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Load Cell Measurement

Basic overview of load cells and strain gages and selecting the correct data aquisition hardware.

by Leo Cordaro

This article is intended to provide the reader with a basic understanding of strain gages, load cells and the data acquisition hardware used to acquire the signal. At the end of the article, you will find additional resources that provide further details. This article will highlight some key aspects of the sensor and some of the important attributes to look for when selecting the data acquisition hardware.

Load cells can be categorized into two major types of cells: mechanical cells and strain gage cells. The primary focus of this article will be on strain gage cells since these tend to be the most commonly used load cells in most industries. However, here is a brief overview of the two mechanical cell types:
  • Hydraulic load cells
These types of load cells measure weight as a change in pressure, typically a fluid inside the cell. The load applied to the sensor is transferred to a piston and in turn compresses a filling fluid within a chamber. As the force increases, pressure of the hydraulic fluid rises, usually linearly. Hydraulic load cells are well suited for a large weight range and hazardous environments.
  • Pneumatic load cells
Similar to hydraulic load cells, pneumatic load cells operate on the force-balance principle, that is, as the load is applied to the sensor, a piston compresses within a chamber. Pneumatic load cells are suited for a wide weight range, and ideal for clean environments (i.e. no potential for hydraulic fluid leaking).
Strain Gage Load Cells

To fully understand the construction of strain gage load cells, we will need to take a detour and discuss strain gages and strain measurement first.

Strain is the result of a force applied to a solid object. More specifically, strain is defined as the fractional change in length of an object, as depicted in the figure below.

There are two types of strain that can be applied to an object 1.) tensile or compressive and 2.) uniaxial force. For example, assume you have a steel bar cantilevered to the edge of your desk. When you push down on part of the bar hanging off the edge of the desk, you are applying a tensile force to the top of the bar. A compressive force would result when you pull the bar upwards toward you. Both of these strains cause a fractional change in length on the steel bar. On the other hand, a uniaxial force would occur if you grabbed the steel bar with both of your hands and pulled it axially (in the direction parallel to the bar). This would cause the girth (diameter) of the bar to shrink, and the length of the bar to increase (both being very small displacements and require some big muscles). The magnitude of these displacements is material dependent and indicated by the material’s Poisson Ratio (for example, the Poisson’s Ratio for steel ranges from 0.25 to 0.30).

A strain gage is a sensor where the electrical resistance varies in proportion to the amount of strain produced in a device. The picture below is an example of the most commonly used gage, called the bonded metallic strain gage.

All strain gages have a specified gage factor (GF), and this is a fundamental parameter of the gage’s sensitivity to strain. From a qualitative perspective, the gage factor is defined as the ratio of the fractional change in electrical resistance to the fractional change in length. The gage factor for a bonded metallic strain gage (as shown above) is typically around 2.
Strain measurements are very small, so it is extremely important that the gage is capable of measuring very small changes in resistance. A Wheatstone bridge is normally used in order to measure such small variations in resistance. The picture below is an example Wheatstone bridge:

This circuit is ideal at measuring any changes in the resistors. Based on the type of strain measurement, one, two or all four resistors can be replaced with a strain gage, commonly referred to as quarter, half and full bridge, respectively. Remember, a strain gage measures resistivity change as strain is applied to the device.

Selecting Correct Data Acquisition Hardware

Load cell (and strain gage) measurements involve sensing small changes in resistance. To properly acquire data from these sensors, the signal must be “conditioned” first. Ideally, the data acquisition hardware should provide the signal conditioning to simplify sensor connectivity. There would be no need to connect separate signal conditioning hardware in addition to data acquisition hardware.

It is important to consider the following when using load cells and / or strain gage sensors:

  • Bridge completion - If the sensor is configured as a quarter or half bridge, high-precision reference resistors are required to complete the Wheatstone bridge circuit.
  • Excitation - All load cells require an excitation voltage, as demonstrated in the Wheatstone bridge circuit. Vex values can range anywhere from 0.625 V to 10 V, and the exact voltage required is sensor dependent.
  • Amplification - The output of a strain sensor is small, for example, it’s common to see 10 mV/V (that is, 10 mV of sensor output per volt of excitation voltage). Therefore, if we apply an excitation voltage of 10 V, the output signal to measure in the 100 mV range. Amplifiers are required to boost the signal level to increase measurement resolution.
  • Filtering – Anti-alias filters remove high-frequency noise before the signal is digitized.
  • Offset Nulling – Slight variations in resistance in the sensor and wire lead resistance will generate some nonzero initial offset voltage. Offset nulling can be performed so that the bridge will output zero volts when no strain is applied.
  • Shunt Calibration – Is the process in which a known resistance is applied to one of the legs of the Wheatstone bridge circuit. When this known resistance is “switched” in, the output of the bridge can then be measured and compared to the expected voltage value. This can be useful to correct span errors in the measurement path.

Please visit the additional resources below and feel free to contact us for any additional questions, or application specific questions.

Resources

Knowing how much pressure to apply is critical for product development.

Load measurements used during the manufacturing process