Optical sensor calibration for electro-optical stomatography

We are currently developing a technology called “electrooptical stomatography” to measure and visualize articulatory movements within the vocal tract using electrical contact sensors and optical proximity sensors. To measure tongue movements with the optical sensors in this system, a mapping between the raw sensor values and actual tongue positions has to be determined. This mapping is non-linear and different for every tongue and sensor. The lack of an accurate, reliable calibration method has so far prevented wide-spread use of optical measurements within the vocal tract. Here, we present a calibration method based on a multi-linear regression model that maps the sensor value at a single distance of 0mm to calibration values at 0, 5, 10, 15, 20, 25, and 30mm. The coefficients of the model are determined by a least-squares regression in 25 training data sets (recorded with 5 subjects and 5 sensors). Evaluation in a leave-one-out cross-validation and on five more data sets recorded with another, different subject on 5 additional sensors yields very good results with maximum median position errors close to 1mm. The calibration of the optical sensors can therefore be semi-automatically accomplished based on a single, easily obtainable measurement during direct tongue contact.