Mechanical Engineering - Measurement

Multiple choice test

8/7/2021 Meas 2021 July 8 Part 2 https://forms.office.com/Pages/ResponsePage.aspx?id=K3EXCvNtXUKAjjCd8ope60_E5LGpsJVDo_abJAuvijNUMEhCSTJRR1pXMThOVDFLW … 1/10 Meas 2021 July 8 Part 2 Every question can have more than a single good answer Ciao Luca, quando invierai il modulo, il proprietario potrà vedere il tuo nome e l'indirizzo di posta elettronica. 1 Cross spectrum and cross correlation (2 punti) They are linked through the coherence function There is no mutual link They are the same thing, just expressed with different terms The cross spectrum is the Fourier Transform of the cross correlation function The phase of the two functions has to be the same 2 The product of a spectral line by its complex conjugate gives (2 punti) The imaginary part The real part The phase The amplitude squared The amplitude 8/7/2021 Meas 2021 July 8 Part 2 https://forms.office.com/Pages/ResponsePage.aspx?id=K3EXCvNtXUKAjjCd8ope60_E5LGpsJVDo_abJAuvijNUMEhCSTJRR1pXMThOVDFLW … 2/103 The following are the spectra of different windows, by which it is possible to appreciate the filtering capability, just looking at their shape. The rectangular window is (1 punto) d) b) c) e) a) 4 The following are the spectra of different windows, by which it is possible to appreciate the filtering capability, just looking at their shape. The Flat-top window is (1 punto) 8/7/2021 Meas 2021 July 8 Part 2 https://forms.office.com/Pages/ResponsePage.aspx?id=K3EXCvNtXUKAjjCd8ope60_E5LGpsJVDo_abJAuvijNUMEhCSTJRR1pXMThOVDFLW … 3/10b) e) c) d) a) 5 The image is taken from a quartz accelerometer datasheet. Let’s consider the two lines related to Spectral noise in two different bandwidths: these data express some quantity measured as the output when no acceleration is provided to the accelerometer. These values are given in terms of (2 punti) 8/7/2021 Meas 2021 July 8 Part 2 https://forms.office.com/Pages/ResponsePage.aspx?id=K3EXCvNtXUKAjjCd8ope60_E5LGpsJVDo_abJAuvijNUMEhCSTJRR1pXMThOVDFLW … 4/10Autocorrelation Covariance RMS values Autospectrum Peak values Spectrum Power spectral density 6 I have the spectrum of a signal on top and the situation after having sampled it. Which of the following are true? (3 punti) 8/7/2021 Meas 2021 July 8 Part 2 https://forms.office.com/Pages/ResponsePage.aspx?id=K3EXCvNtXUKAjjCd8ope60_E5LGpsJVDo_abJAuvijNUMEhCSTJRR1pXMThOVDFLW … 5/10In the picture a) represents the sampling frequency The situation represented in the second picture is a good sampling situation Sampling is equivalent to convolving the original signal by a sequence of pulses In the picture b) represents the sampling frequency The situation represented in the second picture represents aliasing Sampling is equivalent to multiplying the original signal by a sequence of pulses 7 The function given in the picture, defined in the frequency domain (4 punti) 8/7/2021 Meas 2021 July 8 Part 2 https://forms.office.com/Pages/ResponsePage.aspx?id=K3EXCvNtXUKAjjCd8ope60_E5LGpsJVDo_abJAuvijNUMEhCSTJRR1pXMThOVDFLW … 6/10Is a cosine function a=1/2T where T is the window length b=2/T where T is the window length Is the Fourier transform of the Hanning window a=1/T where T is the window length Is the sinc function Is the Fourier transform of the rectangular window 8 In case I apply the inverse Fourier transform to get the time history back from its spectral representation, can I perform this operation if I have the power spectrum? (3 punti) no I can't. when i perform the power spectrum i lost the information about the phase 8/7/2021 Meas 2021 July 8 Part 2 https://forms.office.com/Pages/ResponsePage.aspx?id=K3EXCvNtXUKAjjCd8ope60_E5LGpsJVDo_abJAuvijNUMEhCSTJRR1pXMThOVDFLW … 7/109 I have the time record given on top, which is made of the two harmonic components given in the second image. Which of the following apply? (4 punti) The Hanning window is probably the best choice to properly analyse the signal in the frequency domain I can use the rectangular window without any problem At least one of the two harmonics will be affected by leakage Both harmonics are affected by leakage The chosen window length prevents from any kind of leakage 10 I have a sequence of pulses equally spaced every 2s. Its spectrum is (3 punti) 8/7/2021 Meas 2021 July 8 Part 2 https://forms.office.com/Pages/ResponsePage.aspx?id=K3EXCvNtXUKAjjCd8ope60_E5LGpsJVDo_abJAuvijNUMEhCSTJRR1pXMThOVDFLW … 8/10a) c) b) 11 FRF (3 punti) The phase of H2 is the same as that of H1 In case I have a single record of the input and the output, then Coherence can be less than 1 if I have noise at the input H1 is only imaginary Once I know the spectrum of the input signal Gxx, the estimate H1 can be enough to get the cross spectrum Gxy The different estimates H1 H2 and Ha can be exactly the same under specific conditions Coherence can be obtained from H1 and H2 only if I do not average 8/7/2021 Meas 2021 July 8 Part 2 https://forms.office.com/Pages/ResponsePage.aspx?id=K3EXCvNtXUKAjjCd8ope60_E5LGpsJVDo_abJAuvijNUMEhCSTJRR1pXMThOVDFLW … 9/1012 Is it possible to get the cross spectrum out of the coherence function? (4 punti) Yes, I can easily get the cross spectrum from coherence without knowing anything else Coherence espresses the level of correlation between two signals, at each single frequency No there is no way Yes I can get the complete complex form of the cross spectrum from coherence; I just need the autospectra of the two functions to be considered I can only get the amplitude of the cross spectrum as, even in case I know the autospectra of the two function under analysis, I miss any info about the phase 13 An amplifier has a gain of 0 dB (3 punti) 0 dB means amplifying 100 times The output is 20 times higher than the input, as this is the coefficient in the formula expressing the dB It nulls the output, so it cannot be considered a real amplifier The output is the same as the input In reality such a gain is typical of a filter 14 The following are the spectra of different windows, by which it is possible to appreciate the filtering capability, just looking at their shape. The Hanning window is (1 punto) 8/7/2021 Meas 2021 July 8 Part 2 https://forms.office.com/Pages/ResponsePage.aspx?id=K3EXCvNtXUKAjjCd8ope60_E5LGpsJVDo_abJAuvijNUMEhCSTJRR1pXMThOVDFL … 10/10 Questo contenuto è creato dal proprietario del modulo. I dati inoltrati verranno inviati al proprietario del modulo. Microsoft non è responsabile per la privacy o le procedure di sicurezza dei propri clienti, incluse quelle del proprietario di questo modulo. Non fornire mai la password. Con tecnologia Microsoft Forms | Privacy e cookie | Condizioni per l'utilizzoInvia d) b) c) e) a)