AUTOLAND

A typical autoland system consists of an ILS (integrated glideslope receiver, localizer receiver,

and perhaps GPS receiver as well) radio to receive the localizer and glideslope signals. The

output of this radio will be a "deviation" from center which is provided to the flight control

computer; this computer which controls the aircraft control surfaces to maintain the aircraft

centered on the localizer and glideslope. The flight control computer also controls the aircraft

throttles to maintain the appropriate approach speed. At the appropriate height above the ground

(as indicated by the radio altimeter) the flight control computer will retard the throttles and initiate

a pitch-up maneuver. The purpose of this "flare" is to reduce the energy of the aircraft such that it

"stops flying" and settles onto the runway.

For CAT IIIc, the flight control computer will continue to accept deviations from the localizer and

use the rudder to maintain the aircraft on the localizer (which is aligned with the runway

centerline.) On landing the spoilers will deploy (these are surfaces on the top of the wing towards

the trailing edge) which causes airflow over the wing to become turbulent, destroying lift. At the

same time the autobrake system will apply the brakes and the thrust reversers will activate to

maintain a deceleration profile. The anti-skid system will modulate brake pressure to keep all

wheels turning. As the speed decreases, the rudder will lose effectiveness and the pilot will need

to control the direction of the airplane using nose wheel steering, a system which typically is not

connected to the flight control computer.

From an avionics safety perspective, a CAT IIIc landing is the "worst-case scenario" for safety

analysis because a failure of the automatic systems from flare through the roll-out could easily

result in a "hard over" (where a control surface deflects fully in one direction.) This would happen

so fast that the flight crew may not effectively respond. For this reason Autoland systems are

designed to incorporate a high degree of redundancy so that a single failure of any part of the

system can be tolerated (fail active) and a second failure can be detected – at which point the

autoland system will turn itself off (uncouple, fail passive). One way of accomplishing this is to

have "three of everything." Three ILS receivers, three radio altimeters, three flight control

computers, and three ways of controlling the flight surfaces. The three flight control computers all

work in parallel and are in constant cross communications, comparing their inputs (ILS receivers

and radio altimeters) with those of the other two flight control computers. If there is a difference in

inputs, then a computer can "vote out" the deviant input and will notify the other computers that

"RA1 is faulty." If the outputs don't match, a computer can declare itself as faulty and, if possible,

take itself off line.

When the pilot "arms" the system (prior to capture of either the localizer or glideslope) the flight

control computers perform an extensive series of Built In Tests (BIT). For a CAT III landing, all

the sensors and all the flight computers must be "in good health" before the pilot receives an

AUTOLAND ARM (These are generic indications and will vary depending on equipment supplier

and aircraft manufacturer) indication. If part of the system is in error, then an indication such as

"APPROACH ONLY" would be presented to inform the flight crew that a CAT III landing is not

possible. If the system is properly in the ARM mode, when the ILS receiver detects the localizer,

then the autoland system mode will change to 'LOCALIZER CAPTURE' and the flight control

computer will turn the aircraft into the localizer and fly along the localizer. A typical approach will

have the aircraft come in "below the glideslope" (vertical guidance) so the airplane will fly along

the localizer (aligned to the runway centerline) until the glideslope is detected at which point the

autoland mode will change to CAT III and the aircraft will be flown by the flight control computer

along the localizer and glideslope beams. The antennas for these systems are not at the runway

touch down point however, with the localizer being some distance beyond the runway. However

at a predefined distance above the ground the aircraft will initiate the flare maneuver, maintain

the same heading, and settle onto the runway within the designated touch down zone.

If the autoland system loses redundancy prior to the decision height, then an AUTOLAND FAULT

will be displayed to the flight crew at which point the crew can elect to continue as a CAT II

approach or if this is not possible because of weather conditions, then the crew would need to

initiate a go-around and proceed to an alternative airport.

