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Predictive maintenance & condition monitoring


First, separate the columns of data in Figure 2 into three worksheets within the Excel file, named X, Y, and Z, as shown in Figure 4. Insert a column to the left of the data—this column is for the timestamp of each data value. Since there were 500,000 samples of data


taken over a one second timespan, each data point was taken 2 μs apart. Therefore, in the first cell of the new column, enter 2E-6


representing the first timestamp at 2 μs. The easiest way to fill in the rest of the timestamp column is to use the Series command. In the Search box in Microsoft Excel, type “Series” to bring up the menu options shown in Figure 5.


Figure 3. Format of time and voltage instances.


Hardware and software capable of conducting Fourier analysis can be expensive, so here we show a method of performing Fourier analysis on MEMS data that is essentially no-cost. LTspice is a powerful, and free, circuit


simulator that has the ability to plot the frequency content of any waveform using Fourier analysis, including the vibration data obtained from the MEMS sensor in a condition-based monitoring system. LTspice can produce a Fourier analysis plot when the data is in the format shown in Figure 3, where each vibration data point is paired with its corresponding timestamp.


Figure 5. How to fill many cells in Microsoft Excel.


From here, choose Fill Series or Pattern, then Series… from the dropdown menu. The dialogue box shown in Figure 6


appears, with the Columns and Linear radio buttons selected. Enter a Step value of 2E-6 and a Stop value of 1.


Figure 7. Columns showing the timestamp and corresponding data sample.


Figure 6. Fill the cells with a linearly expanding dataset.


Click OK to fill the left column data timestamps, incrementing from 2 μs to one second. The same can be achieved by filling in the first few values, then dragging the cursor to the bottom cell at the end of the data range—but with 500,000 lines of data, this would be a long drag. The data is now in a format ready to be processed by LTspice, as shown in Figure 7. Select the appropriate number of decimal


places, as shown in Figure 9. If the dataset is large and the sample


Figure 4. Three sheets were created and the X, Y, and Z data were copied to their respective sheets.


Instrumentation Monthly October 2020


interval is low, it is possible that Microsoft Excel might round the timestamps to an inappropriate number of decimal places. If this is the case, highlight the first column,


Figure 8. Reformat the cells to remove any rounding errors.


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