What is MPC and what are the limitations of MPC

MPC (Model predictive control)

  • Key features
    • Process control that relies on dynamic models of the precesses, most often empirical models obtained by system identification
    • Represent the behaviour of complex dynamic systems

 

  • Main advantage
    • Optimizing the current time slot while keeping future time slots into account. This is achieved by optimizing a finite time-horizon, not only the current time slot.
    • MPC has the predictive ability to anticipate future events, which PID and LQR do not have.

 

  • Limitations
    • Performance
    • Model mismatch, imperfect predictions of disturbances
  • Installation & maintenance cost
    • Installation generally requires weeks or months of plant tests, controller building, and post-installation tuning
    • Maintenance
      • Just measuring controller performance is challenging
        • Harris Controller Performance Index is strictly speaking only applicable for the SISO (Single Input Single Output) case
        • Extensions to multivariable control have been proposed, but they are difficult to apply, and some of the underlying assumptions of these techniques are not realized in practice.
        • Tools for determining which of the models (of the tens or hundreds) may be in error are primitive or non-existent, and a common procedure is to simply retest the entire plant (at significant cost) whenever the performance falters too much.
      • Determining whether the controller is driving the plant to the true optimum can also be difficult
      • Lifetime cost for maintenance is high
  • Structural limitation
    • There is no guarantee that the (somewhat arbitrary) mathematical optimizations of MPC translate well into engineering objectives, nor that the rigid structure of MPC controllers matches the engineering design problem at hand.
  • Operator interference
    • Actually, MPC need more interference of operators to check if it is operating adequately
    • Model mismatch, unable to react to unexpected disturbances or differences from design cases

 

Source: http://www.controlartsinc.com/Support/Articles/LimitationsOfMBC.pdf

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