Design of a Robotic Manipulator for a Wheelchair

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Design of a Robotic Manipulator for a Wheelchair
December 8, 2000

• 2000-2001 Gateway Coalition
• Ohio State University
• Sinclair Community College
• Wright State University

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2000-2001 Team Members

• Ohio State University
• Prof. Gary Kinsel
• Corey Johnson
• Tim Kocher
• Curt ODonnell
• Michael Stevens
• Aaron Weaver
• Jeff Webb

Sinclair Community College Prof. Beth
Johnson Brad Cutting Chris Shirkey Tim Tarp
Wright State University Prof. James Menart Shawn
Riley Jason Ruge Lawrence Thomas Eric Yu
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Project Overview History

• Program Background
• 1996-1997
• 1997-1998
• 1999-2000
• 2000 Graduate Design

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Program Background

• National Science Foundation
• Seven Institutions
• Advancement of Engineering Education

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1996-1997 Design

• Cables and Transmission Pulleys
• Keeps motor on base of robot
• Short comings
• Inappropriate motors
• Large size
• High maintenance and manufacturing costs

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1997-1998 Design

• Addition of knuckle joint and rotating base
• Short comings
• Expensive
• Not mounted to wheelchair
• One-directional gripper activation

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1999-2000 Design

• Six degrees of freedom
• Fully functional gripper
• Mounted on wheelchair
• Short comings
• Heavy
• Expensive
• Difficult to manufacture

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2000 Graduate Design

• Chris Fearson
• Ohio State University Graduate Student
• Totally enclosed design
• A little lighter
• Short comings
• Very expernsive
• Hard to mount
• Two-piece clamps

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2000-2001 Design Objectives

• 1.5 kg ( 3 lb) Lift Capacity
• 0.5 m/s Maximum linkage Movement Speed
• Total Assembly Weight Less Than 30 lbs
• Total Manufacturing Cost

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Preliminary Design Calculations

• Estimated Moment Calculations
• Initial calculations
• Shoulder moment 500 in-lbs
• Elbow moment 125 in-lbs
• Updated calculations
• Shoulder moment 473.8 in-lbs
• Elbow moment 169.8 in-lbs
• Application factor 1.5
• Weight to lift (load) 3.3 lbs

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Complete Arm Design
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Design Characteristics

• Freedom of motion
• Shoulder joint
• 360 of twist
• Up to 200 of bend
• Elbow joint
• 280 of bend
• Wrist
• 360 of twist
• 300 of bend
• Both shoulder motors in the base
• Reduces the weight of the lower arm

• Length
• Lower arm 15.5 in.
• Forearm 13.65 in.
• Full extension 35.5 in.
• Width
• Extends 3.25 in. beyond wheelchair width.
• Wheelchair width with arm 27.75 in.
• Typical Door Width 34 in.
• Compact travel position
• 5 in. tall
• 15.5 in. long

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Travel Position
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Full Extension
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Shoulder Assembly

• Design Limitations (clearance, gear size, bearing
  size)
• Mounting Brackets
• Motors Placement
• Gearing
• Bearings

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Design Limitations

• Maximum distance from the side of the chair must
  be less than 6 inches
• Limits the diameter of twist gear and width of
  base plate
• Also limits the size of the twist bearing
• Spacing of the mounting brackets governed by the
  current design of the wheelchair frame

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Mounting Brackets

• Single-piece design
• Rubber lining to protect the finish of the
  wheelchair
• Close tolerances make installation easy
• Simple clamping technique one person can secure
  entire arm to wheelchair

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Motors and Placement

• Twist and bend motors placed on the base
• Design reduces weight in the lower arm
• Bend motor mounted to the twist gear
• Rotates with the arm

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Gearing

• Twist Gearing
• Simple spur gear design
• Gear size cannot be reduced due to shoulder
  bracket position
• Current large pinion gear is due to mounting and
  placement limitations
• Future investigation into use of a idler gear
• Bend Gearing
• Bevel gear design
• Currently 83 ratio

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Bearings

• Twist bearing
• Large bearing
• 3.5 in. O.D.
• 3 in. I.D.
• Concerned with thickness
• Ideally .5 in.
• Current findings gt2 in.
• Investigating Oil Impregnated

• Bend Bearings
• Mounted in the shoulder brackets
• 1 in. O.D.
• .5 in. I.D.
• Flanged

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Lower Arm Assembly

• 2.5 in. Square Aluminum Tubing
• Elbow Motor
• Shaft
• Bearing
• Gear

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2.5 in. Square Aluminum Tubing

• Creates an enclosed and clean design
• Structurally strong, yet fairly light weight
• Requires minimal machining

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Shoulder Shaft

• Diameter .5 in.
• Length 3.625 in.
• Snap-ring attachment

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Bearing and Gearing

• Bearings at elbow side of the lower arm to allow
  free rotation
• Mounted in the square tubing
• Same bearing used in the shoulder for the bend
  motion
• Bevel gears used to move the elbow joint
• Currently the ratio is approximately 83

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Elbow Joint

• Bracket Design
• Elbow Bracket
• Bent 1/8 in Stock Aluminum Plate
• Snap-ring attachment
• Bolted to the forearm
• Degree of Movement
• Design allows for 280 of motion at the elbow

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Forearm Assembly

• Same assembly used in previous design
• Last modified by Chris Fearson
• Components
• 2.5 in. square tubing
• Both wrist motors completely enclosed by tubing
• Mounting for the differential gearing at the wrist

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Forearm Assembly Model
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Differential Gear Set
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Forearm Assembly Model
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Preliminary Finite Element Analysis

• Mounting Brackets
• Stationary Plate
• Lower Arm Tube
• Elbow Links

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FEA Mounting Brackets
100 lb
Front

• Constraints
• Front and top inner surfaces fixed
• Rear bolt hole fixed
• Loading
• 100 lb load applied to top surface of bracket
• Maximum Stress
• Front 272 psi
• Factor of Safety 184
• Rear 338 psi
• Factor of Safety 148

Rear
100 lb
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FEA Stationary Plate

• Constraints
• Fixed at mounting bolt holes
• Loading
• 100 lb load at bearing hole
• 500 lbin moment at bearing hole
• Maximum Stress
• 8000 psi
• Factor of Safety 6.25

100 lb
500 lbin
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FEA Lower Arm Tube

• Constraints
• Fixed at shoulder shaft hole
• Loading
• 100 lb at elbow shaft hole
• 100 lb side load at end (to simulate side impact)
• Maximum Stress
• 36057 psi
• Factor of Safety 1.39

100 lb
100 lb
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FEA Elbow Links

• Constraints
• Fixed at elbow shaft hole
• Loading
• 50 lb total load at distributed over bolt holes
• 170 lbin moment at forearm end
• Maximum Stress
• 17808 psi
• Factor of Safety 2.81

50 lb
170 lbin
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Preliminary Bill of Materials

• Not a complete listing
• No machining costs or estimates
• Twist shoulder bearing still under investigation
• Gears can not be found until motor data is
  complete

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