Plotting Magnitudes

Modal shapes plotting magnitudes

This is the simplest form of modal shape analysis:

Procedure

1. Adjust some magnitude factors if necessary, so that all observers have consistent values. This is useful if different units are used, in this case a conversion factor must be applied. Please refer to Magnitude factors for a complete description of magnitude factors.

2. Make sure the Magnitudes tab is selected and click on the Plot button to produce the plot like the one shown here:

   Figure 42: Plot of magnitudes

Results

Simcenter Amesim produces animation to see how each observer responds.

To start the animation

Animation for modal shapes

Procedure

Click on the Start/pause animation button or select the item in the ModalShapes menu.

Figure 43: ModalShapes menu

Results

Note

The response of the observer:

• will normally be oscillatory if the eigenvalue is complex,

• will normally be a simple decay in magnitude for a real negative eigenvalue,

• will be nothing at all if that observer is not influenced by that frequency.

To add titles for the x- and y-axes

Procedure

1. Right-click on the plot and select Titles, just as you would for a standard plot. However, this does not insert the observer variable titles,

2. To display the observer variable titles, select ModalShapes > Add Observer Titles.

To animate a collection of modal shape plots

If you have more than one modal shape plot displayed you can animate them all by using ModalShapes > Start/pause all animations.

Temporal view of modal shapes

As an aid to understanding modal shapes a temporal view is provided by default. You can remove this temporal view from the graph through a right-click and selecting Remove > All graph(s). To reinstate the temporal view on a modal shape plot proceed as follows.

Procedure

1. Select ModalShapes > Temporal view.

2. The pointer takes the form of a hand: . Click on the modal shape plot concerned.

   Figure 44: Temporal view of modal shapes

Initial view

Note

The initial display of a modal shapes plot is not at t=0. In fact, the display is at the time (t) which corresponds to the greatest amplitude detected amongst the variables analyzed. In this way we obtain a reliable comparison reference point, and avoid mis-interpretation.

In the above illustration, we can see (from the temporal view) that the greatest amplitudes occur at t=0.75 s, so the bars displayed correspond to these values (- 33.33 and 66.67), not the values at t=0.

Magnitude factors

Magnitude factors can be useful when plotting modal shapes of variables in the following cases:

• Variables with different units in multi-domain systems (for example flow rates, mass velocities, and currents).Example: In a braking system, when a piston is controlled by a hydraulic line, the velocity of the piston can be compared with the flow rate in the line by using the cross-section of the line as a magnitude factor.

• Variables associated with a direction, such as linear or rotary speed, hydraulic flow rates etc.Example: In a hydraulic network when 2 flow rates have opposite directions, they will have opposite factors (-1 or +1) depending on the sign convention.

• Variables related by a known proportional coefficient.Example: In a gear box, when comparing 2 rotary mass velocities the gear ratio must be indicated. Refer to the demo Reducer.ame which can be found in the demo folder Infrastructure > AnalysisTools > LinearAnalysis > EigenValuesModalshapes.