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Created page with "= Electromagnetic tracking = '''Electromagnetic tracking''' ('''EMT''') is a pose-estimation technology widely used in [[Virtual reality]] (VR) and [[Augmented reality]] systems, as well as in medical navigation, robotics, and human–computer-interaction research. Unlike [[optical tracking]] or [[inertial tracking]], EMT determines the 3-D position and..."
 
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= Electromagnetic tracking =
{{see also|Terms|Technical Terms}}
{{see also|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.   
[[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 [[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" />
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]] produces a precisely controlled 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 suffers '''no cumulative drift''', while latencies are typically only a few milliseconds.<ref name="Yaniv2009">Yaniv Z., Wilson E., Lindisch D., Cleary K. “Electromagnetic tracking in the clinical environment.” ''Medical Physics'' 36 (3): 876–892 (2009). doi:10.1118/1.3075829.</ref>


== History ==
==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" />   
Commercial EMT began with [[Polhemus]]’s 1969 [[Space-Tracker]], followed by its FASTRAK line in the 1980s and [[Ascension Technology|Ascension]]’s pulsed-DC ‘‘Flock of Birds’’ (1991).<ref name="FASTRAK2018">Polhemus. “FASTRAK® Motion Tracking System — Product Overview.” PDF brochure, 2018. Accessed 30 Apr 2025.</ref><ref name="Flock2002">Ascension Technology Corp. ''Flock of Birds® Installation and Operation Guide,'' Rev B (2002).</ref>   
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" />
During the 1990s EMT migrated from military simulators into VR CAVEs and into medical navigation. Key medical trackers such as [[NDI Aurora]] (2002) made EMT the de-facto standard for surgical tool tracking. Consumer products followed: the [[Razer Hydra]] PC controller (2011) and the “Control” wand for the original [[Magic Leap One]] AR headset (2018) both employ EMT.<ref name="Hayden2018">Hayden S. “Magic Leap One Controller Appears in FCC Filing, Release on Track for 2018.” ''Road to VR,'' 21 Jun 2018.</ref><ref name="Razer2011">Razer Inc. “Thanks to the Razer Hydra, Now You’re Thinking With Motion Control.” Press release, 21 Apr 2011.</ref>


== Principles of operation ==
==Principles of operation==
A typical field generator contains three orthogonal transmitter coils driven in sequence (AC systems) or as brief pulses (pulsed-DC systems). Each receiver contains three orthogonal pick-up coils. As every axis is energised, the induced voltages encode the local magnetic-field vector. Solving the overdetermined set yields the sensor’s 3-D position and orientation.<ref name="AscensionPulsedDC" />   
A field generator contains three orthogonal transmitter coils driven sequentially (AC systems) or in short pulses (pulsed-DC systems). Each receiver holds three orthogonal pick-up coils. As every axis is energized, the induced voltages encode the local magnetic-field vector; an over-determined least-squares solve yields 3-D position and orientation.<ref name="pciBIRD">Ascension Technology Corp. “pciBIRD — Pulsed DC Magnetic Tracking.” Product sheet, accessed 30 Apr 2025.</ref>   
For an ideal magnetic dipole the field magnitude falls off approximately with the inverse **cube** of distance (|B| ∝ 1/''r''³), constraining useful range to roughly one cubic metre around the transmitter.<ref name="DipoleField" />
For an ideal magnetic dipole the far-field magnitude decays with the inverse cube of distance (|B| ∝ 1/''r''³), sharply limiting working volume.<ref name="Jackson1998">Jackson J. D. ''Classical Electrodynamics,'' 3rd ed. Wiley (1998) p. 181.</ref>


== Technical characteristics ==
==Technical characteristics==
; Field generators 
===Field Generators===
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" />
Transmitters are supplied as planar plates, cube frames, or towers. A standard Polhemus FASTRAK source covers ≈ 1.5 m × 1.5 m × 1.5 m at up to 120 Hz.<ref name="PolhemusChart2020">Polhemus. “Motion Tracking Technical Comparison Chart.” PDF, 2020. Accessed 30 Apr 2025.</ref>


; Sensors
===Sensors===
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" />
Modern sensors are extremely small: the Aurora 6DOF “micro” sensor is only 1.8 mm Ø, while its smallest 5DOF sensor is 0.3 mm Ø.<ref name="AuroraTools">Northern Digital Inc. “Aurora Electromagnetic Tracking — Sensors & Tools.” Product page, accessed 30 Apr 2025.</ref> A single Aurora controller can track up to 32 5DOF or 16 6DOF sensors.


