Control Systems/Signal Flow Diagrams
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Signal Flow Diagrams 
Signal Flow Diagrams are another method for visually representing a system. Signal Flow Diagrams are especially useful, because they allow for particular methods of analysis, such as Mason's Gain Formula.
Signal flow diagrams typically use curved lines to represent wires and systems, instead of using lines at right-angles, and boxes, respectively. Every curved line is considered to have a multiplier value, which can be a constant gain value, or an entire transfer function. Signals travel from one end of a line to the other, and lines that are placed in series with one another have their total multiplier values multiplied together (just like in block diagrams).
Signal flow diagrams help us to identify structures called "loops" in a system, which can be analyzed individually to determine the complete response of the system.
Mason's Gain Formula 
Mason's rule is a rule for determining the gain of a system. Mason's rule can be used with block diagrams, but it is most commonly (and most easily) used with signal flow diagrams.
Forward Paths 
A forward path is a path in the signal flow diagram that connects the input to the output without touching any single node or path more than once. A single system can have multiple forward paths.
A loop is a structure in a signal flow diagram that leads back to itself. A loop does not contain the beginning and ending points, and the end of the loop is the same node as the beginning of a loop.
Loops are said to touch if they share a node or a line in common.
The Loop gain is the total gain of the loop, as you travel from one point, around the loop, back to the starting point.
Delta Values 
The Delta value of a system, denoted with a Greek Δ is computed as follows:
- A is the sum of all individual loop gains
- B is the sum of the products of all the pairs of non-touching loops
- C is the sum of the products of all the sets of 3 non-touching loops
- D is the sum of the products of all the sets of 4 non-touching loops
- et cetera.
If the given system has no pairs of loops that do not touch, for instance, B and all additional letters after B will be zero.
Mason's Rule 
If we have computed our delta values (above), we can then use Mason's Gain Rule to find the complete gain of the system:
Where M is the total gain of the system, represented as the ratio of the output gain (yout) to the input gain (yin) of the system. Mk is the gain of the kth forward path, and Δk is the loop gain of the kth loop.