Accuracy map of an optical motion capture system with 42 or 21 cameras in a large measurement volume

doi:10.1016/j.jbiomech.2017.05.006

Journal of Biomechanics

Volume 58, 14 June 2017, Pages 237-240

https://doi.org/10.1016/j.jbiomech.2017.05.006 Get rights and content

Abstract

Optical motion capture is commonly used in biomechanics to measure human kinematics. However, no studies have yet examined the accuracy of optical motion capture in a large capture volume (>100 m³), or how accuracy varies from the center to the extreme edges of the capture volume. This study measured the dynamic 3D errors of an optical motion capture system composed of 42 OptiTrack Prime 41 cameras (capture volume of 135 m³) by comparing the motion of a single marker to the motion reported by a ThorLabs linear motion stage. After spline interpolating the data, it was found that 97% of the capture area had error below 200 μm. When the same analysis was performed using only half (21) of the cameras, 91% of the capture area was below 200 μm of error. The only locations that exceeded this threshold were at the extreme edges of the capture area, and no location had a mean error exceeding 1 mm. When measuring human kinematics with skin-mounted markers, uncertainty of marker placement relative to underlying skeletal features and soft tissue artifact produce errors that are orders of magnitude larger than the errors attributed to the camera system itself. Therefore, the accuracy of this OptiTrack optical motion capture system was found to be more than sufficient for measuring full-body human kinematics with skin-mounted markers in a large capture volume (>100 m³).

Introduction

Optical motion capture (OMC) is used in a variety of fields, including biomechanics (Bell-Jenje et al., 2016, Le and Marras, 2016). The accuracy of various optical motion capture systems has been extensively evaluated in the past (Carse et al., 2013, Ehara et al., 1995, Ehara et al., 1997, Richards, 1999, Thewlis et al., 2013). However, these evaluations are typically performed in small volumes, from 0.005 to 15 m³ (Eichelberger et al., 2016, Windolf et al., 2008). Research on occupational ergonomics interventions, sports biomechanics, and rehab biomechanics often involve activities that span a larger volume, and thus require a more expansive motion capture system. Windolf and Eichelberger found that accuracy varies by location within the capture volume. In relatively large capture volumes (>100 m³), it is expected that accuracy has the potential to vary considerably by location, and a single measurement of accuracy at or near the center of the space is not adequate. No studies could be found that examined how accuracy changed approaching the edges of the capture space. Additionally, most previous studies have compared inter-marker distance to a more precisely known length of the same object. However, by only considering error in the measured distance (1D) between two markers, this technique fails to capture off-axis (3D) errors.

Thus, the aims of the current study were to establish the 3D accuracy of an OMC system for tracking individual markers within a large capture volume that is currently being used for biomechanical research, and to determine what portion of the capture area has acceptable accuracy for full-body biomechanics applications, such as gait analysis and occupational ergonomics.

Section snippets

Materials and methods

The OMC system used in this study was composed of 42 OptiTrack Prime 41 cameras and operated using OptiTrack Motive 1.10.1 Final software (NaturalPoint, Corvallis, Oregon, USA). Each calibration of the OMC system was performed by hand using an OptiTrack CWM-250 calibration wand with a length of 250.018 ± 0.002 mm. A panorama of the motion capture area can be seen in Fig. 1.

A ThorLabs LTS300 (Newton, New Jersey, USA) linear motion stage was used to evaluate the accuracy of the OMC data. The stage

Results

Maps of mean RMSE for each number of cameras are shown in Fig. 3. The majority of the capture area had an interpolated RMSE <200 μm (∼97% for 42 cameras; ∼91% for 21 cameras). Every point had a mean RMSE less than 1 mm, and the only locations that that exceeded 200 μm of error were those in the extreme corners of the capture area.

As expected, there is generally less error toward the center of the capture area. Errors then increase nonlinearly approaching the edges of the capture area, as revealed

Discussion

The results of this study confirm that OMC can be used to measure passive marker locations with accuracy better than 200 μm in the vast majority (97%) of even relatively large capture volumes (>100 m³), given optimal marker visibility conditions. Even when using only half (21) of the cameras, 91% of the capture area was found to achieve the same accuracy.

One objective of this study was to understand the magnitude of error contributed by the OMC system compared to other known sources of error when

Conclusion

The OMC system demonstrated submillimeter mean accuracy at every location in the capture volume, and error was found to be less than 200 μm in 97% of the capture volume (using all 42 cameras). Only very near the edges of the capture volume did error exceed 200 μm. The errors of the OMC system were found to be orders of magnitude smaller than other known sources of error associated with skin-mounted markers (marker placement errors and soft tissue artifact). Therefore, a large OMC system like the

Conflict of interest statement

OptiTrack (NaturalPoint, Corvallis, Oregon USA) provided equipment (including the linear motion stage and calibration wand) and technical assistance to optimize the system for this study.

Acknowledgments

None.

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Short communicationAccuracy map of an optical motion capture system with 42 or 21 cameras in a large measurement volume

Abstract

Introduction

Section snippets

Materials and methods

Results

Discussion

Conclusion

Conflict of interest statement

Acknowledgments

Manual Ther.

Physiotherapy

Gait Posture

Gait Posture

Gait Posture

J. Biomech.

J. Biomech.

Appl. Ergon.

Short communication
Accuracy map of an optical motion capture system with 42 or 21 cameras in a large measurement volume