servoamplifiers are electronic devices that receive aerosurface
commands during atmospheric flight from the flight control system
software and electrically position hydraulic valves in aerosurface
actuators, causing aerosurface deflections.
is driven by a hydraulic actuator controlled by a redundant set
of electrically driven valves (ports). There are four of these valves
for each aerosurface actuator, except the body flap, which has only
three. These valves are controlled by the selected ASAs.
There are four
ASAs located in aft avionics bays 4, 5 and 6. Each ASA commands
one valve for each aerosurface, except the body flap. ASA 4 does
not command the body flap.
to the command channels from the ASAs to the control valves, there
are data feedback channels to the ASAs from the aerosurface actuators.
Each aerosurface has four associated position feedback transducers
that are summed with the position command to provide a servoloop
closure for one of the four independent servoloops associated with
the elevons, rudder and speed brake. The body flap utilizes only
three servoloops. The path from an ASA to its associated servovalve
in the actuators and from the aerosurface feedback transducers to
an ASA is called a flight control channel; there are, thus, four
flight control channels, except for the body flap.
Each of the
four elevons located on the trailing edges has an associated servoactuator
that positions it. Each servoactuator is supplied with hydraulic
pressure from the three orbiter hydraulic systems. A switching valve
is used to control the hydraulic system that becomes the source
of hydraulic pressure for that servoactuator. The valve allows a
primary source of pressure (P1) to be supplied to that servoactuator.
If the primary hydraulic pressure drops to around 1,200 to 1,500
psig, the switching valve allows the first standby hydraulic pressure
(P2) to supply that servoactuator. If the first standby hydraulic
pressure drops to around 1,200 to 1,500 psig, the secondary standby
hydraulic source pressure (P3) is then supplied to that servoactuator.
The yellow hyd press caution and warning light will be illuminated
on panel F7 if the hydraulic pressure of system 1, 2 or 3 is below
2,400 psi and will also illuminate the red backup caution and warning
alarm light on panel F7.
servoactuator receives command signals from each of the four ASAs.
Each actuator is composed of four two-stage servovalves that drive
a modulating piston. Each piston is summed on a common mechanical
shaft, creating a force to position a power spool that controls
the flow of hydraulic fluid to the actuator power ram, controlling
the direction of ram movement, thus driving the elevon to the desired
position. When the desired position is reached, the power spool
positions the mechanical shaft to block the hydraulic pressure to
the hydraulically operated ram, locking the ram at that position.
If a problem develops within a servovalve or it is commanded to
a position different than the positions of the other three within
an actuator, secondary delta pressure should begin to rise to 2,200
psi. Once the secondary delta pressure is at or above 2,200 psia
for more than 120 milliseconds, the corresponding ASA sends an isolation
command to the servovalve, opening the isolation valve, bypassing
the hydraulic pressure to the servovalve, and causing its commanded
pressure to the power spool to drop to zero, effectively removing
it from operation. The pressure differential is sensed by a primary
linear differential pressure transducer across the modulating piston
when the respective FCS channel switch on panel C3 is in auto .
This automatic function prevents excessive transient motion to that
aerosurface, which could result in loss of the orbiter due to slow
The FCS channel
yellow caution and warning light on panel F7 will be illuminated
to inform the flight crew of a failed channel. A red FCS saturation
caution and warning light on panel F7 will be illuminated if one
of the four elevons is set at more than plus 15 degrees or less
than minus 20 degrees.
There are four
FCS channel switches on panel C3- FCS channels 1, 2, 3 and 4; each
has an override, auto and off position. The switch for a channel
controls the channel for the elevons, rudder/speed brake and body
flap, except channel 4, which has no body flap commands. When an
FCS channel switch is in auto and that channel was bypassed, it
can be reset by positioning the applicable switch to override .
When an FCS channel switch is positioned to off , that channel is
In each elevon
servoactuator ram, there are four linear ram position transducers
and four linear ram secondary differential pressure transducers.
The ram linear transducers provide position feedback to the corresponding
servoloop in the ASA, which is then summed with the position command
to close the servoloop. This feedback is then summed with the elevon
ram linear secondary differential pressures to develop an electrohydraulic
valve drive current that is proportional to the error signal in
order to position the ram. The maximum elevon deflection rate is
20 degrees per second.
the elevons are deflected to reduce wing loads caused by rapid acceleration
through the lower atmosphere. In this scheme, the inboard and outboard
elevons are deflected together. By the time the vehicle reaches
approximately Mach 2.5, the elevons have reached a null position,
where they remain. This is accomplished by the initialized-loaded
brake, which consists of upper and lower panels, is located on the
trailing edge of the orbiter's vertical stabilizer. One servoactuator
positions the panels together to act as a rudder; another opens
the panels at the rudder's flared end so it functions as a speed
and speed brake servoactuator receives four command signals from
the four ASAs. Each servoactuator is composed of four two-stage
servovalves that function like those of the elevons. The exception
is that the rudder's power spool controls the flow of hydraulic
fluid to the rudder's three reversible hydraulic motors and the
power spool for the speed brake controls the flow of hydraulic fluid
in the speed brake's three hydraulic reversible motors. Each rudder
and speed brake hydraulic motor receives hydraulic pressure from
only one of the orbiter's hydraulic systems. Each hydraulic motor
has a hydraulic brake. When the motor is supplied with hydraulic
pressure, the motor's brake is released. When the hydraulic pressure
is blocked to that hydraulic motor, the hydraulic brake is applied,
holding that motor and the corresponding aerosurface at that position.
