DIGITAL CONTROL OF PROCESSES.
LOUISIANA STATE UNIV BATON ROUGE
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The dissertation presents the frequency response of closed-loop digital control systems that use conventional digital control algorithms. In particular, proportional-plus-integral PI, proportional-plus-integral-plus-derivative PID, deadbeat, and nonminimal algorithms are investigated. The plant is approximated by a first order lag plus dead-time and the forcing function is a sinusoidal signal on the feedback path. The results are presented in the form of plots of the ratio of the amplitude of the controlled variable to the amplitude of the input signal versus frequency. These plots show a maximum amplitude ratio at low frequencies of the input signal by the inability of the sampler to differentiate between signals that differ in frequency by an interger times the sampling frequency. In order to reduce this error, a low pass filter is proposed to attenuate high frequency disturbances before they reach the sampler. A study of the effect of the filter on the amplitude ratio plots indicates that a filter time constant of one half the sampling frequency yields the best filter design. Of the control algorithms investigated, the PI and PID algorithms with minimum ITAE parameters produce the lowest amplitude ratio at the resonance peak. Author
- Industrial Chemistry and Chemical Processing
- Theoretical Mathematics
- Test Facilities, Equipment and Methods