Technical Specification – Design Phases of the Control System

This is the first article in this series where we try to describe the different design phases of a control system. In PLC-based systems the design engineer must analyze all the technical requirements and make the design that meets the specifications with the lowest possible cost. In medium and large systems it can be a complex task for which many technicians are not sufficiently prepared.

In this first article we deal with the technical requirements of the design.

Index of design phases:

1-Analysis of the technical requirements.

2A-Hardware selection of input / output modules (A)

2B-Hardware selection of input / output modules (B)

3-Grouping and segmentation of signals and racks – Functional distribution

4-CPU selection and architecture design

5-Selection of HMI / SCADA

6-Power Supplies and Circuit Breakers

7-Isolators, field terminals and prefabricated cables

8-Design of the control cabinet.


technical requirements
1-Analysis of the technical requirements

The first thing to do is to analyze in detail the technical specification of the customer. Depending on the sector and the size of the application we can find a simple document, several extensive and demanding technical documents or any other intermediate case.

In any case, the design engineer should not be limited to the exclusively technical content of the specification, but should know other information about the project that can condition the design as it may be everything related to delivery times, penalties for noncompliance, etc.

The most important points to keep in mind throughout the design process are:

– Approved manufacturers (PLC / DCS, SCADA and other components).

– Redundancy requirements (CPU, I / O, networks, servers, etc.)

– Safety requirements (SIL level, hazardous area, firewalls, etc.)

– Requirements for the architecture (if there must be several controllers, number of I / O by type and if they are local and / or decentralized or remote, communication speeds in different networks or with third parties, etc.)

– Requirements for the construction of the cabinet (non-standard mechanical characteristics, front and / or rear access, minimum spare space, entry of field cables with cable glands, shielded cables, use of prefabricated cables, compliance with seismic regulations, use of marshalling cabinets, etc.)

-HMI / SCADA requirements (industrial operating stations, graphics , SOE resolution, historical and alarm management, backup management, access management, number of graphics and monitors, number of communication I/Os, etc.)

-Other requirements (interposing relays in digital I / O, galvanically separated I / O, network type and communication protocol with third parties, maximum number of process variables per server or CPU, digital signals with line monitoring, FAT and SAT, etc.)

All this list above is not exhaustive, but gives an idea of ​​the number of concepts that we must analyze. Some of them can have a very high impact on the cost, such as redundancy and safety requirements.

In the following sections we will delve into many of these technical requirements.

Link to the next phase.