Cascading Logic

Cascading Logic: A Machine Control Methodology for Programmable Logic Controllers explains how to develop a functional machine control program for industrial equipment that operates sequentially. The programming approach begins by breaking the machine into its fundamental components. These small, manageable parts help the programmer focus on major concerns before addressing details. The method then explains how to program each component and how to assemble the components into a complete machine control system. This book aims to give the reader the confidence to make decisions and proceed with certainty that the program works as intended, without odd logic combinations causing unintended actions. The sequential order of events also assists operators and maintenance staff in troubleshooting and maintaining the equipment after deployment. Ladder logic illustrations demonstrate each section of the text. Although the ladder logic examples use the instruction set for the Allen-Bradley SLC 500 PLC, the concepts and techniques are applicable to any brand of PLC.

ABOUT THE AUTHOR ix INTRODUCTION xi Chapter 1 LADDER LOGIC BASICS 1 1.0 Introduction, 1 1.1 Rungs and Branches, 2 1.2 Program Execution and Scan Time, 3 1.3 One Shots, 4 1.4 Always On Bit, 5 1.5 First Scan Bit, 6 1.6 Alternators, 6 1.7 Binary Counters, 7 1.8 One Scan Oscillator, 9 1.9 Latches, 9 1.10 Duty Timers, 12 1.11 DeMorgan's Theorem, 12 1.12 Other Circuits, 14 Chapter 2 SEQUENCES AND MOTION OBJECTS 15 2.0 Introduction, 15 2.1 Functional Grouping, 15 2.2 Documentation, 17 2.3 Sequences, 17 2.4 The Motion Object, 20 2.5 Summary, 27 Chapter 3 OUTPUT LOGIC 29 3.0 Introduction, 29 3.1 Application Considerations, 29 3.2 Spring-Return Valves, 32 3.3 Auxiliary Rungs, 32 3.4 Spring-Return Valve Output Latch, 35 3.5 Recovery, 37 3.6 Double-Ended, Detented Valves, 39 3.7 Double-Ended Valve Output Logic (Spring-Return Configuration), 40 3.8 Double-Ended Valve Output Logic (Double-Ended Configuration), 41 3.9 Three-Position Valves, 42 3.10 Three-Position Cylinders, 45 3.11 Pulse Outputs, 46 3.12 Conclusion, 47 Chapter 4 SIMULATING SENSORS 49 4.0 Introduction, 49 4.1 Cylinders with Only One Sensor, 49 4.2 Cylinders with No Sensors, 50 4.3 Time-Driven Motion Objects, 50 4.4 Output Logic Versus Motion Objects, 52 Chapter 5 CHECKING SENSORS 53 5.0 Introduction, 53 5.1 Problems with Lone Sensors, 54 5.2 Back Checking, 55 5.3 Forward Checking, 57 5.4 Position Bits, 59 5.5 How Often Sensors Are Checked, 60 5.6 Home Position, 61 Chapter 6 CONTROLLING SEQUENCES 65 6.0 Introduction, 65 6.1 Types of Sequences, 65 6.2 Common Control Structure, 66 6.3 Conclusion, 75 Chapter 7 STEP AND SINGLE CYCLE 77 7.0 Introduction, 77 7.1 Why Have Stepping?, 77 7.2 Why Have Single Cycle?, 78 7.3 Mode Selection, 78 7.4 Single-Cycle Control, 80 7.5 Step Mode Control, 82 7.6 Modified Sequence Control, 83 7.7 Mix and Match Modes, 85 7.8 Machine Running, 86 7.9 In Cycle, 87 7.10 Fault Recovery, Debug, and Startup Tools, 87 Chapter 8 SYSTEM CONTROL 91 8.0 Introduction, 91 8.1 Summary of Sequence Control, 91 8.2 Control Hierarchy, 94 8.3 System Control, 96 8.4 Starting Auxiliary Equipment with the Machine, 103 8.5 Program Structure, 103 8.6 Summary, 104 Chapter 9 PARENT/CHILD SEQUENCES 107 9.0 Introduction, 107 9.1 The Dial Machine, 107 9.2 The Dial Sequence, 112 9.3 Typical Station Sequence, 124 9.4 Cycle Stop Points, 128 9.5 Manual Load/Unload Stations, 131 9.6 Dial Mechanisms, 131 9.7 Indexing Stations, 133 9.8 Code Generation and Checking, 134 Chapter 10 DECISION LOGIC 137 10.0 Introduction, 137 10.1 The Pick/Place Example, 137 10.2 Part Inspection, 137 10.3 Continuing Sequences, 142 10.4 Branching Sequences, 143 10.5 Converging Sequences, 143 10.6 Parallel Branches, 144 10.7 Conclusion, 145 Chapter 11 DEPENDENT SEQUENCES 147 11.0 Introduction, 147 11.1 The Example, 147 11.2 The Problem, 149 11.3 The Solution, 152 11.4 Other Problems, 152 11.5 Conclusion, 154 Chapter 12 MACHINE STATES 155 12.0 Introduction, 155 12.1 Machine States, 156 12.2 Machine State Concepts, 156 12.3 Power Off State, 158 12.4 First Scan State, 158 12.5 Fault State, 159 12.6 Manual Mode State, 162 12.7 Ready and Not Ready States, 162 12.8 Running State, 163 12.9 Other Possible States, 164 12.10 Conclusions, 164 Chapter 13 MESSAGE LOGIC 167 13.0 Introduction, 167 13.1 Motion Object, 168 13.2 Message Displays, 169 13.3 Message Display Logic, 169 13.4 Types of Messages, 171 13.5 Message Text, 172 13.6 Message Logic Payback, 173 Appendix A 175 GLOSSARY 185 INDEX 203