Kinematic Mount Design of Line Replaceable Units at the National Ignition Facility

Kinematic Mount Design of Kinematic Mount Design of Line Replaceable UnitsLine Replaceable Unitsat the National Ignition at the National Ignition

FacilityFacilityME 250 - Precision Machine Design - Dr. FurmanME 250 - Precision Machine Design - Dr. Furman

 April 8, 2003 April 8, 2003

Dennis PakDennis PakBehrouz SadrabadiBehrouz Sadrabadi

National Ignition FacilityLocated at Lawrence Livermore National Lab$3.9 Billion DOE Defense Programs project192 laser beams will all converge onto a BB-sized capsulePower output 1000 times the electric generating power of the U.S.

NIF MissionsPrimary mission: support the Stockpile Stewardship Program Maintain safety and reliability of U.S.’s nuclear arsenal 85% of experiments: nuclear weapons physics Will provide experimental data necessary to complete

computer simulations

Inertial fusion energy (IFE) Determine feasibility of IFE as an energy source

Basic scientific research Experiments in high-energy-density physics for

astrophysics, hydrodynamics, plasma physics, material properties, radiation physics

NIF Laser System - LRUsEach beamline contains 40 large optical elementsLight path must have clean room atmosphere - beamlines are sealedOptics needed to be removable for repair and cleaningLine Replaceable Unit (LRU) concept was thus adopted - can be installed and removed with a semi-automated robotic unit

Line Replaceable Units (LRUs)

Kinematic mount is essentially a three-vee couplingTwo vee grooves at bottom of LRU resting on retractable pins on laser structureUpper mount consists of two pin-slot constraints, effectively creating a wide vee

Kinematic Coupling Evolution

The LRU kinematic coupling evolved from the basic three-vee couplingCoupling was rotated to better accommodate tall LRU geometryLower vees were rotated to carry gravity loadUpper vee was spread wide to decrease rotational inertia and increase torsional stiffness

Mount Points in Vertical Plane

Placing mount points into vertical plane provides a more favorable aspect ratio.Provides for smallest footprint - dense packing of LRUs in beamline

Wide Upper Vee - Inertia

Upper vee spread with pin-slot constraintsInstant center of rotation is thus brought closer to the principal axis of LRURotational inertia reduced, vibrational frequency increased

Wide Upper Vee - Stiffness

Primary motivation for wide vee is to maximize torsional stiffness.Torsional stiffness of pin-slot constraint is an order of magnitude greater than LRU structureMaximum potential static twist due to friction remains well below the maximum allowable

Modeling of CouplingKinematic mount modeled as parallel combination of 6 springsLoad vector: 3 forces, 3 momentsDeflection vector: 3 displacements, 3 rotations6 X 6 stiffness matrix for a mount point determined in local coordinate systemStiffness matrices transformed into global coordinates, combined by addition

Optimization:Variable Parameters

Locations of mounts already established by overall geometry of LRUThe remaining geometric attributes were set as variable parameters subject to optimization Lower mount pin angle Lower mount outside vee angle Lower mount inside vee angle Upper mount slot angle

O u tsid e V eeA ngle

In sid e V eeA ngle

Optimization: Centering Ability

LRUs optimized for maximum limiting coefficient of friction -> maximum centering abilityAs LRU contacts engage, minimum limiting C.O.F. occurs when 5 contacts are engagedLimiting C.O.F. for a given set of variable parameters taken to be minimum of six valuesAlthough optimization algorithm could have been used, graphical approach was taken instead

SummaryLRUs needed to be easily installed and removed with a high degree of repeatability by a robotic systemBasic configuration of three-vee coupling was determined by overall geometry of LRUThe remaining, unspecified parameters were determined by optimizing for maximum centering abilityIn general, the performance of a kinematic mount can be maximized through analysis and optimization