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Many people use weighing scale for many reasons: to check their weight in the morning, to dose the food to prepare a meal, to know the weight of a load to carry. But many don’t know that in order to perform such a simple operation they are using LOAD CELLS!
In the past, to know the weight of objects they used to use old weighing scales with the help of some weights, but nowadays we have instruments that automatically measure weight.
For definition, a load cell is an electric component, specifically a transductor, which measures the deformation of a rigid body towards which a determined force was applied. We must specify that a load cell doesn’t measure the object mass, expressed in Kg, but it does measure the strength with which the concerned object deforms the load cell itself. Cells work on the basis of Hooke’s law, which states that an elastic body undergoes a deformation directly proportional to the effort applied to it, in simple words determines the behaviour of elastic materials.
There are 4 types of load cells:
• Pneumatic/Air cells
• Hydraulic cells
• Strain gauge/ Extensimetric cells
• Capacitive cells
Air cells, as the name suggests, use air pressure to work; they’re made of an elastic diaphragm on which rests the support base for the object to be weighed. An air regulator which manages the quantity of air to introduced, and on the other side there’s a pressure gauge. When placing the object on the platform, this will deform the diaphragm compressing the air inside of the cell, the pressure gauge will measure the change in pressure and through an electric impulse sent to a control panel, it will convert it into kilograms.
Hydraulic cell uses a fluid, whether it’s water or oil, for the measurement. The principle is similar to the air cells one, but instead of air it uses a pressurised liquid. Inside the cell there’s an elastic diaphragm, a piston, inside of which there are: a liquid, a support base, and a tubular spring pressure gauge. When the object is placed on it, the piston compresses the liquid inside it, which in turn deforms the diaphragm with the exerted pressure. Pressure is detected by the pressure gauge that can send an electric signal with the help of a pressure sensor.
The next cells are the strain gauge ones, the most frequently used type in industrial context. This load cell simply is a transductor which changes its electrical resistance when it undergoes a deformation. Electrical resistance is proportional to the deformation exerted on the cell, resulting in an accurate measurement, obviously the electrical resistance can be converted in strength and thus in weight. Strain gauge load cell is made of 4 extensometers; the extensometer is a measuring instrument which detects small deformations of a body subjected to mechanical stress. Extensometers inside the cell are in a “Wheatstone” bridge configuration, this setup provides 4 extensometers connected between them in a rhombus shape, and it’s capable of obtaining very accurate measurements with values between 10 and 100k ohm. Extensometers are glued onto a structure which gets deformed when a weight is applied; the structure features cavities to increase the deformation.
In the past, to know the weight of objects they used to use old weighing scales with the help of some weights, but nowadays we have instruments that automatically measure weight.
For definition, a load cell is an electric component, specifically a transductor, which measures the deformation of a rigid body towards which a determined force was applied. We must specify that a load cell doesn’t measure the object mass, expressed in Kg, but it does measure the strength with which the concerned object deforms the load cell itself. Cells work on the basis of Hooke’s law, which states that an elastic body undergoes a deformation directly proportional to the effort applied to it, in simple words determines the behaviour of elastic materials.
There are 4 types of load cells:
• Pneumatic/Air cells
• Hydraulic cells
• Strain gauge/ Extensimetric cells
• Capacitive cells
Air cells, as the name suggests, use air pressure to work; they’re made of an elastic diaphragm on which rests the support base for the object to be weighed. An air regulator which manages the quantity of air to introduced, and on the other side there’s a pressure gauge. When placing the object on the platform, this will deform the diaphragm compressing the air inside of the cell, the pressure gauge will measure the change in pressure and through an electric impulse sent to a control panel, it will convert it into kilograms.
Hydraulic cell uses a fluid, whether it’s water or oil, for the measurement. The principle is similar to the air cells one, but instead of air it uses a pressurised liquid. Inside the cell there’s an elastic diaphragm, a piston, inside of which there are: a liquid, a support base, and a tubular spring pressure gauge. When the object is placed on it, the piston compresses the liquid inside it, which in turn deforms the diaphragm with the exerted pressure. Pressure is detected by the pressure gauge that can send an electric signal with the help of a pressure sensor.
The next cells are the strain gauge ones, the most frequently used type in industrial context. This load cell simply is a transductor which changes its electrical resistance when it undergoes a deformation. Electrical resistance is proportional to the deformation exerted on the cell, resulting in an accurate measurement, obviously the electrical resistance can be converted in strength and thus in weight. Strain gauge load cell is made of 4 extensometers; the extensometer is a measuring instrument which detects small deformations of a body subjected to mechanical stress. Extensometers inside the cell are in a “Wheatstone” bridge configuration, this setup provides 4 extensometers connected between them in a rhombus shape, and it’s capable of obtaining very accurate measurements with values between 10 and 100k ohm. Extensometers are glued onto a structure which gets deformed when a weight is applied; the structure features cavities to increase the deformation.
The last type of cell is the capacitive one. Capacitive cells work on the capacity of a system to store an electric charge; it’s made of two flat parallel plates towards which an electric current is applied, which gets stored between the two plates: the further apart they are, the bigger the storage capacity is. When a load is placed, the space between the plates decreases and consequently so does the charge; this change can be measured in volts and transformed in kg.
We talked about the various types, now let’s see some application examples.
The first example can be made in a salt production chain. A tank drops salt onto a conveyor belt, which is weighed with the help of a load cell positioned under the belt; when the right weight is meet, the hopper stops, the belt brings the salt towards the funnel which makes salt drop into the previously positioned sack.
Load cell can also be used for the stamping of metal panels. An aluminium sheet is cut and placed under a baler; when the baler closes, a load cell detects the amount of strength applied on the mould, and once the predefined limit is reached, the press stops. A greater force would cause the rupture of the panel, while a lower one wouldn’t bent it.
Determining which load cell is the most suitable for a process is crucial, in order not to make mistakes that would cause disruption in production. A load cell is mainly chosen for its sensitivity and accuracy: capacitive cells are the most sensitive, immediately followed by strain gauge cells with a very high degree of accuracy, air and hydraulic cells are still used today in many processes but are the least accurate.
We discussed the various types of load cells, their characteristics and how they are applied in the industrial field. If you have any questions or curiosity about this topic, please write them in the comments.
We talked about the various types, now let’s see some application examples.
The first example can be made in a salt production chain. A tank drops salt onto a conveyor belt, which is weighed with the help of a load cell positioned under the belt; when the right weight is meet, the hopper stops, the belt brings the salt towards the funnel which makes salt drop into the previously positioned sack.
Load cell can also be used for the stamping of metal panels. An aluminium sheet is cut and placed under a baler; when the baler closes, a load cell detects the amount of strength applied on the mould, and once the predefined limit is reached, the press stops. A greater force would cause the rupture of the panel, while a lower one wouldn’t bent it.
Determining which load cell is the most suitable for a process is crucial, in order not to make mistakes that would cause disruption in production. A load cell is mainly chosen for its sensitivity and accuracy: capacitive cells are the most sensitive, immediately followed by strain gauge cells with a very high degree of accuracy, air and hydraulic cells are still used today in many processes but are the least accurate.
We discussed the various types of load cells, their characteristics and how they are applied in the industrial field. If you have any questions or curiosity about this topic, please write them in the comments.