System Design

Once Functional Analysis has been completed, we know WHAT we want the system to do.  The System Design is the infrastructure (frame, skeleton) for the logic or the code.  System Design brings discipline to the process of specifying HOW the system will accomplish it's tasks.

English language descriptions make poor analysis documents because of ambiguity and redundancy.  Methodologies that are structured, precise, and graphical are necessary.  The methodologies used by IPACT are Sequential Function Charts (SFCs), and Yourdon methodology.

SFCs work best at describing sequential control functions, e.g. turn on Valve A, when 100Kg of syrup has been added, turn off Valve A and start mixer, etc.  State Transition Diagrams (STDs) are a subset of SFCs that have been in use in North America for a longer period of time. 

The basic SFC section presents the SFC methodology, including all the rules for steps and transitions, leveling, etc. and reinforces these methods with exercises.  The advanced SFC section shows how complex, real-world processes can be tackled by using a "top down" hierarchical approach and/or by breaking the process up into smaller, inter-dependent pieces.  The following is an relatively simple example of an SFC.  It represents a small portion of the control logic for control system.

Yourdon methodology works best at describing systems (or parts of systems) that are very data processing intensive or algorithmic in nature, e.g. a self-tuning, adaptive control scheme to regulate the percentage of various components in a continuous blend.  The Yourdon section presents the tools of the methodology, including data flow diagrams (DFDs), mini-specs, and data dictionaries, along with student exercises.  The following is an example DFD.

The techniques we use apply to all implementation platforms when implementing sequencing applications.  The tools apply very well in scenarios in which specifications must be clearly understood amongst multiple members of a project team, or to help guarantee the correct interpretation of customer specifications when projects are performed by other departments or outside vendors.

One thing that IPACT does is rather unique and valuable.  We employ HMI tools to render the SFCs so that the process logic may be visually tracked.  This helps maintenance personnel greatly, because the SFCs break the logic down into manageable steps.  We feel that this offers a tremendous benefit to our customers.  Please click on the following screens to view a demonstration of this concept.

The following is an example HMI that renders process logic in SFC format.  The current process steps are highlighted in green.  This easily identifies the present state of the logic.   Anyone trying to understand or troubleshoot the system is significantly helped  when the system logic is presented in this format.

In practice, most if not all process steps are clickable to bring up the SFC logic for those individual processes.