the tension in the tether to be positive. An arm link is used to control the attitude of the gripper attitude during the deployment phase, and a microgravity experiment was carried out to validate the feasibility of this scheme. Bergamaschi studied the coupling between the tether taut string vibrations and the satellite attitude motion. Wang proposed an attitude and orbit coordinated control method using mobile tether attachment point for tethered robot in approaching phase.

Approaching Control Based on a Movable Platform

The approach control of a tethered space robot system is a very challenging research issue that involves two research areas. One is the noncooperative target detection and tracking, and the other is the rendezvous control using the space tether. In the area of noncooperative target detection and tracking, Du et al. studied the pose measurement problem presented by a large noncooperative satellite based on two collaborative cameras. A nonlinear and recursive identification mechanism in a motion-based detection and tracking algorithm is presented in Doulamis. Thienel et al. designed a nonlinear approach for estimating the body rates of a noncooperative target vehicle and coupled the estimation to a tracking control scheme. Under the assumption that the noncooperative target has been detected and measured by the visual perception system of a TSR, this chapter focuses on the rendezvous control using the space tether. In this area, the tether-mediated orbital rendezvous between the tether tip and the space shuttle was first discussed by Carroll. Several years later, Carroll provided a preliminary design for a tether transport facility capable of providing between 0.6 and 1.2xa0km/s velocity increments to payloads. Stuart originally considered the in-plane cooperative tether-mediated rendezvous between a free-flying spacecraft and the tether tip by combining tether reeling with thrusters on the tether tip. More recently, Blanksby and Trivailo proposed a method to realize a gentle rendezvous by the tension controller. Westerhoff discussed a linear control strategy to minimize errors in rendezvous for a spinning momentum-exchange system, which assumed that the tether is reasonably close to the desired rendezvous position. Williams studied payload capture problems and examined the influence of thermomechanical and tether flexibility effects. However, most of the previous studies have focused on the dynamic and control problem of a long-tether system, which is known as the tethered space satellite system (TSS). Rather than TSS, this chapter proposes a TSR for the on-orbit capture task. The space tether of TSR is only a few hundred meters long, which is much shorter than the traditional TSS. The tether deployment depends on ejection velocity instead of the gravity gradient, and the whole capture operation only takes a matter of minutes. The short dimensions and different deployment procedure distinguish TSR from the traditional TSS. Furthermore, the relative states between the target and TSR are much more important in the studies of the TSR. Therefore the previous achievements are not suitable for TSR. In studies of TSR, Mankala and Agrawal examined dynamic modeling and simulation. Nakamura discussed the collaborative control method, but did not consider the attitude disturbances and relative motions among the platform, tether, robot, and target. The multibody relative motion should not be ignored because of its importance to the direction of tension and the approach instruction. With consideration for the relative motions among platform, target, and gripper, the approach control problem of the TSR is studied in this chapter.

Approaching Control Based on a Distributed Tether Model

Before performing the on-orbit service mission, the TSR should arrive at the desired position in the neighborhood area of the target and keep a stable relative attitude. Therefore how to control the TSR to approach the target and maintain the attitude is one of the key techniques of this system. Given the limited fuel carried on the terminal operation robot, the cost of fuel is regarded as one of the most important factors during the controller design of position and attitude. To reduce the fuel consumption in the approaching phase, various coordinated controllers are investigated in the literature.

Pose Measurement Based on Vision Perception

Autonomous rendezvous and docking (ARVD) is necessary for planned space programs such as space attack and defense, on-orbital servicing, and other rendezvous and proximity operations. Recently, there have been some different solutions to realize measurement, such as microwave radar, laser radar, satellite navigation, and vision-based measurements. All of them have been used or tested for ARVD. The PRISMA mission has demonstrated a navigation technology based on both a passive optical system and an active RF system. The passive optical system has shown relatively precise reconstruction of the distance to target. It has also demonstrated the reconstruction of the “pose” of the target, meaning the reconstruction of its quaternions (based on predefined target geometry). The navigation concept of OLEV is to use ranging for absolute navigation and to hand over to relative navigation at the distance of a two kilometer. For relative navigation, a set of six rendezvous cameras (far, mid- and close-range, and redundant) are used. For DEOS, the sensor package consists of the following individual sensor arrangements: 2 LIDAR heads, 2 far range mono cameras, 1 mid-range stereo camera, 1 close range stereo camera, and 1 docking mono camera. From their reports, we can see the task is far from trivial though much research has worked for decades to address it.

Approaching Control Based on a Tether Releasing Mechanism

Abstract The operation robot should arrive at the appointed position and keep its relative attitude stable before starting on-orbit service. The key technology for this procedure consists of optimal trajectory planning, tracking control, and relative attitude stability. Therefore in this chapter, a coordinated coupling control method for tracking the optimal trajectory is provided by utilizing a tether releasing mechanism, space tether, and thrusters.

Optimal Trajectory Tracking in Approaching

Abstract During the mission of a TSR, the operation robot should arrive at the appointed position and maintain stable relative attitude before on-orbit service. Therefore an optimal mode (minimum fuel or flight time) is necessary for this process. However, the fuel of the operation robot is limited. To save thruster fuel, this chapter proposes a coordinated orbit control and attitude stability method for tracking the optimal approach trajectory based on the space tether and actuators of the operation robot.