introduction autoilot
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AUTOPILOT AVIONICSSMF 3252 06/07-II
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AUTOPILOT AVIONICSSMF 3252 06/07-II
TITLE : AUTOPILOT
OBJECTIVE : To investigate the autopilot system in aircraft
To know the function of autopilot
To know the characteristics of autopilot system
To know overall about the autopilot weather for
the military aircraft or commercial aircraft
BACKGROUND :
The autopilot is the system which furnishes the ‘muscles’ to
move the control, which in turn, position the aircraft in space. The
autopilot is not always a luxury. In military aircraft, which are often
extremely fast and maneuverable, pilot reaction time may be
inadequate. It is then a necessarily to damp out the fast oscillations
which may occur along any axis. Anyone who has flown with one of
these wonders for any length of time can appreciate their value in
decreasing pilot fatigue. They allow the pilot a break from continuous
hand flying, providing time to handle other cockpit duties. Helpful
when VFR, an autopilot really pays off when flying single pilot IFR or
when flying a large, complex aircraft.
Advanced avionics system main mission is to stabilize and
navigate the UAV without human operator intervention. This is
conducted with the integrated autopilot system, the low level control.
Besides this, telemetry communication, payload data signal processing
and transferring operations are handled within the avionics system. It
has the capability to transform all types of tactical mini range fixed
wing planes to fly without a need for a pilot.
DEFINATION OF AUTOPILOT :
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An autopilot is a mechanical, electrical, or hydraulic system
used to guide a vehicle without assistance from a human being. Most
people understand an autopilot to refer specifically to aircraft, but self-
steering gear for ships and boats is sometimes also called by this term.
RESEARCH ASPECT
Fundamental / principles of autopilot
Part of autopilot
Autopilot systems - Boeing 747 Flight Control Autopilot
Modern of autopilot
EDO – Aire Mitchell Systems – No followup Control
Systems
Fluidics – Smith SEP – 6 Autopilot
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The first aircraft autopilot was developed by Sperry Corporation
in 1912. Lawrence Sperry is a Son of famous inventor Elmer Sperry was
demonstrated it two years later in 1914, and proved his credibility of
the invention, by flying the plane with his hands up.
The autopilot connected a gyroscopic attitude indicator and
magnetic compass to hydraulically operated rudder, elevator, and
ailerons. It permitted the aircraft to fly straight and level on a compass
course without a pilot's attention, thus covering more than 80% of the
pilot's total workload on a typical flight. This straight-and-level
autopilot is still the most common, least expensive and most trusted
type of autopilot. It also has the lowest pilot error, because it has the
simplest controls. In the early 1920s, the Standard Oil tanker J.A Moffet
became the first ship to use autopilot.
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SERVOSYSTEM FUNDAMENTALS
Any servo system operates on the same basic principles
of autopilot. There is some kind of comparator unit at the input
through which a positive or negative input signal can be
compared with a feedback signal or opposite polarity. For
example the exact cancellation of the input will occur when the
output element, the rudder, elevator, or airplane itself is doing
exactly what the input signal commanded it to do. If place the
airplane in a turn, the feedback from the rudder will begin to
cancel the input when the rudder is turned far enough to start
the airplane turning, but complete cancellation will not take
place until the airplane is turning fast enough to make the
required turn in the required time. If the airplane tends to turn
too fast, the rate gyro signal changes polarity and slows down
the turn. For example yaw damping maybe selected by a switch
on the autopilot control panel.
AXIS AUTOPILOT
A block diagram for a 3 axis system used in the trident aircraft.
Note the use of the computer-amplifier before the servomotors and
examine the feedback loops. The system in the diagram also has a
mach (speed) hold system, which works like the cruise control on some
automobiles.
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APPROACH TO GLIDE PATH
The airplane is steered so that angles and are reduced to zero
at some distance d from the runway as designed by the transmitter.
The angular displacement can be converted into a distance l from
center by the equations l = d tan . The
l = d tan
The rate at which l is decreasing is given by
l = sin
Where equals approach velocity.
So the pilot knows is approaching the centerline. A panel
instrument will shows the deviation and can show to fast is
approaching the beam center. He knows that if he keeps his approach
rate at or below the present value, he will have a small transient, or
oscillation, about the beam center then crosses it. If he tries to
approach at a larger angle or faster rate, then he ill overshoot the
beam center and a large transient ill occur.
