Damping with complementary feedback

If Td > At, critical damping cannot be achieved. As with complementary feedback, reducing P by one-half produces zero damping, by one-fourth gives 1. -amplitude damping. 

Figure 1.26 shows the required proportional band for j i-amplitude damping for any combination of dead time and capacity. A band of 100 percent (proportional gain of 1.0) is seen to be required for a process whose Td/ri = 1.2. But with (mmplomentary feedback, the same proportional gain could produce criti( al damping. Complementary feedback is, by this token, of advantage in the most difficult processes. 

A proportional-plus-reset controller applied to the same process, and adjusted to produce 22.5 phase lag, can serve as a reference for compaii son. The values of reset time and proportional band required for )- 4-ainplitude damping were calculated for selected ratios of trated error per unit load chan was then found as the PR product, to compare with that obtainable through complementary feedback. This information is plotted in Fig. 4.16, with coordinates... 

The foregoing discussion on complementary feedback was based primarily on critically damped response. With a pure dead-time process, this was the best obtainable. But with less difficult processes, lower damping will enhance recovery from load disturbances due to greater controller gain. 

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