Automation of boiler

Hi i am automating a boiler thru S7-300. I am using FB41 PID to control pressure, level and temp of the boiler (3 sepearate PIDs) i checked on PLC SIM. The PID is not functioning properly. it doesnt bring the LMN value near SET POINT. what should i do???...I have scaled, used PID and then unscaled again for the final output.

From what you wrote, I don't think you're using PID block properly. Did you placed it in OB35? LMN is control output and it is supposed to be sent to final control element (for example control valve) as output from PLC. PV or process value should match Setpoint.
Please, have a look at [url=http://plctalk.net/qanda/showthread.php?t=41422[/url]]this:

This is very good way to learn to use PID FB41. You can play with changing parameters and through PLCSIM observed how PV is changing. Since process is simulated inside program, there is no need for pheriperal inputs and outputs. Just look how I made connections with PID controller process FB (for example FOPDT). In reality, both Process value should be input (scaled) and LMN should be output (scaled). Setpoint could be also an input, depending on your way of changing it.

Good luck with your project.

Thanks for the help Pandiani



Yes I have called PID and unscaling block in OB35 bt the scale block is in OB1. I am sending LMN on unscale block what do you mean by “PV or process value should match Setpoint”?



The main problem is that the LMN always goes towards higher or lower limit specified in the inputs. The PID doesn’t work.



I cant understand the coding on FOPDT.

Can anybody please tell me that which inputs of FB41 PID should i activate and which ones de activate???

Just look at example program I attached on the previous thread.
Purpose of controller is to controlled some proces variable (PV). When operator defines setpoint (desired value), controller must ensure that process value (pressure, temperature, level, flow, or whatever) match setpoint i.e. actual value match desired value.
LMN is just controller output value. This must be sent to final control element (actuator). Look and play with my program for a while.

Raiya_23 said:

The PID is not functioning properly. it doesnt bring the LMN value near SET POINT. what should i do???...I have scaled, used PID and then unscaled again for the final output.

Click to expand...

It is not supposed to bring LMN close to SP_INT.

PV_IN value on the block is the actual value.
The block arranges PID output (LMN) , to make PV_IN equal to SP_INT.

So, probably you didn't enter anything to the actual value and PID output goes up to the high limit because there is no feedback.

Lets say you are controlling a pump motors speed to make sure the pressure on the pipe is 6.0 bars...
You'd need to enter pressure feedback to PV_IN (Process Value input) , 6.0 to SP_INT (Setpoint) and you will get a value between LMN_HLM and LMN_LMN to make the pressure 6.0 bars...

Hope it helps,