AC electromagnetic tracking

Alternating current (AC) electromagnetic tracking is a type of electromagnetic 3D tracking that uses alternating current (AC) electric signals to generate magnetic fields, which are picked up by a tracking sensor.

AC tracking uses analog-to-digital converters (ADCs) to convert the sensed electromagnetic pulses into readable digital data. an SEU does this, and typically feeds the data into a microcontroller whose firmware does the mathematics and sends the pose data to a computer. AC electromagnetic tracking requires a nyquist rate on the order of 3000hz.

The fastest commercial AC electromagnetic tracking systems in the world as of 2025 are the Polhemus Viper 8 and 16, which track up to 960hz.

An example of an AC system is Plume. Plume is based on the following principle: A transmitter generating an oscillating magnetic field with a well defined narrow frequency. The field excites a receiver which is an oscillating circuit tuned to the frequency. The strength of the excitation, measured from the induced voltage, depends on the location of the receiver in the transmitter field, which is the basic effect used for positioning.

The strength of the signal induced in each receiver coil depends on the position within the field of the transmitter, and on the angle between the receiver coil and the field lines at that point.

AC trackers have interference with aluminum, copper, and carbon steel, but not as much with stainless steel or iron.

Conventional alternating-current (AC) based EM position and orientation tracking systems generally operate at a total system frequency between 8 kHz and 40 kHz. More specifically, 14 kHz is a common frequency. At this frequency, the skin depth of aluminum is 0.71 millimeters and the skin depth of titanium is 2.76 millimeters. Any metal in the environment needs to be thinner than the skin depth in order to avoid getting inaccurate tracking.