Download Motors and Actuation

Introduction to Robot Design:
Motors and Actuation
Gui Cavalcanti
5/19/2011
Overview
• A little bit of physics first!
• Roles of actuators
• Types of actuators
• Actuator sizing
• Electric motors
Physics
• All motion requires a force or torque
• Work:
– Force or torque exerted over a distance
– Measured in Joules (J) of energy
• Power:
– Amount of work done in a given time
– Measured in Watts (W) of power
Force
• All motion is generated by forces acting on a mass
• Newton’s Three Laws of Motion
1.
2.
3.
Every body remains in a state of constant velocity unless acted
upon by an outside unbalanced force.
A body of mass M subject to a net force F undergoes an
acceleration A that has the same direction of the force and a
magnitude that is proportional to the force and inversely
proportional to the mass; 𝐹 = 𝑚𝑎
For every action there is an equal and opposite reaction.
Torque
• Torque is a force acting
rotationally through a radius
• Torque is only produced by
force perpendicular to the
radius of force applied
• Angular acceleration is
proportional to torque applied
and inversely proportional to
rotational inertia; 𝜏 = 𝐼𝛼
Work Example
100 lb
100 lb
• Lifting a weight involves pushing against
gravity over a certain distance. No matter
how fast you lift it, you’re expending the
same amount of energy to lift it up.
Work Example
100 lb
100 lb
• Moving an already-lifted weight sideways
requires no additional physical work.
What’s wrong with this statement?
Power Example
100 lb
100 lb
• Lifting the block in 1 second takes a
certain amount of power. Lifting the same
block the same distance in 10 seconds
takes 1/10th the power.
Power Take-Home Message
100 lb
100 lb
• Any sustained source of force can do
almost any task given enough time.
Physics
• Some Forms of Energy:
– Kinetic: 𝐾𝐸 =
1
𝑚𝑣 2
2
• M is mass, V is velocity
– Gravitational Potential: 𝑃𝐸 = 𝑚𝑔ℎ
• G is gravitational acceleration (9.8 m/s/s), H is
height from reference
– Linear Spring: 𝑆𝐸 =
1
𝑘𝑥 2
2
• K is spring rate, X is displacement
Example Problems
• If you drop a 1 kg mass from 1 meter,
how fast is it going when it hits the
ground?
• If you draw a 1 kg mass back 1 meter on
a spring with a K value of 1000 N/m, how
fast is it going when the spring is fully
restored to its normal length?
Example Problems
• If you drop a 1 kg mass from 1 meter, how fast is it
going when it hits the ground?
– KE = PE
–
1
𝑚𝑣 2
2
= 𝑚𝑔ℎ
– 4.43 m/s
• If you draw a 1 kg mass back 1 meter on a spring with a
K value of 1000 N/m, how fast is it going when the
spring is fully restored to its normal length?
– KE = SE
–
1
𝑚𝑣 2
2
1
2
= 𝑘𝑥 2
– 31.6 m/s
Example Problem
• Let’s characterize your knee joint in a squat!
1.
2.
3.
4.
Figure out your weight in kilograms
Measure how long your leg is from your hip to your knee
Figure out how fast you can stand up from a squat by timing it
Use this data to compute
1.
2.
Maximum knee torque in a squat
Average rotational velocity during standing
• Extension: What do you think would make good
‘envelope’ values if you had to replace your muscles
with an actuator? Why?
