An operating point of a dynamic system defines the states and root-level input signals of the model at a specific time. For example, in a car engine model, variables such as engine speed, throttle angle, engine temperature, and surrounding atmospheric conditions typically describe the operating point.
A steady-state operating point of a model, also called an equilibrium or trim condition, includes state variables that do not change with time.
A model can have several steady-state operating points. For example, a hanging damped pendulum has two steady-state operating points at which the pendulum position does not change with time. A stable steady-state operating point occurs when a pendulum hangs straight down. When the pendulum position deviates slightly, the pendulum always returns to equilibrium. In other words, small changes in the operating point do not cause the system to leave the region of good approximation around the equilibrium value.
When using optimization search to compute operating points for nonlinear systems, your initial guesses for the states and input levels must be near the desired operating point to ensure convergence.
When linearizing a model with multiple steady-state operating points, it is important to have the right operating point. For example, linearizing a pendulum model around the stable steady-state operating point produces a stable linear model, whereas linearizing around the unstable steady-state operating point produces an unstable linear model.
This topic shows how to setup a steady-state operating point in Sensitivity Analysis tool. To improve the fit between the model and measured data, the model must be set to steady-state when parameters are evaluated.
Open the Sensitivity Analysis tool and setup your requirements using the steps outlined in Identify Key Parameters for Estimation (GUI).
In the toolstrip, click Options and select
Operating Point Options from the drop
The following Operating Point dialog box opens.
The Evaluate at steady-state
option is checked by default when you open the operating point dialog.
Select the appropriate requirement to change the parameters for from the
Requirement: drop down menu. Use the
and Outputs tabs to specify the known
parameters, bounds and deviations. For instance, there is one state in
the above figure. Use the operating point dialog to specify that this
state should be treated as an unknown, and it should be set to steady
state. During sensitivity analysis, the operating point computation will
vary this state to set it at steady-state.
You can also sync operating point specifications from your Simulink® model or another requirement using the
with specification from: drop-down list. After you
make your selection, click on the button to copy the parameters.
Design Optimization™ software uses optimization methods to search for operating
points in a model. Use the Options tab of the
dialog to specify these optimization methods. These options specify the
optimization algorithm, tolerances, and stopping conditions. For
instance, the option
Gradient descent with
projection is often used to find the operating point
for systems that use physical modeling. For more information, click on
Having specified the operating point parameters, continue with the sensitivity analysis workflow as described in Identify Key Parameters for Estimation (GUI).