Compelling Benefits of Micron Instruments’ Semiconductor Strain Gages

Strain Basics

Strain (ε) is defined as the amount of deformation per unit length of an object when a load is applied. Strain is calculated by dividing the total deformation of the original length by the original length:

ε = ΔL / L

The magnitude of measured strain is generally very small and, as such, is expressed as micro-strain (μ-strain), which is ε x 10-6.

Strain can be positive (tensile) or negative (compressive).

When a material is compressed in one direction, it usually tends to expand in the other two directions perpendicular or parallel to the direction of flow. This is called the Poisson effect. Poisson's ratio (η) is a measure of this effect. The Poisson ratio is the fraction (or percent) of expansion divided by the fraction (or percent) of compression, for small values of these changes. Conversely, if the material is stretched rather than compressed, it usually tends to contract in the directions that are transverse to the direction of stretching.

Strain Gages

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

Ideally, a strain gage would change resistance only due to the deformations of the surface to which the sensor is attached. However, in real applications, temperature, material properties, the adhesive that bonds the gage to the material surface, and the stability of the material metal all affect the measured resistance.

The Temperature Coefficient of Resistance (TCR) and the Thermal Coefficient of Gage Factor (TCGF) are important measures of the thermal effects in the silicon matrix of the strain gage inhibiting the flow of electrons, and defined Gage Terms and Definitions.

Wheatstone Bridges are important electrical circuits when using semiconductor strain gages. A Wheatstone Bridge is an electrical circuit used to measure an electrical resistance by balancing legs of a bridge circuit, one leg of which includes the unknown component. Wheatstone Bridges are used in one-quarter configurations where there is one gage and three precision resistors; half-bridge configurations with two gages, and full-bridge configurations with four gages. The voltage output of a full-bridge is twice that of a half-bridge, and 4 times that of a quarter bridge. And, as discussed below, the use of a half-bridge or full-bridge configuration is required for temperature compensation. Information on Wheatstone Bridges is widely available on the Internet.

Micron Instruments’ Semiconductor Strain Gages

Semiconductor strain gages 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. Semiconductor gages 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. Semiconductor strain gage 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.

Micron Instruments’ Semiconductor Strain Gages vs. Metal-Foil Gages

Perhaps the most popular strain gage is the foil-type gage, produced by photo-etching techniques, and using similar metals to the wire types (alloys of copper-nickel, nickel-chromium, nickel-iron, platinum-tungsten, etc. Foil gages are considerably less expensive than semiconductor gages, and can be useful in less demanding applications.

However, Micron Instruments’ Semiconductor Strain Gages offer significant benefits that make them the obvious choice for a diversity of applications, including medical, industrial, robotic, precision instruments, infrastructure, aerospace, and defense applications.

Here are some of the most compelling benefits of Micron Instruments’ Semiconductor Strain Gages.

Size

Many applications, e.g., implantable medical sensors, offer extremely small space for gage or bridge placement. Micron Instrument’s Semiconductor Strain Gages typically require less than 2% of the area needed for a metal-foil gage, and are as small as .018” long (with an active area of .011”) and .0004” thick.

Sensitivity and Signal Output

The Gage Factor (GF) for metal-foil gages is typically 1 to 4. In contrast, Micron Instruments’ Semiconductor Strain Gages have a GF as high as 200 – roughly two orders of magnitude!

A consequence of having a low GF is the need to place a metal-foil gage in very high strain regions to achieve a sufficiently strong output signal.
Micron Instruments’ Semiconductor Strain Gages can operate effectively as low as 50 μ-strain, and as high as 3,000 μ-strain. There is even a “crash gage” available that operates up to 9,000 μ-strain.

Micron Instruments’ Semiconductor Strain Gages will also operate for an infinite number of cycles, provided that operating strain is kept within limits, as discussed below.

Where metal-foil gages have a typical full-scale sensor output at 500 μ-strain of 2mV/V, Micron Instruments’ Semiconductor Strain Gages deliver a typical full-scale output of 20mV/V when temperature compensated.

Life Cycles

Metal-foil gages usually fail from fatigue after 10 thousand to ten million cycles. Micron Instruments’ Semiconductor Strain Gages will operate for an infinite number of cycles provided that operating strain is kept under 500 μ-strain and the maximum full-scale strain is kept under the one μ-strain precision elastic limit for the material the gage is being bonded to.

Precision Temperature Compensation and Gage Matching

Semiconductor Strain Gages have large temperature coefficients of resistance (TCR) making single gage strain measurements difficult unless used at a constant temperature. Micron Instruments’ Semiconductor Strain Gages are predominantly used in half-bridge and full-bridge configurations, which compensate for temperature and deliver highly accurate strain output. Micron Instruments uses advanced instrumentation for precision measurement of gage slope and intercept. This temperature characterization is then used to carefully match gage sets for use in half or full bridges.

Resistance

Metal-foil gages typically offer an impedance range of 120Ω to 5,000Ω. This can be limiting for wireless sensing applications, especially passive wireless sensors that require over-the-air power. High impedance gages reduce the required power at a fixed voltage, enabling stronger RF signals at greater transmission range.

Micron Instruments is driving the high end of strain gage impedance, currently as high as 25,000Ω, and expected to reach 50,000Ω in the near future. This makes Micron Instruments’ Semiconductor Strain Gages an ideal choice for wireless sensing applications.

Expert 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.

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’ Semiconductor Strain Gages for creating industry-leading customer products.

Micron Instruments also provides expert gaging services to ensure our gages are properly placed and adhered onto your parts. Again, over 40 years of experience has allowed us to develop proprietary methods of adhering gages to parts that minimize creep and facilitate a long and stable operating life.

At Micron Instruments, we turn Customer Objectives and Requirements into Great Products.

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