Transcript BEST_Engineering_Wok..

Collin County Boosting Engineering Science and
Technology
Workshop
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AGENDA
•Introduction
•Lessons Learned
•Design Process
•Engineering Mechanics
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Time
Save time:
1. Complete any known tasks prior to kickoff.
2. Organize all tools, parts, and supplies.
3. Establish a secure work area.
4. Establish and enforce chain of command to prevent unnecessary
rework.
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Strategy
STRATEGY IS AS IMPORTANT AS A FUNCTIONAL
MACHINE:
1. Complete machine early in order to get practice.
2. Participate in Mall Day but also visit other hubs Mall Day.
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Documentation
DOCUMENTATION IS ESSENTIAL
1. If it isn’t documented, it didn’t happen – document everything.
2. Have team notebooks and an overall master notebook.
3. Complete engineering notebook in stages as they occur.
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Schedule
WEEK 1:
Concept Selected
WEEK 2:
Mock-up Complete
Course Complete
WEEK 3:
Prototype Robot Complete
WEEK 4:
“Production” Robot Complete
Begin Drawings
WEEK 5:
Drive Practice, Strategy Development
WEEK 6:
Drive Practice
Complete Drawings
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BEST Design Process
The four main phases of design are:
Phase
What You Get
Example
Conceptual Design
Concept
Four wheels, scoop, scissor
arm
Preliminary Design
Model or mockup
Cardboard model of concept
Detailed Design
Prototype
Robot from kit parts
Production Design
Product
Refined robot from kit parts
In the COCO BEST suggested schedule, you have 1 week for each phase.
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Conceptual Design
STEP 1: LIST ALL REQUIREMENTS FOR THE ROBOT.
This list is generated after reviewing the rules and developing a general strategy.
Draw a picture of the playing field and sketch strategies.
Example of requirements: Meet weight requirements
Meet size requirements
Negotiate course
Have high reach
Easy to operate
Pick up gamepieces
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Conceptual Design
STEP 2: BREAK DOWN LIST INTO NEEDS AND WANTS
Need or Want
Requirement
Meet weight requirements
Need
Meet size requirements
Need
Negotiate course
Want
Have high reach
Want
Easy to operate
Want
Pick up gamepieces
Need
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Conceptual Design
STEP 3: SET DESIGN TARGETS FOR EACH REQUIREMENT:
Requirement
Need or Want
Design Target
Meet weight requirements
Need
Less than 24 lbs
Meet size requirements
Need
Less than 23x23x23
Negotiate course
Want
Climb 5 inch ledge
Have high reach
Want
Reach 50 inches
Easy to operate
Want
One function per
motor
Pick up gamepieces
Need
Pick up soup can and
lawn chair
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Conceptual Design
STEP 4: SELECT THE RELATIVE IMPORTANCE FOR ALL WANTS
USING PAIRWISE COMPARISON
Negotiate
Course
Negotiate Course
1 1
High Reach
0
Easy to Operate
0
High Reach
Easy to
Operate
Total
Weight
2
2/3
1
1
1/3
0
0
0/3
Total
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Conceptual Design
STEP 5: LIST ALL ROBOT FUNCTIONS
MOVE TO SCORING AREA
OBTAIN GAME PIECE
SECURE GAME PIECE
LIFT GAME PIECE
…
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Conceptual Design
STEP 6: DEVELOP CONCEPTS FOR EACH FUNCTION
FUNCTION
CONCEPT (MAKE SKETCH)
A
MOVE TO SCORING
AREA
Chassis with wheels, chassis with
treads, frame with wheels
B
OBTAIN GAME PIECE
Jaw, scoop, velcro
C
SECURE GAME PIECE
Spring, lock, rubber band
D
LIFT GAME PIECE
Lever arm, fork lift, scissor lift
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Conceptual Design
STEP 7: ASSIGN A LETTER AND NUMBER TO EACH CONCEPT
(Make a sketch of each)
A1 – Chassis with wheels
A2 - Chassis with treads
A3 - Frame with wheels
B1 – Jaw
B2 - Scoop
B3 – Velcro
C1 – Spring
C2 – Lock
C3 – Rubber Band
D1 – Level arm
D2 – Fork lift
D3 – Scissor lift
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Conceptual Design
STEP 8: EVALUATE CONCEPTS USING:
• Feasibility – can this be done?
• Go / No Go – does it meet all needs?
• Decision Matrix – does it meet wants?
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Conceptual Design
Feasibility
Concept
Feasible?