; Performance
===Performance===
* *Static accuracy* (typical laboratory, undistorted): FASTRAK ≈ 0.76 mm RMS, 0.15° RMS.<ref name="PolhemusSpecs" />
Static laboratory accuracy for FASTRAK is ≈ 0.76 mm RMS and 0.15° RMS;<ref name="PolhemusChart2020" /> update rates range 50–120 Hz; latency is 3–10 ms. Real-world performance degrades near conductive or ferromagnetic objects, high-current devices, or at distances > 1 m, where the field drops rapidly.
* *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===
AC trackers (Polhemus, NDI) supply strong continuous fields but are susceptible to eddy-current distortion. Pulsed-DC trackers (Ascension “Bird”) reduce such distortion at the cost of lower refresh rates.<ref name="pciBIRD" />


; AC vs. pulsed-DC excitation 
==Comparison with Other Tracking Modalities==
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 ==
{| class="wikitable"
{| class="wikitable"
! Modality !! Advantages !! Limitations
! Modality !! Key strengths !! Key limitations
|-
|-
| '''EMT''' || No line-of-sight, drift-free absolute pose, works in darkness/inside body || Limited range; sensitive to metallic distortion; single-room volume
| '''EMT''' || Drift-free absolute pose; works through occlusion & darkness; sensors millimetres in size || Limited to ≲ 1 m volumes; distorted by nearby metal; magnetic interference
|-
|-
| [[Optical tracking]] || Sub-millimetre accuracy; room-scale; immune to metal || Requires direct visibility and lighting; occlusion failures
| [[Optical tracking]] || Sub-mm accuracy, room-scale, unaffected by metal || Requires clear line-of-sight & lighting; suffers from occlusion
|-
|-
| [[Inertial tracking]] || High update (>1 kHz); no external infrastructure || Unlimited drift; cannot provide absolute position
| [[Inertial tracking]] || kHz update; no external infrastructure || Unlimited drift; cannot give 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" />
Hybrid camera-IMU systems such as [[Microsoft HoloLens]] and [[Meta Quest Pro]] achieve room-scale tracking; EMT is sometimes fused in research prototypes to re-localise after optical dropout.<ref name="Yaniv2009" />


== Applications ==
==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" />
* '''Consumer / gaming''' – The [[Razer Hydra]] offered ≈ 1 mm / 1° precision over a 1-m radius base station; Magic Leap One’s hand-held controller transmits three AC fields at 28.5–42.4 kHz for EMT.<ref name="Hayden2018" />
* '''Medical navigation''' – NDI Aurora and Ascension 3D Guidance track needles, catheters, and endoscopes without radiation.<ref name="Yaniv2009" />
* '''Medical navigation''' – [[NDI Aurora]] and Ascension 3D Guidance track needles, catheters, and endoscopes during minimally invasive procedures where cameras cannot see.<ref name="Yaniv2009" />
* '''Industrial & research''' – Tool tracking in welding simulators, motion capture under clothing, robot hand-guiding inside metallic workcells.
* '''Industrial / research''' – Welding simulators, robot hand-guiding in metallic cells, marker-less motion capture under clothing, and ergonomics studies use EMT where optical solutions fail.


