HKIE Professional Assessment - Control, Automation & Instrumentation Discipline cheat sheet

PID equation (ideal/parallel): u(t) = Kp e(t) + Ki ∫e dt + Kd de/dt, with Ki = Kp/Ti and Kd = Kp·Td; Laplace form C(s) = Kp(1 + 1/(Ti s) + Td s).

Closed-loop transfer function for negative feedback: T(s) = G(s)H(s)/(1 + G(s)H(s)); characteristic equation 1 + G(s)H(s) = 0 determines stability.

Routh-Hurwitz criterion: necessary condition all coefficients of characteristic polynomial positive; sufficient condition first column of Routh array has no sign changes.

Bode stability margins: gain margin = −20 log|G(jωpc)| where phase = −180°; phase margin = 180° + ∠G(jωgc) where |G(jωgc)| = 1 (0 dB).

Ziegler-Nichols ultimate gain method: with P-only control at ultimate gain Ku and period Pu, PID settings Kp = 0.6Ku, Ti = 0.5Pu, Td = 0.125Pu.

Second-order step response: percent overshoot ≈ 100·exp(−ζπ/√(1−ζ²)); settling time (2% band) ≈ 4/(ζωn).

Final value theorem: steady-state error e_ss = lim_{s→0} s·E(s) for a stable closed loop; type 0 gives finite error to step, type 1 to ramp, type 2 to parabola.

For a first-order plus dead time process G(s) = K e^{−θs}/(τs+1), the θ/τ ratio > 1 indicates dead-time-dominant dynamics where PID tuning is difficult and model-based control (Smith predictor) may be needed.

Anti-windup is required whenever integral action is present and the actuator can saturate; common methods are back-calculation and conditional integration (integrator clamping).

4–20 mA current loop: 4 mA represents live zero (0%), 20 mA represents 100%; current signalling is preferred over voltage because loop current is independent of cable resistance within compliance limits.

Maximum loop resistance R_max = (Vsupply − Vmin_transmitter)/0.020 A; for a 24 V supply and 12 V minimum transmitter voltage, R_max = 600 Ω.

RTD (Pt100) resistance: R(T) ≈ R0(1 + αT) with α ≈ 0.00385 /°C; at 100 °C R ≈ 138.5 Ω. Three-wire or four-wire connections compensate lead resistance.

Thermocouple Seebeck effect: voltage depends on the temperature difference between measuring and reference junctions; cold-junction compensation is mandatory.

Orifice plate flow: Q ∝ √(ΔP); square-root extraction is needed to linearise the signal before control or totalisation.

Differential pressure level measurement: ΔP = ρgh; for closed tanks, the low-pressure leg must be filled and the transmitter zero elevated or suppressed to compensate the wet leg.

Accuracy, repeatability, hysteresis, and drift are distinct specifications; total probable error is often taken as the root-sum-square of independent contributions.

Intrinsic safety: energy-limiting barriers or galvanic isolators limit voltage and current so that no spark or thermal effect can ignite the hazardous atmosphere; entity parameters (Uo, Io, Co, Lo) must match the hazardous-area device.

PLC scan cycle: read inputs → execute program → update outputs → housekeeping/communications; worst-case response time is at least one full scan plus input filter delay.

Ladder logic contacts in series implement AND, contacts in parallel implement OR, and normally-closed contacts implement NOT; seal-in (latching) circuits use a parallel contact around a start pushbutton.

Safety-related outputs must not rely on standard PLC logic alone; safety functions belong in a safety PLC or hardwired safety relay with diagnostics.

PID function blocks in PLCs require bumpless transfer between manual and automatic, output limiting, and anti-windup; cycle time must be shorter than the process dominant time constant.

Sequential control is best expressed as a state machine (SFC/grafcet); each step has actions and transition conditions, and the sequence must define safe states on fault.

Industrial Ethernet (PROFINET, EtherNet/IP) is non-deterministic by nature; use managed switches, VLANs, and prioritisation (QoS) for control traffic.

Redundancy: hot standby requires bumpless switchover; redundant controllers, power supplies, and networks must be tested for switchover time against the process tolerance.

