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Fixed sampling and synchronous sampling are two different digital signal processing methods that can be used to calculate spectral maps and orders during a rotating machinery measurement.
Fixed sampling acquires data at a fixed data rate, while synchronous sampling acquires data at a rate proportional to the speed of the rotating machinery.
The results of each technique are shown in the colormaps in Figure 1 below. Fixed sampling gives a global overview of the speed sweep dynamics, while synchronous sampling has narrower and well defined orders.
Both techniques result in a colormap with three difference axes: frequency, rpm, and amplitude. However, the calculations involved in each technique are performed differently.
This article covers:
Note: Sometimes synchronous sampling is also called order tracking. The Simcenter Testlab software module that performs synchronous sampling is called ‘Order Tracking’.
How do fixed and synchronous sampling techniques work?
Both fixed sampling and synchronous sampling techniques calculate a series of spectrums at a defined tracking increment over a speed sweep.
Time versus Revolution Data Blocks
The difference in the two techniques is how the spectrums at each increment are calculated.
See Figure 2 for a comparison of the time data used for spectral calculations at four different speeds of the rotating system.
Once the block of data is captured, either an order or frequency spectrum is calculated.
Frequency Spectrum versus Order Spectrum
Figure 3 shows the difference between the results of a Fourier transform for fixed sampled data versus the synchronously sampled data. The fixed sampling data produces a frequency spectrum, while synchronous sampling produces an order spectrum.
A key difference between the order spectrum and frequency spectrum is how data is spaced along the x-axis (see Figure 5):
A result of this is that order content can appear “smeared” when fixed sampled, but not when order tracked.
Smearing in Fixed and Synchronous Sampling
In fixed sampling, the Fourier Transform requires a fixed time frame of data. During the fixed time frame, the speed of the rotating system changes. As the speed changes, the frequency of the order content also changes.
As the order content changes within the same data frame, there is a leakage, sometimes referred to as smearing, of the order content as shown in Figure 4.
For example, if the fixed time frame is two seconds, and the rpm changes from 1000 rpm to 1060 rpm over this time period, the frequency of 1st order would change 1 Hertz (60 rpm = 1 rev per second = 1 Hz). This would create a 1 Hz smear. For other orders, the change would be even greater. For example, 4th order would create a 4 Hz smear during the 2 seconds.
This smearing effect is worse at higher orders (example: 54th, 72nd order) than lower orders (1st, 2nd, 3rd).
The order spectrum does not have this smearing. The constant order resolution keeps spectral lines aligned with order frequencies, which prevents them from being smeared.
To reduce the smearing induced from the fixed sampling approach with respect to a changing rpm, three actions can be taken:
There are several key settings that can affect the processed results.
Table 1 below lists the equations and parameters used in the two techniques.
These same settings are illustrated in the diagrams in Figure 5.
The next section illustrates the differences in the calculated results between fixed sampling and synchronous sampling results.
Results and Differences of Fixed Sampling and Synchronous Sampling
This section shows how the results differ between fixed sampling and synchronous sampling. See Table 2 below for a summary of the key differences between each of the techniques:
Fixed Sampling Global View
A key benefit of fixed sampling is the “global view” that it provides as shown in Figure 6.
Fixed sampling highlights both resonance information alongside with the order information. In a fixed sampled map, resonances appear as vertical lines at fixed frequencies.
Synchronous Sampling: Narrow Orders
The benefit of using synchronous sampling is that a much finer order resolution can be achieved.
Look at Figure 7 below for an example of this. The orders are narrower and better defined on the synchronous sampling colormap plot (right) compared to the fixed sampling plot (left).
Because orders tend to be smeared more on fixed sampled data, caution must be used when performing order cuts. The width of the order cut is very important: when cutting an order from fixed sampled data, care must be taken to ensure that the width of the order is selected to properly contain the order content.
Order 10.5 is cut from both the fixed sampled and synchronous sampled plots from above (see Figure 8, below).
The same order width (0.2) is used for both cuts (Figure 8, left graph). Notice that the fixed sampled order (red) appears to be lower level than the order tracked order (green). This is because the order content for the fixed sample order has smeared beyond the 0.2 order width bandwidth. Therefore, the order cut is not capturing all of the energy content of the order.
However, if the order width of the fixed sampled data is increased to 0.4 order width, the results between fixed and synchronous sampling compare much more closely (Figure 8, right graph).
This is especially important when a rotating system has closely spaced orders. If significant smearing is present in a fixed sampling technique, it can be impossible to separate the orders and get the correct results, because the adjacent orders smear together.
For example, if a piece of rotating machinery made two orders: 10.1 and 10.3, a smearing of 0.4 order would not produce correct results for these orders.
Synchronous Sampling: Fast Speed Sweeps
Another area in which synchronous sampling has an advantage is fast speed sweeps. The faster the rpm changes, the more the frequency content changes over time.
The synchronous sampling techniques reduces the smearing of the orders that are caused by a fast speed sweep as shown in Figure 9.
In Figure 9, the difference between fixed sampling and synchronous sampling on a 4 second “fast” sweep and a 20 second “slow” runup is shown. For the same data orders, the fixed sampling data for the four second runup shows order smearing. The higher number orders also have more smear.
Either by slowing down the speed sweep, or by switching from fixed sampling to synchronous sampling, the order smearing can be reduced.
The next section explains how to perform fixed and synchronous sampling on speed sweep data in Simcenter Testlab.
Simcenter Testlab Settings
Both fixed sampling and synchronous sampling can be done in Simcenter Testlab. Both methods can be used simultaneously while processing the same data.
When using Signature Acquisition or Signature Throughput processing, the fixed sampling approach is used by default. However, synchronous sampling can also be turned on via the ‘Order Tracking’ add-in and used in parallel with the default fixed sampling.
Enable synchronous sampling by selecting “Tools -> Add-ins -> Order Tracking” from the main Simcenter Testlab menu as shown in Figure 10.
The Order Tracking add-in requires 20 tokens.
Fixed Sampling versus Order Tracking Settings
In Figure 11, the Fixed Sampling and Order Tracking settings in Signature Throughput Processing are shown. The Fixed Sampling settings are under the tab called “FS Acquisition” while the Order Tracking settings are under the tab called “OT Acquisition”.
There is a fixed relationship between frequency, rpm, and maximum order, as shown in Equation 1:
When the maximum frequency of the acquisition, or bandwidth, is used in Equation 1, the maximum order and corresponding rpm can also be determined. These settings and relationships are also described in Table 1 above.
For example, the software menus shown in Figure 11 indicate that 16th order can be measured up to 24000 rpm with a given frequency bandwidth of 6400 Hz. This uses the equation: [6400 Hz = (16 * rpm)/60]. This maximum rpm is automatically calculated (Figure 11, right graph). To go to a higher rpm, and still measure 16th order, the frequency bandwidth would need to be increased.
Engineers will often calculate both fixed sampled and order tracked results to get the benefits of both types of data processing.
Displays: Switching Axes between Order and Frequency
In Simcenter Testlab, it is possible to switch between frequency and order on the plot axis.
For a fixed sampled map, simply right click on the X Axis, and select “X axis -> Frequency or Derived Order” as shown in Figure 12.
For an order tracked map, simply right click on the X-axis and make the selection for either “Order” or “Derived Frequency” as shown in Figure 13.
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