engineering design process, prototyping, & …robo2008/lectures/designprocesslinkages.pdf4 grasp...

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Nora Ayanian

University of Pennsylvania

Engineering Design Process, Prototyping, & Linkages/Mechanisms

SAAST Robotics 2008

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Administrative Information

Contact info: Nora Ayanian

[email protected]

Class website: www.seas.upenn.edu/~robo2008

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Engineering Design Process

SAAST Robotics 2008

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The Accreditation Board for Engineering and Technology defines the engineering design as:

… the process of devising a system, component or process to

meet desired needs. It is a decision-making process (often

iterative), in which the basic sciences, mathematics, and engineering sciences are applied to convert resources

optimally to meet a stated objective. Among the fundamental elements of the design process are the

establishment of objectives and criteria, synthesis, analysis, construction, testing, and evaluation.

[Wikipedia]

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Identify the

Problem Research the

Problem

Develop

Possible

Solutions

Choose Best

SolutionConstruct

Prototype

Test and

Evaluate

Solution

Communicate

and Document

Solution

Redesign

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Identify the Problem

More than establishing the need, DEFINING the need

Collect data, run experiments, perform computations

Formulate the problem in clear, unambiguous terms

Do not assume anything about the solution

Establish criteria for success

Always include specifications a design solution must meet

May be modified later

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What problem are you trying to solve? � Problem Statement

Design a better mousetrap

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What problem are you trying to solve? � Problem Statement

What tasks or functions are needed to accomplish this?

• The design must be low cost.

• The design should be safe, particularly with small children.

• The design should not be detrimental to the environment.

• The design should be aesthetically pleasing.

• The design should be simple to operate, with minimum human effort.

• The design must be disposable (you don’t reuse the trap).

• The design should not cause undue pain and suffering for the mouse.

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Identify the


Problem

Develop

Possible

Solutions

Choose Best

SolutionConstruct

Prototype

Test and

Evaluate

Solution

Communicate

and Document

Solution

Redesign

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Research the Problem

Collect all information pertinent to the problem:

• Is the problem real and its statement accurate?

• Is there really a need for a new solution or has the problem already been solved?

• What are the existing solutions to the problem?

• What is wrong with the way the problem is currently being solved?

• What is right about the way the problem is currently being solved?

• What companies manufacture the existing solution to the problem?

• What are the economic factors governing the solution?

• How much will people pay for a solution to the problem?

• What other factors are important to the problem solution?

• Safety, Aesthetics, Environmental Issues, etc.

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Identify the


Problem

Develop

Possible

Solutions

Choose Best

SolutionConstruct

Prototype

Test and

Evaluate

Solution

Communicate

and Document

Solution

Redesign

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Develop Possible Solutions

Brainstorm

Be creative – think outside the box

Multiple solutions to the same problem

DO NOT RULE OUT ANY SOLUTIONS!!!

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Identify the


Problem

Develop

Possible

Solutions

Choose Best

SolutionConstruct

Prototype

Test and

Evaluate

Solution

Communicate

and Document

Solution

Redesign

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Choose Best Solution

Analyze design solutions based on the following factors:

• Functional analysis – will it function the way it should?

• Ergonomics/ease of use – how easy/hard is it for humans to interact with design?

• Safety – is this a safe design?

• Mechanical/Strength Analysis – will the mechanical

components hold up during operation?

• Electrical System Analysis – how about the electrical components?

• Manufacturability/Testability – Can the design be made and tested with resources at hand?

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Identify the


Problem

Develop

Possible

Solutions

Choose Best

SolutionConstruct

Prototype

Test and

Evaluate

Solution

Communicate

and Document

Solution

Redesign

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Construct Prototype• Prototypes may not be fully tested or may not work or

operate as intended

• Purpose: Test the design under solution under real conditions

Test and Evaluate SolutionDesign tests to tell you the following:

• What works?

• What doesn’t work?

• What can be fixed?

• What has to be redesigned?

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Identify the


Problem

Develop

Possible

Solutions

Choose Best

SolutionConstruct

Prototype

Test and

Evaluate

Solution

Communicate

and Document

Solution

Redesign

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Communicate/Document Solution Performance

Record:

• Details of design

• Manufacturing methods

• Testing results

Redesign

Design is an iterative process!

Redesign solution based on results

In most cases you will have to redesign

[Seyyed Khandani, Education Transfer Plan – Design Process, IISME, Solectron, 2005]

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Prototyping

SAAST Robotics 2008

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Prototyping: Construction

Adhesives

• Permanent

• Especially good for small or light objects

Threaded fasteners

• Wood screws

• Machine screws

• Self-tapping screws

• Dry-wall screws

• Set screws

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Prototyping: Adhesive Types

Non-filling

• Solvents

• Acrylic cement

• Glues

• Cyanoacrylate “Superglue”

• Elmer’s glue

• Rubber Cement

Filling

• Hot melt glue (hot glue)

• Epoxy (if mix is optimal can be very strong)

• Gelled C-A “Zap-a-gap”

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Prototyping: Adhesive Loading

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Prototyping: Threaded Fasteners �� Machine Screws

Heads:Pan

Countersunk

ButtonScrew drives:

Slot headPhillips

Hex screw head

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Prototyping: Threaded Fasteners �� Machine Screws

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Prototyping: Standard Screw Sizes

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Prototyping: Standard Screw Sizes

Most common

2-564-40

6-32¼ - 20

Only use even numbers!

