Control Quadruple-Tank Using Passivity-Based Nonlinear MPC

This example shows how to design a passivity-based controller for a quadruple-tank using nonlinear model predictive control (MPC).

Overview

The dynamics for a quadruple tank system can be written as in [1]:

$$\dot{\mathit{x}}=\mathit{f}\left(\mathit{x}\right)+\mathit{g}\left(\mathit{x}\right)\mathit{u}$$,

where $\mathit{x}$ denotes the heights of the four tanks and $\mathit{u}$ denotes the flows of the two pumps. These dynamics are implemented in stateFcnQuadrupleTank.m.

The control objective is to select the flow of the pumps $\mathit{u}$ such that $\left(\mathit{x},\mathit{u}\right)$ moves towards the equilibrium $\left({\mathit{x}}_{\mathit{s}},{\mathit{u}}_{\mathit{s}}\right)$.

To enforce closed-loop stability, the controller includes a passivity constraint.

To define the passivity constraint, first define the state error vector:

$$\mathit{e}=\mathit{x}-{\mathit{x}}_{\mathit{s}}$$.

As in [1], define the storage function as $\mathit{V}=\frac{1}{2}({\mathit{e}}_3^2+{\mathit{e}}_4^2)$ and take the derivative of $\mathit{V}$ to obtain the relationship:

$\dot{\mathit{V}}\le {{\mathit{u}}_{\mathit{p}}}^{\prime }{\mathit{y}}_{\mathit{p}}$.

This relationship means that, the system is passive from ${\mathit{u}}_{\mathit{p}}=\left[{\mathit{u}}_2;{\mathit{u}}_1\right]-{\mathit{u}}_{\mathit{s}}$ to ${\mathit{y}}_{\mathit{p}}=\left[{\mathit{e}}_3;{\mathit{e}}_4\right]$. The relationship for the passivity input ${\mathit{u}}_{\mathit{p}}$ and the passivity output ${\mathit{y}}_{\mathit{p}}$ are described in the helper function getPassivityInputQuadrupleTank.m and getPassivityOutputQuadrupleTank.m.

To enforce closed-loop stability, define the passivity constraint as follows:

${{\mathit{u}}_{\mathit{p}}}^{\prime }{\mathit{y}}_{\mathit{p}}\le -\rho {{\mathit{y}}_{\mathit{p}}}^{\prime }{\mathit{y}}_{\mathit{p}}$ with $\rho >0$.

For a nonlinear MPC controller, you define the passivity constraint by setting the Passivity property of the nonlinear MPC object.

Design Nonlinear MPC Controller

Create a nonlinear MPC object with four states, four outputs, and two inputs.

nlobj = nlmpc(4,4,2);

Zero weights are applied to one or more OVs because there are fewer MVs than OVs.

Specify the quadruple-tank dynamics function as the state function of the prediction model.

nlobj.Model.StateFcn = "stateFcnQuadrupleTank";

The default cost function of a nonlinear MPC problem is a standard quadratic cost function. For this example, keep a quadratic cost function and specify nonzero weights for the first two output variables [1].

nlobj.Weights.OutputVariables = [0.1 0.1 0 0];

Specify the passivity property fields of the nonlinear MPC object.

nlobj.Passivity.EnforceConstraint = true;
nlobj.Passivity.InputFcn = "getPassivityInputQuadrupleTank";
nlobj.Passivity.OutputFcn = "getPassivityOutputQuadrupleTank";
nlobj.Passivity.OutputPassivityIndex = 1;

Closed-Loop Simulation

Specify the initial conditions of the states.

x0 = [25;16;20;21];

Specify the equilibrium point for the quadruple-tank model.

xs = [28.1459,17.8230,18.3991,25.1192]';
us = [37,38]';

Open the Simulink® model.

mdl = "quadrupleTankNLMPC";
open_system(mdl)

Run the model.

sim(mdl);

View the errors for the quadruple-tank states.

open_system(mdl + "/Quadruple-tank/state_error")

The errors go to zero and the closed-loop system is stable.

To view the performance of the nonlinear MPC controller without the passivity constraint, remove it from the controller.

nlobj.Passivity.EnforceConstraint = false;

Run the simulation.

sim(mdl);

Without the passivity constraint, the closed-loop system becomes unstable with the same controller design parameters.

References

[1] Raff, Tobias, Christian Ebenbauer, and Frank Allgöwer. “Nonlinear Model Predictive Control: A Passivity-Based Approach.” In Assessment and Future Directions of Nonlinear Model Predictive Control, edited by Rolf Findeisen, Frank Allgöwer, and Lorenz T. Biegler, 358:151–62. Berlin, Heidelberg: Springer Berlin Heidelberg, 2007. https://doi.org/10.1007/978-3-540-72699-9_12.