What is a load cell? How does a load cell work?

Design and function of a load cell

Load cells are used to measure weight. They are an integral part of our daily life. "In your car or at the cheese counter in the supermarket – we encounter load cells everywhere," says HBM Product Manager Stefan Schmidt. Of course they are usually not immediately recognizable, because they are hidden in the inner workings of instruments. Load cells generally consist of a spring element on which strain gauges have been placed. The spring element is usually made of steel or aluminum. That means it is very sturdy, but also minimally elastic. As the name "spring element" suggests, the steel is slightly deformed under load, but then returns to its starting position, responding elastically to every load. These extremely small changes can be acquired with strain gauges. Then finally the deformation of the strain gauge is interpreted by analysis electronics to determine the weight. To understand this last point, let us consider strain gauges in more detail: They are electrical conductors firmly attached to a film in a meandering pattern. When this film is pulled, it – and the conductors – get longer. When it is contracted, it gets shorter. This causes the resistance in the electrical conductors to change. The strain can be determined on this basis, as resistance increases with strain and diminishes with contraction. The strain gauges are firmly attached to the spring element, and therefore undergo the same movements it does. These strain gauges are arranged in what is called a bridge circuit, or more precisely a Wheatstone bridge circuit (see diagram). This means that four SGs are connected "in a ring" and the measuring grid of the force being measured is aligned accordingly. If an object is placed on the load cell or suspended from it, the object's weight can be determined. The intended load for a load cell is always aligned in the direction of the center of the earth, in other words in the direction of gravity. Only that force component of the load should be acquired. That is not the case for force sensors, which are similar in design, and are also frequently specified as "load cells": They are usually designed to acquire loads that occur in all directions. The direction of the earth's gravitational force is not relevant to how they are installed.

Environmental effects on load cells

One special feature of load cells is that the environment in which they are used plays a decisive role – in a number of ways.

 

Ambient temperatures

 

Every material changes with temperature, expanding in response to heat and contracting in response to cold. Of course the same applies to load cells and their strain gauges. This also changes the electrical resistance of the conductor. Yet load cells must measure the correct weight everywhere in the world, regardless of the ambient temperature. To achieve this, a sophisticated temperature compensation mechanism is built into every HBM load cell. Load cells must be able to withstand various effects. "Consider a truck scale: These scales are exposed to the elements: rain, dirt or heat – they have to be able to withstand outdoor ambient conditions. And we are talking world-wide: A truck scale in Siberia, for example, is exposed to different effects than one in South Africa. But they do have one thing in common: They must be designed for environments with severe weather and must therefore be correspondingly rugged," says Stefan Schmidt.

Using load cells

Of course load cells are often installed in scales. But there are also many other applications. Think of bottling plants or systems for filling bottles and cans by weight – with a load cell under every one. Or sorting systems for distributing candies or potatoes into bags so that they all have the same weight at the end of the process.

"It's astonishing how many applications require load measurements." - Stefan Schmidt

And there are also unusual uses for load cells, as Schmidt points out: "For example, our PW15iA single point load cell is used in the development of high-performance triathlon apparel and swim wear: It measures the water resistance of suits for world class swimmers."

As part of its current research to demonstrate the effect of water resistance on various suits for world class swimmers, Huub Ltd. is introducing its new M16 system for measuring water resistance to the market. The system uses load cells type PW15iA from HBM.