Load Cells & DP Cells Presented To Dr. –ING Naveed Ramzan Presented By Muhammad Atir Hashmi 07-Chem-22

Load Cells It is a transducer which takes input as (force) mechanical signal and converts into an output as electrical signal. The other names of Load Cells are Load Transducer or Load Sensors. There are different types of Load Cell; most common type is Strain Gauge based Load Cell.

Working Principle A load cell is made by bonding strain gauges to an elastic material. For accurate measurement, strain gauges are bonded to the position on the Elastic material where the impact of the stress will be the largest. The strain gauge utilizes this principle and detects a strain by changes in electrical resistance.

Advantages Rugged and compact construction No moving parts Highly Accurate Wide range of measurement, can be used for static and dynamic loading

Disadvantages Mounting is difficult Calibration is a tedious procedure

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

Strain Gauge Load Cells This is the most common type of load cell. Such a load cell is used for measuring compressive forces. The length of the load sensing element is made short so that it does not buckle under the maximum allowable load.

Different Shapes of Strain Gauges

Approximate Capacities of Different Shapes of Strain Gauges

Hydraulic Load Cells Hydraulic load cells are force-balance devices, measuring weight as a change in pressure of the internal filling fluid. The liquid (usually oil) has a preload pressure. Application of the force to the loading member increases the fluid pressure which is measured by a pressure transducer or displayed on a pressure gauge dial via Bourdon tube.

Pneumatic Load Cells Pneumatic load cells also operate on force-balance principle. The force is applied to one side of a diaphragm of flexible material and balanced by pneumatic pressure on the other side. The counteracting pressure is proportional to the force and is displayed on a pressure dial.

Inductive and Reluctance-Based Load Cells These cells are based on the measurement of displacement of a ferromagnetic core caused to a force-summing device, like a diaphragm or bellows, by the applied force. The former changes the inductance of a solenoid coil due to the movement of its iron core while the latter changes the reluctance of a very small air gap.

Magnetoelastic Load Cells We know that the magnetic permeability of a ferromagnetic material changes when subjected to a mechanical stress. The load cell is build from a stack of ferromagnetic laminations forming a load-bearing column. A set of primary and secondary transformer coils, oriented at 90o with each other, are wound through holes in the column.

Piezoelectric Load Cells A pre-tensioned bolt, which allows the measurement of forces in both tension and compression, is used. Mounting of a load washer in this way is illustrated in the fig. below. The pre-loading ensures an optimum linearity and facilitates calibration after mounting. The force that acts upon the stack is transmitted to each of the three discs with the same magnitude and direction. These discs were cut along the specified axes. Each produces a charge proportional to the force component specific to it. The charge is collected via electrodes inserted into the stack.

Fibre-Optic Load Cells Like a wire strain gauge, a fibre-optic strain gauge can be fabricated using optical fibres. If this fibre-optic strain gauge 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 a monochromatic light is used to feed the two gauges experiencing different strain levels then the phase difference between the two beams emerging from the gauges, in number of half wavelengths, is a measure of the supplied force.

Resonant Wire Load Cells The resonant-wire load cell consists of a ferromagnetic wire that is excited into resonant transverse vibrations by a drive coil. A pick-up coil detects these vibrations. The resonant frequency is a measure of the tension of the wire and hence, applied force at that instant.

Selection Criteria Capacity Basic type and mode of operation Number of load cells Method of mounting Performance or accuracy level-related to system requirements Sealing level and material of construction Approval requirements (metrological, safety, environmental) Cost

Applications There are different applications of Load Cells. Some are shown in next slides.

Dual Tank Level Controller

Bag Filling Machine

Tank / Silo / Hopper

Food Packaging

DP Cells A DP cell is a differential pressure cell. It is used to measure the differential pressure between two input points. It consists of a sensor, a transducer and a transmitter combined in a single device.

Working Principle In DP cell a diaphragm is present which remains in normal condition when the forces on both sides of diaphragm are equal. The unequal forces (pressure difference) create deformation in the diaphragm. By the extent of deformation, the differential pressure is calculated.

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 discuss the Diaphragm Capsules which are the main sensing element of the DP Cell.

Diaphragm Capsules Most DP cells have a diaphragm capsule as their sensor. Capsules consist of two circular metallic diaphragms formed into a disc and an internal backing plate. The diaphragms often have concentric corrugations to optimize their flexibility/rigidity. The space between the diaphragms and the backing plate is filled with silicone oil.

Pneumatic DP Cell The capsule is held between two flanged castings which form chambers on either side. These are designated as the high and low pressure sides of the DP cell. Air is supplied to keep the force bar in horizontal position and in this way differential pressure is calculated.

Electronic DP Cell The general features of construction of electronic and pneumatic DP cells are much the same. These cells convert the deflection of the diaphragms into a change in resistance, inductance or capacitance which is then measured using an integrated circuit as appropriate.

Applications Level Measurement Density Measurement

Continued….. Flow Rate Measurement Interface Measurement

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=0CEYQ6AEwBA#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

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