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LMS Test.Lab Displays: XY Graph

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LMS Test.Lab Displays: XY Graph


Need to plot force versus displacement?  Or pressure versus velocity?


In LMS Test.Lab, the XY plot makes it easy to graph the independent data from two different channels of data against one another. What does this mean exactly?


Consider two channels of data that were acquired at the same time:

  • Force versus time
  • Displacement versus time

Using the XY graph, the force can be plotted against the displacement.


As shown in Figure 1, the displacement data is placed on the X axis of the XY graph, and the force data is put on the Y axis. 

XY_Disp_Force.pngFigure 1: A LMS Test.Lab XY graph plots the independent variables of two time histories against each other

The resulting XY graph is:

  • Force versus displacement (values indicated on the X and Y axes)
  • The dependent data, time, is indicated below the XY plot (from 0 to 1 second)

New insights into the data are achieved by viewing it the XY graph, versus viewing the time data separately.  For example, the 90 degree phase difference shows as a circle/oval in the XY graph.


LMS Test.Lab XY plot


To open a XY plot in LMS Test.Lab, click on the XY icon once as shown in Figure 2.

XY_Open.pngFigure 2: Press the XY icon to open a XY graph

A new XY graph is created in which time histories can be plotted against each other.



First, drag a time channel history into the bottom area of the XY graph (Figure 3).  The displacement (mm) vs time data is placed on the X axis.



XY_X.pngFigure 3: Drop displacement time history data on X axis

Drag a second time history into the side area of the XY graph (Figure 4).  In this example, force (N) versus time data is used and force is displayed on the Y axis.



XY_Y.pngFigure 4 – Drop force time data on Y axis

The two channels will be plotted against each other in the XY plot as shown in Figure 5.


XY_final.pngFigure 5 – XY graph shows force versus displacement

That is all there is to it!


The time graph located below the XY graph can be used to display only a subsection of the data. This can be done interactively:

  • Zoom In/Out – Hold the mouse pointer over the time axis. Hold down the Control Key and use the scroll wheel to zoom in and out.
  • Scroll – Scroll thru time by holding down the shift key while scrolling left and right

Check out the ‘mouse and keyboard shortcuts’ knowledge base article for more display shortcuts.


Shaft Centerline


A shaft centerline plot shows changes in the radial positon of a rotating shaft.  This is done by mounting proximity probes at 90 degree angles on the bearing of a rotating shaft (Figure 6).

proximity.pngFigure 6 – Displacement time histories from proximity probes are mounted at 90 degrees and plotted in the XY

By plotting the two displacements against each other (Figure 7), different issues in the rotating system can be identified. An issue can manifest itself with a distinct pattern in the XY graph.

shaft.pngFigure 7 – XY graph used as shaft centerline plot

The shaft centerline plot shows the XY position of the shaft.  If plotted against the clearance of a bearing, it can be seen if the shaft is rotating about the center of the bearing or not.


PV Diagram and the angle domain


In a combustion engine, plotting the cylinder pressure (P) versus the enclosed cylinder volume (V) results in a PV diagram (Figure 8).



PV.pngFigure 8 – Left: Pressure (Top) and Volume (Bottom), Right: PV Diagram

For a PV diagram, the individual channels are functions of angle, rather than time.  The PV diagram is done over one ‘cycle’, or two revolutions (720 degrees total). In a four stroke combustion engine, a complete combustion cycle takes two revolutions of the engine (Figure 9).


combust.pngFigure 9 – Two revolutions for one complete cycle

There are Intake, Compression, Power, and Exhaust strokes which takes two revolutions to complete.


The enclosed area in the PV diagram is used to determine the Mean Effective Pressure (MEP) of an engine.  It gives insight into how much power the engine produces.




The XY graph is used to plot the independent variables of two functions or channels against one another. The functions or channels could have dependent variables of:

  • Time
  • Angle

Plotting the two sets of data in a XY can produce insights that cannot be easily seen from the respective pieces of data themselves.


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