Block Diagram Reduction
Signal-Flow Graphs
Block Diagram Reduction
Signal-Flow Graphs
Unit 4: Block Diagram Reduction
1
Block Diagram Reduction
Cascade Form
Parallel Form
Engineering 5821:
Control Systems I
Feedback Form
Moving Blocks
Example
Faculty of Engineering & Applied Science
Memorial University of Newfoundland
February 15, 2010
ENGI 5821
Block Diagram Reduction
Signal-Flow Graphs
Unit 4: Block Diagram Reduction
Cascade Form
Parallel Form
Feedback Form
Moving Blocks
Example
Block Diagram Reduction
1
Signal-Flow Graphs
ENGI 5821
Unit 4: Block Diagram Reduction
First we summarize the elements of block diagrams:
Subsystems are represented in block diagrams as blocks, each
representing a transfer function. In this unit we will consider how
to combine the blocks corresponding to individual subsystems so
that we can represent a whole system as a single block, and
therefore a single transfer function. Here is an example of this
reduction:
We now consider the forms in which blocks are typically connected
and how these forms can be reduced to single blocks.
Reduced Form:
ENGI 5821
Unit 4: Block Diagram Reduction
Block Diagram Reduction
Signal-Flow Graphs
Cascade Form
Parallel Form
Feedback Form
Moving Blocks
Example
Cascade Form
When multiple subsystems are connected such that the output of
one subsystem serves as the input to the next, these subsystems
are said to be in cascade form.
The algebraic form of the final output clearly shows the equivalent
system TF—the product of the cascaded subsystem TF’s.
ENGI 5821
When reducing subsystems in cascade form we make the
assumption that adjacent subsystems do not load each other.
That is, a subsystem’s output remains the same no matter what
the output is connected to. If another subsystem connected to the
output modifies that output, we say that it loads the first system.
Consider interconnecting the circuits (a) and (b) below:
The overall TF is not the product of the individual TF’s!
Unit 4: Block Diagram Reduction
Block Diagram Reduction
Signal-Flow Graphs
We can prevent loading by inserting an amplifier. This amplifier
should have a high input impedance so it does not load its source,
and low output impedance so it appears as a pure voltage source
to the subsystem it feeds into.
Cascade Form
Parallel Form
Feedback Form
Moving Blocks
Example
Parallel Form
Parallel subsystems have a common input and their outputs are
summed together.
If no actual gain is desired then K = 1 and the “amplifier” is
referred to as a buffer.
The equivalent TF is the sum of parallel TF’s (with matched signs
at summing junction).
ENGI 5821
Unit 4: Block Diagram Reduction
Feedback Form
Systems with feedback typically have the following form:
We can easily establish the following two facts:
Noticing the cascade form within the feedforward and feedback
paths we can simplify:
E (s) = R(s) ∓ C (s)H(s)
C (s) = E (s)G (s)
We can now eliminate E (s) to obtain,
Ge (s) =
G (s)
1 ± G (s)H(s)
Moving Blocks
A system’s block diagram may require some modification before
the reductions discussed above can be applied.
We may need to move blocks either to the left or right of a
summing junction:
Or we may need to move blocks to the left or right of a pickoff
point:
Block Diagram Reduction
Signal-Flow Graphs
Cascade Form
Parallel Form
Feedback Form
Moving Blocks
Example
Example
Reduce the following system to a single TF:
We can now recognize the parallel form in the feedback path:
First we can combine the three summing junctions together...
ENGI 5821
We now have G1 cascaded with a feedback subsystem:
Unit 4: Block Diagram Reduction
Example 2
Reduce the following more complicated block diagram:
Steps:
Rightmost feedback loop can be reduced
Create parallel form by moving G2 left
Reduce parallel form involving 1/G2 and unity
Push G1 to the right past the summing junction to create a
parallel form in the feedback path
Reduce parallel form on left
Reduce feedback form on left
Recognize cascade form on right
Signal-Flow Graphs
We can convert the cascaded, parallel, and feedback forms into
signal-flow graphs:
Signal-flow graphs are an alternative to block diagrams. They
consist of branches which represent systems (a) and nodes which
represent signals (b). Multiple branches converging on a node
implies summation.
V (s) = R1 (s)G1 (s) − R2 (s)G2 (s) + R3 (s)G3 (s)
C1 (s) = V (s)G4 (s)
C2 (s) = V (s)G5 (s)
C3 (s) = V (s)G6 (s)
e.g. Convert the following block diagram to a signal-flow graph: