Transcript Mechanical Actuation System

Electrical Actuation System
Lecture 8
(Chapter 9)
Introduction
• The electrical systems used as
actuators for control are:
1. Switching devices – mechanical
switches and solid-state switch
2. Solenoid type device
3. Drive system – motors (AC, DC,
Stepper)
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7.2: Mechanical Switches
• Mechanical switches – devices used as
sensors to give input to system
• E.g. – to switch on motor, heating
element, current for actuating solenoid
valve in hydraulic and pneumatic system
• Example - relay
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7.2.1: Relay
• An electrical switch that opens and closes
under the control of another electrical circuit
• When a current flows through the coil, the
resulting magnetic field attracts an armature
that is mechanically linked to a moving contact
• The movement either makes or breaks a
connection with a fixed contact
• When the current to the coil is switched off, the
armature is returned by a force approximately
half as strong as the magnetic force to its
relaxed position
• Usually this is a spring, but gravity is also used
commonly in industrial motor starters.
Sources: Wikipedia, the free encyclopedia
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Figure 9.1
(a) A relay, (b) a driver circuit
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• A relay consists of two
separate and completely
independent circuit
• The bottom - drives the
electromagnet
• When the switch is on, the
electromagnet attracts the
armature (blue)
• The armature – acts as a
switch in the second circuit
• When the electromagnet is
energized, the armature
completes the second
circuit and the light is on
• When the electromagnet is
not energized, the spring
pulls the armature SME
away
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Application – Elevator control
system
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Relays are still used in the control panels
of current elevator designs
In the examples, relays are used:
to control the elevator floor and door
signals
for fire mode operation
to control the elevator’s car ventilation
fan, lighting, and motor cooling fan.
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control the elevator
floor and door
signals.
for fire mode
operation.
to control the elevator’s car
ventilation fan, lighting, and
motor cooling fan.
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7.3: Solid-state switches
•
1.
2.
3.
4.
Examples of solid-state devices to
electronically switch circuit:
Diode
Thyristor and triac
Bipolar transistor
Power MOSFET
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Diode operation: (a) Current flow is prmitted; the diode is forward biased. (b) Current
flow is prohibited; the diode is reversed biased.
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7.3.1: Diode
• a semiconductor device, allows current to flow through it in
only one direction
• Some applications of diode:
 as a rectifier that converts AC (Alternating Current) to DC
(Direct Current) for a power supply device
 to separate the signal from radio frequencies
 as an on/off switch that controls current
Several different types of single diodes
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Figure 9.3
(a) Diode characteristic, (b) half-wave rectification
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Bridge rectifier
• 4 diodes in a single package is called a
BRIDGE or BRIDGE RECTIFIER
• To achieve full-wave rectification
• Convert AC voltage to DC pulsating
voltage
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7.3.2: Thyristor and triacs
• Thyristor - A semiconductor device with four layers of alternating N
and P-type material
• Act as a switch, conducting when their gate receives a current pulse,
and continue to conduct for as long as they are forward biased (that
is, as long as the voltage across the device has not reversed)
• Thyristors are mainly used where high currents and voltages are
involved, and are often used to control alternating currents, where
the change of polarity of the current causes the device to
automatically switch off; referred to as Zero Cross operation
• Thyristors can also be found in power supplies for digital circuits,
where they can be used as a sort of "circuit breaker" or "crowbar" to
prevent a failure in the power supply from damaging downstream
components.
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Figure 9.4
(a) Thyristor characteristic, (b) thyristor circuit
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• A TRIAC, or TRIode for Alternating Current is an
electronic component approximately equivalent to two
silicon-controlled rectifiers (SCRs/thyristors) joined in
inverse parallel (paralleled but with the polarity
reversed) and with their gates connected together
• bidirectional electronic switch which can conduct
current in either direction when it is triggered (turned
on). It can be triggered by either a positive or a
negative voltage being applied to its gate electrode.
