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[[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]].   
[[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]] 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>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>
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" />


==History==
==History==
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>Polhemus. “FASTRAK® Motion Tracking System – Product Overview.” Polhemus.com, accessed 30 April 2025.</ref><ref>Ascension Technology Corp. ''Flock of Birds® User Manual'', Rev G (1999).</ref>   
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" />
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>Lang B. “Magic Leap One Controller Appears in FCC Filing, Suggests 2018 Headset Launch.” ''Road to VR'', 20 Sept 2017.</ref><ref>Razer Inc. “Thanks to the Razer Hydra, Now You’re Thinking With Motion Control.” Press release, 11 Apr 2011.</ref>
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" />


==Principles of operation==
==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.<ref>Ascension Technology Corp. “Pulsed DC Magnetic Tracking Technology Overview.” White paper, 2003.</ref>   
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 working volume.<ref>Jackson J.D. ''Classical Electrodynamics'', 3 rd ed. Wiley (1998) p. 181.</ref>
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.


==Technical characteristics==
==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.<ref>Polhemus. “Motion Tracking Technical Comparison Chart.” PDF, 2020.</ref>
===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.


===Sensors===
===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.<ref>Northern Digital Inc. “Aurora Electromagnetic Tracking – Sensors & Tools.” NDigital.com, accessed 30 April 2025.</ref> A single Aurora controller can track up to 32 5DOF or 16 6DOF sensors.
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.


===Performance===
===Performance===
Static laboratory accuracy for FASTRAK is ≈0.76 mm RMS and 0.15° RMS<ref>Polhemus. “Motion Tracking Technical Comparison Chart.”</ref>; 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.
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 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>Ascension Technology Corp. “Pulsed DC Magnetic Tracking Technology Overview.”</ref>
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==
==Comparison with other tracking modalities==
{| class="wikitable"
{| class="wikitable"
! Modality !! Key strengths !! Key limitations
! Modality !! Key strengths !! Key limitations
Line 29: Line 34:
| '''EMT''' || Drift-free absolute pose; works through occlusion & darkness; sensors millimetres in size || Limited to ≲1 m volumes; distorted by nearby metal; magnetic interference
| '''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
| [[Optical tracking]] || Sub-mm accuracy; room-scale; unaffected by metal || Requires clear line-of-sight & lighting; occlusion problems
|-
|-
| [[Inertial tracking]] || kHz update; no external infrastructure || Unlimited drift; cannot give absolute position
| [[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.<ref>Yaniv Z. et al., 2009.</ref>
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" />


==Applications==
==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.<ref>Lang B., 2017.</ref>
* '''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''' – [[NDI Aurora]] and Ascension 3D Guidance track needles, catheters, and endoscopes during minimally invasive procedures where cameras cannot see.<ref>Yaniv Z. et al., 2009.</ref>
* '''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, robot hand-guiding in metallic cells, marker-less motion capture under clothing, and ergonomics studies use EMT where optical solutions fail.
* '''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.


==Strengths and limitations==
==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.   
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 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>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).</ref>
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" />


==Notable commercial systems==
==Notable commercial systems==
* '''[[Polhemus]] FASTRAK / LIBERTY''' – AC; ≤0.76 mm RMS position, 0.15° orientation; up to 120 Hz.  
* '''[[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 Ø0.3–1.8 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 typical.  
* '''[[Ascension Technology|Ascension]] trakSTAR / Flock of Birds''' – Pulsed-DC; up to 144 Hz   
* '''[[Razer Hydra]] / Sixense STEM''' – Consumer dual-wand game controller (2011).  
* '''[[Razer Hydra]] / Sixense STEM''' – Dual-wand consumer game controllers (2011)   
* '''[[Magic Leap One]] “Control”''' – Hand-held AR controller generating 28.5–42.4 kHz AC fields.
* '''[[Magic Leap One]] “Control”''' – Hand-held AR controller emitting 28.5–42.4 kHz AC fields


==References==
==References==
<references/>
<references />

Revision as of 10:44, 30 April 2025

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 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.[1]

History

Commercial EMT traces back to Polhemus Navigation Sciences’ 3SPACE tracker, demonstrated by Raab et al. in 1979 and shipped in the early 1980s.[2] Polhemus’s FASTRAK line followed in 1991,[3] and Ascension Technology (now NDI) introduced the pulsed-DC Flock of Birds in 1990.[4] 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.[1] Consumer products followed: the dual-wand Razer Hydra PC controller (2011)[5] 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.[6]

Principles of operation

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.[7] For an ideal magnetic dipole the far-field magnitude decays with the inverse cube of distance (|B| ∝ 1/r³), sharply limiting the working volume.

Technical characteristics

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;[8] larger TX4 sources extend range at reduced precision.

Sensors

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

Performance

Static laboratory accuracy for FASTRAK is 0.76 mm RMS position and 0.15° RMS orientation;[8] update rates are 50–120 Hz and end-to-end latency is ≈4 ms.[3] 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.[1]

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.[7]

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; occlusion problems
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; recent research prototypes fuse EMT with visual SLAM to re-localise after optical dropout and to guide flexible tools.[10]

Applications

  • Consumer / gaming – The Razer Hydra offers ≈1 mm / 1° precision over a 1 m radius base station;[5] Magic Leap One’s hand-held controller transmits three AC fields at 28.5–42.4 kHz for EMT-based 6-DOF tracking.[6]
  • Medical navigation – Aurora and Ascension 3D Guidance track needles, catheters and endoscopes during minimally invasive procedures where cameras cannot see.[1]
  • 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.

Strengths and limitations

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.[11]

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 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

  1. ↑ 1.0 1.1 1.2 1.3 Cite error: Invalid <ref> tag; no text was provided for refs named Yaniv2009
  2. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Raab1979
  3. ↑ 3.0 3.1 Cite error: Invalid <ref> tag; no text was provided for refs named FASTRAKManual1993
  4. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named FlockManual1999
  5. ↑ 5.0 5.1 Cite error: Invalid <ref> tag; no text was provided for refs named RazerHydra2011
  6. ↑ 6.0 6.1 Cite error: Invalid <ref> tag; no text was provided for refs named MagicLeapFCC2017
  7. ↑ 7.0 7.1 Cite error: Invalid <ref> tag; no text was provided for refs named AscensionPulsedDC
  8. ↑ 8.0 8.1 Cite error: Invalid <ref> tag; no text was provided for refs named PolhemusComp2020
  9. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named AuroraSensors
  10. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named MiniEMT2024
  11. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Poulin2002
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