The Fixed step integrator

Fixed step, fixed order methods are much less robust and flexible that the variable step, variable order methods. However, they are extensively used in real time simulations. Note that facilities are available to export a Simcenter Amesim model to a real time environment. However, there are limitations on the models that will run in this environment:

• The equations governing the model normally must be explicit. In other words there must be no constraints or implicit state variables.

• There will be some limit on the size of the model. This limit will depends on the capability of the real time platform.

• The dynamics of the model must be sufficiently slow. Study of this often involves determining the eigenvalues of the system.

• Experienced users often developed very complex high fidelity models in Simcenter Amesim. To run these models in a real time environment normally involves work to reduce the complexity. This process is often called model reduction. After a while the modeler begins to think that the model is sufficiently reduced to run in the target real time environment. At this stage the reduced model can be run in Simcenter Amesim with the fixed step fixed order integration methods provided. Remember that the reference results are the high fidelity results. If this is successful, the export to the real time environment can go ahead.

Note

There is nothing to stop you trying to run all your models with fixed step fixed order integrators. For general simulation this is not recommended. Remember these are explicit methods with very limited stability and there is no error control.

For the rest of this section we will assume that we are doing general simulation with variable step variable order methods.