nserman@fsb.hr Transfer Function Concept Digest
11. BASICS OF CLOSED LOOP DYNAMICS QUALITATIVE ANALYSIS
Closed-loop dynamics is optimized to achieve the closed-loop stability with a satisfactory transients damping and a transients duration to be as short as possible. In terms of the Bode diagram stability margins this means that the stability margins should be:
- positive (provides stability as such),
- sufficiently large (provides adequate transients damping),
- within relevant frequency range (provides faster transients vanishing).
To comply with #3 the shaded area in fig. 11.1 should be as far as its achievable to the right side.
Figure 11.1 Stability margins and relevant frequency range (shaded area) in an open-loop frequency characteristics Bode plot.
Frequency characteristics of a serial connection in Bode diagram are made by a simple summation of the respective block characteristics. This offers a rational ground for qualitatively analyzing of what are the effects of additional block inclusion into the open loop.
Figure 11.2 Closed-loop system under consideration – open loop W(s) = H(s) H0(s) encircled by negative unity feed-back
Transfer function H(s) in fig 11.2 is an additional block. Fig 11.3 shows what would be the effects if that block is a static gain while fig 11.4 shows the same in a case of an integrator.
Figure 11.3 Effect of adding static gain H(s) = K to the component H0(s) when K > 1.
Adding a static gain to the existing component H0(s) results in an amplitude frequency characteristic vertical translation. The transition is upwards for K > 1 and downwards for K < 1, thus directly influencing the stability margins. The red arrows are the stability margins after the inclusion of H(s).
Figure 11.4 Adding of an integrator s 
to the existing component H0(s)
Adding an integrator to the existing component H0(s) rotates the amplitude frequency characteristic around its point at frequency ω0. The phase characteristic is shifted 90° downwards.
