Design of a Robotic Manipulator for a

Wheelchair

2000-2001 Gateway Coalition

Ohio State University

Sinclair Community College

Wright State University

December 8, 2000

2000-2001 Team Members

Ohio State University

Prof. Gary Kinsel

Corey Johnson

Tim Kocher

Curt O’Donnell

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

Project Overview & History

• Program Background

• 1996-1997

• 1997-1998

• 1999-2000

• 2000 Graduate Design

Program Background

• National Science Foundation

• Seven Institutions

• Advancement of Engineering Education

1996-1997 Design

• Cables and Transmission Pulleys– Keeps motor on base

of robot

• Short comings– Inappropriate motors

– Large size

– High maintenance and manufacturing costs

1997-1998 Design

• Addition of knuckle joint and rotating base

• Short comings– Expensive

– Not mounted to wheelchair

– One-directional gripper activation

1999-2000 Design

• Six degrees of freedom

• Fully functional gripper

• Mounted on wheelchair

• Short comings– Heavy– Expensive– Difficult to

manufacture

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

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

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

Complete Arm Design

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

Travel Position

Full Extension

Shoulder Assembly

• Design Limitations (clearance, gear size, bearing size)

• Mounting Brackets

• Motors & Placement

• Gearing

• Bearings

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

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

1999-2000 Mounting Bracket

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

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 ~8:3 ratio

Bearings

• Twist bearing– Large bearing

• 3.5 in. O.D.

• 3 in. I.D.

– Concerned with thickness

• Ideally: ~.5 in.

• Current findings: >2 in.

– Investigating Oil Impregnated

• Bend Bearings– Mounted in the

shoulder brackets

– 1 in. O.D.

– .5 in. I.D.

– Flanged

Lower Arm Assembly

• 2.5 in. Square Aluminum Tubing

• Elbow Motor• Shaft• Bearing• Gear

1999-2000 Lower Arm Link

2.5 in. Square Aluminum Tubing

• Creates an enclosed and clean design

• Structurally strong, yet fairly light weight

• Requires minimal machining

Shoulder Shaft

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

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 8:3

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

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

Forearm Assembly Model

Differential Gear Set

Forearm Assembly Model

Preliminary Finite Element Analysis

• Mounting Brackets

• Stationary Plate

• Lower Arm Tube

• Elbow Links

FEA: Mounting Brackets

• 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

Front

Rear

100 lb

100 lb

FEA: Stationary Plate

• Constraints– Fixed at mounting bolt

holes

• Loading– 100 lb load at bearing hole

– 500 lb*in moment at bearing hole

• Maximum Stress– 8000 psi

– Factor of Safety = 6.25

100 lb

500 lb*in

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

FEA: Elbow Links

• Constraints– Fixed at elbow shaft hole

• Loading– 50 lb total load at

distributed over bolt holes

– 170 lb*in moment at forearm end

• Maximum Stress– 17808 psi

– Factor of Safety = 2.81

50 lb

170 lb*in

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

Part Description Manufacturer Product Number Unit Cost Net Cost

Front Base Mounting Bracket: 1/2" Aluminum (Machined)Back Base Mounting Bracket: 1/2" Aluminum (Machined)

Rubber Inserts For Mounting BracketStationary Plate: 7" x 5" x 1/8" (Machined)

Shoulder Brackets: 2.5" x 3.1" x 0.5" (Machined)Shoulder Spacer: 3" OD, 3/4" Thick (Machined)

Bend Motor Mounting Plate: 2" x 2" x 1/4" (Machined)Drill Motor: FireStorm 18.0 V Black & Decker HP932K-2 $149.00 $298.00

Drill Motor EncoderMotor Shaft Bearing: 1" OD, 7/16 ID, 5/16" Thick

Twist Pinion: 1" OD, 7/16" ID, 3/8" ThickTwist Motor Shaft: 1/4" x 1/2" Key, 7/16" OD, 1-5/16"

Total Length(Machined)

Twist Gear: 1/2" OD, 5/16" ThickTwist Bearing: 3-1/2" OD, 3" ID, 1/2" Thick

Bend Pinion: 3/4" Head OD, 45-Degree Bevel, 1/2" Total Length

Bend Motor Shaft: 1/4" x 1/2" Key, 7/16 OD, 2.56" Total Length

(Machined)

Part Description Manufacturer Product Number Unit Cost Net CostAluminum Tubing: 2-1/2" x 2-1/2" x 18", 1/8" Wall

ThicknessElbow Motor Mount: 2-1/4" x 2-1/4" x 1/4" (Machined)

Flanged Bearing: 1/2" PIC Design E7-S3F $25.46 $101.84Bevel Gear: 2" OD, 1/2" ID, 45-Degree Bevel

Elbow Joint Shaft: 1/2" OD, 3-1/4" Long (Machined)Shoulder Joint Shaft: 1/2" OD, 3-5/8" Long (Machined)

Drill Motor: FireStorm 18.0 V Black & Decker HP932K-2 $149.00 $149.00Drill Motor Encoder

Elbow Motor Shaft: 1/4" x 1/2" Key, 7/16 OD, 1-1/16" Total Length

(Machined)

Elbow Motor Pinion: 3/4" Head OD, 45-Degree Bevel, 1/2" Total Length

Aluminum Tubing: 2-1/2" x 2-1/2" x 12-1/2", 1/8" Wall Thickness

$47.50

Wrist Pillow Block: 5/8" ID (Machined)Elbow Link (Machined)

Wrist BearingWrist Motor Pittman GM8724S027 $150.22 $300.44

Wrist Shaft: 3/16" x 4-1/4" SDP/SI A7X1-06042 $2.05 $2.05Wrist Shaft Coupling SDP/SI A5C9-0606 $5.66 $5.66

Wrist Shaft Ball Bearing: 3/16" SDP/SI S9912Y-E1850FS2 $10.18 $10.18Wrist Differential SDP/SI S9570A-TS3 $304.93 $304.93

Wrist Gear Boston Gear GSS486Y-G $23.30 $46.60Wrist Pinion Boston Gear GSS486Y-P $19.27 $38.54