Vibration Measurement

# Introduction to Vibration measuring devices

## Definition

Basically, vibration is oscillating motion of a particle or body about a fixed reference point. Such motion may be simple harmonic (sinusoidal) or complex (non-sinusoidal). It can also occur in various modes – such as bending or translational modes – and, since the vibration can occur in more than one mode simultaneously, its analysis can be difficult.

### Units of Vibration

The units of vibration depend on the vibrational parameter, as follows:

a) acceleration, measured in g or [m/s2] ;
b) velocity, measured in [m/s] ;
c) displacement, measured in [m].

## Some effects of Vibration

Vibration can cause damage to structures and machine sub-assemblies, resulting in mis-operation, excessive wear, or even fatigue failure. Vibration may have adverse effects on human beings. The primary effects are task-performance
interference; motion sickness; breathing and speech disturbance; and a hand-tool disease known as “white finger”, where the nerves in the fingers are permanently damaged, resulting in loss of touch sensitivity

## Vibration-measuring devices

Vibration Transducers

#### Stroboscope Method

The fixed pointer or stud, shown in figure , is attached to the vibrating surface and is used to give an indication of the displacement only. By using the light of a stroboscope to “freeze” or “slowly move” the stud, quite high-frequency small-amplitude vibrations may be measured. The typical upper range of frequency is quoted at 500 Hz for direct measurement

#### Reed Vibrometer

The variable-length reed vibrometer shown in figure  is used to measure the main frequency component of the vibration. In practice the length l is adjusted until the maximum reed vibration occurs, when its resonant frequency is the same as the frequency of the vibrating mechanism or structure. The length l is calibrated directly in Hz. A small mass may be added to the cantilever if the vibrometer is to be used for very-low frequency investigation, but the scale readings would then need to be corrected for the additional mass. The range of measurement is quoted as 5 Hz to 10kHz.

### Seismic-Mass Transducer

In instrumentation, seismic pickups are used to measure the motion of the surfaces to which they are fixed. They are sensitive to motion along one axis only, so if the motion is three dimensional, three seismic pickups are needed to determine the components of the motion along three mutually perpendicular axes. The principal features of a seismic pickup are shown diagrammatically in figure. The essential component is the seismic mass. This is a body of metal, suspended from a resilient support. This is a support whose deflection is proportional to the force applied to it. The inertia of the seismic mass causes it to lag behind the motion of the casing when the casing is accelerated, causing a
deflection in the support. This deflection forms the input to a transducer, which produces a proportional output signal. In Figure  the transducer is represented by a potentiometer, but any suitable type of transducer may be used The damping shown in figure may consist only of the hysteresis of the support material, or it may be increased by filling the casing with a silicone fluid of suitable viscosity for example. By choosing suitable values for the mass, the stiffness of the support and the damping, and by using an appropriate transducer, the same basic arrangement of seismic pickup can be designed as a displacement pickup, a velocity pickup or an acceleration pickup (accelerometer). The seismic pickup is essentially a damped spring-mass system, and will have a natural frequency , unit rad/s, of vibration given by

• k is spring stiffness (N/m)
• m is the mass (kg)