Force and Load Sensors

Force and load sensors cover electrical sensing devices that are used to measure tension, compression, and shear forces. Tension cells are used for measurement of a straight-line force "pulling apart" along a single axis; typically annotated as positive force.  Compression tension cells are used for measurement of a straight-line force "pushing together" along a single axis; typically annotated as negative force.  Shear is induced by tension or compression along offset axes.  They are manufactured in many different packages and mounting configurations.

Important parameters for force and load sensors include the force and load measurement range and the accuracy.  The measurement range is the range of required linear output.  Most force sensors actually measure the displacement of a structural element to determine force. The force is associated with a deflection as a result of calibration. There are many form factors or packages to choose from: S-beam, pancake, donut or washer, plate or platform, bolt, link, miniature, cantilever, canister, load pin, rod end, and tank weighing.  Shear cell type can be shear beam, bending beam, or single point bending beam.  Force and load sensors can have one of many output types.  These include analog voltage, analog current, analog frequency, switch or alarm, serial, and parallel.

Force and load sensors can be many different types of devices including sensor element or chip, sensor or transducer, instrument or meter, gauge or indicator, and recorder and totalizers.  A sensor element or chip denotes a "raw" device such as a strain gage, or one with no integral signal conditioning or packaging.  A sensor or transducer is a more complex device with packaging and/or signal conditioning that is powered and provides an output such a dc voltage, a 4-20mA current loop, etc.  An instrument or meter is a self-contained unit that provides an output such as a display locally at or near the device. Typically also includes signal processing and/or conditioning.  A gauge or indicator is a device that has a (usually analog) display and no electronic output such as a tension gage.  A recorder or totalizer is an instrument that records, totalizes, or tracks force measurement over time.  Includes simple datalogging capability or advanced features such as mathematical functions, graphing, etc. 

The most common force and load sensor technologies are piezoelectric and strain gauge.  For piezoelectric devices, a piezoelectric material is compressed and generates a charge that is conditioned by a charge amplifier.  For strain gage devices, strain gages (strain-sensitive variable resistors) are bonded to parts of the structure that deform when making the measurement. These strain gages are typically used as elements in a Wheatstone bridge circuit, which is used to make the measurement.  Strain gages typically require an excitation voltage, and provide output sensitivity proportional to that excitation.

Features common to force and load sensors include biaxial measurement, triaxial measurement, and temperature compensation.  Biaxial load cells can provide load measurements along two, typically orthogonal, axes.  Triaxial load cells can provide load measurements along three, typically orthogonal, axes.  Temperature compensated load cells provide special circuitry to reduce/eliminate sensing errors due to temperature variations.  Other parameters to consider include operating temperature, maximum shock, and maximum vibration.

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