摘要(英) |
This thesis aims to study a full-space Lagrange-Newton algorithm for solving nonlinear optimal control problems. These problems represent a wide range of applications in computational science and engineering, such as trajectory optimization problems, industrial robots problems that can be also mathematically formulated as equality-constrained optimization problems.
In this method, the first step is to introduce the Lagrange multiplier into the objective function and then solve the optimization problem by finding the critical solution of the first-order necessary optimality condition (also known as the KKT condition) by the Newton-type method. One of the advantages of the Newton-type method is fast convergence provided that an initial guess is close to the solution. However, such a good initial guess is not easy to obtain. And Newton′s method suffers from the convergence issue when the nonlinearity of the system is not well balanced even some globalization technique is used. One of the drawbacks of the Lagrange-Newton method is that the KKT matrix needs to construct. The computation of the Hessian matrix of the KKT system could be expensive for example using finite-difference approximation. To improve the robustness of Newton′s method, we propose a new three-stage decoupling preconditioner. The key point of the new proposed algorithm is that before performing the global Newton update, in the three-stage decoupling preconditioning phase, we correct the Lagrange multiplier, control variables, state variables in order.
Our numerical results based on several benchmark test problems show that the three-stage decoupling preconditioner helpful for the convergence of the Lagrange-Newton algorithm and can reduce the number of iterations. Besides, we report a series of comparative study to investigate the full-space method with different approaches for constructing the Hessian matrix, including analytical approach, finite differences, automatic differentiation, and the low-rank update based method. We also show numerically that the full-space method is about a few hundred times faster than an optimizer in Matlab toolbox, which is implemented using the reduced-space Lagrange-Newton method. |
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