AN3396| Application Note
Maxim/Dallas > App Notes > SENSOR SIGNAL CONDITIONERS Keywords: Sensor, sensor signal conditioning, sensor conditioner, strain gauge, sensor compensation, strain gauge compensation, pressure sensor calibration, pressure sensor compensation
Nov 11, 2004
APPLICATION NOTE 3396
Driving Strain-Gauge Bridge Sensors with Signal-Conditioning ICs
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 r
ange of zero-strain resistance. Sensor materials and technology account and 350 ) have become prominent through widespread for the broad range, but several values (such as 120 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.
Types and Composition of Foil Gauges
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 350 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 an
d 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 3k to 30k have been developed for this technology, which has been used to manufacture both pressure and force sensors.
Bridge Excitation Techniques
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 +VOUT and -VOUT terminals.
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