Simulink Control Design™ - MathWorks

Estimating Parameter Values

Once you have selected a model structure, you have several options for manually or

automatically adjusting parameter values to achieve a good match between the estimated

model response and your measured or simulated input/output data. For an example that

illustrates all these options, see:

• “Interactively Estimate Plant Parameters from Response Data” (Control System
  Toolbox) (Control System Toolbox)


• “Interactively Estimate Plant from Measured or Simulated Response Data” on page 7-
  73 Simulink Control Design)

PID Tuner does not perform a smart initialization of model parameters when a model

structure is selected. Rather, the initial values of the model parameters, reflected in the

plot, are arbitrarily-chosen middle of the range values. If you need a good starting point

before manually adjusting the parameter values, use the Initialize and Estimate option

from the Plant Identification tab.

Handling Initial Conditions

In some cases, the system response is strongly influenced by the initial conditions. Thus a

description of the input to output relationship in the form of a transfer function is

insufficient to fit the observed data. This is especially true of systems containing weakly

damped modes. PID Tuner allows you to estimate initial conditions in addition to the

model parameters such that the sum of the initial condition response and the input

response matches the observed output well. Use the Estimation Options dialog box to

specify how the initial conditions should be handled during automatic estimation. By

default, the initial condition handling (whether to fix to zero values or to estimate) is

automatically performed by the estimation algorithm. However, you can enforce a certain

choice by using the Initial Conditions menu.

Initial conditions can only be estimated with automatic estimation. Unlike the model

parameters, they cannot be modified manually. However, once estimated they remain

fixed to their estimated values, unless the model structure is changed or new

identification data is imported.

If you modify the model parameters after having performed an automatic estimation, the

model response will show a fixed contribution (i.e., independent of model parameters)

from initial conditions. In the following plot, the effects of initial conditions were

identified to be particularly significant. When the delay is adjusted afterwards, the portion

7 PID Controller Tuning