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Prototyping: Foamcore Mock-ups

Cutting

• Use a straight edge to make cuts

• Use a sharp blade

• Make two passes, the first half way as a guide, the second all the way through.

Glue with hot glue along the edges

• Be careful, the glue can be very hot, and can melt the foam

Dull

Blade

Sharp

Blade

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Clean corners hide the foam:

1. Cut a groove the width of the edge (careful not to cut all the way through)

2. Remove foam, use file if necessary

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3. Glue and hold

� Reinforce, if desired

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Non-regular angles

Curves

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Prototyping: Foamcore Mock-ups �� Paper Flexure joints

Pros

• Quick and easy to prototype

• Works well with small, light forces

Cons

• Not precise (slop in system)

• Short life (fatigue failure)

• Joint limits

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Prototyping: Laser Cutting

• Cuts planar (2D parts) from CAD file

• Plotter with laser instead of pen

• Used mostly with soft materials such as plastics (plexi-glass, acrylic) and wood

• Does not cut metal

• Hints for use:

• Don’t assume thickness of stock material – measure it

• 0.25” � 0.21”

• Don’t assume laser will cut slot of exact thickness. Laser width is about 0.007”

• Laser diameter depends on focus. Cuts from laser will not be perfectly vertical but close to it. Depends on focal length of lens.

• Bond parts together with acrylic cement in joints

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Mechanisms/Linkages

SAAST Robotics 2008

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Mechanisms/Linkages: Degrees of Freedom (DOF)

The number of independent movements of a rigid body or system

How many DOF for an object on a plane?

How many DOF for an object in space?

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Mechanisms/Linkages: Joints �� Kinematic constraints

Rigid bodies can be coupled through kinematic constraints, reducing the DOF of the system.

Two types: Revolute, Prismatic/Slider

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Mechanisms/Linkages: Many Possible Joint Pairs

RPRP

RR

PP

RRRRRP

RPP

PPP

RRRP

RRPP

RPPP

RRRPP

RRPPP

Hinge

Slider/PrismaticCylinder

Universal Joint

XY stage (2 sliders)

Spherical joint

Universal w/slider

Planar constraint

XYZ stage (3 sliders)

Spherical w/slider

Universal joint on XY stage

Hinge on xyz stage

Spherical joint on XY stage

Universal on XYZ stage

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Linkages: Systems of rigid bodies (links) with joints

• Most linkages are planar

• Large variety of different linkages (entire classes devoted to linkage analysis and synthesis)

• Assembly of n planar links � 3n DOF before joined to form mechanism

• Connecting links with joints reduce system DOF

• Pin (revolute) joints and slider joints allow for 1 DOF of relative motion for links they join

Gruebler’s equation for planar mechanisms

# DOF = 3(n-1) -2(j1 + j2)

n = # of links

j1 = # of pin (revolute) joints

j2 = # of slider joints

Mechanisms/Linkages

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# DOF = 3(n-1) -2(j1 + j2)

n = # of links



Mechanisms/Linkages: Gruebler’s equation Ex.

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Mechanisms/Linkages: Gruebler’s equation Ex.


# DOF = 3(n-1) -2(j1 + j2)

n = # of links

j1 = # of revolute joints


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Mechanisms/Linkages: Homework Problem


# DOF = 3(n-1) -2(j1 + j2)

n = # of links



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Mechanisms/Linkages: Four Bar Linkages

• 4 bar linkages used to convert:

• Continuous rotation into continuous rotation

• Continuous rotation into oscillation or reciprocation

• Oscillation into oscillation, reciprocation into reciprocation

• Number of links = n = 4

• Ground is 4th bar

• Coupler is opposite ground

• Driver

• Followerground

coupler

ground

driverfollower

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s = shortest link; l = longest link; p, q = intermediate lengths

Grashof's theorem:

A four-bar mechanism has at least one revolving link if s + l <= p + q.

All three mobile links will rock if s + l > p + q.

All four-bar mechanisms fall into one of these four categories:

Case l + s vers. p + q Shortest Bar Type

1 < Frame Double-crank

2 < Side Crank-rocker

3 < Coupler Double-rocker

4 = Any Change Point

5 > Any Double-rocker

1) When the shortest link is a side link, the mechanism is a crank-rocker mechanism. The shortest link is the crank in the mechanism.

2) When the shortest link is the frame of the mechanism, the mechanism is a double-crank mechanism.

3) When the shortest link is the coupler link, the mechanism is a double-rocker mechanism.

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When the shortest link is…

• a side link, the mechanism is

a crank-rocker mechanism.

• the frame of the mechanism,

the mechanism is a drag-link

mechanism.

• the coupler, the mechanism

is a double-rocker

mechanism.

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Kinematic Inversions of

crank-rocker mechanismParallelogram Linkage

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Adjusting

Screw

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Mechanisms/Linkages: Examples

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Geneva Mechanism

- Intermittent motion mechanism

Scooping Mechanism

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Limit Mechanism

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Different Mechanisms – Similar Tasks

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