Low Cost Super-Nyquist Asynchronous Demodulation for Use in EM Tracking Systems

We present a novel method for demodulating multiple frequency components of an ac magnetic field for use in electromagnetic tracking systems. Our undersampling method allows for a low sampling rate to acquire signals of interest in a range of 20-30 kHz without significant loss of precision. Lower sample rates give additional computation time between each sample, allowing for real-time demodulation without expensive high speed data acquisition and processing hardware. This also allows the use of low speed high precision analog-to-digital converters (ADCs) for sampling high frequency signals. The presented system demodulates 16 frequency components from two signals using an Arduino Due microcontroller and a two channel 16-bit ADC. The demodulated signals are then used to accurately determine the position of a magnetic sensor to within 1 mm in a volume 25 cm × 25 cm × 25 cm. The system accuracy compares well with both commercial and research systems in terms of accuracy, but at a dramatically lower cost.