Step 7 PID With P term only

barryoc

Lifetime Supporting Member
B
Join Date
Nov 2009
Location
Kilkenny
Posts
148
Hello everyone,

From an basic instrumentation course I attended i wanted to try PID control in S7 300 PLC.

I'm using fb41 called in OB35 with both having cycle time of 100ms. My problem arises when I change SP_INT. In theory with P term only the following occurs,

PV = 50%
SP = 50%
MV = 50%

PV = 40%
SP = 50%
MV = 60% or 40% depending on Direct or Reverse Acting.

I have tried as much as i possible can to replicate this in S7 300 but with no success. The only success i have had is when i set gain to -1.0 and the SP to 0.0, LMN HLM 100, LMN LLM 0. The input is scaled in FC105, 0 - 100. With this i get the expected results on the PV and the MV but they make no sense as the SP is 0. I have read that the SP is -100 to 100% or a real value. How do i enter a percentage on the SP_INT input?

Any guidance would be appreciated.

Regards
Barry.
 
It is suggested that you should use % for Setpoint feedback and output but you can use real units if you wish, you just need to be aware of what the limits are.

Highlight fb41 in ob35 and press f1 to bring up the help. Use the S7-PID config tool (All Programs\Simatic\Step 7\Standard PID Control parameter Assignment). N.B. If you hard code the gains and other settings for FB41 you will not be able to change them from the config tool.

Nick
 
barryoc said:
Hello everyone,

From an basic instrumentation course I attended i wanted to try PID control in S7 300 PLC.

I'm using fb41 called in OB35 with both having cycle time of 100ms. My problem arises when I change SP_INT. In theory with P term only the following occurs,

PV = 50%
SP = 50%
MV = 50%

PV = 40%
SP = 50%
MV = 60% or 40% depending on Direct or Reverse Acting.

I have tried as much as i possible can to replicate this in S7 300 but with no success. The only success i have had is when i set gain to -1.0 and the SP to 0.0, LMN HLM 100, LMN LLM 0. The input is scaled in FC105, 0 - 100. With this i get the expected results on the PV and the MV but they make no sense as the SP is 0. I have read that the SP is -100 to 100% or a real value. How do i enter a percentage on the SP_INT input?

Any guidance would be appreciated.

Regards
Barry.
Click to expand...

1: The MAN input must be 0 (=AUTO).
2: What kind of process do you have? Loopback?
3: If you use loopback: try with a very low gain, f.x. 0.1
4: If you want better help, post a snapshot to show your settings.

Kalle
 
barryoc said:
Hello everyone,

From an basic instrumentation course I attended i wanted to try PID control in S7 300 PLC.

I'm using fb41 called in OB35 with both having cycle time of 100ms. My problem arises when I change SP_INT. In theory with P term only the following occurs,

PV = 50%
SP = 50%
MV = 50%

PV = 40%
SP = 50%
MV = 60% or 40% depending on Direct or Reverse Acting.

I have tried as much as i possible can to replicate this in S7 300 but with no success. The only success i have had is when i set gain to -1.0 and the SP to 0.0, LMN HLM 100, LMN LLM 0. The input is scaled in FC105, 0 - 100. With this i get the expected results on the PV and the MV but they make no sense as the SP is 0. I have read that the SP is -100 to 100% or a real value. How do i enter a percentage on the SP_INT input?

Any guidance would be appreciated.

Regards
Barry.
Click to expand...

You cant get your expected result as your expected result is wrong. Check your math.
 
barryoc said:
Hello everyone,

From an basic instrumentation course I attended i wanted to try PID control in S7 300 PLC.

I'm using fb41 called in OB35 with both having cycle time of 100ms. My problem arises when I change SP_INT.
Click to expand...
I am not a S7 FB41 user but I know about PIDs. What is SP_INT? The set point in integer form? If so then

In theory with P term only the following occurs,

PV = 50%
SP = 50%
MV = 50%
Click to expand...
How can the MV be 50% if there is no integrator and no error? What term produces the output? Is your theory wrong?
 

Similar Topics

N
Question about Siemens Step 7 PID
Is it Gain or Proportional band? The HMI shows it as Proportional Band and the tag name is Gain???
Replies
3
Views
2,161
NetNathan
N
B
pid of moteur DC with step 7
hello , pid of moteur DC with step 7, i'need your help please , give me some information about this project
Replies
1
Views
1,123
boneless
boneless
N
Step 7 ET200s - PID controller - dt problem
How can I get the curret "t" in SCL dt=tnow-tprevious I dont know how to get "tnow". I am not sure should it be in s(seconds) or miliseconds. I...
Replies
4
Views
2,099
KalleOlsen
K
B
Converting Temperature PID from Siemens Step 7 to Portal
I'm in the process of converting over a temperature loop from Step 7 to Portal. The old Step 7 program uses FB58 TCONT_CP for the PID block. In...
Replies
0
Views
4,313
Barry Kirk
B
I
PID in Step-7
Hello guys! Could anyone help me to understand how do i set the PID (FB41) action mode Direct or Reverse in Step-7? Thank you in advance and...
Replies
2
Views
1,713
Indiaum
I
Back
Top Bottom