In addition to the panel, an aircraft ill have the selector type
panel. Approach to the localizer beam, the pilot ill push the button
marked VOR LOC to feed the input of the localizer receiver into the
autopilot for automatic control of the airplane laterally.
SEP-6 THEORY OF OPERATION
The SEP-6 autopilot is an electromechanical system
providing flight stabilization and maneuver control in the three
aircraft axes. Each computer has its own power supply, thus
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preventing a single power failure from rendering the whole
system inoperative. This has the advantages of allowing split-
axis operation as well as independent yaw damping.
The SEP-6 autopilot operates on a ‘rate-rate’ principle in
which control surface rates are mode proportional to aircraft
disturbance rate. Rate gyros in the three control axes are used
to detect aircraft disturbances and the resulting electrical
signals are amplified to drive the electromechanical
servomotors for short-term stabilization in pitch, roll and yaw.
In addition to short-term stabilization, attitudes changes
can be demanded to maneuver the aircraft in pitch and roll.
The maneuver commands are converted to rate commands
which are fed to the appropriate servomotor, together with
long-term stabilization signals derived from the attitude
reference system and various mode sensors. Any resultant
standing errors and long term data shifts are compensated by
integration.
The SEP-6 achieves all commanded functions by pitch and
roll axis control. This means that directional control is
maintained by ailerons throughout all flight maneuvers.
Provision is made to feed an aileron signal into yaw channel
for turn coordination. The pitch channel also provides signals
to control a separate trim servomotor which can be fitted to
the aircraft’s trim system. Autopilot operates in some detail
the pitch, roll and yaw channel.
PITCH CHANNEL
1. Function of pitch channel
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a) This channel provides pitch stabilization by operating the
aircraft controls and pitch trim pitch in response to
attitude and attitude rate signals.
2. Short term damping is derived from pitch and yaw rates,
modified by the roll angle to produce the true pitch rate in
the roll repeater module before being passed to the pitch
shaping module. Maneuver demands are obtained from in
inputs.
3. In the pitch shaping module, a pitch rate demand is
computed from the pitch attitude demand and added to the
true pitch rate signal, which has been phase advanced, it is
also added to the balance integrator signal and the servo
tachometer feedback signal. This combined signal is then
the gain scheduled to suit the particular flight configuration,
dependent on flap and airspeed conditions. The resultant
control signal, passed to the servomotor, also produces a
servo torque signal which is used to drive the trim
servomotor via the trim servo amplifier. It also provides a
trim signal to the pilot’s pitch trim indicator.
4. The glide coupling module computes a pitch demand signal
from deviation information derived from the instrument
landing system (ILS) radio receiver. A gain compensation
circuit reduces the gain as a function of time following glide
slope engagement. For category ll operation the glide signal
is further reduced as a function of radio altitude and is
supplement by a signal derived from pitch altitude. The
pitch “data chaser” produces a pitch error signal, which is
used as a pitch demand in the pitch attitude mode. The
balance integrator produces an error signal, based on pitch
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attitude demand to compensate for long-term data drift
within the channel.
ROLL CHANNEL
1. Function of roll channel is:
a) The roll channel stabilizes the aircraft about its roll axis
and in azimuth by controlling the ailerons in response to
the roll rate.
b) It is also used to control the aircraft azimuth when fed
with compass or radio deviation signals or manual control
demands.
2. Short term damping information is derived by multiplying
the yaw rate and the pitch angle and adding this to the
sensed roll rate to provide a true roll rate signal. The roll
channel contains a cutout to disengage it in case of
excessive roll rate or roll attitude.
3. The demands signal is compared with bank angle, and the
difference is fed as an error signal to the roll shaping
module, which operates in the same manner as the pitch
shaping module but with the demanded rill rate limited to 5
degree.
4. The control signal is amplified and passed to the
servomotor. The servo amplifier also produces a torque
signal to operate the pilot’s aileron trim indicator.
5. The heading data chaser produces a heading error signal
from the compass which is used as a bank demand when in
the heading mode.
