load cell & its application in textile

8/2/2019 Load Cell & Its Application in Textile

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Load Cells & its

application inTextiles


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What is a Load Cell ? Introduction

A load cell is a transducer that converts aload acting on it into an analog electricalsignal. This conversion is achieved bydeformation of strain gauges which arebonded to the load cell beam and wired intoa Wheatstone Bridge configuration.

The heart of any weighing system is the load cell . Load cells are highly accurate transducers.

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Types of Load Cell

Mechanical & Strain Gauge

In order to get maximum benefit from this device, we need to have a betterunderstanding of the different principles used by this device. We will take aquick look at the different type of Load Cell -

Load Cell

Mechanical Strain Gauge

Hydraulic Pneumatic

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Strain Gauge Load Cell

Principle

L L

R R

/

/ Gauge Factor K s =

Where K s is Gauge Factor

R/R = K

s

. L

L

Strain =

The principle of operation of the device is based on the fact that resistance ofan electrical conductor changes with a ratio R/R, if a stress is applied suchthat its length changes by L/L.

Where R is the change in resistance from the unstressed value

L is the change in length from original unstressed length

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Structure - Look inside the cell


The most commonly used Strain gauge loadcells comprises of foil type strain gauge.It has metal foil photo-etched in a gridpattern on the electric insulator of thethin resin and gauge leads attached. This

arrangement is safely mounted on abacking material using some high qualityadhesive.

Foil is normally made ofCopper Nickel alloy.

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How to measure strain Wheatstone Bridge

Since R may be regarded extremely larger than R,

If R1

= R2

= R3

= R4

= R,

Suppose the resistance R 1 is a strain gage and it changes by R due to strain.Then, the output voltage is,

Strain-initiated resistance change is extremely small. Thus, for strain measurementa Wheatstone bridge is formed to convert the resistance change to a voltagechange. The output voltage e o (V) is obtained with the following equation:

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Types of Strain Gauge Load Cell

Quarter Bridge

Type of Wiring Used Half Bridge

Full Bridge

Strain gauge load cells can be classified in two ways - invented in 1938 by Edward E. Simmons and Arthur Ruge

Tension Load CellArea of application Compression Load Cell

Bending Load CellShear Load Cell

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Strain Gauge Load CellQuarter Bridge Circuit

)2

.1(

1..

4

10

s

s EX K

K V V

In a quarter bridge circuit, a strain gauge is connected to a side of the bridgeand a fixed resistor is inserted into each of the other 3 sides.

Let, the nominal resistance of the strain gauge isdesignated as R G, then the strain-induced change inresistance, R, can be expressed as R = RGK s .Assuming that R 1 = R2 and R3 = RG, the bridgeequation above can be rewritten to express V 0 /VEX asa function of strain,

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Strain Gauge Load CellHalf Bridge Circuit

.2

10s

EX

K V V

In a half bridge circuit, two active strain gauges are connected, one in thedirection of axial strain, and the other perpendicular to the principal axis ofstrain (Poisson Gauge).

V0 /VEX can be expressed as,

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Strain Gauge Load CellFull Bridge Circuit

.0 s EX

K V V

In a full bridge circuit, four active strain gauges are connected, two gauges forcompression and two for tension.

V0 /VEX can be expressed as,

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Strain Gauge Load CellErrors & how to compensate

Where

T = Thermally induced strainDummy gauges connected perpendicular to the strain gauge measuring thestrain, can nullify the effect the temperature. The strain has little effect onthe second gauge, called the dummy gauge. However, any changes intemperature will affect both gauges in the same way. Because the temperaturechanges are identical in the two gauges, the ratio of their resistance does notchange, the voltage V

Odoes not change, and the effects of the temperature

change are minimized.

In a quarter bridge circuit, the main disadvantage is the effect of temperature .In such cases,

).( T M sK R R

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Strain Gauge Load CellApplication in Textiles ......

The Tensile Testing Instrument -

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Strain Gauge Load CellApplication in Textiles contd......

Warp Tension Control on Weaving M/C -

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Strain Gauge Load CellOperating range

Measurable Strain 2-4% max

Thermal Output 20 -160 C +- 2/ C

160 -180 C+-

5/ C

Nominal Resistance 120

Nominal Resistance Tolerance +-0.5%

Gauge Factor 2.0 approx

Fatigue life >10,000,000 cycles at +-1500

Foil material Copper nickel alloy

Price varies from $4.9 - $14.9 per strain gauge element

$

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Strain Gauge Load CellAdvantages & Disadvantages

Advantages :-Low costCan be used for both expansion and compression

Highly accurate for small scale applications, Accuracy Rating

0.07% e.g.Weighing Balance, Tensile Testers

Disadvantages :-Strain Gauge load cells can not be used for pressure measurement of the

highly reactive or corrosive materials, which can damage the gauge material.Amplification is required for output display because the signal produced by the

gauge is in the order of milivolts.Not suitable to measure very high loads.

