Chapter 12, Problem 12.11P

To determine

The recent stage of the developments in the instruments and data collection process for earthquake detection and monitoring, such as the force balance accelerometer.

Explanation of Solution

The recent development shows that the servo force balance accelerometer provide better performance and accuracy over the conventional accelerometers. These force balance accelerometer comprises a freely suspended mass controlled by an electrical equivalent mechanical spring unlike the conventional accelerometers. For these reasons, these have wide applications for earthquake detection and monitoring with the utmost accuracy of 0.1% or less.

The servo force balance accelerometers can be broadly classified as the pendulous type, comprising an unbalanced pivoting mass providing angular displacement, and the non-pendulous type, comprising a mass which displaces rectilinearly. The basic operating principle for all these accelerometers is the Newton’s second law of motion which states that in order to accelerate a mass$\text{'m'}$ to an acceleration value$\text{'}a\text{'}$ force equals to this mass times acceleration must be applied. Thus, for pendulous type accelerometer, the following relation has been stated:

$T=\left(ml\right)a$

Where,

$l$ is the distance between axis of rotation and the center of mass.

$m$ is the mass of the pendulum.

These force balance accelerometers are designed for measuring DC and low frequencies, which are encountered during the motion of motor vehicles, aircrafts and ships. These accelerometers can measure levels from as low as$0.0001\text{g}$ to$200\text{g}$ for a frequency ranging from DC to$1\text{kHz}$. The force balance accelerometers are also employed for measuring angles of inclination due to their better performance and accuracy. These sensors as an inclinometer has wide applications in gun sight control, borehole mapping, pipeline leveling, platform leveling and other low level seismic measurement applications.