Confusions about PID controllers
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I have just started working on a project in which it is necessary to control the opening degree of control valves on the steam pipes using a PLC, in order to regulate the steam flow and thus maintain the temperature inside the heating furnace within the desired range. The temperature inside the furnace is also fed back to the PLC via thermocouples, creating a closed-loop control system. I wonder whether, for this purpose, I should use the PID program built into the PLC or an external PID controller. If I use the former, how should it be tuned? And if I use an external controller, how should that one be adjusted? I am also not quite clear about the structure of PID controllers and how they connect and work in conjunction with a PLC. I hope that those with experience in such projects can offer some guidance, or perhaps share some images or documents to help clarify things
Upstairs, it seems that posting pictures on the forum is done by browsing and uploading them; the /IMG reference path doesn’t seem to work.
A heating furnace isn’t it burned by gas? What is the purpose of using steam?
PLCs have dedicated temperature control modules
It depends on what type of PLC the poster is using. If the PLC has a PID control module, it’s best to use that module; in this case, simply set the PV value of the PID module to correspond to the PV value of the thermocouple, and connect the MV output of the PID module to the output of the control valve, thereby completing the hardware setup for this control loop. If the PLC doesn’t have an internal PID module, then an additional regulator will be necessary.
It’s not the kind of heating furnace used in steel mills; it raises the temperature inside the furnace mainly through steam
Use the PLC’s PID module; connect the wires as usual, and invoke the PID module through configuration.
The key to PID is the adjustment of parameters
Using a PLC’s PID is more cost-effective.
If there are no special requirements, using a PLC is fine.
I agree with the views of the comrades above: lol
Hello, expert on the 6th floor! I currently have a system similar to the one mentioned by the original poster; mine is a PID pressure control system. It uses the analog output module of a PLC to control the opening degree of a proportional control valve, thereby maintaining the actual pressure at the set value. The actual pressure is read through an analog input module (the range of the actual sensor is 1000 kilograms, with a signal of 4–20 mA; this value is converted to Newtons in the program). My program is currently under debugging. I would like to ask whether, in the PID calculations, the units and data types of the PV value and SV value need to be consistent. Since the sensor data is in terms of mass, I converted it to newtons; the resulting value is a floating-point number. If the units are consistent, then the unit of the MV value will also be newtons, just like those of PV and SV. Is the calculated MV value sent directly to the analog output module? The adjustment range of the proportional control valve is 0.05Mpa–0.9Mpa and 0–20ma. Furthermore, the characteristic curves of my analog input and output are inconsistent. The corresponding digital values are also inconsistent. What should be done if the MV value is not for direct shipment?
Both the internal PID of the PLC and an external PID controller can meet your requirements; however, the external PID may have a higher failure rate, but it is more convenient for operators on site to use.
It’s similar to a closed-loop feedback system; one can refer to the method of adjusting the motor speed based on the actual DO value, which is also achieved through analog output. I couldn’t find that article in my forum anymore, but the principle is the same – the units must be consistent, and there seems to be some conversion factor for MV values!