Mechanical Design

1
Mechanical Design

• Anne Bergeron
• Mechanical Engineer
• SAIC
• 3Drobotics_at_gmail.com

2
Overview

• Physics review
• Mechanical design
• Motor Characteristics
• Motor curves
• Picking the right motor
• Drive train calculations

• Manipulators
• Articulating Arms
• Telescoping Lifts
• Grippers
• Latches
• Accumulators
• Pneumatics
• Mechanical Tips

3
Work

• Work Force Distance
• Example The arm weighs 10 lbs and moves 3 ft
  vertically. The mechanism that contains the
  balls weighs 5 lbs. The balls weigh 3 lbs. The
  mechanism and balls move 6 ft vertically.
• (use the center point of object to determine
  distance)

4
Power

• Power Work / Time
• (Force Distance) / Time
• Force Velocity
• Torque Angular Velocity
• POWER IS LIMITED!!!!!
• Example Desire the motion to be completed in 3
  seconds.

5
Motor Characteristics

• Stall Torque
• Highest amount of torque a motor can generate,
  the motor will be stalled with this much load.
• Stall Current
• Amount of current drawn when motor is stalled
• Free Speed
• Speed of motor under no load, fastest speed
• Motor Power
• How much mechanical power a motor has

6
Speed-Torque Curve
Stall Torque (T0)
K (slope)
Torque
Speed
Free Speed (Wf)
7
Current-Torque Curve
Stall current
Current
Max breaker current
Max Design Torque
Torque
8
Power-Torque Curve
Max Power
Power
Max design torque
Torque
9
Combined Motor Curves
Power Speed
Current
Max breaker current
Speed-Torque Current-Torque Power-Torque
Torque
10
Motor Equation

• Use to get better estimates from graph
• Equation form YmXb
• Calculate the slope, m
• Substitutions

11
Motor Examples
Motor Equations 1. 2002-04 Chiaphua T
(-2.45/5,342) W 2.45 2. 2003 Fisher-Price
T (-0.38/15,000) W 0.38 3. 2003-04 Bosch
Drill T (-0.87/19,670) W 0.87
12
Figuring out gear ratios

• Example A robot is to be designed to have a top
  speed of 8 ft/sec. The robot will have 4 wheels
  that have a diameter of 8 in and will be using
  one CIM motor for each pair of wheels. Find the
  needed gear ratio.

13
Step one Gather info

• Given
• Wheel diameter(d) 8 in 0.67 ft
• Robot speed (V) 8 ft/sec
• Motor info
• CIM use a 40 amp breaker
• Look at graphs or use formula to find following
• T40 0.80 N-m 0.59 ft-lb
• W40 4045 rpm

14
Step two Find wheel revolutions

• Find distance traveled per revolution of wheel
• Get wheel revolution needed for desired speed

15
Step 3 Find needed gear ratio

• Find ratio to get speed of motor to speed
  required by wheel
• Motor speed (W40) 4045 rpm
• Required wheel speed 228.12 rpm
• This ratio can be achieved using one 171 ratio
  or a combination of smaller steps. Additional
  steps are multiplied.

16
Step 4 Calculate Force

• Find the torque at the wheel
• Find force due to torque per wheel

17
Step 5 Pushing force

• Get the wheel max friction force (µ1) which is
  equal to the max contact force (weight)
• Find torque needed for pushing force
• Greater than torque of motors so not pushing bot.

18
More possibilities

• The same procedure can also be followed using
  Torque or Power as the starting points.
• Can use the wanted pushing force as starting
  point
• Helps to know the coefficient of friction
• Iterations will be needed.
• Multiply in efficiencies when of stages
  determined
• Recalculate with actual ratios
• This can also be used to calculate ratios for
  manipulators as well.

19
Gear Types and Efficiency

• Previous calculations were done under ideal
  conditions.
• Add efficiency in calculation by multiplying in
  with ratio
• Spur gears
• Efficiency 95 - 98

20
Gear Types and Efficiency (cont.)

• Chain and Sprockets
• Efficiency 95 - 98
• Belt and Pulley
• Efficiency 85-98 (timing belt best)

21
Gear Types and Efficiency (cont.)

• Bevel Gears
• Efficiency 90 - 95
• Worm Gears
• Efficiency 40-70

22
Ordering Gears

• Remember clearances
• Match
• Pitch
• Pressure angle
• Good Sources
• McMaster Carr
• Boston Gear
• MSC
• Pic-Design
• Order early!!!

