mpcmoveAdaptive

Compute optimal control with prediction model updating

Syntax

mv = mpcmoveAdaptive(mpcobj,x,Plant,Nominal,ym,r,v)

[mv,info] = mpcmoveAdaptive(mpcobj,x,Plant,Nominal,ym,r,v)

[___] = mpcmoveAdaptive(___,options)

Description

mv = mpcmoveAdaptive(mpcobj,x,Plant,Nominal,ym,r,v) computes the optimal manipulated variable moves at the current time. This result depends on the properties contained in the MPC controller, the controller states, an updated prediction model, and the nominal values. The result also depends on the measured output variables, the output references (setpoints), and the measured disturbance inputs. mpcmoveAdaptive updates the controller state, x, when using default state estimation. Call mpcmoveAdaptive repeatedly to simulate closed-loop model predictive control.

[mv,info] = mpcmoveAdaptive(mpcobj,x,Plant,Nominal,ym,r,v) returns additional details about the solution in a structure. To view the predicted optimal trajectory for the entire prediction horizon, plot the sequences provided in info. To determine whether the optimal control calculation completed normally, check info.Iterations and info.QPCode.

[___] = mpcmoveAdaptive(___,options) alters selected controller settings using options you specify with mpcmoveopt. These changes apply for the current time instant only, enabling a command-line simulation using mpcmoveAdaptive to mimic the Adaptive MPC Controller block in Simulink^® in a computationally efficient manner.

Input Arguments

mpcobj — MPC controller

MPC controller object

MPC controller, specified as an implicit MPC controller object. To create the MPC controller, use the mpc command.

x — Current MPC controller state

mpcstate object

Current MPC controller state, specified as an mpcstate object.

Before you begin a simulation with mpcmoveAdaptive, initialize the controller state using x = mpcstate(mpcobj). Then, modify the default properties of x as appropriate.

If you are using default state estimation, mpcmoveAdaptive expects x to represent x[n|n-1]. The mpcmoveAdaptive command updates the state values in the previous control interval with that information. Therefore, you should not programmatically update x at all. The default state estimator employs a linear time-varying Kalman filter.

If you are using custom state estimation, mpcmoveAdaptive expects x to represent x[n|n]. Therefore, prior to each mpcmoveAdaptive command, you must set x.Plant, x.Disturbance, and x.Noise to the best estimates of these states (using the latest measurements) at the current control interval.

For more information on state estimation for adaptive MPC and time-varying MPC, see State Estimation.

Plant — Updated prediction model

discrete-time state-space model | model array

Updated prediction model, specified as one of the following:

• A delay-free, discrete-time state-space (ss) model. This plant is the update to mpcobj.Model.Plant and it must:

  • Have the same sample time as the controller; that is, Plant.Ts must match mpcobj.Ts

  • Have the same input and output signal configurations, such as type, order, and dimensions

  • Define the same states as the controller prediction model, mpcobj.Model.Plant

• An array of up to p+1 delay-free, discrete-time state-space models, where p is the prediction horizon of mpcobj. Use this option to vary the controller prediction model over the prediction horizon.

  If Plant contains fewer than p+1 models, the last model repeats for the rest of the prediction horizon.

Tip

If you use a plant other than a delay-free, discrete-time state-space model to define the prediction model in mpcobj, you can convert it to such a model to determine the prediction model structure.

If the original plant is	Then
Not a state-space model	Convert it to a state-space model using ss.
A continuous-time model	Convert it to a discrete-time model with the same sample time as the controller, mpcobj.Ts, using c2d with default forward Euler discretization.
A model with delays	Convert the delays to states using absorbDelay.

Nominal — Updated nominal conditions

structure | structure array | []

Updated nominal conditions, specified as one of the following:

• A structure of with the following fields:

  Field	Description	Default
  X	Plant state at operating point	[]
  U	Plant input at operating point, including manipulated variables and measured and unmeasured disturbances	[]
  Y	Plant output at operating point	[]
  DX	For continuous-time models, DX is the state derivative at operating point: DX=f(X,U). For discrete-time models, DX=x(k+1)-x(k)=f(X,U)-X.	[]

• An array of up to p+1 nominal condition structures, where p is the prediction horizon of mpcobj. Use this option to vary controller nominal conditions over the prediction horizon.

  If Nominal contains fewer than p+1 structures, the last structure repeats for the rest of the prediction horizon.

If Nominal is empty, [], or if a field is missing or empty, mpcmoveAdaptive uses the corresponding mpcobj.Model.Nominal value.

ym — Current measured outputs

row vector of length N_ym

Current measured outputs, specified as a row vector of length N_ym vector, where N_ym is the number of measured outputs.

If you are using custom state estimation, ym is ignored. If you set ym = [], then mpcmoveAdaptive uses the appropriate nominal value.

r — Plant output reference values

p-by-N_y array | []

Plant output reference values, specified as a p-by-N_y array, where p is the prediction horizon of mpcobj and N_y is the number of outputs. Row r(i,:) defines the reference values at step i of the prediction horizon.

r must contain at least one row. If r contains fewer than p rows, mpcmoveAdaptive duplicates the last row to fill the p-by-N_y array. If you supply exactly one row, therefore, a constant reference applies for the entire prediction horizon.

If you set r = [], then mpcmoveAdaptive uses the appropriate nominal value.

To implement reference previewing, which can improve tracking when a reference varies in a predictable manner, r must contain the anticipated variations, ideally for p steps.

v — Current and anticipated measured disturbances

p-by-N_md array | []

Current and anticipated measured disturbances, specified as a p-by-N_md array, where p is the prediction horizon of mpcobj and N_md is the number of measured disturbances. Row v(i,:) defines the expected measured disturbance values at step i of the prediction horizon.

