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Ladder-logic Tutorial
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Relay Ladder Logic 39.1
Relay Ladder Logic ControlRelay Ladder Logic Control
►► Logic control is used with relatively simple ON/OFF Logic control is used with relatively simple ON/OFF systems systems -- like pneumaticslike pneumatics
PB-1 CR-1
CR-1A
CR-1B
LS-1
SOL-A
LS-1
Sol. A
120 VAC
Pneumatic System Relay Ladder Logic(RLL) Control
Normally Open SchematicsNormally Open Schematics
y p ( )
Limit Switch
MomentaryContactPushbutton
PressureSwitch
ManualSwitch
Contacts
Normally Closed SchematicsNormally Closed Schematics
Limit Switch
MomentaryContactPushbutton
PressureSwitch
ManualSwitch
Contacts
Output SchematicsOutput Schematics
SolenoidCoil
ControlRelay Coil
Lamp Annunciator(Horn)
Control Relay Control Relay -- Not ActivatedNot Activated
Common
NC contact
NO contact
no current throughcoil
"control relay" not activated
Vpower
No current thru CR
No current can flowthru NO and Common
Current can flowthru NC and Common
Control Relay Control Relay -- ActivatedActivated
Vpower
Common
NC contact
NO contact
current flowingin coil
"control relay" activated
Current thru CR
No current can flowthru NC and Common
Current can flowthru NO and Common
Relay Ladder Logic 39.2
Why is it called “Logic Control?”Why is it called “Logic Control?”
PB-1 LS-2 SOL-2
IF (PB-1 is pressed) AND (LS-2 is activated)THEN (SOL-2 will be turned on)
THEN currentwill flow throughthe output
IF there is continuity acrossthe inputs
““OR” ExampleOR” Example
LS-1
PB-1CR-6
IF (PB-1 is NOT pressed) OR (LS-1 is activated)THEN (CR-6 will be turned on)
““One Shot” One Shot” -- Single StrokeSingle Stroke
►►Pressing the pushbutton PBPressing the pushbutton PB--1 will cause 1 will cause the cylinder to extend and retract one timethe cylinder to extend and retract one time
Pneumatic System “Ladder” Logic Control
LS-1Sol. A
PB-1 CR-1
CR-1A
CR-1B
LS-1
SOL-A
120 VAC
““One Shot” One Shot” -- Step #1Step #1
►► Pressing the momentary contact pushbutton Pressing the momentary contact pushbutton PBPB--1 energizes the control relay CR1 energizes the control relay CR--11
LS-1Sol. A
PB-1 CR-1
CR-1A
CR-1B
LS-1
SOL-A
120 VAC
““One Shot” One Shot” -- Step #2Step #2
►► After control relay CRAfter control relay CR--1 energizes, normally 1 energizes, normally open contacts CRopen contacts CR--1A and CR1A and CR--1B activate1B activate
LS-1Sol. A
PB-1 CR-1
CR-1A
CR-1B
LS-1
SOL-A
120 VAC
““One Shot” One Shot” -- Step #3Step #3
►► Control relay CRControl relay CR--1 is now energized by a 2nd 1 is now energized by a 2nd path, solenoid SOLpath, solenoid SOL--A also activatesA also activates
LS-1Sol. A
PB-1 CR-1
CR-1A
CR-1B
LS-1
SOL-A
120 VAC
Relay Ladder Logic 39.3
““One Shot” One Shot” -- Step #4Step #4
►► PBPB--1 is released, but control relay CR1 is released, but control relay CR--1 is still 1 is still energized by the 2nd path (“hold” circuit)energized by the 2nd path (“hold” circuit)
LS-1Sol. A
PB-1 CR-1
CR-1A
CR-1B
LS-1
SOL-A
120 VAC
““One Shot” One Shot” -- Step #5Step #5
►► Solenoid A shifts the valve spool to the right, Solenoid A shifts the valve spool to the right, and the cylinder begins to extendand the cylinder begins to extend
LS-1Sol. A
PB-1 CR-1
CR-1A
CR-1B
LS-1
SOL-A
120 VAC
““One Shot” One Shot” -- Step #6Step #6
►► Cylinder activates the normally closed limit Cylinder activates the normally closed limit switch LSswitch LS--1, which “kills” the hold circuit for 1, which “kills” the hold circuit for control relay CRcontrol relay CR--11
LS-1Sol. A
PB-1 CR-1
CR-1A
CR-1B
LS-1
SOL-A
120 VAC
““One Shot” One Shot” -- Step #7Step #7
►► With control relay CRWith control relay CR--1 de1 de--activated, the activated, the contacts CRcontacts CR--1A and CR1A and CR--1B return to their 1B return to their normally open statenormally open state
LS-1Sol. A
PB-1 CR-1
CR-1A
CR-1B
LS-1
SOL-A
120 VAC
Solenoid SOL-A is no longer energized
““One Shot” One Shot” -- Step #8Step #8
►► CRCR--1B is now open, SOL1B is now open, SOL--A is deA is de--activated, activated, spring returns valve to default statespring returns valve to default state
LS-1Sol. A
PB-1 CR-1
CR-1A
CR-1B
LS-1
SOL-A
120 VAC
““One Shot” One Shot” -- Step #9Step #9
►► Cylinder begins to retract, and “rolls off” of Cylinder begins to retract, and “rolls off” of LSLS--1, which returns to its N.C. state1, which returns to its N.C. state
LS-1Sol. A
PB-1 CR-1
CR-1A
CR-1B
LS-1
SOL-A
120 VAC
