Running an FFT Measurement

To make an FFT measurement of the system, select Random as the Noise type.

Before performing an FFT measurement, several parameters must be set:

Measurement Type

Amplitude

Standard Firmware:

— Motor Number

MechaWare Firmware:

— Model number of downloaded model

— Active noise block (Custom or User Buffer)

or

— Active motor block (Closed or Open Loop)

Note: Number Samples, Number FFT Points, Overlap %, and Saving Raw Data from the FFT Test must also be set as described below.

Number Samples

Number Samples sets the number of samples of data that will be collected. Keep in mind that the amount of time needed for the test is set by the sample rate. For example, 20,000 samples at a 2,000 Hz sample rate will result in 10 seconds of excitation. Increasing the number of samples will result in a longer test and less noise in the final measurement. The number of samples must be greater or equal to the number of FFT points.

Number FFT Points

Number FFT Points sets the resolution of the FFT measurement. The resolution of the FFT measurement is the Sample Rate / Number FFT Points. Low resolutions of FFT produce the least amount of noise, whereas high resolutions of FFT yield more noise.

For example, at a 2,000 Hz sample rate and 4096 FFT points, the FFT resolution is 0.49 Hz.

Increasing the Number FFT Points will increase FFT resolution and make the measurement noisier. By lowering the FFT resolution, you are essentially averaging more than one frequency together, thereby lowering the noise. (The FFT algorithm is designed to be efficient for most numbers of FFT Points.) The FFT algorithm is most efficient when using powers of 2 for the Number FFT Points.

The parameter "Number FFT Points" must be a number that can be factored to a largest prime number of 37 or smaller. If "Number FFT Points" does not satisfy this requirement, the following error window will be shown.

If you run the Bode Tool in an automated test from the command line and choose "Num FFT Points" with a prime greater than 37, then the Bode Tool will automatically increase "Num FFT Points" for you until the largest prime number is less than 37.

Known Limitation: Number of FFT Points can not exceed 19000 points. Higher values cause truncation of data at high frequencies.

Overlap %

Use the Overlap % parameter to adjust the overlap between FFT segments. This number can be adjusted from 0 to 50. A value of 50% is a good default setting.

Saving Raw Data from the FFT Test

By default, the Bode Tool does not store the raw noise excitation data to files. To save the raw excitation data to a file, check the Save Raw Data to rawNoise.txt box. The data is saved in a file named "rawNoise.txt" in the Bode Tool working directory.

The data is in the following tab delimited format:
Sample counter [tab] excitation signal [tab] input signal [tab] output signal

Here is a section of a sample rawNoise.txt file:

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