fast model predictive control for magnetic plasma control mpc … · 2016. 2. 29. · for magnetic...

Fast Model Predictive Control for Magnetic Plasma Control MPC using fast online 1st-order QP methods

Matija Perne Jožef Stefan Institute

AWP15-ENR-01/JSI-02

Linear dynamic system, optimum

Matija Perne | FMPCFMPC PM | Ljubljana | 25.02.2016 | Page 2

● Discrete-time model:

● Cost minimization:

● Constraints:

Planning the future

Quadratic program

Hessian

Quadratic program

HessianGradient

Quadratic program

Hessian

Affine setGradient

Quadratic program

Hessian

Affine set

Polyhedron

Gradient

Quadratic program

Hessian

Affine set

Polyhedron

Gradient

Dual method

Solving the QP

● Explicit MPC: solving the problem in advance

Solving the QP

– Solution piecewise linear

Solving the QP

(http://www.seas.upenn.edu/~ese680/papers/explicit_linear_mpc.pdf)

Solving the QP

– Active and inactive constraints:

Solving the QP

● Active set methods

Solving the QP

– Online search for active constraints

Solving the QP

– Online search for active constraints

– Iterative

Solving the QP

• Interior point methods

Solving the QP

• Interior point methods– Cost is modified with a steep smooth function

close to polyhedral constraints

Solving the QP

– Modified problem solved with unconstrained optimization method

Solving the QP

– Modification decreased iteratively

Solving the QP

• First order methods

Solving the QP

– Inspired by gradient method

Solving the QP

– Inspired by gradient method

– Simple, low order of convergence

Existence of a solution

Soft constraints

Making the QP convenient

Simplifying the QP

Move blocking, constraint reduction

Simplifying the QP

(http://animalswalls.blogspot.si/2011/08/cat-wallpapers.html)

Gradient method (ordinary)

(F. Borrelli, A. Bemporad, M. Morari, Predictive Control for linear and hybrid systems, 2015.)

unconstrained

constrained

Lipschitz constant

Gradient method (ordinary)

unconstrained

constrained

Gradient method (fast)

unconstrained

constrained

Lagrange multipliers

("Lagrange multiplier." Wikipedia, The Free Encyclopedia, 2015.)

Lagrange and dual functions

(Stephen Boyd & Lieven Vandenberghe, Convex Optimization, Cambridge University Press 2004)

FGMdual

(generalized fast dual gradient method)

FGMdual

● Code generator:

● Code generator:– Problem described in MATLAB

– MATLAB does the offline calculations

– MATLAB outputs the algorithm code in C

– Code compiled, ran, result returned to MATLAB

– FGMdual one of the supported algorithms

● Knowing the code:

● Knowing the code:– MATLAB code is long

– Generated code is confusing

Restart

(B. O’Donoghue, E. Candès, Adaptive Restart for Accelerated Gradient Schemes, Found Comput Math 2013.)

Complexity certification

• Upper bound on the required number of floating point operations that in turn stems from an upper bound on the iteration count

(S. Richter, Computational Complexity Certification of Gradient Methods for Real-Time Model Predictive Control, ETH Zurich 2012)

• Upper bound on the required number of floating point operations that in turn stems from an upper bound on the iteration count

• accuracy => no. of iterations => no. of operations => computing time

• Explicit: fixed complexity

• Explicit: fixed complexity• Active set:

– Finite termination

– Worst case number of iterations huge

– Early termination unexplored

• Interior point:

• Interior point:– Certificates very conservative

• First order:

• First order:– Certificates within few orders of magnitude

• State of the art for first order methods:

• State of the art for first order methods:– Derived for certain (simpler) algorithms

(not generalized, no restarting)

(not generalized, no restarting)– Derived for certain (simpler) constraints

Nesterov

• Convex optimization

(Y. Nesterov, Introductory lectures on convex optimization : a basic course, Springer 2004)

Nesterov

• Convex optimization• Does not mention duality

(Y. Nesterov, Introductory lectures on convex optimization : a basic course, Springer 2004)

Richter

• MPC with some QP

Richter

• MPC with some QP• Duality: partial Lagrange relaxation

(simple state constraints), shows that complete is worse, but complete is what we need

Richter

• MPC with some QP• Duality: partial Lagrange relaxation

(simple state constraints), shows that complete is worse, but complete is what we need

• Duality: dual problem not strongly concave

Temporary conclusion

• What to do? Transform the MPC-obtained QP into dual space

• Follow either Richter or Nesterov with dual QP

• Preconditioning?• Restarting?

fast model predictive control for magnetic plasma control mpc … · 2016. 2. 29. · for magnetic...

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