Aerosurface 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.

Each aerosurface 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.

In addition 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.

Each elevon 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 manual redundancy.

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 bypassed.

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.

During ascent, 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 program.

The rudder/speed 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 brake.

The rudder 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 panels.

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.

The rotary 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.

Three servoactuators 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 ASAs.

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.