Explain the Ziegler-Nichols tuning method for PID controllers.

Ziegler-Nichols provides two methods: (1) Ultimate gain method - with I and D disabled, increase P gain until sustained oscillation occurs (ultimate gain Ku) at ultimate period Pu. PID settings: Kp = 0.6Ku, Ti = Pu/2, Td = Pu/8. (2) Reaction curve method - apply step change in manual, measure process response to get delay time L and time constant T. PID settings: Kp = 1.2T/(K*L), Ti = 2L, Td = 0.5L. These are starting points requiring refinement; often produce aggressive tuning.