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issue: August 2007 APPLIANCE Magazine

Technology Report
Cost-Sensitive Sensors


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A new family of load cells allows designers to use direct-force measurement in many OEM applications where the technology had previously been too expensive.

Traditionally, load cells have been limited in their applications because they are made of precision-machined parts with epoxy-bonded and manually applied strain-gauge structures. However, Measurement Specialties of Hampton, VA, U.S., says it has changed all that by stamping high-cost machined load cells at a much lower cost. “Microfused load cells utilize ultrasmall, low-cost silicon strain gauges manufactured using MEMS technology, whereas traditional load cells use very expensive, large-area metal-foil strain gauges,” explains James Pierson, product manager.

The new load cells avoid manual mixing of epoxies and hand bonding of strain gauges by utilizing an inorganic glass as the bonding agent. “The gauges are bonded at temperatures that far exceed the maximum temperatures tolerated by epoxy materials,” Pierson explains. “This avoids the process variability associated with manual mixing and creates a low-cost, ultrastable bond between the stamped flexure and the strain gauge.” Automated wire bonding eliminates virtually all of the manual labor usually required to hand solder internal electrical junctions, and automated testing and calibration further reduce costs.

Pierson stresses that in this application, the glass is literally the glue. Unlike the organic materials that comprise epoxy materials, the inorganic glass has very low tendencies to oxidize or relax over time. “The ‘age-invariant’ nature of the Microfused bond yields zero stability (very little drift from zero over time) and sensitivity stability benefits that are simply not possible with traditional epoxy-based technologies,” he explains. “For most OEM products, it is logistically impossible to recalibrate sensors periodically to eliminate performance variations caused by age-related changes in sensor performance.”

Microfused products feature sensitivity stability of less than 1% within the first year of manufacture and an additional 1% thereafter. “This means that OEM designers can depend upon a maximum performance variation due to aging influences of less than 2% for the operating life expectancy of their product,” Pierson says.

The load cells feature low deflection and inherently low mass, which enable optimized response time, low-end resolution, and essentially unlimited fatigue life expectancy. “The perfect load cell for many applications has zero mass and infinite stiffness,” Pierson says. “Since the natural frequency of any mechanical system is equal to the root of the ratio of the stiffness divided by the mass, optimizing stiffness and minimizing mass inherently yields a maximized frequency response by definition.”

Pierson notes the new load cells are especially beneficial to the automotive and appliance industries, where it has been common practice “to measure less-expensive ‘secondary parameters’ than more-accurate ‘primary parameters.’” He explains: “A good example of this might be to measure as secondary parameters the current drawn by a drive motor, or the rate of angular acceleration, to infer the payload mass within a washing machine, when the far more accurate method would be to use load cell(s) to measure the primary parameter you wanted in the first place—weight.” 

 

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