Do you remember that project that kept me awake all night? A packaging production line, with the client pushing hard for results. I thought to myself, this is just a simple conveyor control, I can jump right in and write the program. But what happened? I spent three whole days debugging, with problems popping up one after another: sensor signal conflicts, chaotic action sequences, incomplete emergency stop logic… In the end, I almost missed the deadline.
This experience taught me a lesson: the simpler a project seems, the more important it is to clarify the process flow before starting.
What Exactly is a Process Flow Diagram?
In simple terms, a process flow diagram is like the construction blueprint you draw before building a house. You wouldn’t just start building with a pile of bricks, right? The same logic applies; you can’t just write a PLC program as ideas come to you.
In my view, a process flow diagram includes three core elements:
- • Device state transitions: Under what conditions does the system switch from standby to running, and from running to stopping?
- • Signal logic relationships: Which sensor triggers which actuator, and what is the sequence?
- • Exception handling mechanisms: How should the system respond in case of a fault?
Pits I Fell Into Over the Years
When I first entered the industry, I always thought drawing diagrams was a waste of time; it felt much more efficient to just write the program. Until I met Master Zhang, who was recognized in the factory as the “firefighter,” specializing in solving the difficult problems that others couldn’t handle.
Once, while debugging a control system for an injection molding machine, I watched him spend half an hour drawing a flow diagram on paper. At the time, I thought to myself, “This old guy is so slow.” In the end, his program passed the debugging on the first try, while my project, which I hadn’t diagrammed, was still smoking away.
Master Zhang told me: “Young man, the program is for the machine, but the process flow diagram is for people. If you can’t clarify the logic for yourself, how can you expect the machine to follow your commands?“
The Secret Weapon of Experts
Over the years, I’ve observed that all PLC experts share a common habit: they annotate time parameters on their flow diagrams. For example:
- • Cylinder action time: 0.5 seconds
- • Sensor response delay: 50 milliseconds
- • Safety wait time: 2 seconds
Why be so precise? Because timing is the soul of industrial control. Many inexplicable faults can be traced back to timing issues. I’ve seen too many programmers write complete functional code, but because they didn’t consider the timing relationships clearly, they struggled endlessly during on-site debugging.
The Shift from Chaos to Clarity
The benefits of drawing a process flow diagram are not just about clarifying your thoughts; more importantly, it can expose issues you hadn’t considered.
For example, last year I took over a mixing station project. According to the requirements, it was just about controlling several motors to start and stop in sequence, which seemed simple. But when I started drawing the flow diagram, I discovered a major issue: if there is a sudden power outage during mixing, from which state should the system restart?
If this issue isn’t considered during the design phase, discovering it during on-site debugging could lead to significant problems. It might require redesigning the entire state machine logic or even modifying hardware configurations.
My Insights on Flow Diagrams
After years of experience, I’ve summarized a few key points for drawing process flow diagrams:
First, use different colors to distinguish different types of signals. Input signals in blue, output signals in red, and internal logic in green. This way, you can easily see the signal flow.
Second, annotate all boundary conditions. While normal conditions are usually considered, extreme situations are often overlooked: what if a sensor fails? What if an actuator gets stuck?
Third, don’t forget human-machine interaction. At which points do operators need to intervene? What status information needs to be displayed? All of this should be reflected in the flow diagram.
Conclusion
Looking back now, most of the debugging nightmares that kept me working late into the night could have been avoided with a clear process flow diagram. Although it took a bit more time upfront to draw the diagram, it resulted in smooth debugging later on, making it a worthwhile investment.
Remember, a good PLC program is not written; it is designed. And the first step in design is to create that process flow diagram.