A Self-Calibration Method for an Implantable Displacement Sensor

This paper describes a hardware implementation of an automatic calibration process for an implantable sensor used for monitoring the postoperative stability of a hip or knee implant. The RL-Wheatstone bridge, which is the essential part of the sensor, is capable of being self calibrated in vivo by adjusting its electronic component values alternatively until it is balanced. This calibration method extends the measurement range of the sensor by separating the small, elastic movement from the gross, unrecoverable movement, which is suitable for long term in-vivo measurements. The subsystem for calibration is realized using 0.35 mu m CMOS (Complementary Metal-Oxide Semiconductor) technology on two 2.8 mm(2) chips, consisting of two voltage-controlled resistors, a +/- 90 degrees phase detector and a preamplifier. The simulated and on-bench measured results validate its feasibility to calibrate the bridge, which will ensure a detectable range of +/- 150 mu m in subsequent micromotion measurement.