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plc software - 32.1

32. SOFTWARE ENGINEERING

Topics:

Electrical wiring issues; cabinet wiring and layout, grounding, shielding and inductive loads

Controller design; failsafe, debugging, troubleshooting, forcing

Process modelling with the ANSI/ISA-S5.1-1984 standard

Programming large systems

Documentation

Objectives:

To learn the major issues in program design.

Be able to document a process with a process diagram.

Be able to document a design project.

Be able to develop a project strategy for large programs.

32.1INTRODUCTION

A careful, structured approach to designing software will cut the total development time, and result in a more reliable system.

32.1.1 Fail Safe Design

It is necessary to predict how systems will fail. Some of the common problems that will occur are listed below.

Component jams - An actuator or part becomes jammed. This can be detected by adding sensors for actuator positions and part presence.

Operator detected failure - Some unexpected failures will be detected by the operator. In those cases the operator must be able to shut down the machine easily.

Erroneous input - An input could be triggered unintentionally. This could include something falling against a start button.

Unsafe modes - Some systems need to be entered by the operators or maintenance crew. People detectors can be used to prevent operation while people are present.

Programming errors - A large program that is poorly written can behave erratically when an unanticipated input is encountered. This is also a problem with assumed startup conditions.