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دانشيار گروه مهندسي مكانيك ماشينهاي كشاورزيدانشيار گروه مهندسي مكانيك ماشينهاي كشاورزي13911391- - 9292سال تحصيلي سال تحصيلي

)هفتمجلسه جلسه ((درس اندازه گيري و سيستمهاي كنترل درس اندازه گيري و سيستمهاي كنترل

دانشگاه محقق اردبيليدانشكده فناوري كشاورزي و منابع طبيعي

گروه مهندسي مكانيك ماشينهاي كشاورزي

What is Strain ? Strain is the amount of deformation of a body due to an

applied force. More specifically, strain (ε) is defined as thefractional change in length.

Strain can be positive (tensile) or negative (compressive).Although dimensionless, strain is sometimes expressed in unitssuch as in./in. or mm/mm.

In practice, the magnitude of measured strain is very small.Therefore, strain is often expressed as microstrain (µε), whichis ε ×10-6.

What is a Strain Gauge ?

Strain Gauge is a device used to measure

deformation (strain) of an object.

Strain gauges have been developed for the

accurate measurement of strain

Fundamentally, all strain gauges are designed

to convert mechanical motion into an

electronic signal.

Schematic View Of Strain Gauge

The gauge shown here is primarily sensitive to strain

in the X direction, as the majority of the wire length

is parallel to the X axis.

Solder Tags -for

attachment of wires.

Insulated backing

Gauge, wire / foil approx. 0.025 mm thickX

Y

Strain Gauge

The name "bonded gauge" is given to strain gaugesthat are glued to a larger structure under stress (calledthe test specimen).

Gage Length

Gage length is animportant consideration instrain gage selection

The gage length is thedimension of the activegrid as measured insidethe grid end loops.

The gage length (GGG )ranges from 0.008 in (0.2mm) to 4 in (100 mm).

Strain Gauge Operation

This schematic showshow the strain gaugeresistance varies withstrain (deformation).

On applying a force achange in resistance takesplace.

Tension causes resistanceincrease.

Compression causesresistance decrease.

Ω Ω+

Ω-

Grid Pattern

(a) (b) (c) (d)

Uniaxial Gage with a single grid for measuring strain in the grid direction .

Biaxial Rosettes Gage with two perpendicular grids used to determine principal strains when their directions are known.

Three-Element Rosettes Gage with three independent grids in three directions for ascertaining the principal strains and their directions.

Shear Patterns Gage having two chevron grids used in half-bridge circuits for direct indication of shear strains (difference in normal strains) .

Strain Gauge Installation

The Strain Gauge is bonded to the specimen under test, only after the following:

– cleaning the surface using a degreaser

– cleaning it again with a conditioner solution (mild acid that accelerates the cleaning process)

– neutralizing by applying a base (neutralizes any chemical reaction introduced by the Conditioner)

– finally bonding it with a super glue.

The Strain Gauge has 2 leads which exhibit variation in resistance when strain is applied.

The bonded metallic strain gauge

The metallic strain gauge consists of avery fine wire or metallic foil arranged ina grid pattern.

The grid pattern maximizes the amount ofmetallic wire or foil subject to strain inthe parallel direction.

The grid is bonded to a thin backing,called the carrier, which is attacheddirectly to the test specimen.

The strain experienced by the testspecimen is transferred directly to thestrain gauge, which responds with a linearchange in electrical resistance.

Gauge factor is defined as:

Measuring Circuits

In order to measure strain with a bonded resistance

strain gauge, it must be connected to an electric

circuit that is capable of measuring the minute

changes in resistance corresponding to strain

Strain gauge is connected in a Wheatstone bridge

circuit

A strain gauge bridge circuit indicates measured

strain by the degree of imbalance

It provides an accurate measurement of that

imbalance

Wheatstone Bridge In Figure, if R1, R2, R3, and

Strain gauge are equal, and avoltage, VIN, is appliedbetween points A and C, thenthe output between points Band D will show no potentialdifference.

