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Collin County Boosting Engineering Science and Technology

Workshop

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AGENDA

•Introduction

•Lessons Learned

•Design Process

•Engineering Mechanics

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TimeSave 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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ScheduleWEEK 1: Concept Selected

WEEK 2: Mock-up CompleteCourse Complete

WEEK 3: Prototype Robot Complete

WEEK 4: “Production” Robot CompleteBegin Drawings

WEEK 5: Drive Practice, Strategy Development

WEEK 6: Drive PracticeComplete 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 requirementsMeet size requirementsNegotiate courseHave high reachEasy to operatePick up gamepieces

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Conceptual DesignSTEP 2: BREAK DOWN LIST INTO NEEDS AND WANTS

RequirementNeed or Want

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 DesignSTEP 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 DesignSTEP 4: SELECT THE RELATIVE IMPORTANCE FOR ALL WANTS

USING PAIRWISE COMPARISON

Negotiate Course

High Reach Easy to Operate

Total Weight

Negotiate Course 1 1 2 2/3

High Reach 0 1 1 1/3

Easy to Operate 0 0 0 0/3

Total 3

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Conceptual Design

STEP 5: LIST ALL ROBOT FUNCTIONS

MOVE TO SCORING AREAOBTAIN GAME PIECESECURE GAME PIECELIFT GAME PIECE…

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Conceptual DesignSTEP 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 DesignSTEP 7: ASSIGN A LETTER AND NUMBER TO EACH CONCEPT

(Make a sketch of each)A1 – Chassis with wheelsA2 - Chassis with treadsA3 - Frame with wheelsB1 – JawB2 - ScoopB3 – VelcroC1 – SpringC2 – LockC3 – Rubber BandD1 – Level armD2 – Fork liftD3 – 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 DesignFeasibility

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 DesignGo – 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 datums means that concept is the same at meeting the requirement as the datum

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

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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Preliminary DesignSTEP 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 DesignSTEP 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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MachinesWHAT 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 MachinesSIMPLE 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. LEVERS2. PULLEYS3. 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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LeversThere are three classes of levers:

First Class LeversThe 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 LeversThe 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 LeversThe 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

W1 D1 = W2 D2

THE LEVER BALANCE EQUATION FOR A FIRST CLASS LEVER IS :

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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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LeversHow 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 TackleNow 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 TackleNow 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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TorqueTORQUE

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 = r F

Where:T = torque of the motorr = radius of the motor shaft, pulley or whatever is attached to the motor shaftF = the force created by the motor

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Torque

F = T/r

r = T/F

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:

If you want to know the radius needed for your motor shaft, the equation becomes:

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Design ExampleEXAMPLE: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 ExampleSince 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 from fulcrum (in)

W D(in-oz)

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-ozOr 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.