A1 – Chassis with wheels
Yes
A2 - Chassis with treads
Yes
A3 - Frame with wheels
Yes
B1 – Jaw
Yes
B2 - Scoop
Yes
B3 – Velcro
No
C1 – Spring
Yes
C2 – Lock
No
C3 – Rubber Band
Yes
D1 – Level arm
Yes
D2 – Fork lift
No
D3 – Scissor lift
Yes
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Conceptual Design
Go – NoGo (needs only)
Requirement
A1
A2
A3
Meet weight requirements
Yes
Yes
Yes
Meet size requirements
Yes
Yes
Yes
Pick up gamepieces
Yes
Yes
Yes
Requirement
B1
B2
B3 not
feasible
Meet weight requirements
Yes
Yes
Meet size requirements
Yes
Yes
Pick up gamepieces
Yes
Yes
Etc…
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Conceptual Design
Decision Matrix (wants only)
+ means that concept is better at meeting the
requirement than the datum
- means that concept is worse at meeting the
requirement than the datum
s means that concept is the same at meeting the
requirement as the datum
The chart shows A1 to be the preferred
concept for the “A” function (move to scoring
area)
Continue this for all functions. The end
result will be an overall concept.
Example: Chassis with wheels, jaw, rubber
band lock and lever arm.
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Requirement
Weight
A1
A2
A3
Negotiate
course
.66
Datum
-
S
Have high
reach
.33
Datum
+
-
Easy to operate
0
Datum
+
-
Total Plus
2
0
Total Minus
1
2
Overall
1
-2
Weighted Plus
.33
0
Weighted
Minus
.66
.33
Overall
Weighted
-.33
-.33
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Preliminary Design
STEP 1: Take the concept and sketch an overall configuration. Do not
worry about the details at this point. Label the major components.
STEP 2: Sketch each of the major components on a separate sheet. Put
enough information on the sketch so that the component can be made
from a piece of cardboard. Try to keep the overall size requirement in
mind.
STEP 3: Make cardboard pieces from the sketches and assemble.
STEP 4: Evaluate the model and ensure it meets all of the requirements.
Make modifications as needed. Try it on the course and ensure it fits in
the 24x24x24 inch box.
You now have a model of the robot.
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Detailed Design
STEP 1: Disassemble the cardboard model and mark-up each sketch to show the
final dimensions. Also indicate on the sketch the material that will be used to make
the real part.
STEP 2: Create sketches for parts that are not on the model such as wheel mounts,
motor mounts, etc. Consider lifting requirements, torque available from motors, etc.
STEP 3: Create an overall assembly sketch of all parts. Label each part.
You now have a detailed design of the robot.
STEP 4: Fabricate each part from the sketch and assemble the robot.
You now have a prototype robot.
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Production Design
STEP 1: After testing the prototype, make changes as required.
STEP 2: Once the robot is in its final configuration, make detailed drawings of each
part.
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BEST Engineering Mechanics
Purpose:
•Introduce students to the theory of some simple machines.
• Apply the theory of simple machines to robotics design.
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Machines
WHAT IS A MACHINE?
• A device that transmits, or changes, the application of energy.
• Allows for the multiplication of force at the expense of distance.
• A machine does work.
• Work is force applied through a distance.
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Simple Machines
SIMPLE MACHINES:
•
•
Simple machines have existed and have been used for centuries.
Each one makes work easier to do by providing some trade-off between the
force applied and the distance over which the force is applied.
•
We will discuss the following simple machines and relate them to robotics
design:
1. LEVERS
2. PULLEYS
3. GEARS
We will also discuss the concepts of torque as related to robotics design
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Levers
A lever is a stiff bar that rotates about a pivot point called the
fulcrum. Depending on where the pivot point is located, a lever can
multiply either the force applied or the distance over which the force
is applied
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Levers
There are three classes of levers:
First Class Levers
The fulcrum is between the effort and the load. A seesaw is an example of a simple
first class lever. A pair of scissors is an example of two connected first class levers.
Second Class Levers
The load is between the fulcrum and the effort. A wheelbarrow is an example of a
simple second class lever. A nutcracker is an example of two connected second
class levers.
Third Class Levers
The effort is between the fulcrum and the load. A stapler or a fishing rod is an
example of a simple third class lever. A pair of tweezers is an example of two
connected third class levers.
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Levers
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Levers
Force and Effort
To lift a load with the least effort:
•
Place the load as close to the fulcrum as possible.
•
Apply the effort as far from the fulcrum as possible.
