1. Introduction to load cells
A load cell is a “load transducer” which converts the weight or load acting on it into electricalsignals.
A load cell is composed of an aluminum alloy spring element, strain gauges (serving as sensors) and a bridge circuit.
The strain gauges themselves are bonded onto four areas which become considerably distorted in the spring element. The load cell detects the force of the distortion as voltage change.
2. Spring element configuration
The spring element becomes slightly distorted when a weight is applied.
When looking at the movement of the spring element, we can recognize a general parallelogram configuration, indicating the phenomenon of the Roberval mechanism.
In addition, the elastic area of aluminum alloy is used for the bridge, indicating that Hooke’s law is satisfied.
3. Roberval mechanism
In the case of a balance scale, if two equal weights are placed at unequal positions on each side of the scale pan, the scale does not balance, given the unequal distances from the fulcrum.
If the beam is added to form parallelogram, however, the scales will balance irrespective of the positioning of the weights. This is known as the Roberval mechanism.
4. Hooke’s law
Arrange five springs as shown.
Hang one weight, two weights, three weights and so on from the springs, as shown in the diagram.
The phenomenon which the material is deformed linearly based on force is called the Hooke's law. Material which manifest this behavior are elastic.
5. Principal of the load cell
Let's look, once again, at the motion of the spring element.
When a weight is applied, which portions of the bridge become distorted?
Distortion appears to be greatest at the four thinnest points.
When needles are positioned at these thinner points, they indicate that tension bears upon certain points while other points experience compression.
The weight applied to the load cell can be measured by the degree of distortion.
6. Strain gauge
The strain gauge is used to detect such distortion. A fine resistance wire is printed in zig-zag pattern on a small chip.
Distortion in the direction indicated by the arrow, that is, variation in tension and compression is detected as a change in electrical resistance.
Let's examine how the strain gauge varies according to elastic deformation.
The wire on the strain gauge becomes thinner and longer when pulled, while it becomes thicker and shorter when compressed.
When it becomes thinner and longer, the resistance increases.
While the wire becomes thicker and shirter, the resistance decreases.
This is just like water flowing through a tube.
If the tube is stretched, it becomes thinner and longer, making water flow more difficult. If the tube is returned to the original position, water flows easily.
The strain gauge therefore shows increased resistance when stretched. On the other hand, the gauge shows a small resistance when compressed.