Tank Level Tutorial Part 2 - Hint
Note that we are plotting the actual height h2, rather than the measured height h2m. Hence, the noise we see in the actual height comes via the feedback signal through the controller. Obviously, there must be some non-linear effect in operation for the noise to be present when the level is falling but not when the level is rising. Thus, think about what the control valve is being asked to do in both cases and the effect of controller saturation. This effects the transmission of noise via the control signal connecting h2m to h2 via the plant. This reveals quite a bit about sensitivity and complementary sensitivity functions in control.