How does a capacity strain gauge work

Measurement of weight forces with the help of load cells

In this article we will deal with the measurement of weight forces with the help of load cells, in such detail that you can:

  • seehow load cells work;
  • learnhow weight measurements are made in science and industry;
  • understandhow you can incorporate the measurement into your tests.

What is a load cell?

A load cell is essentially a Force converter or Force sensor. It is mainly used to measure weight forces. And although it can also be used to measure other forces such as torque, pressure, etc., we will look at this in this article Weight measurement applications focus.

Example of a load cell
Image source: Daraceleste / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

Although gravity is the weakest of the four fundamental forces in physics or fundamental interactions that hold the universe together, it is the only one that we can directly sense. On earth, gravity acts on everything that has mass and pulls it to the iron core of our planet. Weight (weight) and mass are connected to one another by gravity.

The international unit of measurement for weight is that Kilogram (kg). Even in the United States, where the Anglo-American system of measurements for weight, distance, and volume is still common, these units are now related to the International Metric System (SI).

So is the US pound (Commercial weight) is no longer defined by another Anglo-American unit of measurement, but officially as 0.45359237 kilograms. In 2019, the International Bureau of Weights and Measures decided on new methods for determining the mass of the kilogram using Planck's constant; the reference to a physical reference standard (original kilogram) has been deleted.

If you remove gravity, a body literally weighs nothing, but its mass remains unchanged. When you weigh something, you are actually measuring the weight that acts on this mass under the conditions prevailing on earth. Similarly, a load cell actually determines a mass, but determines the weight resulting from the physical reality on our planet.

Load cells can be based on different functional principles. In the area of ​​intrinsic safety, for example, pneumatic load cells used, and hydraulic load cellsthat do not require electricity are often used in remote, inaccessible locations. In addition, there are also piezoelectric load cells, for example, which provide a high (but not linear) output signal.

We'll focus on here, however Load cells with strain gauges (DMS) that are most widespread worldwide. They are inexpensive, very reliable, and available for a wide variety of force input signals. With their Accuracy of 0.25% and better over the entire measuring range they are now the de facto standard in the weighing industry.

You may even use a strain gauge load cell based device every morning without even realizing it. This could look something like this:

The digital bathroom scale is a strain gauge based load cell device

While classic mechanical scales use levers to distribute the applied load and move a spring connected to a mechanical scale, most modern digital scales use multiple strain gauge-based load cells to calculate weight.
If you tap the weighing plate with your foot, the built-in microcontroller is activated and carries out a zero point calibration of the strain gauges. The digital display now shows 0.0 kg and you can step on the scale and weigh yourself.
High-quality digital bathroom scales are equipped with strain gauge load cells at all four corners, and these are often housed in the “feet” of the scales. Two of these load cells are positioned in pull mode and the other two in pressure mode.
When you step on the scales, the microcontroller records the output signals of the load cells, converts them into a total weight value in the selected unit of measurement and shows this on a measuring display. Since strain gauges, as resistance sensors, are temperature-sensitive, some scale models even use a thermocouple to measure the ambient temperature and take this into account when calculating the weight.
But let's now turn to the use of load cells for scientific data acquisition applications.

How does a load cell with strain gauges work?

A strain gauge (DMS) measures strain based on changes in resistance. For this purpose, a measuring grid film made of resistance wire is attached to a flexible carrier, which also serves as an insulator. A current is passed through the resistance wire. If the test object is now loaded (i.e. compressed or placed under mechanical tension), there is a change in resistance that is proportional to the extent of the deflection.

Full bridge strain gauges

When stretched, the resistance of a conductor increases. On the other hand, if it is compressed, its resistance decreases. This change in resistance can be measured using a Wheatstone bridge, where four strain gauge sensors are arranged in a specific way.

How a strain gauge works (bending exaggerated for clarity)
Image courtesy of WikiCommons
Schematic structure of a Wheatstone bridge

If all four sensors are used for measurements, one speaks of one Full bridge configuration. In the full bridge diagram above, the output voltage of the sensor is measured at C and B while the excitation voltage is applied to A and D.

So if we apply a strain gauge to a measuring body (mechanical housing) and then subject this measuring body to a tension or force - such as a weight force - then the measuring strip measures the relative pressure or tension that this force exerts. The combination of the measuring body and the strain gauge attached to it is called a load cell.

Now let's look at the different shapes and sizes of such load cells and how they are used.

Load cell measurement applications

There is a practically unlimited number of possible applications for both large and small load cells. Here is a small selection:

  • material testing - Measurement of the weight of components during production to maintain manufacturing tolerances
  • Aerospace - Thrust tests on jet engines, measurement of wheel and chassis loads
  • shipping - Monitoring the tension in mooring lines under the load of ships moving in the water
  • Transportation - Torque measurement on electric, gasoline and diesel engines, axle load monitoring, wheel load measurement on trains and trucks, measurement of load weights on motorway weighing stations
  • Industry - Force measurement on gears and pumps, voltage measurement in underwater pipe laying, voltage and force measurement in paper mills and steel mills, hopper and container weighing
  • Medicine / Healthcare - Patient bed scales for hospitals, precision scales for baby and toddler incubators, load measurement on physiotherapy and training equipment
  • Construction - Cable forces in elevators, force measurement according to international standards on scaffolding. Here you can find an interesting application example.
  • entertainment - Assembly of S-Beam load cells to ensure compliance with the prescribed load limits in steel cables used for lifting acrobats and actors
  • Petrochemicals - Force measurement on oil and gas drilling rigs
  • Agriculture and ranching - Measurement of the live weight of livestock on cattle ramps, force measurement on cables and hoists, funnel, can and silo weighing
  • Household / consumer - digital bathroom scales

When we look at the diversity that can already be seen in just a dozen or so application areas, it becomes clear that force and weight measurements, similar to temperature measurements, are essential for tens of thousands of applications in virtually all branches of industry.

Considerations for using load cells

Maximum load capacity

It is important to use a load cell that not only corresponds to the type or direction of the load you are measuring, but that is sized for the maximum load that you can expect.
Load cells, for example, are classified according to their maximum load capacity in kg (or lbs). In order to measure a weight of up to 1000 kg, you should of course use a load cell that is designed for at least one and a half to two times as much.

Two-point calibration and zero adjustment

The calibration sheet, which is supplied with all load cells, informs you about the preferred excitation voltage and the sensitivity of the strain gauges used. You will need this information in order to set up the software to accurately measure the output signal in the desired unit of measure (usually kg or lbs).

In the Dewesoft X software, it is quite easy to create a ZERO calibration of any sensors perform; z. B. by removing all loads and then clicking ZERO in the software (just like your bathroom scale does automatically). You can also perform a two-point calibration with a known reference load to ensure that your load cell is outputting correct, linear values ​​between zero and the reference load you have applied.

Setup and calibration of a load cell in the data acquisition software Dewesoft X

Environmental aspects