Today, Lao Liu shares a practical PLC temperature control case. Through this case, you can not only grasp the essence of PID control but also understand the most common temperature control applications in industrial sites.
1. Core Knowledge Point Analysis
The temperature control system may seem simple, but it actually hides many secrets. The key is to understand the characteristic of “temperature response lag.” For example, when boiling water, the temperature continues to rise even after the fire is turned off, which requires our control strategy to “predict” in advance.
2. Key Points for Hardware Selection
Choosing the S7-1200 PLC is not excessive at all. The main considerations are:
- CPU processing power is sufficient to run the PID algorithm
- Built-in analog functions save the need for additional modules
- Affordable price with expansion capabilities
- Programming software TIA Portal is user-friendly
3. Program Structure Optimization
Those who have seen the code may notice that the parameter settings for the PID control block are quite conservative:
P_Gain := 2.0, I_Time := T#20S, D_Time := T#5SThis is a summary of years of experience; overly aggressive parameters can cause temperature to oscillate wildly. It is recommended to start with small parameters and gradually increase them.
4. Debugging Tips
There is a practical method for debugging temperature control systems:
- Set the I time very large, essentially equating to pure P control
- Adjust the P value until there is slight overshoot
- Gradually decrease the I time to eliminate steady-state error
- Finally, add a small amount of D action to improve response speed
5. Practical Considerations
Key points for the installation of PT100:
- Installation location is crucial:
- Away from airflow interference
- Avoid heating dead zones
- Ensure good thermal conduction
- Signal line handling:
- Must use shielded wire
- Away from high voltage cables
- Reliable grounding is essential
6. Common Problem Solutions
When encountering temperature fluctuations, the troubleshooting order is:
- Check sensor installation
- Verify the rationality of PID parameters
- Check for interference sources
- Confirm heater power matching
7. Expansion Application Ideas
This system can be slightly modified for use in:
- Injection molding machine temperature control
- Constant temperature water tank
- Fermentation tank control
- Industrial oven
Practical suggestions:
- First use a signal generator to replace the sensor for testing the program
- Set an appropriate alarm dead zone to avoid frequent alarms
- Remember to add a soft start function to protect the heater
- Ensure there is sensor fault protection to cut off heating in time
The temperature control system is the foundation of industrial automation; mastering this case will greatly assist in future work. Lao Liu suggests beginners can start by setting up a small system with a heating rod and water tank to experience the fun of PID debugging.
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