Industrial
Automation Glossary
If you want a quick clue to what an industrial
automation term means this is the place for you.
A
Analyser
A special class of instrument that provides component based data rather than basic physical data. Examples are gas
chromatographs and near infra-red analysers.
see also instrument
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B
BRAIN
This is the name of the protocol used by Yokogawa for communications with their smart instrumentation.
It is similar in principle to the HART system but is a slightly different implementation.
Burner Management System
A special application of a control system to the task of safe startup and shutdown of a combustion process.
BMSs are often implemented on special high integrity control systems (such as fail safe controllers) because of the safety requirement in burning
fuels. BMSs should always be designed to a recognised standard such as NFPA.
see also control system
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C
Cascade Control
A control technique that involves one controller manipulating a second controllers setpoint. A common example is a tank level
controller manipulating the setpoint of an inlet flow controller. The main reason for applying this technique is to assign the different process
dynamics to different controllers so that each controller can be optimally tuned for its particular process dynamics rather than have one controller with
a comprimised set of tuning constants. In the tank example the tank level control is typically slow but the flow control is typically
fast. In addition level control is an integrating process but flow control is a regulatory process.
see also integrating process
Combustion Control System
A special application of a control system to the task of the control of a combustion process (compare with BMS). A combustion control
system ensures that fuel rich conditions never occur normally by utilising a cross-limiting control strategy.
see also control system
Controller
A device that minimises the difference between a desired value (often called the setpoint) and a measured value (often called the process
variable) by manipulation of an output.
More sophisticated controllers can use several measured values and/or several manipulated outputs.
Cross-Limiting Strategy
A control system technique where there are two or more control outputs to manipulate but it is required to move one of them first on increasing
demand and the other one first on reducing demand.
The most common applicaition is combustion processes. Here it is required to increase the oxident supply (normally air) before increasing
the fuel when the firing rate is increased and to descrease the fuel supply before descreasing the oxident when the firing rate is decreased.
This ensures oxident rich conditions.
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D
Database
A collection of organised data that can be searched, re-arranged and processed to enhance its value.
Database is a bad description in reality. It should be called an information-base since data is just a collection of numbers, letters, pictures
etc where as information is a usable and valueable collection. For example a list of instrument ranges is just data where as a structured list
of instrument ranges that can be organised by span, bottom range, top range etc is information.
Direct Digital Control [DDC]
The use of a single processing entity to generate outputs for a control system. Often the generation is based on data from many
sources, for example the control system itself, laboratory results, production schedules.
Note that the entity implementing DDC can have a backup in case of failure.
see also supervisory process control
Distributed Control System [DCS]
A control system in which the processing power is distributed amongst several processors. Current systems can have hundreds of
processors.
The original concept of DCS was to ensure that loss of a single processor did not have a major impact on the functionality of the control
system. If you have ten processors gathering data then a failure only results in one tenth of the data being lost. Basically it is the
not putting all the eggs in one basket approach.
Note it is the processing power which is distributed not the physical equipment. However physical distribution of the DCS processors is
often used for both security and wiring cost reduction.
see also Control System
Dynamic Matrix Controller [DMC]
An advanced control technique that uses as a basis a matrix relating input movements to output movements. From this matrix the
controller can determine how to manipulate the outputs to move from one plant state to another. DMC is thus applied to multivariable
control problems.
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E
Expert System
An expert system is a procedure consisting of a set of heuristic type rules instead of process models or other mathematical analysis.
In its simplest implementation an expert system can be thought of as a set of
"if condition X then do Y"
type statements rather than
Y=fn(X)
type statements.
Expert systems get their name from the use of an expert or experts in the formulation of the rules.
Expert systems are often used with fuzzy logic to assist with the interpretation of the status of the process and to generate the control
actions.
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F
Feedback Control
A control strategy where the controller uses a process variable that is affected by the control output.
Feedback control has nothing to do with the physical location of the measurement point and the control point in the process, often the measurement
point is after the control point but not always.
An example of feedback control are manipulation of the inlet flow of a tank to control level. Note that manipulation of the outlet flow would
still be feedback control since the controller is manipulating a device that affects the tank level.
see also Feedforward Control
Feedforward Control
A control strategy where the controller uses a process variable that represents what disturbances will affect the process.
An example of feedforward control is to measure the inlet flow to a tank and manipulate the outlet flow of the tank in order to control the tank
level. The feedforward controller uses a model of the process to achieve this.
Feedforward control is rarely used on its own because of model errors. Normally it is used in conjunction with feedback control.
see also Feedback Control
Fieldbus
Fieldbus is a method of connecting field instrumentation using a communications network linking the field instruments rather than connecting
each field instrument individually to the control system.
Fieldbus uses digital communication protocols which allows information to be communicated between the control system and the field instrument in
addition to the process signal. Instrumentation connected to a fieldbus network is always of the smart instrumenation type.
see also smart instrument
Fuzzy Logic
Fuzzy logic was formalised by Lofty Zadeh to overcome the inadequacies of conventional logic when analysing real world systems.
In its simplest form it can be thought of a method of assinging a degree of membership of an antribute to a particular set instead of the conventional
logic approach which states that an attribute either belongs to a set or does not.
For example consider a reactor with a temperature of 359oC for which we have defined a high reactor temperature as being 360oC. To the
question
is the reactor temperature high ?
conventional logic would provide the answer "no".
