Load Cells General Description

Using 4-arm, 350-ohm bonded foil or 500-ohm bonded semiconductor bridges, these tough stainless-steel load cells yield high accuracy and linearity in any number of industrial and research applications, with exceptional structural resistance to off-axis loading, side-loading, and other extraneous forces (see Load Cell Side and Bending Forces), and with safe overload protection for up to 50% over capacity.

Daytronic provides three basic types of general-purpose DC-excited strain gage load cells:

  • Low-profile (400 Series: 25 - 5000 lb.)
  • Miniature (431, 431M, 434A, and 434AM Series
  • 5 - 1000 lb; 50 - 1000 g)
  • "Pancake-thin" (441 Series: 5 - 50000 lb.)

Load buttons and other load cell accessories are also offered.

How They Work

Strain Gages

The resistance strain gage is an electrical sensing device that varies its resistance as a linear function of the strain experienced by the structural surface to which it is bonded.  "Strain" is the deformation of a solid material as the result of applied forces (internal or external), and is normally expressed in units of microinches per inch (or "microstrain").

A typical strain gage consists of a conductive grid pattern of etched metallic foil, mounted on a thin base of epoxy or fiberglass.  It can then be bonded to a surface in such a way that any subsequent deformation of the surface produces a like deformation of the gages.

When the gage is deformed, its electrical resistance changes.  This fact is explained partly by simple geometry.  That is, when a conductor is stretched lengthwise, its cross-sectional area decreases, with a consequent increases in resistance.  It is also partly explained by changes in the actual resistivity of the gage material when subjected to strain.

For a given amount of unit strain (ΔL/L), the gage will undergo a corresponding change in resistance (ΔR/R).  The ratio of the unit change in resistance to the unit change in length is known as the gage factor (Fg) of the gage:

Fg = (ΔR/R) / (ΔL/L)

Conventional foil gages have standardized nominal resistance values of 120 and 350 ohms, and typically exhibit gage factors between 1.5 and 3.5.  In typical transducer applications, they are subjected to full-scale design strain levels ranging from 500 to 2000 microstrain.

Strain Gage Transducers

In transducers, strain gage configurations are employed to measure weight, pressure, torque, and similar phenomena, by sensing the deformation of calibrated beams, diaphragms, or other flexures to which mechanical force is applied.  Strain gage transducers can be rugged, compact, linear, highly accurate, and readily compensated for wide temperature ranges.  They can be operated with many types of available AC and DC instruments, are are widely used in industrial and research measurement and control systems.

Through proper flexure design and gage placement, a linear relationship can be achieved between the applied force and the sensed strain.  The Wheatstone Bridge circuit shown below is almost universally used in load cells and other strain gage transducers, because it facilitates cancellation of unwanted temperature effects.  (In any reliable load cell, thermal expansion and temperature resistance effects must be made to cancel.  In particular, temperature effects on the modulus of elasticity of the flexure materials must be compensated, using carefully trimmed temperature-sensitive resistors (Rm in the figure)).

If the gages within a load cell are connected in a balanced Wheatstone Bridge circuit, and are excited by a source of AC or DC voltage, the transducer will produce an electrical output which is a direct linear function of the excitation voltage and the magnitude of the applied mechanical input.

Eout(mV) = Ein(V) • K • F/100

where

  • K = Calibration Factor (mV/V, full scale)
  • F = Input variable (% of full scale)

Transducer sensitivity is expressed in terms of millivolts per volt (mV/V).  The exact value of "K" for each instrument is determined by measurement at the time of manufacture and is furnished as part of that instrument's calibration data.  For conventional transducers, this value usually falls between 0.5 and 3.0.

Excitation voltage can be either AC or DC, and is usually limited by heating considerations to a maximum of 10 volts for 120-ohm bridges and 20 volts for 350-ohm bridges (although good practice dictates somewhat lower values).

 
 
 

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