1. Laboratory 4: Reverse Engineering
Pratima Jan 2007

2. Overview Objectives Concepts Materials Procedure Report / Recitation
Closing

3. Objective Take apart a robot
Measure its gear and velocity ratios
Sketch all gear trains
Introduce the concept of reverse engineering, gears and torque

4. Concept: 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

5. Concept: Forces and Torque
Torque equilibrium:
Torque - French for “twist”
- Created by a twisting force F =
Force (lbs. or Newtons) L
Distance from force to point of rotation (ft. or m)
Torque (ft-lbs. or Newton meters)
If F1=F2, but L1<L2, the right side of the beam will tip down, due to unbalanced torque. L1 L2 F1 F2

6. Gears F1 F2 R1 R2 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:

7. Gears 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)

8. Gears
crown gear
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
Worm Gear

9. Gears
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

10. Gears
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 gears First & last gear rotate
in different direction
idler gear

11. Gears 1” 2” 4” n1 n2 n3
For gears with the same size teeth, the ratio is equal to the number of teeth (n) on (or radius of) each gear

12. Compound Gear Train n1 n2 n5 n6 n3 n4 n7 n8
Angular velocities of all gears are equal to that of the shaft on which they are mounted

13. Materials
Assembled robot that a different team built in the previous lab
NXT kit

14. Procedure 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

15. Procedure 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

16. Procedure 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

17. Assignment: 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)

18. Closing 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