If a single failure occurs below decision height AUTOLAND FAULT will be displayed, however at

that point the aircraft is committed to landing and the autoland system will remain engaged,

controlling the aircraft on only two systems until the pilot completes the rollout and brings the

aircraft to a full stop on the runway or turns off the runway onto a taxiway. This is termed "fail

active." However in this state the autoland system is "one fault away" from disengaging so the

AUTOLAND FAULT indication should inform the flight crew to monitor the system behavior very

carefully and be ready to take control immediately. The system is still fail active and is still

performing all necessary cross checks so that if one of the flight control computers decides that

the right thing to do is order a full deflection of a control surface, the other computer will detect

that there is a difference in the commands and this will take both computers off line (fail passive)

at which time the flight crew must immediately take control of the aircraft as the automatic

systems have done the safe thing by taking themselves off line.

During system design, the predicted reliability numbers for the individual equipment which makes

up the entire autoland system (sensors, computers, controls, and so forth) are combined and an

overall probability of failure is calculated. As the "threat" exists primarily during the flare through

roll-out, this "exposure time" is used and the overall failure probability must be less than one in a

million.

Stability augmentation systemsA stability augmentation system (SAS) is another type of automatic flight control system;

however, instead of maintaining the aircraft on a predetermined attitude or flight path, the SAS

will actuate the aircraft flight controls to dampen out aircraft buffeting regardless of the attitude or

flight path. SAS can automatically stabilize the aircraft in one or more axes. The most common

type of SAS is the yaw damper which is used to eliminate the Dutch roll tendency of swept-wing

aircraft. Some yaw dampers are integral to the autopilot system while others are stand-alone

systems.

Yaw dampers usually consist of a yaw rate sensor (either a gyroscope or angular

accelerometer), a computer/amplifier and a servo actuator. The yaw damper uses yaw rate

sensor to sense when the aircraft begins a Dutch Roll. A computer processes the signals from

the yaw rate sensor to determine the amount of rudder movement that is required to dampen out

the Dutch roll. The computer then commands the servo actuator to move the rudder that amount.

The Dutch roll is dampened out and the aircraft becomes stable about the yaw axis. Because

Dutch roll is an instability that is inherent to all swept-wing aircraft, most swept-wing aircraft have

some sort of yaw damper system installed.

There are two types of yaw dampers: series yaw dampers and parallel yaw dampers. The servo

actuator of a series yaw damper will actuate the rudder independently of the rudder pedals while

the servo actuator of a parallel yaw damper is clutched to the rudder control quadrant and will

result in pedal movement when the system commands the rudder to move.

Some aircraft have stability augmentation systems that will stabilize the aircraft in more than a

single axis. B-52s, for example, require both pitch and yaw SAS in order to provide a stable

bombing platform. Many helicopters have pitch, roll and yaw SAS systems. Pitch and roll SAS

systems operate much the same way as the yaw damper described above; however, instead of

dampening out Dutch roll, they will dampen pitch and roll oscillations or buffeting to improve the

overall stability of the aircraft.

Autopilot for ILS landingsInstrument-aided landings are defined in categories by the International Civil Aviation

Organization, or 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 of 200 ft (61 m) and a forward

visibility or Runway Visual Range (RVR) of 550 m. Autopilots are not required. [8]

CAT II - This category permits pilots to land with a decision height between 200 ft and 100 ft (≈

30 m) and a RVR of 300 m. Autopilots have a fail passive requirement.

CAT IIIa -This category permits pilots to land with a decision height as low as 50 ft (15 m) and a

RVR of 200 m. It needs a fail-passive autopilot. There must be only a 10−6probability of landing

outside the prescribed area.

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. This category permits pilots to land with a

decision height less than 50 feet or no decision height and a forward visibility of 250 ft (76 m,

compare this to aircraft size, some of which are now over 70 m long) or 300 ft (91 m) in the

United States. For a landing-without-decision aid, a fail-operational autopilot is needed. For this

category some form of runway guidance system is needed: at least fail-passive but it needs to be

fail-operational for landing without decision height or for RVR below 100 m.

CAT IIIc - As IIIb but without decision height or visibility minimums, also known as "zero-zero".

Fail-passive autopilot: in case of failure, the aircraft stays in a controllable position and the pilot

can take control of it to go around or finish landing. It is usually a dual-channel system.

Fail-operational autopilot: in case of a failure below alert height, the approach, flare and landing

can still be completed automatically. It is usually a triple-channel system or dual-dual system.

A servomechanism, sometimes shortened to servo, is an automatic device that uses error-

sensing negative feedback to correct the performance of a mechanism and is defined by its

function.[1] It usually includes a built-in encoder.[2] A servomechanism is sometimes called

aheterostat since it controls a system's behavior by means of heterostasis.

