State contributions facility

state count

The Performance Analyzer contains a State contributions State contributions facility. The state contributions facility allows you to see which state variables (explicit or implicit or constraint) are slowing down the simulation. The facility is also useful as a quick way of seeing titles of state variables.

The integration process proceeds in a series of time steps until the final time is reached. At each of these steps an iterative process is used to determine the state variable values at the new time. This iterative process must converge for the step to be successful. In addition, after each step, an error test is applied based on the tolerance specified in the Run Parameters dialog box. At a particular step, some variables may easily satisfy the convergence and error tests. Others may only just pass the tests.

At each step, Simcenter Amesim Run records the variable that had the greatest difficulty in satisfying the tests. The State contributions facility is accessible in the Performance Analyzer which is launched in Simulation mode by clicking on the button. It summarizes the information which can be very useful in determining the cause of a slow simulation.

1. To follow our example, reload the QuarterCar.ame file you used in A simple mechanical system.

Figure 40: Quarter car system

Note You can also get this system through Help > Get demo, from the Tutorials directory. Do not forget to set the parameters as described in Setting parameters and running a simulation.

2. To ensure that the procedures described in this example makes sense for your system, switch to Parameter mode and make sure that the following values are set:

Submodel name and type

Main simulation parameters

1

1D translational mass [MECMAS21] mass with friction and endstops

body velocity [m/s] = 0.0 body displacement [m] = 0.0

2

linear spring with 2 ports capable of linear motion [SPR000A]

spring force with both displacements zero [N] = 0.0

3

1D translational mass [MECMAS21] mass with friction and endstops

wheel velocity [m/s] = 0.0 wheel displacement [m] = 0.0

4

linear spring with 2 ports capable of linear motion [SPR000A]

spring force with both displacements zero [N] = 0.0

3. Go to Simulation mode, open the Run Parameters and check that Hold inputs constant is unticked.

4. Run a simulation.

There are 5 state variables.

5. Click to open the Performance Analyzer.

Figure 41: Performance Analyzer

Note The numbers will vary slightly according to the platform on which you are running.

In this example it is mostly the state variable wheel displacement in submodel MECMAS21 (Wheel Mass) which slows down the simulation.

If the simulation is slow, you can click on Update or check the Automatic update box.

You can rearrange the list by clicking on the Controlled column title.

If you double click on an item in the list, Simcenter Amesim Run will identify the corresponding state count finding the submodel component as in the figure below.

Figure 42: Search facility

With a simple dynamic run for 5 s, the velocity and displacement of the car body is shown in the following figure.

Figure 43: Curves of the velocity and displacement of the car

There are two distinct phases to the motion:

1. The car attempts to find an equilibrium position before the step is reached. The situation is as if the car body was lifted up on its suspension so that the springs and tires were relaxed with the tires just touching the road and then the body was suddenly released giving a transient period before the step is encountered.

2. The step is reached and the car attempts to find a new equilibrium position.

In Getting started, you saw how to remove this transient behavior by doing two dynamic runs. In the first run the input(s) are held constant at their initial values to obtain the free response. In the second run the end results of the first run are used as starting values. This is by far the safest and most reliable way of getting an equilibrium position. However, for a big system the initial run may be very long. An alternative is available which is less reliable but often much faster. This involves using a stabilizing run.