Research Papers

Optimal Design and Fabrication of Narrow-Gauge Compliant Forceps

[+] Author and Article Information
M. E. Aguirre

Department of Mechanical and Nuclear Engineering,  The Pennsylvania State University, University Park, PA 16802mea169@psu.edu

G. R. Hayes, C. L. Muhlstein

R. A. Meirom

Department of Materials Science and Engineering,  The Pennsylvania State University, University Park, PA 16802ram414@psu.edu

M. I. Frecker

Department of Mechanical and Nuclear Engineering,  The Pennsylvania State University, University Park, PA 16802mxf36@engr.psu.edu

J. H. Adair

Department of Materials Science and Engineering,  The Pennsylvania State University, University Park, PA 16802jha3@psu.edu

J. Mech. Des 133(8), 081005 (Aug 10, 2011) (10 pages) doi:10.1115/1.4004539 History: Received June 15, 2010; Revised June 24, 2011; Published August 10, 2011; Online August 10, 2011

This paper describes a multidisciplinary project focused on developing design and fabrication methods for narrow-gauge compliant mechanisms expected to be useful in advanced minimally invasive surgery. In this paper, three aspects of the project are discussed: meso-scale fabrication, compliant mechanism design, and experimental determination of mechanical properties and forceps performance. The selected manufacturing method is a lost mold rapid infiltration forming process that is being developed at Penn State University. The process is capable of producing hundreds of freestanding metallic and ceramic parts with feature sizes ranging from sub-10 μm to approximately 300 μm. To fulfill surgical and manufacturing requirements, a contact-aided compliant mechanism design is proposed. A finite element analysis solution, used to evaluate large deformation and contact, is implemented into an optimization routine to maximize tool performance. A case study demonstrates the design and manufacturing processes for a 1 mm diameter austenitic (300 series) stainless steel forceps. Due to manufacturing variables that affect grain size and particle adhesion, the strength of the fabricated parts are expected to vary from the bulk material properties. Therefore, fabricated parts are experimentally tested to determine accurate material properties. Three point bend tests reveal yield strengths between 603 and 677 MPa. Results from the design optimization routine show that material strengths within this range require large instrument aspect ratios between 40 and 50 with anticipated blocked forces as high as 1.5 N. An initial prototype is assembled and tested to compare experimental and theoretical tool performance. Good agreement between the computational and experimental data confirms the efficacy of the processes used to develop a meso-scale contact-aided compliant forceps.

Copyright © 2011 by American Society of Mechanical Engineers
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Figure 1

Small aspect ratio compliant grippers with intricate designs [3,20]

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Figure 2

Assembled meso-scale forceps prototype for NOTES (Left Inset): Close up of instrument tool tip (Right Inset): Grain structure

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Figure 3

Process flow chart

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Figure 4

Scanning electron micrographs of (Left): dried as received 316L stainless steel powder; (Right) dried suspension of the 316L stainless steel slurry at 60 vol. % in ethanol with oleic acid as the dispersant at 5 wt. %

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Figure 5

(a) Ultra thick photoresist deposition method is shown. A known volume of photoresist is deposited at elevated temperature and allowed to self level, (b) the lithography stack used to fabricate molds is shown, and (c) A cross section of a fabricated (2D) mold is shown with nearly vertical side walls and a thickness of 0.1 mm

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Figure 6

Top (from left) SU-8 molds are infiltrated with the metal slurry leaving a thin layer on top of the molds. The thin overburden layer is removed and parts are allowed to dry. Bottom (from left) the molded parts are subjected to two furnace runs, an initial treatment in air removes dispersant and mold material, and then sintering takes place in a reducing atmosphere. Final parts are left freestanding on the original substrate.

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Figure 7

Forceps topology and actuation principle

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Figure 8

Forceps cross-section circumscribed in a 1 mm diameter circle

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Figure 9

Illustration of contact stress-relief

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Figure 10

Parametric forceps model and boundary conditions

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Figure 11

Free deflection (top) and blocked force (bottom) conditions

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Figure 12

Free deflection and blocked force evaluation procedure

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Figure 13

Force convergence history, stress constraint case σy_11  = 2.2 GPa

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Figure 14

Length parameter convergence history, stress constraint case σy_11  = 2.2 GPa

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Figure 15

Influence of material part strength on AR and blocked force

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Figure 16

Optimal designs for stress constraint cases 5, 6, and 11 (shown top to bottom)

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Figure 17

Normalized residual change in forceps geometry

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Figure 18

Normalized tip deflection history

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Figure 19

Normalized tip deflection considering residual changes in geometry

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Figure 20

A series of images from the pull-off force measurement test in which the forceps (a) were made to grip a latex tube, (b) the tube was pulled away from the forceps reaching, (c) a maximum force of 1.2 N and eventually, and (d) the latex overcomes friction and slips out of the forceps jaws

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Figure 21

The latex tube was then pulled away from the forceps [Fig. 2], reaching a maximum force of about 1.2 N [Fig. 2] before overcoming friction and slipping from the forceps jaws [Fig. 2]




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