Modeling of measured disturbances provides feedforward control action. If your plant model does not include measured disturbances, use v = [].

v must contain at least one row. If v contains fewer than p rows, mpcmoveAdaptive duplicates the last row to fill the p-by-N_md array. If you supply exactly one row, therefore, a constant measured disturbance applies for the entire prediction horizon.

If you set v = [], then mpcmoveAdaptive uses the appropriate nominal value.

To implement disturbance previewing, which can improve tracking when a disturbance varies in a predictable manner, v must contain the anticipated variations, ideally for p steps.

options — Override values for selected controller properties

mpcmoveopt object

Override values for selected properties of mpcobj, specified as an options object you create with mpcmoveopt. These options apply to the current mpcmoveAdaptive time instant only. Using options yields the same result as redefining or modifying mpcobj before each call to mpcmoveAdaptive, but involves considerably less overhead. Using options is equivalent to using an Adaptive MPC Controller Simulink block in combination with optional input signals that modify controller settings, such as MV and OV constraints.

Output Arguments

mv — Optimal manipulated variable moves

column vector

Optimal manipulated variable moves, returned as a column vector of length N_mv, where N_mv is the number of manipulated variables.

If the controller detects an infeasible optimization problem or encounters numerical difficulties in solving an ill-conditioned optimization problem, mv remains at its most recent successful solution, xc.LastMove.

Otherwise, if the optimization problem is feasible and the solver reaches the specified maximum number of iterations without finding an optimal solution, mv:

• Remains at its most recent successful solution if the Optimizer.UseSuboptimalSolution property of the controller is false.

• Is the suboptimal solution reached after the final iteration if the Optimizer.UseSuboptimalSolution property of the controller is true. For more information, see Suboptimal QP Solution.

info — Solution details

structure

Solution details, returned as a structure with the following fields.

Uopt — Optimal manipulated variable sequence

(p+1)-by-N_mv array

Predicted optimal manipulated variable adjustments (moves), returned as a (p+1)-by-N_mv array, where p is the prediction horizon and N_mv is the number of manipulated variables.

Uopt(i,:) contains the calculated optimal values at time k+i-1, for i = 1,...,p, where k is the current time. The first row of Info.Uopt contains the same manipulated variable values as output argument mv. Since the controller does not calculate optimal control moves at time k+p, Uopt(p+1,:) is equal to Uopt(p,:).

Yopt — Optimal output variable sequence

(p+1)-by-N_y array

Optimal output variable sequence, returned as a (p+1)-by-N_y array, where p is the prediction horizon and N_y is the number of outputs.

The first row of Info.Yopt contains the calculated outputs at time k based on the estimated states and measured disturbances; it is not the measured output at time k. Yopt(i,:) contains the predicted output values at time k+i-1, for i = 1,...,p+1.

Yopt(i,:) contains the calculated output values at time k+i-1, for i = 2,...,p+1, where k is the current time. Yopt(1,:) is computed based on the estimated states and measured disturbances.

Xopt — Optimal prediction model state sequence

(p+1)-by-N_x array

Optimal prediction model state sequence, returned as a (p+1)-by-N_x array, where p is the prediction horizon and N_x is the number of states in the plant and unmeasured disturbance models (states from noise models are not included).

Xopt(i,:) contains the calculated state values at time k+i-1, for i = 2,...,p+1, where k is the current time. Xopt(1,:) is the same as the current states state values.

Topt — Time intervals

column vector of length p+1

Time intervals, returned as a column vector of length p+1. Topt(1) = 0, representing the current time. Subsequent time steps Topt(i) are given by Ts*(i-1), where Ts = mpcobj.Ts is the controller sample time.

Use Topt when plotting the Uopt, Xopt, or Yopt sequences.

Slack — Slack variable

nonnegative scalar

Slack variable, ε, used in constraint softening, returned as 0 or a positive scalar value.

• ε = 0 — All constraints were satisfied for the entire prediction horizon.

• ε > 0 — At least one soft constraint is violated. When more than one constraint is violated, ε represents the worst-case soft constraint violation (scaled by your ECR values for each constraint).

See Optimization Problem for more information.

Iterations — Number of solver iterations

positive integer | 0 | -1 | -2

Number of solver iterations, returned as one of the following:

• Positive integer — Number of iterations needed to solve the optimization problem that determines the optimal sequences.

• 0 — Optimization problem could not be solved in the specified maximum number of iterations.

• –1 — Optimization problem was infeasible. An optimization problem is infeasible if no solution can satisfy all the hard constraints.

• –2 — Numerical error occurred when solving the optimization problem.

QPCode — Optimization solution status

'feasible' | 'infeasible' | 'unrealiable'

Optimization solution status, returned as one of the following:

• 'feasible' — Optimal solution was obtained (Iterations > 0)

• 'infeasible' — Solver detected a problem with no feasible solution (Iterations = –1) or a numerical error occurred (Iterations = –2)

• 'unreliable' — Solver failed to converge (Iterations = 0). In this case, if mpcobj.Optimizer.UseSuboptimalSolution is false, u freezes at the most recent successful solution. Otherwise, it uses the suboptimal solution found during the last solver iteration.

Cost — Objective function cost

nonnegative scalar

Objective function cost, returned as a nonnegative scalar value. The cost quantifies the degree to which the controller has achieved its objectives. For more information, see Optimization Problem.

The cost value is only meaningful when QPCode = 'feasible', or when QPCode = 'feasible' and mpcobj.Optimizer.UseSuboptimalSolution is true.

Tips

• If the prediction model is time-invariant, use mpcmove.

• Use the Adaptive MPC Controller Simulink block for simulations and code generation.

Version History

Introduced in R2014b