The three hydraulic
motors provide output to the rudder differential gearbox, which
is connected to a mixer gearbox that drives rotary shafts. These
rotary shafts drive four rotary actuators, which position the rudder
The three speed
brake hydraulic motors provide power output to the speed brake differential
gearbox, which is connected to the same mixer gearbox as that of
the rudder. This gearbox drives rotary shafts, which drive the same
four rotary actuators involved with the rudder. Within each of the
four rotary actuators, planetary gears blend the rudder positioning
with the opening of the rudder flared ends.
There are four
rotary position transducers on the rudder differential gearbox output
and one differential linear position transducer in each rudder servoactuator.
The rotary position transducers provide position feedback to the
corresponding servoloop in the ASA. The feedback is summed with
the linear differential pressures that develop the electrohydraulic
valve drive current in proportion to the error signal in order to
position the rudder.
There are also
four rotary position transducers on the speed brake differential
gearbox output and one differential linear pressure transducer in
each speed brake servoactuator.
position transducers provide position feedback to the corresponding
servoloop in the ASA, which is summed with the position command
to close the servoloop. These are then summed with the linear differential
pressures that develop the electrohydraulic valve drive current
in proportion to the error signal to position the speed brake.
If a problem
occurs in one of the four rudder or speed brake servoactuator channels,
the corresponding linear differential pressure transducer will cause
the corresponding ASA to signal a solenoid isolation valve to remove
the pressure from the failed channel and bypass it if that FCS channel
switch is in auto . The FCS channel switches' override and off positions
and the FCS channel caution and warning light function the same
as for the elevons. The hyd press light indicates a hydraulic failure.
The rudder deflection rate is a maximum of 14 degrees per second.
The speed brake deflection rate is approximately 10 degrees per
second. If two of the three hydraulic motors fail in the rudder
or speed brake, about half the design speed output will result from
the corresponding gearbox due to its velocity summary nature.
at the lower aft end of the fuselage are used to position the body
flap; each is supplied with hydraulic pressure from an orbiter hydraulic
system and has a solenoid-operated enable valve controlled by one
of the three ASAs (the fourth ASA is not used for the body flap
commands). Each solenoid-operated enable valve supplies hydraulic
pressure from one orbiter hydraulic system to a corresponding solenoid-operated
pilot valve, which is, in turn, controlled by one of the three ASAs.
When the individual pilot valve receives a command signal from its
corresponding ASA, it positions a common mechanical shaft in the
control valve, allowing hydraulic pressure to be supplied to the
hydraulic motors (normally one pilot valve is enabled and moves
the other two). The hydraulic motors are reversible, allowing the
body flap to be positioned up or down. The hydraulic brake associated
with each hydraulic motor releases the hydraulic motor for rotation.
When the desired body flap position is reached, the control valves
block the hydraulic pressure to the hydraulic motor and apply the
hydraulic brake, holding that hydraulic motor at that position.
Each hydraulic motor provides the power output to a differential
gearbox, which drives a rotary shaft, and four rotary actuators,
which position the body flap. The rotary position transducer associated
with each rotary actuator provides position feedback to the ASAs;
the fourth ASA is used to provide position feedback to the flight
control system software.
If the FCS
channel switches are in auto , the ASAs will isolate a body flap
channel through the solenoid-operated enable valve if the corresponding
solenoid-operated pilot valve malfunctions or the control valve
associated with the pilot valve does not provide the proper response
and allows the hydraulic pressure fluid to recirculate. The FCS
channel switches and FCS channel caution and warning light function
the same as for the elevons. If the hydraulic system associated
with the hydraulic motor fails, the remaining two hydraulic motors
will position the body flap, and the hyd press caution and warning
light will be illuminated. The body flap deflection rate is approximately
4.5 degrees per second.
Each ASA is
hard-wired to a flight MDM. Flight control commands originate from
guidance software or from controllers. These inputs go to the flight
control software, where they are augmented and then routed to the
There are several
subsystem operating programs associated with the ASA commands and
data. The SOPs convert elevon, rudder and speed brake commands from
flight control software from degrees to millivolts; set commands
to body flap valves based on an enable command from body flap redundancy
management and up/down commands from flight control; convert position
feedback to degrees for the elevons, rudder, speed brake and body
flap; compute elevator position from elevon position feedbacks;
calculate body flap and speed brake deflections as percentages;
calculate elevon and rudder positions for display on the surface
position indicator; monitor the FCS channel switches and if any
are positioned to override, set the override command for that ASA;
monitor hydraulic system pressures for failures; and rate-limit
aileron and elevator commands according to the number of failures.
Each ASA is
mounted on a cold plate and cooled by the Freon-21 coolant loops.
Each is 20 inches long, 6.4 inches high and 9.12 inches wide and
weighs 30.2 pounds.
The ASA contractor
is Honeywell Inc., Clearwater, Fla.