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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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How do we express control laws?
Submitted by: waydo
Submitted on: October 1, 2003
Identifier:
L1.2
, H1
...Is it an equation or more of a statement? What should we write on homework 1?
For example, for an elevator would we write
Ydot = k(Ydesired - Ycurrent)
or “If the elevator is below the desired floor go up, else if it is above go down?”
ANSWER:
Broadly speaking, a “control law” is anything that describes how to take control action, so either description could be a control law. In real life control laws take both forms depending on the system at hand. Note that the two alternatives expressed in the question really mean different things - the first says to move the elevator at a rate proportional to the error in location (proportional control), while the second says to simply move in the correct direction (assuming that the elevator moves up or down at a constant rate) (on-off control). Ignoring startup and slowdown transients, I would say that most elevators are better described by the second statement.
Something like a cruise control in a car, however, is probably better described by the first statement - no one would want to ride in a car that controlled its speed by using full throttle when going too slow and none when going too fast; a throttle level proportional to the error in speed would be more appropriate.
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How much detail do we need to provide in question #1 about the control law?
Submitted by: murray
Submitted on: October 2, 2003
Identifier:
H1
A good answer for the cruise control system we studied in class would be:
Sensor: speedometer or wheel speed sensors
Control law: compare desired speed set by pushing button on cruise control level to the actual speed and increase the throttle setting if it is too slow, decrease if too fast
Actuator: motor that pulls on the gas pedal
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Help! I can't find the MATLAB files/ documentation for the homework.
Submitted by: lars
Submitted on: October 3, 2003
Identifier:
H1
The link on the lectures page is only for the PDF. You can find the supporting files on the page for week 1 (click the "topic" link and scroll down).
Here is the direct URL: http://www.cds.caltech.edu/~murray/cds101/W1_intro.shtml
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my hw1ballbeam.mdl is unstable at the default gains
Submitted by: waydo
Submitted on: October 5, 2003
Identifier:
H1
The original version of this file was posted with a bad set of gains. Go back and download it again and it should work. We apologize for any inconvenience.
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Where/when do we get graded homeworks back?
Submitted by: murray
Submitted on: October 6, 2003
Identifier:
L2.1
, H0
, H1
, H2
Graded homework will be handed back one week after they are turned in. When possible, we will put them out on the table outside 74 Jorgensen before lecture. After that, they will be available outside 109 Steele.
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