Limitted Gain PID

[Rayden]

Hi for all,

i work with labview 14 SP1.

i wana develop control loop with Labview PID but parametre Gain is limitted at 128. but i want to introduce a value like 500 or 1200 or 2100.

what i must do thanks for help.

[GerdW]

Hi Rayden,

I use the PID functions quite often and never had any limits on PID parameters.

(When you are you working with FPGA functions you should explicitely tell us.)

Otherwise just attach an example VI showing the problems you encounter!

[Alexander_Sobolev]

128 seems like Int8 limitation... Can you check control representation? Do you have coercion dot on pid vi?

[Rayden]

It is an FPGA PID with a range from -127 to 127  so 16 bits its a cluster of 3 éléments

picture is linked .

[GerdW]

Hi Rayden,

this has been asked some days ago...
You need to scale your process variable and output on your own to a reasonable range! When you need a P value of 2048 I would set gain to 64 and scale after the PID with a factor of 32...

[Rayden]

Hi GerdW

i understood that i multiple ouput of PID block "output_régulateur" by coefficient, its that ?!!! great idea . But i think that is availble only for proportionel Gain and not other gaind like Kp ( integrale gain) or Kd (derivative gain) 😕

[GerdW]

Hi,

Ki and Kd describe time values, independent from gain...

[nathand]

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@GerdW wrote:
Ki and Kd describe time values, independent from gain...

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Not in the FPGA PID, when wiring the gains directly. Also, technically, "Ki" and "Kd" are gains; "Ti" and "Td" are time periods.

Look at the FPGA PID help to see how to calculate the gains from Kc, Ti, and Td. It might be easier to scale the input down, than to scale the gains up.

[Rayden]

Please more détails when ansering

 How to scale the input down  !!! i add multiple bloc before or after PID.

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[GerdW]

Hi Rayden,

you should not use multple "blocks" but just one multiply function… 🙂