Driving Strain-Gauge Bridge Sensors with Signal-Conditioning ICs

Abstract: Strain-gauge sensors - reliable, repeatable, and precise - are used extensively in manufacturing, process control, and the research industries. They transduce (convert) strain into an electrical signal for use in pressure sensors, weight measurements, force and torque measurements, and materials analysis. A strain gauge is simply a resistor, whose value varies with strain in the material to which it is bonded. The article covers the MAX1452 sensor signal conditioner for temperature compensation. The MAX1452's flexible approach to bridge excitation gives the user a substantial amount of design freedom. This article has focused on voltage drive with and without a current boost, but many other bridge-drive configurations can be implemented. Other design considerations include the use of external temperature sensors on the control loop, and achieving sensor linearization (i.e., nonlinearity with respect to the measured parameter) by feeding the OUT signal into this loop.

Available strain gauges feature a large range of zero-strain resistance. Sensor materials and technology account for the broad range, but several values (such as 120Ω and 350Ω) have become prominent through widespread usage. In the past, standard values simplified strain measurements by allowing an easy hookup to a basic magneto-deflection meter that included matching input-resistance networks.

Foil-gauge manufacturing employs a limited number of alloys, chosen to minimize the difference between the temperature coefficients of the gauge and the material under strain. Steel, stainless steel, and aluminum constitute the majority of sensor materials. Beryllium copper, cast iron, and titanium are used as well, but the "majority" alloys drive the high volume, low-cost manufacture of temperature-compatible strain gauges. The 350Ω constantan-foil strain gauge is probably the most common.

Thick- and thin-film gauges, whose reliability and ease of manufacturing are attractive for automotive applications, are usually produced on a ceramic or metal substrate with an insulating material deposited on the surface. The gauge material is deposited on top of the insulating layer by a vapor deposition process. The sensing gauges and interconnect lines are carved into the metal by laser vaporization or by photo-mask and chemical etch techniques. A protective insulating layer is sometimes added to protect the gauges and interconnects.

Gauge materials usually include a proprietary blend of metals chosen to produce the desired gauge resistance, resistance variation with stress, and (for temperature stability) the best temperature-coefficient match between sensor and base metal. Nominal gauge and bridge resistances of 3kΩ to 30kΩ have been developed for this technology, which has been used to manufacture both pressure and force sensors.

A Wheatstone bridge is usually employed in strain-gauge sensors based on foil, thin film, or thick film. The Wheatstone bridge converts the gauge's strain-induced resistance changes into a differential voltage (Figure 1). With excitation voltage applied to the +Exc and -Exc terminals, a strain-proportional differential voltage appears at the +V_OUTand -V_OUTterminals.


Figure 1. Strain gauges wired in a Wheatstone bridge configuration.

In a half-active Wheatstone bridge circuit (Figure 2), only two elements of the bridge are gauges that respond to strain in the material. The output signal for this configuration (typically 1mV/V at full-scale load) is one half that of a fully active bridge.


Figure 2. Strain gauges wired in a half-active Wheatstone bridge configuration.

Another fully active bridge circuit (Figure 3) employs more than four active 350Ω strain gauges. The characteristic bridge resistance is 350Ω and the output sensitivity is 2mV/V, but the material under strain is distributed over a wider area of the gauge.


Figure 3. A 16-gauge Wheatstone bridge configuration.

Temperature adversely affects sensor performance by causing shifts in the zero-load output voltage (also called offset), and changes in the sensitivity under load conditions (also defined as full-scale output voltage). Sensor manufacturers can compensate the first-order effects of these changes by introducing temperature-sensitive resistances into the circuit as shown in Figures 1-3.

As temperature changes, resistors R_FSOTCand R_{FSOTC_SHUNT}modulate the bridge excitation voltage. Typically, the R_FSOTCmaterial has a positive temperature coefficient that reduces the bridge excitation voltage as temperature increases. Sensor outputs become increasingly sensitive to load as the temperature increases. Decreasing the bridge excitation voltage effectively cancels inherent temperature effects by reducing the sensor output. Resistor R_SHUNTis not sensitive to temperature or strain, and is used to trim the amount of TC compensation delivered by R_FSOTC. An R_SHUNTvalue of 0Ω would cancel all effects of R_FSOTC, while a value of infinity (open circuit) would enable the full effect of R_FSOTC. This technique compensates first-order temperature-sensitivity effects quite well, but does not address the more complicated and higher-order nonlinear effects.

Temperature compensation of offset change is accomplished by inserting a temperature-sensitive resistor into one arm of the bridge. These resistors are shown in Figures 1-3 as R_{OTC_POS}or R_{OTC_NEG}. Again, a shunt resistor (R_{OTC_SHUNT}) trims the amount of temperature influence introduced by R_{OTC_POS}or R_{OTC_NEG}. Whether to use R_{OTC_POS}or R_{OTC_NEG}depends on whether the offset has a positive or negative temperature coefficient.

Using current to excite the bridge sensor causes difficulty because the bridge resistance changes with load, and because current overrides or negates the built-in sensitivity-compensation networks (shown as R_FSOTCand R_FSOTC-SHUNTin Figure 2).

Various means are available to circumvent these problems and enable a current excitation drive. An easy approach is to use the MAX1452 in a configuration that delivers a voltage drive. That circuit includes the minimal number of external components needed to provide the high levels of current necessary with voltage excitation. The MAX1452 is a complete and highly integrated signal-conditioning IC that performs sensor excitation, signal filtering and amplification, and temperature linearization of both offset and sensitivity.

The MAX1452 was designed primarily for the silicon Piezo-Resistive Transducers (PRTs) often used in sensing pressure. It incorporates four 16-bit delta-sigma DACs (D-A converters), a temperature sensor, and temperature-indexed coefficient tables for performing temperature compensation and linearization of bridge sensors (Figure 4). Temperature compensation and amplification are accomplished with an analog signal path between the sensing element and the voltage output. This IC readily accommodates foil or thick film strain gauges with the addition of minimal ext