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Technical Briefs

Effects of Camera Switching on Fine Accuracy in a Motion Capture System

[+] Author and Article Information
Laurel Kuxhaus

Department of Bioengineering, University of Pittsburgh, Pittsburgh, PA 15261; Orthopaedic Biomechanics Research Laboratory, Allegheny General Hospital, Pittsburgh, PA 15212

Patrick J. Schimoler

Orthopaedic Biomechanics Research Laboratory, Allegheny General Hospital, Pittsburgh, PA 15212; Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA 15261

Jeffrey S. Vipperman

Department of Mechanical Engineering and Materials Science and Department of Bioengineering, University of Pittsburgh, Pittsburgh, PA 15261

Mark Carl Miller1

Department of Mechanical Engineering and Materials Science and Department of Bioengineering, University of Pittsburgh, Pittsburgh, PA 15261; Orthopaedic Biomechanics Research Laboratory, Allegheny General Hospital, Pittsburgh, PA 15212mcmiller@wpahs.org

1

Corresponding author.

J Biomech Eng 131(1), 014502 (Nov 21, 2008) (6 pages) doi:10.1115/1.3002910 History: Received July 19, 2007; Revised August 26, 2008; Published November 21, 2008

When using optical motion capture systems, increasing the number of cameras improves the visibility. However, the software used to deal with the information fusion from multiple cameras may compromise the accuracy of the system due to camera dropout, which can vary with time. In cadaver studies of radial head motion, increasing the number of cameras used by the motion capture system seemed to decrease the accuracy of the measurements. This study investigates the cause. The hypothesis was that errors in position can be induced when markers are obscured from and then restored to a camera’s viewable range, as can happen in biomechanical studies. Accuracy studies quantified the capabilities of the motion capture system with precision translation and rotation movements. To illustrate the effect that abrupt perceived changes in a marker’s position can have on the calculation of radial head travel, simulated motion experiments were performed. In these studies, random noise was added to simulated data, which obscured the resultant path of motion. Finally, camera-blocking experiments were performed in which precise movements were measured with a six-camera Vicon system and the errors between the actual and perceived motion were computed. During measurement, cameras were selectively blocked and restored to view. The maximum errors in translation and rotation were 3.7 mm and 0.837 deg, respectively. Repeated measures analysis of variance (ANOVAs) (α=0.05) confirmed that the camera-blocking influenced the results. Taken together, these results indicate that camera-switching can affect the observation of fine movements using a motion analysis system with a large number of cameras. One solution is to offer opportunity for user interaction in the software to choose the cameras used for each instant of time.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 6

Marker coordinate components from a trial in which some cameras were obscured and restored to view. The asterisks indicate the onset and release of the obstruction.

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Figure 5

Error between actual and measured movements. Section (a) shows the results for static and translation trials, and (b) shows the results for rotation. The error bars indicate the range of all error values.

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Figure 4

Radial head travel on the capitellum from simulated data. The large circle is the capitellum. Subplot (a) shows the simulated data as if the radial head was traveling in a perfect circle around the center of the capitellum; (b) shows the results when 1 mm of random noise is added to the position of one marker position; and (c) shows the radial head travel when 1 mm of noise is added to all markers used in tracking the radial head.

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Figure 3

Example Continuity Chart. Each labeled group represents the use of data in marker position computation from each of the six cameras. For example, for the rows labeled “DRAD3,” the bottom two cameras are rarely used.

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Figure 2

Diagram of camera position and camera block states for Cases A–E. The observed markers were located where the letters A–E are shown.

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Figure 1

Rms error of radial head markers during movement and the corresponding radial head travel (insets). Note that the pattern in the rms is echoed in the travel.

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