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Technical Briefs

Design and Evaluation of a New General-Purpose Device for Calibrating Instrumented Spatial Linkages

[+] Author and Article Information
Joshua A. Nordquist

Department of Mechanical Engineering, University of California, One Shields Avenue, Davis, CA 95616

M. L. Hull1

Department of Mechanical Engineering, and Biomedical Engineering Program, University of California, One Shields Avenue, Davis, CA 95616mlhull@ucdavis.edu

1

Corresponding author.

J Biomech Eng 131(3), 034505 (Jan 07, 2009) (6 pages) doi:10.1115/1.2965375 History: Received October 16, 2006; Revised June 25, 2008; Published January 07, 2009

Because instrumented spatial linkages (ISLs) have been commonly used in measuring joint rotations and must be calibrated before using the device in confidence, a calibration device design and associated method for quantifying calibration device error would be useful. The objectives of the work reported by this paper were to (1) design an ISL calibration device and demonstrate the design for a specific application, (2) describe a new method for calibrating the device that minimizes measurement error, and (3) quantify measurement error of the device using the new method. Relative translations and orientations of the device were calculated via a series of transformation matrices containing inherent fixed and variable parameters. These translations and orientations were verified with a coordinate measurement machine, which served as a gold standard. Inherent fixed parameters of the device were optimized to minimize measurement error. After parameter optimization, accuracy was determined. The root mean squared error (RMSE) was 0.175 deg for orientation and 0.587 mm for position. All RMSE values were less than 0.8% of their respective full-scale ranges. These errors are comparable to published measurement errors of ISLs for positions and lower by at least a factor of 2 for orientations. These errors are in spite of the many steps taken in design and manufacturing to achieve high accuracy. Because it is challenging to achieve the accuracy required for a custom calibration device to serve as a viable gold standard, it is important to verify that a calibration device provides sufficient precision to calibrate an ISL.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

Illustration of the calibration device. The device has three degrees of freedom with three revolute axes that intersect at the center of rotation (COR). Also illustrated are the fixed (f) and moving (m) coordinate systems. In the illustration the calibration device is in the defined neutral position.

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Figure 2

Illustration of the revolute joint design. Revolute joints in the calibration device were constructed with a double-bearing compression design. The joint compression screw, when tightened, minimized any play in the joint due to machining and assembly tolerances.

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Figure 3

Illustration of the three-bar linkage and necessary parameters for controlling the rotation about the revolute joint. The micrometer varied the distance “micro” to control rotation. Additional revolute joints, which connected the micrometer to bar1 and bar2, utilized brass bushings and a lock nut compression design to minimize play from machining tolerances. The micrometer was installed using set screws and created the third link with a line contact (i.e., end of micrometer contacted cylindrical surface) to ensure proper micrometer measurement. Lastly, the three-bar linkage was energized with a spring (not shown for clarity) across the micrometer to eliminate play.

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Figure 4

Diagram illustrating 17 of the 29 optimization parameters for the calibration device. Figure 3 illustrates the remaining 12. Double arrows pointing in the same direction represent rotations while arrows pointing in opposite directions at a line indicate parameters (e.g., p1f) with an initial value of 0.

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Figure 5

Illustrated are the features of the calibration device measured with the CMM for the purpose of identifying calibration points. (a) is the fixed end of the device and (b) is the moving end of the device.

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