The PLC Programming - Report Example

PLC programming (Software and Hardware) Table of Contents Table of figures 3 PLC programming (Software and Hardware) 5 Electrical ladder diagram 5 Ladder relay instructions 7 Ladder Logic 8 AND logic 9 OR logic 9 NAND Logic 10 NOR logic 11 Latch and unlatch outputs coils 12 Transition contact 14 Time instructions 15 Address assignment 16 Compare instructions 18 PLC examples 19 References 22 Table of figures Figure 1: Simple PLC program 6 Figure 2: PLC program showing input NO and NC contacts and output coils 6 Figure 3: AND Logic 9 Figure 4: OR logic 10 Figure 5: NAND logic 11 Figure 6: NOR logic 12 Figure 7: Latched output coil 13 Figure 8: Latched output coil 13 Figure 9: Latched output coil with set instruction 14 Figure 10: Positive Transition Contact 14 Figure 11: Negative Transition contact 15 Figure 12: Timer Example delay ON 15 Figure 13: Timer example delay OFF 16 Figure 14: Addition Arithmetic function 17 Figure 15: Subtraction Arithmetic function 18 Figure 16: Compare instruction example 19 Figure 17: sample program to control fan 20 Figure 18: Sample Program for car parking 20 Figure 19: sample program for water pumping 20 Figure 20: sample program for reverse and forward motion of three phase motor 21 PLC programming (Software and Hardware) Introduction PLCs are used in various industries to operate various machines safely and continuously. It is important to distinguish that PLCs are faster and can be programmed in any manner to run an industrial machine in any manner. It provides solutions for the continuous operation for any industrial machine. PLCs are considered best solution for the automation. While thinking about the automation, PLCs are considered the best solution. Electrical ladder diagram Electrical ladder diagram is supplied with nearly every electrical machine. It is a tool to evaluate the problems with the machines while troubleshooting the errors on the machine. An electrical ladder diagram consists of numerous electrical schematic symbols that are linked linearly to develop a diagram. An electrical ladder diagram has two basic portions. One portion is the power sections and the other portion is the control sections of the diagram. One line electrical ladder diagram is supplied with the machines by the manufacturers to troubleshoot the problems, if machine shows abnormal behavior of any kind (Rockwell Automation). In programmable logic controllers (PLCs) electrical ladder diagram is important to make ladder logic according to the requirements of operation. Figure 1: Simple PLC program In the above simple sample program, a normally open (NO) relay contact is used to turn on the lights or output coil. The program shows that I: 1 is the input at terminal 1; I denoted the input. On the other hand, O: 0 is the output at output terminal 0. O is used for the output. Different PLCs manufacturers have different programming software. Different software use different indications for the input and output coils. Figure 2: PLC program showing input NO and NC contacts and output coils In the above example, a different program is used for the programming. In this program, physical input relays are termed as X0, X1, X2 and X3, where X0 and X2 are normally open (NO) relays’ contacts when PLC starts, and X1 and X3 are normally closed (NC) relays’ contacts. In the similar manner, Y0, Y1 Y2 and Y3 are physical output coils, where Y2 and Y3 are normally closed output coils. When the PLC starts, the Y2 and Y3 coils remain ON. Here the words normally open refers to a relay does not completes the circuit, when not energized. And NC relay is the relay that completes the circuit when it is not energized. In most of the PLC ladder logic programs either “] [“or “| |” symbols is used for the input relays contacts and “( )” is used to denote the output coil. Ladder relay instructions Different PLC ladder logic programs use different sett of instructions but in most of the programs the ladder logic remains the same. The symbols for each of the functions are given at the menu bar, where user can select the physical input relay contacts or other functions according to the requirements. Let’s consider a program to run a motor through PLC with the help of push button. The motor runs till the push button is pressed. In the program used for “Delta PLC”, termed as “WPL soft” writes the program as: When the physical input at Xo is closed by applying 24 volts from the output of the PLC supply output to the physical input Xo through a push button, the out put coil gets energized. It can be seen on the physical PLC device. The indicator LED light of the specific input and out blinks when it is energized. It is better to use the contacts of the output coil and connect it another relay since it internal coil contacts of the PLC are unable to pass high current. The output coil also works as a NO contact. But when the program allows it to operate, it works according to the program. In the similar manner the program can be extended to run more than one motor. In the below program, when the push button is pressed, the out put is attained, or the motor connected to the output runs. The above program can be altered to make an inverse logic meaning that the output coil is initially closed with respect to the input relay contacts, but when the input relays are energized, the output coil gets open. Ladder Logic Ladder logic corresponds to the logic of a program to run machine simple ladder logic is based on AND logic, OR logic, NOR logic, NAND logic, Ex OR logic, or other combination for AND-OR logic or other logic combinations. The output is respect to the truth table. AND logic AND logic is the logic in which the NO input contacts are arranged in series.