Compensating Load Cell Temperature Effect on Output at Interface, Inc.
“Temperature Effect on Output” is defined as the change in output due to a change in ambient temperature, where output is defined as the algebraic difference between the load cell signal at applied load and the load cell signal at no load.
In strain gage based load cells, the effect is primarily due to the temperature coefficient of modules of elasticity of the force bearing metal. It is common in the industry to compensate for this effect by adding temperature sensitive resistors external to the strain gage bridge which drop the excitation voltage reaching the bridge. This has the disadvantages of adding thermal time constants to the transducer characteristic and of decreasing the output by 10%.
At Interface, external resistors are not used. Rather, by having close control over the fabrication of strain gages, the gages are made to have a temperature coefficient of gage factor which opposes the temperature coefficient of modules and thereby provides temperature compensation without loss of signal. This self temperature compensating feature is dependent upon the alloy and properties of the base strain gage material, the rolling of it into thin foil strips, and heat treatment.
Fortunately, the base material lots and the subsequent foil strip lots are very large, so that a single lot qualified at the foil strip level will constitute several years’ supply. The qualification of a foil strip lot involves performing nominal heat treating of test samples followed by testing at 0°, 75°, and 150°F. The Temperature Effect on Output is then calculated on a chord slope basis and verified to be within the 0.0008%/°F specification for the two resulting temperature intervals.
On a production basis, sub lots are formed from the strip lots and heat treated. Each sub lot is then qualified for Temperature Effect on Output by testing. These sub lots are small enough that tests are being conducted about weekly. The test consists of collecting samples of gages from a specified distribution within the sub lot and installing these gages into load cell flexures representative of the style and material for which the gages are destined. Four flexures are gages, each with 8 gages. They are then tested at temperatures of 75° and 150°F. The test criteria applied to the chord slopes over this interval is 0.0008%/°F.
The sub lots which pass the criteria are made available for production of load cells of the appropriate style and material. Lots which fail are scrapped. Due to good gage uniformity and monitoring of process trends, lot yield is relatively high.
The testing of individual production load cells for Temperature Effect on Output is prohibitively expensive. The above procedure of lot qualification produces load cells which meet specifications with a high degree of confidence at a reasonable cost.