PLC Programming Methods and Applications


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Principle of Programmable Logic Controller:

Gives students a broad knowledge base of sensors and insight into how to select and apply sensors. Shows students the importance of reliability and safety as part of the PLC system design process. Ladder Diagram Fundamentals. The Programmable Logic Controller. Fundamental PLC Programming.

PLC Programming, Programmable Logic Controller – A I C Solutions Ltd

Advanced Programming Techniques. Mnemonic Programming Code. Wiring Techniques. Discrete Position Sensors. Encoders, Transducers, and Advanced Sensors. Motor Controls. System Integrity and Safety. Programmable Logic Controllers provides the student with a general working knowledge of the various PLC brands and models.

Controlling Water Level in the PLC Ladder Logic Program

In the top rung the contacts are normally open and normally closed. Which means if input A is on and input B is off, then power will flow through the output and activate it. Any other combination of input values will result in the output X being off. The second rung of See A Simple Ladder Logic Diagram is more complex, there are actually multiple combinations of inputs that will result in the output Y turning on. On the left most part of the rung, power could flow through the top if C is off and D is on.

Power could also and simultaneously flow through the bottom if both E and F are true. This would get power half way across the rung, and then if G or H is true the power will be delivered to output Y. In later chapters we will examine how to interpret and construct these diagrams. There are other methods for programming PLCs. One of the earliest techniques involved mnemonic instructions. These instructions can be derived directly from the ladder logic diagrams and entered into the PLC through a simple programming terminal.

In this example the instructions are read one line at a time from top to bottom. The first line has the instruction LDN input load and not for input A.

This will examine the input to the PLC and if it is off it will remember a 1 or true , if it is on it will remember a 0 or false. The next line uses an LD input load statement to look at the input. If the input is off it remembers a 0, if the input is on it remembers a 1 note: this is the reverse of the LD.

The AND statement recalls the last two numbers remembered and if the are both true the result is a 1, otherwise the result is a 0. This result now replaces the two numbers that were recalled, and there is only one number remembered.

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The process is repeated for lines and , but when these are done there are now three numbers remembered. The AND in line combines the results from the last LD instructions and now there are two numbers remembered. The OR instruction takes the two numbers now remaining and if either one is a 1 the result is a 1, otherwise the result is a 0. This result replaces the two numbers, and there is now a single number there.

The last instruction is the ST store output that will look at the last value stored and if it is 1, the output will be turned on, if it is 0 the output will be turned off. Even if you have programmed a PLC with ladder logic, it will be converted to mnemonic form before being used by the PLC. In the past mnemonic programming was the most common, but now it is uncommon for users to even see mnemonic programs.

Sequential Function Charts SFCs have been developed to accommodate the programming of more advanced systems. These are similar to flowcharts, but much more powerful. To read the chart, start at the top where is says start. Below this there is the double horizontal line that says follow both paths. As a result the PLC will start to follow the branch on the left and right hand sides separately and simultaneously. On the left there are two functions the first one is the power up function. This function will run until it decides it is done, and the power down function will come after.

On the right hand side is the flash function, this will run until it is done. These functions look unexplained, but each function, such as power up will be a small ladder logic program. This method is much different from flowcharts because it does not have to follow a single path through the flowchart. Structured Text programming has been developed as a more modern programming language. This example uses a PLC memory location i. This memory location is for an integer, as will be explained later in the book.

The first line of the program sets the value to 0. The next line begins a loop, and will be where the loop returns to. The next line recalls the value in location i, adds 1 to it and returns it to the same location. The next line checks to see if the loop should quit.

If i is greater than or equal to 10, then the loop will quit, otherwise the computer will go back up to the REPEAT statement continue from there. Each time the program goes through this loop i will increase by 1 until the value reaches When a process is controlled by a PLC it uses inputs from sensors to make decisions and update outputs to drive actuators, as shown in See The Separation of Controller and Process. The process is a real process that will change over time.

Actuators will drive the system to new states or modes of operation. This means that the controller is limited by the sensors available, if an input is not available, the controller will have no way to detect a condition. The control loop is a continuous cycle of the PLC reading inputs, solving the ladder logic, and then changing the outputs. Like any computer this does not happen instantly.

When power is turned on initially the PLC does a quick sanity check to ensure that the hardware is working properly. If there is a problem the PLC will halt and indicate there is an error. For example, if the PLC power is dropping and about to go off this will result in one type of fault.

If the PLC passes the sanity check it will then scan read all the inputs. After the inputs values are stored in memory the ladder logic will be scanned solved using the stored values - not the current values. This is done to prevent logic problems when inputs change during the ladder logic scan. When the ladder logic scan is complete the outputs will be scanned the output values will be changed. After this the system goes back to do a sanity check, and the loop continues indefinitely. Unlike normal computers, the entire program will be run every scan.

Typical times for each of the stages is in the order of milliseconds.

PLC Ladder Logic vs. Everything Else

PLC inputs are easily represented in ladder logic. In See Ladder Logic Inputs there are three types of inputs shown. The first two are normally open and normally closed inputs, discussed previously. The IIT Immediate InpuT function allows inputs to be read after the input scan, while the ladder logic is being scanned. This allows ladder logic to examine input values more often than once every cycle. Note: This instruction is not available on the ControlLogix processors, but is still available on older models.

In ladder logic there are multiple types of outputs, but these are not consistently available on all PLCs. Some of the outputs will be externally connected to devices outside the PLC, but it is also possible to use internal memory locations in the PLC.


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Six types of outputs are shown in See Ladder Logic Outputs. The first is a normal output, when energized the output will turn on, and energize an output. The circle with a diagonal line through is a normally on output. When energized the output will turn off. This type of output is not available on all PLC types. When initially energized the OSR One Shot Relay instruction will turn on for one scan, but then be off for all scans after, until it is turned off. The L latch and U unlatch instructions can be used to lock outputs on.

When an L output is energized the output will turn on indefinitely, even when the output coil is deenergized. The output can only be turned off using a U output.

PLC Programming Methods and Applications PLC Programming Methods and Applications
PLC Programming Methods and Applications PLC Programming Methods and Applications
PLC Programming Methods and Applications PLC Programming Methods and Applications
PLC Programming Methods and Applications PLC Programming Methods and Applications
PLC Programming Methods and Applications PLC Programming Methods and Applications
PLC Programming Methods and Applications PLC Programming Methods and Applications
PLC Programming Methods and Applications PLC Programming Methods and Applications

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