Download Actuators - University of Detroit Mercy

Actuators
Instructor: Shuvra Das
Mechanical Engineering Dept.
University of Detroit Mercy
Summary
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Actuators
Some actuator examples
Switches
Electric motors
Piezo-actuators
Mechanisms
Flowchart of Mechatronic
Systems
Actuators
• Elements that can execute physical action
• Electromechanical elements - receive input
from controllers
• Controller could be dedicated or embedded
in software
• Software control needs D/A signal
conversion
Role of Actuators
Electrical actuation
signal (from
controller)
Mechanical load
Actuator (pneumatic,
hydraulic, motor, switch,
etc.)
Mechanisms (belts,
pulleys, gear trains, etc.)
Actuators: Typical Actions
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Move a load
Open a valve to increase flow
Rotate a shaft
…….
Actuators: Examples
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Hydraulic or Pneumatic cylinders
Control valves
Electric motors
Switches
Relays
electric motors are most common actuators but for
high power requirements Hydraulic or pneumatic
ones are used.
Types of Actuators
• Hydraulic Actuators
• Mechanical Actuators
• Electrical Actuators
Electrical Actuators
• Switching devices: mechanical systems, relays or
solid-state devices, control signal switches an
electrical device on or off
• Solenoid type devices: current through a coil
activates an iron core that controls a hydraulic or
pneumatic valve
• Drive systems: D.C. and A.C. motors, where a
current through a motor produces rotation
Electromagnetic Relays
• A mechanical switch can be closed or opened as a
result of control signal
• When the coil is energized it pulls the plunger
closing mechanical contact
• Used in activating motors or heating elements
• Demagnetization leads to contact loss
• NO: normally (unenergized) open
• NC: normally (unenergized) closed
Solenoid relays
• Electrical Energy converted to linear mechanical
motion
• De-energized state: Plunger half-way inside the
coil
• Energized state: Plunger pulled in completely
• e.g. car door locks, opening/closing valves
• disadvantage: stroke very short
Solid State Relay
• Input signal: typically 5V DC, 24V DC, or 120V
AC
• Input circuit works like EMR (electro-magnetic
relay)
• Output circuit works like EMR as well
• output circuit can be AC and electronic switch
capable of supporting large currents
• LED and phototransistor pair optically coupled,
i.e. light activates electrical signal in photo
transistor
Solid State Relay
• The amplifier boosts the signal to a suitable level
to trigger the triac
• Triac: electronic switch that supports current in
both directions
• Input => LED => phototransistor => amplifier=>
triac => actuation output
• Separates high power output side from low power
input side
Electronic Vs. Mechanical
Switches
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Advantages(electronic)
No contact-no wear
No contact bounce
No arcing
Faster
Maybe driven by lowvoltage
• Disadvantages (elec.)
• False triggering
through noise
• Failure unpredictable
• when on-not 100%
short; when off -not
100% open
DC Motors
• Current carrying conductor in magnetic field
experiences force (Lorentz effect)
• A conductor moved in a magnetic field generates
(back) emf that opposes the change that produces
it. (Faraday/Lenz’s law)
• Back emf  rate of change of flux
• Current due to back emf in closed circuit will
create a flux opposite the magnetic flux
• motor direction is reversed by reversing the
polarity of voltage
DC Motors
• Armature coil is free to rotate in the magnetic field
• Loop of wire is connected through the commutator
to the brushes (brushes stationary, commutator
rotates)
• Current flows when power is supplied to brushes
DC Motors
• Opposite forces on opposite sides generates a
torque
• Commutator changes current direction when the
plane of wire is vertical
• Torque direction remains unchanged
• Multiple wires are wound in a distributed fashion
over cylindrical rotor of ferromagnetic material
• Multiple loops increases and also evens out the
torque
Armature
Field Coils
Brushes
Commutator
Permanent magnet DC Motors
• Permanent magnet provides a constant value of flux
density.
• For an armature conductor of length L and carrying a
current I the force resulting from a magnetic flux density B
at right angles to the conductor is B I L.
Permanent magnet DC Motors
• With N conductors the force is F=N B I L. The
forces result in a torque about the coil axis of Fc,
if c is the breadth of the coil, T= (NBLc)I .
• Torque is thus written as T= KTI; I=armature
current,KT is based on motor construction.
Permanent magnet DC Motors
• Since the armature coil is rotating in a magnetic
field, electromagnetic induction will occur and a
back emf will be induced. The back emf E is
related to the rate at which the flux linked by the
coil changes. For a constant magnetic field, is
proportional to the angular velocity of rotation.
• Back emf is related to flux and angular rotation (in
rpm) E= KEw; w= motor speed in rpm.
• KT and KE depend on motor construction
Permanent magnet DC Motors
• The motor circuit can be represented as:
R
V
E
• The current in the circuit is I = (V – E)/R
Permanent magnet DC Motors
• Armature current, I= (V – E)/R. R is the armature
resistance and E is back emf.
• The Torque therefore is T= T= KTI = KT (V – E)/R
= KT (V – KEw)/R
• At start-up, back emf is minimum therefore I is
maximum and Torque is maximum. The faster it
runs the smaller the current and hence the torque.
Permanent magnet DC Motors
T= KTI = KT (V – E)/R = KT (V – KEw)/R
T
V
speed
Other types DC motors
• Separately excited armature windings:
– series wound motor
– shunt wound motor
– compound motor
• Non-DC motors: AC motors
Servo motors
• Consists of DC motor, gear train and built in pot
(and circuitry) for shaft position indication
Servo Motors
• A servo motor is a DC or AC component coupled with a
position sensing device.
• A DC servo motor consists of a motor, gear train,
potentiometer, limit stops, control circuit.
• Three wires: ground, power, control signal.
• The control signal is in the form of a pulse width signal.
• As long as the control line keeps receiving the signal the
servo holds the position of the shaft.
• With the change of the coded signal the position of the
shaft changes.
Servo motors
• Input is pulse width modulated signal (PWM)
• Pulse duration is based on a coded number from 0255 (programmed into microcontroller)
• The PWM is used to turn an electric switch on and
off such that a fixed DC source is intermittently
applied to the motor. This reduces the effective
voltage seen by the motor
Servo motors
• The servo has some control circuit and a pot.
Once the final position is reached the circuit turns
the power off.
• The output shaft can travel between 0 and 180.
• A servo expects to see a pulse every 20 ms. The
duration of the pulse determines how far the servo
will travel. A 1.5ms pulse makes it travel by 90
degrees. For a longer pulse the travel is closer to
180 and for a shorter pulse it is closer to 0.
Servo Motors
• When the new position is reached (coresponding
to the duration of PWM signal) motor is shut off
by the control circuitry
• This position is maintained until the PWM signal
input is unchanged
• Most common servos use 5 volts of input supply
Servo motor
• Amount of power to motor
 distance the servo needs
to travel
• Control wire is used to
send the PWM signal
• Servos are usually small
but extremely powerful
for its size
• Futaba S-148 has
42oz.inches of torque
Stepper Motors
• Moves in discrete
steps
• rotor is permanent
magnet
• When electromagnets
are energized the rotor
aligns itself properly
• Step sizes can be
obtained from 0.9
through 90 degrees
Stepper Motors
• Common uses: dot matrix printer paper advance,
positioning read-write heads of disk-drives
• Advantage: Can be used in open-loop control
mode without shaft position recorder (if the
number of steps taken is recorded). No sensors
needed!
• Disadvantage: for heavy loads steps could be
missed; without feedback this cannot be recovered