Load Cell Types

 

Load Cell

A load cell is a transducer that is used to measure force. The load cell is mostly an electromechanical transducer that converts mechanical energy into electrical energy.

The load cell measures static and dynamic forces acting on it. The load cell is consists of an elastic material that deforms according to the force it receives.

The magnitude of this deformation of elastic material is proportional to the magnitude of the force acting on it. Normally strain gauges are used in the load cell.

Load cell types according to working principle

Following methods are used in load cells to measure forces.

1. Strain gauges load cell 
2. Hydraulic load cell
3. Pneumatic load cell
4. Capacitive load cell
5. piezoelectric load cell

1. Strain gauge load cells

 Load cells are used in different industrial applications to measure force or weight. Load cells are made up of strain gauges and a Wheatstone bridge is used to measure the force acting on the load cell.

Load Cells are made up of four strain gauges and these strain gauges change resistance when the load is applied to them, all strain gauges used in the load cell have the same resistance.

 By applying the ohm Law we configure the drop voltage at the output. Mostly each branch contains 350Ω + 350Ω = 700Ω Resistance. The current flow in the branch and the branch voltage are divided by the branch resistance of the Wheatstone bridge.

Construction and Operation:

Two strain gauges are placed opposite each other to respond proportionally to the change in length. Two other measuring devices are placed on the opposite side of the line.

Since a pair of strain gauges mounted on them the wire becomes shorter the wire diameter becomes larger and reduces their resistance. The other pair of strain gauges are positioned that will extend the cable, thereby reducing the diameter and increasing the wire resistance. If we hang the same weight from the bottom of the line, instead of pressing the line, we will place tension/stress on it. Line and strain gauges will act in the opposite direction but still in the same amount of stretching and pressing the cable. 




We can measure the wire strain to the Wheatstone Bridge configuration.

Strain gauge load cell applications

Strain gauge load cells are explosion-proof. Strain gauge load cells are insensitive to temperature variations. There is no possibility of contamination of the process if the diaphragm breaks because it doesn't contain any fluid.
Strain gauge load cells are low-priced so mostly used in the industry. Strain gauge load cells are highly accurate and low price. They have a high-frequency response for dynamic loads. They can fit into a wide variety of load-mounting configurations so these are used in many industrial applications.

Strain gauge load cells are passive transducers. Strain gauge load cell cells require an excitation volt to operate so these are not fit for an intrinsically safe environment 

2. Hydraulic load cell

Hydraulic load cells are made up of a piston and cylinder device that conveys load by a change in pressure.
Hydraulic load cells convert a load or force applied to hydraulic pressure. The measured load is applied to a load platform attached to a piston. The forced balance method is used in hydraulic load cells to measure force and load. Force is balanced when the internal filling fluid pressure changes. 
 A force is applied in a hydraulic system. This force compresses the internal hydraulic fluid in the chamber. The pressure of the hydraulic fluid rises or lowers, proportional to the force applied. 



Hydraulic load cell Parts


Hydraulic load cells are made up of the following components:

1. diaphragm

2. Piston

3. Chamber

4. Hydraulic fluid

5. Pressure gauge

6. Tubes

7. housings assembly


Hydraulic load cell applications

 Hydraulic load cells can typically measure up to 5MN and have an accuracy of about 0.25 to 1.0 % of full scale. Their resolution of hydraulic load cells is typically up to  0.02 percent. These load cells are sensitive to ambient pressure. 

Hydraulic load cells accuracy is within 0.25% of full scale. Because hydraulic load cell sensors have no electric components, they are often used for hazardous applications. Hydraulic load cells are costly compared to other load cells.

Pneumatic load cell


Pneumatic load cells working principle is the same as hydraulic load cells. However, the pressurized fluid in a pneumatic load cell is a type of gas, mostly its compressed air. 

The force we want to measure is applied to a loading platform, connected to one side of the diaphragm, on the opposite side of the diaphragm pressurized gas or air is applied to balance the force. The pressure gauge is connected through a nozzle which allows some air to escape. The pressure of the air flowing through this nozzle is measured, this pressure is directly proportional to the load or force applied to the load cell.





Pneumatic load cell parts


Pneumatic load cells are made up of the following components or parts:

1. Loading platform

2. Chamber filled with pressurized gas.

3. Air regulator

4. Nozzle

5. Pressure gauge

6. Diaphragm connected to a chamber

Pneumatic load cell applications

Pneumatic load cells are used where intrinsically safe requirements. Pneumatic load cells can tolerate temperature changes. These can measure small changes in load so can be used for higher accuracy and small load measurement.
Pneumatic load cells are more accurate as compared to hydraulic load cells. Pneumatic load cells are designed to measure relatively small loads. Pneumatic load cells are used in all temperature environments. For proper working of pneumatic load cells clean and dry air must supply to the load cell.

