5. Physical components in Modelica

Learning objectives

In this session, you’ll learn about physical components: what they are, how to connect them? These components are the main building blocks of most Modelica models.

Main ressources needed:

5.1. Connecting physical components

We start by considering an electrical system, since the representation of these systems in Modelica will fully reminds us of the usual representation on paper.

In OpenModelica’s Diagram View, drag and drop components from the Modelica.Electrical (a subpackage of Modelica Standard Library) to create the simple RLC circuit of Fig. 5.1:

_images/elec_connect_annot.png

Fig. 5.1 connection of a few electrical components from the Modelica.Electrical standard package

Now switch to Text View and look for a set of lines starting with a keyword, connect:

equation
  connect(constantVoltage1.p, inductor1.p) annotation(/*[...]*/)
  connect(capacitor1.p, inductor1.n) annotation(/*[...]*/)
  /*[...]*/

Question

What is the relation between each of these lines to each electrical node?

Bonus: for one connect(...) line of you choice, you can fiddle with the annotation which describes of the connection line is drawn. For example, you can replace the color attribute color = {0, 0, 255} by color = {255, 0, 0} and jump to the diagram view to see the result.

Theoretical Exercise

Using your knowledge of electrical circuits, what physical equations correspond to the connection of components, at each node?

In particular, for the node 2 on Fig. 5.1, list the set of equations which are equivalent to the connection of inductor1.n, capacitor1.p and resistor1.p.

In the general case, when connecting \(n\) ports, how many equations are there?

5.2. Vehicle suspension with components

Now use components from Modelica.Mechanics.Translational to recreate the ODE model of a simplified Vehicle suspension.

Using the same parameters for mass, spring constant and damping, check that both models yields the same result.

Now, as with electrical circuit, what physical equations correspond to the connection of components?

5.3. Acausal connection

In Modelica, the connection of components is a generalized notion of the connection of electrical components in an electrical diagram.

Physical variables can be classified into:

  • potential (or across) variables

  • flow (or through) variables

Also, the connect equations corresponds to classical physical laws

  • potential equation: electrical wires are equipotential (no wire resistance)

  • flow equation: charge conservation at an electrical node

Now, you fill this table with variables and classical laws of the most usual domains:

Domain

Potential variable

Flow variable

Potential equation

Flow equation

electricity

voltage (V)

current (A)

equipotential wires

charge conservation

mechanics, 1D translation

mechanics, 1D rotation

thermal transfers

1D fluid mechanics