To measure weight, we employ load cells. They play a crucial role in our daily lives. According to HBM Product Manager Stefan Schmidt, “we meet load cells everywhere, whether it’s in your automobile or at the cheese counter in the grocery store.” Of fact, because they are concealed in the inner workings of instruments, they are typically not readily apparent.
Strain gauges have typically been mounted on a spring element that makes up load cells. Typically, the spring component is constructed of steel or aluminium. It is hence extremely solid but only slightly stretchy. The steel deforms somewhat under load, as suggested by the term “spring element,” before returning to its initial position and responding elastically to each load. Strain gauges can measure these incredibly subtle changes. Finally, analysis electronics evaluate the strain gauge’s deformation in order to calculate the weight.
Let’s examine strain gauges in more detail in order to comprehend this last point: They are electrical conductors that are tightly wound around a film. This film becomes longer as it is pulled, as do the conductors. It becomes shorter when it is contracted. The resistance in the electrical conductors changes as a result. On the basis of resistance increasing with strain and decreasing with contraction, the strain can be calculated.
Since the strain gauges are firmly linked to the spring element, they move together with it. A bridge circuit, or more specifically a Wheatstone bridge circuit, is how these strain gauges are set up (see diagram). Accordingly, the measuring grid of the force being measured is aligned and four SGs are connected “in a ring.”
The weight of an object can be determined if it is placed on the load cell or suspended from it. A load cell’s intended load is always oriented towards the centre of the earth, or, more precisely, towards the direction of gravity. The load should only have that force component obtained. Force sensors, which are similar in design and frequently referred to as “load cells,” are not like that because they are typically built to capture loads that occur in all directions. They can be installed in any direction, regardless of the direction of the earth’s gravitational pull.
various load cell types
For various uses, numerous types of load cells are available. Typical examples include:
Single point load cells: a load cell is located under a platform that is loaded with a weight from above
Bending beam load cells: several load cells are positioned under a steel structure and are loaded with a weight from above
Compressive force load cells: several high-capacity load cells are positioned under a steel structure that is loaded with a weight from above
Tensile load cells: a weight is suspended from one or more load cells
Many load cells also have unique characteristics, such a unique design or unique material qualities. Depending on the application, that can be crucial, for instance if systems need to be completely cleaned every day. Some load cells can easily sustain this kind of force, but not all of them can.
In addition, load cells can be divided into groups according to how the signal is transmitted: Built-in electronics in digital load cells are utilised to process measurement findings and deliver them in a predetermined format. A measurement amplifier is a separate device needed for analogue load cells.
At the location where the most deformation occurs when force is applied, four strain gauges are positioned on the load cells below. The arrow points in the direction of applying force.
impact of the environment on load cells
Load cells have the unique characteristic that their usage environment affects their performance in a variety of ways.
Ambient temperatures
Every substance undergoes temperature-related changes, expanding in the presence of heat and contracting in the presence of cold. Of course, load cells and associated strain gauges fall within the same category. The conductor’s electrical resistance is likewise altered by this. Nevertheless, load cells must consistently measure the right weight no matter the local climate. Every HBM load cell has an advanced temperature compensation system to do this.
Load cells need to be resilient to a variety of things. “Think about a truck scale: These scales must be able to survive the outdoor ambient conditions because they are exposed to the elements, including rain, dirt, and heat. Additionally, we’re talking globally: For instance, a truck scale in South Africa is subject to different influences than one in Siberia. But they do share one thing: They must be built to withstand locations with extreme weather, thus they must be tough “Stefan Schmidt adds.
Application of force in other directions (“parasitic forces”)
Other loads in addition to the weight may arise depending on the technological environment in which a load cell is located, such as in a system for weighing containers or in a weighing cell beneath a conveyor belt. “Parasitic forces” are forces that are exerting pressure on the load cell in addition to the desired major direction, such as from the side, below, or in another manner. The measurement findings could be erroneous or just plain wrong because the load cell was not designed for this use. Therefore, it is important to take precautions during installation to guarantee that there are as few parasitic pressures as feasible. Users that employ HBM load cell fittings and weighing modules can reduce these auxiliary forces and obtain accurate measurement results.
Using Load Cells
Scales frequently have load cells fitted, of course. However, there are a tonne of other uses as well. Consider systems or bottling facilities that fill bottles and cans by weight, each using a load cell. Or mechanisms for sorting candy or potatoes into bags so they will all weigh the same after the process is finished.
As Schmidt notes, there are also uncommon applications for load cells. “For instance, our PW15iA single point load cell is used in the creation of high-performance triathlon equipment and swim wear: It analyses the water resistance of suits for world-class swimmers,” he says.
Accuracy of load cells
Accuracy is crucial in every application where a load cell is employed. There are various accuracy classes and maximum capacities for load cells (the maximum capacity indicates the maximum intended load). For classes C and D, strain gauge technology is largely used. In some weighers with better accuracy grades, strain gauge load cells with electronic correction are employed.
Weighing is rigorously controlled as a result. Using several scales is acceptable depending on the goods: The weigher does not need to be highly precise for low-value products (such sand or gravel)—accuracy class D is sufficient. On the other hand, pharmaceutical items must adhere to the highest accuracy classes A or B. Meat, fruit, and vegetables, which are among the most popular consumer commodities, are measured with accuracy class C, which already lays strict criteria on the precision of the weighing equipment in use. However, accuracy class C is also necessary for scales used in building materials or mechanical engineering.
The development of load cells has ongoing difficulties due to the tight regulations and criteria for weighing. A variance in accuracy of just a few ppm (parts per million) of the load cell’s measuring range is allowed for load cells in the higher OIML accuracy class.
Stefan Schmidt observes that it is remarkable how precisely a piece of steel with a strain gauge attached can be measured. “However, it is impossible without the required expertise. HBM has consistently worked to raise the accuracy attained by numerous load cells in recent years. a never-ending process of continuous progress.”
legal verification, calibration, and adjustment
A load cell is often not only adjusted and calibrated in the factory but also checked at the installation place to guarantee that it operates on site as it should. The load cell is loaded with known weights during calibration and legal verification processes for control. Depending on the accuracy class, the indicated weight should also be one kilo, not, for instance, 857 grammes, if a kilo is lying on the load cell or weigher. Unwanted outcomes like parasitic forces can also be found in this method.
Trends in load cell technology
Numerous applications and great precision may be found for load cells based on strain gauges. Only with the necessary knowledge, which is itself founded on experience, can this precision be attained. HBM has developed load cells for more than 65 years, and it is constantly coming up with new solutions to meet the demands of the future.
“Customers used to frequently inquire about the future of strain gauge technology ten years ago because it was already being completely utilised. Of course, we asked that same inquiry to ourselves “Schmidt, Stefan continues. However, we discovered during our investigations that strain gauge technology is still far from being completely utilised and that there are still innumerable other opportunities for significantly upgrading both our sensors and the products of our customers. There is therefore no foreseeable end to the technological advancement of load cells. More precise, more robust, faster, and more intelligent