Pid controllers

[Guest Dick]

I want to control level of a tank [10 Meters height] using PID controller [sLC processor]. The I/P is 4-20 mA[level transmitter] and O/P is also 4-20 mA [current to pressure converter] The setpoint is adjustable [4 to 7 meters] from HMI I use SCP instruction at I/P and O/P and the PID data format is used[0 to 16383].    Input SCP I/P     - I:2.0 I/P Min - 4000 I/P Max - 20000 SCL Min - 0 SCL Max - 16383 O/P     - N7:1    Output SCP I/P     - N7:2 I/P Min - 0 I/P Max - 16383 SCL Min - 6242 SCL Max - 31208 O/P     - O:3.0 The PID is also configured Now the question is about scaling adjustable setpoit! What should be I/P Min and I/P Max for setpoint in SCP instruction? I/P     - N7:6 (from HMI) I/P Min - ? I/P Max - ? SCL Min - 0 SCL Max - 16383 O/P     - N8:2 (This is setpoint inside control block of PID)

[chrisbramall 1]

The setpoint should be scaled over the same span as the level signal. If the tank level from the level meter is 10m then 00m =  4000 (I/P) = 0 (PID ) 10m = 20000 (I/P) = 16383 (PID) If you have entered the setpoint from the LOI in m then you would use 0 for the IP/Min setting and 10,000 for the IP/Max

[chrisbramall 1]

Have you thought of posting the code to the level control program to get some feedback. Also I notice that your using engineering units for the input card  but not for the output card. Any particular reason ? Best Regards, Chris Bramall, UK

[Guest Dick]

Many thanks for the response! The output element is driven by 4-20 mA. So when 1746-NO4I (analog O/P module) is used, 6242 corresponds to 4mA and 31208 corresponds to 20mA. Well I agree with your point of scaling the setpoint same as level signal. If a condition arise, like operator can't set any level than between 4 and 7 meters - is this possible using SCP? I beleive there must be a mathematical relation. Ofcourse the above logic can be implimented by checking for the limits and moving only the value within range! Regards!

[chrisbramall 1]

Several of our customers are now specifying the isolated input and output cards from spectrum controls which do offer the option to use engineering scaled inputs and outputs which do make it easier to follow or fault find from the real world to the Software. Best Regards, Chris Bramall, UK

[chrisbramall 1]

on a side note, I take it you are using a panelview to enter the required level ? Why don't you just set the data entry limits to minimum 4m maximum 7m then the operator can't set a level outside the acceptable range. Best regards, Chris.