To obtain an open-loop transfer function from a model, you specify
a loop opening. Loop openings affect only how the software recombines
linearized blocks, not how the software linearizes each block. In
other words, the software ignores openings when determining the input
signal levels into each block, which influences how nonlinear blocks
are linearized. Consider the following model, where you obtain the
transfer function from
with the outer-loop open at
Here, k1, k2, g1, and g2 are nonlinear.
The software linearizes each block at the specified operating
point. At this stage, the software does not break the signal flow
y1. Therefore, the block linearizations include
the effects of the inner-loop and outer-loop feedback signals.
K1, K2, G1, and G2 are the linearized blocks.
Finally, to compute the transfer function, the software enforces
the loop opening at
y1, injects an input signal
e2, and measures the output at
The software returns (I+G2K2)-1G2K2 as the transfer function.