Basic Principle of Strain Gauge Accelerometer

When a cantilever beam attached with a mass at its free end is subjected to vibration,

vibrational displacement of the mass takes place. Depending on the displacement of the

mass, the beam deflects and hence the beam is strained. The resulting strain is proportional

to the vibration displacement of the mass and hence the vibration/acceleration being

measured when calibrated.

Description of Strain Gauge Accelerometer

The main parts of a strain gauge accelerometer are as follows:

A cantilever beam fixed to the housing of the instrument.

A mass is fixed to the free end of the cantilever beam.

Two bounded strain gauges are mounted on the cantilever beam as shown in diagram.

Damping is provided by a viscous fluid filled inside the housing.

Operation of Strain Gauge Accelerometer

The accelerometer is fitted on to the structure whose acceleration is to be measured.

Due to the vibration, vibrational displacement of the mass occurs, causing the cantilever beam to be strained.

Hence the strain gauges mounted on the cantilever beam are also strained and due to this their resistance change.

Hence a measure of this change in resistance of the strain gauge becomes a measure of the extent to which the cantilever beam is strained.

But the resulting strain of the cantilever beam is proportional to the vibration/acceleration and hence a measure of the change in resistance of the strain gauges becomes a measure of vibration/acceleration.

The leads of the strain gauges are connected to a wheat stone bridge whose output is calibrated in terms of vibration/acceleration.

Introduction

A piezoelectric accelerometer utilizes the piezoelectric effect of certain materials to

measure dynamic changes in mechanical variables, such as mechanical shock,

vibration and acceleration. Like other transducers, piezoelectric accelerometers convert

one form of energy into another and provide an electrical signal in response to the

condition, property or quantity. Acceleration acts upon a seismic mass that is restrained

by a spring or suspended on a cantilever beam, and converts a physical force into an

electrical signal.

There are two types of piezoelectric accelerometers: high and low impedance. High

impedance accelerometers have a charge output that is converted into a voltage using

a charge amplifier or external impedance converter. Low impedance units use the same

piezoelectric sensing element as high-impedance units, and incorporate a miniaturized

built-in charge-to-voltage converter and an external power supply coupler to energize

the electronics and decouple the subsequent DC bias voltage from the output signal.

Working Principle of Piezoelectric Accelerometer

A piezoelectric accelerometer consists of a mass attached to a piezoelectric crystal

which is mounted on a case. When the accelerometer body is subjected to vibration, the

mass on the crystal remains undisturbed in space due to inertia. As a result, the mass

compresses and stretches the piezoelectric crystal. This force is proportional to

acceleration in accordance with Newton’s second law, F = ma, and generates a charge.

The charge output is then converted into low impedance voltage output with the help of

electronics.

Benefits of Piezoelectric Accelerometer

The key benefits of piezoelectric accelerometers are:

Wide frequency range

No moving parts

Excellent linearity over their dynamic range

Low output noise

Self-generating - no external power required

Acceleration signal can be integrated to provide velocity and displacement

Applications

Major applications of piezoelectric accelerometers include:

Engine testing - Combustion and dynamic stressing

Ballistics - Combustion, explosion, and detonation

Industrial/factory - Machining systems, metal cutting, and machine health monitoring

Original equipment manufacturer - Transportation systems, rockets, machine tools,

engines, flexible structures, and shock/vibration testers

Engineering - Dynamic response testing, shock and vibration isolation, auto chassis

structural testing, structural analysis, reactors, control systems and materials evaluation

Aerospace - Ejection systems, rocketry, landing gear hydraulics, shock tube

instrumentation, wind tunnel and modal testing.