Overview of the 3 DOF Parallel-Architecture Wrist Mechanism

Stephen Canfield, ProfessorDepartment of Mechanical EngineeringTennessee Technological UniversityNovember 10, 2008

Overview

About Parallel Mechanisms

Multiple load-bearing chains b/n output and base

Kinematics Advantages

Stronger, lighter Disadvantages

Reduced range of motion, kinematics

About the PAW

3-5R Spatial Parallel-architecture mechanism

3dof Pitch, yaw, plunge

Characteristics of the PAW High payload-to-weight, good dynamic

performance Ground-relative actuation All-revolute design Closed-form forward/inverse kinematics

exist Large range of motion in orientation Singularity free workspace (interior),greater

than hemisphere Removes constraint on control algorithms Improved dexterity

Advantages of proposed design

Parallel-architecture: High payload, accuracy, good dynamic performance in light-weight system

Large, singularity-free range of motion Eliminate need for slip-rings Provide accuracy needed for solar concentrators Inherently redundant system

- Application for pointing solar arrays – Video

Gimbal Kinematics, Options

Past applications

Robotic end-effector, compliant design for pointing mirrors, sub 10nm precision manipulator

Point Design Applications: Thruster

Part Layout

Thruster as built

Application for pointing thruster

Design Point: Solar-Array Mount

Solary Array Mount

Solar Array Mount

Table I: Summary of 3D solar-array tracking mechanism

ComponentDescription Mass/ Workspace

Motor Drive: Maxon EC90 Flat Motor 1.5 lb

Drive reduction Harmonic Drive HDC 1M 2.6 lb

Bearings Bronze bushings, ¾, 3/8 ID

Shaft 440c SS

Link components 7075 Al

Mechanism weight * (without actuation) 14.75 lb

3D Mechanism weight (with actuation, includes a 20% factor)

32.5 lb

Working Volume* 5.5 inch radius cylinderHeight is selectable

5.5 inch cylinder

Stowed Volume* 3 high x 7 radius cylinder (inches)

7x3 inch cylinder

Miniature Gimbal:

Compliant mechanism applications

Conventional 3 dof manipulator (pointing task)

Compliant 3 dof manipulator

High precision compact design

From known topology

Compact, 6 dof compliant manipulator

Kinematics

Force Analysis

Closed-form kinetostatic analysis

0 50 100 150 200 250 300 350 4000

20

40

60

80

100

120

140

160

180motor torque profile of rotary move

rotation (deg)

mot

or t

orqu

es

Beta=60 degrees

0 50 100 150 200 250 300 350 400-100

-50

0

50

100

150

200

250motor torque profile of rotary move

rotation (deg)

mot

or t

orqu

es

Beta=30 degrees

Path planning and trajectory control

Embedded electronics

Currently Motorola S-12 family MCU IK ~ 12k in HCS-12 CPU assembly

(unoptimized) 8 channel PWM, Quadrature encoders,

drivers

Summary