As a professional technician in the field of programmable logic controllers (PLCs), understanding the working steps of PLC cycle scanning is essential. PLCs play a crucial role in industrial automation and are widely used for controlling and monitoring various processes and equipment. In this article, we will take a closer look at the steps involved in PLC cycle scanning, using an AC motor relay control circuit as an example.
The PLC cycle scanning process can be broken down into four main steps: input scan, program scan, output scan, and housekeeping. Let's delve into each of these steps in detail.
Input Scan
The first step in the PLC cycle scanning process is the input scan. The PLC's input modules are responsible for reading signals from sensors and other input devices, which are then converted into binary data. These input signals are then sent to the memory of the PLC, where they are stored until the next scan. During the input scan, the PLC compares the current input signals with the previous scan's signals to determine if there have been any changes.
Program Scan
After the input scan, the PLC moves on to the program scan. This is where the PLC's central processing unit (CPU) executes the user's program logic. The program scan involves reading the stored input signals from the previous step and executing the corresponding program instructions. The PLC's program is typically written in a ladder logic diagram, which is a graphical representation of the control logic. The program scan happens rapidly, with the PLC checking each program instruction in sequence.
Output Scan
Next, the PLC moves on to the output scan. In this step, the PLC's output modules are triggered based on the instructions received during the program scan. These output signals are then sent to devices such as motors, pumps, and valves, which perform the desired actions. The output scan is crucial as it controls the actual physical process or equipment, making it a critical step in the PLC cycle scanning process.
Housekeeping
The final step in the PLC cycle scanning process is housekeeping. This step involves updating the PLC's internal memory and performing any necessary maintenance checks. During the housekeeping step, the PLC also checks for any errors or malfunctions and takes the necessary corrective actions. This step ensures the smooth operation of the PLC and helps prevent any potential issues from arising.
Now that we have covered the four main steps of the PLC cycle scanning process let's look at how these steps work together in a control circuit for an AC motor using relays.
Let's say we have a control circuit for an AC motor with a start and stop button, as well as an overload relay for protection. In the input scan step, the PLC's input module will detect the status of the start and stop buttons and send the signals to the PLC's memory. During the program scan, the PLC will check the status of the buttons and execute the corresponding program instructions. If the start button is pressed, the relay controlling the motor will be energized, and the motor will start. If the stop button is pressed, the relay will de-energize, and the motor will stop. The program scan will also continuously monitor the status of the overload relay to ensure the motor's protection. In the output scan step, the PLC will send the appropriate signals to the motor relay based on the program's logic. Finally, during the housekeeping step, the PLC will check for any errors and perform any necessary maintenance tasks to ensure the smooth operation of the motor control circuit.
In conclusion, understanding the working steps of PLC cycle scanning is crucial for any professional technician in the field of PLCs. The input scan, program scan, output scan, and housekeeping are all essential steps that work together to ensure the proper functioning of a PLC and its control circuits. With the rapid advancement of technology, PLCs continue to play a significant role in industrial automation, making it essential for technicians to have a comprehensive understanding of their operation.
So, the next time you work with a PLC, remember the four steps of cycle scanning and how they work together to make automation possible. With a solid understanding of PLCs and their operation, you can confidently tackle any automation task with ease.