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Master Li Talks About PLC Function Blocks in Smart Homes
A few days ago, Xiao Wang came to my house and was amazed to see that I could control the lights, curtains, and air conditioning with my phone, his jaw almost dropped. “Master, how much does this cost?” I smiled and said, “Actually, we can just make a small modification to the PLC system we have in our factory; the key is the function block programming, which is much cheaper than those commercial systems that cost tens of thousands.”
Function Blocks (FB) are the “assistants” in PLCs, packaging a bunch of logic instructions into a module that can be used directly, saving the trouble of rewriting code. It’s like a rice cooker in the kitchen; you don’t need to worry about how it controls temperature and timing, just put in rice, add water, and press a button.
I remember when I was upgrading my smart system last year, the most troublesome part was the temperature control logic. Traditional ladder diagrams were long and cumbersome, and changing a parameter required flipping through several pages. Later, I had a brainwave and solved it all with function blocks.
The temperature control FB encapsulates the core PID algorithm, requiring only a few simple parameters to connect externally. Here is the core part of the temperature control FB I wrote:
// Core code for temperature control FB
FB_TempControl.SetPoint := Room_SetTemp; // Set temperature
FB_TempControl.ActValue := Room_ActTemp; // Actual temperature
FB_TempControl.Execute(); // Execute control algorithm
Heater_Output := FB_TempControl.Output; // Output to heater
Xiao Wang was still confused, “Master, how do you create this function block?”
This brings us to the key point. First, you need to understand the difference between function blocks and regular programs. You can think of a function block as a small program with “memory” that can remember the last running state. For example, your temperature control system needs to remember the last temperature to determine whether to increase or decrease, which requires the use of static variables in function blocks.
The most powerful feature of function blocks is their reusability and instantiation. What does this mean? Just like I have three rooms in my house that all need temperature control, with traditional methods, you would have to copy and paste the code three times. With function blocks, you write the logic once and then create three “instances,” each room using one, running independently without interference. It’s like a mold that can produce multiple identical but non-interfering products.
// Temperature control instances for three rooms
FB_LivingRoom(SetTemp:=22.0, CurrTemp:=LR_Sensor, Output=>LR_Heater);
FB_BedRoom(SetTemp:=20.0, CurrTemp:=BR_Sensor, Output=>BR_Heater);
FB_Kitchen(SetTemp:=21.0, CurrTemp:=KC_Sensor, Output=>KC_Heater);
Last week, Master Sun came to my house for a visit and said he wanted to set up a system for his home too. “How many programs do I have to write?” I told him the secret lies in modular thinking.
The smart home system may seem complex, but it is actually just a combination of several basic function blocks: input processing, control algorithms, and output control. Inputs can be sensor data or mobile commands; the control algorithm is the judgment conditions and execution logic; the output controls the actions of various devices.
Take the automatic curtain control in my house as an example; I built a light control FB that includes priority handling for three modes: light sensing, time judgment, and manual control. The code looks simple, but it handles many complex situations:
// Curtain control FB snippet
IF Manual_Mode THEN
Curtain_Output := Manual_Position; // Manual priority
ELSIF Light_Level < Min_Light AND Time > Morning_Time THEN
Curtain_Output := OPEN; // Automatically open in the morning when light is insufficient
END_IF;
Connecting multiple function blocks to form a complete system is a key skill. It’s like building with blocks, where each function block is responsible for a small part of the functionality, and then you connect them to form a complete control flow. This method makes debugging and maintenance super simple; you can fix issues where they occur without affecting the whole system.
The part of my smart system that surprised Xiao Wang the most was the mobile control. In fact, this is also thanks to function blocks. I wrote a communication FB that is responsible for converting commands from the mobile app into signals that the PLC can understand, and then distributing them to various control function blocks.
Finally, Xiao Wang asked a good question: “Master, how do you debug these function blocks?” This is indeed a key point.
The most important thing in debugging function blocks is to test them independently first, ensuring that each function block works correctly on its own before testing the overall functionality when combined. It’s like fixing a car; you check each part first, then assemble the car to see its overall performance. I usually create a test variable table to simulate various input situations and see if the output meets expectations.
In summary, PLC function block programming is about breaking down complex systems into manageable small modules, each responsible for specific functions, and then combining them to form a complete system. This method not only clarifies the program structure but also greatly improves reusability and maintainability. Next time you want to do a smart control project, consider trying function blocks; it can really save you a lot of trouble!
Remember, learning programming is like learning to drive; just watching won’t get you on the road. Find a small project to work on, make mistakes, correct them, and gradually you’ll become proficient.
