Optoelectronic measurement systems

The optoelectronic measurement systems (OMS) are more accurate than the other systems (see chart). Not surprisingly, the optical systems (e.g. Optotrak or Vicon) are in literature often regarded as the gold standard in motion capture (Corazza, Mündermann, Gambaretto, Ferrigno, & Andriacchi, 2010). An OMS detects light and uses this detection to estimate the 3D position of a marker via time-of-flight triangulation. Accuracy of the systems is dependent on the following parts of the experimental set-up: the locations of the cameras relative to each other, the distance between the cameras and the markers, the position, number, and type of the markers in the field, and the motion of the markers within the capture volume (Maletsky, Sun, & Morton, 2007). Also, there is a trade-off between camera resolution and sample frequency.

OMS are based on fixed cameras and can therefore acquire data only in a restricted area (Begon et al., 2009). The capture volume is dependent on the maximum number of cameras and the field of view of each camera. The largest measured range with OMS is 824 m2, obtained with a Vicon MX13 measurement system (Spörri, Schiefermüller, & Müller, 2016). For this range, 24 cameras were required. This number of cameras results in significant practical difficulties regarding cost, portability, calibration, synchronization, labor, and set-up. Further limitations of the system are the necessity of line-of-sight, which means that the data output will be interrupted when the cameras lose sight of the markers (Panjkota, Stancic, & Supuk, 2009; Spörri et al., 2016). Furthermore, the systems are highly sensitive to alterations in the setup, e.g. due to accidental shifting of a camera (Windolf, Götzen, & Morlock, 2008). The systems are mostly used in dark areas (indoors), because bright sunlight interferes with the measurements (Spörri et al., 2016).

There are two categories within the optoelectronic systems: active marker systems and passive marker systems. Passive systems use markers that reflect light back to the sensor. The Vicon systems in the chart (Figure 2) are examples of passive motion capture systems. Active systems utilize markers that contain the source of light for the sensors (often infrared) (Richards, 1999). In the chart, Optotrak 3020 is an active marker optical system. The benefit of active markers over passive ones is that the measurements are more robust. However, active markers do require additional cables and batteries, so the freedom of movement is more limited (Stancic, Supuk, & Panjkota, 2013). In addition, the maximum sample frequency is lowered when multiple markers are used as the signal of each individual marker needs to have distinguishable frequency by which it can be identified.

Video: van der Kruk, E., et al. “Getting in shape: Reconstructing three-dimensional long-track speed skating kinematics by comparing several body pose reconstruction techniques.” Journal of biomechanics 69 (2018): 103-112.

 
A rather original way of increasing the range of a marker-based optoelectronic measurement system is the rolling motion capture system (Begon et al., 2009; Colloud, Chèze, André, & Bahuaud, 2008). With this method, cameras are placed on a fixed moving frame, to meet the requirement of fixed relative positions between the cameras. The method was applied in a 3D kinematic analysis of rowing, with a three-camera-recording-system mounted on a boat, which stayed next to the rowers (Kersting, Kurpiers, Darlow, & Nolte, 2008); this study showed an accuracy of about 30 mm in mean joint centres. Kersting et al. concluded, however, that the method is very time consuming – mainly due to calibration- and not suitable for general training purposes.
Indoor GPS (iGPS) is a OMS that is not based on markers, but on receivers that are attached to the tracked object or participant (Nikon, 2017). In contrast to what the name may indicate, the (physical) working principle is entirely different from a regular GPS system: the system has a transmitter which uses laser and infrared light to transmit position information from the transmitter to the receiver (Nikon, 2017). This is a one-way procedure. The advantage of this system is that there is practically no limit to the scalability of the system. Therefore it is possible to add as many transmitters as needed to cover a (factory) wide area and an unlimited number of receivers can be used (Khoury & Kamat, 2009). The accuracy of the system, determined on an indoor ice rink (12600m2), was 6.4 mm (van der Kruk, 2013a). Important drawbacks for the application of this system in sport, are the size and weight of the receivers that need to be attached to the athlete (see Table 1).

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