How do you use a ladder logic diagram to represent the logic and sequence of operations in a control system?
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A ladder logic diagram is a type of schematic diagram that resembles a ladder, with two vertical rails and horizontal rungs. Each rung represents a logical operation or a control action, and each rail is connected to a power source. The power flows from left to right along the rungs, and the output devices are activated when the power reaches them. The input devices, such as switches, sensors, and timers, are usually shown on the left side of the rung, and the output devices, such as motors, valves, and lights, are usually shown on the right side of the rung.
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Say you have a 120VAC ladder Phase & N. There is a condition or set of conditions on the left that have to become true to close the circuit and energize the 120V solenoid to the right.
Ladder logic diagrams use basic symbols to represent devices and operations. A line stands for a wire or connection between devices, while a contact is an input device that can be either normally open (NO) or normally closed (NC). A coil represents an output device that can be energized or de-energized when power is interrupted or received. Finally, a box symbolizes a function block with inputs and outputs that can execute specific operations such as arithmetic, logic, or timing.
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You may have a multitude of series (ANDed) &/or parallel (ORed) logic for a complex permissive but you may have only one coil or output, never more than one in series. A coil output in parallel is a duplicate (as in an indicator lamp for a pump start coil). A contact of a coil feeding itself latches the coil closed. Series logic has to be inserted to intervene to unlatch it. This is the basis for many a motor start/stop circuit or latching a transient alarm even if it cleared itself.
A ladder logic diagram follows a few core rules. Firstly, power flows from left to right along the rungs and from top to bottom along the rails. Input devices can be connected in series or parallel, with series requiring all devices to be activated, and parallel needing only one. Output devices can also be connected in series or parallel, with parallel allowing them to activate independently, and series activating them sequentially. Lastly, function blocks can connect to other devices through internal or external variables, with internal variables being local to the function block and external variables being global to the program.
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A processor scans the ladder from left to right and top to bottom. Once it executes the entire program scan, it starts from the beginning. If you erroneously had one set of logic conditions close a coil in the program and another set open it, you shall encounter chattering of that solenoid relay, called a race condition. Document your rungs, coils and contacts in plain english, using the correct tag numbers so people can follow your thoughts when they troubleshoot the ladder logic, and tally your contacts and coils to field instruments and devices
Creating a ladder logic diagram requires you to identify the input and output devices involved in the control system and assign them names or addresses. Furthermore, you need to recognize the logic and sequence of operations that are necessary for the control system, breaking them down into simple steps or conditions. Then, draw the ladder logic diagram using the appropriate symbols and rules, connecting the input and output devices according to the logic and sequence of operations. Finally, test and debug the ladder logic diagram using a simulator or a real PLC, adjusting any necessary changes or corrections.
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In the good old days when memory was scarce or when programmers did not want to replicate identical sequences of code for say five production lines on a single PLC, they would use a single routine with indirect addressing. Personally I find this very hard to troubleshoot and avoid using this wizard technique that comes easy to many chess like skill endowed ladder software coders.
Ladder logic diagrams are used to control common systems. For example, a motor control system can be started and stopped with two pushbuttons and an overload protection device, a conveyor belt system can move objects with the help of a sensor and solenoid valve, and a traffic light system can alternate between red, yellow, and green lights with the use of timers. The timers control the duration of each light, and the lights change in a fixed order.
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ACCU500, small parts counter auto bagger, Boss wanted the machine to be fully automated, manufacturer stated cant be done-not built for automation. I pictured in my head a working idea from start to finished. Built my idea and worked excellent.
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