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MAINTENANCE MANUAL
D. Servo Card
(1)
The servo card is the third major item in the thrust management process. The servo card functions are as follows:
(a)
Decode dual 429 receivers
(b)
Sense voltages
(c)
Debounce discrete inputs
(d)
Control servo excitation
(e)
Annunciate caution
(f)
Control engage line
(g)
Compute servo loop
(h)
Monitor servo loop
(2)
The servo cards 429 receivers are capable of receiving data from both high and low speed buses. The data received for the thrust management function is stored in the on-board RAM after being decoded. This data is used by the thrust limit computation (TLC) function for thrust limit computations as well as the autothrottle stopper test.
(3)
The sense voltage function will record the voltage levels of various voltage sources. Some sensed voltages, mainly those on the servo motor excitation, are used in the autothrottle control function.
(4)
Analog discrete inputs used by the autothrottle require conditioning prior to their use on the servo card or in software. The debounce discrete inputs function uses sequential logic in hardware to debounce discrete inputs. Those signals requiring more than 100 msec are handled in software.
(5)
The monitor servo excitation process has the responsibility of turning the servo motor excitation voltage on and off. This process is used by the stopper test to ensure de-excitation of the servo motor during the takeoff phase. The monitor servo excitation process contains the logic to operate two switches for servo motor excitation. If either of the switches are open the servo motor excitation voltage is false. If both switches are closed the servo motor excitation will be true.
(6)
To ensure that the master FMC can allow servo excitation. It is directly connected to the autothrottle card. This is an additional constraint to the two engage switches described previously. If the master FMC changes its status (for example, the alternate FMC becomes master) the excitation is immediately disengaged by the old master therefore disengagement of the autothrottle.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
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MAINTENANCE MANUAL
(7)
The annunciate caution process ensures that the CAUTION light and the EICAS message "AUTOTHROTTLE DISC" will go on when the autothrottle is disengaged. Autothrottle disengagement can be done manually by the flight crew or automatically because of a fault detected by the FMC. During normal operation of the annunciation light will go on and off accordingly. In the case of a power failure or power interrupt, an alternate power source (28 volt dc battery) is used. This ensures that the CAUTION light and the EICAS message "AUTOTHROTTLE DISC" will operate regardless of the 115 volt ac power status.
(8)
The control engage line process controls two engage lines, engage 1 and engage 2. These engage lines are used by the servo excitation process to ensure excitation engage or disengage. The A/T arm must be armed (in the ARM position) by the pilot or F/O so that both engage lines will be true. These two control engage lines are independently controlled except in the case of the autothrottle arm switch being on or off.
(9)
The engage 1 switch is controlled by the I/O switch while the engage 2 switch is controlled by software generated inputs.
(10)
The compute servo loop process controls the servo amplifier. This amplifier is capable of driving the servo motor control windings. The two engage lines from the control engage line process enable the servo amplifier. If both engage lines are true, then the amplifier will be enabled thereby enabling the servo motor. If either engage lines enable state is low, the amplifier will not operate, setting the throttles in a hold position.
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