Load Cell Enhancement 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.

[1] H. Chelner and R. Mueller, “Compelling Benefits of Micron Instruments’ Semiconductor Strain Gages


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.

 

High Frequency Load Cells

Due to the small size of Micron’s SS-018 SSG - less than ½ mm long - load cell compliant sections can be minimized. This small physical size, associated low mass, and the low operating strain levels, makes it possible to design high frequency load cells.
 
We refer the reader to [Ref. 2]  for a more detailed discussion on high frequency strain gages.

 

High Accuracy Load Cells

Miniature and high sensitivity SSGs make it possible to build very small high accuracy load cells, easily measuring milligrams. This opens up the possibility of many new applications of load cells.
 

Wireless Load Cell Implementations

For many applications, wireless communication for load cells offers obvious advantages: it’s non-intrusive and avoids the many problems with tethered wired sensors. It can also be important to avoid power sources that have limited life. The good news is there are passive wireless technologies, covered by international (ISO) standards, which meet most industry requirements. The transceivers are commercially available and relatively inexpensive.
 
Several passive wireless technologies are options for wireless load cells. One is “passive high-frequency”, or HF, operating at 13.56MHz. Multiple semiconductor companies offer HF transceivers and interrogators that comply with the ISO 15693 standard and can be used with or without a battery. There is also a Near Field Communication (NFC) standard (now used in smart phones) published by the NFC Forum that supports HF tags compliant with the ISO 14443 A and B standards. HF is most useful for short-range applications that do not have high data rate requirements.
 
Another option is UHF Gen 2, which has approved frequency bands in Europe, India, and the Middle East (865 – 868 MHz); the U.S., South America, and some parts of Asia
(902– 928 MHz), and Japan (950– 956 MHz). There are commercially available EPC

[2] H. Chelner and R. Mueller, “HIGH FREQUENCY, HIGH PRESSURE, and HIGH TEMPERATURE SEMICONDUCTOR STRAIN GAGE SENSORS”,

Gen 2 tags that work across all 860 – 956 MHz, and other optimized for specific sub-bands.
 
Both HF and UHF Gen 2 tags are also available with battery-assisted power (BAP), which can increase both communication range and data rate.
 
A major benefit of Micron’s SSGs is the industry’s top high-impedance options, currently up to 50,000 Ω, which dramatically reduces the over-the-air power required by the sensor to deliver the data to the wireless transceiver. Micron’s high-impedance gages are special products that require contacting the company.
 
A related article on wireless SSGs can be found at [Ref. 3] .

 

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 using our Contact Us Form or phone (805-522-4676).

[3 ]H. Chelner and R. Mueller, “Miniature Medical Implantable Wireless Sensing (MMIWS)”,
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