Fuzzy logic however is able to inform you that although the reactor temperature is not above the high value it is nearly there. This is
achieved by providing a degree of membership, between 0 and 1, to indicate how close the temperature is to the high value. A degree of
membership of 1 indicates that a value is definately above the high temperature and 0 indicates definately not high.
Thus to the above question fuzzy logic could give the answer 0.96 to indicate that the temperature is close to the high value (the actual
value depends on how "high" is defined).
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H
Human-Computer Interface [HCI]
The parts of a computer system that interacts with a human. The HCI includes all the hardware (VDUs, keyboards, mice, trackerballs,
annunciators etc) and the software with an necessary configuration needed to drive them.
Design of a good HCI is all important to a control system. Unfortunately an HCI is often "designed" to look good rather than be
good.
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I
Instrument
A physical device that either measures a process condition or manipulates a process variable.
Examples are flow transmitters, level gauges, control valves, speed regulators.
Integrating Process
An integrating process is one where an error in the amount of control action will result in a continually rising or falling process
variable.
An example is level control. If the inlet valve to a tank is open more than it should be to maintain the current level the tank will eventually
overflow.
Compare with Regulatory Process. Whether a process is integrating or regulatory is important in control system design and especially the
tuning of controllers.
see also Regulatory Process
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M
Man-Machine Interface
Replaced by the term Human-Computer Interface.
see also Human-Computer Interface
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O
Open Systems Interconnectivity (OSI) Model
A standard for computer network systems generated by the International Standards Office (ISO 7498).
Originally devised in 1978 in order
"to provide a common basis for the coordination of standards development for the purpose of systems interconnection, while allowing existing
standards to be placed into perspective within the overall Reference Model."
The major outcome of this standard is the OSI model framework which divides a communications system into 7 layers.
1: Application Layer (File transfer, message exchange).
2: Presentation Layer (Data format or representation).
3: Session Layer (Organisation and synchronisation of the data exchange).
4: Transport Layer (Channel for transfer of messages of one application process to another).
5: Network Layer (Optimum routing of messages from one network to another).
6: Data Link Layer (Framing and error correction format of the data).
7: Physical Layer (Electrical and mechanical definition of system).
It should be noted that the OSI model does not actually define a communications standard but defines the functions that a standard should
have. Thus having two different computer systems based on the OSI model does not mean that they will be able to communicate with each
other.
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P
Piping and Instrumentation Diagram [P&ID]
A diagram (normally A0 size) showing all the piping and instrumentation on a plant or section of plant.
Cool control engineers warning! Do not confuse PID and P&ID--it is a sure sign that you are not a proper control engineer!
see also Proportional, Integral and Derivative [PID]
Pneumatic Control Valve
A control valve that uses pneumatic forces (normally air under pressure) to manipulate a control valve to the desired position. This is
required because the control systems cannot produce enough force in their control signals to manipulate the control valve directly.
Normally pnuematic control valves use compressed air to drive the valve in one direction and a compressed spring is used to drive the valve in the
other (the spring is compressed by the valve travel). This allows the valve to be fail safe since if the control signal or air supply fails the
valve will either fully open or fully close due to the action of a spring.
Positioner
A field based device that takes a signal from a control system and ensures that the control device is at the setting required by the control
system. It is a very fast acting feedback controller.
Positioners are fitted to handle wear of the control device and to overcome forces exhurted on the control device by the process. For
example in a conventional pneumatic control valve the amount of air required to drive a valve to 50% will decrease with time as the spring ages
(assuming air to open). Also at higher flow rates more effort is needed to close a valve becase the process more force on the valve body
.
see also pneumatic control valve
Predictive Control
A form of control that predicts what the process status will be at some point in the future and, using this information, modifies control outputs to
get the process state to the desired value. Feedforward is a very basic form of predictive control.
see also Feedforward
Proportional, Integral and Derivative [PID]
The three most common tuning constants for a controller.
Also a type of controller using one or more of these constants.
Cool control engineers warning! Do not confuse PID and P&ID--it is a sure sign that you are not a proper control engineer!
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R
Regulatory Process
An regulatory process is one where an error in the amount of control action will result in a fixed offset between the setpoint value and the
process variable.
A typical example is flow control. In a flow control valve is opened more than it needs to be to maintain a flow then the flow will be greater
than required but the error will be constant.
Compare with Integrating Process. Whether a process is integrating or regulatory is important in control system design and especially the
tuning of controllers.
see also Integrating Process
Robust Multivariable Predictive Control Technology [RMPCT]
A Honeywell product that impliments advanced model based control schemes.
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S
Smart Instrument
Smart instrumentation is any field device that, in addition to transmitting the measurement variable or receiving the control variable, also
transmits other process information and/or diagnostic information about the device.
Many smart instruments also allow the control system to re-programme the device (such as re-ranging it) in situ with minimal impact on the
process.
Split Range Control
A control technique where the output of a controller is divided into 2 or more signals that are applied to different control devices.
There are several reasons for spliting the signals:
there may be a fine control action device and a coarse control action device
one control device may be for lower rates and the other for higher rates
there maybe parallel streams
Traditionally the signal splitting has been performed in the field by the setting up of the positioner. Today with cheaper controllers split
range should be performed in the control system because it gives more flexibility and provides the operator with a better view of the plant
status.
Supervisory Process Control [SPC]
The use of a single processing entity to generate setpoints for a control system. Often the generation is based on data from many
sources, for example the control system itself, laboratory results, production schedules.
Note that the entity implementing SPC can have a backup in case of failure.
see also Direct digital control
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