Abstract
Six degree-of-freedom (6DOF) robots can be used to examine joints and their mechanical properties with the spatial freedom encountered physiologically. Parallel robots are capable of 6DOF motion under large payloads making them ideal for joint testing. This study developed and assessed novel methods for spinal joint testing with a custom-built parallel robot implementing hybrid load-position control. We hypothesized these methods would allow multi-dimensional control of joint loading scenarios, resulting in physiological joint motions. Tests were performed in 3DOF and 6DOF. 3DOF methods controlled the forces and the principal moment within ±10 N and 0.25 N m under combined bending and compressive loads. 6DOF tests required larger tolerances for convergence due to machine compliance, however expected motion patterns were still observed. The unique mechanism and control approaches show promise for enabling complex three-dimensional loading patterns for in vitro joint biomechanics, and could facilitate research using specimens with unknown, changing, or nonlinear load-deformation properties.
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Abbreviations
- Letter superscripts represent the coordinate system that a variable is expressed in. No superscript denotes that the variable is relative to the global coordinate system.:
-
- e:
-
superscript, end-effector coordinate system
- d:
-
superscript, disc (joint) coordinate system
- T:
-
superscript, matrix transpose
- Δ:
-
change in the value of a variable
- ⊗:
-
tensor-product
- ℓ:
-
machine leg length
- b :
-
3 × 1 base plate joint center position
- e e :
-
3 × 1 end-effector joint center position
- R :
-
3 × 3 rotation matrix
- t :
-
3 × 1 translation vector
- J −1 :
-
6 × 6 inverse kinematic Jacobian matrix
- X :
-
6 × 1 end-effector pose matrix (3 translations, 3 rotations)
- L :
-
6 × 1 inverse kinematics calculated leg lengths (ℓ1, ℓ2,...,ℓ6)
- L′:
-
6 × 1 potentiometer measured leg lengths
- F d :
-
6 × 1 joint load matrix (3 forces, 3 moments)
- P d :
-
6 × 1 joint pose matrix
- C d :
-
6 × 6 joint compliance (inverse stiffness) matrix
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Acknowledgments
This study was supported by a grant from the Natural Sciences and Engineering Research Council of Canada. The authors thank Dr. Jean-Pierre Merlet for discussions on parallel robot kinematics and control, and Dr. Jack Callaghan and Kevin Gillespie for helpful advice throughout this project.
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All research accomplished at: Department of Human Health and Nutritional Sciences, University of Guelph, Guelph ON, Canada.
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Walker, M.R., Dickey, J.P. New methodology for multi-dimensional spinal joint testing with a parallel robot. Med Bio Eng Comput 45, 297–304 (2007). https://doi.org/10.1007/s11517-006-0158-6
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DOI: https://doi.org/10.1007/s11517-006-0158-6