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What are load cells?

Load cells are transducers. Therefore electronic components are used to measure the forces applied to an object – usually mechanical components. These sensors, in fact, measure the electrical signal as a function of the deformation that the force exerts on the object. Their use is common in industrial weighing systems and determines the mechanical compression and traction forces. Through the strain gauges installed inside them, the load cells indirectly detect an object’s mechanical deformation by reading it, usually through the Volts or millivolts.

Going deeper into the discussion can lead one to believe that these are particular tools, used only in specific contexts. In our daily life, however, we encounter load cells very often. Or rather, more than you might think. A practical example, perhaps trivial but immediate, maybe that of the scale on which we weigh ourselves from time to time. Or, that scale that is found in all supermarkets, at the fruit and vegetable counter. And broadening this perspective a little – staying for a moment out of the industrial sphere, which sees Nanolever at the forefront – even Amazon Go, the supermarket of the future, bases its existence on load cells.

The fact is that these sensors are practically invisible. They are found inside the machinery used for weighing, whether industrial weighing systems or small food scales. Or scales of a completely different type, such as jewelry, pharmaceutical, and vehicle scales. In short, load cells are used very frequently.

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Use of load cells

The load cells convert the weight force of what is placed on the scale into an electrical signal, allowing you to determine the object’s mass in question. It is, therefore, correct to state that the load cells are the heart of the scale: they are, in fact, fundamental sensors for any weighing system. Not surprisingly, as we have anticipated, the use of load cells also includes very different areas. Their service makes it possible to measure from micrograms to tons. Here is a small list of situations in which load cells find their most common uses.

  • Dosing and sorting of food products and bottle and can filling systems (processing industries)
  • Quality control for the pharmaceutical industry
  • Weighing of means of transport (trucks, train carriages, ships, hoppers, tanks, etc.)
  • Loads on ropes
  • Detection of mechanical stresses
  • Test benches
Celle di carico
Celle di carico

How does a load cell work?

In summary, load cells are transducers, or electronic components capable of measuring the force applied to an object. This is possible due to an electrical signal which is updated as the deformation produced by the force changes. They are highly versatile devices, ideal for industrial weighing systems precisely because of their precision, practicality, and ease of use. Based on the applied force, the cell releases a signal translated into a numerical value indicating the measurement of the mass.

How to choose a load cell?

Observe the environment

It is essential to observe the environment in which the weighing system will have to operate. Then take note of the technical and environmental conditions and any other circumstance capable of influencing the weight measurement. An environment can be hostile due to extreme temperatures and the presence of corrosive chemicals, high vibrations, or the occurrence of unbalanced loads. Essentially, it is important to establish the conditions to determine the proper load cell to ensure the correct functioning of the entire weighing system.

Choice of material

Generally, load cells are tool steel – stainless steel or aluminum. The former is ideal for environments that guarantee dry conditions. Excessive humidity, in fact, risks rusting the steel. It is also the most used material for these transducers because it limits the phenomena of sliding and hysteresis.

On the other hand, aluminum is used for single-point load cells with low capacity and is unsuitable for humid or otherwise adverse conditions. However, like tool steel, aluminum is a popular option because it is also flexible and has good deformability characteristics.

The one that offers its best under challenging conditions is the most dispensative option in terms of materials. We are talking about stainless steel. The latter is, in fact, resistant to corrosive chemicals and excessive humidity. In the most extreme cases, however, it is possible to apply coatings that offer additional protection.

Check accuracy

Linearity and repeatability determine the accuracy of each load cell. Accuracy is expressed as full scale (±% F.S.) and indicates the range of the transducer. Temperatures that are too high can easily affect accuracy. However, most load cells are self-compensating. Further difficulties can be overcome with the realization of customized sensors (O.E.M.).

Environmental conditions

As anticipated, some environmental factors can be decisive in measuring the industrial weight. It is, therefore, necessary to take into account, for example, the operating temperature range and ambient noise (vibrations, which Sigma Low eliminates). Another factor to consider is environmental waterproofing (I.P. classification), then if the operating area of ​​the weighing system requires ATEX certification.

Finally, the internal or external use of the load cells allows you to determine the level of sealing necessary to counteract the infiltration of water or dust. In fact, in cases of external use, the installation of additional mechanical protections can be considered. The hermetically sealed load cells protect, in fact, both from chemicals and humidity present even with high values. These are transducers made of stainless steel and welded barriers to protect the strain gauge from environmental conditions.

Dimensions and scope

Will the load cell be exposed to overloads, excess weight, or other particular conditions of mechanical stress? It would be best if you were sure that the results were reliable and repeatable under any conditions. In fact, in an industrial plant, all the load cells must have the same weighing capacity.

