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FAQ (Frequently Asked Questions)
Category:
CDS 101/110 Fall 2003
Identifiers: FN H0 H1 H2 H3 H4 H5 H6 H7 H8 L0.0 L1.1 L1.2 L10.1 L2.1 L2.2 L3.1 L3.2 L4.1 L4.2 L5.1 L5.2 L6.2 L8.1 L9.1 L9.2
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In HW5, can we use SI units instead of those given?
Submitted by: demetri
Submitted on: November 4, 2003
Identifier:
H5
Although you are welcome to work in whatever units you please (i.e. internal unit conversions in your models), your final answers should be in the units given (i.e. invert any conversions at the end).
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In problem 2 what is the reference input?
Submitted by: demetri
Submitted on: November 4, 2003
Identifier:
H5
This is part of the modeling problem. Recall that a reference input is the quantity which we wish to track. For example, the desired temperature on your thermostat is the reference input. Hopefully this will make it more clear.
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What do you mean by "roll off" in problem 4? How do we calculate the frequency at which this occurs?
Submitted by: waydo
Submitted on: November 6, 2003
Identifier:
H5
When drawing the Bode plot, virtually all systems will have a range of frequencies where the "interesting" stuff happens (resonant peaks, relatively high gain, etc) followed by a steady decrease in gain (linear when plotted on the log-log scale with a slope that depends on the number of poles and zeros of the system) for all higher frequencies. The point at where this begins to occur is sometimes called the rolloff frequency or break point. For an underdamped second order system, you should see a resonant peak followed by the steady decrease in gain; for such a system the resonant peak is where rolloff begins. For an overdamped system or a first order system, there will generally be a long stretch of fairly constant gain that begins to roll off fairly suddenly at a particular frequency. For a simple (first or second order) system this rolloff begins at about the frequency of the pole(s).
I would recommend picking some numbers for b_1 and b_2 that correspond to over- and underdamped and generating Bode plots using MATLAB to get a feel for this before doing your hand sketches. Also look at Franklin and Powell, Chapter 6 (I think - my copy is not the latest edition) for an excellent description of how to generate Bode plots by hand.
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In problem 2, part c how does the controller relate to the system dynamics?
Submitted by: mreiser
Submitted on: November 7, 2003
Identifier:
H5
We should have been explicit about this, y_c, the output of the controller is simply the input, u_e, to our system.
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How do we deal with the hill force in problem 2 HW5?
Submitted by: demetri
Submitted on: November 8, 2003
Identifier:
H5
This seems to have been a major source of confusion. The issue at hand is understanding transfer functions between particular _pairs_ of signals. For example, we ask you to calculate the transfer function from the command input to the system output. Such a calculation should be done assuming all other inputs (including disturbances) are zero.
Of course, you could also calculate a transfer function from the disturbance to the output, but this is a distinct quantity. Whenever we ask for a transfer function, we specify the two signals that it relates.
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What is the value of Kp in problem 2, part b
Submitted by: atiwari
Submitted on: November 9, 2003
Identifier:
H5
Use the same value as given in problem 2, part d
Kp = 500
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