Load cell

A strain gauge is constructed of very fine wire, or foil, set up in a grid pattern and attached to a flexible backing. When the shape of the strain gauge is altered, a change in its electrical resistance occurs. The wire or foil in the strain gauge is arranged in a way that, when force is applied in one direction, a linear change in resistance results. Tension force stretches a strain gauge, causing it to get thinner and longer, resulting in an increase in resistance. Compression force does the opposite. The strain gauge compresses, becomes thicker and shorter, and resistance decreases. The strain gauge is attached to a flexible backing enabling it to be easily applied to a load cell, mirroring the minute changes to be measured.

Since the change in resistance measured by a single strain gauge is extremely small, it is difficult to accurately measure changes. Increasing the number of strain gauges applied collectively magnifies these small changes into something more measurable. A set of 4 strain gauges set in a specific circuit is called Wheatstone bridge.

A Wheatstone bridge is a configuration of four balanced resistors with a known excitation voltage applied as shown below:

Excitation voltage ${\displaystyle V_{\text{EX}}}$ is a known constant and output voltage ${\textstyle V_{o}}$ is variable depending on the shape of the strain gauges. If all resistors are balanced, meaning ${\displaystyle {\frac {R1}{R2}}={\frac {R4}{R3}}}$ then ${\textstyle V_{o}}$is zero. If the resistance in even one of the resistors changes, then ${\displaystyle V_{0}}$will likewise change. The change in ${\textstyle V_{o}}$ can be measured and interpreted using Ohm's law. Ohm's law states that the current (${\textstyle I}$, measured in amperes) running through a conductor between two points is directly proportional to the voltage ${\textstyle V}$ across the two points. Resistance (${\displaystyle R}$, measured in ohms) is introduced as the constant in this relationship, independent of the current. Ohm's law is expressed in the equation ${\displaystyle I=V/R}$.

When applied to the 4 legs of the Wheatstone bridge circuit, the resulting equation is:

${\displaystyle V_{o}=\left({\frac {R3}{R3+R4}}-{\frac {R2}{R1+R2}}\right)V_{\text{EX}}}$

In a load cell, the resistors are replaced with strain gauges and arranged in alternating tension and compression formation. When force is exerted on the load cell, the structure and resistance of the strain gauges changes and ${\textstyle V_{o}}$ is measured. From the resulting data, ${\textstyle V_{o}}$ can be easily determined using the equation above.[1]

There are several types of strain gauge load cells:[2]

• Single Point load cells; used in small to medium platform scales with plattform sizes of 200x200mm up to 1200x1200 mm.
• Planar Beam load cells; used in low profile solutions where space is limited, like medical scales and retail scales.
• Bending Beam load cells; used in pallet, platform and small hopper scales.
• Shear Beam load cells; used in low profile scale and process applications, available in capacities from 100kg up to 50t!
• Dual Shear Beam load cells; used in truck scales, tank and hopper applications.
• S-type load cells; used in tension applications where you will find static and dynamic loads.
• Compression load cells; used in truck scales, large platform scales, weighbridges and hopper scales.
• Ring Torsion load cells; used in high accuracy hoppers, silo's, plattforms and pallet scales.
• Spoke Type load cells; used in low profile, high precision application. High forces varying from 1t-500t.
• Onboard load cells; used for onboard weighing systems on trucks, tractors and other vehicles.
• Loadpins; used in applications for measuring dynamic, static or hoisting forces.
• Weighpads; portable weighpads for the weighing of cars and the measure the center of gravity of planes.
• Specials; all kind of special sensors.

Vibrating wire load cells, which are useful in geomechanical applications due to low amounts of drift, and capacitive load cells where the capacitance of a capacitor changes as the load presses the two plates of a capacitor closer together.

Piezoelectric load cells work on the same principle of deformation as the strain gauge load cells, but a voltage output is generated by the basic piezoelectric material – proportional to the deformation of load cell. Useful for dynamic/frequent measurements of force. Most applications for piezo-based load cells are in the dynamic loading conditions, where strain gauge load cells can fail with high dynamic loading cycles. The piezoelectric effect is dynamic, that is, the electrical output of a gauge is an impulse function and is not static. The voltage output is only useful when the strain is changing and does not measure static values.

However, depending on conditioning system used, "quasi static" operation can be done. Using a charge amplifier with a long time constant allows accurate measurement lasting many minutes for small loads up to many hours for large loads. Another advantage of Piezoelectric load cells conditioned with a charge amplifier is the wide measuring range that can be achieved. Users can choose a load cell with a range of hundred of kilonewtons and use it for measuring few newtons of force with the same signal-to-noise ratio; again this is possible only with the use of a charge amplifier for conditioning.

The bridge is excited with stabilized voltage (usually 10V, but can be 20V, 5V, or less for battery powered instrumentation). The difference voltage proportional to the load then appears on the signal outputs. The cell output is rated in millivolts per volt (mV/V) of the difference voltage at full rated mechanical load. So a 2.96 mV/V load cell will provide 29.6 millivolt signal at full load when excited with 10 volts.

Typical sensitivity values are 1 to 3 mV/V. Typical maximum excitation voltage is around 15 volts.

The full-bridge cells come typically in four-wire configuration. The wires to the top and bottom end of the bridge are the excitation (often labelled E+ and E−, or Ex+ and Ex−), the wires to its sides are the signal (labelled S+ and S−). Ideally, the voltage difference between S+ and S− is zero under zero load, and grows proportionally to the load cell's mechanical load.

Sometimes a six-wire configuration is used. The two additional wires are "sense" (Sen+ and Sen−), and are connected to the bridge with the Ex+ and Ex- wires, in a fashion similar to four-terminal sensing. With these additional signals, the controller can compensate for the change in wire resistance due to e.g. temperature fluctuations.

The individual resistors on the bridge usually have resistance of 350 Ω. Sometimes other values (typically 120 Ω, 1,000 Ω) can be encountered.

The bridge is typically electrically insulated from the substrate. The sensing elements are in close proximity and in good mutual thermal contact, to avoid differential signals caused by temperature differences.

One or more load cells can be used for sensing a single load.

If the force can be concentrated to a single point (small scale sensing, ropes, tensile loads, point loads), a single cell can be used. For long beams, two cells at the end are used. Vertical cylinders can be measured at three points, rectangular objects usually require four sensors. More sensors are used for large containers or platforms, or very high loads.

If the loads are guaranteed to be symmetrical, some of the load cells can be substituted with pivots. This saves the cost of the load cell but can significantly decrease accuracy.

Load cells can be connected in parallel; in that case, all the corresponding signals are connected together (Ex+ to Ex+, S+ to S+, ...), and the resulting signal is the average of the signals from all the sensing elements. This is often used in e.g. personal scales, or other multipoint weight sensors.

The most common color assignment is red for Ex+, black for Ex−, green for S+, and white for S−.

Less common assignments are red for Ex+, white for Ex−, green for S+, and blue for S−, or red for Ex+, blue for Ex−, green for S+, and yellow for S−.[8] Other values are also possible, e.g. red for Ex+, green for Ex−, yellow for S+ and blue for S−.[9]

• ASTM E4 – Practices for Force Verification of Testing Machines
• ASTM E74 – Practice for Calibration of Force Measuring Instruments for Verifying the Force Indication of Testing Machines
• NTEP – National Conference on Weights and Measures (Certificate of Conformance)