PID 313C Step7 Siemens

dahnuguy

Member
Join Date
Mar 2007
Location
SC
Posts
720
Let me begin with a few general details.

I am new to Step7

I have read as many posts here about PIDs as I could.

I do not find an answer to my issue.

I have the Simatic PID Temp COntrol Manual.

I have read it.

I understand the basics of PID, but I do not think this is the issue.

I have no errors on the PLC. I have a setpoint, I have Man_On set to off with a false BOOL. I reset the block FB58 at COM_RST with a "first scan" BOOL.

I enabled the pulse generator as I need to use a temperature in from a thermocouple to control an electric air heater. A separate PID will run a similar loop to heat oil and a third will be used to control air pressure.

If I can get the first one to work I should be OK. I have some experience tuning PID, but this one will not even turn on.

The PV is 21 and the setpoint is 30. It has been at this point for one hour.

No output.

I am sure it is a small thing I have overlooked.

Any ideas?
 
The value of 4.2e-044 assigned to MD110 used as the setpoint is not 30.0 as you stated. It looks like you have loaded an integer value of 30 and stored it in a floating point variable.
 
Last edited:
L D[AR2 said:
The value of 4.2e-044 assigned to MD110 used as the setpoint is not 30.0 as you stated. It looks like you have loaded an integer value of 30 and stored it in a floating point variable.


I would have never caught that, but if you change the "right click representation" to decimal from automatic, then it is 30.

But since it is not 30.0, it probably is an integer.

I just checked and the MD110 variable is defined as a real.

30 gets moved into MD110 with a move instruction.

I will try using 30.0

INSTANT SUCESS!!!!!!!

nice, I would have never found that.

I will skip the whole "Why can't S7 tell 30 is the same as 30.0" thing.

I thought if I defined the destination as a real, then any number I put in it would become a real.

That is exactly the sort of small thing that will hang you up.

Thanks sooooooooooo much.
 

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