Phase Compensation and PID

Discussion in 'General Electronics Chat' started by Tera-Scale, Jun 26, 2012.

  1. Tera-Scale

    Thread Starter Active Member

    Jan 1, 2011
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    What are the main differences between PID controller and lead/lag compensators? When and were are they implemented?


    I am working on position control of a DC motor connected to a pulley and getting position feedback by means of a potentiometer. The motor is given a square wave for obtaining a step response. I deduced the transfer function and designed a PID using the Manual Method and ZN. Now I need to design a phase lead and phase lag compensator. thanks
     
  2. steveb

    Senior Member

    Jul 3, 2008
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    These just have different pole and zero locations. Generally, the PID is good if you don't have a detailed system model and just want a flexible controller that can be tuned to get good operation. The lead/lag would usually be used if you have a detailed system model and determine that you would like to modify the system response. Without a system model, it might be difficult to know how to tune the lead/lag compensator, although it could be tuned by trial and error experimentally.

    Sometimes, you might want to combine methods. I sometimes use a lead/lag or lag/lead to modify the system response, and then put a PI controller on the new system.
     
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  3. crutschow

    Expert

    Mar 14, 2008
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    A lead-lag circuit can be considered a part of a PID loop. The lead is the D (differentiator) part and the lag is the integrator (I) part. The P is the proportional part, of course.
     
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  4. Tera-Scale

    Thread Starter Active Member

    Jan 1, 2011
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    I assume the final bandwidth of the system cascaded with lead/lag compensator is the whole CLTF not the OLTF.. is that correct? Do you have any idea of the matlab commands required? thanks
     
  5. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    The practical PID controller provides "knobs that can be twiddled" to adjust a system response. Hence the evolution of tuning algorithms such as Ziegler-Nichols to optimize the response of systems fitted with PID controllers.

    One thing that was overlooked in earlier comments is that Integral control is introduced to remove steady-state offset error from the closed loop input-output response. If steady-state error is not a problem then one would be disinclined to add potentially destabilizing Integral control.

    A Lead-Lag compensator is probably less accessible in the sense of having twiddle knobs at one's disposal. The L-L compensator is normally designed with a good understanding of the uncompensated control system's open loop behaviour. The Bode plot is often a tool employed in conjuction with the compensator design. Generally one is attempting to control the open loop phase margin without sacrificing DC ['zero frequency'] gain.
     
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