Me 445 Groover Ch9 Programmable Logic Controllers

Ch 9 Discrete Control Usin Us ing g PLCs PLCs an and d PCs Sections: 1. Discrete Process Control 2. Ladder Logic Diagrams 3. Programmable Logic Controllers 4. Personal Computers Using Soft Logic

©2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover. Groover.

Discrete Process Control Discrete process control systems deal with parameters and variables that are discrete and that change values at discrete moments in time. The parameters and variables are typically binary; they can have either of two possible values, 1 or 0. The values mean 

ON or OFF,

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true or false,

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object present or not present,

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high voltage value or low voltage value, and so on,

depending on the application. ©2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover. Groover.

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Discrete Process Control The binary variables in discrete process control are associated with input signals to the controller and output signals from the controller. Input signals are typically generated by binary sensors, such as limit switches or photosensors photosensors that are interfaced to the process. Output signals are generated by the controller to operate the process in response to the input signals and as a function of time. These output signals turn on and off switches, motors, valves, and other binary actuators related to the process.


Discrete Process Control The purpose of the controller is to coordinate the various actions of the physical system, such as transferring parts into a workholder, feeding a machining workhead, and so on. Discrete process control can be divided into two categories: (1) logic control, control , which is concerned with event-driven changes in the system, and (2) sequencing , which is concerned with time-driven time-d riven chang changes es in the system. Both are referred to as switching systems in the sense that they switch th eir output values on and and off in off in response to changes in events or time. ©2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover. Groover.

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Discrete Process Control


Logic Log ic Co Cont ntro roll A logic control system, also referred to as combina comb inationa tionall logic contro controll , is a switching system whose output at any moment is determined exclusively by the values of the current inputs. (1) A logic contro controll system system has no memory and does not consider any previous values of input signals in determining the output signal. signal. (2) Neither does it have any operating characteristics that perform directly as a function of time.


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Logic Control Example from robotics to il lustrate logic control: In a machine-loading application, the robot is programmed to pick up a raw workpart from a known stopping point along a conveyor and place it in a forging press. Three conditions must be satisfied to initiate the loading cycle. 1. The raw workpart must be at the stopping point; 2. The forging press must have completed the process on the previous part; 3. The previous part must be removed from the die. ©2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

Logic Control The first condition can be indicated by means of a simple limit switch that senses the presence of the part at the conveyor stop and transmits an ON signal to the robot controller. The second condition can be indicated by the forging press, which sends an ON signal after it has completed the previous cycle. The third condition might be determined by a photodetector located so as to sense the presence or absence of the part in the forging die. When the finished part is removed from the die, an ON signal is transmitted by the photocell. All tree of these ON signals must be received by the robot controller to initiate the next work cycle.

©2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

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Boolean Algebra and Truth Tables The logic elements form the foundation for a special algebra that was developed around 1847 by George Boole and that bears his name. Its original purpose was to provide a symbolic means of testing whether complex statements of logic were TRUE or FALSE. In fact, Boole called it logical algebra. It was not until about a century later that Boolean algebra was shown to be useful in digital logic systems.


Elements of Logic Control 

Basic elements, called logic gates: AND – output = 1 if all inputs = 1, zero otherwise OR – output = 1 if any input = 1, zero otherwise NOT – output = 1 if (single) input = 0, and vice versa

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Additional elements: NAND – combination of AND and NOT NOR – combination of OR and NOT


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Elements of Logic Control The basic elements of logic control are the logic gates AND, OR, and NOT. In each case, the logic gate is designed to provide a specified output value based on the values of the input(s). For both inputs and outputs, the values can be one of two levels, the binary values 0 or 1. For purposes of industrial control, we define: 

0 (zero) to mean OFF, and

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1 (one) to mean ON.


AND Gate Electrical circuit illustrating the operation of the logical AND gate.

