PID AutoTune

[Vidmas30]

Hi,

How long does it usually takes PID to autotune? And is it necessary to connect plc to process to do PID autotune? I have PLC on my desk and just for curiosity started PID autotuning, status =0, so just interesting: is it calculating something or nothing happens..

[Emil]

PID parameters are unique for each system. Finding "manually" these parameters takes time even to quite experienced PID user.

Autotune procedure performs few On-Off cycles. The number of these cycles (1-3) is set in the module.

How this operates:

At the beginning, the controller set CV to max.

Comming ot setpoint, it switches CV to 0%.

After PV falling to SP minus some hysteresis, it starts the next On-Off cycle.

During this process, PV makes "waves" of overshoot and undershoot around setpoint. These waves are unique for each system. The controller learns these waves and according to their parameters - length, amplitude, asimetric... calculate the best PID parameters.

How long it will take?

This depends on specific PID system and number of cycles chosen. In some systems it can take less than 5 seconds per cycle, in others - more than 5 hours!

Activate Autotune has any sense only in a real closed loop system, on real working conditions.

Activating Autotune "on the desk" and trying to simulate On-Off process - turning some potentiometer in the simulator or moving temperature sensor from glass with hot water to glass with cold water can show if the logic works at all, but PID parameters, calculated this wy have nothing common with those one, neded for the real system to run properly!

[Vidmas30]

Thanks, Emil,

I understood it, when read Pid server's help file:) I have one more question: when assigning operands to Pid server, 32MI's autotune vector has to be the same as in the PLC's PID FB? And what for are operands CVp, CVi, CVd, RSTintgrl? Thank you!

[Emil]

Hi Vidmas,

PID output CV = CVp + CVi + CVd (Proportional + Integral + derivative components of Control Value). Vision gives the option to read these components. You need to set FB PID module "Read Control components". As PID calculations are performed on 32 bit base, the original numbers are quite big and usually not easy for understanding and plot with PID server. That's why in left side you can set Divider. According to my experience, the best value to set there is 100, but other values can be set too.

Read Control Components needs to be activated with continious condition. It can be located on Main Routine and connected direct to the left line.

The values on right of the module will give you CVp, CVi and CVd. Then, in PID server, you will set the addresses of mentioned MI and will get trends of these 3 components, which will give you closer look on what happens inside PID.

Yes, the address of the 32 MI vector must be the same.