System Synthesis from Frequency Response Characteristics

Control System Synthesis from Frequency Response Characteristics 

Generally recommended frequency response characteristics for automatic control systems are given by Oldenburger (03) as follows  

In the absence of other specifications, the problem of control system synthesis is one of adjusting the characteristics of the various loop components until the recommended frequency response characteristics are obtained. All the control components including measuring elements should have negligible lagging and attenuation characteristics over the range of frequencies which are of importance for the control system. The 

Minimizing the overall phase angle of a typical process system, maximizes the frequency at phase crossover and hence maximizes the response speed of the system. Large phase lags in process components, for example resulting from distance-velocity lags, can be counteracted by the phase lead characteristic of derivative control action. 

Improvement of the speed of response of control systems, requires that the frequency at gain crossover be increased by increasing the controller proportional gain. Since such an increase in gain crossover fre- 

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Seat-to-Head TransmissibUity. The transmissibility expresses the response of one part of a mechanical system (e.g., the head or hand) to steady-state forced vibration of another part of the system (e.g., the buttocks), and is commonly expressed as a function of frequency. A synthesis of measured values for the seat-to-head transmissibility of seated persons has been performed for vibration in the vertical direction, to define the idealized transmissibility. The idealiz transmissibility attempts to account for the sometimes large and unexplained variations in the results from different experimental studies conducted under nominally equivalent conditions. The results of one such analysis are shown by the continuous lines in Fig. 10.5 (ISO 5982, 2001). It can be seen by comparing Figs. 10.4 and 10.5 that the characteristic peak of the apparent mass remains in the modulus of the idealized transmissibility. 

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