OPamp PID Controller

Discussion in 'General Electronics Chat' started by justmailmahesh, Aug 11, 2009.

  1. justmailmahesh

    Thread Starter New Member

    Aug 11, 2009
    hi all
    I am designing a SCR based DC power supply and load is inductive i.e. magnet load.
    At this time this supply is in open loop and i am giving control voltage to firing card using a POT.
    Now i have to make it closed loop and control voltage should be generated from PI controller card.
    Pl. help me that what should be value of propotional gain and integrator gain or any example or any reference for PID controller.
    I am also attaching the circuits.
    even a small help would be appreciable.
    Thank you in advance
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    You can download a demo version of a program called PSIM (ver 8.0) from Powersimtech.

    I have attached a copy of a single phase control schematic using PSIM with feedback control of the current. You can play with the PI parameters to optimise the feedback response. It seemed to work quite well with the values shown. This process might help with your final design.
  3. justmailmahesh

    Thread Starter New Member

    Aug 11, 2009
    Thanks T n k
    It is really good tool
    I think it may work
    but i am also interested in proper designing process
    can you tell me how to find Proportional and integrator gain theoritically
    Or if you suggest some study material

  4. steveb

    Senior Member

    Jul 3, 2008
    A quick answer is to study feedback control theory using standard texts. People here can recommend good texts if you ask for that. A more detailed answer for your particular situation is as follows.

    Very often PID controllers are tuned by trial and error, due to a lack of a theoretical model. When one has a theoretical model, often a more specialized feedback compensator is use. However, a PID can often be used even when it is not the optimum approach, and theoretical tuning requires a good theoretical model.

    For your circuit, you have the issue of trying to model the rectifier, which is a time-dependent (i.e commutating conduction paths) nonlinear circuit. There are two issues. One is that you need a model that works for the large signal main voltage frequency in order to optimize the feedback gains for good response. The other is that you would like to have a small signal model valid at other frequencies, to ensure stability with those gains. Often it is not possible to do both of these, and computer models (or experiments) are needed. However, you could proceed by assuming that stability may not be a major concern since the rectifier should prevent instability by blocking small sinusoidal signals at frequencies far from the main frequency, but this would later need to be verified with computer models or experiments.

    Your case has an inductive load (on the rectifier) which produces slow changes in the output current. In this case, a large signal average rectifier model, at the source frequency, can be established theoretically. The rest of the circuit is simple linear circuitry which can be easily modeled with known equations. Once you have a good model, you can use feedback control theory to tune the gains properly. This usually involves looking at the open loop response and identifying how it should be modified with a PID filter circuit with the proper gains.

    I think t_n_k gave some good advice here. For this circuit, it really helps to have a computer tool because it is not straightforward to create a proper model for the rectifier. My advice would be to practice theoretical PID feedback modeling with a simpler circuit (i.e. no rectifier). If you are really in a situation where you need to model that circuit (with rectifier), I can point you in the direction of some references that give the tools to create a "average rectifier model" for the inductive loading situation like yours.
  5. ali_aku_suka_telo

    New Member

    Jul 9, 2011
    for newbie may be could learn how to tune PID parameter via simulation. We can use proteus to build analog PID controller based on Op-amp IC and then simulate it, just like in this link, i hope this link is useful
  6. strantor

    AAC Fanatic!

    Oct 3, 2010