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s7-1200 PID Control help

Created by: Aminadra at: 6/15/2010 10:17 AM (2 Replies)

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6/15/2010 10:17 AM	Rate (0)
Aminadra Posts: 27 Rating: (4)	Hello, i need to maintain a Diesel Engine at a constant speed. i have a magnetic pickup sensor and a frequency to analog converter and i can read the rpm in the plc. however i need some help using the PID function to maintain the constant speed. using the PID block, i assigned the following: Input_per = analog input from the converter setpoint= desired value Input = variable of the user program ? could anyone explain to me the above parameter and how can i make the PID block work? Aprreciating any help! Regards Aminadra
Aminadra Posts: 27 Rating: (4)
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6/15/2010 12:28 PM	Rate (0)
Aristotelis Posts: 147 Rating: (34)	Hi Aminadra i have a magnetic pickup sensor and a frequency to analog converter and i can read the rpm in the plc. however i need some help using the PID function to maintain the constant speed. using the PID block, i assigned the following: Input_per = analog input from the converter setpoint= desired value Input = variable of the user program ? could anyone explain to me the above parameter and how can i make the PID block work? In control, you usually have 2 different kinds of variables: Manipulated variable & Actual value (terms used by the Step 7 Basic v10.5 help). The one you are able to change is the Manipulated variable. The other (Actual value) depends on the Manipulated variable, but you cannot change directly. So, the whole point of control theory is: "How should we change the Manipulated variable, in order for the Actual value to go where we want it to go?" (We may be interested in "How will it go there?", but this is another topic. Finally, we define as Setpoint the: "where we want it to go" point. In PID_Compact, you are using either "Input" or "Input_per", never both. (Are you aware of the two buttons on the upper right hand side of the LAD block? "Commissioning" & "Configuration"? If you press "Configuration" you see a menu where you can change these & other settings) So, the frequency to analog converter is connected to the Analog Input of your PLC, and Input_per is defined as getting the signal from this particular Analog Input. So now the PLC knows what the frequency of the motor is & where to get it. Regarding your output, I think that it is sound if you use "Output_per", which is the direct manipulation of the Analog Output of your PLC, where you have connected the Manipulated variable (which, since you're not mentioning in your post, I do not know what it is, may be a fuel valve etc). Important: Do NOT forget to tune your PID controller by pressing on the "Commissioning" button. Essentially, in this step we are determining the P, I & D (Proportional, Integral & Derivative) characteristics of your controller, in order for the controller to compensate for your system's gain & time constants. Eventually, what is happening is that you get a graph of the "Actual value" versus "time". I will not go into details regarding stability, overshoot etc, but if this graph looks good to you, then you are done. If not, try to play a bit with the P, I & D variables of your controller until you get it right. Hope this helps
Aristotelis Posts: 147 Rating: (34)
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6/15/2010 1:57 PM	Rate (0)
Denilson Pegaia Platinum Expert Joined: 9/23/2005 Last visit: 9/10/2025 Posts: 4416 Rating: (1485)	Hi, some tips could about PID could be found in the TIP: /tf/WW/en/Posts/33957#top
	Denilson Pegaia Siemens Industry Sector Technical Support
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s7-1200 PID Control help

Created by: Aminadra at: 6/15/2010 10:17 AM (2 Replies)

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