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6. The localizer coupling module computes a bank demand
signal which is a function of the localizer deviation signal
and the rate of change of this signal. During the later stages
of ILS interception, yaw rate is added to the signal, and the
gain of the radio rate circuit is changed at glide slope
engagement to obtain optimum performance during the
final stages of the approach. The computation automatically
provides wind drift correction.
7. Maneuver demands are derived from the autopilot
controller, the heading data chaser, radio coupling modules,
or the HSI. These demands are selected from the mode
selector and having been limited in the roll switching
module to provide a demand of either 10 degree or 30
degree are passed to the roll shaping module and the
balance integrator, when the bank angle is less than 3
degree.
8. The VOR coupling module computes a bank demand signal
from the localizer coupling module and from a course error
signal from the HIS. In the later phases of interception, the
course error signal is washed out, thus allowing the aircraft
to take up a drift angle to offset crosswinds. The balance
integrator operates in the same manner as was described
for the pitch channel.
YAW CHANNEL
1. The yaw channel of the Smith SEP-6- autopilot is a self-
contained yaw damper system which is:
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a) Maintains aircraft yaw stability and assist the roll channel
during azimuth maneuvers so as to give full roll-yaw
coordination.
b) The yaw damper can be installed as a parallel or series
system, depending on the type of aircraft and the
principle of operation required.
2. The yaw channels are divided to two parts which is:
a) Parallel yaw damper
When the damper is in parallel, short term damping
information is derived from yaw rate shaping module to
produce a yaw rate demand signal which is added to the
servomotor feedback signal. This control signal is
amplifier in the servo amplifier, which drives the
servomotor. The amplifier also produces a true signal,
which operates the pilot’s yaw trim indicator. Aileron
position information is fed to the yaw shaping module
and backs off the shaped yaw rate term to insure that the
rudder control does not oppose entry into turns. Also a
lateral accelerometer produces a signal which acts as a
monitor for the suppression of sideslip.
b) Series yaw damper
When the damper is in series, the principle of operation is
the same as when it is in parallel, except that a lateral
accelerometer is not used, and the control signal is a
position demand signal to the linear actuator. Actuator
position information is fed back into the yaw shaping
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module in addition to rate feedback. This position signal
is also fed to the pilot’s yaw trim indicator.
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AERODYNAMICS OSCILLATIONS
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An aircraft in flight acts somewhat like a weathervane,
which tries to turn to point in a new direction as the wind changes.
The weathervane will oscillate, or swing back and forth, before finally
settling down to a new direction. If the wind is gusty, then the
oscillations continue indefinitely. This profile in the wind of a fan,
turning it sideways to the airflow. The release it and watch it oscillate
as it turns to point toward the airstreams.
The real aircraft has two oscillation periods which is a long
oscillation time as it veers back and forth across its flight path and the
short time oscillation as it changes its instantaneous heading. A pilot
can control the aircraft as long as oscillation time is longer than his
reaction time. Some fighter aircraft have very fast oscillation times
when the center of gravity approaches the center of wind pressure and
so autopilot is required in order for the pilot to handle the aircraft. The
autopilot controls the short period oscillation, and the pilot
controls the long period oscillation, or the flight path. Under an
aerodynamics oscillation there is damper.
DAMPER
The yaw damper is the part of the autopilot which
smoothes out the short-period yaw oscillations. The pilot will
have as its input some kind of reference, such as a gyroscope
heading signal, and the yaw damper ill tend to keep the
airplane pointed in the correct direction to follow this heading
with the smallest possible oscillation about the airplane’s
vertical, or yaw, axis .
The long term oscillation along the flight path, the
weaving back and forth along the flight line, is usually under
the control of the pilot in small aircraft. In large aircraft, it
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maybe autopilot controlled with a gyroscope reference, or it
may be under the autopilot and pilot’s control.
This kind of autopilot prevents short term oscillations by
sensing the airplane movement and immediately moving the
rudder to counteract them. An arrow a top the case indicates
the direction of turn for which an output voltage is produced.
Types of servomechanisms used in autopilot. The speed
range is 5-5000rpm. The tachometer gradient is 4.6 V per
1000rpm. The servo actuator at B is a product of control
technology, Inc. Its signal input is at 10V dc. The slew speed is
36°/sec. The unit is designed for operation with power sources of 115V
ac (400Hz) or 28V dc. At C is an Electro craft dc servo meter with a
heavy duty gear head. The power is 1/20 hp and the torque 3-200 in lb
for gearing ratios between 5:1 and 5000:1. The power input voltage is
28V. The tachometer gradient is a 4.6V per 1000rpm.
