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    Module 4 Programmable Logic Control Systems Version 2 EE IIT, Kharagpur 1

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    Lesson 20 Formal Modelling of Sequence Control Specifications and Structured RLL Programming Version 2 EE IIT, Kharagpur 2

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    Instructional Objectives After learning the lesson students should be able to A. Describe motivations for formal modelling in the design of sequence control programs for an industrial control problem. B. Describe the major steps in the design of a sequence control program for an industrial cont...

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    that indicates when the piston is fully retracted and a lower limit switch that indicates when the piston is fully extended. Lastly, the process has a master switch which is used to start the process and to shut it down. The control computer for the process has 3 inputs (2 from the limit sensors ...

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    operator can return the piston to the up position by manually operating the hydraulics, or special auto mode logic should be designed to perform this. • As a second example, note that this process does not have a part detect sensor. This implies that the moment the Master switch is on the pres...

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    Therefore, this phase is to be carried out in close consultation with the plant engineers of the user organization. It basically involves studying the system behavioral aspects of the system to: i) identify the feedback inputs from sensors and the external operator inputs from Man Machine Interfa...

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    Process Control Outputs • Up Solenoid: Control output that drives the Up solenoid of the electro-hydraulic direction control valve which in turn drives the piston up. • Down Solenoid: Control output that drives the Up solenoid of the electro-hydraulic direction control valve which in turn d...

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    control engineer. This is therefore not attempted here for reasons of conciseness. However, the learner is encouraged to augment the control logic with additional logic to detect such failures rapidly and initiate activities for fault tolerance. Point to Ponder: 4 A. What would happen in the ...

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    E. The choice of state variables can be subjective and different designers might pick others. The choice also depends on the nature of control actions that one would like to take. Thus, the choice of states is specific to the machine and its operation. F. During its life cycle, the process moves...

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    State No. O/P 1 2 3 4 5 6 Auto Indicator 0 1 1 1 0 1 Part Hold 0 0 1 1 0 0 Up Sol 0 0 0 1 0 0 Down Sol 0 0 1 0 0 0 Figure 20.4 The Output Table for the industrial stamping press From the State Diagram in Fig. 20.3, the following may be noted. A. The states are marked numerically using the nume...

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    timer outputs, if applicable. In the case of transition A, the external condition is simply the pressing of the Auto PB. Note that, in any scan cycle, at most one transition can be enabled. The next block of rungs constitutes the state logic. If the transition logic for any transition is satisfi...

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    5 This is followed by ladder logic to turn on outputs as requires by the steps. This section of ladder logic corresponds to the actions for each step. The rung for each output therefore contains one NO auxiliary contact corresponding to the state in which it is enabled. If an...

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    Conclusion Different methods can be used to design different controllers. The most basic controllers can be developed using simple flowcharts. More complex control problems should be solved with state diagrams. It is also possible to make a concurrent system using two or more state diagrams. How...

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    Answers, Remarks and Hints to Points to Ponder Point to Ponder: 1 A. Before going through the rest of the system attempt to modify the RLL program shown in Fig. 20.2 to ensure that the process always moves to the up position first, and then from there, resumes its press cycle Ans: Without the ...

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    Ans: A hydraulic pressure sensor may be used for sensing the pump pressure before turning the machine into auto mode. This would be useful, particularly if the machine is run in auto mode for long intervals with robotic material handing equipment for part placement and removal. A fault indicator ...

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    B. Mention any one disadvantage of a formal modelling approach, if you can think of it. Ans: The formal modelling may lead to more number of rungs in the RLL program compared to one that does not use it. Consequently the memory and execution time requirements would be higher. However, these are...

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