The term correctly applies only to systems where the feedback or error-correction signals help

control mechanical position, speed or other parameters.[3] For example, an automotive power

window control is not a servomechanism, as there is no automatic feedback that controls position

—the operator does this by observation. By contrast a car's cruise control uses closed loop

feedback, which classifies it as a servomechanism.

Uses[edit]

Position control[edit]

A common type of servo provides position control. Servos are commonly electrical or partially

electronic in nature, using an electric motor as the primary means of creating mechanical force.

Other types of servos use hydraulics, pneumatics, or magnetic principles. Servos operate on the

principle of negative feedback, where the control input is compared to the actual position of the

mechanical system as measured by some sort oftransducer at the output. Any difference

between the actual and wanted values (an "error signal") is amplified (and converted) and used

to drive the system in the direction necessary to reduce or eliminate the error. This procedure is

one widely used application of control theory.

Speed control[edit]

Speed control via a governor is another type of servomechanism. The steam engine uses

mechanical governors; another early application was to govern the speed of water wheels. Prior

to World War II the constant speed propeller was developed to control engine speed for

maneuvering aircraft. Fuel controls for gas turbine engines employ either hydromechanical or

electronic governing.

Other[edit]

Positioning servomechanisms were first used in military fire-control and marine

navigation equipment. Today servomechanisms are used in automatic machine tools, satellite-

tracking antennas, remote control airplanes, automatic navigation systems on boats and planes,

and antiaircraft-gun control systems. Other examples are fly-by-wire systems inaircraft which use

servos to actuate the aircraft's control surfaces, and radio-controlled models which use RC

servos for the same purpose. Many autofocus cameras also use a servomechanism to

accurately move the lens, and thus adjust the focus. A modern hard disk drive has a magnetic

servo system with sub-micrometre positioning accuracy. In industrial machines, servos are used

to perform complex motion, in many applications.

Rotary or linear[edit]

Typical servos give a rotary (angular) output. Linear types are common as well, using

a leadscrew or a linear motor to give linear motion.

Servomotor[edit]

Small R/C servo mechanism1.

1. electric motor

2. position feedback potentiometer

3. reduction gear

4. actuator arm

Main articles: Servomotor and servo (radio control)

A servomotor is a specific type of motor that is combined with a rotary encoder or

a potentiometer to form a servomechanism. This assembly may in turn form part of another

servomechanism. A potentiometer provides a simple analog signal to indicate position, while an

encoder provides position and usually speed feedback, which by the use of a PID controller allow

more precise control of position and thus faster achievement of a stable position (for a given

motor power). Potentiometers are subject to drift when the temperature changes whereas

encoders are more stable and accurate.

Servomotors are used for both high-end and low-end applications. On the high end are precision

industrial components that use a rotary encoder. On the low end are inexpensive radio control

servos (RC servos) used in radio-controlled models which use a free-running motor and a simple

potentiometer position sensor with an embedded controller. The term servomotor generally refers

to a high-end industrial component while the term servo is most often used to describe the

inexpensive devices that employ a potentiometer. Stepper motors are not considered to be

servomotors, although they too are used to construct larger servomechanisms. Stepper motors

have inherent angular positioning, owing to their construction, and this is generally used in an

open-loop manner without feedback. They are generally used for medium-precision applications.

RC servos are used to provide actuation for various mechanical systems such as the steering of

a car, the control surfaces on a plane, or the rudder of a boat. Due to their affordability, reliability,

and simplicity of control by microprocessors, they are often used in small-

scale roboticsapplications. A standard RC receiver (or a microcontroller) sends pulse-width

modulation (PWM) signals to the servo. The electronics inside the servo translate the width of the

pulse into a position. When the servo is commanded to rotate, the motor is powered until the

potentiometer reaches the value corresponding to the commanded position.

A synchro is, in effect, a transformer whose primary-to-secondary coupling may be varied by physically changing the relative orientation of the two windings. Synchros are often used for measuring the angle of a rotating machine such as an antenna platform. In its general physical construction, it is much like an electric motor. The primary winding of the transformer, fixed to the rotor, is excited by analternating current, which by electromagnetic induction, causes currents to flow in three Y-connected secondary windings fixed at 120 degrees to each other on the stator. The relative magnitudes of secondary currents are measured and used to determine the angle of the rotor relative to the stator, or the currents can be used to directly drive a receiver synchro that will rotate in unison with the synchro transmitter. In the latter case, the whole device may be called a selsyn (a portmanteau of self and synchronizing).