SCADA architecture: field devices → RTUs/PLCs → communication network → SCADA server (historian, alarms) → HMI clients; DCS integrates control and HMI in a proprietary, engineered system.

Alarm management per ISA-18.2: alarms must be actionable, prioritised, and rationalised; target fewer than ~6 alarms per operator per hour in steady state and less than 150 alarms per day.

OPC UA provides secure, platform-independent data exchange with built-in authentication and encryption; classic OPC DA relies on DCOM and is a security risk.

Historian data compression (swinging-door or exception reporting) reduces storage; verify that the deadband does not hide important process excursions.

DCS redundancy typically includes redundant controllers, I/O buses, and power; SCADA redundancy is often server-level with hot standby and mirrored historians.

Time synchronisation (NTP/IEEE 1588 PTP) is essential for sequence-of-events recording; without it, alarm and trip timestamps across devices cannot be correlated.

IEC 61508 defines functional safety lifecycle and SIL 1–4; IEC 61511 applies it to the process industry. SIL is a measure of risk reduction, not of equipment quality.

PFDavg targets: SIL 1 = 10⁻¹ to 10⁻², SIL 2 = 10⁻² to 10⁻³, SIL 3 = 10⁻³ to 10⁻⁴, SIL 4 = 10⁻⁴ to 10⁻⁵ (low-demand mode).

Architectural constraints: HFT and SFF determine the maximum SIL for a subsystem; a simplex (1oo1) element with low diagnostic coverage is generally limited to SIL 2.

Proof test interval T: PFDavg ≈ λDU·T/2 for a 1oo1 element; doubling the proof-test interval roughly doubles the average probability of failure on demand.

Safety function components: sensor, logic solver, and final element; the final element often dominates the unreliability, so valve partial-stroke testing is valuable.

Common-cause failure must be addressed with diversity, physical separation, and a beta factor in reliability modelling; redundant identical devices are not fully independent.

SIS independence: the safety function must be independent of the basic process control system; shared sensors or shared final elements require justification and risk assessment.

Fieldbus comparison: PROFIBUS DP (RS-485, up to 12 Mbit/s, 32 nodes per segment without repeater), Foundation Fieldbus H1 (31.25 kbit/s, power and data on the same pair), HART (4–20 mA with FSK digital signal superimposed).

PROFIBUS DP segment rules: max 32 devices per segment, 120 Ω termination at both physical ends, repeaters for longer distance; max 1200 m at 93.75 kbit/s.

HART: digital signal ±0.5 mA FSK at 1200 baud; allows multidrop (current fixed at 4 mA) but then no analogue control signal.

Cybersecurity defence in depth: network segmentation (zones and conduits per IEC 62443), firewalls between IT and OT, DMZ for historians and remote access, no direct internet exposure of PLCs.

IEC 62443 roles: asset owner, system integrator, product supplier; security levels SL 1–4 describe protection against increasingly capable attackers.

Patch management in OT is constrained by availability; compensating controls include virtual patching, application allow-listing, and strict change control.

WirelessHART and ISA100.11a use mesh topologies with time-synchronised channel hopping; battery life and update rate must be engineered.

Valve flow coefficient: Cv = Q √(SG/ΔP) with Q in US gpm and ΔP in psi; Kv (metric) = 0.865 Cv.

Inherent characteristic: linear, equal percentage, and quick opening; equal percentage is preferred for most process control because it gives approximately linear installed characteristic.

Rangeability is the ratio of maximum to minimum controllable flow; typical globe valve rangeability is 30:1 to 50:1.

Fail-safe action: spring-return actuators fail closed (air-to-open) or fail open (air-to-close); the choice is a safety decision, not a control preference.

Cavitation occurs when vena contracta pressure falls below vapour pressure; flashing is when downstream pressure remains below vapour pressure. Both cause damage and noise.

Valve sizing must consider choked flow: for gas, check that pressure drop does not exceed the choked limit (ΔP > 0.5·P1 for air/gas); use the appropriate ISA/IEC sizing equations.

Positioner improves positioning accuracy and speed of response; a smart positioner provides diagnostics such as friction, deadband, and travel deviation.

