Core Technologies of Industrial Control Systems: Real-Time Data Processing and Task Scheduling Mechanism of Mitsubishi PLC

Today, let’s talk about the real-time data processing and task scheduling mechanism in Mitsubishi PLCs. These two concepts sound impressive, but they essentially refer to how PLCs can timely process data and arrange various tasks. This is crucial for industrial automation!

Real-Time Data Processing: PLC’s “Sixth Sense”

Basic Concept

Real-time data processing simply means that a PLC can complete data collection, processing, and output within a specified time. Just like humans, when we see a red light, we stop immediately; this is a form of real-time response. PLCs need this capability too, such as shutting down a heater immediately when detecting high temperatures.

Hardware Foundation

The real-time data processing in Mitsubishi PLCs mainly relies on:

1. High-speed CPU: Just like our brains, fast processing speed is crucial for timely responses.
2. Dedicated ASIC chips: These are like “assistants” for the PLC, responsible for specific tasks.
3. High-speed bus: This can be understood as the “highway” inside the PLC, enabling fast data transmission.

Data Processing Flow

1. Data collection: Reading sensor data through input modules.
2. Data processing: The CPU performs calculations based on the program.
3. Result output: The processed results are transmitted to actuators through output modules.

Note: The sampling rate should be chosen based on the characteristics of the control object. Sampling too slowly may lose important information, while sampling too quickly may increase the system’s burden.

Practical Application Case

Imagine you are operating an injection molding machine. When the mold closes, the PLC needs to precisely control the pressure and speed. The real-time aspect is reflected in:

1. Quickly reading pressure sensor data (in milliseconds)
2. Real-time calculation of the deviation between current pressure and target pressure
3. Timely adjustment of the hydraulic system’s output

If processing is not timely, it could lead to product quality issues, and in severe cases, damage the mold.

Task Scheduling: PLC’s “Time Management Master”

Basic Concept

Task scheduling refers to how the PLC arranges and executes various tasks. Just like we need to manage our time: prioritize important tasks and address urgent ones first.

Mitsubishi PLC’s Scheduling Mechanism

1. Cycle Scanning: The most basic operating mode, like doing things in a fixed order every day.
2. Timed Interrupts: Tasks executed periodically, such as collecting data every 1ms.
3. Event Interrupts: Tasks triggered by specific conditions, such as handling emergencies immediately upon detection.

Code Example

// Main program (cycle scanning)
MAIN
    M100 = X0 // Read input
    Y0 = M100 // Output result

// Timed interrupt program (executed every 10ms)
INT_10ms
    D0 = D0 + 1 // Counter increment

// External interrupt program (triggered when X1 changes from OFF to ON)
INT_X1
    Y1 = ON // Emergency stop

Note: Interrupt programs should be kept as short as possible to avoid affecting the execution of the main program.

Practical Application Case

Let’s take a simple production line as an example:

1. The main program is responsible for the regular control of the conveyor belt.
2. A timed interrupt is executed every 100ms to check the product’s position.
3. When an anomaly is detected (e.g., X1 is triggered), the external interrupt program is executed to stop the production line immediately.

This scheduling mechanism ensures the normal operation of the production line while being able to respond promptly to emergencies.

Common Issues and Solutions

1. Task response is not timely

• Cause: High CPU load or unreasonable interrupt settings
• Solution: Optimize the program and set task priorities reasonably

• Cause: Sampling rate may be improperly set
• Solution: Adjust the sampling rate based on the characteristics of the control object

• Cause: Possible failure of the watchdog timer
• Solution: Check and configure the watchdog timer correctly

Important Reminder: When performing real-time control, safety must be prioritized. For example, set safety interlocks to ensure that the system can safely stop in emergencies.

Practical Recommendations

1. Start with simple tasks and gradually increase complexity.
2. Use a simulator for initial testing before validating on actual hardware.
3. Learn to use Mitsubishi’s GX Works3 software, especially its monitoring features, which are very helpful for debugging.
4. Pay attention to the PLC’s CPU utilization to avoid overload.
5. Regularly back up programs, especially before making significant modifications.

Theory is no match for practical experience. Get a small PLC development board and try the above examples. Don’t be afraid of encountering problems; the process of solving them is true learning.

The core of control systems lies in how to process data quickly and accurately and respond accordingly. Mastering these will bring you one step closer to becoming an industrial control expert!