Example one To develop a Ladder diagram for the ON/OFF temperature controller for the continuous process of heating of mixture. Sequence: On button to start the process when motor start and V1 is open. Then check the level, is it high level is reached or not. If a high level is reached turn ON the stirrer & heater at a time. After mixing & heating turn OFF the stirrer & heater. Finally, release this mixture through valve V2. PLC Programs Example two Develop a Ladder program for wood saw cutter. A wood saw cutter (W), a fan (F) and a lubrication pump (P) turns on when a START push button is pushed. Following is the stopping sequence: STOP push-button stops the saw only before fan stops it should run for an additional five sec to blow the chips away and also the tube pump is to run for eight sec. Additionally, before it stops. if the saw has run more than one min, the fan should stay ON indefinitely . The fan may be turned OFF by pushing a separate fan reset switch . Example three Develop the Ladder logic to fill the tank. Fill the tank up to 80%. When the tank is filled, turn ON the heater to raise the temperature up to seventy deg. when this temperature is reached, turn OFF the heater & open the outlet valve. When the level in the tank falls below 10%, close the output valve. PLC Programs Example four Develop Ladder logic for traffic control . We have to control the traffic of four roads. The green light of each side is over for ten sec only one side at a time and the same time, the respected red light is ON for 30sec. The same sequence is followed by each side. PLC Programs Example five Develop Ladder logic for an automatic paint process. When the ON button is pushed, a stacker starts stacking plywood sheet at position A. stack height is controlled by PLC counter function. When twelve parts (stack) are placed on the conveyor, it turns ON and moves the stack at the position B. A sensor is used to stop the conveyor at B. At point B, paint is applied for 12 sec after painting is completed, the conveyor restart s manually and moves to position C and stops automatically and stack is removed. Sequence: ON button is used to start the process and stacker starts stacking. The sensor is used to count the no . of sheets. After twelve part stacking, conveyor starts and moves the stack to position B. when it reaches position B, the sensor stops the conveyor by sensing the position . Paint is applied to 12.5 seconds duration. After painting, the conveyor starts manually and moves the stack to position C and stops automatically. PLC Programs Example six Develop Ladder logic for Heat/Quench station . Steps: Master PB is used to turn ON the process. The object is placed on the mandrel Both left & right start-up buttons are pressed. Push stop PB to stop the process. The object is raised from down to top by air pressure. A solenoid valve supplies this air to the pneumatic elevating cylinder. The lower limit switch is getting actuated before the object will rise. Lower limit switch actuation indicates that there is an object on the mandrel and that mandrel is down. The mandrel makes contact with a limit switch at the top of travel. The heat comes ON for ten sec and goes OFF. Quench comes ON for eight sec . The object returns down and the upper limit switch becomes indicated. As it gets down, the down limit switch gets actuated. The object is then removed. Example seven To develop the Ladder diagram using analog input for a multi-input tank level controller. Sequence: Add component (A) to tank up to level A. then add component (B) to tank up to level B. Stop feeding tank. Turn above stirrer for ten sec. Open discharge valve, when level again goes to A. Stop the process PLC Programs Example eight Develop Ladder logic for the coal handling process. Process: The coal plant consist of the following main equipment Storage tank: It stores coal in a huge amount & supplies it to the primary crusher as per requirement. Primary crusher: It crushes the coal from a bigger size to smaller size & supplies it to secondary crusher. Secondary crusher: Two crushers are used to decrease a load of crushing .secondary crusher crushes the small-sized crushed coal coming from the primary crusher to powdered form. Coal mill: This is a mixture that mixes coal powder coming from secondary with hot air coming from air pre-heater & forced it into the furnace. Sequence: When we press START, it starts motor M1 & opens the C1 which passes coal from storage tank to primary crusher. When level ML1, reaches O: 0/1 wills ON, it starts motor M2 & M3, which crushes coal in small part & pass to the secondary crusher. When level ML2 reaches O : 0/2 wills ON, it starts coal mill, starting motors M4, M5, M6 & M7. Example nine Develop Ladder logic for the automatic door opening of the room. Process: The process includes the following sequence of events. When the start PB is pressed, the RTD will sense the temperature of the room. This will be considered as I/P to the analog input of PLC. When the temperature of the room is greater than two hundred and fifty c, then AC-I will be latched. When the temperature of the room is greater