23
Ordering Wheels

• Skyway Wheels (www.skywaywheels.com)
• AndyMark (www.andymark.biz)

24
Typical Drive Train Configurations

• 2 powered wheels, 2 castors

25
Typical Drive Train Configurations (cont.)

• 4 wheels, several configurations
• 4 motors
• 2 motors
• 2 gear boxes
• 2 powered wheels
• 4 powered wheels

26
Typical Drive Train Configurations (cont.)

• Threads

27
Typical Drive Train Configurations (cont.)

• 2 centered wheels

28
Typical Drive Train Configurations (cont.)

• Swerve

29
Appendages

• Articulating Arms
• Telescoping Lifts
• Grippers
• Latches
• Accumulators

30
Arm Design

• Arm device for grabbing moving objects using
  members that rotate about their ends
• Thin Walled Tubing (1/16) is your friend
• Every Pivot has to be engineered
• reduce, reuse, recycle -)
• Pivots on Pivots are confusing to drivers
• 4 bars linkages help control end of arm
• Think about operator interface very important

31
Arm Advice

• Dont make it over-complicated
• Feedback Control is HUGE
• Measure Current Position (potentiometers)
• Set Desired Position
• Calculate Error
• Take Action Based on Error (Search Internet for
  PID control)
• Install limits
• Design-in sensors from the start of design

32
Four Bar Linkage

• Pin Loadings can be very highWatch for buckling
  in lower memberCounterbalance if you canKeep CG
  aft

33
Arm Example 67 in 2001
34
Arm Example 234 in 2001
35
Arm Example 71 in 2004
36
Arm Contrast 45 in 04-05
37
Arm Example 330 in 2005
38
Telescoping Lifts

• Extension Lift
• Scissor Lift

39
Extension
40
Extension Lift Considerations

• Should be powered down AND up
• If not, make sure to add a device to take up the
  slack if it jams
• Segments need to move freely
• Need to be able to adjust cable length(s).
• Minimize slop / free-play
• Maximize segment overlap
• 20 minimum
• more for bottom, less for top
• Stiffness is as important as strength
• Minimize weight, especially at the top

41
Extension - Rigging
Cascade
Continuous
42
Extension Continuous Rigging

• Cable Goes Same Speed for Up and Down
• Intermediate Sections sometimes Jam
• Low Cable Tension
• More complex cable routing
• The final stage moves up first and down last

43
Extension Continuous Internal Rigging

• Even More complex cable routing
• Cleaner and protected cables

44
Extension Cascade Rigging

• Up-going and Down-going Cables Have Different
  Speeds
• Different Cable Speeds Can be Handled with
  Different Drum Diameters or Multiple Pulleys
• Intermediate Sections Dont Jam
• Much More Tension on the lower stage cables
• Needs lower gearing to deal with higher forces
• I do not prefer this one!

45
Lift Example 111 (1997)
46
Lift Example 213 (2001)
47
Scissor Lift
48
Scissor Lift Considerations

• Advantages
• Minimum retracted height - can go under field
  barriers
• Disadvantages
• Tends to be heavy to be stable enough
• Doesnt deal well with side loads
• Must be built very precisely
• Stability decreases as height increases
• Loads very high to raise at beginning of travel
• I recommend you stay away from this!

49
Scissor Lift Example 234 (1999)
50
Arm vs. Lift
51
Braking Prevent Back-driving

• Ratchet Device - completely lock in one direction
  in discrete increments - such as used in many
  winches
• Clutch Bearing - completely lock in one direction
  
• Brake pads - simple device that squeezes on a
  rotating device to stop motion - can lock in both
  directions
• Disc brakes - like those on your car
• Gear brakes - applied to lowest torque gear in
  gearbox
• Note any gearbox that cannot be back-driven is
  probably very inefficient

52
Power

• Summary
• All motors can lift the same amount (assuming
  100 power transfer efficiencies) - they just do
  it at different rates
• BUT, no power transfer mechanisms are 100
  efficient
• Inefficiencies (friction losses, binding, etc.)
• Design in a Safety Factor (2x, 4x)

