An order cut shows the energy of a measured signal (typically from an accelerometer or microphone) in a specified order band.
In Figure 1 below, 3rd order is being cut from the colormap and being displayed on the right.
This article will describe:
How is the amplitude of an order cut calculated?
To get the order cut amplitude, the energy around the specified order is summed (RMS) to get a value at each tracking increment (RPM).
The process is as follows:
To cut an order, the order number as well as the bandwidth must be specified. The bandwidth determines what energy is included in the RMS sum of the order.
Setting Order Bandwidth
It is critical to set the order bandwidth appropriately so that the energy of interest is included in the calculation.
If the order bandwidth is set too narrow, some of the order energy will not be included in the calculation.
If the order bandwidth is set too wide, neighboring energy that is not due to the order of interest will be included in the order cut.
In Figure 3 (below), it is clear that changing the width of the order calculation impacts the resulting order cut.
The same order is cut in each case, but the order bandwidth changes.
As seen in Figure 3 (below), it is a bit like Goldilocks and the Three Bears – the order cut needs to be just right!
Results can vary by many dB meaning that you could either be over or under a critical target depending on how the order is processed.
Therefore, each time an order is cut, care should be taken to ensure the bandwidth is set appropriately. It is recommended to do a visual inspection of the colormap to aid in choosing an appropriate bandwidth.
There is no “fixed rule” when setting bandwidth as the smear of order energy is dependent on the rate of change of the speed, the window used in the Fourier Transform, and the frequency resolution used.
An appropriate bandwidth is one in which all of the content of the order of interest is included, and none of the neighboring content is included.
So, how is order bandwidth set?
Different ways to specify order bandwidth
The order bandwidth represents the band over which energy will be summed for each RPM for the order cut.
The order bandwidth can be specified in four modes:
These modes apply in the frequency domain as well as order domain.
Each mode is described in more detail below.
In this case, a fixed order bandwidth is taken around the defined order value. The frequency bandwidth will increase with the RPM value, so larger groups of frequencies will be included in the order cut calculation at higher RPMs. The order bandwidth will remain constant with RPM.
By setting the order bandwidth, the upper and lower order values are automatically determined.
Cutting using the order mode is the most common type of order section.
In this case, a percentage of the defined order value will be used to define the bandwidth. For a particular order cut, the frequency bandwidth will increase with RPM while the order bandwidth remains constant.
Even if the percentage bandwidth is kept the same, the order bandwidth is wider for higher orders.
So, with the same percentage bandwidth, higher orders will have a wider order bandwidth.
Because higher orders can tend to smear more than lower orders, having a progressively higher bandwidth as a function of order is helpful.
Frequency / Lines Mode
The frequency and lines methods are very similar. The frequency method will use a fixed frequency bandwidth around the specified order. The lines method will use a fixed number of frequency lines around the order specified.
The lines method is highly dependent on the frequency resolution used. The finer the frequency resolution, the smaller the frequency bandwidth. The coarser the frequency resolution, the larger the frequency bandwidth.
If the frequency resolution is 1 Hz, both methods will yield the same result. If the frequency resolution is not 1 Hz, the methods will yield different results.
The frequency bandwidth is constant vs RPM.
Cutting orders in Simcenter Testlab
There are two ways to cut an order in Simcenter Testlab:
Signature Throughput Processing
To cut an order with Signature Throughput Processing, go to “Tools” -> “Add-ins” -> “Signature Throughput Processing” (36 tokens).
Enter the “Time Data Processing” workbook. Under the “Section” header, click “Change Settings”.
The “Section Settings” window will open. Navigate to the “Order Sections” tab.
This tab will contain the parameters for the order cut.
To begin, type the orders of interest into the white text box. To cut multiple orders, separate each value by a semicolon.
Orders need not be integer numbers.
A summary of the parameters
There is also the option to set an offset.
The offset controls where the center frequency of the order begins at zero RPM.
By default, the offset is zero. However, sometimes an order does not go through the origin but rather starts at an offset along the frequency axis.
In the above figure, the order does not emanate from the origin, but rather has an offset of 2500Hz.
This can be caused by things like pulse width modulation in electric motors.
It is also possible to cut an order directly from a colormap in the Navigator.
In a colormap, right click to add a cursor. Then select “Add Processing Cursor” -> “Order”.
This will open the “Cursor Processing” window.
It is possible to specify the processing parameters by right clicking on the block and selecting “Parameters”.
The “Order Section parameters” window will open. It is possible to specify the width (bandwidth) and width unit (mode) by which to calculate the order. Press “OK” to accept changes.
The section parameters will update in the “Cursor Processing” window. It is then possible to drag and drop the block into a display.
Moving the Processing cursor on the color map will cause the order to update in the display with the correspond order cut. It is an interactive cursor.
It is also possible to set the offset for the order cursor. Double click on the order cursor to specify an offset.
Using processing cursors makes interacting with easy and intuitive!
Understanding the parameters behind order section processing is important to ensure that the data is meaningful, and that comparison of multiple order sections is valid.