than three hundred and fifty c, then AC-II will be latched. When the temperature of the room is below two hundred c, then Ac-I will be unlatched. If the temperature of the room is less than three hundred c, AC-II will be unlatched. When a person enters or leave the room, then laser beam contact will be break & this gives the change in the LDR, the door will open for 5 sec. PLC Programs Example ten Develop the Ladder program for the process of alternate rotation of the motor. Process: – Here, the motor with two switches is used as shown. When the start button is pushed, the motor starts rotating towards the RLS and LLS. Sequence: When ON button is pushed, the motor move the arm to the right limit switch position and waits for thirty sec. The motor then cycles seventy-five times between the right to left switches and then stops. The OFF button stops the motor at any time or after 75 cycles have been made. PLC Programs Example 11 To develop the Ladder program for an oven using the process bits. Process: – Process sequence is as follows When the START button is pressed, the valve V1 is turned ON till HL (high level) is reached. After HL is reached, V1 will close and heater and indicators are turned ON for thirty sec. After thirty sec, valve V2 will turn ON for twenty sec to draw the hot water from the tank If the STOP button is pressed during any time of the process, the heater will be turned OFF but V2 output will be kept ON for twenty sec. After this, the process will stop. Example twelve Develop Ladder logic for an automated hydrophobic system. PLC will be used to provide the following sequence of light and nutrients. Process: The growing plant (Vat) must have light . A photo-switch which closes on rising light level, monitors natural light during this period. If the light intensity falls below the switch level for more than 10 sec an artificial light is turned on until natural light brightness again. During this period, the nutrient must be cycled into the plant. Valve V1 will turn ON till level L1 is reached. After that, valve V1 will close and nutrients must stay there for twenty sec. After that pump will start to drain the nutrients again to the tank, till level L2, the pump will stop and again V1 will start. Example 13 To develop Ladder logic or the heating process. Sequence: When the START button is pressed, the valve V1 is turned ON till the high level (HL) is reached. After HL is reached, valve V1 will close and heater and indicator turn ON for thirty sec. After thirty sec, valve V2 will turn ON for twenty sec to draw the hot water from the tank. If STOP input is activated during any time, the heater will turn OFF but the valve V2 will be kept ON for twenty sec and after this, the process will stop. Example 14 To develop the Ladder program for the process of wrapping the chocolate boxes. A machine is designed to wrap the chocolate boxes. The boxes are arriving on the conveyor belt. Sequence: The presence of the box is determined by the optical sensor (P). After this, the conveyor is stopped (C) and the box is clamped in place (H) . A wrapping machine (W) is turned ON for ten sec . A sticker cylinder (S) is turned ON or five sec to put the sticker on the box . The clamp (H) is turned OFF and the conveyor C is turned ON . After the box leaves, the system returns to an ideal state Example 15 To develop the Ladder program for the process in which the tank is filled 80% by analog input. The heater starts for a particular time and the output valve opens when 10% of the water level remains. Then again feed the water in the tank. Process: In this process, when the start button is pushed, the tank is starting to fill by inlet valve V1. When the level in the tank reaches 40% by analog input, valve V1 closes, valve V2 opens and fills the tank till level reaches 80% then heater starts for a pre-determined time. When the temperature reaches sixty deg, then heater stops and outlet valve open. When the level in the tank reaches 10%, then the inlet valve V1opens and outlet valve is closed. Example 16 To develop the Ladder diagram and mimic the diagram for the bottle filling process. In this process, the conveyor belt moves the bottles on it . When the bottle is sensed by the bottle present sensor, the conveyor motor stops, and the valve is opened for filling the bottle. When the bottle full indicator is ON, the valve gets closed and conveyor motor starts. Again next bottle is sensed by the sensor and the whole process goes on. PLC Programs Example 17 To draw the Ladder diagram and mimic the diagram for the automatic metal cutting process. In this process, a metal sheet is allowed to move on the conveyor belt. When the metal sheet is sensed by the sensor, a holder holds the sheet at that instant and at the same time cutter cuts the sheet at the desired place. When cutter cuts the sheet, then sheet roll is released and the cut sheet is passed towards the tray which carries that sheet. The whole process repeats until the STOP button is pressed. Author: Tushar Bhaskarwar Here is the Guide on Programmable Logic Controllers. PLC Guide
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