Electromagnetic articulograph based on a nonparametric representation of the magnetic field

Electromagnetic articulograph (EMA) devices are capable of measuring movements of the articulatory organs inside and outside the vocal tract with fine spatial and temporal resolutions, thus providing useful articulatory data for investigating the speech production process. The position of the receiver coil is detected in the EMA device on the basis of a field function representing the spatial pattern of the magnetic field in relation to the relative positions of the transmitter and receiver coils. Therefore, the design and calibration of the field function are quite important and influence the accuracy of position detection. This paper presents a nonparametric method for representing the magnetic field, and also describes a method for determining the receiver position from the strength of the induced signal in the receiver coil. The field pattern in this method is expressed by using a multivariate spline as a function of the position in the device's coordinate system. Because of the piecewise property of the basis functions and the freedom in the selection of the rank and the number of the basis functions, the spline function has a superior ability to flexibly and accurately represent the field pattern, even when it suffers from fluctuations caused by the interference between the transmitting channels. The position of the receiver coil is determined by minimizing the difference between the measured strength of the received signal and the predicted one from the spline representation of the magnetic field. Experimental results show that the error in estimating the receiver position is less than 0.1 mm for a 14 x 14-cm measurement area, and this error can be further reduced by using a spline-smoothing technique.