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Technical Briefs

A Multiaxis Programmable Robot for the Study of Multibody Spine Biomechanics Using a Real-Time Trajectory Path Modification Force and Displacement Control Strategy

[+] Author and Article Information
Denis J. DiAngelo

Department of Orthopedic Surgery and Biomedical Engineering,
The University of Tennessee Health Science Center,
956 Court Avenue, Suite E226,
Memphis, TN 38163

Manuscript received May 23, 2012; final manuscript received March 7, 2013; published online July 3, 2013. Assoc. Editor: William K. Durfee.

J. Med. Devices 7(3), 034502 (Jul 03, 2013) (7 pages) Paper No: MED-12-1075; doi: 10.1115/1.4024645 History: Received May 23, 2012; Revised March 07, 2013

Robotic testing offers potential advantages over conventional methods including coordinated control of multiple degrees of freedom (DOF) and enhanced fidelity that to date have not been fully utilized. Previous robotic efforts in spine biomechanics have largely been limited to pure displacement control methods and slow quasi-static hybrid control approaches incorporating only one motion segment unit (MSU). The ability to program and selectively direct single or multibody spinal end loads in real-time would represent a significant step forward in the application of robotic testing methods. The current paper describes the development of a custom programmable robotic testing system and application of a novel force control algorithm. A custom robotic testing system with a single 4 DOF serial manipulator was fabricated and assembled. Feedback via position encoders and a six-axis load sensor were established to develop, program, and evaluate control capabilities. A calibration correction scheme was employed to account for changes in load sensor orientation and determination of spinal loads. A real-time force control algorithm was implemented that employed a real-time trajectory path modification feature of the controller. Pilot tests applied 3 Nm pure bending moments to a human cadaveric C2–T1 specimen in flexion and extension to assess the ability to control spinal end loads, and to compare the resulting motion response to previously published data. Stable accurate position control was achieved to within ±2 times the encoder resolution for each axis. Stable control of spinal end body forces was maintained to within a maximum error of 6.3 N in flexion. Sagittal flexibility data recorded from rostral and caudally placed six-axis load sensors were in good agreement, indicating a pure moment loading condition. Individual MSU rotations were consistent with previously reported data from nonrobotic protocols. The force control algorithm required 5–10 path iterations before converging to programmed end body forces within a targeted tolerance. Commercially available components were integrated to create a fully programmable custom 4 DOF gantry robot. Individual actuator performance was assessed. A real-time force control algorithm based on trajectory path modification was developed and implemented. Within a reasonable number of programmed path iterations, good control of spinal end body forces and moments, as well as a motion response consistent with previous reported data, were obtained throughout a full physiologic flexion-extension range of motion in the human subaxial cervical spine.

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Figures

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Fig. 1

Spine Robot configuration. Diagram of 4-DOF robotic system developed in the current study. The manipulator was comprised of a series assembly of a vertical translational z-axis, horizontal translational x-axis (via moving carriage), and rotational pitch and roll axes orthogonally applied through a triangular biaxial gimbal joint. Six axis load sensors were mounted to the manipulator/gimbal assembly (GLS) and to the testing frame base (BLS).

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Fig. 2

Testing system and specimen mounting. (Left) Photo of assembled 4-DOF manipulator within tripod support frame. (Right) Antero-lateral view of human cadaveric cervical spine (C2–T1) mounted in the testing frame with midsagittal plane aligned with the robot x–z plane. The potted rostral end was rigidly affixed to the six-axis gimbal load sensor, and potted caudal end affixed to the base six-axis load sensor and frame. Dashed arrow schematically illustrates flexion path.

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Fig. 3

Spinal end body forces. Applied rostral–caudal (Fz) and antero–posterior (Fx) spinal forces were determined with respect to the moving gimbal load sensor reference frame. Spinal end forces were corrected for the influence of pot weight and changes in gimbal load sensor readings due to changes in orientation.

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Fig. 4

Force control algorithm. Algorithm derived to control spinal end forces by issuing a real-time trajectory path modification command every 4 ms (inner RTTPM loop) according to force control criteria. A separate software looping structure was nested inside a programmed point to point continuous motion path loop to achieve this task.

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Fig. 5

Spinal shear forces during force controlled pure moment pilot test. Programmed spinal shear force (Fx = 0 N) at the rostral specimen end (gimbal load sensor: programmed) is shown in comparison with actual shear forces applied (gimbal load sensor: applied) during flexion and extension. Ideal projected forces expected at the specimen caudal end based on programmed Fx and Fz forces (base load sensor: programmed Ideal) are also shown in comparison with shear force values measured caudally (base load sensor: measured).

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Fig. 6

Axial spinal forces during force controlled pure moment pilot test. Programmed axial spinal force (Fx = 5 N) at the rostral specimen end (gimbal load sensor: programmed) is shown in comparison with actual axial forces applied (gimbal load sensor: applied) during flexion and extension. Ideal projected forces expected at the specimen caudal end based on programmed Fx and Fz forces and a 16 N potted specimen weight (base load sensor: programmed ideal) are also shown in comparison with actual axial forces measured caudally (base load sensor: measured).

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Fig. 7

Global spinal flexibility during force controlled pure moment pilot test. Global spinal flexibility determined from applied bending moment values at the rostral end of the test specimen (gimbal load sensor: applied) is shown in comparison with flexibility determined from bending moment values recorded at the caudal specimen end (base load sensor: measured) during flexion and extension.

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Fig. 8

Percent contribution of MSU rotations to global rotation during pure moment pilot test. Individual motion segment unit percent contributions to global spinal rotation during force controlled pure moment pilot test in flexion and extension.

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