== Strengths and limitations ==
==Strengths and limitations==
EMT offers drift-free, low-latency 6DOF tracking through occlusions and tissue, and supports extremely small, embeddable sensors.   
EMT provides drift-free, low-latency 6DOF tracking in darkness, inside the body, or through clothing, with sensors small enough to embed in tools.   
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.
However, accuracy degrades rapidly outside the calibrated volume and in the presence of metal or strong fields. Interference studies show position errors rising from < 1 mm to > 30 mm when powered instruments are held < 30 cm from the receiver.<ref name="Poulin2002">Poulin F., Amiot L-P. “Interference during the use of an electromagnetic tracking system under OR conditions.” ''Journal of Biomechanics'' 35 (6): 733–737 (2002). doi:10.1016/S0021-9290(02)00036-2.</ref>


== Notable commercial systems ==
==Notable commercial systems==
* '''[[Polhemus]] FASTRAK / LIBERTY''' – AC, up to 120 Hz, ≤ 0.76 mm RMS accuracy.   
* '''[[Polhemus]] FASTRAK / LIBERTY''' – AC; ≤ 0.76 mm RMS position, 0.15° orientation; up to 120 Hz.   
* '''[[NDI Aurora]]''' – Medical-grade AC tracking; up to 32 sensors; sensors as small as 0.3 mm.   
* '''[[NDI Aurora]]''' – Medical-grade AC; up to 32 sensors; micro-sensors Ø 0.3–1.8 mm.   
* '''[[Ascension Technology|Ascension]] 3D Guidance / Flock of Birds''' – Pulsed-DC, 144 Hz max.   
* '''[[Ascension Technology|Ascension]] 3D Guidance / Flock of Birds''' – Pulsed-DC; 144 Hz typical.   
* '''[[Razer Hydra]] / Sixense STEM''' – Consumer dual-wand controller (2011).   
* '''[[Razer Hydra]] / Sixense STEM''' – Consumer dual-wand game controller (2011).   
* '''Magic Leap One “Control”''' – Handheld AR controller with 28.5–42.4 kHz AC transmit coils.
* '''[[Magic Leap One]] “Control”''' – Hand-held AR controller generating 28.5–42.4 kHz AC fields.


== References ==
==References==
<references/>
<references/>
[[Category:Terms]]
[[Category:Technical Terms]]
[[Category:Tracking]]
[[Category:Types of tracking]]
[[Category:Motion tracking]]
[[Category:Magnetic technology]]
[[Category:Human–computer interaction]]
[[Category:Medical navigation]]

Latest revision as of 16:11, 1 May 2025

See also: Terms and Technical Terms
See also: Tracking

Electromagnetic tracking (EMT) is a Pose-estimation technology widely used in virtual reality (VR), augmented reality (AR), medical navigation, and human–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 produces a precisely controlled 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 suffers no cumulative drift, while latencies are typically only a few milliseconds.[1]

History

Commercial EMT began with Polhemus’s 1969 Space-Tracker, followed by its FASTRAK line in the 1980s and Ascension’s pulsed-DC ‘‘Flock of Birds’’ (1991).[2][3] During the 1990s EMT migrated from military simulators into VR CAVEs and into medical navigation. Key medical trackers such as NDI Aurora (2002) made EMT the de-facto standard for surgical tool tracking. Consumer products followed: the Razer Hydra PC controller (2011) and the “Control” wand for the original Magic Leap One AR headset (2018) both employ EMT.[4][5]

Principles of operation

A field generator contains three orthogonal transmitter coils driven sequentially (AC systems) or in short pulses (pulsed-DC systems). Each receiver holds three orthogonal pick-up coils. As every axis is energized, the induced voltages encode the local magnetic-field vector; an over-determined least-squares solve yields 3-D position and orientation.[6] For an ideal magnetic dipole the far-field magnitude decays with the inverse cube of distance (|B| ∝ 1/r³), sharply limiting working volume.[7]

Technical characteristics

Field Generators

Transmitters are supplied as planar plates, cube frames, or towers. A standard Polhemus FASTRAK source covers ≈ 1.5 m × 1.5 m × 1.5 m at up to 120 Hz.[8]

Sensors

Modern sensors are extremely small: the Aurora 6DOF “micro” sensor is only 1.8 mm Ø, while its smallest 5DOF sensor is 0.3 mm Ø.[9] A single Aurora controller can track up to 32 5DOF or 16 6DOF sensors.

