what is the calculation delay in PID used for?

Question

Hello everybody!

I am using the PID example giving in buck converter current mode control. First, I could not find where it is found in the library so I used the example given in the models.

secondly, whenever I put the calculation delay in the Digital C-Script-based PI to 0 value, my system crashes and shows errors, anyone can clarify what is it about?

Thank you

Answer 1

Hi EMdude,

The PI controller is not in the library, but you can use the one from the example in your models.

Setting the calculation delay to zero is not a valid input for the C-Script based PI-Contreller. However you may put in a very small value to make it's effect negligible.

Understanding the C-Script requires deeper knowledge of the PLECS system and using it is not recommended for the beginner. The C-Script chapter in the documentation is a good read and also gives some examples.

Cheers

Falk

Comments

Hi Falk, I appreciate your response.

I have another question related to PID which is stupid but I have to ask it because I am cunfused all the time about it. in the example given of  C-Script based PI-Contreller.

I am not sure what I actually should insert in the Kp and Ki variable, is it

Kp=0.45*Ku

Ki=Kp/Ti.

or

Kp=k

Ki= Ti

 

please find the picture attached

cheers,

EMdude

---

If you open the Controller block, you can have a look at the implementation (Ctrl+U) of the digital as well as the analog controller. The digital version is implemented using a C-Script block and the analog one uses gain blocks and a continuous time integrator. Both versions are implemented in the known parallel controller structure (as opposed to serial).

I would suggest you use the analog version if you don't have experience in discrete time domain modelling. The PI-element can be expressed as a n inverted zero in the s-domain. R. W. Erickson has some good lecture notes on bode plots, google it.

G(s) = k_r * (1 + w_c / s)

When packing the Analog PI version of the controller into an equation, we get

G(s) = k_p + k_i / s.

Using coefficient comparison you, can determine the values for k_p and k_i:

k_r  + k_r * w_c / s  = k_p + k_i / s

->> k_p = k_r

->> k_i = k_r * w_c

For the digital version, there is more stuff involved.

Hope this helps.

Falk