Elastic force device

ELASTIC FORCE DEVICES

Presented by- Bhupendra kumar singh

These are important device for the measurement of both static and dynamic forces.

The force applied to the elastic member result in a displacement or strain in the elastic member, which is sensed by mechanical or electro-mechanical mean.

INTRODUCTION

Axially loaded elastic member;-

E= Young modulus K= stiffness

Relations b/w strain and stiffness for some of the member-

Cantilever-type elastic member;-

1- Mechanical method

2-Electromechanical methods (i) LVDT (ii) Piezo-electric transducer (iii) Resistance strain gauge

Method of measure the deflection of the elastic element:-

A mechanical method involving the use of a dial gauge .

The elastic ring, called the proving ring.

Mechanical method:-

Linear variable-differential transformer(LVDT)

The linear variable-differential transformer(LVDT) is the most widely used inductive transducer to translate linear motion into electrical signal

Electromechanical methods

Any physical displacement of the core causes the voltage of one secondary winding to increase while simultaneously, reducing the voltage in the other secondary winding.

The difference of the two voltages appears across the output terminals of the transducers and gives a measure of the physical position of the core and hence the displacement.

principle:

The transducers possess a high sensitivity.

The transducers have low hysteresis and hence repeatability is excellent under all conditions.

They have infinite resolution.

They are simple,light in weight and easy to maintain.

Advantages of LVDT

They are sensitive to stray magnetic fields but shielding is possible.

They are inherently low in power output.

Temperature affects the performance of transducer.

Disadvantages of LVDT

The LVDT can be used in all applications where displacements ranging from fraction of a mm to a few cm have to be measured.

Acting as a secondary transducer it can be used as a device to measure force, weight and pressure.

Uses:

Appearance of an electric potential across certain faces of a crystal when it is subjected to mechanical pressure

Conversely, when an electric field is applied to one of the faces of the crystal it undergoes mechanical distortion.

PIEZOELECTRIC EFFECT

The material that show significant and useful piezoelectric effect fall under three main groups:

1. Natural, Eg: Quartz, Rochelle salt2. Synthetic, Eg: Lithium Sulphate,

Ammonium Dihydrogen Phosphate3. Polarized Ferroelectric Crystals, Eg: Barium

Titanate, Lead Zirconate-Titanate

MATERIALS USED

There are two families of constants which are used to describe the piezoelectric effect:• g constant• d constant

These are written as gij and dij Where, i= direction of electric effect j= direction of mechanical

effect

PIEZOELECTRIC TRANSDUCERS

Applied Piezo: Applications Piezoelectric materials are used in numerous

applications. The direct piezoelectric effect is used in e.g. generators (e.g. gas igniters) sensors (e.g. accelerometers, pressure

sensors) switches (e.g. in control panels) ultrasonic transducers (e.g. in medical

imaging)

APPLICATIONS

A strain gauge is an example of passive transducer that converts a mechanical displacement into a change of resistance.

A strain gauge is a thin, wafer-like device that can be attached to a variety of materials to measure applied strain.

STRAIN GAUGE

The majority of strain gauges are foil types, available in a wide choice of shapes and sizes to suit a variety of applications. They consist of a pattern of resistive foil which is mounted on a backing material.

They operate on the principle that as the foil is subjected to stress, the resistance of the foil changes in a defined way.

STRUCTURE

The strain gauge is connected into a Wheatstone Bridge circuit. The change in resistance is proportional to applied strain and is measured with Wheatstone bridge.

WORKING

The sensitivity of a strain gauge is described in terms of a characteristic called the gauge factor, defined as unit change in resistance per unit change in length, or

Gauge factor is related to Poisson's ratio µ by,

K=1+2 µ

Based on principle of working :• Mechanical • Electrical• Piezoelectric

Based on mounting : • Bonded strain gauge• Unbonded strain gauge

Based on construction : • Foil strain gauge• Semiconductor strain gauge• Photoelectric Strain gauge

TYPES

Advantages There is no moving part. It is small and inexpensive.

Disadvantages It is non-linear. It needs to be calibrated.

ADVANTAGES & DISADVANTAGES

Residual stress Vibration measurement Torque measurement Bending and deflection measurement Compression and tension measurement Strain measurement

APPLICATIONS

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