Mechanical Engineering - Measurement

Multiple choice test

1 1. The Short T ime Four ier Transform of a qua si-periodic stationary signal exibits:  horizontal lines ✔ o vertical lines o none of the other o diagonal lines 2. Which is the met hod providing the lowest excitation frequency in the study of frequency response function? o non of the other o the impact hammer o the hydraulic actuator  the piezzoelectric patch or shaker ✕ 3. The CPDF c urve of a gaussian distribution o is larger than the signal RMS o none of the other  is nondimensional ✔ o is smaller than the signal RMS 4. The cross -correlation at tau = 3 between the signal X = [0; 0; 0; 1; 1; 1; 1; 0; 0; 0 ] and Y= [ 0; 0; 0; 1; 2; 2; 1; 0; 0; 0 ] is; o 1/3 o 0  1/7 ✔ o 2/7 2 5. A purely harmoni c signal has a frequency of 10.51 Hz with amplitude 4 V , the s ignal is acquired with an accele rome ter with full -scal e range of ± 10 V . The ADC used for the acquisition has a f samp of 20 Hz. A configuration that allows to correctly reconstruct the amplitude of the spectral component is: o non e of the other o number of samples = 40 . Rectangular w indow o number of samples = 40 . Hanning window  number of samples = 200. Hanning window ✕ 6. Said B th e number of bits of a converter, the LSB: o decrea ses when the converter range increases o does not depend on B o inc reases when B increases, but also depends on Vmax and Vmin  decreases when B increases ✔ 7. Describe the spectral leakag e using the convolution theorem, in less than 10 lines. 8. The minimum number of bits to sample correctly a signal with maximum amplitude of 5 mV is: o 16  12 ✕ o 24 o none of the other 3 9. The RMS of a sine wave with amplitude ± 1 V is:  0.707 V ✔ o None of the other o 2 / 2(sqrt (3)) V o 1.414 V 10. Which is the met hod providing the largest excitation force (load ) in the study of frequency response function? o the impact hammer  the hydraulic actuator ✔ o none of the other o the piezoelectric patch or shaker 11. The power spectral density:  Depends on the signal sampling frequency also if we respect Nyquist theorem ✕ o Depends on the signal cross correlation o Depends on the signal covariance o None of the other 12. The Kurtosis coefficient: o is negative when the signal has a non -symmetrical probability density function o none of the other o is negative when the signal has a sharper probability density fun ction than a Gaussian distribution  is negative when the signal has a flatter probability density function than a Gaussian distribution ✔ 4 13. If an operator is linear:  it is always additive ✔ o cannot accept multiple inputs o none of the other o the coherence is always 1, independently from the frequency 14. The average between the signals Y1= 3*cos( 2*pi*t + 2 ) and Y2= 3.1*cos( 2*pi*t + 1.5 ) is a vector rotating at 2 Hz with arguments and frequencies:  |S| = 8.20 , arg = -45.55 ° ✕ o |S| = 6.44, arg = 90 ° o |S| = 5.91 , arg = -79.96 ° o None of the other 15. The amplitude and pha se of the autospectrum of the signal Y= 2*cos( 2*pi* 1* t + 1.2 ) at the frequency of 1 Hz is : o None of the other  |Sxx | = 2, arg = 1.2 ✕ o |Sxx | = 4, arg = 1.2 o |Sxx | = 4, arg = 0 16. The asymptotic attenuation of the rectangular window is: o 10 dB/dec ade o 40 dB/decade o non e of the other s  20 dB/decade ✔ 5 17. An acceleration signal is the sum of two harmonic components: th e first has a pulsation of 157 rad/s, the second has a frequency of 3.1 Hz. Both the signals have a peak -to -peak value of 1 m/s 2 and a re acquired with accelerometer s with sensitivity of 98.065 mV/g . A configuration of the system that re duces the measurement errors is : o fsamp = 60 Hz , Number of samples = 600 , Ex ponential window , T = 10 s o fsamp = 60 Hz , Number of samples = 600 , Hanning window , T = 10 s  fsamp = 100 Hz , Number of samples = 600 , Rectangular window , T = 100 s ✕ o fsamp = 30 Hz, Number of samples = 600, Hanning window, T = 20 s 18. If an operator is stable:  it is always linear ✕ o cannot accept multiple inputs o none of the other o the coherence is always 1, independently from the frequency 19. Which window would you use to obtain the exact amplitude of a harmoni c signal with unknown frequency?  Exponential ✕ o Flat top o Rec tangular o Hanning 20. Superposition integral, the core of the linear system theory:  is imaginary odd ✕ o none of the other o is complex periodic o is real even 6 21. The Welch tecnique is: o A time series averaging technique o A statistical correlation method o A method to doble the frequency resolution  A s pectrum averaging technique ✔