YAW DAMPING (SINGLE AXIS)
A dual channel system is incorporated to damp natural
yaw oscillations on channel driving the top rudder section, the other
the bottom. This redundancy insures safety in case of single failures or
local structural damage. The damping signal to the rudder is series
added to pilot controlled outputs, without causing rudder pedal
movement. A transient turn coordination output is also produced. Each
channel receives independently sensed information:
a) The variables sensed are roll altitude from the inertial navigation system.
b) True airspeed from the central air data computers.
c) Yaw rate from integral rate gyros.
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d) A ground only confidence checks tests the electronics and actuators.
e) Test response is shown on the rudder position indicator. Fault
isolation testing is also provided.
The following list states the capabilities:
Yaw damping (single axis)
Full time
For manual of power operated flying controls
Series or parallel systems as required
Rotary or linear actuator available
Pitch and roll autopilot (two axes) Pitch and roll stabilization
Manual maneuvering provided by controls by controls on
the autopilot controller
Heading lock
Autopilot with manometric locks and radio coupling (three
axes):
Yaw damper with pitch and roll autopilot gives
stabilization in three axes
Automatic pitch trim
Barometric sensors provide height and airspeed lock
facilities
Three channel-Engage and trim indication
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Preselect heading control
Variable beam intercept angle for operational flexibility
Automatic capture of VOR and ILS beams
Automatic wind drift correction
Optional facilities
Altitude preselect and vertical speed control
Mach lock
Category ll capability
The VOR is simply a radio beam transmitter which the airplane
autopilot can identify through its coupler. The fan markers are
transmitters located near the airport which transmit a vertical
fan shaped beam, when the airplane passes over them, the
pilots knows how far he is from the airport. Blue, amber and
white lights are lighted automatically on his panel as he
passes over the different markers. Note how complex the
approach may be if the airplane must be prevented from
landing right away and has to stay in a holding pattern.
AUTOPILOT-FLIGHT DIRECTOR
The autopilot function of the AF-FD provides:
a) Fully automatic attitude or path control, and
semiautomatic control via the flight controller
b) The flight directors display the required attitude change
in most autopilot modes and also when the autopilot is
off.
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The systems employs three identical computers each with
autopilot and flight director output. The latter output,
indicating the attitude change which the pilot or autopilot
must affect to properly control the airplane, can be displayed
on the attitude director indicators (ADI). Each pilot can
independently select any one of the computers.
For en route operation, either autopilot channel is used
alone. In the landing mode, both may be employed if their
outputs disagree beyond tolerance limits, a display warns the
pilot to consider switching to single channel mode or manual
control. The independence between channels including the fact
that each governs a different pitch and roll control
servomechanism gives the system capability for category ll
automatic landings.
The most significant improvement in operational
versatility has been made in pitch axis control, where altitude
preselection, airspeed hold, and altitude hold modes have
been made available both for autopilot and flight director. The
full listing of AP-FD functional capabilities is as follows:
No Mode Autopilot Flight Director
1. Pitch modes
Pitch Yes Yes
Pitch heel control Yes Yes
Altitude hold Yes Yes
Airspeed hold Yes Yes
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Altitude select Yes Yes
Turbulence Yes No
Go around No Yes
Vertical speed control
Yes Yes
2. Lateral modes
Heading hold Yes No
Turn knob control Yes No
Heading select Yes Yes
Inertial navigation Yes Yes
VOR/LOC navigation Yes Yes
Back Beam No Yes
3. Combined modes
Localizer capture Yes Yes
G/S capture and auto land with flare
Yes Yes
AVIATION AUTOPILOT CATEGORIES OF LANDING
Instrument aided landings are defined in categories by the ICAO.
These are dependent upon the required visibility level and the degree
to which the landing can be conducted automatically without input by
the pilot.
CAT I - This category permits pilots to land with a decision height
(where the pilot takes over from the autopilot) of 200 ft
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and a forward visibility of 2400 ft. Simplex autopilots are
sufficient.