Performance

Static laboratory accuracy for FASTRAK is ≈ 0.76 mm RMS and 0.15° RMS;[8] update rates range 50–120 Hz; latency is 3–10 ms. Real-world performance degrades near conductive or ferromagnetic objects, high-current devices, or at distances > 1 m, where the field drops rapidly.

AC vs. Pulsed-DC

AC trackers (Polhemus, NDI) supply strong continuous fields but are susceptible to eddy-current distortion. Pulsed-DC trackers (Ascension “Bird”) reduce such distortion at the cost of lower refresh rates.[6]

Comparison with Other Tracking Modalities

Modality Key strengths Key limitations
EMT Drift-free absolute pose; works through occlusion & darkness; sensors millimetres in size Limited to ≲ 1 m volumes; distorted by nearby metal; magnetic interference
Optical tracking Sub-mm accuracy, room-scale, unaffected by metal Requires clear line-of-sight & lighting; suffers from occlusion
Inertial tracking kHz update; no external infrastructure Unlimited drift; cannot give absolute position

Hybrid camera-IMU systems such as Microsoft HoloLens and Meta Quest Pro achieve room-scale tracking; EMT is sometimes fused in research prototypes to re-localise after optical dropout.[1]

Applications

  • Consumer / gaming – The Razer Hydra offered ≈ 1 mm / 1° precision over a 1-m radius base station; Magic Leap One’s hand-held controller transmits three AC fields at 28.5–42.4 kHz for EMT.[4]
  • Medical navigation – NDI Aurora and Ascension 3D Guidance track needles, catheters, and endoscopes during minimally invasive procedures where cameras cannot see.[1]
  • Industrial / research – Welding simulators, robot hand-guiding in metallic cells, marker-less motion capture under clothing, and ergonomics studies use EMT where optical solutions fail.

Strengths and limitations

EMT provides drift-free, low-latency 6DOF tracking in darkness, inside the body, or through clothing, with sensors small enough to embed in tools. However, accuracy degrades rapidly outside the calibrated volume and in the presence of metal or strong fields. Interference studies show position errors rising from < 1 mm to > 30 mm when powered instruments are held < 30 cm from the receiver.[10]

Notable commercial systems

  • 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 3D Guidance / Flock of Birds – Pulsed-DC; 144 Hz typical.
  • Razer Hydra / Sixense STEM – Consumer dual-wand game controller (2011).
  • Magic Leap One “Control” – Hand-held AR controller generating 28.5–42.4 kHz AC fields.

References

  1. ↑ 1.0 1.1 1.2 Yaniv Z., Wilson E., Lindisch D., Cleary K. “Electromagnetic tracking in the clinical environment.” Medical Physics 36 (3): 876–892 (2009). doi:10.1118/1.3075829.
  2. ↑ Polhemus. “FASTRAK® Motion Tracking System — Product Overview.” PDF brochure, 2018. Accessed 30 Apr 2025.
  3. ↑ Ascension Technology Corp. Flock of Birds® Installation and Operation Guide, Rev B (2002).
  4. ↑ 4.0 4.1 Hayden S. “Magic Leap One Controller Appears in FCC Filing, Release on Track for 2018.” Road to VR, 21 Jun 2018.
  5. ↑ Razer Inc. “Thanks to the Razer Hydra, Now You’re Thinking With Motion Control.” Press release, 21 Apr 2011.
  6. ↑ 6.0 6.1 Ascension Technology Corp. “pciBIRD — Pulsed DC Magnetic Tracking.” Product sheet, accessed 30 Apr 2025.
  7. ↑ Jackson J. D. Classical Electrodynamics, 3rd ed. Wiley (1998) p. 181.
  8. ↑ 8.0 8.1 Polhemus. “Motion Tracking Technical Comparison Chart.” PDF, 2020. Accessed 30 Apr 2025.
  9. ↑ Northern Digital Inc. “Aurora Electromagnetic Tracking — Sensors & Tools.” Product page, accessed 30 Apr 2025.
  10. ↑ Poulin F., Amiot L-P. “Interference during the use of an electromagnetic tracking system under OR conditions.” Journal of Biomechanics 35 (6): 733–737 (2002). doi:10.1016/S0021-9290(02)00036-2.
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