Automating Manufacturing Systems - Process Control and ...

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.
32.1 INTRODUCTION
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.