Control System Feedback Loop

Discussion in 'General Electronics Chat' started by Mazaag, Sep 25, 2007.

  1. Mazaag

    Thread Starter Senior Member

    Oct 23, 2004
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    0
    Hey guys,

    I just have a question about control system feedback loops..

    How can we deal with feedback loops which contain inherent delay in them ? such as delay in sending the sensor signal to the controller
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Most of the time delays are a natural consequence of circuit design. It might help if you could describe in a bit more detail, what the delay in the sensor signal is doing to keep your circuit from operating the way you were expecting it to.

    hgmjr
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    There are delays and then there are delays. For flight systems, everything possible is done to minimise input delays. For something like a humidity sensor, several minutes to settling may not be too critical.
     
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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    In systems with longer delays, such as temperature controllers, algorithms can be used for compensation. PID controllers (proportion, integration, derivative) work this way.
     
  5. Mazaag

    Thread Starter Senior Member

    Oct 23, 2004
    255
    0
    Here's the deal guys... I'm basically heating up a resistor by applying voltage, and measuring the temperature of the resistor using a thermocouple to demonstrate a control system. However, there is inherent thermal lag in heating the resistor. The control system itself will NOT be used with a resistor, but with another medium. As a result, the control system parameters CANNOT be tinkered with. However, we would want to demonstrate its functionality on a resistor. So is there a way to EXTERNALLY deal with such thermal delay ? ( we can add controllers or anything else in series to the system,but cannot change the parameters of the original controller )

    Thanks
     
  6. thingmaker3

    Retired Moderator

    May 16, 2005
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  7. Mazaag

    Thread Starter Senior Member

    Oct 23, 2004
    255
    0
    I don't think i was very clear in explaining the situation...

    The control system already exists...its there.. inside the system .. and it is tuned to work on a specific material... we cannot use this material in demo..so we want to use a resistor...however...the resistor doesn't heat up as fast as the other material does...and a result.. a delay is introduced into the system...

    The control system currently present uses a PID to control the termperature of the material ... NOT the resistor... I was wondering if there is an external circuit that I can connect (in series or parallel) to the output probe ( that causes the heating) to be able to compensate for such a delay in the heating up of the resistor.. I am familiar with PIDs .. but not entirely sure how i could fix this problem having minimum info about the internal controller of the system.

    I hope I clarified things a bit...
    Thanks
     
  8. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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    So you've got oscillation? Try a thermostatic controller in parallel, with a setpoint just below the PID setpoint. You'll still overshoot on the first cycle, but the oscillation should dampen quickly by having eliminated the undershoot.
     
  9. techroomt

    Senior Member

    May 19, 2004
    198
    1
    it sounds as though your problem is not a delay in the feedback loop per se, but the heat acceptance rate of the resistor. realize that resistors are designed to not change much under thermal variations. try a different material.
     
  10. Mazaag

    Thread Starter Senior Member

    Oct 23, 2004
    255
    0
    Okay thanks..

    Is "heat acceptance rate" the proper term ?
     
  11. techroomt

    Senior Member

    May 19, 2004
    198
    1
    it wasn't implied to be a term - as i'm sure there are thermal dynamic quantitative expressions applied here.
     
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