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Logic Control
What is Logic control
• Logic control is a control based on a logic
concept, that is the on-off state of variable
and/or equipment
• Logic control is often used to control
combinational and/or sequential events such as
lift control, automatic production line, engine
start-up, etc.
• Originally used device such as switches, relay,
timer, drum, and any other mechanism to enable
changes of the on-off state
SWITCHES
Toggle Hand Switches
~
Single pole single throw (SPST)
Toggle Hand Switches
~
Single pole double throw SPDT switches
Toggle Hand Switches
DPST
DPDT
Hand Switches
3PST
Rotary
Swtich
Push button Hand Switches
Normally open
NO
Normally close
NC
Push-Push Switch
• This looks like a momentary action push switch but
it is a standard on-off switch:
– push once to switch on,
– push again to switch off.
• This is called a latching action.
Microswitch
• usually SPDT
• Microswitches are designed
to switch fully open or
closed in response to small
movements.
• They are available with
levers and rollers attached.
Keyswitch
• A key operated
switch.
• The example shown
is SPST.
Reed Switch
• Usually SPST
• The contacts of a reed switch
are closed by bringing a small
magnet near the switch.
• They are used in security
circuits, for example to check
that doors are closed.
• Standard reed switches are
SPST (simple on-off) but SPDT
(changeover) versions are also
available.
• reed switches have a glass
body which is easily broken!
DIP Switch
• DIP = Dual In-line Parallel
• This is a set of miniature
SPST on-off switches, the
example shown has 8
switches.
• The package is the same
size as a standard DIL
(Dual In-Line) integrated
circuit.
• This type of switch is
used to set up circuits,
e.g. setting the code of a
remote control.
Multi-pole Switch
• The picture shows a 6pole double throw switch,
also known as a 6-pole
changeover switch.
• It can be set to have
momentary or latching
action.
• Latching action means it
behaves as a push-push
switch, push once for the
first position, push again
for the second position
etc.
Multi-way Switch
• Multi-way switches have 3 or more conducting positions.
They may have several poles (contact sets). A popular
type has a rotary action and it is available with a range of
contact arrangements from 1-pole 12-way to 4-pole 3
way.
• The number of ways (switch positions) may be reduced
by adjusting a stop under the fixing nut. For example if
you need a 2-pole 5-way switch you can buy the 2-pole
6-way version and adjust the stop.
Process Operated Switches
• These switches is constructed using one of the
above switches. A process variable will initiate a
displacement to switch the switch
–
–
–
–
–
–
–
Limit switch
Proximity switch
Pressure switch
Level switch
Temperature switch
Flow switch
etc
SWITCH CAPACITY
• On a switch usually there is a label informing the voltage
and current capacity, e.g.:
250 V
5A
• It means that:
– the maximum current allowed to pass the switch is 5 A.
– The maximum voltage across its terminal allowed is 250 volt
I<5 A
~
~V<250 V
RELAY
Relay
NC contact
NO contact
coil
RELAY
• A relay is an electrically
operated switch.
• Current flowing through the coil
of the relay creates a magnetic
field which attracts a lever and
changes the switch contacts.
• The coil current can be on or off
so relays have two switch
positions and they are double
throw (changeover) switches.
• Relay consist of coil and contact
• Usually a relay has 1 coil and
many contacts both NO and NC
Relay
Picture is downloaded from www.kpsec.freeuk.com/components/relay.htm
Relay
•
•
•
•
In electrical diagram relay is symbolized as shown
A relay can have many contacts both NO and NC
The coil of a relay typically passes 30mA for a 12V relay,
The contacts can drive 5A or more depending on the size of
relay
contacts
coil
NO
NO
NO
NO
NC
NC
RELAY SYMBOL WITH 8 CONTACTS
NC
NC
Relay
30 mA
R12
R11
5A
R1
12 V
~ 220V
• Relays allow one circuit to switch a second circuit
which can be completely separate from the first.
• For example a low voltage battery circuit can use a
relay to switch a 220V AC mains circuit.
