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ECE 481 Topic 1
ECE 481 Topic 1
36
Computer Science
Undergraduate 4
05/09/2012

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Term
What is sampled-data system?
Definition
system which has both discrete and continuous time signals
Term
What does it mean by stable if system is open loop?
Definition
transfer function has all poles in open left half plane
Term
what does it mean by stable if system is close loop?
Definition
every possible closed-loop transfer function is stable
Term
is G(S)= 10 stable?
Definition
yes, because the output is always bounded
Term
why is it impossible to build a controller that's improper?
Definition
Term
what's the difference between stable and stablizing?
Definition
stable is for when open loop, stablizing is for when closed loop
Term
what does it mean by a function is linear?
Definition
super position principal holds
Term
what are steps of designing process?
Definition
1. Get the requirement
2. Model the plant (System identification)
3. Get transfer function for the plan
4. Design a controller
5. Simulate controller
6. implement the controller
Term
what is system identification?
Definition
given input and output, find model
Term
what's the main difference between discrete time systems and continuous time systems?
Definition
discrete time signal has a maximum frequency that can be exhibited, W <= 0.5 Ws
Term
describe ADC by giving a system that has input and output. Draw the behavior of input and output
Definition
Term
describe DAC by giving a system that has input and output. Draw the behavior of input and output.
Definition
Term
what's the condition for an ideal sampler?
Definition
no time shift, no error
Term
what's the condition for an ideal hold?
Definition
no time shift
Term
in ece 481, what do we assume regarding S and H?
Definition
1. S and H are synchronized
2. sampling period is constant
3. no signal quantization
Term
are S and H linear?
Definition
yes, so they can be moved around
Term
do S and H commute?
Definition
no, they are not transfer functions because they are not time invariant
Term
describe an SH and HS system.
Definition
Term
What are some causes of non-linearity?
Definition
1. on and off switch (relay)
2. saturator
3. static friction
Term
What are some solutions to non-linearity?
Definition
1. cancel out the non-linearity
2. approximate by linearization
3. use limit cycles
Term
Why does higher frequency mapped into lower frequency?
Definition
because all signals can be decomposed into cos and sin which have property of higher frequency mapped to lower frequency
Term
what are two ways to deal with aliasing?
Definition
1. sample really fast
2. use an anti-aliasing filter
Term
what is a rule of thumb for cutoff frequency for AA filter?
Definition
Wcutoff < Ws/5
Term
what are two disadvantage of sampling really fast to avoid aliasing?
Definition
1. noise might be also included
2. money issue
Term
what are two disadvantage of using AA filter?
Definition
1. money issue
2. phase lag
Term
what are three advantages to gain from fast sampling rate?
Definition
1. good at tracking and reject disturbances
2. sometimes you can only get good output by sampling fast
3. if AA is used, want cutoff for AA to be big, so sampling rate has to be big as well
Term
what are rule of thumb when sampling using sampling really fast and when using AA filter?
Definition
1. sampling fast: 10Ws
2. AA: 25Ws
Term
what are disadvantages to sample that fast?
Definition
1. economy 2. numerical issues
Term
what are three ways to design controllers for sampled-data systems?
Definition
1. design by emulation
2. direct design
3. true sampled-data design
Term
what are advantages of emulation?
Definition
1. easy because don't need to iterate designing C(s), only need to change sample frequency
Term
what are disadvantages of emulation?
Definition
economy issue (might not be able to sample that fast)
Term
what are design steps in emulation?
Definition
1. find C(S) 2. find D[Z] by approximation 3. implement C[Z]
Term
will emulation work for large T?
Definition
no
Term
What are design steps of designing in discrete time?
Definition
1. get Pd[z]
2. get C[z]
3. implement C[z]
Term
What are advantages of doing direct design?
Definition
it works on any T
Term
what are disadvantages of doing direct design?
Definition
1. Sometimes final performance is not that good
2. inter-sample behavior is ignored
3. have to redo if T is changed
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