No.29 How to judge the operating sound? (capturing the signal with
post-trigger function)

When using the FFT analyzer to judge the operating sound of motor or actuator, a loud sound may be generated from the equipment or jig right before capturing the operating sound. If switch or electric relay sound is generated first, and then the motor operates, extra sounds will be captured unless the signal is cut out well. As a result, the judgment will be NG. We will explain how to capture the operating sound only and perform FFT.

The waveform example is shown in Figure 1 when the sound is generated from equipment and motor starts. If FFT analysis is performed including the operating sound of the equipment, sound pressure value (dB) that is larger than the original motor sound will be obtained. Please let us explain how to process the signal after the operating sound of the equipment has faded out.

The motor sound is keeping only for 0.4 s. Set to capture this period. If the frequency range is 20 kHz, the sampling frequency is 51200 Hz= 2000 x 2.56. If the sampling frequency is 2048 points, time window length is 0.04 s which is too short. The maximum length is 0.32 s at 16284 points. It is shorter than 0.4 s, however, this setting is used. The power spectrum is square mean value for 0.32 s.

The way not to capture the unnecessary part is not to take the data for 0.1 s from the position where the operating sound of equipment starts. The post-trigger function is useful. Set the trigger position to the plus side to delay data acquisition. Trigger position x Sampling frequency = The length of delay time. If the delay time (dt) is 0.1s, trigger position is M, M is 5120 = M x 1/ 51200= 0.1. Trigger position is +5120. (+ is post-trigger)

Which trigger function is used? One is the input signal is used as trigger signal, another is the operating signal from equipment is input as a sampling start timing signal.

By using the post-trigger function, you can capture only the signal you want to process.

The following chart is a timing chart when using external trigger on CF-4700. By using the post-trigger function well, only the target sound can be captured and the correct judgment result can be obtained.

(H.K)