Transcript FINAL CONTROL ELEMENT

FINAL CONTROL ELEMENT
FINAL CONTROL ELEMENT
• The final control element adjust the
amount of energy/mass goes into or out
from process as commanded by the
controller
• The common energy source of final control
elements are:
– Electric
– Pneumatic
– Hydraulic
ELECTRIC FINAL CONTROL
ELEMENT
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Electric current/voltage
Solenoid
Stepping Motor
DC Motor
AC Motor
CHANGING CURRENT/VOLTAGE
• Current or voltage can be easily changed to
adjust the flow of energy goes into the process
e.g. in heating process or in speed control
• Heater elements are often used as device to
keep the temperature above the ambient
temperature. Energy supplied by the heater
element is
W = i2rt (i=current, r=resistance, t=time)
• Motor is often used as device to control the
speed
CHANGING CURRENT/VOLTAGE
• Using
– Potentiometer
– Amplifier
– Ward Leonard system
– Switch (on-off action)
Changing Current/Voltage
Using Rheostat
I = V/(R1+R2)
Rheostat
Heater
R1
Power at rheostat
P1 =I2R1
V
R2
Power at heater
P2 =I2R2
I
Disadvantage loss of
power at rheostat
Example of Heating elements
Changing Current/Voltage
Using Amplifier
Potentiometer
V+
Heater
R1
V
amplifier
R2
V−
Disadvantage loss of power at potentiometer (very small)
and at Amplifier
Changing Current/Voltage
Using Ward Leonard System
• Introduced by Harry Ward Leonard in 1891
• Use a motor to rotate a generator at constant speed
• The output of generator voltage is adjusted by changing
the excitation voltage
• Small change in excitation voltage cause large change in
generator voltage
• Able to produce wide range of voltage (0 to 3000V)
• Ward Leonard system is popular system to control the
speed of big DC motor until 1980’s
• Now a days semi conductors switches replaces this
system
Changing Current/Voltage
Using Ward Leonard System
excitation
voltage
MOTOR
GENERATOR
Changing Current/Voltage
Using Switch
• The switch is closed and opened repeatedly
• No power loss at switch
VL
Switch
Switch closed
V
V
VL
LOAD
t
Switch opened
DUTY CYCLE
VL
• T is period time typical in
millisecond order (fix)
• Ton is switch on time (adjustable)
• Toff is switch off time
V
Ton
Toff
T
t
Duty Cycle is:
(Ton/T) 100%
• Of course we can not use mechanical switches to carry
on this task, electronic switches to be used instead.
• E.g. Transistor, Thyristor, or IGBT
• This methods is often called as Pulse Width Modulation
(PWM)
SOLENOID
• When the coil is energized the core will be
pulled in
core
coil
core
coil
SOLENOID
SOLENOID
• When the coil is energized the core will be
pulled in
V
SIMULATE
SOLENOID
• When the coil is energized the core will be
pulled in
V
SIMULATE
SOLENOID
Tubular solenoid
Open frame solenoid
Rotary solenoid
Solenoid
Solenoid Usage
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pushing buttons,
hitting keys on a piano,
Open closed Valve,
Heavy duty contactor
jumping robots
etc
STEPPING MOTOR
The top electromagnet (1) is turned on,
attracting the nearest teeth of
a gear-shaped iron rotor. With the
teeth aligned to electromagnet 1, they
will be slightly offset from
electromagnet
The top electromagnet (1) is turned
off, and the right electromagnet (2)
is energized, pulling the nearest
teeth slightly to the right. This
results in a rotation of 3.6° in this
example.
STEPPING MOTOR
The bottom electromagnet (3) is
energized; another 3.6° rotation occurs.
The left electromagnet (4) is
enabled, rotating again by 3.6°.
When the top electromagnet (1) is again enabled, the teeth in the
sprocket will have rotated by one tooth position; since there are 25
teeth, it will take 100 steps to make a full rotation in this example.
STEPPING MOTOR
• Practical stepping motor can be controlled
for full step and half step.
• Common typical step size is 1.8o for full
step and 0.90 for half step
• Full step is accomplished by energizing 2
adjacent electromagnet simultaneously.
• Half step is accomplished by energizing 1
electromagnet at a time.
Stepping motor
DC Motors
Every DC motor has six
basic parts –
axle,
rotor (a.k.a., armature),
stator,
commutator,
field magnet(s),
For a small motor the magnets is
made from permanent magnet
and brushes.
2 pole motor
Animate
3 pole DC motors
1
The coil for each poles are
connected serially.
2
3
+
−
The commutator consist of
3 sector, consequently
one coil will be fully
energized and the others
will be partially energized.
DC motors
• As the rotor is rotating,
back emf (Ea) will be
produced, the faster the
rotor turn the higher Ea and
the smaller Ia.
• The starting current of
motors will be much higher
then the rating current.
Ia
V
Ea
motor
DC motors
Field winding
Armature winding
For big motors the magnet is
made from coil and core. The
current flowing in the coil is called
If and the current flowing in the
armature is called Ia.
The armature winding and the
field winding are connected to a
common power supply
The armature winding and the
field winding are often connected
in series, parallel, or compound.
