Load Cells & DP Cells 22

8/2/2019 Load Cells & DP Cells 22

1/33

Presented To

Dr. ING Naveed Ramzan

Presented By

Muhammad Atir Hashmi

07-Chem-22

1


2/33

Load Cells It is a transducer which takes input as (force)

mechanical signal and converts into an output aselectrical signal. The other names of Load Cells areLoad Transducer or Load Sensors.

There are different types of Load Cell; most commontype is Strain Gauge based Load Cell.

2


3/33

Working Principle A load cell is made by bonding strain gauges to an

elastic material. For accurate measurement, straingauges are bonded to the position on the Elasticmaterial where the impact of the stress will be thelargest. The strain gauge utilizes this principle anddetects a strain by changes in electrical resistance.

3


4/33

Advantages Rugged and compact construction

No moving parts

Highly Accurate Wide range of measurement, can be used for static

and dynamic loading

4


5/33

Disadvantages Mounting is difficult

Calibration is a tedious procedure

5


6/33

Types of Load Cells There are different types of Load Cells such as:

Strain Gauge Load Cells

Hydraulic Load Cells Pneumatic Load Cells

Inductive and Reluctance-Based Load Cells

Magnetoelastic Load Cells

Piezoelectric Load Cells Fibre-optic Load Cells

Resonant Wire Load Cells

6


7/33

Strain Gauge Load Cells This is the most common

type of load cell. Such aload cell is used for

measuring compressiveforces.

The length of the loadsensing element is made

short so that it does notbuckle under themaximum allowableload.

7


8/33

Different Shapes of Strain Gauges

8


9/33

Approximate Capacities of

Different Shapes of Strain Gauges

9


10/33

Hydraulic Load Cells Hydraulic load cells are force-

balance devices, measuring weight as a change inpressure of the internal fillingfluid. The liquid (usually oil)has a preload pressure.

Application of the force to theloading member increasesthe fluid pressure which is

measured by a pressuretransducer or displayed on apressure gauge dial viaBourdon tube.

10


11/33

Pneumatic Load Cells Pneumatic load cells also

operate on force-balanceprinciple. The force is

applied to one side of adiaphragm of flexiblematerial and balanced bypneumatic pressure on theother side. Thecounteracting pressure isproportional to the forceand is displayed on apressure dial.

11


12/33

Inductive and Reluctance-Based

Load Cells These cells are based on the measurement of

displacement of a ferromagnetic core caused to aforce-summing device, like a diaphragm or bellows, bythe applied force. The former changes the inductanceof a solenoid coil due to the movement of its iron corewhile the latter changes the reluctance of a very smallair gap.

12


13/33

Magnetoelastic Load Cells We know that the magnetic

permeability of aferromagnetic material

changes when subjected to amechanical stress.

The load cell is build from astack of ferromagneticlaminations forming a load-

bearing column. A set ofprimary and secondarytransformer coils, oriented at90o with each other, are woundthrough holes in the column.

13


14/33

Piezoelectric Load Cells A pre-tensioned bolt, which allows

the measurement of forces in bothtension and compression, is used.Mounting of a load washer in this way

is illustrated in the fig. below. Thepre-loading ensures an optimumlinearity and facilitates calibrationafter mounting.

The force that acts upon the stack istransmitted to each of the three discs

with the same magnitude anddirection. These discs were cut alongthe specified axes. Each produces acharge proportional to the forcecomponent specific to it. The chargeis collected via electrodes insertedinto the stack. 14


15/33

Fibre-Optic Load Cells Like a wire strain gauge, a fibre-optic strain gauge can be

fabricated using optical fibres. If this fibre-optic straingauge is bonded to the elastic element of a load cell, an

applied force will cause length changes in the optical fibre. Suppose, we have two fibre-optic gauges bonded to two

different members one of which is strained. Now, if amonochromatic light is used to feed the two gauges

experiencing different strain levels then the phasedifference between the two beams emerging from thegauges, in number of half wavelengths, is a measure of thesupplied force.

15


16/33

Resonant Wire Load Cells The resonant-wire load cell

consists of a ferromagnetic wire that is excited into

resonant transversevibrations by a drive coil. Apick-up coil detects these

vibrations. The resonant

frequency is a measure ofthe tension of the wire andhence, applied force at thatinstant.

