More common algebraic loops in Simcenter Amesim

The previous example is not the most typical kind of algebraic loop encountered within Simcenter Amesim. Most of the time algebraic loops are due to closed loops of components where information exchanged at ports does not allow a direct ordering of components. The Simcenter Amesim formalism is indeed based on information flow and exchange. Each submodel exchanges variables with others, the information fluxes being oriented by the I/O status of exchanged variables:

• input

• input with default

• input unused

• output

In order a submodel to compute all its outputs, each of its inputs must have been computed first. A specific algorithm is implemented within Simcenter Amesim in order to sort submodels such that their corresponding computing function can be called and the embedded variables can be computed. Unfortunately it is not always possible to sort out all the submodels based on information direction on the sketch.

In the following example, we will consider a very simple system that we want to control through a simple PID controller.

Figure 53: A simple plant/control system

This is a very simple picture of a plant/controller model. The plant is here modelled by a supercomponent SC_1 and the controller by a PID component. In order to control the actuator, the control needs to recover some quantities from the plant model and to force some values in the actuator and thus in the plant model. Obviously, here the algebraic loop comes from the information flow generated by such a system. Here, each component receives its inputs from the component located to its left, apart from the summing junction that needs to collect information from both UD00 and the system supercomponent.

Let’s now try to sort the flow of information in this system at a given time t.

1. To be able to compute its output, the plant component SC1 needs to know its input.

2. This input must thus first be computed, so function f must be evaluated at ongoing time t with input x.

3. The x value must be thus computed by the PID component which itself requires evaluation of the output of the summing junction.

4. The input of the summing junction being the output of the plant model…

Note here that information workflow is a one-way kind.

Figure 54: Underlying information flow of plant/controller model

Most of the time, due to Simcenter Amesim port types, flow of information can be 2-way and algebraic loops might be less obvious. Let’s now consider a very simple electrical system.

Figure 55: A simple electrical system

Even if it is not obvious at first sight this system embeds an algebraic loop. Let’s look at the information exchanged at ports using external variables of submodels involved in the loop, i.e. the one way switch, the electrical junction and the resistor.

Figure 56: External variables in electrical components

With a view of the information flow, the loop can now easily be emphasized.

Figure 57: Underlying information flow of the electrical example

As ports can own several variables with different I/O statuses, an algebraic loop may occur without any obvious evidence. The next section will explain the consequences of these loops and introduce tools to graphically highlight algebraic loops within Simcenter Amesim.