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Technical Briefs

Method for Testing Motion Analysis Laboratory Measurement Systems

[+] Author and Article Information
Marko J. Hakkarainen1

Department of Physics and Mathematics, University of Eastern Finland, Kuopio, Finlandmarko.hakkarainen@uef.fi

Timo Bragge, Pasi A. Karjalainen, Mika Tarvainen

Department of Physics and Mathematics, University of Eastern Finland, Kuopio, Finland

Tuomas Liikavainio, Jari Arokoski

Department of Physical and Rehabilitation Medicine, Kuopio University Hospital, Kuopio, Finland

1

Corresponding author.

J Biomech Eng 132(11), 114501 (Oct 12, 2010) (5 pages) doi:10.1115/1.4002368 History: Received February 12, 2009; Revised June 22, 2010; Posted August 16, 2010; Published October 12, 2010; Online October 12, 2010

This paper proposes a method for comparing data from accelerometers, optical based 3D motion capture systems, and force platforms (FPs) in the context of spatial and temporal differences. Testing method is based on the motion laboratory accreditation test (MLAT), which can be used to test FP and camera based motion capture components of a motion analysis laboratory. This study extends MLAT to include accelerometer data. Accelerometers were attached to a device similar to the MLAT rod. The elevation of the rod from the plane of the floor is computed and compared with the force platform vector orientation and the rod orientation obtained by optical motion capture system. Orientation of the test device is achieved by forming nonlinear equation group, which describes the components of the measured accelerations. Solution for this equation group is estimated by using the Gauss–Newton method. This expanded MLAT procedure can be used in the laboratory setting were either FP, camera based motion capture, or any other motion capture system is used along with accelerometer measurements.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Test device used in this study. The device has four retroreflective targets for the camera based motion tracking and two triaxial accelerometers firmly attached into different locations in the shaft.

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Figure 2

Schematic diagram of measured and true acceleration components of one accelerometer and measured force components. All three-dimensional accelerations and forces are reduced to two-dimensional space by combining their x and y components. The acceleration components aN and aT, which include the gravitational acceleration, are measured by the accelerometers. The components an and at are the true normal and tangential accelerations in the measurement point, respectively, and g is the gravitation. Thin arrow indicates direction of the movement. The distance of the two accelerometers from the bottom tip is R1 and R2, respectively, and the angle of test device relative to the x-y plane of the FP is α. The components Fxy and Fz are forces measured by the force plate.

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Figure 3

The angle of rod respect to x-y plane derived from motion capture data (thick black line), from force plate data (thin black line), and from accelerometer data (gray line)

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Figure 4

The effect of improper calibration of the accelerometer signal. Measured acceleration component aT1 was added with constant error of 0.1g (thick gray line) and relative error of −5% (thin gray line). The angle estimates from properly calibrated signal (thin black line) and from motion capture data (thick black line) are plotted for reference.

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