16


17/33

Selection Criteria Capacity

Basic type and mode of operation

Number of load cells

Method of mounting

Performance or accuracy level-related to systemrequirements

Sealing level and material of construction Approval requirements (metrological, safety

environmental)

Cost

17


18/33

Applications There are different applications of Load Cells.

Some are shown in next slides.

18


19/33

Dual Tank Level Controller

19


20/33

Bag Filling Machine

20


21/33

Tank / Silo / Hopper

21


22/33

Food Packaging

22


23/33

DP Cells A DP cell is a differential pressure cell. It is used to

measure the differential pressure between two inputpoints. It consists of a sensor, a transducer and a

transmitter combined in a single device.

23


24/33

Working Principle In DP cell a diaphragm is present which remains in

normal condition when the forces on both sides ofdiaphragm are equal. The unequal forces (pressure

difference) create deformation in the diaphragm. Bythe extent of deformation, the differential pressure iscalculated.

24


25/33

Types of DP Cells There are two main types of DP Cells:

Pneumatic DP Cell

Electronic DP Cell

Before discussing the types of DP cells, let here discussthe Diaphragm Capsules which are the main sensing

element of the DP Cell.

25


26/33

Diaphragm Capsules Most DP cells have a diaphragm

capsule as their sensor.

Capsules consist of two circularmetallic diaphragms formedinto a disc and an internalbacking plate.

The diaphragms often haveconcentric corrugations tooptimize theirflexibility/rigidity.

The space between thediaphragms and the backingplate is filled with silicone oil.

26


27/33

Pneumatic DP Cell The capsule is held between

two flanged castings whichform chambers on either

side. These are designated as the

high and low pressure sidesof the DP cell.

Air is supplied to keep theforce bar in horizontalposition and in this waydifferential pressure iscalculated.

27


28/33

Electronic DP Cell The general features of

construction of electronicand pneumatic DP cells are

much the same. These cells convert the

deflection of thediaphragms into a change inresistance, inductance orcapacitance which is thenmeasured using anintegrated circuit asappropriate.

28


29/33

ApplicationsLevel Measurement Density Measurement

29


30/33

Continued..Flow Rate Measurement Interface Measurement

30


31/33

References http://www.futek.com/product.aspx?t=load

http://www.omega.com/prodinfo/loadcells.html http://en.wikipedia.org/wiki/Load_cell

http://www.sensorland.com/HowPage002.html

http://www.interweigh.ca/loadcells.htm

http://www.aandd.jp/products/weighing/loadcell/introduction/pdf/2-1.pdf

http://books.google.com.pk/books?id=NCEeONKWzX4C&pg=SA2-

PA25&dq=advantages+and+disadvantages+of+Load+Cells&hl=en&ei=EOjWTK_0Jo-qcfvd8a4L&sa=X&oi=book_result&ct=result&resnum=1&ved=0CDAQ6AEwAA#v=onepage&q=advantages%20and%20disadvantages%20of%20Load%20Cells&f=false

http://www.futek.com/apps_load.aspx

http://www.weighing-systems.com/TechnologyCentre/loadcellselect.html

http://books.google.com.pk/books?id=jndF0UNzz3gC&pg=PA272&dq=Types+of+Load+Cells&hl=en&ei=JTvYTIaaJIqlcf7iwdQL&sa=X&oi=book_result&ct=result&resnum=5&ved=0CEYQ6A

EwBA#v=onepage&q=Types%20of%20Load%20Cells&f=false http://wiki.answers.com/Q/What_is_DP_Cell

http://books.google.com.pk/books?id=eEnobcUiXE0C&pg=PT120&dq=DP+Cells&hl=en&ei=CrvZTP2CLYKecK7XocQG&sa=X&oi=book_result&ct=result&resnum=10&ved=0CF4Q6AEwCQ#v=onepage&q=DP%20Cells&f=false

http://www.samson.de/pdf_en/e95380en.pdf

http://www.vega.com/downloads/PR/EN/37619-EN.PDF 31


32/33

32


33/33

Load Cells & DP Cells 22

Documents

Transcript of Load Cells & DP Cells 22