\ Figure 3: AND Logic The truth table for the AND logic is such that: NO input contact (X0)/ I: 0 NO input contact (X1)/I: 1 Output coil (Y0)/ O: 0 0 0 0 0 1 0 1 0 0 1 1 1 In other programs, we can replace the address of the input NO contacts and output relay. OR logic In OR logic NO input contact are placed in parallel so there is always a secondary path instead of one single path. Below is a sample program or the OR logic. Figure 4: OR logic The truth table for the above program is such that: NO input contact (X0)/ I: 0 NO input contact (X1)/I: 1 Output coil (Y0)/ O: 0 0 0 0 0 1 1 1 0 1 1 1 1 NAND Logic In the NAND logic the NC input contacts are used in series instead of NO contact. At the 0 state both the NC relay contact will turn ON the output coil. All other conditions will be zero. A sample program will be such that: Figure 5: NAND logic The truth table for the program is given below: NO input contact (X0)/ I: 0 NO input contact (X1)/I: 1 Output coil (Y0)/ O: 0 0 0 1 0 1 0 1 0 0 1 1 0 NOR logic In the NOR logic the NC input contacts are used in parallel instead of NO contact. A sample program is given below: Figure 6: NOR logic The truth table for the program is given below: NO input contact (X0)/ I: 0 NO input contact (X1)/I: 1 Output coil (Y0)/ O: 0 0 0 1 0 1 1 1 0 1 1 1 0 Latch and unlatch outputs coils In most of the hardware push buttons are used. In most of the programs, when the input is removed or push on the push button is removed, the output coil also opens. On the other hand, with the help of latching, we can have the constant output coil energized with a single push on a push button. The output coil remains energized until is id opened with the help of another button. Latching is the OR logic between input NO contact relay and output coil relay. If the output coil is unlatched to the input relay contacts, the output will get open as the input relay contacts get opened. Below are the example programs: Figure 7: Latched output coil In case if the NO input relay changes its state to open, the output coil still remain closed. Figure 8: Latched output coil In the similar manner we can use the set command to latch the output coil. The example below illustrates the command: Figure 9: Latched output coil with set instruction The output coil can be reset be using the reset command. Transition contact In the PLCs inputs and outputs are constantly scanned for changes. A transition contact is only scanned for one cycle. Transition contacts only generate a single pulse when the coils are energized. A single time pulse is generated with a transition contact is closed. For example, if the transition contact is at the input, like Xo, it will only generate a single pulse, no matter how long the button is pressed. A positive transition pulse changes the state from OFF to ON, while Negative transition contact changes state from ON to OFF. It is used in he similar manner as that of the NO input relay contacts. Figure 10: Positive Transition Contact Figure 11: Negative Transition contact Time instructions Timers are utilized massively during industrial operations. PLCs have different ranges of timers. A timer block in the ladder logic produces delay either to changes the state of the output coil to ON or either OFF or for any other operation. An example of a timer based program is given below (Delta.com.tw). Figure 12: Timer Example delay ON Figure 13: Timer example delay OFF Address assignment Address is assigned to the input NO/NC contact with respect to the physical availability. Consider we have a PLC module with inputs five contacts from 0 to 4, in the similar manner we have outs coils from 0-4. We have to manage the program under physical boundaries. Arithmetic instructions Arithmetic operations are addition, subtraction, multiplication and division. To make a proper logic suiting nay machine we can make utilization of the arithmetic logical operations or arithmetic functional blocks fro the operation. The output is generated with the help of arithmetic operation. The program below shows a program that adds 1 to the data register. And increments 1 till the normally open contact remains closed. On the other hand, the second NO contact resets the data register value. Figure 14: Addition Arithmetic function The below program shows the arithmetic function of subtraction: Figure 15: Subtraction Arithmetic function The programs for the multiplication and division are nearly the same as that for the addition and subtraction. Compare instructions With the help of compare instructions, two value are compared and then output is generated regarded the comparison. In comparison, three states can be evaluated “less than”, “greater than” and “equals to”. Below is the example of a compare instruction. Figure 16: Compare instruction example In the program above, at first, the when the NO contacts are closed, data registers are incremented from present value by 1. Then the compared functional block, compares the two data registers and gives out put as “greater than”, “less than” and “equals to”. Output coils can be operated with the help of each output generated by the compares functional block. PLC examples 1) Sample program to control a fan motor: Figure 17: sample program to control fan 2) Sample program for a car parking entering mechanism: Figure 18: Sample Program for car parking 3) Water pumping from a tank: Figure 19: sample program for water pumping 4) Reverse forward motion of a three phase motor with the help of magnetic contactors: Figure 20: sample program for reverse and forward motion of three phase motor References Delta.com.tw, Welcome To Delta Electronics, Inc. - Automotive Products. N.p., 2014. Web. 29 Nov. 2014. Rockwell Automation, Listen, Think, Solve: Basic PLC Programming. N.p., 2014. Web. 29 Nov. 2014. Read More