This measurement column addresses frequently asked questions received by our customer support center and provides answers to those questions.
When using an FFT analyzer to determine the operating sound of a motor or actuator, a loud noise may be emitted from the device or fixture immediately before the operating sound you want to determine.
When a switch or electrical relay clicks first, followed by the motor starting, if the signal isn't properly isolated, extraneous sounds can be included, resulting in a high value and a false negative (NG) in the detection result. This section explains how to handle such cases. How can we isolate only the operating sound and perform an FFT?
Figure 1 shows an example of a waveform when sound is emitted from the device and the motor starts to rotate.
The device's operating noise is interfering with the signal. If we include this noise in the FFT analysis, we obtain a higher sound pressure value (dB) than the actual motor noise. This method processes the signal after the device noise has subsided.
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Figure 1
The motor sound only lasts for 0.4 seconds. We will configure the settings to capture this sound.
I want to analyze the frequency range up to 20 kHz.
The sample frequency at 20 kHz is 20000 × 2.56 = 51200 Hz.
When the number of FFT samples is 2048, the processing time is 2048 ÷ 51200 = 0.04 s, which is too short. The maximum length that can be set is 16284 samples, which results in 0.32 s.
Although it's shorter than 0.4 s, we'll use this setting. The power spectrum at this time is for 0.32 s.
This is the mean square.
Next, to avoid unwanted input, you can use the device's operating sound as a reference and avoid acquiring data for 0.1 seconds from that point. This is where the post-trigger function comes in handy. Set the trigger position to the positive side to delay data acquisition. You can delay it by a time of "position setting value × sample period".
If the delay time dt is 0.1 s,
0.1 = M × 1/51200, so M = 5120
Set the trigger position to +5120. (+ indicates post-trigger)
How should the trigger signal be handled?
There are two methods: one that uses an audio input signal to trigger the device, and another that uses an operation start signal from the device itself.
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Figure 2
In this way, by using the post-trigger function, you can capture only the signals you want to process.
The diagram below shows the timing of external control on the CF-4700.
By effectively utilizing the post-trigger function, you can capture only the desired sound and obtain accurate detection results.
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Figure 3
Frequency range: 20 kHz
Sample score: 16,384
Trigger position: +5120
Trigger source: External
Trigger level: 50% (External trigger signal voltage is 12V)
EXT IO OUTPUT : ANALYSYS : DONE
(Excerpt from the email newsletter issued on August 28, 2019)
