Electromagnetic tracking: Difference between revisions

[[Electromagnetic tracking]] ('''EMT''') is a [[pose]]-estimation technology widely used in [[virtual reality]] (VR), [[augmented reality]] (AR), medical navigation and human–[[robotics|robot–computer interaction]].   

Unlike camera-based [[optical tracking]] or pure [[inertial tracking]], EMT determines the ''six-degree-of-freedom'' ([[6DOF]]) position and orientation of miniature sensor coils without requiring line-of-sight. A stationary [[field generator]] emits a precisely characterised magnetic field; tri-axial receiver coils measure that field and the system solves for each sensor’s pose every frame. Because each frame is computed independently, EMT exhibits '''no cumulative drift''', while total latencies are typically 2–10 ms.<ref name="Yaniv2009" />

Commercial EMT traces back to Polhemus Navigation Sciences’ '''3SPACE''' tracker, demonstrated by Raab ''et al.'' in 1979 and shipped in the early 1980s.<ref name="Raab1979" />

Polhemus’s ''FASTRAK'' line followed in 1991,<ref name="FASTRAKManual1993" /> and Ascension Technology (now NDI) introduced the pulsed-DC '''Flock of Birds''' in 1990.<ref name="FlockManual1999" />   

During the 1990s EMT migrated from military simulators into VR CAVEs and into image-guided surgery. The medical-grade '''NDI Aurora''' system (2002) made EMT the de-facto standard for tracking needles and catheters.<ref name="Yaniv2009" /> 

Consumer products followed: the dual-wand '''Razer Hydra''' PC controller (2011)<ref name="RazerHydra2011" /> and the “Control” wand for the original '''Magic Leap One''' AR headset (2018) both employ EMT, the latter generating three AC fields at 28.5–42.4 kHz.<ref name="MagicLeapFCC2017" />

A field generator contains three orthogonal transmitter coils driven sequentially (AC systems) or in short pulses (pulsed-DC systems). Each receiver houses three orthogonal pick-up coils. As every axis is energised the induced voltages encode the local magnetic-field vector; an over-determined least-squares solve yields 3-D position and orientation.<ref name="AscensionPulsedDC" />   

For an ideal magnetic dipole the far-field magnitude decays with the inverse cube of distance (|'''B'''| ∝ 1/''r''³), sharply limiting the working volume.

 

===Field generators===

Transmitters are supplied as planar plates, cube frames or compact blocks. A standard FASTRAK '''TX2''' source guarantees its published 0.76 mm RMS / 0.15° RMS accuracy inside a ~0.75 m radius (≈30 in) sphere;<ref name="PolhemusComp2020" /> larger '''TX4''' sources extend range at reduced precision.

Modern sensors are extremely small: Aurora 6-DOF sensors are 1.8 mm Ø, while its smallest 5-DOF sensor measures just 0.3 mm Ø × 2.5 mm long.<ref name="AuroraSensors" />

A single Aurora controller can track up to 16 6-DOF or 32 5-DOF sensors concurrently.

Static laboratory accuracy for FASTRAK is 0.76 mm RMS position and 0.15° RMS orientation;<ref name="PolhemusComp2020" /> update rates are 50–120 Hz and end-to-end latency is ≈4 ms.<ref name="FASTRAKManual1993" /> 

Real-world accuracy degrades near conductive or ferromagnetic objects, active implants or high-current devices, and falls rapidly beyond 0.8–1 m as the field decays.<ref name="Yaniv2009" />

===AC vs pulsed-DC===

AC trackers (Polhemus, NDI) provide strong continuous fields but are sensitive to eddy-current distortion. Pulsed-DC trackers (Ascension “Bird”, ‘‘trakSTAR’’) reduce such distortion at the expense of lower refresh rates.<ref name="AscensionPulsedDC" />

==Comparison with other tracking modalities==

| [[Optical tracking]] || Sub-mm accuracy; room-scale; unaffected by metal || Requires clear line-of-sight & lighting; occlusion problems

Hybrid camera/IMU systems such as [[Microsoft HoloLens]] and [[Meta Quest Pro]] achieve room-scale tracking; recent research prototypes fuse EMT with visual SLAM to re-localise after optical dropout and to guide flexible tools.<ref name="MiniEMT2024" />

* '''Consumer / gaming''' – The Razer Hydra offers ≈1 mm / 1° precision over a 1 m radius base station;<ref name="RazerHydra2011" /> Magic Leap One’s hand-held controller transmits three AC fields at 28.5–42.4 kHz for EMT-based 6-DOF tracking.<ref name="MagicLeapFCC2017" />

* '''Medical navigation''' – Aurora and Ascension 3D Guidance track needles, catheters and endoscopes during minimally invasive procedures where cameras cannot see.<ref name="Yaniv2009" />

* '''Industrial / research''' – Welding simulators, robotic hand-guiding inside metallic cells, marker-less motion capture beneath clothing and ergonomic studies use EMT where optical systems fail.

EMT provides drift-free, low-latency 6-DOF tracking in darkness, inside the body or through clothing, with sensors small enough to embed in tools.   

However, accuracy decreases rapidly outside the calibrated volume and in the presence of metal or strong fields. Operating-room studies report errors rising from <1 mm to >30 mm when powered instruments are within 30 cm of the receiver.<ref name="Poulin2002" />

* '''[[Polhemus]] FASTRAK / LIBERTY''' – AC; ≤0.76 mm RMS position, 0.15° orientation; up to 120 Hz   

* '''[[NDI Aurora]]''' – Medical-grade AC; up to 32 sensors; micro-sensors Ø 0.3–1.8 mm   

* '''[[Ascension Technology|Ascension]] trakSTAR / Flock of Birds''' – Pulsed-DC; up to 144 Hz   

* '''[[Razer Hydra]] / Sixense STEM''' – Dual-wand consumer game controllers (2011)   

* '''[[Magic Leap One]] “Control”''' – Hand-held AR controller emitting 28.5–42.4 kHz AC fields

<references />