Laboratory 4:Laboratory 4:Reverse EngineeringReverse Engineering

OverviewOverview

• Objectives• Concepts• Materials• Procedure• Report / Recitation• Closing

ObjectiveObjective

• Take apart a robot– Measure its gear and velocity ratios– Sketch all gear trains

• Introduce the concept of reverse engineering, gears and torque

Concept: Reverse EngineeringConcept: Reverse Engineering

• Learning how a device works – Beginning with the intact product– Taking it apart – Noting key components

• Uses:– Understanding how a product is made– Testing quality of product

Concept: Forces and TorqueConcept: Forces and Torque

Torque - French for “twist”

- Created by a twisting

force

L1 L2

F1 F2

If F1=F2, but L1<L2, the right side of the beam will tip down, due to unbalanced torque.

Torque equilibrium:

221121 LFLF LFTorque

F=Force (lbs. or Newtons)L=Distance from force to

point of rotation (ft. or m)=Torque (ft-lbs. or Newton

meters)

GearsGears

• Small force (F1) used to lift large weight (F2)

• Penalty for increased torque- small gear rotates

more than large gear

F1

F2

R1 R2

• Indicated in ratio of revolutions/second (gear speed) in equilibrium equation: 2211 RvRv

GearsGears

• Used to increase/decrease torque or speed• Torque and speed are inversely

proportional• Velocity ratio equals inverse of gear ratio• Values for Output and Input are the

diameters of the respective gears

Output = destination of the rotation

(e.g. a wheel)

Input = source of the rotation(e.g. a motor)

Output

InputRatioVelocity

Input

OutputRatioGear

GearsGears

• Crown Gear: gear wheel with teeth set in rim perpendicular to its plane

• Worm Gear: gear with one tooth that spirals down the length of a shaft

• Spur Gear: gear wheel having radial teeth parallel to axle

crown gearWorm Gear

GearsGears

• Rack Gear– A flat rectangular gear that has teeth running

perpendicular to the axis of the gear – Rack gear changes direction of motion

perpendicularly– Also changes motion from a rotation to a linear

translation

Rack Gear

GearsGears

• Idler gear: gear between two other gears to transmit motion from one to other– Changes direction of rotation – No effect on ratios– ODD number of gears

First & last gear rotate in same direction

– EVEN number of gearsFirst & last gear rotate

in different direction

idler gear

GearsGears

1” 2” 4”

n1

n2n3

For gears with the same size teeth, the ratio is equal to the number of teeth (n) on (or radius of) each gear

4

1

4

2

2

1

Output

InputRatioVelocity

3

1

3

2

2

1 n

n

n

n

n

n

Compound Gear TrainCompound Gear Train

n1n2 n5

n6

n3 n4 n7n8

Angular velocities of all gears are equal to that of the shaft on which they are mounted

8

7

6

5

4

3

2

1

Output

InputRatioVelocity

n

n

n

n

n

n

n

n

MaterialsMaterials

• Assembled robot that a different team built in the previous lab

• NXT kit

ProcedureProcedure

• Examining the Robot– Inspect intact robot– Create sketches of robot, complete

with outside dimensions• Front• Top• Most detailed side • Gear train

– Record appearance of robot• Use digital camera (request photo from

TA)

– Hypothesize how robot works

Examining

Disassembling

Understanding

ProcedureProcedure

• Disassembling the Robot– Take apart robot

• Do NOT break it

– Record what you see as robot is dismantled

– Sketch robot and gear train– Label important parts

Examining

Disassembling

Understanding

ProcedureProcedure

• Understanding the Robot– Describe different components– Record functions major parts – Establish how power is

transferred from motor to ground

– Determine Gear Ratio for gear train linking motor to ground• Recall that a wheel is part of a

gear train!

Examining

Disassembling

Understanding

Assignment: ReportAssignment: Report

• Individual report• Title page• Discussion topics in the manual• Include original data with instructor’s

initials– Scan in data and lab notes

(ask TA for assistance)

ClosingClosing

• Disassemble robots and return sorted parts to kits

• Have all original data signed by TA

• Submit all work electronically

• Return all unused materials to TA