Metacontrol Methodology Overview

The “top-down” part of the self-optimizing control structure selection methodology developed by Skogestad ([31]) has the following main steps:

  1. Identify the relevant process variables: Manipulated variables, disturbances, and potential candidate controlled variables (process measurements), and perform a Degree of Freedom (DOF) analysis taking into account both steady and dynamic states of the process.

  2. Define optimal operation: Formulate the problem with objective function and constraints to be used in order to seek an optimal operating point.

  3. (Mathematically) Model the (industrial) process. Metacontrol is currently compatible with Aspen Plus.

  4. Optimize the process model as formulated in step 2.

  5. Implement the active constraints found in the previous step - “active constraint control” ([31])

  6. Evaluate the loss resulting from a constant setpoint policy ([13]) for each possible control configuration for the remaining (unconstrained) degrees of freedom. This is done in Metacontrol using, e.g., local (linear) methods ([13][15][16][2]), where a reduced-space (unconstrained) problem is required to obtain gradients with respect to candidate controlled variables and disturbances, and the Hessian of the objective function evaluated at the optimal point found in step 4. As stated before in the introduction, Metacontrol uses Kriging predictors to evaluate gradient and hessian matrices. Note that the use of local methods requires the specification of disturbance magnitudes and measurement errors associated with each candidate controlled variable of step 1.

The basic structure of Metacontrol showing the two modes of operation is depicted in Figure below. Mode 1 is the complete implementation of steps 1 through 6 of the SOC procedure, where a first metamodel of the process is generated to evaluate the optimal operating point with all degrees of freedom available, while mode 2 is a shortcut taken when the optimal steady-state is known. In both cases, an extra metamodel is produced as the reduced space model using the remaining unconstrained degrees of freedom.

../_images/metacontrol-workflow.svg

Fig. 2 Basic flowchart of the Metacontrol workflow.

The graphical user interface (GUI) of Metacontrol is a paradigm of the steps that are needed to perform the SOC analysis using metamodels, proposed by [3]. The user will simply navigate between easy-to-use sequential tabs, providing information for the problem that he wants to study, and the results will become available in real-time. Over the next session, each tab of Metacontrol will be discussed in detail, in order to teach you how to use it!