How do I select the observer variables to use?

If there is more than one observer variable, in order to make a valid comparison it is better to have them all using the same unit e.g. all linear velocities in m/s. This is the best solution for a magnitude modal shape.

Some variables such as linear and rotary velocity, hydraulic or pneumatic flow rate and electrical flow rate have special significance in that an energy is proportional to the square of these variables in bond graph terms these variables are called flows. It is necessary to introduce a different constant of proportionality for each observer to get an expression for the energy. In this case the units do not need to be the same. This is the solution for energy modal shapes.

The general procedure is as follows:

Procedure

1. Click on the Modal shapes button or select Analysis > Linearization > Modal shapes to produce the Modal Shapes Analysis dialog box. Note that eigenvalues are either real or occur in conjugate pairs.

2. Create the Modal Shapes analysis dialog box as follows:

   Highlight the eigenvalue of interest. Note that, if it is part of a conjugate pair, you get precisely the same result from an eigenvalue and its conjugate.

   Click on the Modal shapes button to produce the Modal Shapes Analysis dialog box (see figure Figure 41).

   Figure 41: Modal Shapes Analysis dialog box

   You can sort your observer variables by highlighting items in the list and clicking on Move to top, Move to bottom, Move up or Move down. Additionally, you can make multiple selections in the list by using Shift + Click (to select a block of variables), or Ctrl + Click (to select individual variables). Click Select all to select all the variables in the list. Alternatively, you can sort them by clicking on the title of any column. You may wish to do this for the following reasons:

   It is very useful to sort observer variables by Magnitude, since this indicates which observer variables have an influence on the current mode/frequency.

   Sorting observer variables by Unit allows you, for example, to get only the pressures, the flow rates and so on.

   Sorting observer variables by Submodel allows you to find all observers that are local to a particular submodel.

3. Select the observers you are interested in using Shift and Ctrl keys.

   This is useful, for example, to view only the observer variables whose magnitude is different from zero and which have the same unit.

   If you use the Move to top, Move to bottom, Move Up or Move Down facility, this will apply to the current selection.

   If you click on the Plot button, this will plot only the selected observer variables.

Results

You can now:

• plot Magnitudes,

• plot Energies.