Cascade Plant or Parallel Plant (串聯或並聯)

by allenlu2007


在一般控制系統或信號處理中,有串聯式或並聯式。What’s the pros and cons?


What’s the problem of using parallel?


Why not multi-integrator?

two pole is fundamental, right?  polynomial factoring (one pole or two conjugated poles) => cascaded plants

But multi-poles plant/filter/controller do exist (e.g. sigma delta): always decompose to cascaded one pole or two poles plant? 

cascaded plant (串聯). How about parallel plant (並聯) for two poles or multi-poles?

cons:  matching!  amplitude matching; phase (delay) matching ==> very difficult to analyze stability !

no signal multiplication but signal addition or selection/switching (not good for small signal) 

Cons: distortion at cross-over point (class B)


How about Pros?

* Robust:  in cascaded system, one plant fails mean all fails.  Sometimes, not even fail, just saturation causes the entire system fail.  (e.g.  CIFB vs. CIFF in sigma delta modulator)

* High dynamic range:  (i) no cascaded gain, each plant has the same dynamic range;  (ii) control some plant (high Q) to saturate first to increase dynamic range)

* Most attractive:  noise cancellation (but be careful not signal cancellation: by out of frequency or operation band (hmos or pmos only for positive/negative signal), or inner loop (like chopper or cross coupling feedback));  offset cancellation (chopper)

* Stability seems to be better if control each path 2nd pole.

* Low latency; fast response