In Boolean algebra, the AND function is expressed as: Y = X1.X2

(9.1)

This is called the logical product of X1 and X2. As a logic statement it means: Y is true if both X1 and X2 are true; otherwise, Y is false. ©2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

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AND Gate The truth table is often used to present the operation of logic systems. A truth table is a tabulation of all of the combinations of input values to the corresponding logical output values. The truth table for the AND gate has four possible combinations for two input binary variables. ©2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

OR Gate The OR function in Boolean algebra notation is given by: Y = X1 + X2

(9.2)

This is called the logical sum of X1 and X2. In logic, the statement says: Y is true if either X1 or X2 is true; otherwise Y is false. ©2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

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NOT Gate The NOT function is referred to as the negation or inversion of the variable. It is indicated by placing a bar above the variable. Y = X1

(9.3)


Truth Tables for NAND and NOR Gates


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Elements of Logic Control The interlock system in our previous robot forging exampie illustrates the AND gate. All three conditions must be satisfied before loading of the forge press is allowed to occur. The OR gate outputs a value of 1 if either of the inputs has a value of 1, and 0 otherwise. A possible use of the OR gate in a manufacturing system is for safety monitoring. Suppose that two sensors are utilized to monitor two different safety hazards. When either hazard is present, the respective sensor emits a positive signal that sounds an alarm buzzer. ©2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

Elements of Logic Control Both the AND and OR gates can be used with two or more inputs. The NOT gate has a single input. The NOT gate reverses the input signal: If the input is 1, then the output is 0; if the input is 0, then the output is 1. The NOT gate can he used to open a circuit upon receipt of a control signal.


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Laws and Theorems of Boolean Algebra

These laws and theorems can often be applied to simplify logic circuits and reduce the number of elements required to implement the logic, with resulting savings in hardware and/or programming time. ©2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

Symbols for Logical Gates: U.S. and ISO


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Logic Control


Logic Control


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Logic Control


Logic Control


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Logic Control


Sequencing A sequencing system uses internal timing devices to determine when to initiate changes in output variables. Washing machines, dryers, dishwashers, and similar appliances use sequencing systems to time the start and stop of cycle elements. Many applications in industrial automation require the controller to provide a prescheduled set of ON/OFF values for the output variables.


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Sequencing 

Outputs are usually generated “open loop” fashion. 

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There is no feedback verification that the control function has actually been executed.

Sequence of output signals is usually cyclical, as in a high production work cycle; the signaIs occur in the same repeated pattern within each regular cycle. Common sequencing devices are: 

Timer – output switches on/off at preset times

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Counter – counts electrical pulses and stores them


Sequencing - Timer 

A timer is a device that switches its output ON or OFF at preset title intervals. Common timers used in industry and in homes switch on when activated and remain on for a programmed length of time.

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Two additional types of timers used in discrete control systems can be distinguished as: (1) delay-off timers and (2) delay-on timers.


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Sequencing - Timer 

A delay-off timer switches power on immediately in response to a start signal, and then switches power off after a specified time delay. Many cars are equipped with this type of device. When you exit the car, the lights remain on for a certain length of time (e.g., 30 seconds), and then automatically turn off.

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A delay-on tuner waits a specified length of time before switching power on when it receives a start signal. To program a timer, the user must specify the length of the time delay.


Sequencing - Counter 

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A counter is a component used to count electrical pulses and store the results of the counting procedure. The instantaneous contents can be displayed and/or used in a process control algorithm. Counters are classified as up counters, down counters, and up/down counters. Counters require two inputs: 1.

Pulse train (series of on/off signals) that is counted the counter.

2.

Signal to reset the counter and restart the counting procedure.


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Sequencing - Counter Resetting the counter means zeroing the count for an up counter and setting the starting value for a down counter. The accumulated count is retained in memory for use if required for the application. 

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An up counter starts at zero and increments its contents (the count total) by one in response to each pulse. When a preset value has been reached, the up counter can be reset to zero. A down counter starts with a preset value and decrements the total by one for each pulse received. An up/down counter combines the two counting operations.


Logic Diagrams The logic network d iagrams are useful for displaying the relationships between logic elements. Another diagramming technique that exhibits the logic and to some extent, the timing and sequencing of the system is the ladder logic diagram. This graphical method has an important virtue in that it is analogous to the electrical circuits used to accomplish the logic and sequence control. In addition, ladder logic diagrams are familiar to shop personnel who must construct, test, maintain, and repair the discrete control system. ©2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

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Ladder Logic Diagram


Ladder Logic Diagrams In a ladder logic diagram, the various logic elements and other components are displayed along horizontal lines or rungs connected on either end to two vertical rails. The diagram has the general conf igur ation of a ladder, hence its n ame. The elements and components are: contacts (representing logical inputs) and loads also known as coils (representing outputs). It is customary in ladder diagrams to locate the inputs to the left of each rung and the outputs to the right. ©2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

