Load Cell Linearization using Micron Instruments' Semiconductor Strain Gages

Herbert Chelner, CEO and Chief Scientist

Dr. Robert A. Mueller, President and General Manager

Load Cells

A load cell is a transducer that converts an input force into a measurable electrical output. Load cells are built with different force measurement devices; however, strain gage load cells are the most common choice.

Strain gages convert mechanical motion into an electronic signal. A change in resistance is proportional to the strain experienced by the sensor. If a wire is held under tension, it gets slightly longer and its cross-sectional area is reduced. This changes its resistance (R) in proportion to the strain sensitivity of the wire's resistance. When a strain is introduced, the strain sensitivity, which is also called the gage factor (GF), is given by:

GF = (ΔR / R) / ε

Strain gauge-based load cells measure material movement when the material of the load cells deforms appropriately. The gages are bonded onto a beam or structural member that deforms when weight is applied. The change in resistance of the strain gauge provides an electrical value change that is calibrated to the load placed on the load cell.

In most cases, four strain gauges (in a full Wheatstone bridge configuration) are used to obtain maximum sensitivity and temperature compensation. Two of the gauges are placed in tension (T1 and T2), and two in compression (C1 and C2), and are wired with compensation adjustments. Gages are mounted in areas that exhibit strain in compression or tension. When weight is applied to the load cell, gauges C1 and C2 compress decreasing their resistances. Simultaneously, gauges T1 and T2 are stretched increasing their resistances. The change in resistances causes increased current to flow through C1 and C2 and decreased current to flow through T1 and T2. The associated voltage difference through 2 differential outputs provides the desired measurement.

Micron Instruments’ Semiconductor Strain Gages

Semiconductor strain gages (SSGs) make use of the piezo-resistive effect exhibited by certain semiconductor materials such as silicon and germanium in order to obtain greater sensitivity and higher-level output. SSGs can be produced to have either positive or negative changes when strained. They can be made physically small while still maintaining a high nominal resistance. SSG bridges may have 100 times the sensitivity of bridges employing metal films, but are temperature-sensitive and therefore require temperature compensation.

Micron Instruments’ Semiconductor Strain Gages are micro machined from a solid single grown crystal of "P" doped Silicon. This results in a two terminal resistive device that has a minimum of molecular slippages or dislocations permitting repeatable use to high strain levels.

Foil Gage vs. Semiconductor Strain Gage Implementation of Load Cells

We address the relative differences between Foil Gages and SSGs in [Ref. 1].

Foil load cells often have a small non-linearity in the order of 0.25 % of full scale. The sensitivity drops slightly with load. This usually due to the stain range required to get 20 millivolts of full-scale signal, and also depends on the metal the gage is normally bonded to.

Bonding a low resistance SSG in the compression region of the load cell, and wiring it in series with the excitation, can correct the sensitivity loss. As a load is applied, the gage resistance decreases, increasing the voltage to the foil bridge. Selecting the right resistance and gage factor SSG can reduce the non-linearity to less than 0.01 % of full scale – potentially a 25x improvement!

When an over-range or high output signal is required, SSGs should be considered. If the normal 20 millivolts full scale output signal of the foil gage bridge is sufficient, the SSG can be over-ranged five times full scale with no change in performance, twenty times full scale with a minor zero load offset, and 100 times full scale before failure.

If high output is required, the SSG will provide five times full scale (100 millivolts) and still work under the one micro-strain precision elastic limit (infinite life) with five volts excitation or up to 400 millivolts output at the normal strain range of the foil load cells. With 10 volts excitation, a one-volt (1000 millivolt) signal is obtainable with some non- linearity of 0.25 % of full scale, which is normal for a typical foil gage.

[1] H. Chelner and R. Mueller, “Compelling Benefits of Micron Instruments’ Semiconductor Strain Gages”, in Featured Articles, www.microninstruments.com.

Linearization of foil and wire load cells using SSGs

Typically, the foil or wire strain gage load cell millivolt output decreases non-linearly with load to the extent of up to 0.5% of full scale. For instance a 1,000 lb. load cell with a sensitivity of 2.0 mv per lb. for the first 100 pounds could show an error of 0.2 mv at 1,000 lb., which is 1.0 % or ten pounds. The best-fit straight line would be approximately +/- 0.5% or +/- 5.0 pounds.

One solution for linearization is to bond an SSG in the compression region of the load cell. In compression, the gage decreases in resistance non-linearly. When the SSG is connected in series with the bridge, the result is a non-linear increase of voltage to the bridge, increasing the bridge sensitivity with load and decreasing its normal but repeatable non-linearity.

The trick is to trim the SSG to exactly compensate for the load cell typical non-linearity. This can be done by shunting the SSG with a resistor to adjust the amount of non- linearity correction.

Customers report that when they control the compressive stress to the gage by precisely locating it on their part and applying a fixed five percent shunt resistor (if required), the load cells are automatically corrected to within 0.1% FS. Better correction can be affected empirically on an individual basis by varying the shunt resistor.

Expert Load Cell Design Consultation and Gaging Services

Virtually every gaging application requires careful consideration of gage selection, gage placement, adhesives, and curing cycles. This depends heavily on the part to be gaged, material, available area, stress fields from FEAs, operating temperature range, operating strain range, sensitivity requirements, frequency response requirements, etc. Expert gaging is often required to extract the unique benefits of load cells built with Micron’s SSGs.

Micron Instruments has over 40 years of success in guiding our customers to making the right choices at this critical stage of the product development cycle. Our experts are available to offer consultation on how to best use Micron Instruments’ SSGs for creating industry-leading load cells, and to provide expert gaging services to ensure our gages are properly placed and adhered onto your parts. Want to Learn More? Interested in Collaborating? Whether you’re interested in pursuing this type of design, or another measurement related application or design, please contact us for a free, confidential consultation either by email or phone (805-522-4676).