Introduction Of Load Cell

- May 27, 2011-

 A load cell is a device that is used to measure weight or force. W hen a force is applied to it in a specific manner, a l oad cell 

produces an output signal that is proportional to the applied force. Strain gage load cells are at the heart of the majority of weighing 

and force measurement devices produced today. One end of a load cell is typically supported on a rigid structure while the other 

end supports a load-receiving device through which the load or force is applied. L oad cells can be used individually or in 

combinations in weighing devices, as dictated by the geometry of the object to be weighed.

    Strain gauge load cells are by far the most common form of load cell commercially available today, and are briefly described here.

 And the load cells our factory manufactures,of course  is strain gauge load cells.The figure below is one of the common metallic foil 

strain gage. 

This consists of a metallic foil etched into parallel grid lines forming a circuit between the solder pads that are used to complete the circuit. T he foil is bonded to an insulating backing material that, in turn, is bonded to the surface of the load cell(please see the figure below,it shows the arrangement of strain gauge on a load cell). A Strain gage type load cell consists of a spring element that is

selectively weakened to create regions of relatively high strain; this is where the strain gages are applied.

Just as showed on this figure,two gages are illustrated on the top surface and two corresponding gages on the bottom surface are

not shown. In this arrangement two gages measure tensile strains and two measure compressive strains as load is applied to the load cell  The strain gages are wired together to form a Wheatstone bridge, just like this(we usually call load cell as such structure a single point load cell):

Note that several other resistors are typically included in the circuit, for example, a resistor may be added to temperature compensate the cell’s performance, these are not shown for simplicity. A stable excitation voltage is applied to opposite corners of the Wheatstone bridge, and a signal is measured across the others, points A and B in the figure above. With no load applied to the cell, all gages have the same resistance and hence there is no voltage difference between points A and B. As load is applied to the cell, the resistance of the tension gages increases, while that of the compression gages decreases. The bridge now becomes “unbalanced” and a voltage difference (signal) proportional to applied load can be measured across points A and B. E lectronic weight indicators are readily available which, at their most basic, supply the excitation voltage for the bridge, measure the output signal, and provide a digital display of the applied load. Some load cells and indicators have provision for sense wires as shown in the figure above which allows the indicator to measure and adjust for the actual excitation voltage applied to the cell, this is particularly important with long cable runs.


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