Structures, Mechanisms, Control and Systems
This domain tests whether you can analyse simple structures and mechanisms and describe control systems using input–process–output thinking. Questions ask you to identify forces, calculate moments or ratios, explain triangulation, and distinguish open-loop from closed-loop control.
Core rules
- Structural forces: tension, compression, shear, bending and torsion. A body is in equilibrium when forces and moments balance.
- Triangulation resists distortion because a triangle is rigid under load; bracing converts a rectangle into triangles.
- Moments: moment = force × perpendicular distance from the pivot. Levers are class 1, 2 or 3 depending on pivot, load and effort order.
- Mechanisms: gears, pulleys, belts, cams and linkages change speed, torque or motion type. Velocity ratio for a gear pair = driven teeth ÷ driver teeth; ideal torque is multiplied by the same ratio.
- Systems: input, process, output. Open-loop control has no feedback; closed-loop uses a sensor and feedback to reduce error.
Worked example
A 12-tooth driver gear drives a 36-tooth gear. Velocity ratio = 36 ÷ 12 = 3. The driven gear turns at one-third of the driver's speed and, ignoring friction, delivers three times the torque. Reasoning: more teeth means a larger pitch circle, so the same tangential force acts at a larger radius, increasing torque. If the output must reverse direction at a set position, a limit switch and controller form closed-loop control; a timer alone would be open-loop and would drift.
Traps
- Inverting the gear ratio or forgetting that direction reverses in a simple gear pair.
- Using the slant distance instead of the perpendicular distance in a moment calculation.
- Confusing a sensor (input) with an actuator (output).
- Calling a timed system closed-loop when it has no feedback.