The document describes a vibration measuring instrument used to measure displacement of vibrating systems. It consists of a frame with a seismic mass supported by a spring and damper. The mass vibrates along with the vibrating body and its displacement is measured relative to a scale on the frame. For large frequency ratios of the vibrating body to the instrument, the instrument can accurately measure the amplitude. With sufficient damping, it provides a good approximation over a wide frequency range, allowing high frequencies to be measured using a low frequency instrument.

2. Introduction
• The primary purpose of the vibration measuring instrument is to give an output signal
which represents the vibration phenomenon.
• The phenomenon may be displacement, velocity or acceleration of a vibrating system.
• Vibration measuring devices having mass, spring, dash pot, etc. are known as seismic
instruments.

3. K
C
• It is used to measure the displacement of a vibrating
System
• It consists of a frame in which the seismic mass (m) is
supported by means of a spring (k) and damper (c)
• The frame is mounted on a vibrating body and
vibrates along with it
• A scale is fixed on the frame and a pointer on the
seismic mass
Displacement measuring instrument / Vibrometer
Vibrating Body

6. Let us consider equation
Let us assume ω/ωn=r in the above equation
Y
Y

7. Amplitude Ratio(Z/Y):
Characteristics of above equation is plotted as shown in fig.

8. • From figure, it can be seen that for large values of r(=ω/ωn), the ratio Z/Y approaches unity
irrespective of the value of damping.
• It implies that provided r is large, the amplitude recorded is approximately equal to the amplitude of
vibrating body.

9. • Further more, it can be seen from figure that if damping factor(r) is about 0.70 or a little higher, it is
possible to have a better approximation of relation (Z/Y=1) over a large range of frequency ratio.
• So, Vibrometer (AKA low frequency transducer) can be used to measure the high frequency ω of a
vibrating body by having the instruments of low natural frequency ωn usually ranging from 1-5 Hz.

10. Phase Difference:
The phase angle plot shown below indicates the phase lag of the seismic mass with respect to vibrating base of machine