Do block diagrams provide any value in analyzing non-LTI systems? For LTI systems, block diagrams permit a powerful algebra for manipulation/reduction of a system. Are there classes of non-LTI systems (e.g. restricted classes of hybrid systems, restricted classes of non-linear systems, zero-order hold systems) where the structure of a block diagram also facilitates analysis?
Edit: I don't think I was precise enough with my original question. What I really want to understand is whether block diagrams are useful for proving properties (e.g. stability) of non-LTI systems.
The answer is yes.
For example, Simulink code can be written in the this form. And block diagrams are used in the linear systems containing discrete and continuous parts, including holds and converters, in exactly the same manner as they are used in continuous-time linear systems.
Block diagrams are always more useful as a tool to describe and visualize a system than as a construction subject to explicit algebraic rules. For precise analysis and manipulation, system equations are almost invariably more precise and transparent. An example: in linear systems Mason's rules can be used to manipulate block diagrams and obtain transfer functions. Ultimately stability and other properties will be studied by looking at transfer functions. The value of formal block diagram manipulation methods is limited. Moreover extension to general, possibly nonlinear situations, is not easy.