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Explain how force is measured using load cell

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Load Cell : Load Cell is a specific type of transducer or sensor capable of transforming force (load) into a measurable electrical output. Load cells measure tension, compression, or shear. A typical load cell device comprises of four strain gauges in a Wheatstone bridge configuration. 

Working: Force conversion into an electrical signal by a load cell is usually carried out indirectly and in two stages. The first stage conversion is done through a mechanical arrangement in which the force being sensed deforms the strain gauge. This strain gauge then converts the deformation (strain) to electrical signals. One can also get load cells with one or two strain gauges. The electrical signal generated by strain gauge is in the order of a few millivolts which needs further amplification by an instrumentation amplifier. The output of the transducer is fed to an algorithm calculator to calculate the force applied to the transducer.

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Load cell are primarily intended for measurement of weight of bodies such as slowly moving vehicles. Load cell utilize an elastic member as primary transducer and strain gauges as secondary transducer. They are as well designed for various applications where concentrated forces can be conveyed to the load cells through mechanical linkages.Those cells meant for weighing are provided with supports for hanging the body to be weighted A column type load cell primarily consists of either slender rod, robust column of rectangular or circular cross-section, or even the square cylinder shown in figure 


These columns of regular configurations enable the measurements of deformations with reasonable accuracy, though ultimately they are calibrated against standard weights. It is essential to recognize the importance of transmitting the force uniformly over the entire cross-section A of this load cell. it is also essential to recognize that there is no other force working on the column apart from the one under measurement, acting along the axis of the column. Due to the stress F/A, the surface of the column undergoes compressional Ɛa strain along its axis and tensile strain along its circumference Ɛt. These strains are measured conveniently by the resistance type strain gauges, by locating them suitably on the outside surfaces. The strains are given by 

Ɛa = F/AE= Ɛ1= Ɛ3 

 Ɛt = - µF/AE=Ɛ2=Ɛ4. 

The size of the column permits the attachment or bonding of the strain gauges as shown and so utilization of the axial displacement is not considered any more efficient. The column is held rigidly in vertical position on a vibration-free mounting and the force is transmitted through a piston or any other means, to the entire sectional area of the column. They are designed to measure up to 2000 T. Distributing the four strain gauges around the periphery such that the tensile and compressional strain gauges are alternated. it is possible to achieve both temperature compensation and immunity to bending stress due to forces inadvertently applied at an angle to the axis of the column. Some-times, the column is held vertically by guard plates so as to increase its stiffness in the radial direction.

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