To accurately accomplish on-orbit tasks using Space Manipulator Systems (SMS), advanced model-based controllers, dependent on the knowledge of SMS parameters, can be employed. However, these parameters may change on orbit for several reasons. Also, during an SMS task, excitation of flexible appendages, such as solar panels, or fuel sloshing may introduce significant end-effector errors. Therefore, controllers robust to parametric uncertainty and disturbances are needed. A robust controller attractive due to its small computational effort is the Linear Parameter Varying (LPV) gain-scheduled controller. However, its design for spatial SMS is not trivial and has not been studied yet. Therefore, the aim of this work is to study and compare robust controllers and examine their applicability to SMS. An LPV plus H∞controller is compared with a Model-Based PD, and a Model-Based PD plus H∞controller, in the presence of parametric uncertainty, noisy measurements and disturbances, using a planar example. The criteria considered include: (i) Design Complexity, (ii) Trajectory Errors, (iii) Required Torques, and (iv) Computational Effort.

Space manipulator systems in orbit are subject to link flexibilities since they are designed to be lightweight and long reaching. Often, their joints are driven by harmonic gear-motor units, which introduce joint flexibility. Both of these types of flexibility may cause structural vibrations. To improve endpoint tracking, advanced control strategies that benefit from the knowledge of system parameters, including those describing link and joint flexibilities, are required. In this paper, first, the equations of motion of space manipulator systems whose manipulators are subject to both link and joint flexibilities are derived. Then, a parameter estimation method is developed, based on the energy balance during the motion of a flexible space manipulator. The method estimates all system parameters including those that describe both link and joint flexibilities and can reconstruct the system full dynamics required for the application of advanced control strategies. The method, developed for spatial systems, is illustrated by a planar example.

The parameter estimation of space manipulator systems on orbit is studied, whose manipulators are subject to joint flexibilities. To improve path planning and tracking capabilities, advanced control strategies that benefit from the knowledge of system parameters are required. These parameters include the system inertial parameters as well as the stiffness and damping parameters, which describe joint flexibilities. During operation some of these parameters may change or be unknown. Estimation methods based on the equations of motion are sensitive to noise, while methods based on the angular momentum conservation, while they are tolerant to noise, they cannot estimate the parameters that describe joint flexibilities. A parameter estimation method, based on the energy balance, applied during the motion of a space flexible-joint manipulator system in the free-floating mode, is developed. The method is tolerant to noise and can reconstruct the system full dynamics. It is shown that the parameters estimated by the proposed method can describe the system dynamics fully. The application of the developed method is valid for spatial systems; it is illustrated by a planar 7 degrees of freedom (DoF) example system.

To accomplish tasks with high accuracy, advanced control strategies that benefit from the knowledge of system parameters are required. However, during operation some of them may change, or be unknown. In this paper, a novel parameter estimation method is proposed, which is based on the conservation of the angular momentum of a space manipulator system in the free-floating mode. The estimated parameters are combinations of spacecraft, manipulator and payload parameters and render the system full dynamics identified and applicable to model-based control. The algorithm requires only measurements of joint angles and rates, and spacecraft attitude and angular velocity. No information about spacecraft and joint accelerations or joint torques, which include substantial noise, is required. Thus, in contrast to other methods using the equations of motion, the proposed method is insensitive to sensor noise. Moreover, it does not require the prior knowledge of any system parameters and can be applied to free-floating systems with more than one manipulators. The application of the proposed method is illustrated by a 3D example.

Free-floating space manipulator systems have spacecraft actuators turned off and exhibit nonholonomic behavior due to angular momentum conservation. Such systems are subject to path dependent Dynamic Singularities (DS) that complicate their path planning. Due to the existence of DS its workspace is restricted. The Cartesian space path planning of free-floating space robots is studied and a novel path planning technique allowing the end-effector to follow a desired path avoiding any DS is proposed. Since the path is predefined, the method yields the appropriate initial system configurations that avoid dynamically singular configurations during the motion. Therefore, it allows effective use of the entire robot workspace. The proposed method is applicable to both planar and spatial systems and it is demonstrated using straight-line paths. The application of the method is illustrated by two examples.

Free-floating space manipulator systems, have spacecraft actuators turned off and exhibit nonholonomic behavior due to angular momentum conservation. A path planning methodology for planar free– floating space manipulator systems is developed that allows simultaneous manipulator end-point and spacecraft attitude control using manipulator actuators only. The method is based on mapping the angular momentum to a space where it can be satisfied trivially. Smooth and continuous functions such as polynomials are employed driving the system to a desired configuration. It is shown that the method allows for smooth configuration changes in finite and prescribed time, without requiring small cyclical motions. Limitations are discussed and examples are presented.