Another important aspect of being evaluated is the following: the re-entry of the mechanical constraints. Depending on how the automatic line is structured, it is possible to adapt different types of load cells. However, in some cases, only cells of a specific size may be needed. Finally, the installation of the cell represents a further primary detail to be taken into account. It is a fundamental foresight to ensure precise and effective weighing.

Types and principles of measurement

As anticipated, there are different load cells: types based on very different measurement principles, for example, capacitive, strain gauge, solid-state, and electromagnetic compensation systems. Other types of sensors frequently used are load cells with foil in bending, cells with compression force, and, finally, those in traction. Each of these is based on specific operating principles. The choice of one type to the detriment of the other is determined according to the needs of companies. They are, therefore, purely functional choices.

In any case, most of these sensors are electronic. And immediately after, in order of importance – frequency of use -, we find the hydraulic ones – or if you prefer hydrostatic ones. Generally, the latter is used outdoors to avoid some problems that, in particular specific conditions, can occur with more traditional load cells.

Single point or off-center load cells

The most common scales are off-center ones. These weighing systems have several peculiarities that make them attractive in weight measurement. These scales allow you to use a plate to deposit the mass to be weighed wherever the point where you want to place the object is located. The center of gravity coincides with the issue of the application of the plate to the cell. The latter, of course, is in line with the axis of application of the force envisaged by the project.

These cells are also small in size, which makes them easy to place in machines and tools. Their dimensions and geometries remain so even when the full scale varies, increasing its value. This means that the external dimensions of these cells remain identical when, for example, passing from a capacity of 1 kg to 30 kg. What changes is only the thickness of the cell excavation folder.

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Electromagnetic compensation scales

Electromagnetic compensation scales are particularly sophisticated tools, enough to deserve a little separate paragraph. These scales do not use load cells and are very useful for detecting extremely accurate measurement accuracies at high speed. 

Their operation is similar by analogy to that of an arm scale. On the one hand, is the weight. On the other hand, a magnetized cylinder, which enters inside a solenoid – an electromagnetic coil, attracts that magnetized cylinder until the arm realigns to zero. And which therefore balances the weight felt on the opposite side. A current is introduced inside the coil to vary the magnetic field, determining the number of micrograms – the weight – to be transduced.

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Load cells and strain gauges

In summary, a load cell is a geometric metal structure in which the elastic field of its deformation is exploited. But in what sense is its ability to deform itself used? Through strain gauges. The deformation signal (tension) is transduced into numerical values ​​representing the weight measurement by applying them. Each time a product is positioned above the cell, it is thus possible to determine its weight value.

Extensometers are therefore essential to measure this deformation. In the most common cause, the strain gauge is of the resistive type and is positioned where the sensor undergoes the most significant deformation. Therefore it deforms exactly as cells deform. And in the deformation phase, the electrical resistance changes. Because the compression – or elongation – of the resistance filament also modifies the resistance as opposed to the flow of electrons by the material it is composed of.

The principle of strain gauge transduction

The principle of transduction is quite simple to explain. Every time a mass is placed on the sensor, the sensor deforms due to the weight associated with the mass, which is, in turn, dependent on the force of Earth’s gravity.

The functioning of strain gauges

In almost most cases, strain gauges are positioned on materials that can undergo deformation if subjected to a force (for reasons of elasticity). As we have seen, the cells under load undergo a certain deformation. However, once the product to be weighed has been removed, the metals return to the starting position. It is precisely at this moment that strain gauges play a role. Their presence is, in fact, essential to detect the variations and deformations interpreted by the electronics onboard the weighing system.

It may be helpful at this point to offer a brief overview of the strain gauges. These are measuring instruments used to detect small dimensional deformations. Strain gauges are attached to a film: when pulled, it ends up stretching along with the conductors. When it contracts, it shrinks. The consequence of these variations is a change in resistance in the conductors. And the deformation is measured according to this principle, since the opposition, in this case, increases – while, vice versa, it increases in the contraction phase. 

Given the close link between strain gauges and the components to which they are connected, strain gauges can be defined as integral with the latter. Electrical resistance is measured via a Wheatstone bridge circuit. The strain gauge wire follows the deformation of the component to which it is connected. At the same time, the measurement of the potential difference allows tracing the value of the deformation.

Use of resistive strain gauges

In the points of most significant deformation of the load cells, one or more strain gauges are glued. Strain gauge measurement experts call – depending on the number of strain gauges used – the configurations: full-bridge, half-bridge, or quarter bridge (if, respectively, four, two, or only one strain gauge are used).

 These terms should not be misleading because the circuit used for the measurement is always incomplete. The complete bridge is formed using resistors glued in areas where no deformation is created. Or where they are present inside the signal conditioning instrumentation.

