Tolerance Analysis of Assemblies Using Kinematically-Derived Sensitivities Paul Faerber Motorola Corporation – Lawrenceville, GA Presented by: Jeff Dabling BYU – Graduate Student

Differences (Kinematic) Dimensions not allowed to vary Requires sensitivity to velocity inputs Multiple Position Analysis Similarities Both use vector loops Both use kinematic joints Both use sensitivities Differences (Tolerance) Dimensions allowed to vary Requires sensitivity to dimensional variation Single position analysis Tolerance Analysis Kinematic Analysis

Research Objectives Model static assemblies with a kinematic modeler Extract tolerance sensitivities from a kinematic solver Perform tolerance analysis on a mechanism in multiple positions Assess difficulty of adding tolerance analysis to commercial kinematic software

Tolerance Analysis of Mechanisms Tolerance models for each position of interest Current Method: Solid Model New Position Solid Model New Position Solid Model New Position Original Solid Model

Merging Kinematic and Tolerance Analyses Multiple Positions Tolerance models for each position Sensitivities Kinematic/ Dynamic Model

Kinematic Analysis bar mechanism Vector loop equation Resulting velocity equations Matrix formulation rererere iiii ()()()()     rrr rrr sin cos   A r r        sin cos   Kinematic Solution Outputs Input

Tolerance Analysis 4-bar mechanism Vector loop equations Linearized equations

Tolerance Analysis, continued 4-bar mechanismMatrix Formulation Tolerance Analysis Solution (non-statistical)  A d dr B d d                               A rrr rrr            sin cos sin      B rrr rrr            cos   dr 4           d3d3 d4d4 BA d2d2 dr 1 dr 2 dr 3 S i,j d2d2 dr 1 dr 2 dr 3 1                                   Outputs Inputs

Estimated Tolerance Accumulation 4-bar mechanism RSS

Observations Kinematic Analysis Solution Tolerance analysis solution (non-statistical)

Kinematic Analysis of an Equivalent Variational Mechanism A r r r r B                                B rr rr         sin cos                                                  BA r r r r J r r r r ij        , Kinematic analysis solution Matrix formulation Resulting velocity equations Vector loop equation (Dimension r i not constant)

Comparisons Tolerance analysis solution (non-statistical) Kinematic analysis of equivalent mechanism solution Are the sensitivities the same?

Transformation to Relative Angles Relative rotationsDifferential rotationsPsuedo-velocities Kinematic analysis solution Tolerance analysis solution (after substitution)  d dt J d J dr dt J dr dt J dr dt J dr dt   (),,,,, d JJ d JJ dr dt JJ dr dt JJ dr dt JJ dr dt  ()()()()(),,,,,,,,,,

Uses kinematic elements to represent dimensional variations in a kinematic model of the assembly r i are kinematic inputs r i are proportional to dimensional tolerances Equivalent Variational Mechanisms

Vector loops identical for both types of analyses Use tolerance analysis techniques to develop vector loops for assemblies Use these vector loops as a starting point in developing EVM Equivalent Variational Mechanisms

The Stack Blocks Assembly Frame Block Cylinder Gap p b c 2 a r d n e

Creating Vector Loop Assembly Models Kinematic JointsNetwork Graph Part and Feature Reference Frames

Dimensional Variations Each vector represents a link in the EVM Angular Variations Linear Variations Stacked Blocks Model Dimensional Variations

Equivalent Kinematic Joints Stacked Blocks Model Dimensional Variations Kinematic Variations

Stacked Blocks Model (completed) Dimensional Variations Kinematic Variations Fixed Joints Pin Joints EVM Stack Blocks Assembly

EVM Modeling Techniques Extracting Sensitivities from Kinematic Solver Unit velocities are applied to each independent joint, one at a time. Resulting dependent variables represent the row of the tolerance sensitivity matrix corresponding to that joint. Independent variables: r 2, r 3, r 4,  2 Dependent variables:  2,  3,  4 r 2 = 1 r 3 = 0 r 4 22 22 33 44

Variation Results for Stack Blocks Gap Sensitivities

Tolerance Analysis Using Equivalent Variational Mechanisms Variations in link lengths are allowed by including slider elements in each link. Sensitivities are used to form RSS expressions used in statistical tolerance analysis. Kinematic modeler moves to the next location, and the process is repeated. Dependent variables:  2,  3,  4 Independent variables: r 2, r 3, r 4,  2

Modeling Techniques for Mechanisms Apply independent velocities as a reciprocating time function Each new time step (second) is a new analysis point at the nominal link-lengths Controlling the period of the function controls the resolution of the analysis, which affects the accuracy of the analysis Cosine Function Full magnitude at time = 0 Period equal to the time step

Variation Results for Four-bar Mechanism Sensitivities vary with position (r 1,  1 can also vary) Greatest sensitivity at 270º

Contributions Defined relationship between kinematic and tolerance analyses Developed method for creating and analyzing equivalent variational mechanisms (EVM) Equivalent 2-D kinematic joints presented Demonstration of method on static assemblies, as well as kinematic Demonstrated using commercial kinematic software, ADAMS Method for extracting kinematic sensitivities Method for returning the model to its nominal dimensions at each time step

Recommendations for Future Research Investigate relationship between the higher kinematic derivatives (acceleration and jerk) and the higher statistical moments (skewness and kurtosis) Integrate with commercial kinematics CAD applications – Develop a user interface – Study degree of freedom problems Extend into three-dimensional assemblies Include form tolerances in this method