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Actuators Machine element design
Actuators
Seoul National University
School of Aerospace and Mechanical Engineering
Hyeong-Joon Ahn
2004. 6
Machine element design
Actuators
Actuator
Energy conversion
devices
Including power
amplifier
Electromechanical
DC motor
AC motor
Step motor
Brushless DC motor
Piezoelectric actuators
Pneumatic & hydraulic
Other actuators
Machine element design
Actuators
Characteristics of Actuator
Characteristics of actuating system is criteria of selecting the
kind of actuator.
Weight, Power-to-Weight Ratio, Operating Pressure
The weight of the actuating system and Power-to-Weight Ratio
is important.
Pneumatic and hydraulic system are composed of two system,
actuating system and power system.
The weight of the actuating system and Power-to-Weight Ratio
is important.
Stiffness vs. Compliance
The weight of the actuating system and Power-to-Weight Ratio
is important.
Pneumatic and hydraulic system are composed of two
system, actuating system and power system.
The weight of the actuating system and Power-to-Weight Ratio
is important
.
Machine element design
Actuators
Motor: Background
The first electric motor built in 1833 was a DC motor. It was simple to control
speed and to meet the demands of various applications.
In 1899, the first AC motor was designed, The AC motor was more simple and
robust than the DC motor. However, the fixed speed and torque characteristics of
the first AC motors have not been suitable for all applications.
AC motors convert electric energy into mechanical energy by means of
electromagnetic induction. The principle of electromagnetic induction is: If a
conductor is moving across a magnetic field, a voltage is induced. If the
conductor is part of a closed circuit , there will be a current induced.
In the motor, the magnetic field is placed in the stationary part (stator). The
conductors influenced by the electromagnetic forces are located in the rotating
part (rotor).
AC motors can be divided up into two types: induction and synchronous motors.
In principle the stator works in the same way in both motor types. They only
distinguish themselves in the way the rotors are built up and are moving
according to the magnetic field.
With synchronous motors the rotor and the magnetic field are running at the
same speed; with induction motors the rotor and the magnetic field are running at
different speeds. Machine element design
Actuators
Magnetic actuator
Solenoid
Voice coil actuator
Machine element design
Actuators
DC Motors
Permanent magnet
Permanent magnets are used to generate the stator magnetic
field. Electrical current is supplied directly into the armature
winding of the rotor through the brushes and commutators.
Electro magnet
Shunt wound
A stator (field) winding is used as electromagnet. Stator winding is
connected in parallel with the armature winding
.
Series wound
A stator (field) winding is used as electromagnet. Stator winding is
connected in series with the armature winding.
Compound wound
Two stator (field) windings are used as electromagnet. The stator
windings are connected, one in series and one in parallel, with the
armature winding.
Separate wound
A stator (field) winding is used as electromagnet. Both the stator and
armature fields are individually energized.
Machine element design
Actuators
AC Motors
Single-phase
Induction
Single stator winding with squirrel-cage rotor. No external connection to the rotor.
Torque generation is based on the electromagnetic induction between the stator
and rotor. AC current provides the commutation of the fields. Rotor speed is
slightly slower than the rotating stator field (slip).
Synchronous
Permanent magnet rotor or rotor winding with slip ring commutation. Rotating
speed is synchronized with the frequency of the AC source.
Poly-phase
Induction
Similar to single-phase induction motor but with multiple stator windings. Self-
starting.
Synchronous
Similar to single-phase synchronous motor but with multiple stator windings for
smoother operation.
Universal
Essentially a single-phase AC induction motor with similar electrical
connection as a series wound DC motor. Can be driven by either
AC or DC source.
Machine element design
Actuators
Stepper Motors
Permanent magnet
Permanent magnet rotor with stator windings to provide
matching magnetic field. By applying different sequence
(polarity) of coil current, the rotor PM field will align to match
induced stator field.
Variable reluctance
Teethed ferromagnetic rotor with stator windings. Rotor motion
is the result of the minimization of the magnetic reluctance
between the rotor and stator poles.
Hybrid
Multi-toothed rotor with stator winding. The rotor consists of two
identical teethed ferromagnetic armatures sandwiching a
permanent magnetic. Machine element design
Actuators
Brushless DC Motors
Poly-phase
Synchronous
Essentially a poly-phased AC synchronous motor but using electronic
commutation to match rotor and stator magnetic fields. Electronic
commutation enables using a DC source to drive the synchronous
motor.
Machine element design
Actuators
Piezoelectric Actuator (1)
Piezoelectric materials can be made as either sensor of
actuators in general, because the mechanical-electrical
energy conversion is dual for piezoelectric materials.
If a power amplifier charges
the two electrodes of a piece
of piezoelectric material, it will
cause a deformation of the
piezoelectric material along a
certain axis. Such an effect
leads to the development of
piezoelectric actuators.
Machine element design
Actuators
Piezoelectric Actuator (2)
The dynamic model of a piezoelectric actuator is equivalent to a
capacitor to hold the electrical charge due to the supply of a high-
voltage power amplifier.
Since any non-conducting materials have some degree of weak
conductivity, the capacity of a piezoelectric actuator is also
equivalently in parallel with a resistance with a very high resistance
value.
A piezoelectric actuator is usually driven by a power amplifier with a
relatively high output voltage. The output resistance Ro of the power
amplifier is usually significantly smaller than the leakage resistance of
the piezoelectric actuator.
The dynamic model of the piezoelectric actuator driven by a power
amplifier is a 1
st
-order RC circuit with a dynamic equation
where C is the capacitance of the piezoelectric actuator; V
out
and I
out
are the
amplifier output voltage and current respectively.
Machine element design
Actuators
Piezoelectric Actuator (3)
Let V
act
denote the actual voltage applied to the piezoelectric
actuator
The electrical charge and the deformation of the piezoelectric
actuator is proportional to V
act.
Mathematical derivation gives
Which is a 1
st
order low-pass filtered voltage signal.
When used as an actuator, the physical size f the piezoelectric
material must be larger (thicker) than what it would be when used
as a sensor, since the actuator may be required to drive a certain
loading in its applications. The thicker the physical size, the larger
electrical charge it is able to hold; and the heavier loading it is able
to drive.
Besides, the degree to which a piezoelectric material deforms under
external electrical charging is very limited. For this reason ,
piezoelectric actuators are usually used in applications where the
loading is not heavy and the required displacement is relatively
small. Machine element design
Actuators
Application 1
(a) Suppression of oscillations. Piezoactive materials-based dampers
convert mechanical oscillations into electrical energy. Generated energy is
then shunted to dissipate the energy as heat, i.e., oscillation energy is
eliminated.
(b) Microrobot. Robot platform legs are piezoactuators. By applying voltage
to the electrodes, piezolegs are lengthened, shortened, or bent in any
direction in a fine movement.
(c) Micropump. Diaphragm is actuated by piezoactuator, input and output
check valves are subsequently opened for liquid or gas pumping.
Advantages are fast switching and high compression rate.
Machine element design
Actuators
Application 2
(d) Microgripper. Piezoactuator works on contraction for gripping motion
based on the compliant mechanism. Gripper is of very small size and
almost any required geometrical shape.
(e) Micromanipulator. Due to the unlimited resolution, piezoactuators are
used in numerous positioning applications.
(f) Microdosage device. Piezoactuators allow high precision dosage of a
wide variety of liquids in a range of nanoliters for various applications.
Machine element design
Actuators
Hydraulic actuator
linear movement
big forces without gears
actuators are simple
in mobile ma