Weighing scales (e.g., kitchen and bathroom scales) of a proposed type would incorporate sensory devices like the one described in "Low-Power, Microprocessor-Controlled Strain-Gauge Circuit" (NPO-19750), NASA Tech Briefs, Vol. 21, No. 1 (January 1997), page 45. Unlike other weighing scales based on strain gauges, these would not require electrochemical batteries or external power supplies; instead, the proposed scales could be powered adequately by solar batteries like those included on some pocket electronic calculators.
In a weighing scale of the proposed type, power consumption much lower than that of conventional strain-gauge circuits would be achieved by a unique combination of novel and conventional features. As explained in more detail in the earlier article, the intermittency of the operation of the microprocessor-controlled strain-gauge circuit would contribute a major portion of the reduction in time-averaged power. A further reduction would be effected by use of a strain gauge of unusually high electrical resistance — about 3 kΩ instead of the customary value of about 120 Ω. Ordinarily, the use of the lower resistance would be dictated by the need to minimize noise pickup on strain-gauge-output wires, where the noise voltage can be comparable to the strain-gauge output voltage. In this case, however, the strain-gauge circuit would be very small and one could mount the rest of the strain-gauge circuit very close to the strain gauge; this would make it possible to shorten the electrical connections and thereby reduce noise pickup, enabling the use of the higher gauge resistance without incurring excessive noise.
A preliminary estimate shows that with a 3-kΩ strain gauge and associated circuitry operating at a supply potential of 3 V at a sampling rate of twice per second with sampling periods 100 µs long, the overall time-averaged power consumption would be about 12 µW. A typical calculator-type solar battery supplies a current of about 10 µA at a potential of 3 V (power of about 30 µW) under full illumination. Thus, one such battery would provide at least 50 percent power margin to allow for reduced illumination.
As explained in the earlier article, the strain-gauge circuit would automatically correct for variations in the supply voltage. The microprocessor in the strain-gauge circuit could also be programmed to correct for nonlinearity in the strain-gauge response, and to provide a digitally controlled analog offset to correct for tare weight.
This work was done by Shannon P. Jackson and Harold Kirkham of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Electronics & Computers category.