Engineering Exploration: Robotics (Mechatronics) Pre-workshop Assignment - Prof. Tamara Wr, Assignments of Engineering

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ENGE1024^ Fall 200

Engineering Exploration Pre-workshop assignment

Robotics (Mechatronics)

Please use

OneNote

to answer the questions.

Due:

5 PM, Tuesday, October 14, 2008

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ENGINEERINGEDUCATION

This workshop is developed by:

Dr. Pushkin Kachroo, P.E. http://www.ee.vt.edu/~pushkin

Bradley

Department

of^

Electrical

and

Computer

Engineering

in^ consultation

with:

Drs. Vinod K Lohani, Jenny Lo, and G V Loganathan** *^ Engineering

Education

Dept.,

**^

Civil

and

Env.

Eng.

Dept.

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Water-Electricity Analogy - I

Water-pump

Water moving up to the bucket because of the water pump

Height of the water in the bucketdictates the speed of waterflow

Water flow can be used to rotate a wheel

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Water-Electricity Analogy - II

DCMotor

Battery is like the water pump forcing current to flow through the circuit

Voltage is like water height 9 Volts

Electric current flowing through the wires is like water flow (electric charge is flowing through)

0 Volts

Motor running due to electriccurrent is the same idea asa wheel rotating due tothe water falling on it

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ENGINEERINGEDUCATION

Robot Description

(1) Motor rotation

(2) Gear rotation

(3) Wheel rotation

(1) Motor rotation causes (2) Gear rotation causes (3) Wheel rot(1) Motor rotation causes (2) Gear rotation causes (3) Wheel rotation

ation

Greater the voltage appliedto the motor, higher itsangular speed

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Why We Need Gears – Part I Can lift or push only 50 N

Rock weighs 500 N

Solution^ fulcrum

Effort (50 N) Load (500 N)

Length ratio1:

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ENGINEERINGEDUCATION

What is Torque

Apply force

F

here, the nut mightnot move

Apply same force

F

here, the nut mightmove

l^1

l^2

Torque

: measure of a force's tendency to produce rotation

When the force is applied at a farther distance from the center of rotation, it is easier to produce rotation. Torque is the product of the force and the distance from the point application of the force and the center of rotation. (note: the force and the length ‘vector” are perpendicular to each other). From the figure: F

λ< F^2

λ^1

Center ofrotation

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Why We Need Gears – Part II

Motors have low torque but high speed in air

Solution

When you put the wheels on theground, the friction from theground makes the wheels tostop spinning

Use gears so that the low torque andhigh speed at the motor shaft is transferredto wheel as high torque/low speedThe high torque at the wheel can counterthe ground friction so that now the wheelsmove when they are put on the ground.

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ENGINEERINGEDUCATION

Gear Ratios – speed transfer

The point of contact between the twogears moves with the same linear velocity(v) downwards. From Physics we knowthat the linear velocity v for small gear willbe:

r^11 v^

× =^ ω v v

Likewise, for the big gear: Radius r^1

r^22 v^

×

Radius r^2

=^ ω

(1)

/ /^

(2) rr

Equations (1) and (2) show that:

or^

= ω ω

r r^

× =×^ ω ω

Gear Ratio

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Gear Ratios – torque transfer

Motor applies torque to the small gear.The small gear pushes on the big gear with a force

F.^ Therefore, torque applied will be:

rF × = τ F F

The force

F^ produces a torque on the big gear. Radius r^1

rF ×

Radius r^2

= τ

(1)^ (2)

Equations (1) and (2) show that:^ τ τ 21 =^^ rr^21

/ /^

rr

or^

= τ τ

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HW#7– Question 2: Torque and Speed Take the spur-gear example as shown in previous slides. The radius ofsmall gear (i.e., input gear) is 0.50cm, and the radius of the big gear (i.e., outputgear) is 50. cm. 2A : What is the gear ratio? 2B : If the input torque is 0.25 N•m, how much is the torqueproduced at the output? 2C : If the input gear rotates at 50 revolutions/ s, how fastdoes the output gear rotate? Give the answer in radians/s. 2D : Create free hand sketches of both gears. Don’t worry aboutthe scale.

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Robot Movements

Robot Moves Forward when Both Wheels Move Forward

" Now, let's consider the possible movements of the Robot."

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HW#7 – Question 3: Robot Movements Make and fill all the entries of the table that show some of the possible movementsof both wheels (there are 25 total combinations possible) and describe therobot motion for each. Note that first two entries are already filled. Motor Left

Motor Right

Robot Movement

FF^

FF^

Moves forward fast

BF^

FF^

Spins counterclockwise about the center of the wheel axle

FF^

BF NM^

FF FS^

FF NM^

NM BF^

BS

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Sensors:

Switches

Picture

Representation

Front Switch sensors Rear Switch sensors

Application