CAT II - This category permits pilots to land with a decision height
of 100 ft and a forward visibility of 1200 ft. Autopilots have
a fail passive requirement.
CAT IIIa - A full blind landing capability on autopilot. Pilot assumes
control on touch down. The failure rate of the automatic
system must be better than 1 in 10 million.
CAT IIIb - As IIIa but with the addition of automatic roll out after
touchdown incorporated with the pilot taking control some
distance along the runway. Obviously for this category
some form of runway guidance system is needed.
CAT IIIc - As IIIb but with the inclusion of automatic taxi control
enabling runway to terminal without pilot intervention. No
current aircraft has this capability.
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Will examine now some complete autopilot systems. We
define a system as that complete aggregation of elements
which are necessary to cause the airplane to follow some
desired flight pattern.
The autopilot is an analog system using solid state
devices throughout. Direct-coupled circuits are used to
minimize the numbers of components, and all signal switching
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is solid state thus improving the reliability. The system utilizes
standard syncho transmissions and is therefore compatible
with many other systems
AUTOTHROTTLE
The auto throttle system of the AFCS maintains a
preselected airspeed during descent, approach and landing,
thus reducing pilot workload during these critical phases. Auto
throttle speed command, selected on the AP-FD glareshield
panel, is displayed on the pilot’s airspeed indicators. The
difference between the actual and commanded airspeeds
constitutes the error signal to a computer driving a servomotor
coupled by control cable to the throttles. The error signal is
displayed on each ADI. The flight mode annunciator provides
fault isolation testing. Operating any of four switches on the
throttle levers disengages the autothrottle.
An additional autothrottle function is provided. When the
autopilot is in autoland mode and flare altitude is reached, the
autopilot sends the autothrottle computer a signal which
initiates an automatic retarding of the throttles to their aft
stops.
AUTOMATIC THROTTLE –SEP-6
The automatic throttle system requires inputs from the
longitudinal accelerometer, the airspeed error signal, and
pitch attitude signal. These three signals are mixed in the
computer and used to control the servothrottle motors. The
computer also operates a warning flag signal on the pilot’s
display.
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The automatic throttle provides precise control of
airspeed during approach and terminal phases and automatic
closure (decrease) of fuel flow during flare out just before
touchdown.
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MINI AUTOPILOT V1.0.B
- Embbeded Power PC real time solution - Advanced system: Configurable to Fixed Wing and Rotor Wing Platforms- Waypoint navigation (GPS, D-GPS, INS)- Special fail safe navigation mode - Dynamic real time gains, limits etc. adjustments- Dual extended Kalman Filtering for precision navigation- Compressed digital image transferring - Autonomous Hand Launch, Autonomous flight, Autonomous belly landing, Autonomous parachute deployment capability - Manuel steering, Manuel control through secure digital link- Return home mode for communication loss, gps signal loss etc.- Communication relay mode (In Development) - Auto target coordinate detection through INS support- Fault tolerant embedded software
Weight (Sensors, Flight Control system, Radio Modem): 160 grams Dimensions: 12 cm x 9 cm x 4cm
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Features / Specifications:
Software:
Real time operating systemMultithreaded software (Stabilization, Navigation, Payload, Comm. Etc.)
Fault Tolerant
Fault Detection (Continuous Testing of the components)
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Real time flight data loading, gain adjustments and telemetry transfer
Digital Image Transferring
Hardware:
Advanced Integration: All functions in a single chip!3 Axis Gyros (300 deg/sec), Accelerometers (10g),
Magnetometers
Comm. Unit (>60 Miles LOS)
GPS Unit (4 Hz Update Rate)
Ultrasonic Range Finder Unit
Temp. Calibrated Pressure Ports
Digital Camera Unit
Voltage Regulator
13 Servo Output
Flight Control:
Full Autonomous Take Off And Landing (Catapult, Hand Launch, Wheel Take Off)Autonomous Cruise
Manuel Steering
Manuel Flight Mode
Return Home Option on Lost Communication
Advanced Attitude Estimation Techniques Through GPS
Physical:
Size: 180 mm * 125 mm * 55 mmWeight: 550 grams
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Features/Specifications:
A total integrated solution including 3 axis sensor suite, pressure ports, GPS, Ultrasonic Range Finder, Radio Modem, High Level Control PC, High Resolution CameraIndustrial PC Based Hardware
Real Time Operating System
Multithreaded software development
Autonomous Stabilization and Navigation
Real time flight data loading, gain adjustments and telemetry transfer
Home return in case of Lost Communications (Selective)
High Resolution Digital Image Transferring and Onboard storing (80 Gb)
LOS 90 km communication range (256 kbaud/sn.)-
Highly integrated, robust and secure solution for command,
control and monitoring. Microcontroller based control system handles
the management of data transfer of telemetry, payload and uplink
command. The operator interface runs on a Windows based PC system.
