Mechanical Engineering - Measurement

Multiple choice test

1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) Teaory sorusu fakat ekran alıntısını almamış 16) 17) 18) 19) 20) 21) 1 1. The Short Time Fourier Transform of a quasi-periodic stationary signal exibits:  horizontal lines ✔ o vertical lines o none of the other o diagonal lines 2. Which is the method 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 curve 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 harmonic signal has a frequency of 10.51 Hz with amplitude 4 V, the signal is acquired with an accelerometer with full-scale 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 none of the other o number of samples = 40. Rectangular window o number of samples = 40. Hanning window  number of samples = 200. Hanning window ✕ 6. Said B the number of bits of a converter, the LSB: o decreases when the converter range increases o does not depend on B o increases when B increases, but also depends on Vmax and Vmin  decreases when B increases ✔ 7. Describe the spectral leakage 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 method 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 function 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 phase 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/decade o 40 dB/decade o none of the others  20 dB/decade ✔ 5 17. An acceleration signal is the sum of two harmonic components: the 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/s2 and are acquired with accelerometers with sensitivity of 98.065 mV/g. A configuration of the system that reduces the measurement errors is: o fsamp = 60 Hz, Number of samples = 600, Exponential 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 harmonic signal with unknown frequency?  Exponential ✕ o Flat top o Rectangular 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 spectrum averaging technique ✔ 1 1. Describe the spectral leakage using the convolution theorem, in less than 10 lines. The leakage is a result of not having the precise frequency of the signal in our discrete frequencies, multiple of 1/T. It is caused by the convolution in frequency from the window spectra and the signal spectrum. Since in time we have to simply multiply them, the discrete frequencies are instead result of the multiplication in frequency of the circular convolution spectrum and one of the signal, since to periodicize the signal we have to make the convolution in time between those two functions. The windowing affects leakage in represented in the spectrum, since once the window spectrum is centered on the signal bin, the discrete frequencies has to be taken until they reach the window spectrum. The leakage can be reduced using a hanning window, which almost cancels the side lobes due to higher attenuation than the rectangular one. 2. The Fourier Transform of a rectangular window: o is periodic  is complex ✕ o none of the others o is real 3. The 99th percentile of a Gaussian distribution with mean= 0 and variance= 1 is: o -1.475 o -0.034 o 0.134  None of the other ✔ 2 4. A purely harmonic signal has a frequency of 10.51 Hz with amplitude 4 V, the signal is acquired with an accelerometer with full-scale 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 Number of samples = 200. Hanning window o Number of samples = 40. Hanning window  None of the other ✔ o Number of samples = 40. Rectangular window 5. If an operator is linear: o the coherence is always 1, independently from the frequency o cannot accept multiple inputs  it is always additive ✔ o none of the other 6. About the Fourier transform we can say that: o the Fourier transform of a real even function is an imaginary odd function  the Fourier transform of a real odd function is an imaginary odd function ✔ o the Fourier transform of a real even function is a real odd function o the Fourier transform of a real odd function is a real odd function 7. A linear system, characterized by multiple inputs with coherence smaller than 1 in specific frequency ranges, describe by: o the real component of the transfer function o the imaginary component of the transfer function o the imaginary component of the impulse response  none of the other ✔ 3 8. The asymptotic attenuation of the rectangular window is: o 40 dB/decade o 10 dB/decade  20 dB/decade ✔ o none of the others 9. The LSB of a converter with Vmax= 2.5 V and Vmin= -2.5 V and B= 12 is:  1.22 mV ✔ o 2440uV (u = micro) o None of the others o 4.88 mV 10. The modulus of a double-sided spectrum:  none of the other ✔ o depends on the signal cross correlation o depends on the signal covariance o decreases with the signal sampling frequency 11. Which of the following tools could be useful in case I would like to remove non-periodic noise from a single signal? o cross-correlation o none of the others o trigger acquisition  autocorrelation ✔ 4 12. The phase of a double-sided spectrum of a real signal: o is zero for odd signals o is symmetrical with respect to f=0 o is defined only for positive frequencies  is odd ✔ 13. In a Short Time Fourier Transform, the x and y axes show respectively: o amplitude and frequency o none of the other  time and frequency ✔ o time and amplitude 14. The CPDF curve of a Gaussian distribution: o none of the other o is larger than the signal RMS o is always increasing  is not decreasing ✔ 15. The skewness of a sine wave with RMS = 1 m/s2 , standard deviation of 1 m/s2 and a frequency of 10 Hz is: o None of the other  0 ✔ o 1 o 2 5 16. Which is the method providing the largest excitations force (load) in the study of