Load cell

A load cell converts a force such as tension, compression, pressure, or torque into a signal (electrical, pneumatic or hydraulic pressure, or mechanical displacement indicator) that can be measured and standardized. It is a force transducer. As the force applied to the load cell increases, the signal changes proportionally. The most common types of load cells are pneumatic, hydraulic, and strain gauge types for industrial applications. Typical non-electronic bathroom scales are a widespread example of a mechanical displacement indicator where the applied weight (force) is indicated by measuring the deflection of springs supporting the load platform, technically a "load cell".

Strain gauge load cells are the kind most often found in industrial settings. It is ideal as it is highly accurate, versatile, and cost-effective. Structurally, a load cell has a metal body to which strain gauges have been secured. The body is usually made of aluminum, alloy steel, or stainless steel which makes it very sturdy but also minimally elastic. This elasticity gives rise to the term "spring element", referring to the body of the load cell. When force is exerted on the load cell, the spring element is slightly deformed, and unless overloaded, always returns to its original shape. As the spring element deforms, the strain gauges also change shape. The resulting alteration to the resistance in the strain gauges can be measured as voltage. The change in voltage is proportional to the amount of force applied to the cell, so the amount of force can be calculated from the load cell's output.

A strain gauge is constructed of very fine wire, or foil, set up in a grid pattern and attached to a flexible backing. When the shape of the strain gauge is altered, a change in its electrical resistance occurs. The wire or foil in the strain gauge is arranged in a way that, when force is applied in one direction, a linear change in resistance results. Tension force stretches a strain gauge, causing it to get thinner and longer, resulting in an increase in resistance. Compression force does the opposite. The strain gauge compresses, becomes thicker and shorter, and resistance decreases. The strain gauge is attached to a flexible backing enabling it to be easily applied to a load cell, mirroring the minute changes to be measured.

Since the change in resistance measured by a single strain gauge is extremely small, it is difficult to accurately measure changes. Increasing the number of strain gauges applied collectively magnifies these small changes into something more measurable. A set of 4 strain gauges set in a specific circuit is an application of a Wheatstone bridge.

A Wheatstone bridge is a configuration of four balanced resistors with a known excitation voltage applied.

Excitation voltage ${\displaystyle V_{\text{EX}}}$ is a known constant, and output voltage ${\displaystyle V_{\text{o}}}$ is variable depending on the shape of the strain gauges. If all resistors are balanced, meaning ${\displaystyle R1/R2=R4/R3}$, then ${\displaystyle V_{\text{o}}}$ is zero. If the resistance in even one of the resistors changes, then ${\displaystyle V_{\text{o}}}$ likewise changes. The change in ${\displaystyle V_{\text{o}}}$ can be measured and interpreted using Ohm's law. Ohm's law states that the current (${\displaystyle I}$, measured in amperes) running through a conductor between two points is directly proportional to the voltage ${\displaystyle V}$ across the two points. Resistance (${\displaystyle R}$, measured in ohms) is introduced as the constant in this relationship, independent of the current. Ohm's law is expressed in the equation ${\displaystyle I=V/R}$. When applied to the 4 legs of the Wheatstone bridge circuit, the resulting equation is $${\displaystyle V_{\text{o}}=\left({\frac {R3}{R3+R4}}-{\frac {R2}{R1+R2}}\right)V_{\text{EX}}.}$$

In a load cell, the resistors are replaced with strain gauges and arranged in alternating tension and compression formation. When force is exerted on the load cell, the structure and resistance of the strain gauges changes, and ${\displaystyle V_{\text{o}}}$ is measured. From the resulting data, ${\displaystyle V_{\text{o}}}$ can be easily determined using the equation above.

There are several types of strain gauge load cells: