Category: CDS 101/110 Fall 2004

Identifiers: H0 H1 H2 H3 H4 H5 H6 H7 H8 L0.0 L1.1 L1.2 L2.1 L2.2 L2.3 L3.1 L3.2 L4.1 L4.2 L5.1 L5.2 L6.1 L7.1 L9.1

Questions

• When is the recitation for CDS 110a?

• What is "b" on the speed control slide?

• How do you *set* gain on the speed control slide?

• What is the definition of a system? When is something a system and when is it not?

• Aren't most continuous, even mildly complex processes feedback systems?

• How is "open loop" a feedback mechanism? I don't get where the feedback happens?

• What is the reference for the biomolecular regulatory network diagram (slide 7)

• Can a control system include a human operator as a component?

• When is the recitation for CDS 110a?
  Submitted by: waydo
  Submitted on: September 27, 2004
  Identifier: L1.1

  Stay tuned! We will be handing out a signup sheet on Wednesday and should have recitation assignments early next week.

  [Back to Top]

• What is "b" on the speed control slide?
  Submitted by: haomiao
  Submitted on: September 27, 2004
  Identifier: L1.1

  b is the coefficient of drag. The drag force on the vehicle due to atmospheric drag, friction, and everything else is lumped together and abstracted as a linear force directly proportional to the vehicle speed (faster you go, harder you get pushed back), so the -bv term in the force equation is the retarding force on the vehicle.

  [Back to Top]

• How do you *set* gain on the speed control slide?
  Submitted by: ddomitilla
  Submitted on: September 27, 2004
  Identifier: L1.1

  The variable "k" is usually referred to as the gain of the controller, and it establishes how strong the control action is. Usually, k should be large compared to b and u_hill in order to have small regime error in presence of disturbances and small sensitivity to parameter variations ( disturbance rejection and robustness). In general, k should not be too high as it may create overshoot and thus affect performance. The example proposed has a first order repsonse, so overshoot is not possible and thus in theory the best k is the highest you can provide. Gain setting is actually an important problem when you design a real control feedback system. Generally speaking, gain is an artificial amplifier to make certain features of the information prominent. For example, when you want to track a desired speed, then the difference between the desired speed and actual speed is the feature that you want your system cares. Then in this case, the gain will make this feature more 'obvious' to the system, ie. the system can react more agily to the difference of the two speeds. But there is no free dinner in this world. As we increase the gain higher and higher, the reaction ability of the system increases too much such that it over-reacts to a even tiny change of signal, thus making the whole system not efficient in terms of too much overshoot or oscillation or even becoming unstable. We will learn how to define a system to be stable or unstable in the coming lectures, really important concepts in control. Also we will learn the topic of how to set 'smart' gains in real system design. In summary, diverse requirements of the system will shrink your searching space of gain greatly.

  [Back to Top]

• What is the definition of a system? When is something a system and when is it not?
  Submitted by: asa
  Submitted on: September 27, 2004
  Identifier: L1.1

  Very generally, a system is something that can be placed in a physical or metaphorical "box". Slightly more concretely, we might consider a system to be a single object or interacting collection of objects with some number and type of inputs, and some number and type of outputs. We will be most interested in dynamical systems (those which change over time or whose outputs change over time).
  
  Whether something is a system or not depends on what interests you about it. From a dynamical perspective, a fixed, stationary metal block might not be very interesting, with no obvious inputs or outputs, and therefore wouldn't make a good system. However, if we're interested in thermal or electrical properties, it might be an interesting system.

  [Back to Top]

• Aren't most continuous, even mildly complex processes feedback systems?
  Submitted by: stephenc
  Submitted on: September 27, 2004
  Identifier: L1.1

  Short answer: Yes. Many processes, complex or otherwise, incorporate some form of feedback. The fundamental requirement for feedback is that the system in consideration is somehow modulated, controlled, or changed by the output it produces. From an engineering perspective, usually you are looking to control a given system. This is accomplished by measuring the output, performing some kind of decision algorighm based on the output and design specs, and finally implementing an action that influences the input of the system.

  [Back to Top]

• How is "open loop" a feedback mechanism? I don't get where the feedback happens?
  Submitted by: aotang
  Submitted on: September 27, 2004
  Identifier: L1.1

  Yes, you are right. When we say "open loop", that means there is no feedback. I guess you are confused by the figure 1.1 in chapter 1. Authors really want to show you the most general form of a control system there. It can be a feedback control system, like the one shown in subfigure (a), or a non-feedback control system, like the one shown in subfigure (b), an open loop, where you can only use system 1 to control system 2 but not vice versa. A little bit side knowledge here. As you will learn later, when you try to design or analysize a closed loop system, the corresponding open loop system is very likely to be the target to study first.

  [Back to Top]

• What is the reference for the biomolecular regulatory network diagram (slide 7)
  Submitted by: murray
  Submitted on: September 27, 2004
  Identifier: L1.1

  This is taken from reference 11 in the course textbook:

  D. Hanahan and R.A. Weinberg.
  The hallmarks of cancer.
  Cell, 100:57--70, 2000.

  [Back to Top]

• Can a control system include a human operator as a component?
  Submitted by: waydo
  Submitted on: September 28, 2004
  Identifier: L1.1

  Absolutely! Any time the output of one system influences a second, which in turn influences the first, we have a feedback system. If the purpose of the second system and the feedback connection is to alter the dynamics of the first, we have a control system. If one of the systems is a human operator, we have a "human-in-the-loop" system. A very early example of this is the Wright Flyer, described in the reading. The aircraft itself was unstable, and the pilot was a critical component of stabilizing the system and making controlled flight possible. The difficulty of analyzing human-in-the-loop system lies in modeling the behavior of the operator, which in general is much more complex (and non-smooth) than an engineered system. This is a very active area of current research.

  [Back to Top]