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 electromagnetic tracking requires a sampling 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.

Eelectronics

Data-acquisition electronics measures the currents in the three transmitter coils, and measures the voltages induced in the three receiver coils.

Receiver coil signals can be measured all at the same time, or one by one. Many designs have used one operating frequency, driving the transmitter coils sequentially. Use of one frequency simplifies handling frequency-dependent effects. However, simultaneous measurements improve signal-to-noise ratio. Simultaneous AC systems use multiple-frequency designs that drive the three transmitter coils simultaneously, with sinewaves at three distinct frequencies. This improves signal-to-noise ratio by lengthening measurement time.

• Operating frequencies are typically 30 Hz to 15000 Hz. 1000 Hz, 1300 Hz, and 1600 Hz are a good starting point. Higher frequencies give higher induced voltages. Lower frequencies reduce error-causing eddy-current effects.

• The transmitter coils are usually series tuned with capacitors.

• The transmitter-coil currents must be measured. The currents vary slowly due to coil heating, so currents can be measured periodically.

• Some designs use DC pulses to drive the transmitter coils, instead of AC frequencies. This simplifies driver design, but makes receiver signal recovery more difficult. DC pulse-driven transmitter coils must be driven sequentially.

Analog-to-digital converters

Dynamic range is a consideration. Analog to digital converters (ADCs) that have 24-bit resolution, those originally meant for audio, have enough dynamic range.

An electronics setup that has six ADCs can measure three transmitter-coil currents and three receiver-coil voltages continually and simultaneously. Add three more ADCs for each additional receiver coil trio.

A four-ADC electronics can use one channel to measure the currents periodically over time (The currents change slowly as the transmitter coils warm up.), and three channels to measure the three voltages continually and simultaneously.

A two-ADC system can measure currents sequentially with one ADC and voltages sequentially with the other ADC.

A single-ADC electronic system can measure the currents and voltages sequentially.

Papers

• C. L. Dolph, "A current distribution for broadside arrays which optimizes the relationship between beam width and sidelobe level," Proc. IRE, Vol. 35, pp. 335-348, June, 1946. The original Dolph-Chebyshev window article. This window is capable of 140 dB rejection of out-of-band signals.

• The window in Figure 10 of Albert Nuttall's paper exhibits sidelobe peak, four DFT bins from the central peak, 91 dB down from the central peak (The window and its first through fifth derivatives are all continuous for all t, giving 42 dB/octave rolloff of the sidelobes) and is (for symmetrical limits |t|<=L/2, and zero for all t outside the limits):

w(t) = (1/L)(10/32 + 15/32 cos(2pi t/L) + 6/32 cos(4pi t/L) + 1/32 cos(6pi t/L))

• U.S. Patent 4,109,199 describes the use of a calibration coil in the receiver to calibrate the gains of the electronics.

• More elaborate algorithms provide higher accuracy at the expense of much more computation. by modeling the non-dipole and/or non-concentric parts of the coils. Expired U.S. Patent 5,307,072 is an early example.

• C.A. Nafis, V. Jensen, L. Beauregard, P.T. Anderson, "Method for estimating dynamic EM tracking accuracy of Surgical Navigation tools", SPIE Medical Imaging Proceedings, 2006, reports low-cost accuracy-testing methods using a known-flat nonmagnetic surface (such as a granite surface plate).

• A 6DOF tracker using four-coil printed-circuit transmitter and receiver (optimized for academic originality) is discussed in: Peter Traneus Anderson, "A Source of Accurately Calculable Quasi-Static Magnetic Fields", dissertation presented to the Faculty of the Graduate College of the University of Vermont, October 2001, stored here as three files: Media:AndersonPeterDissertation.pdf is the main body. Media:AndersonPeterDissertationReadme.pdf contains copyright license, additional comments, and four figures that are blank in the main body. Media:AndersonPeterDissertationFig14r1.jpg is the color original photo of two of the figures. Expired U.S. Patent 1,172,017 discloses a direct-conversion radio receiver. Peter intended to include this reference as reference 16 in his dissertation, but was unable to find the patent number before Google Patents existed, so made do with existing indirect reference 16.

• A 6DOF tracker using two transmitter coils (instead of three) can be built; Frederick Raab calls this two-state excitation in his 1981 paper. Two-state trackers are severely limited, as they cannot track near the axis of the missing transmitter coil and cannot track near the plane of the two existing transmitter coils.