Engine Control Question 1

[FatCat]

Hi there,

I'm currently an engineering student doing a research paper and I have a question regarding engine control systems:

What is the problem with using PID controllers for controlling engine idle? From what I've read, many state that PID's cannot do non-linear systems very well but I've found that they can do non-linear systems quite well, so what is the real problem with using PID's?

 

[FireLover]

PID control loops are used to control idle. However, they are typically more than just a PID loop. Usually a feed forward term is added, along with an adaptive scheme.
Basically the feed forward term provides the base idle air control valve command signal in open loop form. That is, based on the desired engine speed, inlet air temperature, ambient air pressure, coolant temperature, etc., a base command signal is determined (via lookup tables and/or a formula). Then a PID loop provides a term that is added to the feed forward term to maintain the desired speed. The feed forward portion essentially takes out the non-linearity, and the PID loop just trims out the errors in the feed forward term and changes over time.

The adaptive term is a third term that remembers what PID correction was required under a certain set of conditions (engine speed, air temp., etc.) and adds it to the feed forward term as well. This is a very slow loop to account errors and slow system degradation over time. It allows the PID term to remain near to 0 most of the time.