Roles of Actuators
• Actuator:
– A mechanical system that combines a source
of motion, a power transmission system and a
feedback device to create desired, controlled
motions
Types of Actuators
• Pneumatic
– Use pneumatic (air) pressure to generate
motion in (generally) a linear fashion
• Hydraulic
– Use hydraulic pressure to generate motion in
a (generally) linear fashion
• Electric
– Use electromagnetism to generate motion in a
rotational or linear fashion
Pneumatic Overview
• Common Actuator Forms:
– Pistons
– Vane motors
• Power Source:
– Compressors
• Gas-engine powered
• Electric motor powered
• Typical Use:
– High-force, high-speed equipment
• Jackhammers, impact wrenches
– Two-position, “Bang-Bang” equipment
• Factory Automation
Pneumatic System
• Required Pieces:
– Compressor
• Automatic Cut-off
– Relief Valve
– High-Pressure Storage Tank
– Regulator
– Valves
– Pneumatic Actuators
Pneumatic System
Compressor
Power Source
Regulator
Storage Tank
Relief Valve
Pneumatic Pros and Cons
Pros
• Easy to order custom,
cheap actuators
• Easy to create a
functional system with
the right pieces
• Can create very high
forces and speeds
• Fairly inexpensive
Cons
• Very difficult to control
incremental motion
• Very power inefficient for
mobile systems
• Compressors are always
loud, as a general rule
• Compressed air tanks
can easily become bombs
• Very few hobby-level
resources available
Hydraulic Overview
• Common Actuator Forms:
–
–
–
–
Pistons
Vane Motors/Pumps
Piston Motors/Pumps
Gear Motors/Pumps
• Power Sources
– Pumps
• Gas-engine powered
• Electric motor powered
• Common Uses
– High-force, low-speed equipment
• Bobcats, Earthmovers, Diggers
Hydraulic System
• Required Pieces:
– Pumps
• Variable displacement
• Fixed displacement
– Accumulators (Optional)
– Return Fluid Tank (Optional)
– Valves
– Hydraulic Actuators
Hydraulic Pros and Cons
Pros
• Easy to order custom
actuators
• Incredibly high force
density
• Easy to create a
functional system with
the right pieces
• Can create very high
forces and speeds
Cons
• EXPENSIVE
• Difficult to assemble,
bleed, and work with
• Very power inefficient
• Very dangerous to work
around leaking hydraulics
• Almost no hobby-level
resources available
Electric Motor Overview
•
Common Actuator Forms:
– Straight rotational motor
• AC
• DC
– Gearmotor
• Motor + Gearbox
– Servomotor
• Motor + Gearbox + Feedback device
– Linear motor
• “Unrolled” linear motor
• Linear actuator
•
Power Source:
– Batteries
– AC Line Voltage
– Alternators on Engines
AC Motors
• Design:
– Stator windings are fed
alternating current
– Iron rotor “squirrel cage” has
electric fields induced into it
– Constantly lags slightly behind the
changing field, causing torque
• Features:
– Tend to have one fixed speed
• Generally 3600, 1800, 1200, or 900
rpm
– Asking for too much torque at
speed causes motors to stall, not
slow down
DC Brushed Motors
•
Design:
– Many different magnetic coils exist
on the rotor, get independently
energized by brushes touching a
commutator
– Energized coils are attracted to
nearest magnet
– As motor turns, brushes suddenly
touch a different set of coils
•
Features
– Most common type of motor. Can
be found everywhere, in everything
– Incredibly easy to use and design
around
– Incredibly inexpensive
– Two wires
Hobby Servos
•
Design:
– Small brushed or brushless
motor attached to a 150:1 to
200:1 gear train
– Output is on a potentiometer or
encoder
– Signal sent to hobby servo is a
position command
– Motor controller inside servo
reads feedback device and
positions motor appropriately
•
Features:
–
–
–
–
Out of the box position control
Motors for every budget
Incredible ease of control
Wide range of hobby accessories and
development
DC Brushless Motors
•
Design:
– Many magnetic coils exist on the
stator, while the rotor is made of
individual magnets
•
Stator can be inside or outside the rotor
– Electricity is routed to the stator in
a well-controlled pattern to create
motion
– As motor turns, sensors detect
position of motor and feed it back
to the motor controller
•
Features
– Highest power density of any
electric motor
– Fastest and longest-lived type of
electric motor
– Three wires
Stepper Motors
•
Design:
– Four coils get individually
energized in the stator and attract
an iron gear-shaped rotor to line
up as closely as possible
– Coils are actively switched by
controller
– Can be used with or without
sensors
•
Features
– Easiest motor to command position
control with – can rely on counting
‘steps’ to figure out where motor is
if unloaded
– Second-most common type of
motor, found in office appliances
everywhere
Electric System
• Required Pieces:
– Power Source
• Battery
• Line Voltage/Inverter
• Gas-powered Generator
– Specific Motor Controller
– Gearboxes/Gear Reduction
– Motors
DC Motor Curves