greater than 1/rd rad/sec. The value of Td effected the frequency response of the system as

indicated in Figure 8.21. The gain, Kc, was used to raise or lower the entire diagram as in Figure

8.22.

 Merritt (1967) pointed out some general criteria that are applicable to any "good" servo

system design. He stated that a range of frequencies with a loop gain greater than unity is a

mandatory requirement for any closed-loop system and gain values greater than 5 are usually

required. This requirement is amplified by the accuracy requirements placed on the system.

Also, the open-loop Bode diagram should have an asymptotic slope of -1 or -20 db per decade at

the crossover frequency to insure satisfactory stability. The system bandwidth is always

limited by the requirement that the system not react to high frequency noise and by the

stability requirements.


High K,


log scale


0 slope


Figure 8.22. Bode diagram of a controlled second-order system indicating the effects changes in
 the controller gain, Kc.