Question 1 of 7: Controls — Human Factors, Coding, and the Control-Response Ratio

Question 1: Controls — Human Factors, Coding, and the Control-Response Ratio (20 marks: i–6, ii–7, iii–7)

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.

(i) Factors That Influence Human Control

How well an operator can control a machine depends jointly on the control device, the task, and the operator. The control-device factors are the type of control chosen for the required movement (rotary knob, lever, pedal, joystick, pushbutton), its physical dimensions and resistance (spring, friction, viscous, or inertial), and whether its direction of motion matches population stereotypes (clockwise-to-increase, up-for-on, right-for-right) — a mismatch increases error rate and response time regardless of skill. The display/feedback relationship matters equally: the control-response (C/R) ratio governs the trade-off between speed and precision (part iii), and the operator needs adequate visual, auditory, or kinesthetic (proprioceptive) feedback confirming the control has moved and the system has responded. System-dynamics factors — whether the control produces a position, rate, or acceleration response, and how much lag exists between input and observable output — strongly affect controllability; a high-order (acceleration) control with long lag is intrinsically harder to track than a simple position control. Finally, operator factors (training, fatigue, anthropometric fit of hand/foot to the control, and environmental stressors such as vibration, cold, or gloves that reduce tactile sensitivity and dexterity) set the practical ceiling on achievable control quality.

(ii) Primary Coding Methods for Control Identification

Controls are coded so an operator can identify the correct one quickly, by feel if necessary, and without reading a label under time pressure. Shape coding gives each control a distinct, identifiable-by-touch shape (e.g., the different aircraft-cockpit knob shapes for flaps vs. landing gear) so the correct control can be found without looking. Size coding uses two or three discriminable diameters/lengths so controls can be told apart by feel alone; more than about three sizes are not reliably discriminated. Colour coding gives fast visual identification and can carry a meaning (red = stop/emergency, green = start) but is useless in low light or for colour-deficient operators and needs a redundant code. Location/position coding exploits a fixed, memorised spatial arrangement of controls (always place the same function in the same relative position across a product line) so the operator's motor memory finds it without visual search. Operation-method (mode-of-movement) coding distinguishes controls by how they are actuated — a rotary twist vs. a push vs. a two-stage push-then-turn — which also helps prevent accidental activation of critical controls. Label/legend coding is the most direct but slowest, requiring visual attention and adequate light and print size. In practice, critical controls use combination (redundant) coding — e.g., shape and colour and position together — so that if one code fails the others still let the operator find the right control.

(iii) The Optimum Control-Response (C/R) Ratio

The C/R ratio (also written C/D, control-display ratio) is the ratio of the distance the control moves to the resulting distance the display pointer, cursor, or system output moves: C/R = (control movement)/(display or output movement). It measures the "gain" of the control loop and governs a fundamental speed/precision trade-off in acquiring a target value.

At a low C/R ratio, a small control movement produces a large display/output movement (high "gain"). Coarse positioning is fast — little physical travel is needed — but the same small control movement produces a large output change, so it is easy to overshoot the target and slow, careful fine-adjustment time is added. At a high C/R ratio, a large control movement is needed to produce the same output change (low gain): coarse travel time is longer, but each small correction moves the output only a little, so precise fine-tuning is fast and overshoot is rare. Because travel time rises and adjustment time falls in opposite directions as C/R increases, their sum (total acquisition time) traces a U-shaped curve with a genuine minimum — the optimum C/R ratio. There is no single universal optimum value: it depends on the type of control (knob, lever, joystick), the display size, the viewing distance, the tolerance to which the output must be set, and any lag in the system, so the optimum for a given control-display combination is established experimentally (e.g., Jenkins & Connor's classic knob and lever studies).