Electric Motors in Robot Transmissions and Arms

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Electric Motors in Robot Transmissions and Arms

• by Alan Holmes
• Hybrid Electric Car Engineer

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Presentation Contents

• Background
• Electric Motors
• Electricity
• Mechanical Motion
• Construction and Types of Electric Motors
• Brush-Type DC Motor Example
• Arm Design
• Transmission Design

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Presentation Contents

• Background
• Electric Motors
• Electricity
• Mechanical Motion
• Construction and Types of Electric Motors
• Brush-Type DC Motor Example
• Arm Design
• Transmission Design

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Electric Motors Introduction

• Electric motors convert energy and power
• Electrical power to mechanical power
• Electricity (electron motion)
• Voltage V
• Current i
• Mechanical motion
• Rotary
• Torque T around a point
• Speed N (or ?) around a point
• Linear
• Force F in a straight line
• Speed v in a straight line

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Electricity Basic Ideas

• Electricity is a flow of electrons
• Voltage V measured in volts
• Force of each electron
• Voltage can push electrons through things
• 48V or more can be hazardous
• Current i measured in amps
• Amount of electron flow
• Current can do useful work
• Current causes heating

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Electricity different types

• DCDirect Currentbatteries
• Electricity flows in one direction
• Positive ( red) and negative ( black)
• Comes from battery or DC generator
• 12V is used in cars (and robots)
• ACAlternating Currentwall outlet
• Electricity flows back and forth (sine wave)
• /- (black, red) and 0 (white, green)
• Comes from inverter or AC generator
• 110V-120V used in homes (in America)

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Electrical Power

• Power voltage x current
• Example high voltage and low current
• 240W 120V x 2A
• Small wires but shock danger to humans
• Short circuits are a burn hazard!
• Example low voltage and high current
• 240W 12V x 20A
• Minimal shock hazard but larger wires
• Short circuits are a burn hazard!

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Mechanical Motion

• Rotary torque (T) and speed (N)
• Linear force (F) and speed (v)
• Torque
• Twisting around a point or shaft
• Same as a force acting at some distance
• Torque force x distance
• Examples
• 1 pound-inch 1 lb x 1 in
• pound-inch or inch-pound
• 1 newton-meter 1 N x 1 m

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Rotary Mechanical Units

• Torque force x distance
• Foot-pounds (1/12)
• Inch-pounds (1)
• Inch-ounces (16)
• Newton-meters (0.113)
• Newton-millimeters (113)
• Speed
• RPM revolutions per minute (1000)
• Radians per second 2 x pi / 60 (104.7)

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Rotary Mechanical Power

• Power torque x speed
• Horsepower (in-lb torque x rpm / 63025.)
• Watt (Nm x rad/s)
• Example high torque and low speed
• 0.1 hp 630 in-lb x 10 rpm / 63025
• 75 W 71.2 Nm x 1.047 rad/s
• Example low torque and high speed
• 0.1 hp 6.3 in-lb x 1000 rpm / 63025
• 75 W 0.712 Nm x 104.7 rad/s

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Linear Mechanical Power

• Power force x speed
• horsepower (lb x in/s / 6600)
• watts (N x m/s)
• Example high force and low speed
• 0.1 hp 55 lb x 12 in/s / 6600
• 75 W 245 N x 0.3048 m/s
• Example low force and high speed
• 0.1 hp 0.55 lb x 1200 in/s / 6600
• 75 W 2.45 N x 30.48 m/s

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Electrical to Mechanical Conversion

• Electric Motor converts power
• Electricity IN
• Voltage (V)
• Current (i)
• Electrical power V x i
• Rotary Motion OUT
• Torque (T)
• Speed (N)
• Mechanical power T x N

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Rotary to Linear Transformation

• Wheel, Pulley, Sprocket or Arm
• Rotary Motion IN
• Torque (T)
• Speed (N)
• Mechanical power T x N
• Linear Motion OUT
• Force (F)
• Speed (v)
• Mechanical power F x v

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Presentation Contents

• Background
• Electric Motors
• Electricity
• Mechanical Motion
• Construction and Types of Electric Motors
• Brush-Type DC Motor Example
• Arm Design
• Transmission Design

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Construction of Electric Motors