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Hydraulic Load CellPrinciple

This type of load cell works on the force balance principle .

FORCE BALANCE PRINCIPLE :-

The inertial force produced by a seismic ground motion shifts the mass from itsoriginal equilibrium position, and the change in position or velocity of the massis then interpreted into an electric signal. This principle is for low range loadcells.

For long range load cells the inertial force isbalanced with an electrically generated forceso that the mass moves as low as possible.

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Hydraulic Load CellHow it works a look inside the load cell

Hydraulic load cell consists of a fluid which act as a forcetransfering medium. When the pressure or weight by thevessel or column is applied on the diaphragm of the loadcell it sense that force and then transfers this force tothe fluid which is filled in the casing of this load cell.Then this force is transferred to the piezoelectric crystalby the fluid by Pascals law.

So when the force is transferred by the fluid it disturbs the internal

structure of the piezoelectric crystal and due to this change in the structureof the piezoelectric crystal a potential difference is generated across thepiezoelectric crystal. This potential difference is detected by the electricdetector and electric signal is transferred to the display unit to display themagnitude of the applied force, weight or pressure.

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Hydraulic Load CellTypical Example Weigh Bridges

Different Components :

Typical capacities range from 30 to 100tonnes, in lengths of 9, 15 and 18 metres.Usually four (4) to ten (10) hydrostatic loadcells located underneath the load receivingelement (weigh bridge), depending upon weighbridge length and capacity. Each hydrostaticload cell is connected to the SummingTotalizer with high pressure copper orstainless steel tubing. The hydrostatic loadcell output, measured in pounds per squareinch (PSI) is transmitted to a SummingTotalizer .

The function of the Summing Totalizer is to sum the pressure inputs from eachload cell and convert the sum to a Millivolt (mV) signal, directly proportional tothe sum of the hydrostatic load cell outputs. The mV output is captured,

digitized and displayed on a digital weight indicator.1730.09.2011


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Hydraulic Load Cell Weigh Bridges The Hydrostatic Load Cell

A small amount of fluid is required (less thanone (1) pint of fluid used in a 70 foot truckscale). Each hydrostatic load cell type isdesigned with an exact "acting area" - a knownvalue in units of square inches. For example,when a known weight is applied to ahydrostatic load cell with a known "actingarea", the output in PSI is predictable.A 50,000 lb. weight applied to an "actingarea" of 50 square inches will have an outputof 1,000 PSI.

)(Pr)(

PSI essureOutput In ActingArea

Weight

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The Summing Totalizer converts the output pressureof each hydrostatic load to a force, sums the forcesand converts the sum of the forces to a millivoltoutput signal. The Summing Totalizer is comprised of aseries of hydrostatic receiver cells, each accepting

the PSI from its assigned hydrostatic load cell. ThePSI input is converted to a vertical force measured inpounds and summed in the Summing Totalizer.

Hydraulic Load Cell Weigh Bridges The Summing Totalizer

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Hydraulic Load CellThe Cost Factor .....

32.3 metre by 3 metre 120 tonne Weighbridge 33,950.00







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Hydraulic Load CellOperating range

Reference : environmental systems & services, Australia, www.geosystems.com.au

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Hydraulic Load CellAdvantages & Disadvantages

Advantages :-Linear output and independent of amount of fluid or temperatureCan be used in any environment and for all type of materials

Highly accurate for large scale applications, Accuracy = Rating 0.25%

Disadvantages :-The diaphragm has a limitation of Max. 1000psiExpensive and complex to use

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Capacitive Load CellPrinciple

The capacitance C of an ideal parallel plate capacitor is given by,Where = Electric permittivity of the

dielectric materialA = Area of the plated = Distance between the plates

d

AC

When load is applied the distance betweenthe capacitor place changes, resulting in achange in capacitance. This change is

converted into suitable output to measurethe applied load by using following equation,

)1(

]21[

E d

E A

C

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References

www.engineering-resource.com

www.revretransducers.com

Application Note 078 Strain Gauge Measurement, National instruments, 1998

Data Pack E, 1997, 232 -595, RS components

www.tecsis.de , Hydraulic Load Cell

www.weightron.com

Kim.Y, Kim.H, Lee.J ,Silicon based Capacitive Load Cell for Tensile Load

Measurement, DTIP 2009, April

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http://www.engineering-resource.com/http://www.revretransducers.com/http://www.tecsis.de/http://www.weightron.com/http://www.weightron.com/http://www.tecsis.de/http://www.revretransducers.com/http://www.engineering-resource.com/http://www.engineering-resource.com/http://www.engineering-resource.com/


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Load Cells & its application in Textiles

load cell & its application in textile

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