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THE LOAD CELL

The beginning of resistance measuring

In 1843 an English physicist called Sir Charles Wheatstone (born Feb 6 1802, Gloucester, England- 1802 to 1875) popularized the “wheatstone bridge” by creating a devise bridge circuit that could measure electrical resistances. This bridge circuit was used for measuring the resistance changes that occur in strain gauges and it soon became very popular in laboratories. In the 1940s the first bonded resistance wire strain gauge was developed. Together with the technology developed strain gauges it was able for produced both mechanical scale components and stand-alone load cells to be produced.

Wheatstone was appointed professor of experimental philosophy at King’s College, London, in 1834, the same year that he used a revolving mirror in an experiment to measure the speed of electricity in a conductor. The same revolving mirror, by his suggestion, was later used in measurements of the speed of light.
The year of 1938 the commercialization of the bonded resistance stain gage was introduced, which was a sensor for measuring static and dynamic strains. Since its introduction, this sensor has been used not only in the USA where it was first developed, but all over the world, particularly in Europe where the most advanced technology has developed in the weighing industry. In addition to being used to measure strain, it was applied to measure many quantities that can be related to strain, such as force, pressure, acceleration, and torque. The rapid development of these sensors was due to its characteristic which consisted of small and simple to use, and when it is connected to an electric circuit, strain is transformed into a proportional signal which remains constant with time.

  
Sir Charles Wheatstone and the Wheatstone bridge

 

Installation and Maintenance

A load cell converts a force (usually a gravitational force from an object being weighed) to a strain that can then be converted to an electrical signal by a strain gauges. A load cell will typically have a circuit with four gauges, two in compression and two in tension and six external connections. The span and zero adjust on test are factory set to ensure the bridge is within limits.
Through regular maintenance one must ensure that dust does not close gaps and cause the proof-ring to carry part of the load cell. This can cause the load cells to cause circuit damage, which will in turn result in the load cell reading below the load weight.
Multi-cell weighing systems can be used, with the reading from each cell being summed electronically through a connecting junction box which can connect up to 10 load cells together. In a hopper system three cells inherently spread the load across all cells. With four cell systems the support structure must ensure that all cells are in contact with the load at all times.
Usually the load cells are the only rout to ground from the weigh platform. It is advisable to provide a flexible earth strap (metallic belt that goes from the top to the bottom of the load cell) this will ensure that if there is an electric dysfunction it will go through the strap rather than through the load cell). It will also add safety as well as providing a route for any welding current, which might arise from repairs or later modification.
A weigh system is usually more than a collection of load cells, an indicator (display) it is a tearing weigh system and is usually installed. The gross weight is the weight from the tank (the material of the hopper it self) Each time the tare command is pressed, the gross weight is stored, and this stored value subtracted from the gross weight to give the net weight. In this type of weight system it has also the advantage that it can be linked into a supervisory PLC or computer control network.


The more industrialized operations include batching feeder systems where the material can be fed from a vibrating feeder into a weigh hopper. The system is first tared, then a two-speed system used with a changeover from fast to dribble feeding modes will reach the programmed target weight.





 

 

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