HKIE Professional Assessment - Control, Automation & Instrumentation Discipline flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

PID parallel form

The ideal PID controller in parallel form is u(t) = Kp e(t) + Ki ∫e dt + Kd de/dt, where Ki and Kd are independent gains and the Laplace form is C(s) = Kp + Ki/s + Kd s.

Routh-Hurwitz stability criterion

A necessary condition for stability is that all coefficients of the characteristic polynomial are positive. A sufficient condition is that the first column of the Routh array has no sign changes.

Gain margin and phase margin

Gain margin is the negative of the gain in dB at the phase crossover where phase equals −180°. Phase margin is 180° plus the phase at the gain crossover where gain is 0 dB.

Ziegler-Nichols ultimate gain tuning

With proportional-only control at the ultimate gain Ku and ultimate period Pu, PID settings are Kp = 0.6Ku, Ti = 0.5Pu, and Td = 0.125Pu. Use with caution on dead-time-dominant processes.

4–20 mA live zero

In a 4–20 mA loop, 4 mA represents 0% and 20 mA represents 100%. The live zero allows detection of a broken wire, which reads 0 mA and is distinguishable from a valid zero signal.

Maximum loop resistance

For a 4–20 mA loop, R_max = (Vsupply − Vmin_transmitter)/0.020 A. With a 24 V supply and 12 V minimum transmitter voltage, the maximum loop resistance is 600 Ω.

Orifice plate square-root relationship

Differential pressure across an orifice is proportional to the square of flow: Q ∝ √ΔP. The transmitter signal must be square-root extracted to obtain a linear flow indication.

Intrinsic safety entity parameters

Intrinsically safe circuits are verified by matching entity parameters: the barrier output voltage Uo, current Io, capacitance Co, and inductance Lo must not exceed the hazardous-area device ratings Ui, Ii, Ci, and Li.

PLC scan cycle

A PLC repeats: read inputs, execute the program, update outputs, then perform housekeeping and communications. Worst-case response time includes at least one full scan plus input filter delay.

Seal-in circuit

A seal-in or latching circuit places a normally-open contact in parallel with the start pushbutton so that the output remains energised after the button is released, until a stop or fault contact opens.

Bumpless transfer

Bumpless transfer ensures that when a controller switches between manual and automatic, the output does not jump. The integral term is initialised to the current output to prevent a step change.

ISA-18.2 alarm rationalisation

ISA-18.2 requires each alarm to be actionable, prioritised, and rationalised. A common target is fewer than six alarms per operator per hour and fewer than 150 alarms per day in steady state.

OPC UA security

OPC UA provides platform-independent data exchange with built-in authentication, signing, and encryption. Unlike classic OPC DA, it does not rely on DCOM, which is difficult to secure across networks.

Time synchronisation for sequence of events

Accurate time synchronisation using NTP or IEEE 1588 PTP is required so that alarms and trips recorded on different devices can be ordered correctly in a sequence-of-events analysis.

SIL PFDavg ranges

In low-demand mode, SIL 1 corresponds to PFDavg between 10⁻¹ and 10⁻², SIL 2 between 10⁻² and 10⁻³, SIL 3 between 10⁻³ and 10⁻⁴, and SIL 4 between 10⁻⁴ and 10⁻⁵.

Proof test interval effect

For a simplex element, PFDavg ≈ λDU·T/2, where T is the proof-test interval. Doubling the proof-test interval approximately doubles the average probability of failure on demand.

Common-cause failure beta factor

Redundant identical devices are not fully independent. The beta factor models the fraction of failures that are common cause; diversity, physical separation, and different technologies reduce beta.

SIS independence from BPCS

A safety instrumented function must be independent of the basic process control system. Sharing sensors or final elements between control and safety requires justification through risk assessment.

PROFIBUS DP segment rules

A PROFIBUS DP segment supports up to 32 devices without a repeater, requires 120 Ω termination at both physical ends, and reaches 1200 m at 93.75 kbit/s, with shorter distances at higher speeds.

HART digital signal

HART superimposes a frequency-shift-keyed digital signal of about ±0.5 mA at 1200 baud on the 4–20 mA analogue loop, allowing configuration and diagnostics without interrupting the process signal.

IEC 62443 zones and conduits

IEC 62443 segments an industrial control system into zones of similar security requirements, connected by conduits that enforce and monitor traffic. This limits lateral movement of an attacker.

Valve flow coefficient Cv

Cv is the number of US gallons per minute of water at 60 °F that flows through a valve with a pressure drop of 1 psi. The metric equivalent is Kv, where Kv = 0.865 Cv.

Equal percentage valve characteristic

In an equal percentage valve, equal increments of stem travel produce equal percentage changes in flow. It is often chosen because the installed characteristic becomes approximately linear.

Cavitation versus flashing

Cavitation occurs when the vena contracta pressure falls below vapour pressure and bubbles collapse downstream. Flashing occurs when downstream pressure remains below vapour pressure and bubbles persist.

Fail-safe valve action

Spring-return actuators provide fail-safe action: air-to-open valves fail closed on loss of air, and air-to-close valves fail open. The choice is a safety decision based on the process hazard.

BACnet MS/TP

BACnet MS/TP is a master-slave/token-passing protocol over RS-485 at speeds from 9.6 to 76.8 kbit/s. It is widely used for field devices in building automation systems.

Economiser free cooling

An air-side economiser uses outdoor air for cooling when its enthalpy or temperature is below the return air condition, reducing mechanical cooling energy while maintaining supply air setpoint.

CO₂ demand-controlled ventilation

Demand-controlled ventilation modulates outdoor air based on measured CO₂ concentration, often targeting 800–1000 ppm, to maintain indoor air quality while reducing fan and conditioning energy.

Cause-and-effect matrix

A cause-and-effect matrix cross-references trips, alarms, and outputs to define the required response of the safety and control systems. It is the basis for SIS testing and commissioning.

Factory acceptance test

A factory acceptance test verifies the control system against its specification before shipment, including I/O checks, graphics, logic, and interfaces. It reduces site rework and schedule risk.

Earned value indices

Cost performance index CPI = EV/AC and schedule performance index SPI = EV/PV. Values below 1.0 indicate cost overrun or schedule slippage relative to the plan.

IEC 61511 scope

IEC 61511 is the functional safety standard for safety instrumented systems in the process industry. It covers the safety lifecycle from hazard assessment through design, operation, and decommissioning.