BACnet is the dominant building automation protocol; objects and services model devices; MS/TP is RS-485 at 9.6–76.8 kbit/s, BACnet/IP runs over Ethernet.

AHU control: supply air temperature control via cooling/heating coil valves, static pressure control via supply fan VFD, and economiser free cooling when outdoor air enthalpy is suitable.

Chiller sequencing and staging: lead/lag rotation, minimum run times, and demand limiting; condenser water temperature reset improves chiller efficiency.

VAV terminal control: airflow measured by differential pressure across an inlet probe; minimum and maximum airflow setpoints must respect ventilation requirements.

Indoor air quality: CO₂ demand-controlled ventilation typically targets 800–1000 ppm; ASHRAE Standard 62.1 gives ventilation rates per person and per floor area.

Fire and smoke control: stairwell pressurisation, smoke exhaust, and damper control must interface with the fire alarm system and be tested under a documented cause-and-effect matrix.

Energy metering and sub-metering: tenant billing, M&V per IPMVP, and trend logging of kWh, kW, and run hours support energy performance contracts.

Project phases: feasibility → basic engineering (FEED) → detailed engineering → procurement → construction → commissioning → handover; each phase has defined deliverables and gate reviews.

Instrument index, I/O list, loop diagrams, cable schedules, termination diagrams, and cause-and-effect matrices are core deliverables; the C&E matrix is the basis for SIS testing.

Factory acceptance test (FAT) verifies the system against specification before shipment; site acceptance test (SAT) verifies installed performance and interfaces.

Loop checking confirms continuity, polarity, calibration, and correct signal scaling from field device to control system; it is documented per loop.

Commissioning sequence: mechanical completion → punch list → cold commissioning (water/air) → hot commissioning (process fluids) → performance test.

Change control: any change after design freeze must be assessed for cost, schedule, safety, and cybersecurity impact; undocumented changes are a major source of project failure.

Earned value management: CPI = EV/AC and SPI = EV/PV; values below 1.0 indicate cost or schedule overrun.

IEC 61511 (process industry SIS), IEC 61508 (generic functional safety), IEC 62443 (industrial cybersecurity), IEC 61131-3 (PLC programming languages), IEC 61850 (substation automation).

ISA-18.2 (alarm management), ISA-88 (batch control), ISA-95 (enterprise-control integration), ISA-101 (HMI design), ISA-84 (SIS, aligned with IEC 61511).

Hazardous area classification: IEC 60079 series; Zone 0/1/2 for gas, Zone 20/21/22 for dust; equipment marking includes Ex ia/ib/d/e and temperature class T1–T6.

ATEX Directive 2014/34/EU for equipment and 1999/92/EC for workplace; in Hong Kong, the Factories and Industrial Undertakings (Dangerous Substances) Regulations and local fire services requirements apply.

Electrical installations: IEC 60364 and local wiring regulations (e.g., HKIE/EMSD guidance and the Electricity Ordinance in Hong Kong).

Pressure equipment: ASME VIII or PED 2014/68/EU depending on jurisdiction; instrument air quality per ISO 8573-1.

Documentation standards: P&ID symbols per ISA-5.1; instrument identification letters per ISA-5.1 (e.g., FT = flow transmitter, FIC = flow indicating controller).

HKIE Code of Conduct: members must act with integrity, competence, and objectivity, and must not misrepresent their qualifications or experience.

Duty to public safety and health overrides commercial pressure; if a design is unsafe, the engineer must report it through the appropriate channel, including to the client and, if necessary, the authorities.

Conflict of interest must be declared in writing and managed; accepting undisclosed benefits from suppliers is a breach of professional conduct.

Confidentiality: client information must not be disclosed without consent except where required by law or where public safety is at risk.

Legal liability: designers owe a duty of care; negligence requires a duty, a breach, causation, and damage. Contracts may limit liability but cannot exclude liability for death or personal injury.

Statutory duties in Hong Kong: Occupational Safety and Health Ordinance, Factories and Industrial Undertakings Ordinance, and the Electricity Ordinance impose duties on employers and designers.

Continuing professional development (CPD) is a requirement of HKIE membership; records should be kept and declared honestly.