FULL-BRIDGE CAPACITIVE EXTENSOMETER

Capacitive transducers have proven to be very effective sensors of sma ll displacements, because of inherent stability and noninvasive high r esolution. The most versatile ones have been those of a differential t ype, in which two elements are altered in opposite directions in respo nse to change of the system parameter being monitored. Oftentimes, thi s differential pair has been incorporated into a bridge circuit, which is a useful means for employing synchronous detection to improve sign al to noise ratios. Unlike previous differential capacitive dilatomete rs which used only two active capacitors, the present sensor is a full -bridge type, which is well suited to measuring low-level thermal expa nsions. This analog sensor is capable of 0.1 mum resolution anywhere w ithin a range of several centimeters, with a linearity of 0.1%. Its us er friendly output can be put on a strip chart recorder or directed to a computer for sophisticated data analysis.