Yesterday, I received a request for help from a colleague who said that the intelligent building automation system he was responsible for was always “misbehaving”—the corridor lights would turn on and off unpredictably, and the conference room air conditioning was fluctuating between hot and cold. I immediately recognized that this was the fault of the “polling” programming method. Today, let’s discuss how to use the PLC’s “event-driven” programming concept in building automation to make your control system both energy-efficient and intelligent.
Event-driven programming is essentially a “wait for notification before acting” approach. Unlike traditional polling programming (constantly asking “Is there anything? Is there anything?”), in an event-driven model, the PLC only executes the corresponding processing program when a specific event occurs, greatly improving system response speed and resource utilization. For example, polling is like you constantly asking a friend, “Have you arrived? Have you arrived?” while event-driven is like saying, “Just send me a message when you arrive.”
In the field of building automation, common types of events include: sensor signals triggered by personnel entering, alarms for temperature exceeding thresholds, scheduled equipment maintenance programs, and alarm signals in emergencies. These events are essentially changes in signals, which can be the on/off state of digital inputs (like access control sensors) or threshold triggers of analog signals (like temperature exceeding limits).
So, how can we implement event-driven programming in PLCs? There are mainly two methods:
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Interrupt method: When a specific event occurs (e.g., input signal changes from low to high), the PLC interrupts the currently executing program and executes a pre-defined interrupt service routine. After processing, it returns to the original position to continue execution.
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Status monitoring method: Set conditional judgment statements in the main program to continuously check the status of specific signals, and once the conditions are met, execute the corresponding processing code.
Taking the Siemens S7-1200 PLC as an example, here is a simple event-driven program snippet for controlling the conference room lights:
// When personnel enter the conference room (rising edge trigger)
IF "Human Sensor"(P) THEN
// Turn on the lights
"Conference Room Lights" := TRUE;
// Start a 10-minute timer
"TimerDB".TON(IN := TRUE,
PT := T#10M);
END_IF;
// Turn off the lights when the timer expires and no personnel activity is detected
IF "TimerDB".Q AND NOT "Human Sensor" THEN
"Conference Room Lights" := FALSE;
"TimerDB".TON(IN := FALSE);
END_IF;
In this code, we used rising edge detection (P) to capture the “personnel entering” event. The action to turn on the lights is triggered only when the human sensor changes from no signal to a signal (0→1), rather than continuously checking the status. This is a typical event-driven mindset.
In practical applications, combining event-driven programming with state machines is the most effective programming model. A state machine divides the system into multiple states (such as idle, running, alarm, etc.) and transitions between these states based on different events. This approach is particularly suitable for complex building automation control scenarios.
For example, an intelligent conference room system can be designed with the following states:
Idle state: Monitoring personnel entry events
Preparation state: Automatically adjusting temperature, brightness, etc., after personnel enter
Usage state: Monitoring for no personnel events and reservation end events
Cleaning state: Notifying the cleaning system after the meeting ends
Each state corresponds to different event response logic, resulting in clear code structure and easy maintenance.
Of course, there are several pitfalls to avoid when using event-driven programming:
Event backlog issue: If the event handler takes too long to execute, it may prevent new events from being processed in a timely manner. The solution is to optimize the processing code or set high-priority interrupts for critical events.
During debugging, you can add status indicator lights in the PLC program, allowing different indicator lights to light up for each event trigger, making it easier to visually assess the system’s operational status. When I was debugging an automation system for an office building, I quickly identified a false trigger issue with the sensor using this method.
Finally, for those who want to delve deeper, I recommend starting with simple single-event handling and gradually transitioning to complex multi-event coordination. You can try implementing a small lighting control system using event-driven methods, incorporating human sensing, light sensing, and timed control from three event sources, to experience the different handling methods for various events.
PLC’s event-driven programming may seem complex, but it is actually a key to making control systems smarter. Mastering this concept will not only make your building automation system respond faster but also significantly reduce energy consumption. If you have any questions, feel free to leave a message!