Uses[edit]

Synchro systems were first used in the control system of the Panama Canal in the early 1900s to

transmit lock gate and valve stem positions, and water levels, to the control desks.[1]

View onto the connection description of a synchro transmitter

Fire-control system designs developed during World War II used synchros extensively, to

transmit angular information from guns and sights to an analog fire control computer, and to

transmit the desired gun position back to the gun location. Early systems just moved indicator

dials, but with the advent of the amplidyne, as well as motor-driven high-powered hydraulic

servos, the fire control system could directly control the positions of heavy guns.[2]

Smaller synchros are still used to remotely drive indicator gauges and as rotary position sensors

for aircraft control surfaces, where the reliability of these rugged devices is needed. Digital

devices such as the rotary encoder have replaced synchros in most other applications.

Selsyn motors were widely used in motion picture equipment to synchronize movie

cameras and sound recordingequipment, before the advent of crystal

oscillators and microelectronics.

Large synchros were used on naval warships, such as destroyers, to operate the steering gear

from the wheel on the bridge.

Synchro system types[edit]

There are two types of synchro systems: Torque systems and control systems. In a torque

system, a synchro will provide a low-power mechanical output sufficient to position an indicating

device, actuate a sensitive switch or move light loads without power amplification. In simpler

terms, a torque synchro system is a system in which the transmitted signal does the usable work.

In such a system, accuracy on the order of one degree is attainable. In a control system, a

synchro will provide a voltage for conversion to torque through an amplifier and a servomotor.

Control type synchros are used in applications that require large torques or high accuracy such

as follow-up links and error detectors in servo, automatic control systems (such as an autopilot

system). In simpler terms, a control synchro system is a system in which the transmitted signal

controls a source of power which does the usable work. Quite often, one system will perform

both torque and control functions. Individual units are designed for use in either torque or control

systems. Some torque units can be used as control units, but control units cannot replace torque

units.[3]

Synchro functional categories[edit]

A synchro will fall into one of eight functional categories. They are as follows:[4]

1. Torque Transmitter (TX). Input: Rotor positioned mechanically or manually by the information

to be transmitted. Output: Electrical output from stator identifying the rotor position supplied to a

torque receiver, torque differential transmitter or a torque differential receiver.

2. Control Transmitter (CX). Input: Same as TX. Output: Electrical output same as TX but

supplied to a control transformer or control differential transmitter.

3. Torque Differential Transmitter (TDX). Input: TX output applied to stator; rotor positioned

according to amount data from TX that must be modified. Output: Electrical output from rotor

(representing an angle equal to the algebraic sum or difference of rotor position angle and

angular data from TX supplied to torque receivers, another TDX or a torque differential receiver.

4. Control Differential Transmitter (CDX). Input: Same as TDX but data supplied by CX. Output:

Same as TDX but supplied to only a control transformer or another CDX.

5. Torque Receiver (TR). Input: Electrical angle position data from TX or TDX supplied to stator.

Output: Rotor assumes position determined by electrical input supplied.

6. Torque Differential Receiver (TDR). Input: Electrical data supplied from two TX's, two TDX's or

from one TX and one TDX (one connected to the rotor and one connected to the stator). Output:

Rotor assumes position equal to the algebraic sum or difference of two angular inputs.

7. Control Transformer (CT). Input: Electrical data from CX or CDX applied to stator. Rotor

positioned mechanically or manually. Output: Electrical output from rotor (proportional to sine of

the difference between rotor angular position and electrical input angle.

8. Torque Receiver-Transmitter (TRX). This synchro was designed as a torque receiver, but may

be used as either a transmitter or receiver. Input: Depending on the application, same as TX.

Output: Depending on the application, same as TX or TR.

Operation[edit]

On a practical level, synchros resemble motors, in that there is a rotor, stator, and a shaft.