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Ladder Logic Diagrams A diagram in which various logic elements and other components are displayed along horizontal rungs connected on either end to two vertical rails 

Types of elements and components: 1. Contacts - logical inputs (usually), e.g., limit switches, photo-detector 2. Loads - outputs, e.g., motors, lights, alarms, solenoids 3. Timers - to specify length of delay 4. Counters - to count pulses received


Symbols for Common Elements Used in Ladder Logic Diagrams


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Ladder Logic Diagrams


Ladder Logic Diagrams – NAND, NOR X1

NAND

X2

C

Y

C

C X1 NOR X2 Y C


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Features of Ladder Logic Diagrams Example 9.5 illustrates several important features of a ladder logic diagram. 1. The same input can be used more than once in the diagram. In our example, the relay contact C was used as an input on both the second and third rungs of the ladder. This feature of using a given relay contact in several different rungs of he ladder diagram to serve multiple logic functions provides a substantial advantage for the programmable controller over hardwired control unit. (With hardwired relays, separate contacts would have to be built into the controller for each logic function.)


Features of Ladder Logic Diagrams 2. A second feature of Example 9.5 is that it is possible for an output (load) on one rung of the diagram to be an input (contact) for another rung. The relay C was the output on the top rung in Figure 9.11, but that output was used as an input elsewhere in the diagram. This same feature was illustrated in the push-button ladder diagram of Example 9.4.


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Ladder Logic Diagram


Advantages of Ladder Logic Diagrams 

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Familiar to shop personnel who must construct, test, maintain, and repair the control system, Analogous to the electrical circuits used to accomplish logic and sequence control, Principal technique for pr ogramming PLCs.


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Programmable Logic Controller (PLC) A Programmable Logic Controller (PLC) can be defined as a microcomputer-based controller that uses stored instructions in programmable memory to implement logic, sequencing, timing, counting and arithmetic functions through digital or analog input/output (I/O) modules, for controlling machines and processes. PLCs are primarily associated with discrete manufacturing industries to control individual machines, machine cells, transfer lines, material handling equipment, and automated storage systems.


Programmable Logic Controller (PLC) Examples of applications in process industries include chemical processing, paper mill operations, and food production. Before the PLC was introduced around 1970, hard-wired controllers composed of relays, coils, counters, timers, and similar components were used to implement this type of industrial control. Today, many older pieces of equipment are being retrofitted with PLCs to replace the original hardwired controllers, often making the equipment more productive and reliable than when it was new. ©2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

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Programmable Logic Controller (PLC)


Advantages of PLCs Compared to Relay Control Panels 

Programming a PLC is easier than wiring a relay control panel,

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PLC can be reprogrammed,

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PLCs take less floor space,

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Greater reliability, easier maintenance,

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PLC can be connected to computer systems (CIM),

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PLCs can perform a greater variety of control functions.


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Components of a PLC


PLC Components 1. Processor – is the central processing unit (CPU) of the programmable controller. Executes logic and sequencing functions by operating on the PLC inputs to determine the appropriate output signals. The CPU Consists of one or more microprocessors similar to those used in PCs and other data processing equipment. The difference is that they have a real-time operating system and are programmed to facilitate I/O transactions and execute ladder logic functions. ©2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

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PLC Components 2. Input/output module – connections to process; provides the connections to the industrial equipment or process that is to be controlled. Inputs to the controller are signals from limit switches, push-buttons, sensors, and other on/off devices; Outputs from the controller are on/off signals to operate motors, valves, and other devices required to actuate the process. In addition, many PLCs are capable of accepting continuous signals from analog sensors and generating signals suitable for analog actuators. The Size of a PLC is usually rated in terms of the number of its I/O terminals. ©2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

PLC Components 3. Memory unit – contains the programs of logic, sequencing, and I/O operations. 4. Power su pply – converts 115 - 240 V (AC) to DC voltages of ± 5 V compatible with process equipment. 5. Programming device The PLC is programmed by means of a programming device. The programming device is usually detachable from t he PLC cabinet so that it can he shared among different controllers.


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PLC Components – Programming Device Different PLC manufacturers provide different devices, ringing from simple teach-pendant type devices, similar to those used in robotics, to special PLC programming keyboards and displays. Personal computers can also be used to program PLCs. A PC used for this purpose sometimes remains connected to the PLC to serve a process monitoring or supervisory function and for conventional data processing applications related to the process. ©2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

Typical PLC Operating Cycle 1. Input scan – inputs are read by processor and stored in memory. 2. Program scan – control program is executed. 

