Basic Principle of Strain Gauge Accelerometer. Description of Strain Gauge Accelerometer

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Ferdinand King
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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.

1 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.

2 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.

3 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.

Lecture 20. Measuring Pressure and Temperature (Chapter 9) Measuring Pressure Measuring Temperature MECH 373. Instrumentation and Measurements

Lecture 20. Measuring Pressure and Temperature (Chapter 9) Measuring Pressure Measuring Temperature MECH 373. Instrumentation and Measurements MECH 373 Instrumentation and Measurements Lecture 20 Measuring Pressure and Temperature (Chapter 9) Measuring Pressure Measuring Temperature 1 Measuring Acceleration and Vibration Accelerometers using