Ordinarily, slip rings and brushes connect the rotor to external power. A synchro transmitter's

shaft is rotated by the mechanism that sends information, while the synchro receiver's shaft

rotates a dial, or operates a light mechanical load. Single and three-phase units are common in

use, and will follow the other's rotation when connected properly. One transmitter can turn

several receivers; if torque is a factor, the transmitter must be physically larger to source the

additional current. In a motion picture interlock system, a large motor-driven distributor can drive

as many as 20 machines, sound dubbers, footage counters, and projectors.

Synchros designed for terrestrial use tend to be driven at 50 or 60 hertz (the mains frequency in

most countries), while those for marine or aeronautical use tend to operate at 400 hertz (the

frequency of the on-board electrical generator driven by the engines).

Single phase units have five wires: two for an exciter winding (typically line voltage) and three for

the output/input. These three are bussed to the other synchros in the system, and provide the

power and information to align the shafts of all the receivers. Synchro transmitters and receivers

must be powered by the same branch circuit, so to speak; the mains excitation voltage sources

must match in voltage and phase. The safest approach is to bus the five or six lines from

transmitters and receivers at a common point. Different makes of selsyns, used in interlock

systems, have different output voltages. In all cases, three-phase systems will handle more

power and operate a bit more smoothly. The excitation is often 208/240 V 3-phase mains power.

Many synchros operate on 30 to 60 V AC also.

Synchro transmitters are as described, but 50 and 60-Hz synchro receivers require rotary

dampers to keep their shafts from oscillating when not loaded (as with dials) or lightly loaded in

high-accuracy applications.

A different type of receiver, called a control transformer (CT), is part of a position servo that

includes a servo amplifier and servo motor. The motor is geared to the CT rotor, and when the

transmitter's rotor moves, the servo motor turns the CT's rotor and the mechanical load to match

the new position. CTs have high-impedance stators and draw much less current than ordinary

synchro receivers when not correctly positioned.

Synchro transmitters can also feed synchro to digital converters, which provide a digital

representation of the shaft angle.

Synchro variants[edit]

So called 'brushless synchros' use rotary transformers (that have no magnetic interaction with

the usual rotor and stator) to feed power to the rotor. These transformers have stationary

primaries, and rotating secondaries. The secondary is somewhat like a spool wound with magnet

wire, the axis of the spool concentric with the rotor's axis. The "spool" is the secondary winding's

core, its flanges are the poles, and its coupling does not vary significantly with rotor position. The

primary winding is similar, surrounded by its magnetic core, and its end pieces are like thick

washers. The holes in those end pieces align with the rotating secondary poles.

For high accuracy in gun fire control and aerospace work, so called multi-speed synchro data

links were used. For instance, a two-speed link had two transmitters, one rotating for one turn

over the full range (such as a gun's bearing), while the other rotated one turn for every 10

degrees of bearing. The latter was called a 36-speed synchro. Of course, the gear trains were

made accordingly. At the receiver, the magnitude of the 1X channel's error determined whether

the "fast" channel was to be used instead. A small 1X error meant that the 36x channel's data

was unambiguous. Once the receiver servo settled, the fine channel normally retained control.

For very critical applications, three-speed synchro systems have been used.

So called multispeed synchros have stators with many poles, so that their output voltages go

through several cycles for one physical revolution. For two-speed systems, these do not require

gearing between the shafts.

Differential synchros are another category. They have three-lead rotors and stators like the stator

described above, and can be transmitters or receivers. A differential transmitter is connected

between a synchro transmitter and a receiver, and its shaft's position adds to (or subtracts from,

depending upon definition) the angle defined by the transmitter. A differential receiver is

connected between two transmitters, and shows the sum (or difference, again as defined)

between the shaft positions of the two transmitters. There are synchro-like devices called

transolvers, somewhat like differential synchros, but with three-lead rotors and four-lead stators.

A resolver is similar to a synchro, but has a stator with four leads, the windings being 90 degrees

apart physically instead of 120 degrees. Its rotor might be synchro-like, or have two sets of

windings 90 degrees apart. Although a pair of resolvers could theoretically operate like a pair of

synchros, resolvers are used for computation.

A special T-connected transformer arrangement invented by Scott ("Scott T") interfaces between

resolver and synchro data formats; it was invented to interconnect two-phase AC power with

three-phase power, but can also be used for precision applications.