Automatic environmental compensation of capacitance based proximity sensors

Abstract

Improved capacitive sensor operation is achieved with improved discrimination between environmental drift and apparent drift attributable to human proximity to the sensor. A proximity algorithm detects conditions interpreted as indicating a user is close to, but not touching, a sensor. When such proximity is detected, ambient value calibration is halted, thereby avoiding treating the human's proximity as environmental drift requiring compensation and preventing miscalculation of calibration. The proximity algorithm employs two moving-average filters (implemented in hardware or software) to monitor the CDC output values over time and to make appropriate adjustments to a signal representing the ambient, while distinguishing environmental drift from proximity-induced pseudo-drift. Accurate ambient values allow for improved proximity detection by providing this environmentally compensated average value to an adaptive threshold algorithm which features a fast attack, slow decay peak detection to automatically track and compensate for different response characteristics (typically finger sizes).