Weight measurement

Weight measurement

1. Weight Measurement

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Objectives
Upon completion of this topic, you will be able
to know about strain gauge load cell
•Introduction
•Construction
•Principle of operation for weight measurement
•Applications
•Advantages

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Introduction
•Load cell Strain Gauge is an accurate, stable and
reliable load cell or force transducer
•Strain gauge load cells represent the most practical
means of weighing

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Principle Introduction
•If a strain gauge is strained its cross section varies
•A relationship between resistance and the force
causing the deformation of the SG is established.

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Figure 1 schematic diagram of Strain gauge load cell
Diagram

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Principle of Operation
•It converts weight or force into electrical output which
is provided by strain gauges.
•Four strain gages are used to obtain maximum
sensitivity and temperature compensation
•Two of the gauges are usually in tension, and two in
compression

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Construction
•Wired with compensation adjustments as shown in
Figure 1
•It consists of a steel cylinder which has four identical
strain gauges mounted upon it
•The gauges R
1
& R
4
are along the direction of applied
force.

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Construction
•The gauges R
2
& R
3
are attached circumferentially at
the lateral direction of R
1
& R
4
•These four resistance strain gauges are connected
electrically as a four arms of Wheatstone bridge

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•The circuit diagram of Wheatstone bridge with
Compensation is also shown.

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Figure 2 Wheatstone Circuit without Compensation

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General Purpose Strain Gauges

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Principle of Operation
•When there is no load on the load cell, the four strain
gauges have the same resistance.
•Under this condition the bridge is balanced.
•And the output voltage is zero.

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Principle of Operation
•When the load is applied, the resistance of the
gauges R
1
& R
4
increases
•At the same time, the resistance of the R
2
& R
3

gauges decreases.
•Under this condition, the bridge is unbalanced.

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Principle of Operation
•Some output voltage is produced
•This output voltage is the measure of applied load.

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Advantages
•Temperature compensation is good
•Increases the bridge sensitivity
•Applicable for dynamic forces

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Applications
•Its largest use is in retailing shops.
•Postal and shipping scales
•Crane scales
•Laboratory scales

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Applications
•In on-board weighing for trucks
•Agricultural applications
•Petrochemical applications

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Applications
Thrust measurement on
•Rocket
•Jet engine test standards
•Lunching pads
•Wind tunnels and other branch of Aeronautical research.

21. Torque Measurement
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Definition
•Torque is defined as that force which tends to produce
rotation.
•It is the moment due to tangential force
• T=LW
• Where,
•T=torque
• L=length of the force arm
• W=force

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•Transducer is a device which converts a non-electrical
quantity into electrical.
•Torque is a rotational force.
•Torque transducers measure the torsional force applied
between the two sides of a moving body.
Definition

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It Is Characterized By
•A high torsional stiffness
•Large capacity
•High resistance to bending moments
•Minimal friction error and low error sensitivity due to the
absence of moving parts.

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Fig.1
Diagram Of Strain Gauge Type Torque
Transducer

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•During torsion of a cylinder, strain (tensile or
compressive) exists at 45
0
C to the axis
•This can be measured by bonded strain gauges
•The output can be increased by using 4 gauges, so that,
adjacent arms have strains of opposite nature
Torque Meters

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•The device is useful for both static and dynamic torque
measurements
•Torques over a range of 100 to 100,000 N-cm can be
measured with this device
Torque Meters

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