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:
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.
The resulting XY graph is:
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.
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.
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.
The two channels will be plotted against each other in the XY plot as shown in Figure 5.
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:
Check out the ‘mouse and keyboard shortcuts’ knowledge base article for more display shortcuts.
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).
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.
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).
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).
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:
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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