5/26/2011 9:28 PM | |
Joined: 1/28/2009 Last visit: 9/10/2024 Posts: 6849 Rating: (1365) |
Tunning a controller ensures us that the response of the algorithm to the SETPOINT change or process's disturbance has in range over(under)shoot, rise time and settling time.Sudden Jump in output of a PID controller is due to mode change (auto to manual or vice versa) or gain change in runtime. The topic is discussed in "Controlling with SIMATIC" published by SIEMENS . The reason explained and remedies provided(section 5.2 of book).
This is explained in the following topic by dchartier and me. Please note that , You may select either of SP>PV or PV>SP. For example, in a tank which start from 0 to setpoint to be filled is like SP>PV while colling a process from ambient down to a setpoint is like PV>SP. If you select the action type wrongly , the controller may not function properly.If you have a process in which SP>PV so error is positive and PID functions properly.But if you wrongly chose PV>SP, you have always negative value and and PID may generates 0 in LMN. Determine reverse or direct action of PIDhope to be helpful hdhosseini |
5/28/2011 11:35 AM | |
Joined: 1/28/2009 Last visit: 9/10/2024 Posts: 6849 Rating: (1365) |
Error in PID algorith is calculated by SP-PV.So, if you havePV>SP ,error generated by algorithm is always negative and it may not have proper result. You should select the negative Gain to be multiplied by negative error and lead to a positive value feed to algorithm.
The speed of response or let say rise time is one of the characters of |
5/28/2011 12:40 PM | |
Posts: 53 Rating: (0) |
Hi Hdhosseini, |
5/28/2011 9:02 PM | |
Joined: 1/28/2009 Last visit: 9/10/2024 Posts: 6849 Rating: (1365) |
Please check the attachment,Sorry for drawing my own curve for step response of a process( I am afraid of copyright infringement). As shown in the most important part SP>PV and the control valve is opening. This is the time in whicj the overshoothappens PV>SP, so this is the nature of PID controller to make the error zero.The negative error will force the control valve to close.
I will check you codes again,please provide trends to judge better.Did you modify the codes or they are finalized? regards hdhosseini |
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