HKIE Professional Assessment - Control, Automation & Instrumentation Discipline key terms

A control arrangement in which the measured output is subtracted from the setpoint to produce an error signal that drives the controller, reducing deviation and making the loop self-correcting.

A control algorithm that combines a term proportional to the current error, a term integrating past error, and a term anticipating error trend, giving fast response with offset removal and damping.

The condition where the integral term accumulates while the final element is saturated, so the controller output overshoots badly when the error finally reverses; prevented by anti-windup limits or back-calculation.

A technique that aligns controller output with the manipulated variable at the moment of switching between manual and automatic, or between cascade modes, so the final element does not jump.

A structure where a primary controller's output becomes the setpoint of a faster secondary controller, isolating the main process from disturbances in an inner loop such as flow or valve position.

Control that acts on a measured disturbance before it affects the controlled variable, using a process model to compute corrective action; it complements rather than replaces feedback correction.

The transport delay between a change at the controller output and the first detectable response in the measurement; large dead time limits achievable loop gain and stability.

The ratio of steady-state change in the measured variable to a unit change in controller output, indicating how strongly the process responds and therefore how much controller gain is tolerable.

Selecting controller gain, integral time and derivative time so the loop meets stability and response requirements; open-loop reaction-curve and closed-loop ultimate-gain methods are common starting points.

The amount by which open-loop phase exceeds minus 180 degrees at the gain crossover frequency; it indicates relative stability and tolerance to delay in a feedback loop.

Coordinating two final elements so one handles the lower half of the controller output and the other the upper half, used for heating/cooling or for sequenced valve capacity.

Agreed conventions, notably the ISA-5.1 instrument identification and loop diagram scheme, that give every tag a function letter code and consistent graphical representation on P&IDs.

The complete set of field instrument, wiring or network, controller function and final element that together regulates one controlled variable; its boundaries define testing and documentation scope.

A schematic showing process equipment, piping, instruments with tag numbers, and control loops; it is the master reference for instrument scope, interlock logic and commissioning checks.

A field device that converts a sensor signal into a standardised output, commonly 4-20 mA or a digital protocol, for transmission to a control system or indicator.

A wiring arrangement in which a single pair carries both the 24 V DC supply and the 4-20 mA signal, with the transmitter regulating loop current to represent the measurement.

The practice of representing zero measurand by a nonzero signal such as 4 mA, so that a broken wire or dead transmitter reads below zero and is distinguishable from a genuine zero value.

A quantified statement of the range within which the true value lies, combining contributions from calibration reference, repeatability, drift, installation effects and environment, usually at a stated confidence level.

Accuracy describes closeness to the true value while precision describes repeatability of successive readings; a device can be precise yet consistently offset and therefore inaccurate.

An unbroken chain of comparisons, each with stated uncertainty, linking the instrument under test through working and reference standards to a national or international metrology standard.

A record stating the instrument identity, test points, as-found and as-left readings, reference standard used, environmental conditions and uncertainty, supporting traceability and audit requirements.

A flow measurement method where a restriction creates a pressure drop related to flow by a square-root law, requiring square-root extraction and attention to straight pipe runs and tap locations.

A meter that vibrates a tube and measures the phase shift caused by mass flow, giving direct mass flow and density; it is largely independent of fluid profile and conductivity.

Non-contact level measurement using time-of-flight of microwave reflections from the product surface, affected by dielectric constant, foam, and false echoes requiring suppression mapping.

Two common temperature sensors: a thermocouple generates a small voltage at a junction of dissimilar metals, while an RTD changes resistance with temperature and is generally more stable and linear.

The repeating sequence of reading inputs, executing the user program, and updating outputs; scan time affects response to fast events and must suit interlock timing requirements.

A graphical programming language representing relay contacts and coils, standardised in IEC 61131-3, widely used for discrete interlocks and motor control because electricians read it easily.

An integrated architecture of controllers, operator stations and I/O distributed around a plant, optimised for continuous modulating control, history and alarm management rather than fast discrete interlocking.

A system that gathers telemetry from remote sites and issues commands from a central master station, typically over wide-area communications, with emphasis on visibility rather than local loop control.

Rugged field device at a remote site that acquires inputs, stores data, executes local logic and communicates with a SCADA master, often using low-bandwidth or intermittent links.