The torque characteristic will be
different for each connection.
The figure shows a parallel
connection
SERIES DC MOTOR
Field and armature winding are
series connected, this type of
motor is called series DC motor
DC motors
Field and armature winding are
parallel connected, this type of
motor is called shunt DC motor
DC MOTOR
Compound DC motor is DC
motor having 2 field winding
the first one is connected
parallel to the armature
winding and the other is
connected series
DC MOTOR
Torque: T = KΦIa
– K is a constant
– Φ magnetic flux
– Ia is armature current
• Magnetic flux is constant if it is from
permanent magnet
• It is depend on the If if it is produced by
current
DC MOTOR TORQUE-SPEED
CURVE
Torque: T = KΦIa
SERIES DC MOTOR TORQUESPEED CURVE
Torque:
T = KΦIa
T= KIa2
SHUNT DC MOTOR TORQUESPEED CURVE
Torque: T = KΦIa
COMPOUND DC MOTOR
TORQUE-SPEED CURVE
SYNCHRONOUS AC MOTOR
N
The rotating field.
When alternating current is
applied to the field coil the
magnetic field will also
alternating. Therefore the
permanent magnet will
rotate
S
o
311
-311
~
THREE PHASE SYNCHRONOUS AC MOTOR
R S T
S
R
S
T
N
4 pole 3Φ motor
R
S
T
SYNCHRONOUS AC MOTOR
SYNCHRONOUS AC MOTOR USING
EXTERNAL EXITER
R
S
T
The magnetic flux of permanent magnet is low for a bigger
motor we have to use externally exited magnetic field
ASYNCHRONOUS AC MOTOR
• When instead of exited, the rotor coil is
shorted an induced current will be
generated and the rotor will be
magnetized and start to turn.
• The faster the speed the smaller the
induced current and finally the current
will cease at synchronous speed and so
does the rotation
• This motor will turn at speed less the its
synchronous rotation that is why it
called asynchronous motor
• This motor is also called induction motor
Iinduced
Calculating Motor Speed
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A squirrel cage induction motor is a constant speed device. It cannot
operate for any length of time at speeds below those shown on the
nameplate without danger of burning out.
To Calculate the speed of a induction motor, apply this formula:
Srpm = 120 x F
P
Srpm = synchronous revolutions per minute.
120 = constant
F
= supply frequency (in cycles/sec)
P
= number of motor winding poles
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Example: What is the synchronous of a motor having 4 poles connected to
a 60 hz power supply?
Srpm = 120 x F
P
Srpm = 120 x 60
4
Srpm = 7200
4
Srpm = 1800 rpm
Calculating Braking Torque
• Full-load motor torque is calculated to determine the required
braking torque of a motor.
To Determine braking torque of a motor, apply this formula:
T = 5252 x HP
rpm
T
= full-load motor torque (in lb-ft)
5252 = constant (33,000 divided by 3.14 x 2 = 5252)
HP = motor horsepower
rpm = speed of motor shaft
• Example: What is the braking torque of a 60 HP, 240V motor
rotating at 1725 rpm?
T = 5252 x HP
rpm
T = 5252 x 60
1725
T = 315,120
1725
T = 182.7 lb-ft
Calculating Work
• Work is applying a force over a distance. Force is any cause that
changes the position, motion, direction, or shape of an object. Work
is done when a force overcomes a resistance. Resistance is any
force that tends to hinder the movement of an object.If an applied
force does not cause motion the no work is produced.
• To calculate the amount of work produced, apply this formula:
• W=FxD
• W = work (in lb-ft)
F = force (in lb)
D = distance (in ft)
• Example: How much work is required to carry a 25 lb bag of
groceries vertically from street level to the 4th floor of a building 30'
above street level?
• W=FxD
W = 25 x 30
W = 750 -lb
Pneumatic Actuator
Pneumatic Actuator
Reverse-Acting
Actuator
I/P Converter
• A "current to pressure" converter (I/P) converts an analog
signal (4-20 mA) to a proportional linear pneumatic output (315 psig).
• Its purpose is to translate the analog output from a control
system into a precise, repeatable pressure value to control
pneumatic actuators/operators, pneumatic valves, dampers,
vanes, etc.
Air supply
30 psi
Current
4 to 20 mA
I/P
Pneumatic 3 to 15 psi
Supplied to actuator
Sample of I/P Converter
Generation and distribution
of pneumatic pressure
• Compressor is needed for pneumatic system
PC
PS
Regulator valve
Tank
To I/P
compressor
100 psi
30 psi
Hydraulic Actuator
Hydraulic Actuator
Hydraulic Actuator
Advantage
ELECTRIC
PNEUMATIC
HYDRAULIC
Accurate position
Suit to advance
control
No tubing
Inexpensive
Fast
No pollution
No return line
No stall damage
Large capacity
Locking capability
Self lubricating
Easy to control
Smooth operation
Low accuracy
Noise pollution
Difficult speed control
Need infrastructure
Expensive
Leakage problems
Difficult speed control
Need return line
Need infrastructure
Disadvantage
Low speed
Expensive
Unsafe
Need brake
overheating