The system gives support for multi-UAV monitoring from a single
ground station thanks to the network enabling communication
systemsVV
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General Specifications and Features:
Ground Control System
Microcontroller based hardware
Real-time operating system environment
Multithreaded software architecture
Multi UAV command, control and monitoring support
Communications Management
HARDWARE IN THE LOOP SIMULATOR
Development of modern control, guidance and navigation
systems mostly depend on the control system and the modeling,
simulation, real-time testing of the system dynamics. Successfully
applied modeling and simulations can decrease the time required for
hardware prototype development in incredible amounts. Also, it
decreases the risks that might be encountered during testing and
system integration processes.
Simulation of the UAVs dynamic behavior in conditions that are
very close to real-flying environment is required very much for the
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development of the control system and the algorithms and also
evaluation of the vehicles performance under different conditions.
Conforming to these principles, flight control system algorithms and
guidance control system developed at Baykar Machine are initially
applied to the platform in the simulation environment and tests with
tuning continued till satisfactory results are taken.
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Modern autopilots generally divide to a several part which is:
A flight into taxi Take-off
Ascent
Level
Descent,
Approach and landing phases
Autopilots exist that automate all of these flight phases except
the taxiing. Landing on runway and controlling the aircraft on rollout
for example keeping it on the centre of the runway is CAT 3b landing,
used on the majority of major runways today. Landing, rollout and taxi
control to stand is CAT 3c. This is not usually used to date but may be
used in the future. An autopilot is often an integral component of a
Flight Management System.
Modern autopilots use computer software to control the aircraft.
The software reads the aircraft's current position, and controls a flight
control system to guide the aircraft. In such a system, besides classic
flight controls, many autopilots incorporate thrust control capabilities
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that can control throttles to optimize the air-speed, and move fuel to
different tanks to balance the aircraft in an optimal attitude in the air.
The autopilot reads its position and the aircraft's attitude from an
inertial guidance system. Inertial guidance systems accumulate errors
over time. They will incorporate error reduction systems such as the
carousel system that rotates once a minute so that any errors are
dissipated in different directions and have an overall nulling effect.
Error in gyroscopes is known as drift.
The six dimensions are usually roll, pitch, yaw, altitude, latitude
and longitude. Aircraft may fly routes that have a required
performance factor, therefore the amount of error or actual
performance factor must be monitored in order to fly those particular
routes. The longer the flight the more error accumulates within the
system.
Radio aids such as DME, DME updates and GPS may be used to
correct the aircraft position. Inertial reference units, i.e. gyroscopes,
are the basis of aircraft on board position determining, as GPS and
other radio update systems depend on a third party to supply
information. IRU's are completely self-contained and use gravity and
earth rotation to determine their initial position (earth rate). They then
measure acceleration to calculate where they are in relation to where
they were to start with. From acceleration one can get speed and from
speed one can get distance. As long as one knows the direction (from
accelerometers) the IRU's can determine where they are (software
dependent).
The Digital Autopilot is an all digital, self calibrating, two axis
Autopilot system designed specifically for the EFIS/One & EFIS/Lite. It
incorporates the latest in small, high torque mini DC motors, a high
quality aerospace grade gearbox, a high resolution position encoder
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and a magnetic clutch all enclosed in single aircraft quality aluminum
housing.
Automatic Throttle
The automatic throttle system, inputs from the
longitudinal accelerometer, the airspeed error signal, and
pitch attitude signal. These three signals are mixed in the
computer and used to control the servo throttle motors. The
computer also operates a warning flag signal on the pilot’s
displays.
The automatic throttle provides precise control of
airspeed during approach and terminal phases and automatic
closure (decrease) of fuel flow during flare out just before
touchdown.
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