• There is no electrical connection inside the relay
between the two circuits, the link is magnetic and
mechanical. N
Ladder diagram
Ladder Diagram
• To make such as previous diagram easier
to read a ladder diagram is used
+
S
R1
R11
Basic logic
+
AND LOGIC
s1
s2
L
Lamp L will light if switch s1 and s2 are on.
In logic on usually symbolized as 1 and off as 0.
s1
0
s2
0
L
0
0
1
0
1
0
0
1
1
1
Mathematically written as
L = S1 AND S2
-
Basic logic
+
OR LOGIC
s1
L
s2
Lamp L will light if switch s1 OR s2 are on.
s1
0
s2
0
L
0
0
1
1
1
0
1
1
1
1
Mathematically written as
L = S1 OR S2
-
Basic logic
NOT LOGIC
+
s1
R1
R11
L
Lamp L will light if not R1 is on
R1
0
L
1
1
0
Mathematically written as
L = NOT(R1)
-
Combinational logic
Suppose you want to design a safe car with the following criteria:
The gear box (GB) will not engage unless:
1. The hand brake (HB) is released and the doors (D1-D4) are locked or
2. The safety system is disable by switching on override switch (OS) for
maintenance purpose
Mathematically the above logic is written as
GB = (HB AND D1 AND D2 AND D3 AND D4) OR OS
HB
D1
D2
D3
D4
GB
OS
Motor Start Stop (sequential logic)
• The following ladder diagram is used to
switch a motor on and off
S1
S2
R1
start
stop
R11
R12
R13
motor
Latching action
R14
Auto start of water pump
• Suppose that the motor is
used to drive water pump
and we want that the
pump can run or stop
automatically depending
on the water level
• In addition we also want
to override the automatic
control using manual start
and sop control
LS
Auto start of water pump
• Off, manual and auto motor control
O
M
A
S2
S1
R1
start
stop
motor
R11
LS
Permissive circuits
• Often it is desired that a piece of equipment is
allowed to start if several conditions are met.
• For example overload switch and over
temperature switch must be closed in order the
motor can be started
• Each process condition is called a permissive,
and each permissive switch contact is wired in
series, so that if any one of them detects an
unsafe condition, the circuit will be opened.
Auto start of water pump with
protection
• Suppose we want to protect the motor against over load
and over temperature
Permissive
circuits
O
M
S0
A
S2
S1
R1
start
stop
OL
R11
LS
motor
OT
Interlock circuits
• Often it is desired that only one piece of
equipment is allowed to start if all other
equipments are in off condition.
• For example push button circuit used in Quiz
show program where several contestant have to
answer a question.
• The first one who pushes the push button will
disable the other’s push button switch
• This circuit is called interlock since acting one
circuit will lock the others to function
Push Button In Quiz Show program
A
B
R12
R21
R11
R1
R2
LA
R22
LB
R13
R23
Push Button In Quiz Show program
A
R21
R31
B
R11
R31
C
R11
R21
R12
R12
R22
R13
R23
R33
R1
R2
R3
LA
LB
LC
Push Button In Quiz Show program
Instead of pushing
the PB continually
it is desired that
just pushing once
is enough for the
contestant to claim
that they are the
first team pushing
the button
The presenter must
push the reset
button to reset the
system back to
original state
Reset
A
R21
B
R11
R1
R12
R2
R22
R13
R23
R13
R23
LA
LB
Interlock
Another example of interlock is the forward circuit of motor
must prevent the reverse circuit, otherwise the motor will
damage
Note:
Motor contactor (or "starter") coils are typically designated by the letter "M"
in ladder logic diagrams.
Time delay relay
• If the motor is carry a high inertia load it is dangerous to reverse the
direction of the motor instantaneously.
• Time delay relay can be installed to prevent such occurrence to
happen
Fail safe design
• Consider an alarm system
as shown.
• It can be design in 2 ways
• Both ways work exactly in
the same manner
• The second design
however gives fail save
design.
• Murphy’s law is true. If
something can go wrong
it will.