2 Chapter 2: LADDER DIAGRAM 3.1 Schematic symbols and drawing procedures3.2 Two-wire and Three-wire control circuit3.3 AC & DC Motor control circuits3.3.1 Start-Stop3.3.2 Hiccup3.4 Overload protection scheme
3 Introduction:What is Ladder diagram?Ladder diagrams are specialized schematics commonly used to document industrial control logic systems.It is a type of graphic language for automatic control systems.It is a drawing program of a switching circuit.It is a language or method to create a program. The first PLCs were programmed with a technique that was based on relay logic wiring schematics.Basic concept of this diagram is similar to the electrical wiring. However in ladder logic, the symbol has changed and been standardized.Called "ladder" diagrams because they resemble a ladder, with two vertical lines representing the power trails (supply power) and circuits are connected in horizontal lines called rungs. Connection between the element in the rung called link.*PLC = programmable logic controller
4 Basic concept of ladder diagram is similar to electrical wiring.
6 Ladder diagram operation principle InputInterfaceOutput+24V-0VBetween these two rails, a horizontal straight line was drawn with two symbols. These two symbols refer to the input and output devices, which are used in the actual process/system.On the left, we put all kinds of input. While on the right, we place all types of the outputs.Once we complete one line of the program it seems like a ladder. This horizontal line which places the input and output make one rung.
7 Ladder diagram symbols Ladder diagram uses standard symbols to represent the circuit components and functions found in a control system.
10 Ladder diagram application Conveyor control using Allen-Bradley PLC.
11 How do we know which relay contact is actuated by which relay coil? ActivityHow do we know which relay contact is actuated by which relay coil?
12 How do you think the system works? 12How do you think the system works?
13 Draw a simple ladder diagram for the circuit below. ActivityDraw a simple ladder diagram for the circuit below.
14 ActivityPredict how the operation of this motor control circuit will be affected as a result of the following faults. Consider each fault independently (i.e. one at a time, no multiple faults):"Stop" pushbutton switch fails open.Relay contact CR1-1 fails open.Relay contact CR1-2 fails open.Relay coil CR1 fails open.
15 "Stop" pushbutton switch fails open: Motor cannot start, lamp never energizes. Relay contact CR1-1 fails open: Motor starts and lamp energizes when "Start" button is pressed, but both immediately de-energize when it is released.Relay contact CR1-2 fails open: "Motor run" lamp turns on and off as expected, but the motor itself never runs.Relay coil CR1 fails open: Motor cannot start.
16 Digital Logic Functions Ladder diagrams are capable of performing logic functions similar to the logic gates used in digital electronics. We can construct simple logic functions for our electrical wiring diagram, using multiple contacts. There are 6 logic functions that can be developed using combination of switches:ANDORNOTNORNANDEX-OR (XOR)
17 Digital Logic Functions: AND Logic Solenoid coil is not energized unless both switch A & B are closed.
18 Digital Logic Functions: OR Logic Solenoid coil is not energized until either switch A & B is closed.
19 Digital Logic Functions: NOT Logic The logical inversion, or NOT, function can be performed on a contact input simply by using a normally-closed contact instead of a normally-open contact.
20 Digital Logic Functions: NAND Logic There is no output when both A and B have an input.
21 Digital Logic Functions: NOR Logic There’s no output when either A & B or both are opened.
22 Digital Logic Functions: XOR Logic There’s no output when both 1 & 2 have input.
23 ActivityWhat type of logic function behavior (AND, OR, NAND, or NOR) does the circuit exhibit? AND Logic
24 ActivityWhat type of logic function behavior (AND, OR, NAND, or NOR) does the circuit exhibit? AND Logic
25 ActivityWhat type of logic function behavior (AND, OR, NAND, or NOR) does the circuit exhibit? OR Logic
26 Process SequenceThere are several method to describe the process sequence of a ladder diagram:GrafsetMotion diagramDisplacement diagramSimple narrativeBoolean
27 Process sequence: Grafset Initial step / Reset = Double boxActionsConsequence of actions
28 Process sequence: Grafset ExampleSolution1st: Identify input & output LA – level ALB – level BES – empty switchTIM – timer contact (on for 5 mins)SB – start buttonINPUTVA – valve AVB – valve BVC – valve CH – heaterST – stirrerOUTPUTThe tank system has the narrative description below:Fill the tank to level A from valve A.Fill the tank to level B from valve B.Start a timer, heater and stirrer for 5 minutes.Open output valve C until the empty switch engages.
