Fluid Power Research Update: Two-Degree-of-Freedom, Magnetic-Resonance-Compatible Pneumatic Stepper Module

By David Comber, Vanderbilt University, originally run in CCEFP Newsblast

CCEFP Research Project 2G: Fluid Powered Surgery and Rehabilitation via Compact Integrated Systems
Research Project Leaders: Prof. Eric Barth, Vanderbilt University. Prof. Robert Webster, Vanderbilt University, Prof. Jun Ueda, Georgia Institute of Technology, Vito Gervasi, R&D Manager, Rapid Prototyping Center, Milwaukee School of Engineering

pneumatic prototypeFluid power actuators (hydraulic and pneumatic) are well suited for electromagnetically sensitive environments like magnetic resonance imaging (MRI) machines. Such actuators would enable intra-operative MRI guidance of robotically steerable needles. Yet, a technical barrier to using fluid power in MRI-guided surgical systems is the absence of commercial, off-the-shelf fluid power actuators that are sterilizable and intrinsically safe.

Towards filling this technology gap, researchers at Vanderbilt University and Milwaukee School of Engineering have designed and built an additively manufactured pneumatic stepper actuator. Designed using corrugated diaphragm theory, one helix-shaped bellows and one toroid-shaped bellows provide pure rotation and pure translation, respectively. The entire actuator module functions as a two degree-of-freedom needle driver; that is, the two bellows directly translate and rotate the base of one tube of a steerable needle. Several of these modules can be cascaded together as a complete actuation unit for steerable needles comprising multiple, concentric tubes. For needle tip translations and rotations, mechanical stops limit the bellows’ movements to maximum unplanned step sizes of 0.5 mm and 0.5 degrees, which are acceptably safe in the event of a systems failure. Additively manufactured by laser sintering of nylon powder, the prototype device is compact and hermetically sealed for sterilizability. The linear bellows produced peak forces of 7.4 lbf and -6.0 lbf for needle insertion and retraction, respectively. The rotary bellows produced peak torques of ±0.60 lbf-in. A precision, sub-step controller allows translations and rotations less than full step increments, such that mean steady-state errors of 0.013 mm and 0.29 degrees were achieved with the prototype shown above.

For more information about other CCEFP research projects, visit the project overview page.

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