• Stator main stationary part, in housing
• Iron core and copper windings OR
• Permanent magnets
• Rotor main rotating part, on shaft
• Iron core and copper windings OR
• Iron core and aluminum bars OR
• Permanent magnets OR
• Permanent magnets in iron core OR
• Iron core alone

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Example of Motor Construction

• Stator in housing
• permanent magnets
• Rotor on shaft
• iron core with copper windings
• Commutator
• Copper bars on end of shaft
• Carbon brushes in end of housing

• Brush DC Motor

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Operation of Electric Motors

• Most run by action of two magnetic fields
• Stator
• Rotor
• One magnetic field is stationary
• Permanent magnets OR
• Constant flow of current through coil or coils
• One magnetic field is rotating or changing
• AC through coil or coils OR
• Switched current flow through coil or coils

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Switching for DC Electric Motors

• Commutator
• Switches DC current and magnetic field
• Brush commutator
• Carbon brushes attached to housing
• Copper bars attached to rotor
• Easy to make but brushes eventually wear out
• Most small DC motors
• Electronic commutator
• Switching circuit, usually with sensor
• Used with permanent magnets on rotor
• Computer fan motors, run from internal DC supply

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Common Types of Electric Motors

• Brush DC
• 99 of DC motors are brush-type.
• Some AC motors are brush-type AC/DC motors.
• Brushless DC (Electronic Commutation)
• DC motors needing long life or high efficiency
• AC Induction
• 90 of AC motors are induction-type.
• Construction (FYI)
• Stator with windings
• Rotor with iron core and aluminum bars
• Bars look like a squirrel cage / hamster wheel

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Presentation Contents

• Background
• Electric Motors
• Electricity
• Mechanical Motion
• Construction and Types of Electric Motors
• Brush-Type DC Motor Example
• Arm Design
• Transmission Design

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Brush-Type DC Motor Example

• Motor performance chart or curves and specs

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Brush DC Motor Example CIM Motor from Robot
Kits
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Brush DC Motor Operation

• Output speed N varies with motor voltage V
• In CIM motor performance chart, voltage is
  always 12 V.
• Speed control can be done by varying the voltage
  with PWM.
• PWM is pulse width modulation done with
  electronic switching.
• Input current i varies with output torque T
• In CIM motor performance chart, i(T) is a
  straight line
• Current is proportional to total torque
• Some friction torque in motor, so line has an i
  intercept
• Power torque x speed
• Goes up with increasing torque
• Goes down with decreasing speed
• Maximum with best combination of torque and speed
• Efficiency is 59 at rated power

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Brush DC Motor Example CIM Motor from Robot
Kits
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CIM Brush DC Motor Summary

• Power
• Maximum Output Power 286 watts mechanical power
• Rated Output Power 204 watts
• Rated at speed of 4,300 rpm (at 12 V)
• Rated at torque of 64 oz-in (in-oz) 4 in lb
• Rated at current of 28.7 amps (12 V x 28.7 A
  344 W electrical input)
• Torque
• Maximum torque (stall torque) 276 oz-in 17.25
  in-lb
• Maximum torque requires 112.4 amps!
• Torque at 40 A 90 oz-in 5.6 in-lb
• Speed
• Maximum (12 V) No-load speed of motor alone
  5,600 rpm
• Maximum (12 V) speed of motor at 40 A 3,600 rpm

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Presentation Contents

• Background
• Electric Motors
• Electricity
• Mechanical Motion
• Construction and Types of Electric Motors
• Brush-Type DC Motor Example
• Arm Design
• Transmission Design

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Arm Design

• Transforms mechanical motion
• From Motor
• Rotary motion
• High speed (1000s of RPM)
• Low torque (A few inch-pounds)
• To Robot
• Rotary motion (or Linear motion)
• High force (10s of pounds)
• Low speed (A few RPM)

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Example of Arm Operation

• Arm on rotating joint
• Lifting object under the force of gravity

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Torque for Arm Operation

• Torque on joint force x distance
• Force from gravity always points straight down
• Distance is perpendicular to force
• Greatest horizontal distance from pivot point
• Load is straight out from pivot
• Maximum distance
• Maximum torque needed
• Total torque needed
• To lift load
• To lift arm, too!

