Electromagnetic 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[10]; 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.[11]
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.[12]
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.[13]
- Medical navigation – NDI Aurora and Ascension 3D Guidance track needles, catheters, and endoscopes during minimally invasive procedures where cameras cannot see.[14]
- 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.[15]
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 Ø0.3–1.8 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
- ↑ 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.
- ↑ Polhemus. “FASTRAK® Motion Tracking System – Product Overview.” Polhemus.com, accessed 30 April 2025.
- ↑ Ascension Technology Corp. Flock of Birds® User Manual, Rev G (1999).
- ↑ Lang B. “Magic Leap One Controller Appears in FCC Filing, Suggests 2018 Headset Launch.” Road to VR, 20 Sept 2017.
- ↑ Razer Inc. “Thanks to the Razer Hydra, Now You’re Thinking With Motion Control.” Press release, 11 Apr 2011.
- ↑ Ascension Technology Corp. “Pulsed DC Magnetic Tracking Technology Overview.” White paper, 2003.
- ↑ Jackson J.D. Classical Electrodynamics, 3 rd ed. Wiley (1998) p. 181.
- ↑ Polhemus. “Motion Tracking Technical Comparison Chart.” PDF, 2020.
- ↑ Northern Digital Inc. “Aurora Electromagnetic Tracking – Sensors & Tools.” NDigital.com, accessed 30 April 2025.
- ↑ Polhemus. “Motion Tracking Technical Comparison Chart.”
- ↑ Ascension Technology Corp. “Pulsed DC Magnetic Tracking Technology Overview.”
- ↑ Yaniv Z. et al., 2009.
- ↑ Lang B., 2017.
- ↑ Yaniv Z. et al., 2009.
- ↑ 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).