frequency response function? o none of the other o the impact hammer o the piezoelectric patch or shaker  the hydraulic actuator ✔ 17. The synchronous sampling is: o common in monitoring impulse phenomena o achieved acquiring a signal with a sampling rate independent by the observed phenomenon o common in monitoring high frequency signals  none of the other ✔ 18. Which is the method providing the lowest excitation frequency in the study of frequency response function? o the impact hammer o none of the other  the hydraulic actuator ✔ o the piezoelectric patch or shaker 19. The average of spectra with a deterministic phase and random modulus tends to: o the real sampling frequency  none of the other ✔ o the phase is correctly detected, but the modulus tends to zero o the cross-spectral average 6 20. If an operator is linear: o the coherence is always 1, indepedently from the frequency o cannot accept multiple inputs o none of the other  it is always casual ✔ 21. The probability density function p(x) of a signal at level x indicates: o the probability that the instantenous value of the signal in between x and x + delta, where delta is 1  the probability that the instantenous value of the signal in between x and x + delta, where delta infinitesimal ✔ o the cumulative probability at x + delta o the cumulative probability at x 19. The Cres Factor: o is useful to detect the maxima of a signal if the noise is incorrelated with the signal itself o gives information about the flatness of a signal’s PDF  none of the other ✔ o is useful to detect peaks only in periodic signals 20. If an operator is stable: o the response of the system can never go to infinite o is additive o it is always linear  its response is limited if the input is limited ✔ 21. The Fourier Transform of a real, even signal is:  real, even ✔ o real, odd o imaginary, even o imaginary, odd 1 1. The Kurtosis coefficient:  none of the other ✕ o is negative when the signal has a sharper probability density function than a Gaussian distribution o is negative when the signal has a flatter probability density function than a Gaussian distribution o is negative when the signal has a non-symmetrical probability density function 2. Which window would you use to obtain amplitude of a random signal with unknown frequency components if you are not able to tune the acquisition parameters?  Hanning ✔ o Flat top o Exponential o Rectangular 3. In a Short Time Fourier Transform, the x and y axes show respectively: o none of the other o amplitude and frequency  time and frequency ✔ o time and amplitude 4. The autocorrelation of a signal at T=0 is 7 (units missing). The signal RMS is: o 3.18 o 1.13 o 7  2.64 ✔ 2 5. The CPDF curve of a gaussian distribution: o is nondimensional o is larger than the signal RMS o none of the other  is smaller than the signal RMS ✕ 6. Increasing the acquisition time T has positive effects on leakage because: o it allows to avoid aliasing o it allows to completely solve the problem of leakage o when T increases, the spectrum affected by leakage becomes as high as the correct spectrum  the change in resolution decreases the effect of both long-term and short-term leakage ✔ 7. The Fourier Transform of a real, odd signal is: o real, odd o real, even o imaginary, even  imaginary, odd ✔ 8. Superposition integral, the core of the linear system theory: o depends on the frequency resolution o none of the other  can be used to predict the frequency response of a linear system ✕ o is always imaginary 3 9. The average between the two 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: o |S| = 5.91, arg = -79.96° o |S| = 8.20, arg = -45.55° o |S| = 6.44, arg = 90°  None of the other ✔ 10. The sampling property of the Dirac function: o is periodic o it is always linear also if the signal is nonlinear o none of the other o cannot accept multiple inputs 11. The LSB of a converter with Vmax= 10 V, Vmin= 0 V and B= 12 is: o 1.22 mV o 4.88 mV o None of the other  2440 uV (u = micro) ✔ 12. The Fourier Transform of a rectangular window: o is real o non of the others  is periodic ✕ o is complex 4 13. Describe the spectral leakage using the convolution theorem, in less than 10 lines. 14. Which window would you use to obtain the spectrum of the transient (impulse) response of a lightly damped 2DOF mechanical system? o Rectangular o Flat top o Exponential o Hanning 15. The convolution: o can be used to predict the time response of LTI systems o generates aliasing o is the inverse of Fourier Transform of the transfer function  can be used create periodic signals from sampled frequency responses ✕ 16. The Fourier Transform of Dirac function: o none of the other o is complex  it is always linear ✔ o the modulus depends on the delay tau 17. The Fourier Transform of a real, even signal is: o imaginary, odd  real, even ✔ o imaginary, even o real, odd 5 18. The LSB of a converter with Vmax= 2.5 V, Vmin= -2.5 V and B= 12 is: o 4.88 mV o None of the others  1.22 mV ✔ o 2440 uV (u = micro) 19. Which window would you use to analyse the spectrum of the impulse response of a lightly damped 3DOF mechanical system? o Hanning o Flat Top o Exponential o Rectangular 20. The 60th percentile of a Gaussian distribution with mean=0 and variance=1 is: o -0.034  none of the other o 1.475 ✔ o 2.134 21. The faster signal to identify the response of a linear time invariant is: o the gaussian noise  all the signals can be used, but we must consider the nonlinear effect of leakage ✕ o a single, short lasting, harmonic signal o a practical implementation of the Dirac function