ISO 8625-4:2011

INTERNATIONAL ISO
STANDARD 8625-4
First edition
2011-10-15
Aerospace — Fluid systems —
Vocabulary —
Part 4:
General terms and definitions relating to
control/actuation systems
Aéronautique et espace — Systèmes de fluides — Vocabulaire —
Partie 4: Termes et définitions généraux relatifs aux systèmes de
commande/d'actionnement
Reference number
©
ISO 2011
©  ISO 2011
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ii © ISO 2011 – All rights reserved

Contents Page
Foreword . iv
Introduction . v
Scope . 1
Terms and definitions . 1
4.1  Control system classification . 1
4.2  Control system technology (control engineering) . 3
4.3  Control system performance (servomechanism) . 9

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
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International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 8625-4 was prepared by Technical Committee ISO/TC 20, Aircraft and space vehicles, Subcommittee
SC 10, Aerospace fluid systems and components.
ISO 8625 consists of the following parts, under the general title Aerospace — Fluid systems — Vocabulary:
 Part 1: General terms and definitions relating to pressure
 Part 2: General terms and definitions relating to flow
 Part 3: General terms and definitions relating to temperature
 Part 4: General terms and definitions relating to control/actuation systems
iv © ISO 2011 – All rights reserved

Introduction
ISO 8625 contains only those terms which can be applied to general equipment and systems. Terms which
are only used for specific applications and specific components are to be incorporated into the relevant
product specifications and product standards.
Terms and definitions for components and systems which are associated with other systems (such as
electromechanical actuation systems or electronic control units) are incorporated only on a very general basis,
provided they have direct interfaces with fluid systems.

INTERNATIONAL STANDARD ISO 8625-4:2011(E)

Aerospace — Fluid systems — Vocabulary —
Part 4:
General terms and definitions relating to control/actuation
systems
Scope
This part of ISO 8625 defines general terms relating to control/actuation systems in the field of aerospace fluid
systems and components.
Terms and definitions
For the purposes of ISO 8625, terms have been given a two-element number: the first element refers to the
number of the part of ISO 8625 in which the term is defined and the second element refers to the reference
number of the term within that part.
EXAMPLE
4.3.38
velocity vs. force/torque characteristics
the term “velocity vs. force/torque characteristics” is defined in ISO 8625-4
Terms are presented according to the alphabetical order of terms in English.
4.1 Control system classification
4.1.1
adaptive control system
control system which improves system performance by changing system parameters in response to varying
operational conditions
4.1.2
AFCS
automatic flight control systems
systems consisting of electrical, mechanical and hydraulic components that generate and transmit automatic
control commands, which provide pilot assistance through automatic or semiautomatic flight path control or
which automatically control airframe response to disturbances
NOTE 1 This classification includes automatic pilots, stick or wheel steering, autothrottles, structural mode control and
similar mechanizations.
NOTE 2 AFCS functions include, but are not limited to, airspeed hold, automatic navigation, all weather landing,
automatic terrain following, altitude hold, heading hold, altitude select, heading select, attitude hold (pitch and roll), lateral
acceleration and sideslip limiting, automatic instrument, low approach mach hold, automatic carrier landing, automatic
vectoring modes.
4.1.3
autobrake
automatic/electronic control of braking of a specific energy level
4.1.4
autoland
automatic/electronic control that takes the aircraft all the way to a full land
4.1.5
autothrottle
control means which sets a given position to maintain thrust for a given attitude/speed combination
4.1.6
bistable control system
control system in which the control output is fully on in either polarity
NOTE When the time is modulated by the input, the system is called pulse width modulated (PWM). The terms
“ON-OFF control” and “bang-bang control” are sometimes used. These types of controls pertain to digital controls.
4.1.7
closed-loop control system
control system in which measurement of the output parameter is used to make system corrections so as to
maintain a desired output based on input commands
4.1.8
CAS
control augmentation system
vehicle flight control system wherein the control system responds to the error between commanded vehicle
motion and the actual vehicle motion as well as to surface position command inputs
4.1.9
control and stability augmentation system
combination of CAS (4.1.8) and SAS (4.1.20)
4.1.10
control authority
total amount of control surface or force effector deflection available to a flight control system
NOTE The prefixes “pilot”, “CAS” or “SAS” are often used to define that part of the total available to the pilot, the CAS
or the SAS, respectively.
4.1.11
control system
system in which deliberate guidance or manipulation is used to achieve a prescribed value of a variable
NOTE A control system has at least one input and one output.
4.1.12
digital control system
control system which uses digital signals and wherein the control information is digital
4.1.13
flight control systems
systems that enable the controlled flight of aircraft, helicopters and missiles
NOTE “Manual flight control systems” transmit pilot commands through mechanical components, though they often
incorporate electrical components to augment the pilot commands. The term is also sometimes used for those systems
which transmit pilot commands to the surfaces mechanically, without power or force assistance.
2 © ISO 2011 – All rights reserved

