Measure inside machines eliminating errors due to vibrations

Load cells

Measurements up to 1 mg of precision obtained in industrial environments


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The operating principle of resistive strain gauge load cells

The load cells

Everything you need to know about a load cell

The load cells are the active part of a sensor for the transduction of forces among which the weight measurement is the most common use. There are different types of load cells, several of these are based on very different measurement principles, for example capacitive, strain gauge, solid-state and electromagnetic compensation systems.

Strain gauge load cells

The strain gauge load cells are built using a metal material, often aluminium is used for example avional or, for special cases, steel alloys are used.

How does a load cell work? The transduction principle is quite simple to understand when a mass is weighed on the load cell, the cell deforms as a result of the weight associated with the mass-dependent on the action of the acceleration of earth’s gravity.

The deformation undergone by the load cell is measured with a device called a strain gauge. In the most common case, the strain gauge is of the resistive type which is glued into the load cell where it undergoes the greatest deformation.

The strain gauge deforms together with the load cell, it deforms exactly as the cell deforms. In deformation, the electrical resistance changes because the compression or elongation of the resistance filament changes the resistance that the material, of which it is composed, opposes to the flow of electrons.

The Wheatstone Bridge

The strain gauges used in load cells form a simple resistive circuit that takes the name of who perfected the instrument invented by Samuel Hunter Christie in 1833: the Wheatstone bridge.

A Wheatstone bridge is a particularly clever idea for making electrical resistance measurements.

In strain gauges, the Wheatstone bridge is used to determine the change in resistance of one or more branches of the bridge.

In the load cells, the strain gauges are powered in two opposite vertices with a voltage that is normally chosen from 3 to 12 Volts, while in the remaining two the voltage difference of the order of microvolts is measured.

Use of strain gauges to sensor the load cells

One or more strain gauges are glued at the points where the load cell deforms most. The experts in strain gauges call, according to the number of strain gauges used, constructions made as full-bridge, half-bridge or quarter bridge, if respectively four strain gauges are used, two or only one. These terms must not mislead because the circuit used for the measurement is always incomplete and the complete bridge is formed by means of resistors that are glued in areas where no deformation is created or are present inside the instrumentation used for the signal conditioning.

 

The compensation of the temperature of the load cells

Temperature causes variations in the electrical resistance of conductive materials. All the increase in temperature increases the electrical resistance and therefore the increase in electronic noise which determines a lower accuracy of the measurement.

In addition to influencing electronic agitation, the variation in temperature determines the variations in the volumes of bodies and liquids.

In the case of strain gage measurements carried out using load cells, the temperature variations, if not compensated, determine important measurement errors.

On the market, load cells are self-compensated in temperature. These are load cells that use self-compensated strain gauges, created to compensate for the expansive changes in the materials with which the load cells are made.

In addition to the use of self-compensated strain gauges, the connection to a Wheatstone bridge also allows for temperature compensation. By connecting two strain gauges to a Wheatstone bridge, the signal is doubled, therefore a deformation occurs as a function of temperature, the deformation appears on both strain gauges with the same sign so as to cancel each other’s effects.

Single point or off-centre load cells

Off-centre load cells are the most widespread, they have several peculiarities that interest them, let’s see them in order.

They must use a plate to deposit the mass to be weighed where whatever is the point where the object is placed, the centre of gravity coincides with the point of application of the plate to the cell which is exactly in line with the axis of application of the strength foreseen by the project. They are small in size and therefore relatively easy to place in machines and tools.

The dimensions of the load cells are also such when the full scale changes by increasing its value. It means that the external dimensions of these identical load cells passing from a capacity of 1 Kg to, for example, 30 Kg, what changes are only the thickness of the cell excavation folder.

They are among the most accurate resistive strain gauge load cells currently on the market.

The Sigma Low load cells are available on an off-centre cell for the measurement part of the mass to be weighed, while the apparent weight is measured by a diced system.