Open-loop frequency response test

An open-loop frequency response test on an unknown system produced the following results ... 

The Nyquist stability criterion is similar to the Bode criterion in that it determines closed-loop stability from the open-loop frequency response characteristics. Both criteria provide convenient measures of relative stability, the gain and phase margins, which will be introduced in Section J.4. As the name implies, the Nyquist stability criterion is based on the Nyquist plot for GqiXs), a polar plot of its frequency response characteristics (see Chapter 14). The Nyquist stability criterion does not have the same restrictions as the Bode stability criterion, because it is applicable to open-loop unstable systems and to systems with multiple values of co or cOg. The Nyquist stability criterion is the most powerful stability test that is available for linear systems described by transfer function models. 

To test the hypothesis that dead time has no effect on the open-loop process attenuation for a capacity-dominated process, again perform a frequency response test using the sine wave input. Run the 40 min (frequency 0.025 min ), 25°C ainilitude disturbance through the process of Figure W3.5 with the tank level set to 50 per cent. Has the attenuation changed from the value you calculated earlier ... 

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