Electromagnetic tracking
Electromagnetic tracking
Electromagnetic tracking (EMT) is a 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 (Template:Small) 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.[1]
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’s ‘‘Flock of Birds’’ pulsed-DC trackers.[2][3] 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).[4][5]
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.[6] For an ideal magnetic dipole the field magnitude falls off approximately with the inverse **cube** of distance (|B| ∝ 1/r³), constraining useful range to roughly one cubic metre around the transmitter.[7]
Technical characteristics
- 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.[8]
- 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.[9] Up to **32** 5DOF sensors or **16** 6DOF sensors may be tracked simultaneously by a single Aurora controller.[9]
- Performance
- *Static accuracy* (typical laboratory, undistorted): FASTRAK ≈ 0.76 mm RMS, 0.15° RMS.[8]
- *Update rate*: 50–120 Hz for most commercial units; some research rigs exceed 240 Hz.
- *Latency*: 3–10 ms sensor-to-host.[2]
Performance degrades in the presence of conductive or ferromagnetic materials, electrical motors, or large ambient fields.[10]
- 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.[6]
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).[1]
Applications
- Consumer & gaming – Razer Hydra (1 mm / 1° precision), Magic Leap One controller (28.5–42.4 kHz AC coils).[5][4]
- Medical navigation – NDI Aurora and Ascension 3D Guidance track needles, catheters, and endoscopes without radiation.[1]
- 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/r³) and (b) distortion from nearby metals or active electronics, which can introduce centimetre-scale bias if unmodelled.[10] 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 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
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