Ten years ago, when I first joined an automotive parts factory as an automation engineer, I faced a suddenly halted production line. The factory manager anxiously asked, “Why does the equipment respond so slowly after pressing the start button? Sometimes it doesn’t respond at all!” At that time, I couldn’t explain it well, but later I realized that it was all related to the PLC scan cycle. Today, let me help you thoroughly understand this concept in the time it takes to enjoy a cup of coffee.
The “Heartbeat” of PLC Operation
Imagine that the PLC is like the “brain” of the factory, constantly receiving information, making decisions, and issuing commands. This repetitive process is what we refer to as the scan cycle.
Specifically, during each scan cycle, the PLC primarily completes three steps:
- Input Scan: Read the status of all input points (Are buttons pressed? Does the sensor detect an object?)
- Program Execution: Perform logical operations based on ladder diagrams or other programming languages
- Output Refresh: Update the status of all output points (Start the motor? Open the valve?)
Lessons from a Coffee Machine
Let’s compare the PLC to a waiter in a coffee shop. In each scan cycle, he must:
- Check if all customers are raising their hands to place an order (Input Scan)
- Remember all orders and decide the order of preparation (Program Execution)
- Deliver the prepared coffee to the corresponding customer (Output Refresh)
If the waiter works too slowly (long scan cycle), customers may have to wait a long time to be noticed after raising their hands; if he is too hurried (short scan cycle), he may frequently check on customers but not have enough time to think and prepare the coffee.
“Paranormal Events” Caused by Scan Cycles
I remember once on a packaging line, an operator complained that after pressing the emergency stop button, the conveyor belt continued to run for a short distance. The technician mysteriously said, “This is because the PLC’s soul hasn’t reacted yet!”
In reality, this is the delay caused by the scan cycle. If the moment the emergency stop button is pressed coincides with the PLC completing the input scan, then this signal will have to wait until the next cycle to be processed, resulting in a seemingly “slow response”.
How Long is the Scan Cycle?
The scan cycle of different PLC models ranges from a few milliseconds to several tens of milliseconds. The main influencing factors include:
- Program Size: The more complex the logic, the longer the execution time
- Number of I/O Points: The more inputs and outputs, the longer the scan time
- PLC Performance: Like a computer CPU, better performance leads to faster processing
- Special Functions: PID control, communication, etc., will increase processing time
In a project I debugged for an injection molding machine, the originally used small PLC had a scan time of 15ms, which often led to missed counts during high-speed counting.After upgrading to a medium-sized CPU, the scan cycle was reduced to 3ms, and the problem was resolved.
The Engineer’s “Secret Weapon”
To tackle the challenges posed by scan cycles, we have several practical techniques:
- Interrupt Technology: Use interrupt functions for critical signals, eliminating the need to wait for the normal scan cycle
- Task Grouping: Assign different execution frequencies to tasks of varying priorities
- Direct I/O Access: Immediately read inputs or update outputs at certain points in the program
- Scan Time Monitoring: Regularly check the system scan time to detect anomalies and intervene early
The Art of Scan Cycles
Over the years, I have found that mastering the scan cycle is like the art of managing time. Beginners often focus only on whether the program logic is correct, while experienced engineers consider how to optimize the program structure for scan efficiency.
When writing timing control logic, a clear understanding of the scan cycle can help you avoid many pitfalls.Each scan may miss transient signals, and using edge detection and state holding can cleverly solve this problem.
Conclusion
Returning to the initially halted production line, the problem lay in the scan cycle. A complex communication program consumed a lot of processing time, causing the response of normal logic to slow down. By optimizing the program structure, we reduced the scan cycle from 28ms to 8ms, and the equipment finally regained its responsive behavior.
I hope this article serves as a rich cup of coffee, providing you with a clearer understanding of the PLC scan cycle. Remember, understanding the scan cycle not only helps you diagnose faults but also enables you to design more efficient and reliable automation systems.