Tim Holsgrove

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The Upgrade of a Dynamic Six-Axis Spine Simulator
The previously developed six-axis spine simulator was upgraded through support from the Higher Education Investment Fund (HEIF), the Enid Linder Foundation, the Universtiy of Bath Alumni Fund, and University of Bath Department of Mechanical Engineering.

The two hydraulic axes previously present were replaced with electro-mechanical systems to create a standalone electro-mechanical, six-axis testing machine. Accuracy at higher testing speeds were achieved through upgrades to the hardware and software of the control system, which was also adapted to provide either position or load control in all six axes.

The control system and positional accuracy has been upgraded with the use of an incremental encoder interface board, and a new dedicated quad-core processor for real-time testing. The system uses digital encoder signals to drive the motors through a timing and digital I/O board. The six-axes load cell data is acquired through an A/D board.

The rotations are applied through zero-backlash Harmonic Drive Gear assemblies. Shear translations are applied from motors through zero-backlash couplings to ball screw assemblies with built-in preload to eliminate backlash. The axial translation axis uses a zero-backlash Harmonic Drive Gear and motor assembly connected to a preloaded ball screw via a zero-backlash coupling.
Axis Description ROM Speed Resolution Load
TX A-P shear +/-90 mm +/-20 mm/s 0.001 mm +/-500 N
TY Lateral shear +/-90 mm +/-20 mm/s 0.001 mm +/-500 N
TZ Compression-tension +/-90 mm +/-10 mm/s 0.013 um +/-4000 N
RX Lateral bending +/-45 deg +/-45 deg/s 0.002 deg +/-50 Nm
RY Flexion-extension +/-45 deg +/-45 deg/s 0.002 deg +/-50 Nm
RZ Axial rotation +/-45 deg +/-45 deg/s 0.002 deg +/-50 Nm
An initial study demonstrating the capability of the system to minimise off-axis loading using load control to simulate phsyiologic loading has been completed, and further studies investigating complex spinal loading are underway. The simulator has the capacity to replicate complex multi-axis loading, which can be used both to understand the natural spine, investigate spinal injuries and degeneration, or to assess the efficacy of spinal devices and instrumentation.

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