53
Grippers

• Gripper grabbing game object
• How to grip
• How to hang on
• Speed
• Control

54
How to grip

• Pneumatic linkage grip
• 1 axis
• 2 axis
• Motorized grip
• Roller grip
• Hoop grip
• Pneumatic grip

55
Pneumatic linear grip

• Pneumatic Cylinder extends retracts linkage to
  open and close gripper
• 254 robot 2004, 1-axis
• 968 robot 2004, 1-axis
• Recommended

56
Pneumatic linear grip

• Pneumatic Cylinder, pulling 3 fingers for a
  2-axis grip
• 60 in 2004
• Recommended

57
Motorized Linear Grip

• Slow
• More complex (gearing)
• Heavier
• Doesnt use pneumatics
• 49 in 2001
• Not
• recommended

58
Roller Grip

• Slow
• Allows for misalignment when grabbing
• Wont let go
• Extends object as releasing
• Simple mechanism
• 45 in 98 and 2004
• Recommended

59
Roller Grip Example 45 (1998)
60
Roller Grip Example 121 (1998)
61
Hoop grip

• Slow
• Needs aligned
• Cant hold on well
• 5 in 2000
• Not
• recommended

62
Pneumatic Grip

• Vacuum
• generator cups to grab
• Slow
• Not secure
• Not easy to control
• Simple
• Problematic
• Not
• recommended

63
Hang on!

• Friction High is needed (over 1.0 mu)
• Rubber, neoprene, silicone, sandpaper
• Force Highest at grip point
• Force multiple x object weight (2-4x)
• Linkage, toggle mechanical advantage
• Extra axis of grip More control
• Best grip roller gripper

64
Speed

• Quickness covers mistakes
• Quick to grab
• Drop re-grab
• 292 example
• Fast
• Pneumatic gripper
• Not fast
• Roller, motor gripper, vacuum

65
Grip control

• Holy grail of gripping
• Get object fast
• Hang on
• Let go quickly
• This must be done under excellent control
• Limit switches
• Auto-functions
• Ease of operation

66
Latches

• Spring latches
• Hooks / spears
• Speed Control

67
Latch example 267

• Pneumatic Latch
• 2001 game
• Grabs pipe
• No smart mechanism

68
Latch example 469

• Spring-loaded latch
• Motorized release
• Smart Mechanism
• 2003

69
Latch example 118

• Spring-loaded latch
• Pneumatic release
• Smart mechanism
• 2003

70
Latching advice

• Dont depend on operator to latch, use a smart
  mechanism
• Spring loaded (preferred)
• Sensor met and automatic command given
• Have a secure latch
• Use an operated mechanism to let go
• Be able to let go quickly
• Pneumatic lever
• Motorized winch, pulling a string

71
Accumulation

• Accumulator rotational device that pulls
  objects in
• Types
• Horizontal tubes - best for gathering balls from
  floor or platforms
• Vertical tubes - best for sucking or pushing
  balls between vertical goal pipes
• Wheels - best for big objects where alignment is
  pre-determined
• When it comes to gathering balls, there is
  nothing more efficient
• If set up in the proper orientation, will not
  knock the ball away, just suck it in

72
Conveying Gathering

• Conveyor - device for moving multiple objects,
  typically within your robot
• Types
• Continuous Belts
• Best to use 2 running at same speed to avoid
  jamming
• Individual Rollers
• best for sticky balls that will usually jam on
  belts and each other
• When it comes to gathering balls, there is
  nothing more efficient
• If set up in the proper orientation, will not
  knock the ball away, just suck it in

73
Conveyors

• Why do balls jam on belts?
• Sticky and rub against each other as they try to
  rotate along the conveyor
• Solution 1
• Use individual rollers
• Adds weight and complexity
• Solution 2
• Use pairs of belts
• Increases size and complexity
• Solution 3
• - Use a slippery material for the non-moving
  surface (Teflon sheet works great)

74
Roller example 111
75
Accumulator example 173 47
76
Pneumatics
77
Pneumatics vs. MotorsSome, but not all important
differences

