Design Guide for PLC Systems
We offer here some suggestions for best practice when designing automation control systems.
Project Execution Plan
- A statement of purpose – why are we doing this, what are the benefits, who are the stakeholders and how will success be determined?
- Who is on the team? What are their roles, where are the lines of responsibility?
- What is the budget, and where are the areas that will stress it?
- Is there to be any development of new tools / procedures / methods etc to be undertake on the project?
- What are the risks, how bad might the be, and what can you do to address them?
This is a major section – the more thought that goes into considering what could go wrong and what to do about it, the better.
In industrial automation projects, Process Description and Functional Description are two essential but distinct types of documentation. Here’s a breakdown of each and how they differ.
Process Description
A Process Description outlines what the process is, focusing on the sequence of operations, physical flow, and interactions involved in the production or manufacturing process. It is typically written from the perspective of the process engineers or operators and describes the technical or physical behavior of the system under normal and abnormal operating conditions.
Contents typically include:
- Description of equipment and machinery involved, including a detailed device list with identity tags
- Material flow and handling
- Process steps and sequences (e.g., mixing, heating, cooling)
- Operating conditions (temperatures, pressures, speeds, etc.)
- Interlocks, alarms, and safety considerations
- Start-up and shutdown procedures
Purpose:
To provide a clear understanding of how the physical process works and what needs to be automated. This serves as a foundational document for control system design, operator training, and safety analysis.
Functional Description
A Functional Description specifies how the automation system should behave, detailing the logic and control strategy used to automate the process described in the Process Description. It’s often written from the perspective of control engineers or programmers and defines the behavior of programmable logic controllers (PLCs), distributed control systems (DCS), or SCADA systems.
Contents typically include:
- Control logic sequences and flowcharts
- Input/output (I/O) definitions
- PLC tags and variable names
- HMI interactions and screen logic
- Alarms and event logic
- Interlocks and safety logic (mapped to control logic)
- Communication with other systems
Purpose:
To provide a blueprint for coding the control system, enabling software engineers to implement automation systems that control the real-world process as specified. They are essentially translating the language from the written word to a software model.
🔍 Key Differences between the PD and FD
| Aspect | Process Description (PD) | Functional Description (FD) |
|---|---|---|
| Focus | Physical process & operations | Control system behavior |
| Perspective | Process/operations engineer | Automation/control engineer |
| Describes | What happens in the plant | How it’s controlled via automation |
| Used for | Understanding the real-world process | Designing and programming control systems |
| Typical Output | Narrative with process steps | Logic diagrams, pseudo-code, function blocks |
🧠 Simple Analogy:
- Process Description is like describing a recipe—ingredients, steps, tools, and results.
- Functional Description is like explaining how the robot chef should follow that recipe—turn on the stove, stir for 2 minutes, monitor temperature, etc.
Hardware Design
- We recommend buying the DC versions and using a quality AC to DC power supply. This is on the basis that the power supply is a crucial part of the product’s design and the most exposed to electrical disturbance. We find that Mean Well supplies are extremely reliable and value for money, but Weidmuller, and the Allen-Bradley 1606 range are also reliable.
- Preferably, a surge filter (as opposed to a simple surge diverter) is also remommended to take out not only voltage excursions, but the damaging fast voltage spikes with high dV/dt that damage electronics. Erico (now owned by nVent) have a good range of surge protection products.
- When developing Digital Input circuit drawings, define the asserted state of the input, and use fail-safe principles as a starting point. That is, what does a logical 1 represent? Here are some examples.
- A tank Low Level switch must indicate that the tank is empty if it fails (that is, if the wire falls off for example). So a 0 represents empty, therefore 1 must represent that the sensor is Covered.
- A tank High Level switch on the other hand has to indicate that the tank is full if it fails (so that it will stop being filled). So a 0 represents full, then 1 must represent that the sensor is Uncovered.
- A similar approach is used for Pressure, Temperature and Flow Switches.