Smart load cells: an industrial application

Abstract

This paper describes a data acquisition solution using a single-chip RISC type microcontroller with very few other active and passive components around, taking advantage of the ratiometric functioning of the load cells. The need for thermally stable circuits and components is minimized through the use of the same amplification chain for both signal and reference, together with software calibration. The amplification and filtering is done through switched-capacitors techniques, controlled by the microcontroller. Moreover, this option allows the choice of the proper scale according to the platform, with periodically and automatically calibration. The analog-to-digital conversion is done using the single ramp approach controlled by the microcontroller, which does all the digital processing required as well the external transmission of the data for a local network following the nine-bit protocol. The conversion time was around 13 ms. For an industrial platform of 8 smart load cells the weighing tests show errors below 100 gr. in 400 kg. The paper also describes an example of software calibration of a multi-load cell weighbridge using one processor per smart load cell.