z
0
Pi
I
v o
n
IrI
u
17-2
ElectronicFeedback Systems
Blackboard 17.1
Blackboard 17.2
Blackboard 17.3
ConditionalStability 17-3
NON LINEAR
Viewgraph 17.1
Viewgraph 17.2
17-4
ElectronicFeedback Systems
Viewgraph 17.3
950 Hz, a = 24
390 H_
Viewgraph 17.4
Iaf-)
a = 2.7
I
106,
Bode Plot for a = 80
Magnitude
s
102
S1
104~
W
Viewgraph 17.5
4
erf->i.)
10
102
103
104
-90*.
-150'
-180*
106
w
for ao=
-120
1.0
ConditionalStability 17-5
Iaf(jo)I
106
Gain Phase plot for ao = 80
S1
-+
10,
104
= 10w
*
o = 2.5 X 101
10
w = 6.3 X
Saf
1
(j)
= 104
o
= 10,
Viewgraph 17.6
1 7-6
Electronic Feedback Systems
Demonstration Photograph
17.1 Conditional-stability
demonstration
Demonstration Photograph
17.2 Close-up of conditionallystable system
ConditionalStability 17-7
This lecture introduces the idea of conditional stability and uses a
demonstration system to illustrate important concepts. In certain
systems, a loop transmission that rolls off faster than 1/s2 over a
range of frequencies is used to achieve high desensitivity while
retaining a relatively low crossover frequency. If the frequency
range of fast roll-off is broad enough, the phase angle may become
more negative than -180* over a range of frequencies below crossover. Such systems can be well behaved when they are operating
in their linear region, yet become unstable when saturation lowers
crossover frequency to a region of negative phase margin.
Comments
Describing-function analysis indicates the potential for this
type of behavior, predicts oscillation parameters in systems where
instability is possible, and can also be used to determine appropriate nonlinear compensation methods.
Textbook: Sections 6.3.4 and 6.3.5.
Reading
Problem
Problem 17.1 (P6.9)
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RES.6-010 Electronic Feedback Systems
Spring 2013
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