Magnetic tracking systems are electromagnetic systems that calculate both the position and orientation by the relative flux of three orthogonal coils on both the transmitter and receiver (sensor on the subject). In magnetic tracking systems, the tracking sensors (receivers) are placed on the subject to measure the spatial relationship relative to the magnetic transmitter. The tracking sensors output 6 degrees of freedom (DOF), 3 each for the translations and orientations. Thus, no post-processing is required to compute rotations of the body segments. By placing 1 sensor on each body segment of interest, one can obtain full rigid body motion for each underlying skeletal segment in real-time. This makes magnetic trackers very popular for real-time performance analysis; it has become a very popular motion tracking technology for the golf industry.
There are two different types of magnetic tracking systems. The first uses direct current (DC) electromagnetic fields (Ascencion), while the second uses alternating current (AC) electromagnetic fields (Polhemus). Each system is susceptible to interference, with AC systems being very sensitive to aluminum and copper, whereas DC systems aver very sensitive to iron and steel. The capture volumes for magnetic tracking systems are much smaller than for optical systems. This precludes their use in certain projects that require a large capture volume. However, this can be advantageous for projects that require only a small capture volume, as the amount of interference is reduced due to the smaller volume. It is important to note that sensor response is non-linear, especially near the edges of the capture volume.
Magnetic tracking sensors do not suffer from occlusion from any non-metallic objects placed between the transmitter and the receiver. However, the magnetic sensors require cables to run from each sensor to the transmitter, so complex motions in high-speed activities may not be able to be recorded with magnetic tracking systems. Manufacturers are now offering some wireless capabilities for magnetic sensors for any unencumbered motion needs; however this is typically limited to up to only 4 sensors and the sampling rate in these product offerings is often less than 50 Hz which is not adequate for any high-speed motions. Another consideration is that the batteries needed for the sensors typically only last up to 4 hours without having to be recharged. This may or may not be a limiting factor dependent upon the project scope.
The sampling rate for magnetic tracking systems typically range from 120 – 240 Hz. While this is lower than most optical tracking systems, it is usually adequate for most projects except for extremely high-speed motions. Magnetic tracking systems data output tends to be a little noisier than optical tracking output. However, that disadvantage is offset by the fact that magnetic tracking systems provide real-time 6 DOF output. Magnetic tracking systems can capture multiple subjects simultaneously if multiple setups are used. Magnetic tracking systems cost much less than optical tracking systems and are fairly easy to setup and use out of the box.
Magnetic Tracking Systems Advantages
- 6 DOF position and orientation output are immediately output without post-processing.
- Tracking sensors do not suffer from line-of-sight occlusions from non-metallic objects.
- Real-time feedback allows for real-time applications.
- Multiple subjects can be captured simultaneously with multiple magnetic tracking setups.
- Magnetic tracking systems are solid-state systems that do not suffer from drift errors over time.
- Magnetic tracking systems are less expensive than optical tracking systems.
Magnetic Tracking Systems Disadvantages
- Magnetic tracking systems are susceptible to magnetic and electrical interference.
- Wiring and batteries for magnetic tracking sensors can impede the capture subjects’ movements for certain activities.
- Magnetic sensors have a lower sampling rate than most optical systems.
- Magnetic 6 DOF output data is typically noisier than optical system 3D positional outputs.
- Magnetic tracking systems capture volumes are much smaller than optical tracking systems.