Characteristics Of Piezoresistive Sensors
Characteristics Of Piezoresistive Sensors
The core element of a piezoresistive sensor is the strain gauge. The characteristics of a strain gauge are mainly defined by the gauge dimensions, resistance, gauge factor, temperature coefficient, resistivity, and thermal stability.
Gauge dimensions and shape are very important in choosing a right type of strain
gauge for a given application.
Gauge resistance is defined as the electrical resistance measured between two metal tabs or leads. Gauge resistance is an important design and application parameter since it determines both the output signal amplitude of the gauge (ΔV/V = ΔR/R) and the dissipation power (V2/R).
Gauge factor or strain sensitivity is defined as the ratio of (ΔR/R) and the strain ε. The real GF plots of common gauge materials are shown in Figure 1, where the GF is the slope of the curve. Both Ferry alloys and Constantan alloys have relatively high and constant GF values, indicating a well-behaved and consistent pattern. The 10% rhodium–platinum alloy exhibits a desirable and high GF feature between 0% and 0.4% range of strain ε, but its performance degrades above 0.4% strain ε point. Pure nickel even demonstrates a negative GF for small strain (ε < 0.5%). Table 1 presents the gauge factors and ultimate elongations for several materials.
The GF values of semiconductor materials are much larger than the GF values of metals. Therefore, the majority of piezoresistive strain gauges used today are made of semiconductor materials. Table 2 gives the typical GF range of main types of strain gauges.
Hysteresis of a strain gauge is defined as the ratio (in percent) of the difference between the output signals of the gauge (obtained with increasing and decreasing strain loading at identical strain values) divided by the maximum output signal. Figure 2 shows the hysteresis of a strain-gauge-type blood pressure sensor.
Creep is defined as the relative variation of the measured strain over time, Δt, when the gauge is under a constant stress:
For instance, Vishay’s Transducer-Class® strain gauge sensors have a creep less than ±0.02% of full scale (FS) during a 20-minute test. Creep is caused by the nonideal elastic behaviors of piezoresistors and adhesive materials when bonded to the measured object. Most gauges can be adjusted in design to exhibit either a positive or a negative creep under load. Spring element materials in piezoresistive sensors exhibit only positive creep under load.
Temperature characteristics of strain gauges are often described by two coefficients:
- Temperature coefficient of resistance (TCR), defined as the relative resistance variation of the measuring element per degree of temperature variation: “Free” means the measuring element is unbounded or unembedded into any other material. TCR is an intrinsic characteristic of the gauge material; hence it is a basic design criterion.
- Temperature coefficient of sensitivity (TCS), defined as the relative variation of the gauge factor GF per degree of temperature variation:
Figure 3 shows the performance of a uniaxial metal foil strain gauge tested on a mild steel.
Other characteristics of strain gauges include maximum permitted RMS excitation voltage (the maximum RMS value of the applied voltage), maximum elongation (the strain value at which the linearity deviation exceeds ±5%), linearity (the maximum difference between the actual output signal and the ideal output signal under the same strain), resolution (the smallest strain variation that can be resolved by the gauge), fatigue life (the number of load cycles supported by the gauge without significantly changing its characteristics), frequency response (the maximum sinusoidal strain variation frequency that can be resolved by the gauge), and smallest bending radius (the smallest value of the radius that the gauge will withstand in bending in one direction without significant changes in its characteristics).
Other important characteristics that must be considered when selecting a strain gauge include its stability. This is because the most desirable strain gauge materials are also sensitive to temperature variations and tend to change resistance as they age. Therefore, temperature and drift compensation must be included in many strain gauges.