Electromagnetic tracking: Difference between revisions

= Electromagnetic tracking =

'''[[Electromagnetic tracking]]''' ('''EMT''') is a [[Pose (computer vision)|pose]]-estimation technology widely used in [[Virtual reality|[[Virtual reality]]]] (VR) and [[Augmented reality|[[Augmented reality]]]] systems, as well as in medical navigation, robotics, and human–computer-interaction research.   

Unlike [[Optical tracking|[[optical tracking]]]] or [[Inertial tracking|[[inertial tracking]]]], EMT determines the 3-D position and orientation ({{small|[[Six degrees of freedom|6DOF]]}}) of miniature sensor coils **without requiring line-of-sight**. A stationary ‘‘field generator’’ produces a known magnetic field; small tri-axial receiver coils measure that field, and the system solves for the sensor’s pose each frame. Because every frame is computed independently, EMT suffers **no cumulative drift** and achieves latencies of only a few milliseconds.<ref name="Yaniv2009" />

== History ==

Early research prototypes appeared in the late 1960s, but commercialisation began with [[Polhemus]]’s 1969 “Space-Tracker,” followed by its 1980s FASTRAK line and [[Ascension Technology|Ascension]]’s ‘‘Flock of Birds’’ pulsed-DC trackers.<ref name="PolhemusWebsite" /><ref name="FlockManual" />   

During the 1990s EMT migrated from military flight simulators into VR CAVEs and medical navigation. Today it remains the dominant method for tracking surgical tools (e.g. [[NDI Aurora]]), and has seen niche consumer use in the [[Razer Hydra]] game controller (2011) and the Magic Leap One AR controller (2018).<ref name="MagicLeapFCC" /><ref name="RazerHydra" />

 

== Technical characteristics ==

Transmitters are supplied in planar plates, cube frames, or tower geometries. Polhemus FASTRAK’s standard source covers about a 1.5 m × 1.5 m × 1.5 m volume, updating sensors at up to 120 Hz.<ref name="PolhemusSpecs" />

Modern 6DOF sensors can be extremely small: NDI lists a micro 6DOF sensor only 1.8 mm in diameter, while its smallest 5DOF sensor is 0.3 mm × 2.5 mm.<ref name="NDIAuroraSensors" /> Up to **32** 5DOF sensors or **16** 6DOF sensors may be tracked simultaneously by a single Aurora controller.<ref name="NDIAuroraSensors" />

* *Static accuracy* (typical laboratory, undistorted): FASTRAK ≈ 0.76 mm RMS, 0.15° RMS.<ref name="PolhemusSpecs" />

* *Update rate*: 50–120 Hz for most commercial units; some research rigs exceed 240 Hz. 

* *Latency*: 3–10 ms sensor-to-host.<ref name="PolhemusWebsite" /> 

 

Performance degrades in the presence of conductive or ferromagnetic materials, electrical motors, or large ambient fields.<ref name="Poulin2002" />

; AC vs. pulsed-DC excitation  

AC trackers (Polhemus, NDI) deliver high SNR but are more susceptible to eddy-current distortion. Pulsed-DC units (Ascension “Bird”) trade some refresh rate for improved metal immunity.<ref name="AscensionPulsedDC" />

== Comparison with other tracking modalities ==

! Modality !! Advantages !! Limitations

| '''EMT''' || No line-of-sight, drift-free absolute pose, works in darkness/inside body || Limited range; sensitive to metallic distortion; single-room volume

| [[Optical tracking]] || Sub-millimetre accuracy; room-scale; immune to metal || Requires direct visibility and lighting; occlusion failures

| [[Inertial tracking]] || High update (>1 kHz); no external infrastructure || Unlimited drift; cannot provide absolute position

Hybrid camera + IMU systems used in [[Microsoft HoloLens]] or [[Meta Quest Pro]] achieve room-scale tracking; EMT is sometimes integrated to recover when optical tracking fails (e.g. catheter tips occluded inside the body).<ref name="Yaniv2009" />

== Applications ==

* '''Consumer & gaming''' – [[Razer Hydra]] (1 mm / 1° precision), Magic Leap One controller (28.5–42.4 kHz AC coils).<ref name="RazerHydra" /><ref name="MagicLeapFCC" />

* '''Medical navigation''' – NDI Aurora and Ascension 3D Guidance track needles, catheters, and endoscopes without radiation.<ref name="Yaniv2009" />

* '''Industrial & research''' – Tool tracking in welding simulators, motion capture under clothing, robot hand-guiding inside metallic workcells.

== Strengths and limitations ==

EMT offers drift-free, low-latency 6DOF tracking through occlusions and tissue, and supports extremely small, embeddable sensors.   

Its chief drawbacks are (a) rapid accuracy loss beyond ≈1 m from the transmitter (field ∝ 1/''r''³) and (b) distortion from nearby metals or active electronics, which can introduce centimetre-scale bias if unmodelled.<ref name="Poulin2002" /> Calibration mapping and hybrid sensor fusion mitigate but do not eliminate these effects.

== Notable commercial systems ==

* '''[[Polhemus]] FASTRAK / LIBERTY''' – AC, up to 120 Hz, ≤ 0.76 mm RMS accuracy.   

* '''[[NDI Aurora]]''' – Medical-grade AC tracking; up to 32 sensors; sensors as small as 0.3 mm.   

* '''[[Ascension Technology|Ascension]] 3D Guidance / Flock of Birds''' – Pulsed-DC, 144 Hz max.   

* '''[[Razer Hydra]] / Sixense STEM''' – Consumer dual-wand controller (2011).   

* '''Magic Leap One “Control”''' – Handheld AR controller with 28.5–42.4 kHz AC transmit coils.

== References ==