Load Cell Theory and Design
Load cell is a conversion device for converting a weight signal or a pressure signal into an electric quantity signal.Laod cell adopts metal resistance strain gauges to form a measuring bridge,using the principles that under the action of tension ,the metal resistance wire will elongate and slim,and the resistance will increase.That is to say,it is caused by the effect that the metal resistance will change along with resistance varies.
Construction principle of the load cell: Metal resistance has the property of hindering the flow of current, that is, with the resistance, the resistance varies according to the type of metal. The same resistance wire, in general, the more slender, the greater the resistance. When the metal resistance wire stretches by external force , its resistance value will increase or decrease within a certain range. Therefore, the wire (or film) is sticked onto a measured object, the wire or film is very slender or very thin, and the paste is very perfect, then, when the measured object stretches by external force , the metal resistance wire (or film) will be stretched in proportion,and its resistance value will be a corresponding change. Load cell is to paste the metal resistance strain gauge onto the metal weighing beam to measure the weight of the signal.
Weight measuring types:normal force measurement(column type load cell ,single point type load cell),shear force measurement(double shear beam type load cell ,some S type load cell,spoke type load cell and so on) can be divided into compression type(column type load cell,disk type load cell) ,tension type(some S type load cell,round type load cell,and compression and tension type(some column type load cell,spoke type load cell,S type load cell,etc.)
Applications of load cells with different structures:column type load cell--large tonnage truck scale, hopper scale, tank scale, testing machine, force monitoring and measurement. S type load cell--hopper scale, tank scale, packaging machine, materials testing machine, etc .; double shear beam load cell- truck scale, track scale, etc .; single point type load cell- weighing balance, price scale, platform scale, industrial field weight control and measurement.
Load cell circuit components:when using a load cell to measure,we need to know the changes of resistance when the strain gaauge is loaded.Usually strain gauges are used to form bridge circuits(wheastone bridge),to convert the resistance changes caused by strain gauge into voltage changes to measure.
Why there must be internal compensation circuit for the load cell?In order to improve its performance,especially to improve the temperature characteristics,usually there must be zero and sensitivity temperature compensation in strain circuit.That is to say,in addition to strain gauge,there are still various compensation resistance.The purpose of zero compensation is to minimize the changes of bridge zero while the temperature changes;the purpose of sensitivity compensation is to reduce the changes of output voltage while temperature changes,that is to say,the compensating elastic modulus of elasticity and strain gauge sensitivity coefficient change with temperature changes.Therefore, two resistors in series with the same temperature compensation as the bridge are connected in series,while the other resistors in the circuit are used to adjust the initial balance, rated output and input resistance of the bridge will be adjusted to the specified value.
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