30 Process sequence: Motion Diagram The input and the output are separated with the input at the upper side and the output is at the bottom side.
31 Process sequence: Displacement Diagram Presented in form of graph indicates the position of the cylinders and the displacement.Step 1 : Cyl 1 and Cyl 2 extend.Step 2 : Cyl 1 retracts while Cyl 2 remains extend for 3 seconds.Step 3 : Cyl 1 remains retracts for 3 seconds and Cyl 2 retracts.Step 4 - Step 6 : The position of Cyl 1 and Cyl 2 similar as Step 1 – Step 3.
32 Process sequence: Simple Narrative ExampleA push button is used to turn on the conveyor belt A that brings the bottles to the filing line. Sensor 1 that is placed at the center of the conveyor A will stop the conveyor belt A upon detecting the bottle and open the valve for filing for 2 second. After that the conveyor belt will be on again until the bottle is sensed by sensor 2 at the end of conveyor belt A. Conveyor belt A once again stop and the solenoid is energized for 3 seconds in order for the piston to push the bottle to conveyor belt B. The process repeats until stop button is pressed.
33 Process sequence: Simple Narrative Solution1st: Identify input & output 2nd: Simplify statement Start buttonSensor 12 sec timer contactSensor 23 sec timer contactStop buttonINPUTConveyor beltValve2 sec timer coil3 sec timer coilSolenoid (piston)OUTPUTSB turns on conveyor A.Sensor 1 stops conveyor A, opens valve. Timer 1 on for 2 sec.After timer 1 times up (after 2 sec), valve closed and conveyor A on.Sensor 2 stops the conveyor, energizes solenoid (piston). Timer 2 on for 3 sec.Timer 2 times up (after 3 sec), solenoid de-energized and conveyor A on again.Stop button reset all.
34 Process sequence: Boolean Another method to describe process sequence.Is a logic expression using gates.SolutionExampleD = DoorD = LockD’ = UnlockS1 = Sensor 1S1 = SenseS1’ = Not senseS2 = Sensor 2S2 = SenseS2’= Not senseS2 = Sensor 2S2 = SenseS2’= Not senseSB = Start buttonSB = Pressed ,SB’ = Not pressedCNT = CounterRB = Reset buttonRB = ResetRB’ = Not resetThe door of the automatic train will be locked if start button is pushed and will unlocked when sensor 1 detects it’s arrival at station 1. Sensor 2 that is placed at the entrance door of the train will count the number of train passengers on that day.D = SB.S1’D’ = SB’.S CNT = S2.S1.RB’
35 Draw a Grafset and Motion diagram for the circuit below. ActivityDraw a Grafset and Motion diagram for the circuit below.
36 AC & DC Motor control circuit START – STOP circuitWhen PB is pressed, relay coil will energized and relay contact will close.When PB is depressed, relay coil is still energized since current can go through the relay contact unless disturbed by stop button.For most organizations, technical debt comes with the territory, an unavoidable outcome of decades of technology spend. Understanding, containing, and mitigating technical debt can be a platform, not only for a stronger IT foundation, but for a renewed level of trust and transparency with the business.Ladder diagram
37 VIBRATING / JOG circuit AC & DC Motor control circuitVIBRATING / JOG circuitFor most organizations, technical debt comes with the territory, an unavoidable outcome of decades of technology spend. Understanding, containing, and mitigating technical debt can be a platform, not only for a stronger IT foundation, but for a renewed level of trust and transparency with the business.When PB is pressed, output IR is activated.After several microsecond, the normally closed IR is open then it will stop the output IR.This set-up is applicable to move an object which needs to be precisely located.Ladder diagram