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Load cells: temperature and electrical resistance

Temperature causes variations in the electrical resistance of conductive materials. And as the temperature increases, the electrical resistance also increases. Hence the increase in electronic noise determines a lower accuracy of the measurement of industrial weight. In addition to affecting electronic agitation, the temperature variation imposes variations in the volumes of bodies and liquids. Strain gauges are sensitive to changes in heat, so that if these are not compensated, they can cause even critical measurement errors.

Electrical stresses

The electrical resistance is variable concerning the temperature: by increasing the latter, the electrical resistance also increases (and vice versa). Therefore, the temperature introduces measurement errors – which are corrected by the same operating principle as the Wheatstone bridge. In this regard, some resistances of the electronic configuration are used precisely to compensate for the temperature variation during weight measurement.

In the vast majority of cases, the use of weighing systems takes place at room temperature. Never below 10 degrees, and rarely above 40. For a Wheatstone bridge, canceling the effect of temperature is intrinsic. So the problem does not arise (if you have the right tools). 

What is the only aspect that can make temperature drift problematic in extreme precision measurements? The answer may appear obvious to the trained eye. The resistances that form the bridge do not all have the same precision, and therefore are not the same. It is then clear that if they are not, they will measure responses in slightly different temperatures.

Variazioni di temperatura ed elettroni di conduzione

Tutte le misure di grande precisione risentono della temperatura. I metalli, per esempio, si comprimono e dilatano facilmente a seconda del caldo o del freddo. La variazione è minima, certo, però avviene. (E un discorso simile vale anche per i liquidi). Le misure elettriche, in sostanza, sono quelle che risentono maggiormente della temperatura. Questo perché all’interno di un conduttore la variazione di temperatura modifica sostanzialmente il moto caotico degli elettroni. Cioè, maggiore è la temperatura, maggiore è il loro movimento caotico – che disturba il moto degli altri elettroni. E con l’aumentare del rumore di fondo, diminuisce la capacità della bilancia di misurare più precisamente possibile. A tal proposito i materiali super-conduttori vengono impiegati, nei sistemi di pesatura industriali, a bassissime temperature. In questo modo diventa infatti possibile limitare il moto degli elettroni – lasciando scorrere solamente quelli di conduzione.

Celle di carico: come controbilanciare la temperatura

La maggioranza delle celle di carico sul mercato è di tipo auto-compensante in temperatura. Si tratta di sensori che utilizzano estensimetri anche loro auto-compensanti, nati per equilibrare le modifiche espansive dei metalli tipici delle celle di carico. Oltre all’uso di questo tipo di estensimetri, anche il collegamento ad un ponte di Wheatstone consente di gestire quest’aspetto. Questo perché collegando due estensimetri ad un ponte di Wheatstone si ottiene il raddoppio del segnale. Pertanto, se si verifica una deformazione in funzione della temperatura, la deformazione appare a entrambi gli estensimetri con stesso segno. In questo modo si annullano reciprocamente gli effetti.

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The effects of parasitic forces

Parasitic forces are forces that have no effect in the direction desired by the load cell. On the contrary, they can come from the sides, from below, or from another direction. Their origin – or their intensity- is closely related to the functioning of the automatic machine. The risk is to design an automated machine that acts incorrectly on the load cells, with parasitic forces capable of negatively influencing the weight measurement result. The load cells have not been realized for this purpose: it is necessary, in positioning them, to make sure that there are not too many external influences. 

However, Nanolever’s patented Sigma Low weighing system eliminates this problem at its root. The positioning of the load cells is not affected in any way by external forces. So much so that it was born as a solution to measurement errors induced by the vibrations of industrial machinery.

Accuracy of the load cells

Electronically corrected strain gauge load cells can achieve very high levels of accuracy. Based on the type of product you want to weigh, you can choose a specific scale and a kind of load cells. Some weighing systems may be appropriate for measuring sand and gravel; others are suitable for fruit and vegetables. In contrast, still, others are suitable for measuring the weight of precious metals or medicines.

Installation and legal verification

Once installed, any load cell must also be verified against its actual functions: calibrating it is not enough. Calibration and legal verification are, in fact, processes for which it is essential to set up a control. It is possible to notice any problems such as parasitic forces or that the accuracy class corresponds to the one established.

Weighing technologies: the Sigma Low system

Nanolever has patented a weighing system to solve the harmful problem of vibrations, typical of industrial machinery. The latter vibrating can distort the result and induce processing industries to commit substantial wastes of raw material. As for the measurement of the mass to be weighed, the Sigma Low weighing system is based on an off-center cell. The apparent weight, on the other hand, is measured by a dedicated system. To learn more, click here. And do not hesitate to contact us for any need relating to weighing systems.