# Earthquake Engineering Questions and Answers – Seismic Theory – Accelerometer

This set of Earthquake Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Seismic Theory – Accelerometer”.

1. Which of the following is measured by setting the tuning ratio nearly zero?
a) Displacement
b) Velocity
c) Acceleration
d) Damping

Explanation: To measure accelerations, we set the tuning ratio nearly zero i.e., making the natural frequency ωn >> ω. Whereas to measure the displacement or velocity pickup have a very low value of natural frequency ωn.

2. Which of the following is an ideal condition?
a) Phase shift is same for waves of all frequencies
b) Phase shift is a logarithmic function of frequency
c) Phase shift is a linear function of frequency
d) Phase shift is different for waves of different frequencies

Explanation: If the phase shift is a linear function of frequency, then the resulting output signal will have the same shape as the input signal with a small, constant phase shift. A transducer element with damping of about 70% of critical damping possesses a phase shift frequency curve which is almost a straight line.

3. For what value of damping, the phase shift frequency curve is a good approximation of a straight line?
a) 5% of critical damping
b) 40% of critical damping
c) 70% of critical damping
d) 90% of critical damping

Explanation: A transducer element having a damping of about 70% of critical possesses a phase shift frequency curve that is a good approximation to a straight line. If the phase shift is a linear function of frequency, then the resulting output signal will have the same shape as the input signal with a small, constant phase shift.

4. Which of the following can be used in place of a mechanical sensor to pick up the ground vibration?
a) Force balance accelerometer
b) Servo seismometer
c) Accelerograms
d) Odometer

Explanation: A Force Balance Accelerometer (FBA) or servo-accelerometer is used instead of mechanical sensor, to pick up the ground vibration. Significant reduction of cross-axis sensitivity and broadening the frequency range of measurement are few advantages of FBA’s over mechanical accelerometers.

5. What happens if the phase shift is same for all frequencies?
a) Gives a distorted signal
b) Output signal is simply shifted a little in time
c) Output signal has same shape of the input signal with small phase shift
d) Output signal is a logarithmic function of input signal

Explanation: If the phase shift is same for waves of all frequencies, then the resulting signal is simply shifted a little in time. This little shift is of no consequence for the calculations of structural response. If the phase shift is different for different frequencies, we obtain a distorted signal.

6. The measurements of analog instruments are more reliable and accurate in comparison with those of digital accelerometers.
a) True
b) False

Explanation: The measurements of digital accelerometers are more reliable and accurate in comparison with those of analog instruments. The availability of pre-event data substantially reduces the uncertainties associated with the initial displacement and velocity of the ground motion.

7. What is used to sense the relative displacement of the transducer mass M?
a) Variable capacitance
b) Coil
c) Diaphragm
d) Sensing axis

Explanation: Variable capacitance with a sensitivity D is used to sense the relative displacement of the transducer mass M caused by applied acceleration. It is converted to voltage output. This voltage is sent to an amplifier of gain k and a velocity sensing pickup.

8. When does the force of gravity on the seismic mass is same as the force of inertia due to acceleration of 9.81 m/s2?
a) Angle between the sensing axis and vertical is 300 degrees
b) Angle between the sensing axis and vertical is 180 degrees
c) Angle between the sensing axis and vertical is 90 degrees
d) Angle between the sensing axis and vertical is 0 degrees

Explanation: When angle between the sensing axis and vertical is 0, the force of gravity on the seismic mass is same as the force of inertia due to acceleration of 9.81 m/s2. The accelerometer is mounted on a tilting table and the angle is measured. At any other angle, the corresponding acceleration is 9.81 cos θ m/s2.

9. Which of the following is not an advantage of Force Balance Accelerometer (FBA) over mechanical accelerometers?
a) Broadening the frequency range of measurement
b) It is possible to alter the natural frequency
c) Increase in cross-axis sensitivity
d) It is possible to alter the damping of the transducer

Explanation: When we use Force Balance Accelerometer (FBA) over mechanical accelerometers, there is significant reduction of cross-axis sensitivity. This is because of the practically zero relative movement of the mass. Other advantages of FBA are the broadening of frequency range of measurement and the possibility to alter the natural frequency and damping of the transducer.

10. What is the change in acceleration, if the change in angle is 180 degrees?
a) g
b) 2g
c) 3g
d) 0.5g

Explanation: 180-degree rotation produces a change of 2g in acceleration. For 90-degree rotation there is change of 1g in acceleration. For any other angle, the corresponding acceleration is found out by 9.81 cos θ m/s2.

Sanfoundry Global Education & Learning Series – Earthquake Engineering.

To practice all areas of Earthquake Engineering, here is complete set of Multiple Choice Questions and Answers.

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