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Example of Torque for Arm

• Load
• Force 10 lbs
• Distance 30 in
• Torque 300 in lbs
• Arm
• Force weight 10 lbs
• Distance 18 in
• from center of gravity of arm to pivot of arm
• Torque 180 in lbs
• Total torque 480 in lbs

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Mechanical Motion Transformation

• Gear, chain or pulley mechanical ratio
• Diameter or number of teeth on one, d1 or n1
• Diameter or number of teeth on other, d2 or n2
• R d1 / d2 or n1 / n2
• Torque ratio mechanical ratio
• Larger has more torque
• Not exactly equal because efficiency lt 100
• Speed ratio is inverse of mechanical ratio
• Smaller turns faster

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Transformation of Arm Motion

• Total torque required for arm 480 in-lbs
• Motor torque available 5.6 in-lbs
• Torque ratio needed
• 480 in-lbs / 5.6 in-lbs 85.7
• If n1 11 teeth (smallest), then n2 943 teeth!
• Very high ratio must be taken in stages
• Total ratio is the product of ratios for all
  stages
• R total R stage I x R stage II
• 85.7 91 9.5 x 9.5 ( 105 / 11 x 105 / 11)
• Speed ratio is the inverse of mechanical ratio
• N arm N motor / (R stage I x R stage II )
• 40 rpm 3,600 rpm / ( 105 / 11 x 105 / 11 )

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Transformation of Arm Motion

• Stages with higher torque must be stronger
• Larger gears (follow gear specifications) OR
• Larger chain and sprockets or cable and pulleys
• Use linear transformation to find load on chain
  or cable.
• Efficiency affects output torque and required
  ratio!
• If efficiency were 100, then Power OUT Power
  IN
• Efficiency of gears, chains or cables is less
  than 100
• 99 to 75 or lower per stage, depending on
  construction
• Power OUT Power IN x efficiency
• Speed ratio is not affected by efficiency
• Torque OUT Torque IN x R x efficiency
• 480 in-lb 5.6 in-lb x 9 x 97 x 10 x 97

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Presentation Contents

• Background
• Electric Motors
• Electricity
• Mechanical Motion
• Construction and Types of Electric Motors
• Brush-Type DC Motor Example
• Arm Design
• Transmission Design

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Transmission Design

• Transforms mechanical motion
• From Motor
• Rotary motion
• High speed (1000s of RPM)
• Low torque (A few inch-pounds)
• To Robot
• Linear motion
• High force (10s of pounds)
• Low speed (A few feet per second)
• Variable transformation (Shifting Gears)

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Example of Robot Driving

• Robot driven by wheels (or tracks)
• Pushing against something heavy
• Reaction force F on robot from ground
• 120 lbs for example

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Example of Robot Driving

• Force F produces torque T on wheel
• T F x wheel radius
• 960 in-lb 120 lb x 4 in
• Motors to drive robot produce small torque
• T 5.6 in-lb x 2 11.2 in lb

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Transformation of Wheel Motion

• Total torque required for wheels 960 in-lbs
• Motor torque available 11.2 in-lbs
• Torque ratio needed
• 960 in-lbs / 11.2 in-lbs 85.7
• If n1 11 teeth (smallest), then n2 943 teeth!
• Very high ratio must be taken in stages
• Total ratio is the product of ratios for all
  stages
• R total R stage I x R stage II
• 85.7 91 9.5 x 9.5 ( 105 / 11 x 105 / 11)
• Speed ratio is the inverse of mechanical ratio
• N wheel N motor / (R stage I x R stage II )
• 40 rpm 3,600 rpm / ( 105 / 11 x 105 / 11 )

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Example of Robot Transmission

• Speed across floor is very slow
• V N wheel x 2 x pi x wheel radius
• 1000 in/min 40 rpm x 2 x pi x 4 in
• 1000 in/min 16 in/s 1.4 ft/s 1 mph
• Possible improvements
• Reduce drive ratio to increase speed
• R 15 gives 6 mph and 20 lb driving force
• Add multiple-ratio drive system (shifting)
• One ratio for high force R 91
• One ratio for high speed R 15

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The End

• Thanks!
• Good luck in the design and construction of your
  robot.