PID controller in 16F737


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  1. #1
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    Default PID controller in 16F737

    Hi, I want to control an oven with a pic.
    I basicaly have build the oven, including the temp-sensing with a NTC.
    The oven will be used for making SMD PCB's.
    Therefor it has to follow a temperature profile in time.
    I want to do this with a PID-controller.
    I calculated the PID parameters (gain, I, D) by the step response.

    I also tried to build the PID algorithm.
    Only, in case of overshoot, I will have a negative error (wish temp minus real temp).
    I wrote a routine to handle negative values; to multiply and subtract negative values (see below).
    As this is quite complex, does anyone know if there is a more simple (faster) method to implement a PID for this application?

    code;
    AminB: '*** PROCEDURE A - B ***
    tekA=NCD(A)
    tekB=NCD(B)
    IF (tekA = 16 AND tekB = 16) OR (tekA <>16 AND tekB<>16) Then 'check if A and B have same sign
    abswaardeAminB = ABS(A-B)
    IF tekA = 16 Then
    IF NCD(A-B) = 16 Then
    tekAminB = 16
    Else
    tekAminB = 0
    EndIF
    Else
    tekAminB = NCD(A-B)
    EndIF
    Else
    abswaardeAminB = ABS(A) + ABS(B)
    IF NCD(A-B) = 16 Then
    tekAminB = 16
    Else
    tekAminB = 0
    EndIF
    EndIF
    'assembly of the value
    IF tekAminB = 16 Then
    C= 0 - abswaardeAminB 'negative word
    Else
    C = abswaardeAminB
    EndIF
    Return

    AmaalB: '*** AmaalB ***
    IF NCD(A) = 16 XOR NCD(B) = 16 Then
    C = ABS(A) * ABS(B)
    C = 0 - C
    Else
    C = ABS(A) * ABS(B)
    EndIF
    Return

    regelkring:
    errlastlast = errlast
    errlast = err
    err = wenswaarde - adval2 'wish temp minus real temp
    A = 2
    B = errlast
    GoSub AmaalB
    A = C
    B = errlastlast
    GoSub AminB
    A = err
    B = C
    GoSub AminB
    B = C
    A = D100 'd*100
    GoSub AmaalB
    DTERM = C
    A = I100 'I*100
    B = err
    GoSub AmaalB
    A = C 'ITerm
    B = 0 - DTERM
    GoSub AminB
    ID = C 'IDterm
    A = err
    B = errlast
    GoSub AminB 'C bevat Pterm
    A = 100
    B = C
    GoSub AmaalB
    A = C
    B= 0 - ID
    GoSub AminB 'P + ID = PID
    A = gain10 'gain * 10
    B = C
    GoTo AmaalB
    IF NCD(C) = 16 Then
    C = ABS(c)/1000 'PID term
    C = 0 - C
    Else
    C = C / 1000
    EndIF
    A = duty
    B = 0 - C
    GoSub AminB
    duty = C
    IF duty > 100 AND NCD(duty)<>16 Then duty = 100
    IF NCD(duty) = 16 Then duty = 0

    Return

  2. #2
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    Hi,

    As it's a slow process ... Have a look here :

    http://www.parallax.com/dl/docs/books/edu/ic.pdf

    Can be directly used with PBP ... just few mods for your sensor ( will a NTC fit your temp range ??? I'm not so sure ... )

    Good reading

    Alain
    ************************************************** ***********************
    Why insist on using 32 Bits when you're not even able to deal with the first 8 ones ??? ehhhhhh ...
    ************************************************** ***********************
    IF there is the word "Problem" in your question ...
    certainly the answer is " RTFM " or " RTFDataSheet " !!!
    *****************************************

  3. #3
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    I knows this document and will have a look at it.
    My intention was to have a PID which I can use afterwards for any process I want.
    The NTC is no problem. It comes with the original oven, and I measured the curves to translate the R-value towards C.

  4. #4
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    Quote Originally Posted by joeri
    I knows this document and will have a look at it.
    My intention was to have a PID which I can use afterwards for any process I want.
    The NTC is no problem. It comes with the original oven, and I measured the curves to translate the R-value towards C.
    What are the resistance values over the range desired? Will you use it for non lead solder, and will it (oven) be quick enough to heat up so as not to over stress the components?
    I use a Cuisinart digital convection oven, and it is a little on the slow side to hit 225c. No problems though, other than pizza tasting a little metallic.

    Ron

  5. #5
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    at 100C I have 11 kOhm; at 150C I have 3 kOhm; at 260C I have 500Ohm. I use a current source which drives a small current through the NTC, then I amplify the voltage over the NTC with a opamp and bring it into the analog pic input. So far, so good.

    The oven is a samsung with 1500W. I added two heat resistors from a waffle machine (another 1000W), so total of 2,5KW.
    This boosts quite fast. Also, I have a fan to equalize the temp inside the oven. The complete oven is packed in rockwool.

    But my problem is the PID. Any experience on how to work with negative errors (overshoot)?

    The calculate PID values are:
    gain= 10
    I= 0,04
    D = 100

    So it seems I have a PD controller. Due to the slow process, that might be ok. Or not?
    Last edited by joeri; - 19th June 2006 at 20:20.

  6. #6
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    Quote Originally Posted by joeri
    at 100C I have 11 kOhm; at 150C I have 3 kOhm; at 260C I have 500Ohm. I use a current source which drives a small current through the NTC, then I amplify the voltage over the NTC with a opamp and bring it into the analog pic input. So far, so good.

    The oven is a samsung with 1500W. I added two heat resistors from a waffle machine (another 1000W), so total of 2,5KW.
    This boosts quite fast. Also, I have a fan to equalize the temp inside the oven. The complete oven is packed in rockwool.

    But my problem is the PID. Any experience on how to work with negative errors (overshoot)?

    The calculate PID values are:
    gain= 10
    I= 0,04
    D = 100

    So it seems I have a PD controller. Due to the slow process, that might be ok. Or not?
    It sounds like the rock wool is overkill, plus you want to cool things down pretty quickly once soldered. I crack the door of the toaster oven slightly for my high tech solution.
    I really think the PID may be overkill. If it were me, I would take the current value at a given time and compare it to a desired constant value with ...

    heat_loop:
    for time = 1 to 10

    If curr_temp_val < set_temp_val then
    relay = 0; A lower number means the temp is too high.
    else relay = 1 ; temp too low
    endif
    pause 1000
    next time
    goto get_next_heat_increment ; Put next temp constant into set_temp_val and start again.

    Otherwise, the relay pulls in to increase temperature.You can check/regulate the temp this way until the next time increment is reached. At this point, update set_temp_val and compare. You can make the time increments every 10 seconds, or every second for that matter. A heavily insulated unit will require you to cut off power to the elements early, or you will have substantial overshoot. I'd say to use the rockwool for the hydroponic "tomatoes", and use less insulation or an automatic vent to the outside.

  7. #7
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    I will keep it in mind for the 'tomatoes'.
    I have a fan in the oven that can cool things down quickly.
    Also, I made a LCD-screen in the oven to read out the temp.
    Thanks for the help, but I want to implement the PID in order to have the controller ready when I want to use it for more sophisticated processes..

  8. #8
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    hi:
    take a look at these links. they should get you started.

    http://www.web-ee.com/Schematics/Tem...controller.htm


    http://www.parallax.com/dl/docs/prod...eb-PC-v1.0.pdf


    the documentation from parallax is great.

    latter
    sam

  9. #9
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    Thank you for your support.
    I continued with my code and got it up and running.
    So this subject can be closed.

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