* Integrator--Circuit in an FBW flight control system that reduces response errors over time. It "remembers" the pilot's command and continues to move the control surfaces until the desired response is achieved and no further "error signal" is present.
Integrators remember the pilot's request as of some time ago, which may differ significantly from the pilot's instantaneous request during rapid control inputs. This may cause system lag and instability.
To provide immediate response to pilot input, the computer provides a direct path to the elevator via the proportional line (called the "feed forward gain" in the B-777). For precision over time, an integrator produces a control surface command until the feedback signal is equal to the pilot's command signal.
Pure integral control, or too much integrator gain (K), causes excessive lag in the aircraft response, hence the use of the proportional circuit. This arrangement, called "proportional plus integral" control, is found in most fly-by-wire designs, including the B-777 and the A320.
In a block diagram, "1/s" or "K/s" denotes an integrator, the "K" indicating some gain value. FBW engineers must "tune" the integrator gain to prevent excessive lag.
Lag causes delay in changing directions--for example, nose-up to nose-down, which is a classic cause of pilot-involved oscillation, or PIO. Engineers can mathematically analyze control laws for such instabilities.
Last edited by Paris Hilton; 25th March 2007 at 04:45.