4.1.14
fly by wire (FBW) system
control by wire (CBW) system
control system wherein control information and signals are transmitted completely by electrical means
4.1.15
fly by light (FBL) system
control by light (CBL) system
control system wherein control information is transmitted by light through a fibre optic cable
NOTE A true FBL system does not have FBW or mechanical backup, nor FBW or mechanical override.
4.1.16
hydraulic boost
use of hydraulic power actuation to reduce the pilot effort needed for control of a vehicle wherein the actuator
output force or torque is in direct proportion to the manual, mechanically applied, input force or torque
4.1.17
integral control system
control system, which uses an integrator in the control loop elements to provide an output response to the
error signal, and for which the control effort is proportional to the integral of the error
4.1.18
open-loop control
control system in which an output is produced in direct response to a command, without feedback from the
output being used to affect the system response
4.1.19
proportional control system
control system which uses proportional control elements in its forward or feedback control paths, or both, to
provide an output in response to the error signal
4.1.20
SAS
stability augmentation system
portion of a flight control system that improves the handling characteristics by modifying the aerodynamic
response of the vehicle
NOTE The SAS generally has limited authority. SAS signals are normally introduced by a series servo, the operation
of which does not have an impact on the pilot's command signal.
4.1.21
tristable control system
control system in which the power to control the load is fully on in one polarity, off, or fully on in the other
polarity (tristable)
NOTE When the time is modulated by the input, the system is called pulse width modulated (PWM). The terms
“ON-OFF control” and “bang-bang control” are sometimes used. These types of controls pertain to digital controls.
4.2 Control system technology (control engineering)
4.2.1
backlash
uncontrolled load motion due to clearance in actuation elements, including the load attach point, usually
expressed in terms of absolute load motion
4.2.2
bandwidth
frequency range over which the actuation system has acceptable dynamic response
NOTE This spectrum extends from a base frequency up to a specified frequency, which is usually the frequency
where the open-loop amplitude ratio has unity gain (0 dB) in other than single order systems. For a first order system, this
is the frequency where the closed-loop response is down 3 dB and the phase lag is 45° [see also decibel (4.2.6)].
4.2.3
closed-loop frequency response
frequency response between command input and control system output with the feedback signal summed
algebraically with command
NOTE Actuation system response for a closed-loop system is usually specified as closed-loop frequency response.
4.2.4
command
input which represents the desired output of the control system
4.2.5
control passband
frequency range over which the control responds without attenuation
4.2.6
decibel
dB
unit of measure used to express amplitude ratio of output to control input
NOTE Decibels = 20 log (amplitude out/amplitude in).
4.2.7
dynamic impedance
impedance, a complex quantity, associated with the output deflections of an active, closed-loop actuation
system caused by externally applied dynamic forces, usually sinusoidal, over a specific frequency range
NOTE Dynamic impedance at the surface includes the effects of the surface attachment spring, its load mass and its
viscous friction. The impedance at the actuator will not include these factors.
4.2.8
error signal
algebraic difference between the command input and the output feedback
4.2.9
feedback element
component in a closed-loop system that provides the feedback signal of the output quantity, or a function of
the output that can be compared with the reference input
4.2.10
forward loop control elements
elements situated between the error signal and the controlled variable
4.2.11
frequency response
complex ratio of the actuation system output to the command input while the input is cycled sinusoidal at a
constant amplitude and the frequency is varied
NOTE Frequency response is usually presented as a log frequency plot of normalized amplitude ratio, expressed in
dB, and input to output phase angle degrees versus frequency.
4 © ISO 2011 – All rights reserved

4.2.12
gain crossover
point of the plot of the open-loop transfer function at which the magnitude is unity (LmG (j) = 0 dB)
NOTE The frequency at gain crossover is called the phase margin frequency,  .

4.2.13
gain margin
measure of system stability defined as the gain required to raise the open-loop amplitude ratio to 0 dB (unit
gain) at the frequency corresponding to 180° of phase lag
4.2.14
hysteresis
difference in actuation system output for the same input command level during a complete cycle of input
command when cycled throughout the full range of travel
NOTE It is necessary that the cycling rate be significantly below the control bandpass so that velocity error signals
are not included in this parameter.
4.2.15
input
independent variable supplied to the control system
4.2.16
linearity
degree to which the normal output curve conforms to a straight line under specified load conditions, usually
expressed as a percentage of full range, or sometimes of rated output, which is typically half full range
4.2.17
load natural frequency
undamped resonant frequency of the load
...