• Cylinders use up their power source rather
  quickly
• the 2 air tanks we are allowed do not hold much
• Motors use up very little of the total capacity
  of the battery
• Cylinders are great for quick actuations that
  transition to large forces
• Motors have to be geared for the largest forces
• Our ability to control the position of mechanisms
  actuated by cylinders is very limited
• We are not given dynamic airflow or pressure
  controls
• We are given much more versatile electronic
  controls for motors
• Since air is compressible, cylinders have
  built-in shock absorption
• Cylinders used with 1-way valves are great for
  Armageddon devices - stuff happens when power is
  shut off
• This could be good or bad - use wisely

78
Components

• Compressor
• Pressure gages
• Helps diagnose problems
• Cylinders
• custom sizes
• Flow controls

79
Components (cont.)

• Tanks
• Regulators
• 60 PSI
• Relieving
• At least one required after tanks
• Inlet labeled
• Solenoid Valves

80
Fittings

• Flow Control
• Plug Valve

81
Tank
Relay
Regulator
Tank
Gage
Fuse Box
Valve
Compressor
Control System
Piston
82
Layout of Test Board
83
Solenoid Valves

• Opens and closes air flow to cylinders
• Single Solenoid
• Primary position set (extended or retracted)
• Returns to primary position
• Double Solenoid
• Pulse one solenoid, extend
• Pulse other solenoid, retract
• Maintains position

84
Wiring Solenoids

• Single Solenoid
• Connect the red to M of relay
• Connect the black to M- of relay
• Double Solenoid (as two singles)

Single Solenoid
Single Solenoid
85
Double Solenoid
M
M-
Double Solenoid
M
M
Double Solenoid
M-
M-
86
Pistons

• Several sizes available
• Bore sizes ¾, 1 ½, 2
• Stroke Lengths ½ to 12
• Force Pressure Area
• Stroke length

Stroke Length
Bore Size
87
Forces
88
Basic Mechanical Tips

• Know your limitations
• Machining
• Design
• KISS
• Keep track of weight
• Spreadsheet
• Estimates and actuals
• Include materials
• Have 5-10 pound buffer
• Assign per subsystem
• Get a good scale
• Think about maintenance during design
• Access to parts
• Determine high maintenance parts

89
Basic Mechanical Tips (cont.)

• Keep the center of gravity low
• Battery/Compressor
• Wheel Base
• Prototype ideas
• Create design drawings
• 2D or 3D
• CAD or paper
• Keep at building site
• Standardize hardware
• Metric or standard
• 1-2 Sizes (1/4-20, 10-32)
• Lots of lengths

90
Basic Mechanical Tips (cont.)

• Avoid set screws
• Too much traction can be bad
• Be aware of robot systems when drilling or
  machining parts on the robot
• Avoid cantilevered shafts
• Avoid stalling your motors
• Use the right tool for the job

91
Basic Mechanical Tips (cont.)

• Make spares
• Get a base and drive train done quickly
• Dont forget about pneumatics

92
Structural Material

• Metals
• Iron
• Steel
• Aluminum
• Forms
• Extruded
• Plating
• Angle
• Tubing
• Circular
• Square

• Wood
• Fiberglass
• Lexan (no Plexiglass)
• Carbon fiber

93
Profiles
Angle 1x1 1/8 thick
Square 1x1 1/16 thick Same weight, much
more strength and stiffness Takes more space
Extruded 1x1 1/16 thick Heavier, much more
strength and stiffness Takes more space Easy to
assemble and connect to
94
Extruded

• Item America www.itemamerica.com
• 80/20 www.8020.net
• Bosh www.boschrextroth.com
• IPS www.industrialprofile.com

95
Questions?

• Thanks to
• Andy Baker (45)
• Chris Hussman (330)
• Joe Johnson (47)
• Raul Olivera (111)
• www.chiefdelphi.com
• www.firstrobotics.net
• www.firstrobotics.uwaterloo.ca

96
Activity

• The Task
• Design a robot drive train using AndyMark
  gearbox.
• Specifications
• Robot Speed 8 ft/sec
• Use 4 Chiaphua motors (Chip)
• Weight of robot130 lbs

97
Steps

• Decide on a wheel size
• Find gear ratio needed from gearbox to wheels
• Calculate the pushing force of robot
• Is it a pushing robot?
• Conversion factors
• 1 oz-in 0.0052083 pound foot