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Homework Help: Questions in electrical engineering

  1. Apr 14, 2012 #1
    Hi all
    can please help me
    Decide whether each of these statemtents is true of false

    1 ) An on-off temperature controller must have

    A ) An error signal input which switches the controller on or off (False )
    B ) An output signal to switch on or off the correction element ( true )

    2 ) Oscillations of the variable being controlled occur with
    on -off temperature controller becuse

    A There is a time delay in switching off the correction element
    when variable reaches the set value (True )
    B) there is a time delay in switching on the correction element
    when the variable falls below the set value . (False )

    3 ) with a proprtional contoller
    A ) the controller output is proportional to the error (True )
    B ) the controller gain is proptioal to the error (False )
    4 )
    A steady state error will not occur when there is a change to
    the set value with a control system operating in the mode
    A) proportional
    B) proportional plus derivative
    D) proportional plus integral

    The answer is C
    5 ) With a controller operating with a derivative control mode

    A) the controller output will be constant when the error is constant (False)
    B ) the controller output will be zero when there error is constant (True )

  2. jcsd
  3. Apr 14, 2012 #2
    though i dont know the context, level of detail, system you've been studying etc...
    heres a few pointers.

    if that statement is false, how does the controller know when to switch?

    I think those two statements must be equivalent, I would be very surprised if they could have opposite, uh, truthfulnes to each other.

    can you see that your answer here contradicts your correct answer to 5? they cant both be true.
  4. Apr 15, 2012 #3
    Hi earlofwessex

    for controller , Is only send the signal to correction element and correction element responsible for the on/off ؟

    This is what I thought !!


    For Q 2 both is true because there is lag


    For Q 4 is proportional mode ??
  5. Apr 15, 2012 #4
    to be frank, I have no idea what a correction element refers to in this case.
    any controller must be either open loop or have feedback (or feed-forward, but that is unlikely to be the only form in this case), a bang-bang controller working in open loop mode would be the equivalent of a (human controlled) switch, and probably wouldn't be classed as a controller at all. the answer to question 1 largely depends on the given definitions for "controller" and "correction element". I suppose if the wording were interpreted literally the statement would imply that the controller itself was being switched on or off, ie, active or redundant, which would make no sense whatsoever.

    the answer to 4 is simple, do you know what each term (proportional, integral, derivative) does individually? why would we introduce these concepts into a controller design?
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