Question 1 of 4: Displays and Controls — Yogurt Mixing Panel

Question 1: Displays and Controls — Yogurt Mixing Panel (40 marks: a–20, b–10, c–10)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Part (a) — Digital Control System Design

The system must let one trained operator monitor eight variables (temperature, pressure, volume, inflow/outflow, mixture colour, density, plus mixing-arm temperature and speed) and intervene when colour or density drifts – a supervisory-control task, not a fully automated one, so the panel design goal is fast anomaly detection and low-error manual correction rather than continuous manual operation. Four design principles drive the layout below (Sanders & McCormick, displays/controls chapters): (1) functional grouping – vat-thermal variables (temperature, mixing-arm temp/speed) grouped left, flow/packaging variables (pressure, volume, inflow/outflow) grouped centre, and quality-judgment variables (colour, density) grouped right, matching the operator's own three-stage mental model of the process; (2) population stereotypes and compatible mapping – every increase/decrease control turns clockwise-to-increase and every numeric display trends left-to-right in time, so a trained operator on any station reads the panel the same way; (3) redundant coding for the two safety/quality-critical variables (temperature and colour/density) – a digital numeric readout PLUS an analogue-style bar-graph trend and a colour-coded status light (green/amber/red against the 2°C target band), because a single-format digital number is slow to read as a trend and easy to misread under time pressure; (4) large, glove-compatible physical controls for the highest-consequence adjustments (temperature setpoint, mixer speed, emergency stop) rather than small touchscreen icons, because Part (c) establishes the operator works in a cold, PPE-heavy environment where fine touchscreen gestures are unreliable – the touchscreen is used for monitoring/trend review and low-consequence setpoint entry (volume, inflow/outflow), while temperature, speed and stop remain dedicated hard controls with tactile detents.

Part (b) — Human Perceptual and Cognitive Components

Monitoring and manipulating this panel is a closed-loop human-information-processing task, and every stage of that loop is exercised: sensation and perception – visual perception of the numeric/graphical displays and the colour-coded status lights, and colour perception specifically for the mixture-colour comparison in Part (a)'s quality group (a task made harder under the panel's own ambient lighting, which must render true colour, not a tinted work light); auditory perception for any alarm tone signalling an out-of-band temperature or pressure excursion. Attention – selective attention to scan across the three functional zones on a duty cycle appropriate to how fast each variable can drift (temperature slowly, pressure/flow quickly), and divided attention when a quality deviation (colour/density) demands a correction while thermal and flow variables must still be tracked. Working memory – holding the current reading, the target band, and the direction/magnitude of any needed correction simultaneously while reaching for the right control, and comparing the just-adjusted value against its target after the change (a classic working-memory-limited comparison task, which is why the redundant trend graph in Part (a) is provided rather than relying on the operator to remember a prior numeric reading). Long-term memory and mental models – the trained operator's schema of how the process normally behaves (e.g. "temperature drifts up slowly if the vat lid is opened; density lags a thickener adjustment by roughly a minute"), which lets small deviations be correctly attributed to a cause instead of triggering an unnecessary or wrong correction. Decision-making and diagnosis – recognition-primed decision making for familiar deviations (matching the current pattern to a known cause-and-fix from experience) versus slower analytical diagnosis for a novel or compound deviation. Response selection and motor execution – selecting and operating the correct control (helped by the compatible mapping and grouping from Part (a)) and confirming the action's effect via the display, closing the perception-cognition-action loop.

Part (c) — Physical Work Environment

Considerations and environmental factors. Three physical-environment stressors are stated and must be characterized together, since the operator is exposed to all three simultaneously at one fixed station, not sequentially: (1) thermal environment – ambient held at or below 4°C to protect product quality, a sustained cold-workplace exposure for a full shift, not an occasional cold-room entry; (2) noise – continuous mechanical noise from the conveyor drive, mixer motor and pneumatic filling system, likely compounded by reflective/hard surfaces typical of a washable food-processing room, which raises reverberant sound levels above what the source noise alone would produce; (3) whole-body vibration (WBV) – transmitted through the floor from the same three sources into a standing operator, a continuous low-frequency exposure rather than a transient one.

Adverse effects on human operators. Cold exposure at or below 4°C for a sustained shift risks localized cooling of the hands (reduced hand-skin temperature below roughly 16°C measurably degrades fine motor dexterity and tactile sensitivity – directly relevant here since Part (a)'s touchscreen and trim-knob operations both need intact hand function), general thermal discomfort and, over a full cold shift with inadequate PPE/breaks, cold stress (shivering, reduced core temperature, elevated frostbite risk at extremities). Sustained noise exposure above the occupational action level (Canadian jurisdictions typically use an 85 dBA/8h criterion with a 3 dB exchange rate) risks permanent noise-induced hearing loss with cumulative exposure, and even sub-threshold continuous noise degrades verbal communication (masking a spoken hand-off between operators) and adds to overall mental workload, which is a direct performance risk on a monitoring task that already taxes attention and working memory (Part (b)). Continuous floor-transmitted WBV is associated with fatigue and, over chronic exposure, musculoskeletal complaints (particularly low-back discomfort in a standing operator); it can also degrade the fine control inputs Part (a)'s panel calls for by inducing a low-level tremor in a standing, unsupported operator. Critically, the three stressors interact: cold-reduced dexterity, noise-elevated workload and vibration-degraded control precision compound on the SAME manual-correction task (Part (a)/(b)), so the combined error risk on a critical colour/density adjustment is higher than any single stressor's effect taken alone – this is the strongest argument for the panel's large, glove-compatible hard controls and redundant coding recommended in Part (a), and it should also drive mandatory scheduled warm-up breaks, hearing protection and anti-fatigue/vibration-damping flooring at the operator's station.