PID Control System Design and Automatic Tuning using MATLAB/Simulink. Liuping Wang

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Название PID Control System Design and Automatic Tuning using MATLAB/Simulink
Автор произведения Liuping Wang
Жанр Отраслевые издания
Серия
Издательство Отраслевые издания
Год выпуска 0
isbn 9781119469407



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      with images being the Laplace transform of the error signal images. With this, the Laplace transfer function of the PI controller is expressed as

      (1.16)equation

      The example below is used to illustrate closed-loop control with a PI controller. For comparison purpose, we use the same plant as that used in Example 1.1.

Block diagram depicting PI control system.

      Example 1.2

       Assume that the plant is a first order system with the transfer function:

      (1.17)equation

       the PI controller has the proportional gain , and the integral time constant and 0.5 respectively. Examine the locations of the closed-loop poles. With the reference signal as a unit step signal, find the steady-state value of the closed-loop output .

      Solution. We calculate the closed-loop transfer function between the reference and output signals:

      (1.18)equation

       With given in (1.16) and in (1.17), we have

      (1.19)equation

       The closed-loop poles of this system are determined by the solutions of the closed-loop characteristic equation,

      (1.20)equation

       which are

      (1.21)equation

       If the quantity

equation

       then there are two identical real poles located at

equation

       If the quantity

equation

       then there are two real poles located at

equation

       If the quantity

equation

       then there are two complex poles located at

equation

       The closed-loop system is stable as long as is positive and .

       Applying the final value theorem, we calculate

      (1.22)equation

       where the steady-state value is equal to the reference signal, and it is independent of the value of integral time constant . Figure 1.6 shows for the same as in Example 1.1 the closed-loop step response with and , respectively. It is seen that as reduces, the closed-loop response speed becomes faster. Nevertheless, the steady-state responses with both values are equal to one.

      It is often the case that the output of a PI control system exhibits overshoot to a step reference signal. The percentage of overshoot increases as higher control performance demanded. This may cause a conflict in the PI control performance specifications: on the one hand a fast control system response is desired, and yet on the other hand, the overshoot is not desirable when step reference changes are performed. The overshooting problem in reference change could be reduced by a small change in the configuration of the PI controller. This small change is to put the proportional control on the output signal images, instead of the feedback error images. More specifically, the control signal images is calculated using the following relation,

      (1.23)equation

Graph depicting Time (sec) on the horizontal axis, Output on the vertical axis, and two curves plotted for Kc 8, 80.

      Figure 1.6 Closed-loop step response of a PI control system (Example 1.2).

Block diagram depicting IP controller structure.

      Applying a Laplace transform to this equation leads to the Laplace transform of the controller output in relation to the reference and the output as

      (1.24)equation

       Assume that the plant is described by the transfer function:

      (1.25)equation

       and the PI controller has the parameters: , 2. Find the closed-loop transfer function between the reference signal and the output signal for the original PI controller structure (see Figure 1.5) and