John Y. Hung

Posicast-based Control

Posicast was originally proposed by O. J. M. Smith to cancel the oscillatory behavior of lightly damped systems. One of the earliest textbook descriptions of Posicast can be found in Smith's 1958 book, "Feedback Control Systems," McGraw-Hill, pp. 331-345. Smith showed how accurate knowledge of the system damping and damped natural frequency could be used to design a feedforward dynamic compensator that cancels overshoot in the system step response.


Fig. 1. Step response of a lightly damped system is characterized by
the overshoot $\delta$ and time period $T_d$.


Fig. 2. Posicast as originally proposed by O. J. M. Smith.

Posicast was further examined in the early 1960's by researchers trying to minimize vibration in various types of lightly damped systems, including flexible structures, and vibration control. Application of Posicast to nonlinear dynamics has also been proven effective when the nonlinearity is slowly time varying. It is also well-known that Posicast is very sensitive to inaccurate knowledge of the damped resonant frequency; such performance sensitivity is common to many feedforward control methods that rely on dynamic cancellation or model inversion.

Hung recently suggested that the Posicast technique be used within a feedback loop, instead of the classical feedforward configuration. In this way, sensitivity to parameter uncertainty is reduced. Posicast-based feedback control has been successfully implemented for several power electronic systems.


Fig. 3. Posicast-based feedback control proposed by Hung

Related publications

  1. Q. Feng, R. M. Nelms, and J. Y. Hung, "Posicast-based digital control of the buck converter," IEEE Transactions on Industrial Electronics, vol. 53, no. 3, pp. 759-767, June 2006.
  2. Q. Feng, J. Y. Hung, and R. M. Nelms, "Digital control of a boost converter using Posicast," Proceedings of 18th Annual IEEE Applied Power Electronics Conference and Exposition, Miami, FL, February 2003, pp. 990-995.
  3. J. Y. Hung, "Feedback control with posicast," IEEE Transactions on Industrial Electronics, vol. 50, no. 1, pp. 94-99, February 2003.
  4. Q. Feng, R. M. Nelms, and J. Y. Hung, "The application of Posicast control to dc-dc converters," Proceedings of 35th Intersociety Energy Conversion Engineering Conference, Washington, DC, July 2002, paper #20160.

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Last updated 14 Feb 2008