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Cost-Reducing Techniques in Event-Driven Programming for PLC Industrial Automation Systems

Last month, the packaging production line that Xiao Li was responsible for encountered issues again; the equipment inexplicably stopped every three to four hours, almost delaying the delivery schedule. The factory manager was in a panic, and the maintenance personnel were in and out for several days, but in the end, they still needed my help as an experienced electrician. Upon reviewing their PLC program, I immediately understood where the problem lay—typical polling programming, with excessive ineffective scans, causing the CPU load to remain above 90% year-round, making it prone to triggering watchdog protection during fluctuations.

Today, I will share with you all about event-driven PLC programming, a cost-effective and worry-free solution.

Traditional PLC programming mostly uses polling methods, similar to a security guard patrolling without regard to whether there is an issue, checking each location in sequence. For example, if there are 20 workstations on the production line, you need to write 20 scanning segments, even if only 3 workstations are active at the time, wasting CPU resources scanning the other 17. Over time, this not only bloats the program and reduces operational efficiency but also adversely affects hardware lifespan and energy consumption.

The core idea of event-driven programming is akin to equipping each workstation with a “call button”; whoever has an issue rings the bell, and the PLC only processes the parts that have changed. This way, when the equipment is idle, it consumes almost no resources, and it responds immediately when an event occurs, significantly improving system efficiency and stability.

First, let’s look at a common implementation of traditional polling PLC programs:

// Traditional polling method - executes even if the sensor state has not changed

IF Sensor_1 = TRUE THEN

Motor_1 := TRUE;

ELSIF Timer.Q THEN

Motor_1 := FALSE;

END_IF;

Now, let’s examine the event-driven approach:

// Event-driven method - executes only when the sensor state changes

IF Sensor_1 AND Sensor_1_LastState = FALSE THEN

Motor_1 := TRUE;

Alarm := FALSE;

END_IF;

From my over ten years of factory experience, switching to event-driven programming can reduce costs by more than 30%, primarily reflected in several aspects:

First is hardware investment. You may not know that for the same production task, an event-driven program can be completed with a lower-spec PLC. For the chemical plant’s line, the original budget was to purchase a mid-to-high-end PLC costing over 50,000, but I helped them rewrite the program logic using event-driven methods, and in the end, we managed it with an economical model costing just over 20,000, saving enough money to buy two more frequency converters.

Secondly, energy consumption. Although PLCs do not consume much power, the entire automation system is connected to a host of devices like frequency converters, servos, and valves. During traditional polling, even if the production line is idling, these devices remain in standby mode consuming energy; whereas event-driven systems can allow devices to enter deeper sleep modes when no events occur, reducing average energy consumption by 15%-20%. After the transformation of Xiao Zhang’s factory’s injection molding line, the monthly electricity bill dropped by nearly two thousand yuan.

Maintenance costs are also significantly reduced. Think about it, with lower CPU loads, less heat is generated, and the lifespan of components naturally increases. We have a 12-year-old Siemens PLC in our factory that uses an event-driven architecture, and it has had no issues to date; meanwhile, the polling-based PLC in the adjacent workshop failed its CPU after just four years.

Speaking of practical applications, let me share a few useful tips:

First, use edge-triggered instead of state detection. For example:

// Using R_TRIG (rising edge) and F_TRIG (falling edge) function blocks

R_TRIG_1(CLK := Sensor_1);

IF R_TRIG_1.Q THEN

// This part of the code executes only when the sensor changes from OFF to ON

END_IF;

Second, make good use of interrupt functions. Modern PLCs support interrupt handling, allowing critical sensors to trigger interrupts directly without waiting for the main loop to scan. This is particularly suitable for scenarios requiring rapid responses, such as safety protection and precise positioning.

Third, manage reasonable partitioning. Different functional modules have different response priorities, and it is crucial to manage the system by partitioning based on response speed. For instance, safety-related tasks should use interrupt handling, general control can use event triggers, and non-urgent tasks like data logging can use timed polling.

Once, I went to a pharmaceutical factory to help; their filling line was always unstable in precision. Upon investigation, I found that all modules were mixed in programming, causing high-priority actions to be delayed by lower-priority ones. I helped them restructure the program architecture, converting all critical control points to event-driven, which immediately stabilized the process and increased capacity by about 8%.

Remember, using event-driven programming effectively is not just a technical issue; it directly impacts the company’s efficiency. Especially now, with energy and equipment costs so high, every bit saved counts! However, I must also remind you that not all situations are suitable for pure event-driven approaches; safety-related monitoring functions still require necessary periodic checks. It is better to consume a bit more resources than to have safety hazards.

Alright, that’s all for today. Remember, the core of industrial automation is to be both stable and reliable, as well as economically efficient. Next time you encounter a problem like Xiao Li’s, you’ll know how to handle it, right?