Gimbal Actuator FDI
The OMS gimbal actuator FDI detects and identifies off-nominal
performance of the pitch and yaw gimbal actuators of the OMS engines.
The OMS gimbal actuator FDI is divided into two processes. The
first determines if the actuators should move from their present
position. If the actuators must move, the second part determines
how much they should move and whether the desired movement has
The first part checks the actuators' gimbal deflection error
(which is the difference between the commanded new position and
the actuators' last known position) and determines whether the
actuators should extend or retract or if they are being driven
against a stop. If the actuators are in the desired position or
being driven against a stop, the first part of the process will
be repeated. If the first part determines that the actuator should
move, the second part of the actuator FDI process is performed.
The second part of the actuator FDI process checks the present
position of each actuator against its last known position to determine
whether the actuators have moved more than a threshold amount.
If the actuators have not moved more than this amount, an actuator
failure is incremented by one. Each time an actuator fails this
test, the failure is again incremented by one. When the actuator
failure counter reaches an I-loaded value of four, the actuator
is declared failed and a fault message is output. The actuator
failure counter is reset to zero any time the actuator passes
the threshold test.
The first and second parts of the actuator FDI process continue
to perform in this manner. The actuator FDI process can detect
full-off gimbal failures and full-on failures indirectly. The
full-on failure determines that the gimbal has extended or retracted
too far and commands reverse motion. If no motion occurs, the
actuator will be declared failed. The flight crew's response to
a failed actuator is to select the secondary actuator electronics
by item entry on the maneuver CRT display.