Once triggered, the device continues to conduct until
the current through it drops below a certain threshold
value, such as at the end of a half-cycle of alternating
current (AC) mains power
• TRIAC a very convenient switch for AC circuits
• Low power TRIACs are used in many applications
such as light dimmers, speed controls for electric fans
and other electric motors, and in the modern
computerized control circuits of many household small
and major appliances
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Figure 9.5
Triac characteristic
Several thyristors
and triacs
Symbols and pin
placements for:
a - thyristor,
b - triac,
c - diac
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Figure 9.6
Voltage control: (a) thyristor, (b) triac
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Figure 9.7
Thyristor d.c. control
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7.3.3: Bipolar Transistor
• A Bipolar Transistor
essentially consists of a pair
of PN Junction Diodes that
are joined back-to-back
• Two kinds of Bipolar
sandwich, the NPN and PNP
varieties
• The three layers of the
sandwich are conventionally
called the Collector, Base,
and Emitter
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NPN and PNP Transistors
• The vertical line represents the base (B), the angular line
with the arrow on it represents the emitter (E), and the
other angular line represents the collector (C)
• The direction of the arrow on the emitter distinguishes
(graphically) the NPN from the PNP transistor
• If the arrow points in, (Points iN) the transistor is a PNP
• On the other hand if the arrow points out, the transistor is
an NPN (Not Pointing iN).
NPN
PNP
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Transistor Configuration
• 3 basic configurations:
common emitter (CE),
common base (CB),
and common collector
(CC)
• common - denote the
element that is common
to both input and output
circuits
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To identify a specific transistor configuration
is to follow three simple steps:
1. Identify the element (emitter, base, or
collector) to which the input signal is
applied.
2. Identify the element (emitter, base, or
collector) from which the output signal is
taken.
3. The remaining element is the common
element, and gives the configuration its
name.
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Transistor as a switch
• If the circuit uses the transistor as a switch, then biasing is arranged
to operate on regions of the output curves known as saturation (red)
and cut-off (yellow).
• "cut-off" region - are zero input base current, zero output collector
current and maximum (supply rail) collector voltage
• "saturation“ - BJT will be biased, maximum base current is applied,
maximum collector current flow and minimum collector emitter
voltage.
•
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Figure 9.9
(a) Transistor symbols, (b), (c), (d), (e) transistor switch
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7.3.4: MOSFET
• Acronym for metal-oxide semiconductor fieldeffect transistor, a common type of transistor in
which charge carriers, such as electrons, flow
along channels
• The width of the channel, which determines how
well the device conducts, is controlled by an
electrode called the gate, separated from
channel by a thin layer of oxide insulation
• The insulation keeps current from flowing
between the gate and channel
• MOSFETs are useful for high-speed switching
applications and also on integrated circuits in
computers.
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MOSFET
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MOSFET and Motor Control
• This power MOSFET is
working at a higher switch
frequency of 20 kHz
• The MOSFET transistor in
this motor driver requires a
special driver circuit between
the PWM circuit and the
MOSFET itself because
switching the gate voltage of
the MOSFET transistor
requires high transient
current (2 A) due to relatively
high capacitive load
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7.4: Solenoid
• Solenoids are actuators capable of linear motion
• They can be electromechanical (AC/DC), hydraulic, or
pneumatic driven - all operating on the same basic
principles
• Give it energy and it will produce a linear force
• E.g. - for pushing buttons, hitting keys on a piano, valve
operators, and even for jumping robots
• DC solenoids operate on the same basic principles as a
DC motor
• The difference between a solenoid and a motor is that a
solenoid is spring loaded and cannot rotate.
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How Solenoids Work
• Inside a solenoid is
motor wire coiled in a
special way (see image
above)
• When an electric current
is sent through this wire
(energized), a magnetic
field is created
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• The inner shaft of a solenoid
is a piston like cylinder made
of iron or steel, called the
plunger or slug (equivalent
to an armature)
• The magnetic field then
applies a force to this
plunger, either attracting or
repelling it
• When the magnetic field is
turned off, a spring then
returns the plunger to its
original state
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Push and Pull Type Solenoids
• In pull type solenoids, the plunger is
normally outside the solenoid because the
spring naturally forces the plunger out. Yet
when energized, the force 'pulls' the
plunger into the solenoid
• Push type solenoids are the opposite, in
that the spring forces the plunger into the
solenoid, but when energized the plunger
is 'pushed' out.
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3/2 valve (cont.)
• 3/2-valve usually for controlling
single-acting cylinder
• Usually use poppet valve
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5/2 valve (cont.)
• 5/2-valve usually for controlling
double-acting cylinder
• Usually use slide valve
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Mechanical Considerations
• One very important thing to
consider with a solenoid is the
stroke
• The stroke distance (maximum
distance a plunger can travel) is
sufficient for your application
• Also able to handle the sudden
non-linear high speeds and high
forces expected from such an
actuator
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End of Lecture 8