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Digital Stab Trim Update B727 & B737 |
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Present simulation techniques used to
model the Boeing 727 and 737 stabilizer trim utilize a maintenance intensive
system of hydraulic motors, clutches, limit switches, brakes and other gear
driven mechanisms to provide proper operation. By digitizing the stabilizer
trim system model, a significant reduction in actual physical linkage and
moving components has been realized. We have simplified the mechanical
system, provided an easily adjustable detailed mathematical model and added
an automatic calibration feature to provide the customer a state of the art
system, which requires a minimum of maintenance. A simple chain driven
electric motor replaces a pair of complicated hydraulic loader assemblies,
and all of the aircraft components are modeled in our high-speed DCL
computational system. Our digital stab trim update provides a simple,
reliable, virtually maintenance free electromechanical system utilizing state
of the art, commercial off the shelf hardware. We have removed the two
hydraulic/mechanical loaders, and replaced them with a single main drive
electric actuator consisting of a high tech DC motor assembly and a sprocket
set connected by a chain. The stabilizer trim indicator on the pedestal has
been separated from the main drive to provide ATG and IP enhancements. The
indicator is driven by a DC servo motor gearbox in combination with a PWM
servo amplifier. All of the aircraft stab trim mechanical components (clutch,
manual override, aero force, physical limits and motor operation in main,
cruise and auto pilot trim) are modeled digitally at an iteration rate of 2048
Hz using simple straight forward techniques in the 'C' programming language.
A Digital Control Loading (DCL) computer designed to our stringent
specifications provides an extra measure of reliability when mounting is
required onboard the simulator. An Intel Pentium processor insures that we
have the computational power required to realistically model the dynamic
characteristics of the actual aircraft system being simulated. The only
moving parts on this computer are the two air circulation fans, as the hard
drive is actually a solid-state flash memory device without any moving parts.
Every single component used in this system is a commercially available off
the shelf (COTS) item. We have produced a product, which ensures a long
service life and provides alternative sources for interchangeable parts
replacement. |
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Stab Trim System Description |
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The electric actuator assembly
consists of a high performance DC servo motor fitted with a resolver to
provide velocity and position feedback for inner loop control, actuator
housing, and a chain and sprocket assembly. This unit does not require any
setup or adjustments An 80 VDC power supply assembly houses an isolation
transformer, a reservoir capacitor, a full wave rectifier, and a PWM DC servo
amplifier. A dual color status lamp on the amplifier indicates the state of
readiness; a green light is the normal mode for operation. A red light
indicates an amplifier fault or inhibit. A power relay, two 25 amp 250V fuses
and a red indicator light are housed in the safety relay assembly, which is a
metal box mounted to the frame near the high voltage DC power supply. A
watchdog timer in conjunction with the control loading turn on logic actuates
this relay and the corresponding red indicator light. If the computer should
halt, or if hydraulic power is removed from the control loading system
through the pedestal switch or the IOS, this relay will disconnect power to
the main drive motor and extinguish the indicator light. An electric DC servo
motor with a 12.5:1 gearbox is used to drive the stab trim position
indicator. The motor assembly drives the indicator through a set of bevel
gears. Position feedback is provided through a 50:1 miniature gearbox
attached to a Midori 5Kohm film type potentiometer. No setup is required for
the indicator assembly. A PWM DC servo amplifier provides the control current
required to drive the indicator motor assembly. This amplifier contains a red
fault light which when lit, indicates a problem with the unit. An Isobar
premium surge suppresser mounted to the DCL computer provides AC
distribution, surge protection and diagnostic indicators which monitor line
voltage and polarity of the AC input. The DCL computer and the indicator
drive amplifier are connected to this unit The Analog Interface Module (AIM)
is mounted on the DCL computer, it provides an expandable interface which is
easily modified in the event that any changes are deemed necessary. A 6VDC
relay is installed to provide an inhibit signal to the indicator amplifier.
The DCL computer is an industrial rack mount computer with a 300 watt power
supply, a 14 slot backplane, and two cooling fans. This chassis is
specifically designed to endure the rigors of onboard simulator operation.
All circuit cards are secured in their respective slots with multiple hold
down attachment points. A filter is mounted in front of the intake fan on the
front of the computer behind the intake grill. This filter should be checked
bi-monthly to insure that there is no significant blockage present A Pentium
850 MHz single board computer (SBC) is utilized for computational processing.
Our custom high speed executive provides a 2048 Hz frame rate which allows
ample processing time for stab trim modeling and I/O control. Debug and plot
routines are provided running as embedded background tasks. A Flash drive
hard disk is provided with this system. This flash drive is basically a 16
megabyte solid state EEPROM chip set with no moving parts. The executable
program (aastab.exe) resides on this flash ROM which is drive C. The
autoexec.bat file has been modified such that the executable program will
start immediately after boot up. A high speed PCI video card is used to
display debug and plotting information, system status, and timing information
via the monochrome monitor provided |
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