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Actuators
MEE 390: Instrumentation and
Measurements
MEE 390
Lecture 9
Topics
• Introduction
• Types of Actuators
• Electric Motors /solenoids
• H-Bridge
• Smart Materials
MEE 390
Lecture 9
What is an Actuator?
• An actuator is a device that converts a signal
variable into a physical variable
• Physical variables might be
– Force
– Displacement
– Pressure (etc…)
•
Signal variable might be
– Voltage
– Heat
– Current (etc…)
• It is often called a transducer
• In Mechatronics we are mainly interested in
electrically controlled actuators
MEE 390
Lecture 9
Solenoids / Relays
• The term solenoid refers specifically to a magnet designed to
produce a uniform magnetic field in a volume of space
• Electromechanical solenoids consist of an electromagnetically
inductive coil, wound around a movable steel slug
...
• A Relay is an electromechanical solenoid that turns an
electrical switch on and off
...
Lecture 9
Lorentz Forces
• The forces inside
a motor that
cause the rotor to
rotate are called
Lorentz Forces:
F
MEE 390
I B
Thanks Wikipedia!
Lecture 9
Brushed DC Motors
A simple DC electric motor
...
The left side of the
armature is pushed away from the
left magnet and drawn toward the
right, causing rotation
...
The
process then repeats
...
• By reversing the current in the moving coil of a
motor's armature, a steady rotating force
(torque) is produced
...
• Key Characterisitics:
–
–
–
–
–
High construction complexity
Diagram courtesy of MicroMo
Low reliability (brush wear)
Low efficiency
Terrible EMI (brushes create sparks and ozone)
Driven by Rheostat, H-bridge or Chopper
controllers
– Easy to drive, even sensorless
– Lowest total system cost for basic motion
MEE 390
Lecture 9
Brushless DC Motors
• They have no Commutator (brushes)
• The electric control is done using external electric circuitry
...
• Key characteristics:
– Brushless motors are typically 85-90% efficient, whereas DC
motors with brushgear are typically 75-80% efficient
– Less noise
– Less heating
– High accuracy since they rely on feedback hall-effect sensors
– Long life
– Difficult to implement and control
MEE 390
Lecture 9
Torque / Speed / Power
• Max efficiency: The maximum efficiency of the motor in
converting electrical power to mechanical power
...
• The core windings are typically supported by being impregnated
with epoxy resins
• Key benefits:
–
–
–
–
–
–
–
–
–
–
MEE 390
No magnetic cogging
High acceleration and deceleration due to the low moment of inertia
Low electromagnetic interference
Low inductance
High efficiency
Linearity between voltage and speed
Low starting voltage
Smooth and accurate speed, position, and torque control
Small torque ripple due to the multi-segment commutator
Compact design
Lecture 9
The H-Bridge
•
Electric device used to control the direction and speed of a motor
Motor Positive
S1
S2
DC Motor
S3
S4
Motor Ground
MEE 390
The BJT H-bridge
Vcc
...
Same as Brushless motor but
doesn’t require a controller
• An outside stationary stator having coils supplied with AC
current produce a rotating magnetic field
• An inside rotor attached to the output shaft that is given a
torque by the rotating field
...
• The induction motor turns slightly slower than the supply
frequency
...
(e
...
1 step = 6o)
•
When commutated electronically, the motor's position
can be controlled precisely, without any feedback
mechanism
•
A driver usually fires the coils in a given sequence to
produce either a full step or a half step
MEE 390
Lecture 9
Types of Stepper Motors
• Unipolar motors: they are the easiest
to operate, and each coil will require a
single transistor
...
It is a unique controller
in which it controls a parameter by commanding the time-based
derivative of that parameter (Similar to PD control)
•
Servomotor is a motor that is position controlled to specific
angular positions (or velocity) by sending it a servo a coded signal
•
Sevosignal is similar to a PWM signal without regard to the down
time
...
MEE 390
Lecture 9
Electrostatic motor
• Operates on the repulsion of electric
charge as compared to magnetic
fields
• Requires a high voltage power supply
• Frequently used in MEMS
MEE 390
Nanomotor constructed at
UC Berkeley
...
–Aluminum does not have this structure, therefore
a mechanical stress doesn’t generate any
electric field
...
Does the converse property exist? If we apply
a voltage, will we measure a deflection?
MEE 390
Lecture 9
Piezoelectric Effect
• Piezoelectric materials are an example of a
material that exhibits electromechanical coupling
...
• Passing an electrical signal through an electronic
amplifier will produce deflection in the
piezoelectric material