Capacitive load cell

 In capacitive load cells change in capacitance is measured proportional to change in force or load applied. 
Two plates are used having a gap between them. Load is applied on one plate so the gap narrows and capacitance changes, by measuring the change in capacitance we can measure load.



 

Capacitive load cell parts

Capacitive load cells are made up of the following parts.

1. Loading platform

2. Insulted housing having fixed and movable plates of capacitance.

3. Dielectric material, maybe air

4. Electrical wiring

5. Rigid rod connected to movable plate and loading platform. 

Capacitive load cell applications

Capacitive load cells are very sensitive. These are accurate over a wide range of forces. 

They are simple in design.

Capacitive load cells are very cost-effective than other types of load cells. 

 Mostly used for food and medical weighing applications where hygiene is an issue.

Capacitive load cells operate using an electric charge they are not a good choice to be used in flammable environments.

 If dielectric materials are sensitive to temperature, then the accuracy may be affected by temperature.

piezoelectric load cell

The piezoelectric type load cell working principle is based on the piezoelectric effect. This is a natural property of materials such as quartz crystal and some ceramics. When the polarized crystal is stressed or deforms piezoelectricity is produced.
By using this property of piezoelectric material we measure the force applied. Piezoelectric sensors are also used to measure pressure.
Metallic electrodes are bonded to the surface of piezoelectric material to transfer charge to measure. These electrodes are placed perpendicular to the applied force to be measured used.




 The output voltages are proportional to the applied force. This type of load cell can measure both compression and Tensions.

A piezoelectric load cell is an active transducer. It does not require an external power source to generate an output signal. This is best to use where an external power source is inconvenient. Some amplification of output signal is required evcauseto signal is evryrequired small.


Piezoelectric type Load cell parts

Piezoelectric load cells are made up of the following parts.

1. Loading platform

2. Metallic electrode bonded to the piezoelectric material

3. Piezoelectric material

4. wiring


Piezoelectric load cell applications

The piezoelectric effect happens for dynamic forces, this does not work for static forces. These types of load cells are used to measure transit forces.

 Piezoelectric load cells have a high-frequency response.

Types of Load cells according to shapes and forces applied

Here are expressed some types of load cells :

Single point load cell

Miniature load cell

Beam load cell

Platform Load cell

Canister load cell

Tension and compression load cell

Rod type load cell

RTN load cell.

Single point load cell

These are hermetically sealed load cells. This type of load cell can measure off-center loads. These are used in weighing scales, counting scales, balances, medical weighing, bag fillers. 




Miniature load cell

Miniature and sub miniature load cells are compression load cells. These are designed to perform in high-capacity loads. These are used for test benches, industrial applications, and prototype structures.




Beam Load Cell

Bending beam load cells are low capacity load cells. Beam load cells are made up of aluminum alloy. The capacity of the bending beam load cell is from 1 kg to 500 kg.

These are used for OEM force and industrial applications. These are very cost-effective.

Bending beam load cells are of two types, 

1. Shear beam load cell 

2. bending beam load cells.

These are low-profile construction load cells and are used in restricted areas. These load cells are very cost-effective. These are used for static and dynamic weighing measurements. Beam load cells are used in industrial measurement and tank weighing.




Platform Load cell

These load cells are hermetically sealed, highly accurate, and best for watertight applications. Mostly used in food processing, weighing applications, and automatic weighing stations.




Canister Load Cell

Canister type load cell is used for single weighing and multi weighing applications. Canister-type load cells are also hermetically sealed and water-resistant. 
These are heavy-duty design load cells. 
Canister type load cell sensors are sealed so these are better to use in harsh environments.
Canister type load cells are best for axial compression applications. 



Canister type load cells are used for submerged operations like marine weighing, underwater platforms, and pit flooding environments. 

Tension and compression load cell

Most of the load cells are compression load cells. These load cells are highly accurate, have a high capacity, and are mostly compact design.

 These load cells are used in applications of highly reliable requirements.




These are used for hopper weighing, platform scales, truck scales and electronics weigh scales.

Rod Type Load Cells

This type is widely used for loads ranging up to 10 kg. 

The bending movements that occur in the stem due to the force cause stress to occur near the stem mooring.



By placing two strain gauges R1 and R3 will get a strain pull while R2 and R4 are used to measure force.

RTN Load cell

RTN stands for Ring TorsioN. 
RTN load cells are used for a wide range of load measurements from 1 Tonn to 470 Tonn. 
RTN load cells are compact type load cells. 





These load cells are very robust. RTN protection is very high in IP68 and IP 69.
 These are used in large platform scales, like weighbridges.



 


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