A table cross-referencing each initiating cause with the resulting outputs and actions, used to specify, test and document interlock and shutdown behaviour unambiguously.

A defined protective action, with a specified integrity level, that brings the process to a safe state when a detected condition exceeds limits; each SIF is implemented by a safety instrumented system.

The collection of sensors, logic solver and final elements that executes safety instrumented functions independently of the basic process control system, designed and maintained to a target integrity.

A discrete target measure of risk reduction assigned to a safety function, expressed as a range of probability of failure on demand or failure rate, with requirements on hardware, diagnostics and process safety management.

A periodic test that reveals undetected dangerous failures in a safety function and restores its integrity, with test interval and coverage directly affecting achieved probability of failure on demand.

The staged process from hazard and risk assessment through safety requirements specification, design, installation, validation, operation and decommissioning, with verification and documentation at each stage.

A protection concept limiting energy in hazardous-area circuits so that no spark or hot surface can ignite the atmosphere, using barriers or galvanic isolators and entity or system parameter checks.

The zoning of areas by likelihood and duration of flammable atmosphere, together with gas or dust group and temperature class, which determines permitted apparatus marking and installation methods.

A protection method in which an enclosure contains any internal ignition without transmitting flame to the surrounding atmosphere and keeps surface temperatures below the class limit.

Digital fieldbus standards replacing point-to-point wiring: Foundation Fieldbus supports distributed function blocks in devices, while PROFIBUS DP is widely used for discrete and drive communication.

Ethernet adapted for plant use with rugged cabling, deterministic protocols and redundancy, carrying controller, HMI and drive traffic; network topology and segmentation strongly affect availability.

Layering technical, procedural and physical controls across zones and conduits so that compromise of one layer does not expose the whole operational technology estate, following IEC 62443 principles.

A buffer network between plant control networks and enterprise IT that terminates and inspects traffic, hosts replicated data, and prevents direct connections across the boundary.

Protecting control systems from unauthorised access and disruption while preserving availability of physical processes, requiring asset inventory, patching strategy, access control and incident response.

The relationship between valve opening and flow capacity, commonly equal-percentage or linear, chosen so installed characteristic combined with process and piping effects gives acceptable loop gain.

Selecting valve capacity so it passes design flow at the available pressure drop with adequate margin, avoiding excessive oversizing that causes poor low-lift control and instability.

The spring-driven position a valve takes on loss of air or signal, air-to-open or air-to-close, chosen so the process fails safe while respecting the safety function's required direction.

Liquid flow conditions where pressure falls below vapour pressure: flashing stays vapour downstream, while cavitation collapses bubbles and causes noise, vibration and erosion, requiring different valve selection.

A device that compares actuator travel with the control signal and adjusts air supply to reduce positioning error, improving response to friction and packing load.

A supervisory system monitoring and controlling building services such as air handling, chillers, lighting and energy metering, usually integrating multiple proprietary protocols through gateways.

An open building automation protocol standardised internationally, defining objects, services and network layers so controllers and workstations from different vendors can interoperate.

Ventilation control that adjusts fresh-air quantity to measured occupancy indicators such as carbon dioxide, reducing energy while maintaining indoor air quality.

A project document defining responsibilities, inspection and test activities, hold points, acceptance criteria and records needed to demonstrate that delivered work meets specified requirements.

Staged verification of a control system: FAT proves functionality at the vendor's works against an agreed procedure, and SAT confirms correct operation after installation and integration on site.

The structured process of checking, energising, testing and proving installed systems, including loop checks and interlock tests, before handover and operation.

A structured study identifying hazards, estimating consequence and likelihood, and defining risk reduction measures, providing the basis for safety requirements and design decisions.

The disciplined design and review of alarms against rationalisation criteria so that operators receive timely, meaningful and prioritised alerts rather than an unmanageable flood.

A controlled process for reviewing, approving, implementing and recording modifications to plant, control systems or procedures so that hazards and documentation stay aligned with the as-built state.

The ethical obligations of an engineering institution member, including honesty, competence, public safety, avoiding conflicts of interest and reporting matters that endanger people or the environment.

The professional and legal obligation to exercise reasonable skill so that designed systems are safe to construct, operate and maintain, including foreseeable misuse and lifecycle considerations.