However, if R4 is changed tosome value which does notequal R1, R2, and R3, thebridge will become unbalancedand a voltage will exist at theoutput terminals.

The variable strain sensor hasresistance Rg, while the otherarms are fixed value resistors.

Wheatstone Bridge

The sensor, however, can occupy one, two, or

four arms of the bridge, depending on the

application.

The total strain, or output voltage of the circuit

(Vout) is equivalent to the difference between

the voltage drop across R1 and R4, or Rg.

It is given by Vout = Vcd – Vcb

Force vs. Mass measurements

Balance systems compare weight of the known etalon with

the force applied to the measuring arm:

FeF

• If the measured force, F is the weight of a stationary object,

then we balance the mass of the object with the mass of the

etalon.

Force Measurement By An Elastic

Element

Elastic elements are often employed for force

measurements via stress/strain Hooke’s law:

F Ky=

Where K is the deflection constant and y is the

deflection at some characteristic point. Table 13.1 in

the textbook lists some characteristic cases:

Force Measurement By An Elastic Element

Important note: Do not confuse the deflection constant K with the

Bridge Constant, also labeled by K!

Load cells مقدمه اي بر اندازه گيري گشتاور چرخشيمقدمه اي بر اندازه گيري گشتاور چرخشي

روشهاي اندازه گيري گشتاور چرخشيروشهاي اندازه گيري گشتاور چرخشي روشهاي اندازه گيري گشتاور چرخشيروشهاي اندازه گيري گشتاور چرخشي

روشهاي اندازه گيري گشتاور چرخشيروشهاي اندازه گيري گشتاور چرخشي روشهاي اندازه گيري گشتاور چرخشيروشهاي اندازه گيري گشتاور چرخشي

روشهاي اندازه گيري گشتاور چرخشيروشهاي اندازه گيري گشتاور چرخشي دينامومترهاي پيچشيدينامومترهاي پيچشي

روشهاي اندازه گيري گشتاور چرخشيروشهاي اندازه گيري گشتاور چرخشي حلقه هاي تماسي لغزشي در مبدل گشتاورحلقه هاي تماسي لغزشي در مبدل گشتاور

حلقه هاي تماسي لغزشي در مبدل گشتاورحلقه هاي تماسي لغزشي در مبدل گشتاور روشهاي اندازه گيري گشتاور چرخشيروشهاي اندازه گيري گشتاور چرخشي

استفاده از حلقه هاي لغزشي جهت انتقال داده هاي گشتاوراستفاده از حلقه هاي لغزشي جهت انتقال داده هاي گشتاور پيچش سنجپيچش سنج

استفاده از سنسورهاي مجاورتي در اندازه گيري گشتاوراستفاده از سنسورهاي مجاورتي در اندازه گيري گشتاور دينامومتر تسمه ايدينامومتر تسمه اي

دينامومتر دنده ايدينامومتر دنده اي دينامومتر دنده ايدينامومتر دنده اي

دينامومتر دنده ايدينامومتر دنده اي دينامومتر دنده ايدينامومتر دنده اي

دينامومتر رانشيدينامومتر رانشي دينامومتر جذبيدينامومتر جذبي

دينامومتر جريان گردابيدينامومتر جريان گردابي دينامومتر جذبي مكانيكيدينامومتر جذبي مكانيكي

دينامومتر جذبي مكانيكيدينامومتر جذبي مكانيكي دينامومتر جذبي مكانيكيدينامومتر جذبي مكانيكي

دينامومتر جذبي فراوددينامومتر جذبي فراود ترمز پرونيترمز پروني

دينامومتر جذبي هيدروليكيدينامومتر جذبي هيدروليكي دينامومتر جذبي هيدروليكيدينامومتر جذبي هيدروليكي

تعيين محدوده كاري دينامومترهاي جذبيتعيين محدوده كاري دينامومترهاي جذبي