Types and Structures of Strain Gauges
Types and Structures of Strain Gauges
Types of strain gauges include wire strain gauges, foil strain gauges, single-crystal semiconductor strain gauges, and thin-film strain gauges.
Wire Strain Gauges
A wire strain gauge is the original type of resistive strain gauge. The first bonded, metallic wire-type strain gauges were developed in 1938. They are still extensively used in high-temperature environments today. The wire is made of metals or their alloys with a typical diameter of 6 ~ 30 μm. Common wire materials are CrNi alloys for standard applications and PtW alloys for high-temperature applications. The measuring grid is made by either flat winding or wrapping around a metallic wire, and is then bonded to, or completely embedded in, the substrate or carrier. The carrier or backing materials are reinforced epoxies (for standard applications), selfadhesive glass fiber-reinforced Teflon (“peel-off” backing, for high-temperature applications), and sheet brass or polycarbonate (for encapsulated strain gauges in rough ambient conditions).
Metal Foil Strain Gauges
A foil strain gauge (see Figure 1) is the most widely used type. A very thin metal foil pattern (2~5 μm thick, usually Constantan or Nichrome V) is deposited onto a thin insulating backing or carrier (10~30 μm thick, usually epoxy, polyimide, or polycarbonate). The measuring grid pattern including the metallic terminal tabs is produced by the photoetching process. The entire gauge is typically 5~15 mm long.
The main advantages of foil strain gauges over wire gauges are their better heat dissipation, low transverse sensitivity, better flexibility (the smallest bending radius is 0.3 mm), and easy creep compensation (the positive creep of the elastic sensing element can be compensated by the negative creep of the carrier material of the gauge). The disadvantages include limited working temperature due to properties of the carrier materials and adhesives, as well as technical limitations in miniaturization.
Single-Crystal Semiconductor Strain Gauges
This type of strain gauge is manufactured from a thin strip of semiconductor cut from a single crystal of silicon or germanium, doped with accurate amounts of impurities to obtain either n-or p-type. The output of a semiconductor gauge is very high compared to a wire or foil gauge. Semiconductor gauges can provide both positive and negative gauge factors depending on whether the gauges are n-type or p-type. The typical gauge factor of a semiconductor is −100 ~ +170, although −115 ~ +205 are achievable. The output of semiconductor gauges is usually nonlinear with strain (p-type gauges have better linearity in tension while n-type gauges are more linear in compression), but they exhibit no creep or hysteresis and have an extremely long fatigue life. Semiconductor gauges are highly sensitive to temperature; thus they require a high level of temperature compensation. They also present large TCR due to their specific resistance–temperature curves. Semiconductor gauges are widely used in small sensors such as force, acceleration, and pressure sensors since their sensing elements can be micromachined out of a single piece of silicon. Figure 2 illustrates a semiconductor strain gauge—a resistance strip (fabricated from a semiconductor single crystal) fixed on an insulating substrate that supports both the strip and the terminals on its surface.
Thin-Film Strain Gauges
In thin-film strain gauges, the thin film (resistance element) is produced by sputtering or evaporating metals, alloys, or semiconductors (silicon or germanium) onto the carrier material (as a substrate) in vacuum. Several stages of sputtering and evaporation may be needed to form up to eight layers of material in thin-film polycrystalline structure. The whole surface is finally coated with an appropriate sealant. The final properties of the thin film are strongly influenced by various parameters of the deposition process (e.g., ultimate vacuum, substrate temperature, and rate of film growth). These processes are highly cost effective when strain gauges are produced in large quantities. The structure of a thin-film gauge is shown in Figure 3. The gauge factor of a thin-film strain gauge is higher than those of wire or foil gauges, but lower than those of single-crystal semiconductor gauges. For instance, a germanium thin-film strain gauge has a GF of 32∼39.
The nominal resistance of each type of strain gauges is shown in Table 1.
Gauge Spring Element
A spring element serves as a reacting element for the applied load and provides an isolated and uniform strain field, where the strain gauges are placed. It should exhibit good linearity, low hysteresis, low creep, and low relaxation. A typical sensor spring is designed for 0.1 mm or less of deflection, which requires an extremely low compliance and high-precision spring.