8. When Td = 0, the PD controller reduces to a proportional controller, G s c( ) = K. In this case, use the Nyquist plot to determine the limiting value of K for closed-loop stability.

a. K = 0.5

b. K = 1.6

c. K = 2.4

d. K = 4.3

For Problems 8 and 9, consider the block diagram in Figure 9.69 where G(s)= 9 (s+1)(s+3s+9) and the controller is the proportional-plus-derivative (PD) controller G(s) = K(1+Tas).