Class Of Reference Trajectories Research Articles

Adaptive iterative learning control for robot manipulators

In this paper, we propose some adaptive iterative learning control (ILC) schemes for trajectory tracking of rigid robot manipulators, with unknown parameters, performing repetitive tasks. The proposed control schemes are based upon the use of a proportional-derivative (PD) feedback structure, for which an iterative term is added to cope with the unknown parameters and disturbances. The control design is very simple in the sense that the only requirement on the PD and learning gains is the positive definiteness condition and the bounds of the robot parameters are not needed. In contrast to classical ILC schemes where the number of iterative variables is generally equal to the number of control inputs, the second controller proposed in this paper uses just two iterative variables, which is an interesting fact from a practical point of view since it contributes considerably to memory space saving in real-time implementations. We also show that it is possible to use a single iterative variable in the control scheme if some bounds of the system parameters are known. Furthermore, the resetting condition is relaxed to a certain extent for a certain class of reference trajectories. Finally, simulation results are provided to illustrate the effectiveness of the proposed controllers.

An optimal guidance approximation for quasi-circular orbital rendezvous

Three-dimensional guidance about a time-optimal rendezvous flight path is examined within the framework of a quasi-circular orbital dynamics assumption. The guidance scheme, optimal in the same sense as the nominal trajectory, is based formally upon the approximate construction of a field of neighbouring optimal rendezvous paths, following a method developed by Kelley in an earlier paper1. For purposes of illustration, a class of reference trajectories, generated by a low magnitude, circumferentially directed thrust vector, has been adopted for the guidance analysis. Such trajectories are time optimal, under appropriate circumstances, for transfer between neighbouring co-planar circular orbits. The application considered permits an analytical representation of the extremal field which in turn leads to a closed form linear feedback control solution with time-varying gains. Some suggestions are also given for possible modifications which might enhance system accuracy and the range of operability during practical implementation of low-thrust rendezvous guidance.