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12/1/2010 3:24 PM | |
Joined: 9/27/2006 Last visit: 10/17/2024 Posts: 12289 Rating: (2687) |
Hello srv; As confirmed by the following FAQ, to reverse the action of a PID control FB, simply put in a negative value for the GAIN parameter: Reversing the direction of control action of the controller "CONT_C" (SFB41/FB41) This is due to the fact that the PID algorithm used if a "dependant gain" type, where the value of the proportional gain is used in every calculation (of P, I and D action). So a negative value of GAIN will implement a negative output value, therefore reversing the action. See the diagrams in the following FAQ: What is the meaning of the parameters "GAIN", "TN" and "TV" with the CTRL_PID block? Hope this helps, Daniel Chartier Edit: Thanks to hdhosseiny for pointing out that the links I posted did not updateproperly and showing the correct address for my first link. Here is the right address for my second one: http://support.automation.siemens.com/WW/view/en/5800170 D.C. |
Last edited by: dchartier at: 12/1/2010 5:15 PMLast edited by: dchartier at: 12/1/2010 5:13 PMLinks not updated properly. |
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12/1/2010 4:45 PM | |
Joined: 1/28/2009 Last visit: 10/11/2024 Posts: 6852 Rating: (1365) |
Dear member This is common notation of a closed control loop in academic literature.For following diagram the error which is fed to PID algorith is calculated by (SP-PV). In this case positive gain cause no change in error sign. + SP--->.---PID---SYSTEM---- -^ | |PV | ------------------------------- But for this diagram, -(SP-PV) is fed to PID so you need a negative gain to multiply it to error in 3 parallel branches of PID algorithm. - SP--->.---PID---SYSTEM---- +^ | |PV | ------------------------------- despite the fact that negative gain is discussed briefly in control academic reference books, it is seems to be a strange parameter (But it is mathematically OK).As a user of modular PID control package ,I prefer calculating error out of PID algorithm {(sp-pv)in case of direct and (pv-sp) in case of reverse action} and also process this value further with DEADBAND or A_DEAD_B function and then put the result in the ER parameter of PID algorithm .
check this link (http://support.automation.siemens.com/WW/view/en/37306605) regards |
This contribution was helpful to3 thankful Users |
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