Input values stored in memory are used in the control logic calculations to determine values of outputs

3. Output scan – output values are updated to agree with calculated values. Time to perform the three steps (scan time) varies between 1 and 25 ms. ©2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

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Typical PLC Operating Cycle - Scan 

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One of the potential problems that can occur during the scan cycle is that the value of an input can change immediately after it has been sampled. Since the program uses the input value stored in memory, any output values that are dependent on that input are determined incorrectly. There is obviously a potential risk involved in this mode of operation.


Typical PLC Operating Cycle - Scan 

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However, the risk is minimized because the time between updates is so short that it is unlikely that the output value being incorrect for such a short time will have a serious effect on process operation. The risk becomes most significant in processes in which the response times are very fast and where hazards can occur during the scan time. Some PLCs have special features for making “immediate” updates ol output signals when input variables are known to cycle back and forth at frequencies faster than the scan time. ©2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

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Additional PLC Capabilities 

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An alo g c on tr ol – PID control available on some PLCs for continuous processes. Ar it hm eti c f un ct ion s – permits more complex control algorithms to be implemented than conventional logic and sequencing elements. Matrix funct ions – e.g., linear programming for optimal control. Data processing and reporting – business applications. 

Blurs the distinction between PLCs and PCs.


PLC Programming A standard for PLC programming was published by the International Electrotechnical Commission in 1992, entitled: International Standard for Programming Contro llers (IEC 1131-3). This standard specifics three graphical languages and two text-based languages for programming PLCs. IEC 1131-3 also states that the five languages must be able to interact with each other to allow for all possible levels of control sophistication in any given application. ©2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

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PLC Programming 

Graphical languages: 1. Ladder l ogic diagrams – most widely used, 2. Function block di agrams – instructions composed of operation blocks that transform input signals, 3. Sequential functi on charts – series of steps and transitions from one state to the next (Europe).



Text-based languages: 1. Instruction list – low-level computer language 2. Structured text – high-level computer language


PLC Programming


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PLC Programming 1. Ladder logic diagrams (LD) The most widely used PLC programming language today involves ladder diagrams (LDs). Direct entry of the ladder logic diagram into the PLC memory requires the use of a keyboard and monitor with graphics capabilit y to display symbols representing the components and their interrelationships in the ladder logic diagram. The programmer inputs the ladder logic circuit diagram rung by rung into the PLC memory with the monitor displaying the results for verification. ©2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

PLC Programming 2. Function Block Diagrams (FBD) A function block diagram provides a means of inputting high-level instructions. Instructions are composed of operational blocks. Each block has one or more inputs and one or more outputs. Within a block, certain operations take place on the inputs to transform the signals into the desired outputs.


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PLC Programming 3. Sequential Function Charts (SFC) Also called the Grafcet Method graphically displays the sequential functions of an automated system as a series of steps and transitions from one state of the system to the next. It has become a standard method for documenting logic control and sequencing in much of Europe. However, its use in the United States is more limited.


PLC Programming 4. Instruction List (IL) Instruction list programming provides a way of entering the ladder logic diagram into PLC memory The programmer uses a low-level computer language to construct the ladder logic diagram by entering statements that specify the various components and their relationships fur each rung of the ladder diagram.


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PLC Programming – Instruction List Let us explain this approach by introducing a hypothetical PLC instru ction set. Our PLC “language” is a composite of various manufacturers’ languages. It contains fewer features than must commercially available PLC’s. The instruction set for our PLC is presented in Table 9.10.


PLC Programming – Instruction List


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PLC Programming 5. Struct ured Text (ST) A high-level computer-type language, likely to become more common in the future to program PLCs and PC for automation and control applications. The principal advantage ol a high-level language is its capability to perform data processing and calculations on values other than binary. Ladder diagrams and low level PLC languages are usually quite limited in their ability to operate on signals that are other than ON/OFF types. ©2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

PLC Programming – Structured Text The capability to perform data processing and computation permits the use of more complex control algorithms, communication with other computer-based systems, display of data on a monitor, and input of data by a human operator. Another advantage is the relative ease with which a complicated control program can be interpreted by a user. Explanatory comments can be inserted into the program to facilitate interpretation.


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Me 445 Groover Ch9 Programmable Logic Controllers

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