Stress-free strain gaging

March 4, 2004
Signal-conditioning IC simplifies strain-gage bridge sensor setup.
A MAX1452 signal-conditioning IC provides bridge excitation and all necessary signal conditioning such as filtering, amplification, and temperature linearization.
A Wheatstone bridge is usually employed in strain-gage sensors based on foil, or thin or thick film. The bridge converts strain-induced resistance changes into a differential voltage. An excitation voltage applied to the + and -Exc terminals induces a strain-proportional differential voltage at the + and - VOUT terminals.

Mark A. Parsons
Maxim Integrated Products
Sunnyvale, Calif.

Strain-gage sensors are used in many applications including manufacturing, process control, and research. They convert strain into an electrical signal for use in pressure sensors, weight, force, and torque measurements, and material analysis.

A common way to use a strain gage is in a bridge configuration. Usually this requires an excitation signal such as a voltage source and a number of external components. But a new IC minimizes the number of components, simplifying design.

The MAX1452 is an integrated signal-conditioning IC that performs sensor excitation, signal filtering, amplification, and temperature linearization of both offset and sensitivity. The chip's flexible approach to bridge excitation gives a substantial amount of design freedom. Many other bridge-drive configurations can be implemented besides the voltage drive with and without a current boost. Other design considerations include using an external temperature sensor on the control loop and achieving sensor linearization via output feedback.

A strain gage is a resistor whose value varies with strain in the material to which it's bonded. The majority of strain gages are passive-resistive devices constructed by depositing or etching a wire or foil sensing grid on a substrate known as the carrier matrix. A Wheatstone bridge detects an imbalance between resistances in each bridge arm, producing a voltage proportional to the amount of strain.

But sensor performance is affected by temperature changes, causing shifts in the zero-load output or offset voltage, and changes in the sensitivity under load conditions known as full-scale output voltage. Sensor manufacturers can compensate offset change by placing temperature-sensitive resistances into one arm of the bridge circuit. Another way is to use current to excite the bridge sensor. However, this raises issues because the bridge resistance changes with load, and because current overrides or negates built-in sensitivity-compensation networks. A current excitation drive is one way to avoid these problems. Another is to use the MAX1452 in a configuration that delivers a voltage drive.

Available strain gages feature a large range of zero-strain resistance. There is a broad range of sensor materials, but several values such as 120 and 350 Ω have become prominent through wide usage. Traditionally, standard values simplified strain measurements by allowing an easy hookup to a basic magneto-deflection meter that included matching input-resistance networks.

Foil-gage manufacturing employs a limited number of alloys, which are chosen to minimize the difference between the temperature coefficients of the gage 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 gages.

Reliable and easy-to-manufacture thick and thin-film gages are usually produced on a ceramic or metal substrate with an insulating material deposited on the surface. The gage material is deposited on top of the insulating layer by vapor deposition. The sensing gages and interconnect lines are carved into the metal by laser vaporization or by photomask and chemical-etch techniques. A protective insulating layer is sometimes added to protect the gages and interconnects.

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

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