Page 93 - Thomson, William Tyrrell-Theory of Vibration with Applications-Taylor _ Francis (2010)

80 Harmonically Excited Vibration Chap. 3

Figure 3.11-2. Response of a vibration-measuring instrument.

displacement Z approaches V regardless of the value of damping as indicated in
Fig. 3.11-2. The mass m then remains stationary while the supporting case moves
with the vibrating body. Sueh instruments are called seismometers.
One of the disadvantages of the seismometer is its large size. Because Z = Y,
the relative motion of the seismic mass must be of the same order of magnitude as
that of the vibration to be measured.
The relative motion z is usually converted to an electric voltage by making
the seismic mass a magnet moving relative to coils fixed in the case, as shown in
Fig. 3.11-3. Because the voltage generated is proportional to the rate of cutting of
the magnetic field, the output of the instrument will be proportional to the velocity
of the vibrating body. Sueh instruments are called velometers. A typical instrument

Figure 3.11-3.