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Levers
THE LEVER BALANCE EQUATION FOR A FIRST CLASS LEVER IS :
W1 D1 = W2 D2
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Levers
If more weights are to be added, simply add them to the required side of
the equation. For example, to add an additional weight (W3), a distance
(D3) to the right of the fulcrum makes the equation :
W1 D1 = W2 D2 + W3 D3
THIS CAN BE DEMONSTRATED USING A RULER AS A LEVER AND
COINS AS WEIGHTS
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Levers
How many levers can you find in the loader?
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Block and Tackle
PULLEYS / BLOCK AND TACKLE
A block and tackle is an arrangement of rope and
pulleys that allows you to trade force for distance.
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Block and Tackle
Imagine that you have the arrangement of a 100 pound
weight suspended from a rope, as shown. If you are
going to suspend the weight in the air then you have to
apply an upward force of 100 pounds to the rope. If the
rope is 100 feet long and you want to lift the weight up
100 feet, you have to pull in 100 feet of rope to do it.
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Block and Tackle
Now imagine that you add a pulley. Does this
change anything? Not really. The only thing that
changes is the direction of the force you have to
apply to lift the weight. You still have to apply 100
pounds of force to keep the weight suspended,
and you still have to reel in 100 feet of rope to lift
the weight 100 feet
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Block and Tackle
Now add another pulley. This actually does change
things in an important way. You can see that the
weight is now suspended by two ropes rather
than one. That means the weight is split equally
between the two ropes, so each one holds only half
the weight, or 50 pounds. That means that if you
want to hold the weight suspended in the air, you
only have to apply 50 pounds of force (the
ceiling exerts the other 50 pounds of force on the
other end of the rope). If you want to lift the weight
100 feet higher, then you have to reel in twice as
much rope - 200 feet of rope must be pulled in.
This demonstrates a force-distance tradeoff. The
force has been cut in half but the distance the
rope must be pulled has doubled.
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Gears
Gears are generally used for one of three different reasons:
1.To reverse the direction of rotation
2.To increase or decrease the speed of rotation
3.To move rotational motion to a different axis
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Gears
You can see effects 1, 2 and 3 in the figure
-The two gears are rotating in opposite directions.
-The smaller gear spins twice as fast as the larger gear because the diameter of
the gear on the left is twice that of the gear on the right. The gear ratio is therefore
2:1 pronounced, "Two to one").
-The axis of rotation of the smaller gear is to the right of the axis of rotation for the
larger gear.
-If D is the motor and 2D is being driven, 2D has twice the torque. (Same effect
can be accomplished with a belt).
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Torque
TORQUE
A force applied to a body that causes it to rotate creates torque.
The motors supplied in your kits are designed for a specific torque and are listed as:
Large motors - 216 in-oz at 56 rpm (a little less than 1 revolution per second)
Small motors – 34 in –oz at 113 rpm (a little less than 2 revolutions per second)
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Torque
The equation for torque for the motors is:
T=rF
Where:
T = torque of the motor
r = radius of the motor shaft, pulley or whatever is attached to the motor shaft
F = the force created by the motor
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Torque
Since the torque is pretty much a constant ( you are stuck with the motors
provided in the kit), and you probably want to know the force your motor
can produce, the equation can be written as:
F = T/r
If you want to know the radius needed for your motor
shaft, the equation becomes:
r = T/F
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Design Example
EXAMPLE:
Let’s take the concepts we have learned and design an arm that will
lift a 1 lb game piece.
Let’s assume that our mockup resulted in the following:
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Design Example
Since the arm is a lever, let’s use the lever equation to figure out how
much force is on the string. The weights and distances from the fulcrum
for everything on the right is :
Item
Weight (oz)
Distance
WD
from
(in-oz)
fulcrum (in)
Rocket
16
30
480
Grabber
8
30
240
Arm
8
10
80
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Design Example
If we add 480+240+80 we get 800 in-oz. This is the right side of the lever
equation. The equation becomes:
W1 x 5 inches = 800 in-oz
Or W1 = 160 oz
This means that the force in the string is 160 oz except, we have two strings
sharing the load because of the pulley arrangement so therefore the force in
the string is only 80 oz.
Let’s put in a safety factor of 1.5 so that the force in the string is now 80 * 1.5
= 120 oz. This will ensure that the motor will lift the required weight even on
low batteries etc.
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Design Example
Now we need to calculate the motor shaft size that will
create a 120 oz force. The equation for the shaft radius is
r = T/F or r = 34 / 120 = .283 inches.
This means our shaft needs to have a radius of .283 inches
or a diameter of .566 inches.
What else could you do to improve things?
A counterweight , but not too much or the arm will not lower.
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